SBCL 1.3.13 User Manual

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This manual is part of the SBCL software system. See the README file for more information.

This manual is largely derived from the manual for the CMUCL system, which was produced at Carnegie Mellon University and later released into the public domain. This manual is in the public domain and is provided with absolutely no warranty. See the COPYING and CREDITS files for more information.

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1 Getting Support and Reporting Bugs

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1.1 Volunteer Support

Your primary source of SBCL support should probably be the mailing list sbcl-help: in addition to other users SBCL developers monitor this list and are available for advice. As an anti-spam measure subscription is required for posting:

Remember that the people answering your question are volunteers, so you stand a much better chance of getting a good answer if you ask a good question.

Before sending mail, check the list archives at either


to see if your question has been answered already. Checking the bug database is also worth it See Reporting Bugs, to see if the issue is already known.

For general advice on asking good questions, see

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1.2 Commercial Support

There is no formal organization developing SBCL, but if you need a paid support arrangement or custom SBCL development, we maintain the list of companies and consultants below. Use it to identify service providers with appropriate skills and interests, and contact them directly.

The SBCL project cannot verify the accuracy of the information or the competence of the people listed, and they have provided their own blurbs below: you must make your own judgement of suitability from the available information - refer to the links they provide, the CREDITS file, mailing list archives, CVS commit messages, and so on. Please feel free to ask for advice on the sbcl-help list.

(At present, no companies or consultants wish to advertise paid support or custom SBCL development in this manual).

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1.3 Reporting Bugs

SBCL uses Launchpad to track bugs. The bug database is available at

Reporting bugs there requires registering at Launchpad. However, bugs can also be reported on the mailing list sbcl-bugs, which is moderated but does not require subscribing.

Simply send email to and the bug will be checked and added to Launchpad by SBCL maintainers.

1.3.1 How to Report Bugs Effectively

Please include enough information in a bug report that someone reading it can reproduce the problem, i.e. don't write

     Subject: apparent bug in PRINT-OBJECT (or *PRINT-LENGTH*?)
     PRINT-OBJECT doesn't seem to work with *PRINT-LENGTH*. Is this a bug?

but instead

     Subject: apparent bug in PRINT-OBJECT (or *PRINT-LENGTH*?)
     In sbcl-1.2.3 running under OpenBSD 4.5 on my Alpha box, when
     I compile and load the file
                                         (LET ((*PRINT-LENGTH* 4))
                                           (PRINT X Y)))))
          X Y)
     then at the command line type
     the program loops endlessly instead of printing the object.

A more in-depth discussion on reporting bugs effectively can be found at

1.3.2 Signal Related Bugs

If you run into a signal related bug, you are getting fatal errors such as signal N is [un]blocked or just hangs, and you want to send a useful bug report then:

  1. Compile SBCL with ldb support (feature :sb-ldb, see base-target-features.lisp-expr) and change #define QSHOW_SIGNAL 0 to #define QSHOW_SIGNAL 1 in src/runtime/runtime.h.
  2. Isolate a smallish test case, run it.
  3. If it just hangs kill it with sigabrt: kill -ABRT <pidof sbcl>.
  4. Print the backtrace from ldb by typing ba.
  5. Attach gdb: gdb -p <pidof sbcl> and get backtraces for all threads: thread apply all ba.
  6. If multiple threads are in play then still in gdb, try to get Lisp backtrace for all threads: thread apply all call backtrace_from_fp($ebp, 100). Substitute $ebp with $rbp on x86-64. The backtraces will appear in the stdout of the SBCL process.
  7. Send a report with the backtraces and the output (both stdout and stderr) produced by SBCL.
  8. Don't forget to include OS and SBCL version.
  9. If available, include information on outcome of the same test with other versions of SBCL, OS, ...

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2 Introduction

SBCL is a mostly-conforming implementation of the ANSI Common Lisp standard. This manual focuses on behavior which is specific to SBCL, not on behavior which is common to all implementations of ANSI Common Lisp.

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2.1 ANSI Conformance

Essentially every type of non-conformance is considered a bug. (The exceptions involve internal inconsistencies in the standard.) See Reporting Bugs.

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2.2 Extensions

SBCL comes with numerous extensions, some in core and some in modules loadable with require. Unfortunately, not all of these extensions have proper documentation yet.

System Definition Tool
asdf is a flexible and popular protocol-oriented system definition tool by Daniel Barlow. see the asdf manual, for more information.
Foreign Function Interface
sb-alien package allows interfacing with C-code, loading shared object files, etc. See Foreign Function Interface.

sb-grovel can be used to partially automate generation of foreign function interface definitions. See sb-grovel.

Recursive Event Loop
SBCL provides a recursive event loop (serve-event) for doing non-blocking IO on multiple streams without using threads.
Metaobject Protocol
sb-mop package provides a metaobject protocol for the Common Lisp Object System as described in Art of Metaobject Protocol.
Extensible Sequences
SBCL allows users to define subclasses of the sequence class. See Extensible Sequences.
Native Threads
SBCL has native threads on x86/Linux, capable of taking advantage of SMP on multiprocessor machines. See Threading.
Network Interface
sb-bsd-sockets is a low-level networking interface, providing both TCP and UDP sockets. See Networking.
Introspective Facilities
sb-introspect module offers numerous introspective extensions, including access to function lambda-lists and a cross referencing facility.
Operating System Interface
sb-ext contains a number of functions for running external processes, accessing environment variables, etc.

sb-posix module provides a lispy interface to standard POSIX facilities.

Extensible Streams
sb-gray is an implementation of Gray Streams. See Gray Streams.

sb-simple-streams is an implementation of the simple streams API proposed by Franz Inc. See Simple Streams.

sb-profile is a exact per-function profiler. See Deterministic Profiler.

sb-sprof is a statistical profiler, capable of call-graph generation and instruction level profiling, which also supports allocation profiling. See Statistical Profiler.

Customization Hooks
SBCL contains a number of extra-standard customization hooks that can be used to tweak the behaviour of the system. See Customization Hooks for Users.

sb-aclrepl provides an Allegro CL -style toplevel for SBCL, as an alternative to the classic CMUCL-style one. See sb-aclrepl.

CLTL2 Compatibility Layer
sb-cltl2 module provides compiler-let and environment access functionality described in Common Lisp The Language, 2nd Edition which were removed from the language during the ANSI standardization process.
Executable Delivery
The :executable argument to Function sb-ext:save-lisp-and-die can produce a `standalone' executable containing both an image of the current Lisp session and an SBCL runtime.
Bitwise Rotation
sb-rotate-byte provides an efficient primitive for bitwise rotation of integers, an operation required by e.g. numerous cryptographic algorithms, but not available as a primitive in ANSI Common Lisp. See sb-rotate-byte.
Test Harness
sb-rt module is a simple yet attractive regression and unit-test framework.
MD5 Sums
sb-md5 is an implementation of the MD5 message digest algorithm for Common Lisp, using the modular arithmetic optimizations provided by SBCL. See sb-md5.

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2.3 Idiosyncrasies

The information in this section describes some of the ways that SBCL deals with choices that the ANSI standard leaves to the implementation.

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2.3.1 Declarations

Declarations are generally treated as assertions. This general principle, and its implications, and the bugs which still keep the compiler from quite satisfying this principle, are discussed in Declarations as Assertions.

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2.3.2 FASL Format

SBCL fasl-format is binary compatible only with the exact SBCL version it was generated with. While this is obviously suboptimal, it has proven more robust than trying to maintain fasl compatibility across versions: accidentally breaking things is far too easy, and can lead to hard to diagnose bugs.

The following snippet handles fasl recompilation automatically for ASDF-based systems, and makes a good candidate for inclusion in the user or system initialization file (see Initialization Files.)

     (require :asdf)
     ;;; If a fasl was stale, try to recompile and load (once).
     (defmethod asdf:perform :around ((o asdf:load-op)
                                      (c asdf:cl-source-file))
        (handler-case (call-next-method o c)
           ;; If a fasl was stale, try to recompile and load (once).
           (sb-ext:invalid-fasl ()
              (asdf:perform (make-instance 'asdf:compile-op) c)

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2.3.3 Compiler-only Implementation

SBCL is essentially a compiler-only implementation of Common Lisp. That is, for all but a few special cases, eval creates a lambda expression, calls compile on the lambda expression to create a compiled function, and then calls funcall on the resulting function object. A more traditional interpreter is also available on default builds; it is usually only called internally. This is explicitly allowed by the ANSI standard, but leads to some oddities; e.g. at default settings, functionp and compiled-function-p are equivalent, and they collapse into the same function when SBCL is built without the interpreter.

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2.3.4 Defining Constants

SBCL is quite strict about ANSI's definition of defconstant. ANSI says that doing defconstant of the same symbol more than once is undefined unless the new value is eql to the old value. Conforming to this specification is a nuisance when the “constant” value is only constant under some weaker test like string= or equal.

It's especially annoying because, in SBCL, defconstant takes effect not only at load time but also at compile time, so that just compiling and loading reasonable code like

     (defconstant +foobyte+ '(1 4))

runs into this undefined behavior. Many implementations of Common Lisp try to help the programmer around this annoyance by silently accepting the undefined code and trying to do what the programmer probably meant.

SBCL instead treats the undefined behavior as an error. Often such code can be rewritten in portable ANSI Common Lisp which has the desired behavior. E.g., the code above can be given an exactly defined meaning by replacing defconstant either with defparameter or with a customized macro which does the right thing, e.g.

     (defmacro define-constant (name value &optional doc)
       `(defconstant ,name (if (boundp ',name) (symbol-value ',name) ,value)
                           ,@(when doc (list doc))))

or possibly along the lines of the defconstant-eqx macro used internally in the implementation of SBCL itself. In circumstances where this is not appropriate, the programmer can handle the condition type sb-ext:defconstant-uneql, and choose either the continue or abort restart as appropriate.

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2.3.5 Style Warnings

SBCL gives style warnings about various kinds of perfectly legal code, e.g.

This causes friction with people who point out that other ways of organizing code (especially avoiding the use of defgeneric) are just as aesthetically stylish. However, these warnings should be read not as “warning, bad aesthetics detected, you have no style” but “warning, this style keeps the compiler from understanding the code as well as you might like.” That is, unless the compiler warns about such conditions, there's no way for the compiler to warn about some programming errors which would otherwise be easy to overlook. (Related bug: The warning about multiple defuns is pointlessly annoying when you compile and then load a function containing defun wrapped in eval-when, and ideally should be suppressed in that case, but still isn't as of SBCL 0.7.6.)

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2.4 Development Tools

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2.4.1 Editor Integration

Though SBCL can be used running “bare”, the recommended mode of development is with an editor connected to SBCL, supporting not only basic lisp editing (paren-matching, etc), but providing among other features an integrated debugger, interactive compilation, and automated documentation lookup.

Currently SLIME1 (Superior Lisp Interaction Mode for Emacs) together with Emacs is recommended for use with SBCL, though other options exist as well.

SLIME can be downloaded from

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2.4.2 Language Reference

CLHS (Common Lisp Hyperspec) is a hypertext version of the ANSI standard, made freely available by LispWorks – an invaluable reference.


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2.4.3 Generating Executables

SBCL can generate stand-alone executables. The generated executables include the SBCL runtime itself, so no restrictions are placed on program functionality. For example, a deployed program can call compile and load, which requires the compiler to be present in the executable. For further information, See Function sb-ext:save-lisp-and-die.

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2.5 More SBCL Information

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2.5.1 SBCL Homepage

The SBCL website at has some general information, plus links to mailing lists devoted to SBCL, and to archives of these mailing lists. Subscribing to the mailing lists sbcl-help and sbcl-announce is recommended: both are fairly low-volume, and help you keep abreast with SBCL development.

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2.5.2 Online Documentation

Documentation for non-ANSI extensions for various commands is available online from the SBCL executable itself. The extensions for functions which have their own command prompts (e.g. the debugger, and inspect) are documented in text available by typing help at their command prompts. The extensions for functions which don't have their own command prompt (such as trace) are described in their documentation strings, unless your SBCL was compiled with an option not to include documentation strings, in which case the documentation strings are only readable in the source code.

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2.5.3 Additional Documentation Files

Besides this user manual both SBCL source and binary distributions include some other SBCL-specific documentation files, which should be installed along with this manual on your system, e.g. in /usr/local/share/doc/sbcl/.

Licence and copyright summary.
Authorship information on various parts of SBCL.
Covers installing SBCL from both source and binary distributions on your system, and also has some installation related troubleshooting information.
Summarizes changes between various SBCL versions.

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2.5.4 Internals Documentation

If you're interested in the development of the SBCL system itself, then subscribing to sbcl-devel is a good idea.

SBCL internals documentation – besides comments in the source – is currently maintained as a wiki-like website:

Some low-level information describing the programming details of the conversion from CMUCL to SBCL is available in the doc/FOR-CMUCL-DEVELOPERS file in the SBCL distribution, though it is not installed by default.

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2.6 More Common Lisp Information

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2.6.1 Internet Community

The Common Lisp internet community is fairly diverse: news://comp.lang.lisp is fairly high volume newsgroup, but has a rather poor signal/noise ratio. Various special interest mailing lists and IRC tend to provide more content and less flames. and contain numerous pointers places in the net where lispers talks shop.

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2.6.2 Third-party Libraries

For a wealth of information about free Common Lisp libraries and tools we recommend checking out CLiki:

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2.6.3 Common Lisp Books

If you're not a programmer and you're trying to learn, many introductory Lisp books are available. However, we don't have any standout favorites. If you can't decide, try checking the Usenet news://comp.lang.lisp FAQ for recent recommendations.

If you are an experienced programmer in other languages but need to learn about Common Lisp, some books stand out:

Practical Common Lisp, by Peter Seibel
An excellent introduction to the language, covering both the basics and “advanced topics” like macros, CLOS, and packages. Available both in print format and on the web:
Paradigms Of Artificial Intelligence Programming, by Peter Norvig
Good information on general Common Lisp programming, and many nontrivial examples. Whether or not your work is AI, it's a very good book to look at.
On Lisp, by Paul Graham
An in-depth treatment of macros, but not recommended as a first Common Lisp book, since it is slightly pre-ANSI so you need to be on your guard against non-standard usages, and since it doesn't really even try to cover the language as a whole, focusing solely on macros. Downloadable from
Object-Oriented Programming In Common Lisp, by Sonya Keene
With the exception of Practical Common Lisp most introductory books don't emphasize CLOS. This one does. Even if you're very knowledgeable about object oriented programming in the abstract, it's worth looking at this book if you want to do any OO in Common Lisp. Some abstractions in CLOS (especially multiple dispatch) go beyond anything you'll see in most OO systems, and there are a number of lesser differences as well. This book tends to help with the culture shock.
Art Of Metaobject Programming, by Gregor Kiczales et al.
Currently the prime source of information on the Common Lisp Metaobject Protocol, which is supported by SBCL. Section 2 (Chapters 5 and 6) are freely available at

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2.7 History and Implementation of SBCL

You can work productively with SBCL without knowing or understanding anything about where it came from, how it is implemented, or how it extends the ANSI Common Lisp standard. However, a little knowledge can be helpful in order to understand error messages, to troubleshoot problems, to understand why some parts of the system are better debugged than others, and to anticipate which known bugs, known performance problems, and missing extensions are likely to be fixed, tuned, or added.

SBCL is descended from CMUCL, which is itself descended from Spice Lisp, including early implementations for the Mach operating system on the IBM RT, back in the 1980s. Some design decisions from that time are still reflected in the current implementation:

SBCL also inherited some newer architectural features from CMUCL. The most important is that on some architectures it has a generational garbage collector (“GC”), which has various implications (mostly good) for performance. These are discussed in another chapter, Efficiency.

SBCL has diverged from CMUCL in that SBCL is now essentially a “compiler-only implementation” of Common Lisp. This is a change in implementation strategy, taking advantage of the freedom “any of these facilities might share the same execution strategy” guaranteed in the ANSI specification section 3.1 (“Evaluation”). It does not mean SBCL can't be used interactively, and in fact the change is largely invisible to the casual user, since SBCL still can and does execute code interactively by compiling it on the fly. (It is visible if you know how to look, like using compiled-function-p; and it is visible in the way that SBCL doesn't have many bugs which behave differently in interpreted code than in compiled code.) What it means is that in SBCL, the eval function only truly “interprets” a few easy kinds of forms, such as symbols which are boundp. More complicated forms are evaluated by calling compile and then calling funcall on the returned result.

The direct ancestor of SBCL is the x86 port of CMUCL. This port was in some ways the most cobbled-together of all the CMUCL ports, since a number of strange changes had to be made to support the register-poor x86 architecture. Some things (like tracing and debugging) do not work particularly well there. SBCL should be able to improve in these areas (and has already improved in some other areas), but it takes a while.

On the x86 SBCL – like the x86 port of CMUCL – uses a conservative GC. This means that it doesn't maintain a strict separation between tagged and untagged data, instead treating some untagged data (e.g. raw floating point numbers) as possibly-tagged data and so not collecting any Lisp objects that they point to. This has some negative consequences for average time efficiency (though possibly no worse than the negative consequences of trying to implement an exact GC on a processor architecture as register-poor as the X86) and also has potentially unlimited consequences for worst-case memory efficiency. In practice, conservative garbage collectors work reasonably well, not getting anywhere near the worst case. But they can occasionally cause odd patterns of memory usage.

The fork from CMUCL was based on a major rewrite of the system bootstrap process. CMUCL has for many years tolerated a very unusual “build” procedure which doesn't actually build the complete system from scratch, but instead progressively overwrites parts of a running system with new versions. This quasi-build procedure can cause various bizarre bootstrapping hangups, especially when a major change is made to the system. It also makes the connection between the current source code and the current executable more tenuous than in other software systems – it's easy to accidentally “build” a CMUCL system containing characteristics not reflected in the current version of the source code.

Other major changes since the fork from CMUCL include

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3 Starting and Stopping

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3.1 Starting SBCL

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3.1.1 From Shell to Lisp

To run SBCL type sbcl at the command line.

You should end up in the toplevel REPL (read, eval, print -loop), where you can interact with SBCL by typing expressions.

     $ sbcl
     This is SBCL, an implementation of ANSI Common Lisp.
     More information about SBCL is available at <>.
     SBCL is free software, provided as is, with absolutely no warranty.
     It is mostly in the public domain; some portions are provided under
     BSD-style licenses.  See the CREDITS and COPYING files in the
     distribution for more information.
     * (+ 2 2)
     * (exit)

See also Command Line Options and Stopping SBCL.

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3.1.2 Running from Emacs

To run SBCL as an inferior-lisp from Emacs in your .emacs do something like:

     ;;; The SBCL binary and command-line arguments
     (setq inferior-lisp-program "/usr/local/bin/sbcl --noinform")

For more information on using SBCL with Emacs, see Editor Integration.

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3.1.3 Shebang Scripts

Standard Unix tools that are interpreters follow a common command line protocol that is necessary to work with “shebang scripts”. SBCL supports this via the --script command line option.

Example file (hello.lisp):

     #!/usr/local/bin/sbcl --script
     (write-line "Hello, World!")

Usage examples:

     $ ./hello.lisp
     Hello, World!
     $ sbcl --script hello.lisp
     Hello, World!

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3.2 Stopping SBCL

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3.2.1 Exit

SBCL can be stopped at any time by calling sb-ext:exit, optionally returning a specified numeric value to the calling process. See Threading for information about terminating individual threads.

— Function: exit [sb-ext] &key code abort timeout

Terminates the process, causing sbcl to exit with code. code defaults to 0 when abort is false, and 1 when it is true.

When abort is false (the default), current thread is first unwound, *exit-hooks* are run, other threads are terminated, and standard output streams are flushed before sbcl calls exit(3) -- at which point atexit(3) functions will run. If multiple threads call exit with abort being false, the first one to call it will complete the protocol.

When abort is true, sbcl exits immediately by calling _exit(2) without unwinding stack, or calling exit hooks. Note that _exit(2) does not call atexit(3) functions unlike exit(3).

Recursive calls to exit cause exit to behave as it abort was true.

timeout controls waiting for other threads to terminate when abort is nil. Once current thread has been unwound and *exit-hooks* have been run, spawning new threads is prevented and all other threads are terminated by calling terminate-thread on them. The system then waits for them to finish using join-thread, waiting at most a total timeout seconds for all threads to join. Those threads that do not finish in time are simply ignored while the exit protocol continues. timeout defaults to *exit-timeout*, which in turn defaults to 60. timeout nil means to wait indefinitely.

Note that timeout applies only to join-thread, not *exit-hooks*. Since terminate-thread is asynchronous, getting multithreaded application termination with complex cleanups right using it can be tricky. To perform an orderly synchronous shutdown use an exit hook instead of relying on implicit thread termination.

Consequences are unspecified if serious conditions occur during exit excepting errors from *exit-hooks*, which cause warnings and stop execution of the hook that signaled, but otherwise allow the exit process to continue normally.

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3.2.2 End of File

By default SBCL also exits on end of input, caused either by user pressing Control-D on an attached terminal, or end of input when using SBCL as part of a shell pipeline.

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3.2.3 Saving a Core Image

SBCL has the ability to save its state as a file for later execution. This functionality is important for its bootstrapping process, and is also provided as an extension to the user.

— Function: save-lisp-and-die [sb-ext] core-file-name &key toplevel executable save-runtime-options purify root-structures environment-name compression

Save a "core image", i.e. enough information to restart a Lisp process later in the same state, in the file of the specified name. Only global state is preserved: the stack is unwound in the process.

The following &key arguments are defined:

The function to run when the created core file is resumed. The default function handles command line toplevel option processing and runs the top level read-eval-print loop. This function returning is equivalent to (sb-ext:exit :code 0) being called.

toplevel functions should always provide an abort restart: otherwise code they call will run without one.

If true, arrange to combine the sbcl runtime and the core image to create a standalone executable. If false (the default), the core image will not be executable on its own. Executable images always behave as if they were passed the –noinform runtime option.
If true, values of runtime options –dynamic-space-size and –control-stack-size that were used to start sbcl are stored in the standalone executable, and restored when the executable is run. This also inhibits normal runtime option processing, causing all command line arguments to be passed to the toplevel. Meaningless if :executable is nil.
If true (the default on cheneygc), do a purifying gc which moves all dynamically allocated objects into static space. This takes somewhat longer than the normal gc which is otherwise done, but it's only done once, and subsequent GC's will be done less often and will take less time in the resulting core file. See the purify function. This parameter has no effect on platforms using the generational garbage collector.
This should be a list of the main entry points in any newly loaded systems. This need not be supplied, but locality and/or gc performance may be better if they are. This has two different but related meanings: If :purify is true - and only for cheneygc - the root structures are those which anchor the set of objects moved into static space. On gencgc - and only on platforms supporting immobile code - these are the functions and/or function-names which commence a depth-first scan of code when reordering based on the statically observable call chain. The complete set of reachable objects is not affected per se. This argument is meaningless if neither enabling precondition holds.
This has no purpose; it is accepted only for legacy compatibility.
This is only meaningful if the runtime was built with the :sb-core-compression feature enabled. If nil (the default), saves to uncompressed core files. If :sb-core-compression was enabled at build-time, the argument may also be an integer from -1 to 9, corresponding to zlib compression levels, or t (which is equivalent to the default compression level, -1).
Present only on Windows and is meaningful only with :executable t. Specifies the subsystem of the executable, :console or :gui. The notable difference is that :gui doesn't automatically create a console window. The default is :console.

The save/load process changes the values of some global variables:

*standard-output*, *debug-io*, etc.
Everything related to open streams is necessarily changed, since the os won't let us preserve a stream across save and load.
This is reinitialized to reflect the working directory where the saved core is loaded.

save-lisp-and-die interacts with sb-alien:load-shared-object: see its documentation for details.

On threaded platforms only a single thread may remain running after sb-ext:*save-hooks* have run. Applications using multiple threads can be save-lisp-and-die friendly by registering a save-hook that quits any additional threads, and an init-hook that restarts them.

This implementation is not as polished and painless as you might like:

This isn't because we like it this way, but just because there don't seem to be good quick fixes for either limitation and no one has been sufficiently motivated to do lengthy fixes.

— Variable: *save-hooks* [sb-ext]

A list of function designators which are called in an unspecified order before creating a saved core image.

Unused by sbcl itself: reserved for user and applications.

In cases where the standard initialization files have already been loaded into the saved core, and alternative ones should be used (or none at all), SBCL allows customizing the initfile pathname computation.

— Variable: *sysinit-pathname-function* [sb-ext]

Designator for a function of zero arguments called to obtain a pathname designator for the default sysinit file, or nil. If the function returns nil, no sysinit file is used unless one has been specified on the command-line.

— Variable: *userinit-pathname-function* [sb-ext]

Designator for a function of zero arguments called to obtain a pathname designator or a stream for the default userinit file, or nil. If the function returns nil, no userinit file is used unless one has been specified on the command-line.

To facilitate distribution of SBCL applications using external resources, the filesystem location of the SBCL core file being used is available from Lisp.

— Variable: *core-pathname* [sb-ext]

The absolute pathname of the running sbcl core.

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3.2.4 Exit on Errors

SBCL can also be configured to exit if an unhandled error occurs, which is mainly useful for acting as part of a shell pipeline; doing so under most other circumstances would mean giving up large parts of the flexibility and robustness of Common Lisp. See Debugger Entry.

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3.3 Command Line Options

Command line options can be considered an advanced topic; for ordinary interactive use, no command line arguments should be necessary.

In order to understand the command line argument syntax for SBCL, it is helpful to understand that the SBCL system is implemented as two components, a low-level runtime environment written in C and a higher-level system written in Common Lisp itself. Some command line arguments are processed during the initialization of the low-level runtime environment, some command line arguments are processed during the initialization of the Common Lisp system, and any remaining command line arguments are passed on to user code.

The full, unambiguous syntax for invoking SBCL at the command line is:

sbcl runtime-option* --end-runtime-options toplevel-option* --end-toplevel-options user-options*

For convenience, the --end-runtime-options and --end-toplevel-options elements can be omitted. Omitting these elements can be convenient when you are running the program interactively, and you can see that no ambiguities are possible with the option values you are using. Omitting these elements is probably a bad idea for any batch file where any of the options are under user control, since it makes it impossible for SBCL to detect erroneous command line input, so that erroneous command line arguments will be passed on to the user program even if they was intended for the runtime system or the Lisp system.

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3.3.1 Runtime Options

--core corefilename
Run the specified Lisp core file instead of the default. Note that if the Lisp core file is a user-created core file, it may run a nonstandard toplevel which does not recognize the standard toplevel options.
--dynamic-space-size megabytes
Size of the dynamic space reserved on startup in megabytes. Default value is platform dependent.
--control-stack-size megabytes
Size of control stack reserved for each thread in megabytes. Default value is 2.
Suppress the printing of any banner or other informational message at startup. This makes it easier to write Lisp programs which work cleanly in Unix pipelines. See also the --noprint and --disable-debugger options.
Disable the low-level debugger. Only effective if SBCL is compiled with LDB.
There are some dangerous low level errors (for instance, control stack exhausted, memory fault) that (or whose handlers) can corrupt the image. By default SBCL prints a warning, then tries to continue and handle the error in Lisp, but this will not always work and SBCL may malfunction or even hang. With this option, upon encountering such an error SBCL will invoke ldb (if present and enabled) or else exit.
--script filename
As a runtime option this is equivalent to --noinform --disable-ldb --lose-on-corruption --end-runtime-options --script filename. See the description of --script as a toplevel option below. If there are no other command line arguments following --script, the filename argument can be omitted.
When platform support is present, provide hints to the operating system that identical pages may be shared between processes until they are written to. This can be useful to reduce the memory usage on systems with multiple SBCL processes started from similar but differently-named core files, or from compressed cores. Without platform support, do nothing.
Ensures that no sharing hint is provided to the operating system.
Reverts the sharing hint policy to the default: only compressed cores trigger hinting. Uncompressed cores are mapped directly from the core file, which is usually enough to ensure sharing.
Print some basic information about SBCL, then exit.
Print SBCL's version information, then exit.

In the future, runtime options may be added to control behaviour such as lazy allocation of memory.

Runtime options, including any –end-runtime-options option, are stripped out of the command line before the Lisp toplevel logic gets a chance to see it.

Previous: Runtime Options, Up: Command Line Options

3.3.2 Toplevel Options

--sysinit filename
Load filename instead of the default system initialization file (see Initialization Files.)
Don't load a system-wide initialization file. If this option is given, the --sysinit option is ignored.
--userinit filename
Load filename instead of the default user initialization file (see Initialization Files.)
Don't load a user initialization file. If this option is given, the --userinit option is ignored.
--eval command
After executing any initialization file, but before starting the read-eval-print loop on standard input, read and evaluate the command given. More than one --eval option can be used, and all will be read and executed, in the order they appear on the command line.
--load filename
This is equivalent to --eval '(load "filename")'. The special syntax is intended to reduce quoting headaches when invoking SBCL from shell scripts.
When ordinarily the toplevel "read-eval-print loop" would be executed, execute a "read-eval loop" instead, i.e. don't print a prompt and don't echo results. Combined with the --noinform runtime option, this makes it easier to write Lisp "scripts" which work cleanly in Unix pipelines.
By default when SBCL encounters an error, it enters the builtin debugger, allowing interactive diagnosis and possible intercession. This option disables the debugger, causing errors to print a backtrace and exit with status 1 instead. When given, this option takes effect before loading of initialization files or processing --eval and --load options. See sb-ext:disable-debugger for details. See Debugger Entry.
--script filename
Implies --no-userinit --no-sysinit --disable-debugger --end-toplevel-options.

Causes the system to load the specified file instead of entering the read-eval-print-loop, and exit afterwards. If the file begins with a shebang line, it is ignored.

If there are no other command line arguments following, the filename can be omitted: this causes the script to be loaded from standard input instead. Shebang lines in standard input script are currently not ignored.

In either case, if there is an unhandled error (e.g. end of file, or a broken pipe) on either standard input, standard output, or standard error, the script silently exits with code 0. This allows e.g. safely piping output from SBCL to head -n1 or similar.

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3.4 Initialization Files

SBCL processes initialization files with read and eval, not load; hence initialization files can be used to set startup *package* and *readtable*, and for proclaiming a global optimization policy.

Neither initialization file is required.

Previous: Initialization Files, Up: Starting and Stopping

3.5 Initialization and Exit Hooks

SBCL provides hooks into the system initialization and exit.

— Variable: *init-hooks* [sb-ext]

A list of function designators which are called in an unspecified order when a saved core image starts up, after the system itself has been initialized.

Unused by sbcl itself: reserved for user and applications.

— Variable: *exit-hooks* [sb-ext]

A list of function designators which are called in an unspecified order when sbcl process exits.

Unused by sbcl itself: reserved for user and applications.

Using (sb-ext:exit :abort t), or calling exit(3) directly circumvents these hooks.

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4 Compiler

This chapter will discuss most compiler issues other than efficiency, including compiler error messages, the SBCL compiler's unusual approach to type safety in the presence of type declarations, the effects of various compiler optimization policies, and the way that inlining and open coding may cause optimized code to differ from a naive translation. Efficiency issues are sufficiently varied and separate that they have their own chapter, Efficiency.

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4.1 Diagnostic Messages

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4.1.1 Controlling Verbosity

The compiler can be quite verbose in its diagnostic reporting, rather more then some users would prefer – the amount of noise emitted can be controlled, however.

To control emission of compiler diagnostics (of any severity other than error: see Diagnostic Severity) use the sb-ext:muffle-conditions and sb-ext:unmuffle-conditions declarations, specifying the type of condition that is to be muffled (the muffling is done using an associated muffle-warning restart).

Global control:

     ;;; Muffle compiler-notes globally
     (declaim (sb-ext:muffle-conditions sb-ext:compiler-note))

Local control:

     ;;; Muffle compiler-notes based on lexical scope
     (defun foo (x)
       (declare (optimize speed) (fixnum x)
                (sb-ext:muffle-conditions sb-ext:compiler-note))
       (values (* x 5) ; no compiler note from this
           (declare (sb-ext:unmuffle-conditions sb-ext:compiler-note))
           ;; this one gives a compiler note
           (* x -5))))
— Declaration: muffle-conditions [sb-ext]

Syntax: type*

Muffles the diagnostic messages that would be caused by compile-time signals of given types.

— Declaration: unmuffle-conditions [sb-ext]

Syntax: type*

Cancels the effect of a previous sb-ext:muffle-conditions declaration.

Various details of how the compiler messages are printed can be controlled via the alist sb-ext:*compiler-print-variable-alist*.

— Variable: *compiler-print-variable-alist* [sb-ext]

an association list describing new bindings for special variables to be used by the compiler for error-reporting, etc. Eg.

           ((*PRINT-LENGTH* . 10) (*PRINT-LEVEL* . 6) (*PRINT-PRETTY* . NIL))

The variables in the car positions are bound to the values in the cdr during the execution of some debug commands. When evaluating arbitrary expressions in the debugger, the normal values of the printer control variables are in effect.

Initially empty, *compiler-print-variable-alist* is Typically used to specify bindings for printer control variables.

For information about muffling warnings signaled outside of the compiler, see Customization Hooks for Users.

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4.1.2 Diagnostic Severity

There are four levels of compiler diagnostic severity:

  1. error
  2. warning
  3. style warning
  4. note

The first three levels correspond to condition classes which are defined in the ANSI standard for Common Lisp and which have special significance to the compile and compile-file functions. These levels of compiler error severity occur when the compiler handles conditions of these classes.

The fourth level of compiler error severity, note, corresponds to the sb-ext:compiler-note, and is used for problems which are too mild for the standard condition classes, typically hints about how efficiency might be improved. The sb-ext:code-deletion-note, a subtype of compiler-note, is signalled when the compiler deletes user-supplied code after proving that the code in question is unreachable.

Future work for SBCL includes expanding this hierarchy of types to allow more fine-grained control over emission of diagnostic messages.

— Condition: compiler-note [sb-ext]

Class precedence list: compiler-note, condition, t

Root of the hierarchy of conditions representing information discovered by the compiler that the user might wish to know, but which does not merit a style-warning (or any more serious condition).

— Condition: code-deletion-note [sb-ext]

Class precedence list: code-deletion-note, compiler-note, condition, t

A condition type signalled when the compiler deletes code that the user has written, having proved that it is unreachable.

Previous: Diagnostic Severity, Up: Diagnostic Messages

4.1.3 Understanding Compile Diagnostics

The messages emitted by the compiler contain a lot of detail in a terse format, so they may be confusing at first. The messages will be illustrated using this example program:

     (defmacro zoq (x)
       `(roq (ploq (+ ,x 3))))
     (defun foo (y)
       (declare (symbol y))
       (zoq y))

The main problem with this program is that it is trying to add 3 to a symbol. Note also that the functions roq and ploq aren't defined anywhere.

Next: , Up: Understanding Compiler Diagnostics The Parts of a Compiler Diagnostic

When processing this program, the compiler will produce this warning:

     ; file: /tmp/foo.lisp
     ; in: DEFUN FOO
     ;     (ZOQ Y)
     ; --> ROQ PLOQ
     ; ==>
     ;   (+ Y 3)
     ; caught WARNING:
     ;   Asserted type NUMBER conflicts with derived type (VALUES SYMBOL &OPTIONAL).

In this example we see each of the six possible parts of a compiler diagnostic:

  1. file: /tmp/foo.lisp’ This is the name of the file that the compiler read the relevant code from. The file name is displayed because it may not be immediately obvious when there is an error during compilation of a large system, especially when with-compilation-unit is used to delay undefined warnings.
  2. in: DEFUN FOO’ This is the definition top level form responsible for the diagnostic. It is obtained by taking the first two elements of the enclosing form whose first element is a symbol beginning with “‘def’”. If there is no such enclosing “‘def’” form, then the outermost form is used. If there are multiple ‘def’ forms, then they are all printed from the outside in, separated by ‘=>’'s. In this example, the problem was in the defun for foo.
  3. (ZOQ Y)’ This is the original source form responsible for the diagnostic. Original source means that the form directly appeared in the original input to the compiler, i.e. in the lambda passed to compile or in the top level form read from the source file. In this example, the expansion of the zoq macro was responsible for the message.
  4. --> ROQ PLOQ’ This is the processing path that the compiler used to produce the code that caused the message to be emitted. The processing path is a representation of the evaluated forms enclosing the actual source that the compiler encountered when processing the original source. The path is the first element of each form, or the form itself if the form is not a list. These forms result from the expansion of macros or source-to-source transformation done by the compiler. In this example, the enclosing evaluated forms are the calls to roq and ploq. These calls resulted from the expansion of the zoq macro.
  5. ==> (+ Y 3)’ This is the actual source responsible for the diagnostic. If the actual source appears in the explanation, then we print the next enclosing evaluated form, instead of printing the actual source twice. (This is the form that would otherwise have been the last form of the processing path.) In this example, the problem is with the evaluation of the reference to the variable y.
  6. caught WARNING: Asserted type NUMBER conflicts with derived type (VALUES SYMBOL &OPTIONAL).’ This is the explanation of the problem. In this example, the problem is that, while the call to + requires that its arguments are all of type number, the compiler has derived that y will evaluate to a symbol. Note that ‘(VALUES SYMBOL &OPTIONAL)’ expresses that y evaluates to precisely one value.

Note that each part of the message is distinctively marked:

Each part of the message is more specific than the preceding one. If consecutive messages are for nearby locations, then the front part of the messages would be the same. In this case, the compiler omits as much of the second message as in common with the first. For example:

     ; file: /tmp/foo.lisp
     ; in: DEFUN FOO
     ;     (ZOQ Y)
     ; --> ROQ
     ; ==>
     ;   (PLOQ (+ Y 3))
     ; caught STYLE-WARNING:
     ;   undefined function: PLOQ
     ; ==>
     ;   (ROQ (PLOQ (+ Y 3)))
     ; caught STYLE-WARNING:
     ;   undefined function: ROQ

In this example, the file, definition and original source are identical for the two messages, so the compiler omits them in the second message. If consecutive messages are entirely identical, then the compiler prints only the first message, followed by: ‘[Last message occurs repeats times]’ where repeats is the number of times the message was given.

If the source was not from a file, then no file line is printed. If the actual source is the same as the original source, then the processing path and actual source will be omitted. If no forms intervene between the original source and the actual source, then the processing path will also be omitted.

Next: , Previous: The Parts of a Compiler Diagnostic, Up: Understanding Compiler Diagnostics The Original and Actual Source

The original source displayed will almost always be a list. If the actual source for an message is a symbol, the original source will be the immediately enclosing evaluated list form. So even if the offending symbol does appear in the original source, the compiler will print the enclosing list and then print the symbol as the actual source (as though the symbol were introduced by a macro.)

When the actual source is displayed (and is not a symbol), it will always be code that resulted from the expansion of a macro or a source-to-source compiler optimization. This is code that did not appear in the original source program; it was introduced by the compiler.

Keep in mind that when the compiler displays a source form in an diagnostic message, it always displays the most specific (innermost) responsible form. For example, compiling this function

     (defun bar (x)
       (let (a)
         (declare (fixnum a))
         (setq a (foo x))

gives this error message

     ; file: /tmp/foo.lisp
     ; in: DEFUN BAR
     ;     (LET (A)
     ;     (DECLARE (FIXNUM A))
     ;     (SETQ A (FOO X))
     ;     A)
     ; caught WARNING:
     ;   Asserted type FIXNUM conflicts with derived type (VALUES NULL &OPTIONAL).

This message is not saying “there is a problem somewhere in this let” – it is saying that there is a problem with the let itself. In this example, the problem is that a's nil initial value is not a fixnum.

Previous: The Original and Actual Source, Up: Understanding Compiler Diagnostics The Processing Path

The processing path is mainly useful for debugging macros, so if you don't write macros, you can probably ignore it. Consider this example:

     (defun foo (n)
       (dotimes (i n *undefined*)))

Compiling results in this error message:

     ; in: DEFUN FOO
     ;     (DOTIMES (I N *UNDEFINED*))
     ; ==>
     ;   (PROGN *UNDEFINED*)
     ; caught WARNING:
     ;   undefined variable: *UNDEFINED*

Note that do appears in the processing path. This is because dotimes expands into:

     (do ((i 0 (1+ i)) (#:g1 n))
         ((>= i #:g1) *undefined*)
       (declare (type unsigned-byte i)))

The rest of the processing path results from the expansion of do:

     (block nil
       (let ((i 0) (#:g1 n))
         (declare (type unsigned-byte i))
         (tagbody (go #:g3)
           #:g2    (psetq i (1+ i))
           #:g3    (unless (>= i #:g1) (go #:g2))
           (return-from nil (progn *undefined*)))))

In this example, the compiler descended into the block, let, tagbody and return-from to reach the progn printed as the actual source. This is a place where the “actual source appears in explanation” rule was applied. The innermost actual source form was the symbol *undefined* itself, but that also appeared in the explanation, so the compiler backed out one level.

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4.2 Handling of Types

One of the most important features of the SBCL compiler (similar to the original CMUCL compiler, also known as Python) is its fairly sophisticated understanding of the Common Lisp type system and its conservative approach to the implementation of type declarations.

These two features reward the use of type declarations throughout development, even when high performance is not a concern. Also, as discussed in the chapter on performance (see Efficiency), the use of appropriate type declarations can be very important for performance as well.

The SBCL compiler also has a greater knowledge of the Common Lisp type system than other compilers. Support is incomplete only for types involving the satisfies type specifier.

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4.2.1 Declarations as Assertions

The SBCL compiler treats type declarations differently from most other Lisp compilers. Under default compilation policy the compiler doesn't blindly believe type declarations, but considers them assertions about the program that should be checked: all type declarations that have not been proven to always hold are asserted at runtime.

Remaining bugs in the compiler's handling of types unfortunately provide some exceptions to this rule, see Implementation Limitations.

CLOS slot types form a notable exception. Types declared using the :type slot option in defclass are asserted if and only if the class was defined in safe code and the slot access location is in safe code as well. This laxness does not pose any internal consistency issues, as the CLOS slot types are not available for the type inferencer, nor do CLOS slot types provide any efficiency benefits.

There are three type checking policies available in SBCL, selectable via optimize declarations.

Full Type Checks
All declarations are considered assertions to be checked at runtime, and all type checks are precise. The default compilation policy provides full type checks.

Used when (or (>= safety 2) (>= safety speed 1)).

Weak Type Checks
Declared types may be simplified into faster to check supertypes: for example, (or (integer -17 -7) (integer 7 17)) is simplified into (integer -17 17).

Note: it is relatively easy to corrupt the heap when weak type checks are used if the program contains type-errors.

Used when (and (< safety 2) (< safety speed))

No Type Checks
All declarations are believed without assertions. Also disables argument count and array bounds checking.

Note: any type errors in code where type checks are not performed are liable to corrupt the heap.

Used when (= safety 0).

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4.2.2 Precise Type Checking

Precise checking means that the check is done as though typep had been called with the exact type specifier that appeared in the declaration.

If a variable is declared to be (integer 3 17) then its value must always be an integer between 3 and 17. If multiple type declarations apply to a single variable, then all the declarations must be correct; it is as though all the types were intersected producing a single and type specifier.

To gain maximum benefit from the compiler's type checking, you should always declare the types of function arguments and structure slots as precisely as possible. This often involves the use of or, member, and other list-style type specifiers.

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4.2.3 Getting Existing Programs to Run

Since SBCL's compiler does much more comprehensive type checking than most Lisp compilers, SBCL may detect type errors in programs that have been debugged using other compilers. These errors are mostly incorrect declarations, although compile-time type errors can find actual bugs if parts of the program have never been tested.

Some incorrect declarations can only be detected by run-time type checking. It is very important to initially compile a program with full type checks (high safety optimization) and then test this safe version. After the checking version has been tested, then you can consider weakening or eliminating type checks. This applies even to previously debugged programs, because the SBCL compiler does much more type inference than other Common Lisp compilers, so an incorrect declaration can do more damage.

The most common problem is with variables whose constant initial value doesn't match the type declaration. Incorrect constant initial values will always be flagged by a compile-time type error, and they are simple to fix once located. Consider this code fragment:

     (prog (foo)
       (declare (fixnum foo))
       (setq foo ...)

Here foo is given an initial value of nil, but is declared to be a fixnum. Even if it is never read, the initial value of a variable must match the declared type. There are two ways to fix this problem. Change the declaration

     (prog (foo)
       (declare (type (or fixnum null) foo))
       (setq foo ...)

or change the initial value

     (prog ((foo 0))
       (declare (fixnum foo))
       (setq foo ...)

It is generally preferable to change to a legal initial value rather than to weaken the declaration, but sometimes it is simpler to weaken the declaration than to try to make an initial value of the appropriate type.

Another declaration problem occasionally encountered is incorrect declarations on defmacro arguments. This can happen when a function is converted into a macro. Consider this macro:

     (defmacro my-1+ (x)
       (declare (fixnum x))
       `(the fixnum (1+ ,x)))

Although legal and well-defined Common Lisp code, this meaning of this definition is almost certainly not what the writer intended. For example, this call is illegal:

     (my-1+ (+ 4 5))

This call is illegal because the argument to the macro is (+ 4 5), which is a list, not a fixnum. Because of macro semantics, it is hardly ever useful to declare the types of macro arguments. If you really want to assert something about the type of the result of evaluating a macro argument, then put a the in the expansion:

     (defmacro my-1+ (x)
       `(the fixnum (1+ (the fixnum ,x))))

In this case, it would be stylistically preferable to change this macro back to a function and declare it inline.

Some more subtle problems are caused by incorrect declarations that can't be detected at compile time. Consider this code:

     (do ((pos 0 (position #\a string :start (1+ pos))))
       ((null pos))
       (declare (fixnum pos))

Although pos is almost always a fixnum, it is nil at the end of the loop. If this example is compiled with full type checks (the default), then running it will signal a type error at the end of the loop. If compiled without type checks, the program will go into an infinite loop (or perhaps position will complain because (1+ nil) isn't a sensible start.) Why? Because if you compile without type checks, the compiler just quietly believes the type declaration. Since the compiler believes that pos is always a fixnum, it believes that pos is never nil, so (null pos) is never true, and the loop exit test is optimized away. Such errors are sometimes flagged by unreachable code notes, but it is still important to initially compile and test any system with full type checks, even if the system works fine when compiled using other compilers.

In this case, the fix is to weaken the type declaration to (or fixnum null) 2.

Note that there is usually little performance penalty for weakening a declaration in this way. Any numeric operations in the body can still assume that the variable is a fixnum, since nil is not a legal numeric argument. Another possible fix would be to say:

     (do ((pos 0 (position #\a string :start (1+ pos))))
         ((null pos))
       (let ((pos pos))
         (declare (fixnum pos))

This would be preferable in some circumstances, since it would allow a non-standard representation to be used for the local pos variable in the loop body.

Previous: Getting Existing Programs to Run, Up: Handling of Types

4.2.4 Implementation Limitations

Ideally, the compiler would consider all type declarations to be assertions, so that adding type declarations to a program, no matter how incorrect they might be, would never cause undefined behavior. However, the compiler is known to fall short of this goal in two areas:

These are important issues, but are not necessarily easy to fix, so they may, alas, remain in the system for a while.

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4.3 Compiler Policy

Compiler policy is controlled by the optimize declaration, supporting all ANSI optimization qualities (debug, safety, space, and speed).3

For effects of various optimization qualities on type-safety and debuggability see Declarations as Assertions and Debugger Policy Control.

Ordinarily, when the speed quality is high, the compiler emits notes to notify the programmer about its inability to apply various optimizations. For selective muffling of these notes See Controlling Verbosity.

The value of space mostly influences the compiler's decision whether to inline operations, which tend to increase the size of programs. Use the value 0 with caution, since it can cause the compiler to inline operations so indiscriminately that the net effect is to slow the program by causing cache misses or even swapping.

— Function: describe-compiler-policy [sb-ext] &optional spec

Print all global optimization settings, augmented by spec.

— Function: restrict-compiler-policy [sb-ext] &optional quality min

Assign a minimum value to an optimization quality. quality is the name of the optimization quality to restrict, and min (defaulting to zero) is the minimum allowed value.

Returns the alist describing the current policy restrictions.

If quality is nil or not given, nothing is done.

Otherwise, if min is zero or not given, any existing restrictions of quality are removed. If min is between one and three inclusive, it becomes the new minimum value for the optimization quality: any future proclamations or declarations of the quality with a value less then min behave as if the value was min instead.

This is intended to be used interactively, to facilitate recompiling large bodies of code with eg. a known minimum safety.

See also :policy option in with-compilation-unit.

experimental interface: Subject to change.

— Macro: with-compilation-unit [cl] options &body body

Affects compilations that take place within its dynamic extent. It is intended to be eg. wrapped around the compilation of all files in the same system.

Following options are defined:

:override Boolean-Form
One of the effects of this form is to delay undefined warnings until the end of the form, instead of giving them at the end of each compilation. If override is nil (the default), then the outermost with-compilation-unit form grabs the undefined warnings. Specifying override true causes that form to grab any enclosed warnings, even if it is enclosed by another with-compilation-unit.
:policy Optimize-Declaration-Form
Provides dynamic scoping for global compiler optimization qualities and restrictions, limiting effects of subsequent optimize proclamations and calls to sb-ext:restrict-compiler-policy to the dynamic scope of body.

If override is false, specified policy is merged with current global policy. If override is true, current global policy, including any restrictions, is discarded in favor of the specified policy.

Supplying policy nil is equivalent to the option not being supplied at all, ie. dynamic scoping of policy does not take place.

This option is an SBCL-specific experimental extension: Interface subject to change.

:source-namestring Namestring-Form
Attaches the value returned by the Namestring-Form to the internal debug-source information as the namestring of the source file. Normally the namestring of the input-file for compile-file is used: this option can be used to provide source-file information for functions compiled using compile, or to override the input-file of compile-file.

If both an outer and an inner with-compilation-unit provide a source-namestring, the inner one takes precedence. Unaffected by :override.

This is an SBCL-specific extension.

:source-plist Plist-Form
Attaches the value returned by the Plist-Form to internal debug-source information of functions compiled in within the dynamic extent of body.

Primarily for use by development environments, in order to eg. associate function definitions with editor-buffers. Can be accessed using sb-introspect:definition-source-plist.

If an outer with-compilation-unit form also provide a source-plist, it is appended to the end of the provided source-plist. Unaffected by :override.

This is an SBCL-specific extension.


            ;; Prevent proclamations from the file leaking, and restrict
            ;; SAFETY to 3 -- otherwise uses the current global policy.
            (with-compilation-unit (:policy '(optimize))
              (restrict-compiler-policy 'safety 3)
              (load "foo.lisp"))
            ;; Using default policy instead of the current global one,
            ;; except for DEBUG 3.
            (with-compilation-unit (:policy '(optimize debug)
                                    :override t)
              (load "foo.lisp"))
            ;; Same as if :POLICY had not been specified at all: SAFETY 3
            ;; proclamation leaks out from WITH-COMPILATION-UNIT.
            (with-compilation-unit (:policy nil)
              (declaim (optimize safety))
              (load "foo.lisp"))

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4.4 Compiler Errors

Next: , Up: Compiler Errors

4.4.1 Type Errors at Compile Time

If the compiler can prove at compile time that some portion of the program cannot be executed without a type error, then it will give a warning at compile time.

It is possible that the offending code would never actually be executed at run-time due to some higher level consistency constraint unknown to the compiler, so a type warning doesn't always indicate an incorrect program.

For example, consider this code fragment:

     (defun raz (foo)
       (let ((x (case foo
                   (:this 13)
                   (:that 9)
                   (:the-other 42))))
         (declare (fixnum x))
         (foo x)))

Compilation produces this warning:

     ; in: DEFUN RAZ
     ;     (CASE FOO (:THIS 13) (:THAT 9) (:THE-OTHER 42))
     ; ==>
     ;   (COND)
     ; caught WARNING:
     ;   This is not a FIXNUM:
     ;   NIL

In this case, the warning means that if foo isn't any of :this, :that or :the-other, then x will be initialized to nil, which the fixnum declaration makes illegal. The warning will go away if ecase is used instead of case, or if :the-other is changed to t.

This sort of spurious type warning happens moderately often in the expansion of complex macros and in inline functions. In such cases, there may be dead code that is impossible to correctly execute. The compiler can't always prove this code is dead (could never be executed), so it compiles the erroneous code (which will always signal an error if it is executed) and gives a warning.

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4.4.2 Errors During Macroexpansion

The compiler handles errors that happen during macroexpansion, turning them into compiler errors. If you want to debug the error (to debug a macro), you can set *break-on-signals* to error. For example, this definition:

     (defun foo (e l)
       (do ((current l (cdr current))
            ((atom current) nil))
           (when (eq (car current) e) (return current))))

gives this error:

     ; in: DEFUN FOO
     ;     (DO ((CURRENT L (CDR CURRENT))
     ;        ((ATOM CURRENT) NIL))
     ; caught ERROR:
     ;   (in macroexpansion of (DO # #))
     ;   (hint: For more precise location, try *BREAK-ON-SIGNALS*.)
     ;   DO step variable is not a symbol: (ATOM CURRENT)

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4.4.3 Read Errors

SBCL's compiler does not attempt to recover from read errors when reading a source file, but instead just reports the offending character position and gives up on the entire source file.

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4.5 Open Coding and Inline Expansion

Since Common Lisp forbids the redefinition of standard functions, the compiler can have special knowledge of these standard functions embedded in it. This special knowledge is used in various ways (open coding, inline expansion, source transformation), but the implications to the user are basically the same:

When a function call is open coded, inline code whose effect is equivalent to the function call is substituted for that function call. When a function call is closed coded, it is usually left as is, although it might be turned into a call to a different function with different arguments. As an example, if nthcdr were to be open coded, then

     (nthcdr 4 foobar)

might turn into

     (cdr (cdr (cdr (cdr foobar))))

or even

     (do ((i 0 (1+ i))
       (list foobar (cdr foobar)))
       ((= i 4) list))

If nth is closed coded, then

     (nth x l)

might stay the same, or turn into something like

     (car (nthcdr x l))

In general, open coding sacrifices space for speed, but some functions (such as car) are so simple that they are always open-coded. Even when not open-coded, a call to a standard function may be transformed into a different function call (as in the last example) or compiled as static call. Static function call uses a more efficient calling convention that forbids redefinition.

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4.6 Interpreter

By default SBCL implements eval by calling the native code compiler.

SBCL also includes an interpreter for use in special cases where using the compiler is undesirable, for example due to compilation overhead. Unlike in some other Lisp implementations, in SBCL interpreted code is not safer or more debuggable than compiled code.

— Variable: *evaluator-mode* [sb-ext]

Toggle between different evaluator implementations. If set to :compile, an implementation of eval that calls the compiler will be used. If set to :interpret, an interpreter will be used.

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5 Debugger

This chapter documents the debugging facilities of SBCL, including the debugger, single-stepper and trace, and the effect of (optimize debug) declarations.

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5.1 Debugger Entry

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5.1.1 Debugger Banner

When you enter the debugger, it looks something like this:

     debugger invoked on a TYPE-ERROR in thread 11184:
       The value 3 is not of type LIST.
     You can type HELP for debugger help, or (SB-EXT:QUIT) to exit from SBCL.
     restarts (invokable by number or by possibly-abbreviated name):
       0: [ABORT   ] Reduce debugger level (leaving debugger, returning to toplevel).
       1: [TOPLEVEL] Restart at toplevel READ/EVAL/PRINT loop.
     (CAR 1 3)

The first group of lines describe what the error was that put us in the debugger. In this case car was called on 3, causing a type-error.

This is followed by the “beginner help line”, which appears only if sb-debug:*debug-beginner-help-p* is true (default).

Next comes a listing of the active restart names, along with their descriptions – the ways we can restart execution after this error. In this case, both options return to top-level. Restarts can be selected by entering the corresponding number or name.

The current frame appears right underneath the restarts, immediately followed by the debugger prompt.

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5.1.2 Debugger Invocation

The debugger is invoked when:

When the debugger is invoked by a condition, ANSI mandates that the value of *debugger-hook*, if any, be called with two arguments: the condition that caused the debugger to be invoked and the previous value of *debugger-hook*. When this happens, *debugger-hook* is bound to NIL to prevent recursive errors. However, ANSI also mandates that *debugger-hook* not be invoked when the debugger is to be entered by the break function. For users who wish to provide an alternate debugger interface (and thus catch break entries into the debugger), SBCL provides sb-ext:*invoke-debugger-hook*, which is invoked during any entry into the debugger.

— Variable: *invoke-debugger-hook* [sb-ext]

This is either nil or a designator for a function of two arguments, to be run when the debugger is about to be entered. The function is run with *invoke-debugger-hook* bound to nil to minimize recursive errors, and receives as arguments the condition that triggered debugger entry and the previous value of *invoke-debugger-hook*

This mechanism is an sbcl extension similar to the standard *debugger-hook*. In contrast to *debugger-hook*, it is observed by invoke-debugger even when called by break.

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5.2 Debugger Command Loop

The debugger is an interactive read-eval-print loop much like the normal top level, but some symbols are interpreted as debugger commands instead of being evaluated. A debugger command starts with the symbol name of the command, possibly followed by some arguments on the same line. Some commands prompt for additional input. Debugger commands can be abbreviated by any unambiguous prefix: help can be typed as ‘h’, ‘he’, etc.

The package is not significant in debugger commands; any symbol with the name of a debugger command will work. If you want to show the value of a variable that happens also to be the name of a debugger command you can wrap the variable in a progn to hide it from the command loop.

The debugger prompt is “frame]”, where frame is the number of the current frame. Frames are numbered starting from zero at the top (most recent call), increasing down to the bottom. The current frame is the frame that commands refer to.

It is possible to override the normal printing behaviour in the debugger by using the sb-ext:*debug-print-variable-alist*.

— Variable: *debug-print-variable-alist* [sb-ext]

an association list describing new bindings for special variables to be used within the debugger. Eg.

           ((*PRINT-LENGTH* . 10) (*PRINT-LEVEL* . 6) (*PRINT-PRETTY* . NIL))

The variables in the car positions are bound to the values in the cdr during the execution of some debug commands. When evaluating arbitrary expressions in the debugger, the normal values of the printer control variables are in effect.

Initially empty, *debug-print-variable-alist* is typically used to provide bindings for printer control variables.

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5.3 Stack Frames

A stack frame is the run-time representation of a call to a function; the frame stores the state that a function needs to remember what it is doing. Frames have:

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5.3.1 Stack Motion

These commands move to a new stack frame and print the name of the function and the values of its arguments in the style of a Lisp function call:

— Debugger Command: up

Move up to the next higher frame. More recent function calls are considered to be higher on the stack.

— Debugger Command: down

Move down to the next lower frame.

— Debugger Command: top

Move to the highest frame, that is, the frame where the debugger was entered.

— Debugger Command: bottom

Move to the lowest frame.

— Debugger Command: frame [n]

Move to the frame with the specified number. Prompts for the number if not supplied. The frame with number 0 is the frame where the debugger was entered.

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5.3.2 How Arguments are Printed

A frame is printed to look like a function call, but with the actual argument values in the argument positions. So the frame for this call in the source:

     (myfun (+ 3 4) 'a)

would look like this:

     (MYFUN 7 A)

All keyword and optional arguments are displayed with their actual values; if the corresponding argument was not supplied, the value will be the default. So this call:

     (subseq "foo" 1)

would look like this:

     (SUBSEQ "foo" 1 3)

And this call:

     (string-upcase "test case")

would look like this:

     (STRING-UPCASE "test case" :START 0 :END NIL)

The arguments to a function call are displayed by accessing the argument variables. Although those variables are initialized to the actual argument values, they can be set inside the function; in this case the new value will be displayed.

&rest arguments are handled somewhat differently. The value of the rest argument variable is displayed as the spread-out arguments to the call, so:

     (format t "~A is a ~A." "This" 'test)

would look like this:

     (FORMAT T "~A is a ~A." "This" 'TEST)

Rest arguments cause an exception to the normal display of keyword arguments in functions that have both &rest and &key arguments. In this case, the keyword argument variables are not displayed at all; the rest arg is displayed instead. So for these functions, only the keywords actually supplied will be shown, and the values displayed will be the argument values, not values of the (possibly modified) variables.

If the variable for an argument is never referenced by the function, it will be deleted. The variable value is then unavailable, so the debugger prints ‘#<unused-arg>’ instead of the value. Similarly, if for any of a number of reasons the value of the variable is unavailable or not known to be available (see Variable Access), then ‘#<unavailable-arg>’ will be printed instead of the argument value.

Note that inline expansion and open-coding affect what frames are present in the debugger, see Debugger Policy Control.

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5.3.3 Function Names

If a function is defined by defun it will appear in backtrace by that name. Functions defined by labels and flet will appear as (FLET name) and (LABELS name) respectively. Anonymous lambdas will appear as (LAMDBA lambda-list).

Up: Function Names Entry Point Details

Sometimes the compiler introduces new functions that are used to implement a user function, but are not directly specified in the source. This is mostly done for argument type and count checking.

With recursive functions, an additional external frame may appear before the frame representing the first call to the recursive function. This is a consequence of the way the compiler works: there is nothing odd with your program. You may also see cleanup frames during the execution of unwind-protect cleanup code, and optional for variable argument entry points.

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5.3.4 Debug Tail Recursion

The compiler is “properly tail recursive.” If a function call is in a tail-recursive position, the stack frame will be deallocated at the time of the call, rather than after the call returns. Consider this backtrace:

     (BAR ...)
     (FOO ...)

Because of tail recursion, it is not necessarily the case that FOO directly called BAR. It may be that FOO called some other function FOO2 which then called BAR tail-recursively, as in this example:

     (defun foo ()
       (foo2 ...)
     (defun foo2 (...)
       (bar ...))
     (defun bar (...)

Usually the elimination of tail-recursive frames makes debugging more pleasant, since these frames are mostly uninformative. If there is any doubt about how one function called another, it can usually be eliminated by finding the source location in the calling frame. See Source Location Printing.

The elimination of tail-recursive frames can be prevented by disabling tail-recursion optimization, which happens when the debug optimization quality is greater than 2. See Debugger Policy Control.

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5.3.5 Unknown Locations and Interrupts

The debugger operates using special debugging information attached to the compiled code. This debug information tells the debugger what it needs to know about the locations in the code where the debugger can be invoked. If the debugger somehow encounters a location not described in the debug information, then it is said to be unknown. If the code location for a frame is unknown, then some variables may be inaccessible, and the source location cannot be precisely displayed.

There are three reasons why a code location could be unknown:

In the last two cases, the values of argument variables are accessible, but may be incorrect. For more details on when variable values are accessible, Variable Value Availability.

It is possible for an interrupt to happen when a function call or return is in progress. The debugger may then flame out with some obscure error or insist that the bottom of the stack has been reached, when the real problem is that the current stack frame can't be located. If this happens, return from the interrupt and try again.

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5.4 Variable Access

There are two ways to access the current frame's local variables in the debugger: list-locals and sb-debug:var.

The debugger doesn't really understand lexical scoping; it has just one namespace for all the variables in the current stack frame. If a symbol is the name of multiple variables in the same function, then the reference appears ambiguous, even though lexical scoping specifies which value is visible at any given source location. If the scopes of the two variables are not nested, then the debugger can resolve the ambiguity by observing that only one variable is accessible.

When there are ambiguous variables, the evaluator assigns each one a small integer identifier. The sb-debug:var function uses this identifier to distinguish between ambiguous variables. The list-locals command prints the identifier. In the following example, there are two variables named X. The first one has identifier 0 (which is not printed), the second one has identifier 1.

     X  =  1
     X#1  =  2
— Debugger Command: list-locals [prefix]

This command prints the name and value of all variables in the current frame whose name has the specified prefix. prefix may be a string or a symbol. If no prefix is given, then all available variables are printed. If a variable has a potentially ambiguous name, then the name is printed with a “#identifier” suffix, where identifier is the small integer used to make the name unique.

— Function: var [sb-debug] name &optional identifier

This function returns the value of the variable in the current frame with the specified name. If supplied, identifier determines which value to return when there are ambiguous variables.

When name is a symbol, it is interpreted as the symbol name of the variable, i.e. the package is significant. If name is an uninterned symbol (gensym), then return the value of the uninterned variable with the same name. If name is a string, sb-debug:var interprets it as the prefix of a variable name that must unambiguously complete to the name of a valid variable.

identifier is used to disambiguate the variable name; use list-locals to find out the identifiers.

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5.4.1 Variable Value Availability

The value of a variable may be unavailable to the debugger in portions of the program where Lisp says that the variable is defined. If a variable value is not available, the debugger will not let you read or write that variable. With one exception, the debugger will never display an incorrect value for a variable. Rather than displaying incorrect values, the debugger tells you the value is unavailable.

The one exception is this: if you interrupt (e.g., with <C-c>) or if there is an unexpected hardware error such as “‘bus error’” (which should only happen in unsafe code), then the values displayed for arguments to the interrupted frame might be incorrect.4 This exception applies only to the interrupted frame: any frame farther down the stack will be fine.

The value of a variable may be unavailable for these reasons:

Since it is especially useful to be able to get the arguments to a function, argument variables are treated specially when the speed optimization quality is less than 3 and the debug quality is at least 1. With this compilation policy, the values of argument variables are almost always available everywhere in the function, even at unknown locations. For non-argument variables, debug must be at least 2 for values to be available, and even then, values are only available at known locations.

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5.4.2 Note On Lexical Variable Access

When the debugger command loop establishes variable bindings for available variables, these variable bindings have lexical scope and dynamic extent.5 You can close over them, but such closures can't be used as upward function arguments.

You can also set local variables using setq, but if the variable was closed over in the original source and never set, then setting the variable in the debugger may not change the value in all the functions the variable is defined in. Another risk of setting variables is that you may assign a value of a type that the compiler proved the variable could never take on. This may result in bad things happening.

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5.5 Source Location Printing

One of the debugger's capabilities is source level debugging of compiled code. These commands display the source location for the current frame:

— Debugger Command: source [context]

This command displays the file that the current frame's function was defined from (if it was defined from a file), and then the source form responsible for generating the code that the current frame was executing. If context is specified, then it is an integer specifying the number of enclosing levels of list structure to print.

The source form for a location in the code is the innermost list present in the original source that encloses the form responsible for generating that code. If the actual source form is not a list, then some enclosing list will be printed. For example, if the source form was a reference to the variable *some-random-special*, then the innermost enclosing evaluated form will be printed. Here are some possible enclosing forms:

     (let ((a *some-random-special*))
     (+ *some-random-special* ...)

If the code at a location was generated from the expansion of a macro or a source-level compiler optimization, then the form in the original source that expanded into that code will be printed. Suppose the file /usr/me/mystuff.lisp looked like this:

     (defmacro mymac ()
     (defun foo ()

If foo has called myfun, and is waiting for it to return, then the source command would print:

     ; File: /usr/me/mystuff.lisp

Note that the macro use was printed, not the actual function call form, (myfun).

If enclosing source is printed by giving an argument to source or vsource, then the actual source form is marked by wrapping it in a list whose first element is ‘#:***HERE***’. In the previous example, source 1 would print:

     ; File: /usr/me/mystuff.lisp
     (DEFUN FOO ()

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5.5.1 How the Source is Found

If the code was defined from Lisp by compile or eval, then the source can always be reliably located. If the code was defined from a fasl file created by compile-file, then the debugger gets the source forms it prints by reading them from the original source file. This is a potential problem, since the source file might have moved or changed since the time it was compiled.

The source file is opened using the truename of the source file pathname originally given to the compiler. This is an absolute pathname with all logical names and symbolic links expanded. If the file can't be located using this name, then the debugger gives up and signals an error.

If the source file can be found, but has been modified since the time it was compiled, the debugger prints this warning:

     ; File has been modified since compilation:
     ;   filename
     ; Using form offset instead of character position.

where filename is the name of the source file. It then proceeds using a robust but not foolproof heuristic for locating the source. This heuristic works if:

If the heuristic doesn't work, the displayed source will be wrong, but will probably be near the actual source. If the “shape” of the top-level form in the source file is too different from the original form, then an error will be signaled. When the heuristic is used, the source location commands are noticeably slowed.

Source location printing can also be confused if (after the source was compiled) a read-macro you used in the code was redefined to expand into something different, or if a read-macro ever returns the same eq list twice. If you don't define read macros and don't use ## in perverted ways, you don't need to worry about this.

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5.5.2 Source Location Availability

Source location information is only available when the debug optimization quality is at least 2. If source location information is unavailable, the source commands will give an error message.

If source location information is available, but the source location is unknown because of an interrupt or unexpected hardware error (see Unknown Locations and Interrupts), then the command will print:

     Unknown location: using block start.

and then proceed to print the source location for the start of the basic block enclosing the code location. It's a bit complicated to explain exactly what a basic block is, but here are some properties of the block start location:

In other words, the true location lies between the printed location and the next conditional (but watch out because the compiler may have changed the program on you.)

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5.6 Debugger Policy Control

The compilation policy specified by optimize declarations affects the behavior seen in the debugger. The debug quality directly affects the debugger by controlling the amount of debugger information dumped. Other optimization qualities have indirect but observable effects due to changes in the way compilation is done.

Unlike the other optimization qualities (which are compared in relative value to evaluate tradeoffs), the debug optimization quality is directly translated to a level of debug information. This absolute interpretation allows the user to count on a particular amount of debug information being available even when the values of the other qualities are changed during compilation. These are the levels of debug information that correspond to the values of the debug quality:

Only the function name and enough information to allow the stack to be parsed.
> 0
Any level greater than 0 gives level 0 plus all argument variables. Values will only be accessible if the argument variable is never set and speed is not 3. SBCL allows any real value for optimization qualities. It may be useful to specify 0.5 to get backtrace argument display without argument documentation.
Level 1 provides argument documentation (printed argument lists) and derived argument/result type information. This makes describe more informative, and allows the compiler to do compile-time argument count and type checking for any calls compiled at run-time. This is the default.
Level 1 plus all interned local variables, source location information, and lifetime information that tells the debugger when arguments are available (even when speed is 3 or the argument is set).
> 2
Any level greater than 2 gives level 2 and in addition disables tail-call optimization, so that the backtrace will contain frames for all invoked functions, even those in tail positions.
Level 2 plus all uninterned variables. In addition, lifetime analysis is disabled (even when speed is 3), ensuring that all variable values are available at any known location within the scope of the binding. This has a speed penalty in addition to the obvious space penalty.
> (max speed space)
If debug is greater than both speed and space, the command return can be used to continue execution by returning a value from the current stack frame.
> (max speed space compilation-speed)
If debug is greater than all of speed, space and compilation-speed the code will be steppable (see Single Stepping).

As you can see, if the speed quality is 3, debugger performance is degraded. This effect comes from the elimination of argument variable special-casing (see Variable Value Availability). Some degree of speed/debuggability tradeoff is unavoidable, but the effect is not too drastic when debug is at least 2.

In addition to inline and notinline declarations, the relative values of the speed and space qualities also change whether functions are inline expanded. If a function is inline expanded, then there will be no frame to represent the call, and the arguments will be treated like any other local variable. Functions may also be “semi-inline”, in which case there is a frame to represent the call, but the call is to an optimized local version of the function, not to the original function.

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5.7 Exiting Commands

These commands get you out of the debugger.

— Debugger Command: toplevel

Throw to top level.

— Debugger Command: restart [n]

Invokes the nth restart case as displayed by the error command. If n is not specified, the available restart cases are reported.

— Debugger Command: continue

Calls continue on the condition given to debug. If there is no restart case named continue, then an error is signaled.

— Debugger Command: abort

Calls abort on the condition given to debug. This is useful for popping debug command loop levels or aborting to top level, as the case may be.

— Debugger Command: return value

Returns value from the current stack frame. This command is available when the debug optimization quality is greater than both speed and space. Care must be taken that the value is of the same type as SBCL expects the stack frame to return.

— Debugger Command: restart-frame

Restarts execution of the current stack frame. This command is available when the debug optimization quality is greater than both speed and space and when the frame is for is a global function. If the function is redefined in the debugger before the frame is restarted, the new function will be used.

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5.8 Information Commands

Most of these commands print information about the current frame or function, but a few show general information.

— Debugger Command: help
— Debugger Command: ?

Displays a synopsis of debugger commands.

— Debugger Command: describe

Calls describe on the current function and displays the number of local variables.

— Debugger Command: print

Displays the current function call as it would be displayed by moving to this frame.

— Debugger Command: error

Prints the condition given to invoke-debugger and the active proceed cases.

— Debugger Command: backtrace [n]

Displays all the frames from the current to the bottom. Only shows n frames if specified. The printing is controlled by *debug-print-variable-alist*.

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5.9 Function Tracing

The tracer causes selected functions to print their arguments and their results whenever they are called. Options allow conditional printing of the trace information and conditional breakpoints on function entry or exit.

— Macro: trace [cl] &rest specs

trace {Option Global-Value}* {Name {Option Value}*}*

trace is a debugging tool that provides information when specified functions are called. In its simplest form:

                 (TRACE NAME-1 NAME-2 ...)

The NAMEs are not evaluated. Each may be a symbol, denoting an individual function, or a string, denoting all functions fbound to symbols whose home package is the package with the given name.

Options allow modification of the default behavior. Each option is a pair of an option keyword and a value form. Global options are specified before the first name, and affect all functions traced by a given use of trace. Options may also be interspersed with function names, in which case they act as local options, only affecting tracing of the immediately preceding function name. Local options override global options.

By default, trace causes a printout on *trace-output* each time that one of the named functions is entered or returns. (This is the basic, ansi Common Lisp behavior of trace.)

The following options are defined:

:report Report-Type
If Report-Type is trace (the default) then information is reported by printing immediately. If Report-Type is nil, then the only effect of the trace is to execute other options (e.g. print or BREAK).
:condition Form
:condition-after Form
:condition-all Form
If :condition is specified, then trace does nothing unless Form evaluates to true at the time of the call. :condition-after is similar, but suppresses the initial printout, and is tested when the function returns. :condition-all tries both before and after.
:break Form
:break-after Form
:break-all Form
If specified, and Form evaluates to true, then the debugger is invoked at the start of the function, at the end of the function, or both, according to the respective option.
:print Form
:print-after Form
:print-all Form
In addition to the usual printout, the result of evaluating Form is printed at the start of the function, at the end of the function, or both, according to the respective option. Multiple print options cause multiple values to be printed.
:wherein Names
If specified, Names is a function name or list of names. trace does nothing unless a call to one of those functions encloses the call to this function (i.e. it would appear in a backtrace.) Anonymous functions have string names like "DEFUN FOO".
:encapsulate {:DEFAULT | t | NIL}
If t, the tracing is done via encapsulation (redefining the function name) rather than by modifying the function. :default is the default, and means to use encapsulation for interpreted functions and funcallable instances, breakpoints otherwise. When encapsulation is used, forms are *not* evaluated in the function's lexical environment, but sb-debug:arg can still be used.
:methods {T | NIL}
If t, any function argument naming a generic function will have its methods traced in addition to the generic function itself.
:function Function-Form
This is a not really an option, but rather another way of specifying what function to trace. The Function-Form is evaluated immediately, and the resulting function is traced.

:condition, :break and :print forms are evaluated in a context which mocks up the lexical environment of the called function, so that sb-debug:var and sb-debug:arg can be used. The -after and -all forms can use sb-debug:arg.

— Macro: untrace [cl] &rest specs

Remove tracing from the specified functions. Untraces all functions when called with no arguments.

— Variable: *trace-indentation-step* [sb-debug]

the increase in trace indentation at each call level

— Variable: *max-trace-indentation* [sb-debug]

If the trace indentation exceeds this value, then indentation restarts at 0.

— Variable: *trace-encapsulate-default* [sb-debug]

the default value for the :encapsulate option to trace

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5.10 Single Stepping

SBCL includes an instrumentation based single-stepper for compiled code, that can be invoked via the step macro, or from within the debugger. See Debugger Policy Control, for details on enabling stepping for compiled code.

The following debugger commands are used for controlling single stepping.

— Debugger Command: start

Selects the continue restart if one exists and starts single stepping. None of the other single stepping commands can be used before stepping has been started either by using start or by using the standard step macro.

— Debugger Command: step

Steps into the current form. Stepping will be resumed when the next form that has been compiled with stepper instrumentation is evaluated.

— Debugger Command: next

Steps over the current form. Stepping will be disabled until evaluation of the form is complete.

— Debugger Command: out

Steps out of the current frame. Stepping will be disabled until the topmost stack frame that had been stepped into returns.

— Debugger Command: stop

Stops the single stepper and resumes normal execution.

— Macro: step [cl] form

The form is evaluated with single stepping enabled. Function calls outside the lexical scope of the form can be stepped into only if the functions in question have been compiled with sufficient debug policy to be at least partially steppable.

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5.11 Enabling and Disabling the Debugger

In certain contexts (e.g., non-interactive applications), it may be desirable to turn off the SBCL debugger (and possibly re-enable it). The functions here control the debugger.

— Function: disable-debugger [sb-ext]

When invoked, this function will turn off both the sbcl debugger and ldb (the low-level debugger). See also enable-debugger.

— Function: enable-debugger [sb-ext]

Restore the debugger if it has been turned off by disable-debugger.

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6 Efficiency

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6.1 Slot access

6.1.1 Structure object slot access

Structure slot accessors are efficient only if the compiler is able to open code them: compiling a call to a structure slot accessor before the structure is defined, declaring one notinline, or passing it as a functional argument to another function causes severe performance degradation.

6.1.2 Standard object slot access

The most efficient way to access a slot of a standard-object is by using slot-value with a constant slot name argument inside a defmethod body, where the variable holding the instance is a specializer parameter of the method and is never assigned to. The cost is roughly 1.6 times that of an open coded structure slot accessor.

Second most efficient way is to use a CLOS slot accessor, or slot-value with a constant slot name argument, but in circumstances other than specified above. This may be up to 3 times as slow as the method described above.


     (defclass foo () ((bar)))
     ;; Fast: specializer and never assigned to
     (defmethod quux ((foo foo) new)
       (let ((old (slot-value foo 'bar)))
         (setf (slot-value foo 'bar) new)
     ;; Slow: not a specializer
     (defmethod quux ((foo foo) new)
       (let* ((temp foo)
              (old (slot-value temp 'bar)))
         (setf (slot-value temp 'bar) new)
     ;; Slow: assignment to FOO
     (defmethod quux ((foo foo) new)
       (let ((old (slot-value foo 'bar)))
         (setf (slot-value foo 'bar) new)
         (setf foo new)

Note that when profiling code such as this, the first few calls to the generic function are not representative, as the dispatch mechanism is lazily set up during those calls.

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6.2 Dynamic-extent allocation

SBCL has fairly extensive support for performing allocation on the stack when a variable is declared dynamic-extent. The dynamic-extent declarations are not verified, but are simply trusted as long as sb-ext:*stack-allocate-dynamic-extent* is true.

— Variable: *stack-allocate-dynamic-extent* [sb-ext]

If true (the default), the compiler respects dynamic-extent declarations and stack allocates otherwise inaccessible parts of the object whenever possible. Potentially long (over one page in size) vectors are, however, not stack allocated except in zero safety code, as such a vector could overflow the stack without triggering overflow protection.

If dynamic extent constraints specified in the Common Lisp standard are violated, the best that can happen is for the program to have garbage in variables and return values; more commonly, the system will crash.

In particular, it is important to realize that dynamic extend is contagious:

     (let* ((a (list 1 2 3))
            (b (cons a a)))
        (declare (dynamic-extent b))
        ;; Unless A is accessed elsewhere as well, SBCL will consider
        ;; it to be otherwise inaccessible -- it can only be accessed
        ;; through B, after all -- and stack allocate it as well.
        ;; Hence returning (CAR B) here is unsafe.

This allows stack allocation of complex structures. As a notable exception to this, SBCL does not as of propagate dynamic-extentness through &rest arguments – but another conforming implementation might, so portable code should not rely on this.

     (declaim (inline foo))
     (defun foo (fun &rest arguments)
       (declare (dynamic-extent arguments))
       (apply fun arguments))
     (defun bar (a)
       ;; SBCL will heap allocate the result of (LIST A), and stack allocate
       ;; only the spine of the &rest list -- so this is safe, but unportable.
       ;; Another implementation, including earlier versions of SBCL might consider
       ;; (LIST A) to be otherwise inaccessible and stack-allocate it as well!
       (foo #'car (list a)))

There are many cases when dynamic-extent declarations could be useful. At present, SBCL implements stack allocation for


     ;;; Declaiming a structure constructor inline before definition makes
     ;;; stack allocation possible.
     (declaim (inline make-thing))
     (defstruct thing obj next)
     ;;; Stack allocation of various objects bound to DYNAMIC-EXTENT
     ;;; variables.
     (let* ((list (list 1 2 3))
            (nested (cons (list 1 2) (list* 3 4 (list 5))))
            (vector (make-array 3 :element-type 'single-float))
            (thing (make-thing :obj list
                               :next (make-thing :obj (make-array 3)))))
       (declare (dynamic-extent list nested vector thing))
     ;;; Stack allocation of arguments to a local function is equivalent
     ;;; to stack allocation of local variable values.
     (flet ((f (x)
              (declare (dynamic-extent x))
       (f (list 1 2 3))
       (f (cons (cons 1 2) (cons 3 4)))
     ;;; Stack allocation of &REST lists
     (defun foo (&rest args)
       (declare (dynamic-extent args))

Future plans include

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6.3 Modular arithmetic

Some numeric functions have a property: N lower bits of the result depend only on N lower bits of (all or some) arguments. If the compiler sees an expression of form (logand exp mask), where exp is a tree of such “good” functions and mask is known to be of type (unsigned-byte w), where w is a “good” width, all intermediate results will be cut to w bits (but it is not done for variables and constants!). This often results in an ability to use simple machine instructions for the functions.

Consider an example.

     (defun i (x y)
       (declare (type (unsigned-byte 32) x y))
       (ldb (byte 32 0) (logxor x (lognot y))))

The result of (lognot y) will be negative and of type (signed-byte 33), so a naive implementation on a 32-bit platform is unable to use 32-bit arithmetic here. But modular arithmetic optimizer is able to do it: because the result is cut down to 32 bits, the compiler will replace logxor and lognot with versions cutting results to 32 bits, and because terminals (here—expressions x and y) are also of type (unsigned-byte 32), 32-bit machine arithmetic can be used.

As of SBCL 0.8.5 “good” functions are +, -; logand, logior, logxor, lognot and their combinations; and ash with the positive second argument. “Good” widths are 32 on HPPA, MIPS, PPC, Sparc and x86 and 64 on Alpha. While it is possible to support smaller widths as well, currently this is not implemented.

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6.4 Global and Always-Bound variables

— Macro: defglobal [sb-ext] name value &optional doc

Defines name as a global variable that is always bound. value is evaluated and assigned to name both at compile- and load-time, but only if name is not already bound.

Global variables share their values between all threads, and cannot be locally bound, declared special, defined as constants, and neither bound nor defined as symbol macros.

See also the declarations sb-ext:global and sb-ext:always-bound.

— Declaration: global [sb-ext]

Syntax: (sb-ext:global symbol*)

Only valid as a global proclamation.

Specifies that the named symbols cannot be proclaimed or locally declared special. Proclaiming an already special or constant variable name as global signal an error. Allows more efficient value lookup in threaded environments in addition to expressing programmer intention.

— Declaration: always-bound [sb-ext]

Syntax: (sb-ext:always-bound symbol*)

Only valid as a global proclamation.

Specifies that the named symbols are always bound. Inhibits makunbound of the named symbols. Proclaiming an unbound symbol as always-bound signals an error. Allows the compiler to elide boundness checks from value lookups.

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6.5 Miscellaneous Efficiency Issues

FIXME: The material in the CMUCL manual about getting good performance from the compiler should be reviewed, reformatted in Texinfo, lightly edited for SBCL, and substituted into this manual. In the meantime, the original CMUCL manual is still 95+% correct for the SBCL version of the Python compiler. See the sections

Besides this information from the CMUCL manual, there are a few other points to keep in mind.

Finally, note that Common Lisp defines many constructs which, in the infamous phrase, “could be compiled efficiently by a sufficiently smart compiler”. The phrase is infamous because making a compiler which actually is sufficiently smart to find all these optimizations systematically is well beyond the state of the art of current compiler technology. Instead, they're optimized on a case-by-case basis by hand-written code, or not optimized at all if the appropriate case hasn't been hand-coded. Some cases where no such hand-coding has been done as of SBCL version 0.6.3 include

If your system's performance is suffering because of some construct which could in principle be compiled efficiently, but which the SBCL compiler can't in practice compile efficiently, consider writing a patch to the compiler and submitting it for inclusion in the main sources. Such code is often reasonably straightforward to write; search the sources for the string “deftransform” to find many examples (some straightforward, some less so).

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7 Beyond the ANSI Standard

SBCL is derived from CMUCL, which implements many extensions to the ANSI standard. SBCL doesn't support as many extensions as CMUCL, but it still has quite a few. See Contributed Modules.

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7.1 Reader Extensions

SBCL supports extended package prefix syntax, which allows specifying an alternate package instead of *package* for the reader to use as the default package for interning symbols:



       'foo::(bar quux zot) == '(foo::bar foo::quux foo::zot)

Doesn't alter *package*: if foo::bar would cause a read-time package lock violation, so does foo::(bar).

SBCL also extends the reader to normalize all symbols to Normalization Form KC in builds with Unicode enabled. Whether symbols are normalized is controlled by

— Function: readtable-normalization [sb-ext] readtable

Returns t if readtable normalizes strings to nfkc, and nil otherwise. The readtable-normalization of the standard readtable is t.

Symbols created by intern and similar functions are not affected by this setting. If sb-ext:readtable-normalization is t, symbols that are not normalized are escaped during printing.

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7.2 Package-Local Nicknames

SBCL allows giving packages local nicknames: they allow short and easy-to-use names to be used without fear of name conflict associated with normal nicknames.

A local nickname is valid only when inside the package for which it has been specified. Different packages can use same local nickname for different global names, or different local nickname for same global name.

Symbol :package-local-nicknames in *features* denotes the support for this feature.

— Macro: defpackage [cl] name [[option]]* ⇒ package

Options are extended to include


          (defpackage :bar (:intern "X"))
          (defpackage :foo (:intern "X"))
          (defpackage :quux (:use :cl) (:local-nicknames (:bar :foo) (:foo :bar)))
          (find-symbol "X" :foo) ; => FOO::X
          (find-symbol "X" :bar) ; => BAR::X
          (let ((*package* (find-package :quux)))
            (find-symbol "X" :foo))               ; => BAR::X
          (let ((*package* (find-package :quux)))
            (find-symbol "X" :bar))               ; => FOO::X

— Function: package-local-nicknames [sb-ext] package-designator

Returns an alist of (local-nickname . actual-package) describing the nicknames local to the designated package.

When in the designated package, calls to find-package with the any of the local-nicknames will return the corresponding actual-package instead. This also affects all implied calls to find-package, including those performed by the reader.

When printing a package prefix for a symbol with a package local nickname, the local nickname is used instead of the real name in order to preserve print-read consistency.

See also: add-package-local-nickname, package-locally-nicknamed-by-list, remove-package-local-nickname, and the defpackage option :local-nicknames.

Experimental: interface subject to change.

— Function: package-locally-nicknamed-by-list [sb-ext] package-designator

Returns a list of packages which have a local nickname for the designated package.

See also: add-package-local-nickname, package-local-nicknames, remove-package-local-nickname, and the defpackage option :local-nicknames.

Experimental: interface subject to change.

— Function: add-package-local-nickname [sb-ext] local-nickname actual-package &optional package-designator

Adds local-nickname for actual-package in the designated package, defaulting to current package. local-nickname must be a string designator, and actual-package must be a package designator.

Returns the designated package.

Signals a continuable error if local-nickname is already a package local nickname for a different package, or if local-nickname is one of "CL", "COMMON-LISP", or, "KEYWORD", or if local-nickname is a global name or nickname for the package to which the nickname would be added.

When in the designated package, calls to find-package with the local-nickname will return the package the designated actual-package instead. This also affects all implied calls to find-package, including those performed by the reader.

When printing a package prefix for a symbol with a package local nickname, local nickname is used instead of the real name in order to preserve print-read consistency.

See also: package-local-nicknames, package-locally-nicknamed-by-list, remove-package-local-nickname, and the defpackage option :local-nicknames.

Experimental: interface subject to change.

— Function: remove-package-local-nickname [sb-ext] old-nickname &optional package-designator

If the designated package had old-nickname as a local nickname for another package, it is removed. Returns true if the nickname existed and was removed, and nil otherwise.

See also: add-package-local-nickname, package-local-nicknames, package-locally-nicknamed-by-list, and the defpackage option :local-nicknames.

Experimental: interface subject to change.

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7.3 Package Variance

Common Lisp standard specifies that “If the new definition is at variance with the current state of that package, the consequences are undefined;” SBCL by default signals a full warning and retains as much of the package state as possible.

This can be adjusted using sb-ext:*on-package-variance*:

— Variable: *on-package-variance* [sb-ext]

Specifies behavior when redefining a package using defpackage and the definition is in variance with the current state of the package.

The value should be of the form:

            (:WARN [T | packages-names] :ERROR [T | package-names])

specifying which packages get which behaviour -- with t signifying the default unless otherwise specified. If default is not specified, :warn is used.

:warn keeps as much state as possible and causes sbcl to signal a full warning.

:error causes sbcl to signal an error when the variant defpackage form is executed, with restarts provided for user to specify what action should be taken.


            (setf *on-package-variance* '(:warn (:swank :swank-backend) :error t))

specifies to signal a warning if swank package is in variance, and an error otherwise.

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7.4 Garbage Collection

SBCL provides additional garbage collection functionality not specified by ANSI.

— Variable: *after-gc-hooks* [sb-ext]

Called after each garbage collection, except for garbage collections triggered during thread exits. In a multithreaded environment these hooks may run in any thread.

— Function: gc [sb-ext] &key full gen &allow-other-keys

Initiate a garbage collection.

The default is to initiate a nursery collection, which may in turn trigger a collection of one or more older generations as well. If full is true, all generations are collected. If gen is provided, it can be used to specify the oldest generation guaranteed to be collected.

On CheneyGC platforms arguments full and gen take no effect: a full collection is always performed.

7.4.1 Finalization

Finalization allows code to be executed after an object has been garbage collected. This is useful for example for releasing foreign memory associated with a Lisp object.

— Function: finalize [sb-ext] object function &key dont-save

Arrange for the designated function to be called when there are no more references to object, including references in function itself.

If dont-save is true, the finalizer will be cancelled when save-lisp-and-die is called: this is useful for finalizers deallocating system memory, which might otherwise be called with addresses from the old image.

In a multithreaded environment function may be called in any thread. In both single and multithreaded environments function may be called in any dynamic scope: consequences are unspecified if function is not fully re-entrant.

Errors from function are handled and cause a warning to be signalled in whichever thread the function was called in.


            ;;; GOOD, assuming RELEASE-HANDLE is re-entrant.
            (let* ((handle (get-handle))
                   (object (make-object handle)))
             (finalize object (lambda () (release-handle handle)))
            ;;; BAD, finalizer refers to object being finalized, causing
            ;;; it to be retained indefinitely!
            (let* ((handle (get-handle))
                   (object (make-object handle)))
              (finalize object
                        (lambda ()
                          (release-handle (object-handle object)))))
            ;;; BAD, not re-entrant!
            (defvar *rec* nil)
            (defun oops ()
             (when *rec*
               (error "recursive OOPS"))
             (let ((*rec* t))
               (gc))) ; or just cons enough to cause one
              (finalize "oops" #'oops)
              (oops)) ; GC causes re-entry to #'oops due to the finalizer
                      ; -> ERROR, caught, WARNING signalled

— Function: cancel-finalization [sb-ext] object

Cancel any finalization for object.

7.4.2 Weak Pointers

Weak pointers allow references to objects to be maintained without keeping them from being garbage collected: useful for building caches among other things.

Hash tables can also have weak keys and values: see Hash Table Extensions.

— Function: make-weak-pointer [sb-ext] object

Allocate and return a weak pointer which points to object.

— Function: weak-pointer-value [sb-ext] weak-pointer

If weak-pointer is valid, return the value of weak-pointer and t. If the referent of weak-pointer has been garbage collected, returns the values nil and nil.

7.4.3 Introspection and Tuning

— Variable: *gc-run-time* [sb-ext]

Total cpu time spent doing garbage collection (as reported by get-internal-run-time.) Initialized to zero on startup. It is safe to bind this to zero in order to measure gc time inside a certain section of code, but doing so may interfere with results reported by eg. time.

— Function: bytes-consed-between-gcs [sb-ext]

The amount of memory that will be allocated before the next garbage collection is initiated. This can be set with setf.

On gencgc platforms this is the nursery size, and defaults to 5% of dynamic space size.

Note: currently changes to this value are lost when saving core.

— Function: dynamic-space-size [sb-ext]

Size of the dynamic space in bytes.

— Function: get-bytes-consed [sb-ext]

Return the number of bytes consed since the program began. Typically this result will be a consed bignum, so if you have an application (e.g. profiling) which can't tolerate the overhead of consing bignums, you'll probably want either to hack in at a lower level (as the code in the sb-profile package does), or to design a more microefficient interface and submit it as a patch.

— Function: gc-logfile [sb-ext]

Return the pathname used to log garbage collections. Can be setf. Default is nil, meaning collections are not logged. If non-null, the designated file is opened before and after each collection, and generation statistics are appended to it.

— Function: generation-average-age [sb-ext] generation

Average age of memory allocated to generation: average number of times objects allocated to the generation have seen younger objects promoted to it. Available on gencgc platforms only.

Experimental: interface subject to change.

— Function: generation-bytes-allocated [sb-ext] generation

Number of bytes allocated to generation currently. Available on gencgc platforms only.

Experimental: interface subject to change.

— Function: generation-bytes-consed-between-gcs [sb-ext] generation

Number of bytes that can be allocated to generation before that generation is considered for garbage collection. This value is meaningless for generation 0 (the nursery): see bytes-consed-between-gcs instead. Default is 5% of the dynamic space size divided by the number of non-nursery generations. Can be assigned to using setf. Available on gencgc platforms only.

Experimental: interface subject to change.

— Function: generation-minimum-age-before-gc [sb-ext] generation

Minimum average age of objects allocated to generation before that generation is may be garbage collected. Default is 0.75. See also generation-average-age. Can be assigned to using setf. Available on gencgc platforms only.

Experimental: interface subject to change.

— Function: generation-number-of-gcs-before-promotion [sb-ext] generation

Number of times garbage collection is done on generation before automatic promotion to the next generation is triggered. Default is 1. Can be assigned to using setf. Available on gencgc platforms only.

Experimental: interface subject to change.

— Function: generation-number-of-gcs [sb-ext] generation

Number of times garbage collection has been done on generation without promotion. Available on gencgc platforms only.

Experimental: interface subject to change.

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7.5 Metaobject Protocol

7.5.1 AMOP Compatibility of Metaobject Protocol

SBCL supports a metaobject protocol which is intended to be compatible with AMOP; present exceptions to this (as distinct from current bugs) are:

7.5.2 Metaobject Protocol Extensions

In addition, SBCL supports extensions to the Metaobject protocol from AMOP; at present, they are:

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7.6 Extensible Sequences

ANSI Common Lisp has a class sequence with subclasses list and vector on which the “sequence functions” like find, subseq, etc. operate. As an extension to the ANSI specification, SBCL allows additional subclasses of sequence to be defined 6. Users of this extension just make instances of sequence subclasses and transparently operate on them using sequence functions:

     (coerce (subseq (make-instance 'my-sequence) 5 10) 'list)

From this perspective, no distinction between builtin and user-defined sequence subclasses should be necessary. Providers of the extension, that is of user-defined sequence subclasses, have to adhere to a “sequence protocol” which consists of a set of generic functions in the sequence package.

A minimal sequence subclass has to specify standard-object and sequence as its superclasses and has to be the specializer of the sequence parameter of methods on at least the following generic functions:

— Generic Function: length [sb-sequence] sequence

Returns the length of sequence or signals a protocol-unimplemented error if the sequence protocol is not implemented for the class of sequence.

— Generic Function: elt [sb-sequence] sequence index

Returns the element at position index of sequence or signals a protocol-unimplemented error if the sequence protocol is not implemented for the class of sequence.

— Generic Function: (setf elt [sb-sequence])

Replaces the element at position index of sequence with new-value and returns new-value or signals a protocol-unimplemented error if the sequence protocol is not implemented for the class of sequence.

— Generic Function: adjust-sequence [sb-sequence] sequence length &key initial-element initial-contents

Return destructively modified sequence or a freshly allocated sequence of the same class as sequence of length length. Elements of the returned sequence are initialized to initial-element, if supplied, initialized to initial-contents if supplied, or identical to the elements of sequence if neither is supplied. Signals a protocol-unimplemented error if the sequence protocol is not implemented for the class of sequence.

— Generic Function: make-sequence-like [sb-sequence] sequence length &key initial-element initial-contents

Returns a freshly allocated sequence of length length and of the same class as sequence. Elements of the new sequence are initialized to initial-element, if supplied, initialized to initial-contents if supplied, or identical to the elements of sequence if neither is supplied. Signals a protocol-unimplemented error if the sequence protocol is not implemented for the class of sequence.

make-sequence-like is needed for functions returning freshly-allocated sequences such as subseq or copy-seq. adjust-sequence is needed for functions which destructively modify their arguments such as delete. In fact, all other sequence functions can be implemented in terms of the above functions and actually are, if no additional methods are defined. However, relying on these generic implementations, in particular not implementing the iterator protocol can incur a high performance penalty See Iterator Protocol. When the sequence protocol is only partially implemented for a given sequence subclass, an attempt to apply one of the missing operations to instances of that class signals the following condition:

— Condition: protocol-unimplemented [sb-sequence]

Class precedence list: protocol-unimplemented, type-error, error, serious-condition, condition, t

This error is signaled if a sequence operation is applied to an instance of a sequence class that does not support the operation.

In addition to the mandatory functions above, methods on the sequence functions listed below can be defined.

There are two noteworthy irregularities:

— Generic Function: emptyp [sb-sequence] sequence

Returns t if sequence is an empty sequence and nil otherwise. Signals an error if sequence is not a sequence.

In the spirit of dolist, generic sequences can be traversed using the macro

— Macro: dosequence [sb-sequence] (element sequence &optional return) &body body

Executes body with element subsequently bound to each element of sequence, then returns return.

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7.6.1 Iterator Protocol

The iterator protocol allows subsequently accessing some or all elements of a sequence in forward or reverse direction. Users first call make-sequence-iterator to create an iteration state and receive functions to query and mutate it. These functions allow, among other things, moving to, retrieving or modifying elements of the sequence. An iteration state consists of a state object, a limit object, a from-end indicator and the following six functions to query or mutate this state:

— Function: step function sequence iterator from-end

Moves the iterator one position forward or backward in the associated sequence depending on the iteration direction.

— Function: endp function sequence iterator limit from-end

Returns non-nil when the iterator has reached the end of the associated sequence with respect to the iteration direction.

— Function: element function sequence iterator

Returns the sequence element associated to the current position of the iteration.

— Function: setf element function new-value sequence iterator

Destructively modifies the associates sequence by replacing the sequence element associated to the current iteration position with a new value.

— Function: index function sequence iterator

Returns the position of the iteration in the associated sequence.

— Function: copy function sequence iterator

Returns a copy of the iteration state which can be mutated independently of the copied iteration state.

An iterator is created by calling:

— Generic Function: make-sequence-iterator [sb-sequence] sequence &key from-end start end

Returns a sequence iterator for sequence or, if start and/or end are supplied, the subsequence bounded by start and end as nine values:

1. iterator state 2. limit 3. from-end 4. step function 5. endp function 6. element function 7. setf element function 8. index function 9. copy state function

If from-end is nil, the constructed iterator visits the specified elements in the order in which they appear in sequence. Otherwise, the elements are visited in the opposite order.

Note that make-sequence-iterator calls make-simple-sequence-iterator when there is no specialized method for a particular sequence subclass. See Simple Iterator Protocol. The following convenience macros simplify traversing sequences using iterators:

— Macro: with-sequence-iterator [sb-sequence] (&rest vars) (sequence &rest args &key from-end start end) &body body

Executes body with the elements of vars bound to the iteration state returned by make-sequence-iterator for sequence and args. Elements of vars may be nil in which case the corresponding value returned by make-sequence-iterator is ignored.

— Macro: with-sequence-iterator-functions [sb-sequence] (step endp elt setf index copy) (sequence &rest args &key from-end start end) &body body

Executes body with the names step, endp, elt, setf, index and copy bound to local functions which execute the iteration state query and mutation functions returned by make-sequence-iterator for sequence and args. step, endp, elt, setf, index and copy have dynamic extent.

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7.6.2 Simple Iterator Protocol

For cases in which the full flexibility and performance of the general sequence iterator protocol is not required, there is a simplified sequence iterator protocol consisting of a few generic functions which can be specialized for iterator classes:

— Generic Function: iterator-step [sb-sequence] sequence iterator from-end

Moves iterator one position forward or backward in sequence depending on the iteration direction encoded in from-end.

— Generic Function: iterator-endp [sb-sequence] sequence iterator limit from-end

Returns non-NIL when iterator has reached limit (which may correspond to the end of SEQUENCE) with respect to the iteration direction encoded in from-end.

— Generic Function: iterator-element [sb-sequence] sequence iterator

Returns the element of sequence associated to the position of iterator.

— Generic Function: (setf iterator-element [sb-sequence])

Destructively modifies sequence by replacing the sequence element associated to position of iterator with new-value.

— Generic Function: iterator-index [sb-sequence] sequence iterator

Returns the position of iterator in sequence.

— Generic Function: iterator-copy [sb-sequence] sequence iterator

Returns a copy of iterator which also traverses sequence but can be mutated independently of iterator.

Iterator objects implementing the above simple iteration protocol are created by calling the following generic function:

— Generic Function: make-simple-sequence-iterator [sb-sequence] sequence &key from-end start end

Returns a sequence iterator for sequence, start, end and from-end as three values:

1. iterator state 2. limit 3. from-end

The returned iterator can be used with the generic iterator functions iterator-step, iterator-endp, iterator-element, (SETF ITERATOR-ELEMENT), iterator-index and iterator-copy.

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7.7 Support For Unix

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7.7.1 Command-line arguments

The UNIX command line can be read from the variable sb-ext:*posix-argv*.

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7.7.2 Querying the process environment

The UNIX environment can be queried with the sb-ext:posix-getenv function.

— Function: posix-getenv [sb-ext] name

Return the "value" part of the environment string "name=value" which corresponds to name, or nil if there is none.

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7.7.3 Running external programs

External programs can be run with sb-ext:run-program. 7

— Function: run-program [sb-ext] program args &key env environment wait search pty input if-input-does-not-exist output if-output-exists error if-error-exists status-hook external-format directory

run-program creates a new process specified by the program argument. args are the standard arguments that can be passed to a program. For no arguments, use nil (which means that just the name of the program is passed as arg 0).

The program arguments and the environment are encoded using the default external format for streams.

run-program will return a process structure. See the cmu Common Lisp Users Manual for details about the process structure.

Notes about Unix environments (as in the :environment and :env args):

The &key arguments have the following meanings:
a list of STRINGs describing the new Unix environment (as in "man environ"). The default is to copy the environment of the current process.
an alternative lossy representation of the new Unix environment, for compatibility with cmu cl
Look for program in each of the directories in the child's $PATH environment variable. Otherwise an absolute pathname is required.
If non-NIL (default), wait until the created process finishes. If nil, continue running Lisp until the program finishes.
Either t, nil, or a stream. Unless nil, the subprocess is established under a pty. If :pty is a stream, all output to this pty is sent to this stream, otherwise the process-pty slot is filled in with a stream connected to pty that can read output and write input.
Either t, nil, a pathname, a stream, or :stream. If t, the standard input for the current process is inherited. If nil, /dev/null is used. If a pathname, the file so specified is used. If a stream, all the input is read from that stream and sent to the subprocess. If :stream, the process-input slot is filled in with a stream that sends its output to the process. Defaults to nil.
:if-input-does-not-exist (when :input is the name of a file)
can be one of: :error to generate an error :create to create an empty file nil (the default) to return nil from run-program
Either t, nil, a pathname, a stream, or :stream. If t, the standard output for the current process is inherited. If nil, /dev/null is used. If a pathname, the file so specified is used. If a stream, all the output from the process is written to this stream. If :stream, the process-output slot is filled in with a stream that can be read to get the output. Defaults to nil.
:if-output-exists (when :output is the name of a file)
can be one of: :error (the default) to generate an error :supersede to supersede the file with output from the program :append to append output from the program to the file nil to return nil from run-program, without doing anything
:error and :if-error-exists
Same as :output and :if-output-exists, except that :error can also be specified as :output in which case all error output is routed to the same place as normal output.
This is a function the system calls whenever the status of the process changes. The function takes the process as an argument.
The external-format to use for :input, :output, and :error :STREAMs.
Specifies the directory in which the program should be run. nil (the default) means the directory is unchanged.
Windows specific options:
:escape-arguments (default T)
Controls escaping of the arguments passed to CreateProcess.

When sb-ext:run-program is called with wait equal to NIL, an instance of class sb-ext:process is returned. The following functions are available for use with processes:

— Function: process-p [sb-ext] object

t if object is a process, nil otherwise.

— Function: process-input [sb-ext] instance

The input stream of the process or nil.

— Function: process-output [sb-ext] instance

The output stream of the process or nil.

— Function: process-error [sb-ext] instance

The error stream of the process or nil.

— Function: process-alive-p [sb-ext] process

Return t if process is still alive, nil otherwise.

— Function: process-status [sb-ext] process

Return the current status of process. The result is one of :running, :stopped, :exited, or :signaled.

— Function: process-wait [sb-ext] process &optional check-for-stopped

Wait for process to quit running for some reason. When check-for-stopped is t, also returns when process is stopped. Returns process.

— Function: process-exit-code [sb-ext] process

The exit code or the signal of a stopped process.

— Function: process-core-dumped [sb-ext] instance

t if a core image was dumped by the process.

— Function: process-close [sb-ext] process

Close all streams connected to process and stop maintaining the status slot.

— Function: process-kill [sb-ext] process signal &optional whom

Hand signal to process. If whom is :pid, use the kill Unix system call. If whom is :process-group, use the killpg Unix system call. If whom is :pty-process-group deliver the signal to whichever process group is currently in the foreground.

Next: , Previous: Support For Unix, Up: Beyond the ANSI Standard

7.8 Unicode Support

SBCL provides support for working with Unicode text and querying the standard Unicode database for information about individual codepoints. Unicode-related functions are located in the sb-unicode package.

SBCL also extends ANSI character literal syntax to support Unicode codepoints. You can either specify a character by its Unicode name, with spaces replaced by underscores, if a unique name exists 8 or by giving its hexadecimal codepoint preceded by a “U”, an optional “+”, and an arbitrary number of leading zeros. You may also input the character directly into your source code if it can be encoded in your file. If a character had an assigned name in Unicode 1.0 that was distinct from its current name, you may also use that name (with spaces replaced by underscores) to specify the character, unless the name is already associated with a codepoint in the latest Unicode standard (such as “BELL”).

For example, you can specify the codepoint U+00E1 (“Latin Small Letter A With Acute”) as

7.8.1 Unicode property access

The following functions can be used to find information about a Unicode codepoint.

— Function: general-category [sb-unicode] character

Returns the general category of character as it appears in UnicodeData.txt

— Function: bidi-class [sb-unicode] character

Returns the bidirectional class of character

— Function: combining-class [sb-unicode] character

Returns the canonical combining class (CCC) of character

— Function: decimal-value [sb-unicode] character

Returns the decimal digit value associated with character or nil if there is no such value.

The only characters in Unicode with a decimal digit value are those that are part of a range of characters that encode the digits 0-9. Because of this, `(decimal-digit c) <=> (digit-char-p c 10)` in #+sb-unicode builds

— Function: digit-value [sb-unicode] character

Returns the Unicode digit value of character or nil if it doesn't exist.

Digit values are guaranteed to be integers between 0 and 9 inclusive. All characters with decimal digit values have the same digit value, but there are characters (such as digits of number systems without a 0 value) that have a digit value but no decimal digit value

— Function: numeric-value [sb-unicode] character

Returns the numeric value of character or nil if there is no such value. Numeric value is the most general of the Unicode numeric properties. The only constraint on the numeric value is that it be a rational number.

— Function: mirrored-p [sb-unicode] character

Returns t if character needs to be mirrored in bidirectional text. Otherwise, returns nil.

— Function: bidi-mirroring-glyph [sb-unicode] character

Returns the mirror image of character if it exists. Otherwise, returns nil.

— Function: age [sb-unicode] character

Returns the version of Unicode in which character was assigned as a pair of values, both integers, representing the major and minor version respectively. If character is not assigned in Unicode, returns nil for both values.

— Function: hangul-syllable-type [sb-unicode] character

Returns the Hangul syllable type of character. The syllable type can be one of :l, :v, :t, :lv, or :lvt. If the character is not a Hangul syllable or Jamo, returns nil

— Function: east-asian-width [sb-unicode] character

Returns the East Asian Width property of character as one of the keywords :n (Narrow), :a (Ambiguous), :h (Halfwidth), :w (Wide), :f (Fullwidth), or :na (Not applicable)

— Function: script [sb-unicode] character

Returns the Script property of character as a keyword. If character does not have a known script, returns :unknown

— Function: char-block [sb-unicode] character

Returns the Unicode block in which character resides as a keyword. If character does not have a known block, returns :no-block

— Function: unicode-1-name [sb-unicode] character

Returns the name assigned to character in Unicode 1.0 if it is distinct from the name currently assigned to character. Otherwise, returns nil. This property has been officially obsoleted by the Unicode standard, and is only included for backwards compatibility.

— Function: proplist-p [sb-unicode] character property

Returns t if character has the specified property. property is a keyword representing one of the properties from PropList.txt, with underscores replaced by dashes.

— Function: uppercase-p [sb-unicode] character

Returns t if character has the Unicode property Uppercase and nil otherwise

— Function: lowercase-p [sb-unicode] character

Returns t if character has the Unicode property Lowercase and nil otherwise

— Function: cased-p [sb-unicode] character

Returns t if character has a (Unicode) case, and nil otherwise

— Function: case-ignorable-p [sb-unicode] character

Returns t if character is Case Ignorable as defined in Unicode 6.3, Chapter 3

— Function: alphabetic-p [sb-unicode] character

Returns t if character is Alphabetic according to the Unicode standard and nil otherwise

— Function: ideographic-p [sb-unicode] character

Returns t if character has the Unicode property Ideographic, which loosely corresponds to the set of "Chinese characters"

— Function: math-p [sb-unicode] character

Returns t if character is a mathematical symbol according to Unicode and nil otherwise

— Function: whitespace-p [sb-unicode] character

Returns t if character is whitespace according to Unicode and nil otherwise

— Function: soft-dotted-p [sb-unicode] character

Returns t if character has a soft dot (such as the dots on i and j) which disappears when accents are placed on top of it. and nil otherwise

— Function: hex-digit-p [sb-unicode] character &key ascii

Returns t if character is a hexadecimal digit and nil otherwise. If :ascii is non-NIL, fullwidth equivalents of the Latin letters A through f are excluded.

— Function: default-ignorable-p [sb-unicode] character

Returns t if character is a Default_Ignorable_Code_Point

— Function: grapheme-break-class [sb-unicode] char

Returns the grapheme breaking class of character, as specified in uax #29.

— Function: word-break-class [sb-unicode] char

Returns the word breaking class of character, as specified in uax #29.

— Function: sentence-break-class [sb-unicode] char

Returns the sentence breaking class of character, as specified in uax #29.

— Function: line-break-class [sb-unicode] character &key resolve

Returns the line breaking class of character, as specified in uax #14. If :resolve is nil, returns the character class found in the property file. If :resolve is non-NIL, centain line-breaking classes will be mapped to othec classes as specified in the applicable standards. Addinionally, if :resolve is :east-asian, Ambigious (class :AI) characters will be mapped to the Ideographic (:ID) class instead of Alphabetic (:AL).

7.8.2 String operations

SBCL can normalize strings using:

— Function: normalize-string [sb-unicode] string &optional form filter

Normalize string to the Unicode normalization form form. Acceptable values for form are :nfd, :nfc, :nfkd, and :nfkc. If filter is a function it is called on each decomposed character and only characters for which it returns t are collected.

— Function: normalized-p [sb-unicode] string &optional form

Tests if string is normalized to form

SBCL implements the full range of Unicode case operations with the functions

— Function: uppercase [sb-unicode] string &key locale

Returns the full uppercase of string according to the Unicode standard. The result is not guaranteed to have the same length as the input. If :locale is nil, no language-specific case transformations are applied. If :locale is a keyword representing a two-letter iso country code, the case transforms of that locale are used. If :locale is t, the user's current locale is used (Unix and Win32 only).

— Function: lowercase [sb-unicode] string &key locale

Returns the full lowercase of string according to the Unicode standard. The result is not guaranteed to have the same length as the input. :locale has the same semantics as the :locale argument to uppercase.

— Function: titlecase [sb-unicode] string &key locale

Returns the titlecase of string. The resulting string can be longer than the input. :locale has the same semantics as the :locale argument to uppercase.

— Function: casefold [sb-unicode] string

Returns the full casefolding of string according to the Unicode standard. Casefolding removes case information in a way that allows the results to be used for case-insensitive comparisons. The result is not guaranteed to have the same length as the input.

It also extends standard Common Lisp case functions such as string-upcase and string-downcase to support a subset of Unicode's casing behavior. Specifically, a character is both-case-p if its case mapping in Unicode is one-to-one and invertable.

The sb-unicode package also provides functions for collating/sorting strings according to the Unicode Collation Algorithm.

— Function: unicode< [sb-unicode] string1 string2 &key start1 end1 start2 end2

Determines whether STRING1 sorts before STRING2 using the Unicode Collation Algorithm, The function uses an untailored Default Unicode Collation Element Table to produce the sort keys. The function uses the Shifted method for dealing with variable-weight characters, as described in uts #10

— Function: unicode= [sb-unicode] string1 string2 &key start1 end1 start2 end2 strict

Determines whether STRING1 and STRING2 are canonically equivalent according to Unicode. The start and end arguments behave like the arguments to STRING=. If :strict is nil, UNICODE= tests compatibility equavalence instead.

— Function: unicode-equal [sb-unicode] string1 string2 &key start1 end1 start2 end2 strict

Determines whether STRING1 and STRING2 are canonically equivalent after casefoldin8 (that is, ignoring case differences) according to Unicode. The start and end arguments behave like the arguments to STRING=. If :strict is nil, UNICODE= tests compatibility equavalence instead.

— Function: unicode<= [sb-unicode] string1 string2 &key start1 end1 start2 end2

Tests if STRING1 and STRING2 are either UNICODE< or UNICODE=

— Function: unicode> [sb-unicode] string1 string2 &key start1 end1 start2 end2


— Function: unicode>= [sb-unicode] string1 string2 &key start1 end1 start2 end2

Tests if STRING1 and STRING2 are either UNICODE= or UNICODE>

The following functions are provided for detecting visually confusable strings:

— Function: confusable-p [sb-unicode] string1 string2 &key start1 end1 start2 end2

Determines whether STRING1 and STRING2 could be visually confusable according to the idna confusableSummary.txt table

7.8.3 Breaking strings

The sb-unicode package includes several functions for breaking a Unicode string into useful parts.

— Function: graphemes [sb-unicode] string

Breaks string into graphemes acording to the default grapheme breaking rules specified in uax #29, returning a list of strings.

— Function: words [sb-unicode] string

Breaks string into words acording to the default word breaking rules specified in uax #29. Returns a list of strings

— Function: sentences [sb-unicode] string

Breaks string into sentences acording to the default sentence breaking rules specified in uax #29

— Function: lines [sb-unicode] string &key margin

Breaks string into lines that are no wider than :margin according to the line breaking rules outlined in uax #14. Combining marks will always be kept together with their base characters, and spaces (but not other types of whitespace) will be removed from the end of lines. If :margin is unspecified, it defaults to 80 characters

Next: , Previous: Unicode Support, Up: Beyond the ANSI Standard

7.9 Customization Hooks for Users

The toplevel repl prompt may be customized, and the function that reads user input may be replaced completely.

The behaviour of require when called with only one argument is implementation-defined. In SBCL, require behaves in the following way:

— Function: require [cl] module-name &optional pathnames

Loads a module, unless it already has been loaded. pathnames, if supplied, is a designator for a list of pathnames to be loaded if the module needs to be. If pathnames is not supplied, functions from the list *module-provider-functions* are called in order with module-name as an argument, until one of them returns non-NIL. User code is responsible for calling provide to indicate a successful load of the module.

— Variable: *module-provider-functions* [sb-ext]

See function documentation for require.

Although SBCL does not provide a resident editor, the ed function can be customized to hook into user-provided editing mechanisms as follows:

— Function: ed [cl] &optional x

Starts the editor (on a file or a function if named). Functions from the list *ed-functions* are called in order with x as an argument until one of them returns non-NIL; these functions are responsible for signalling a file-error to indicate failure to perform an operation on the file system.

— Variable: *ed-functions* [sb-ext]

See function documentation for ed.

Conditions of type warning and style-warning are sometimes signaled at runtime, especially during execution of Common Lisp defining forms such as defun, defmethod, etc. To muffle these warnings at runtime, SBCL provides a variable sb-ext:*muffled-warnings*:

— Variable: *muffled-warnings* [sb-ext]

A type that ought to specify a subtype of warning. Whenever a warning is signaled, if the warning is of this type and is not handled by any other handler, it will be muffled.

Next: , Previous: Customization Hooks for Users, Up: Beyond the ANSI Standard

7.10 Tools To Help Developers

SBCL provides a profiler and other extensions to the ANSI trace facility. For more information, see Macro common-lisp:trace.

The debugger supports a number of options. Its documentation is accessed by typing help at the debugger prompt. See Debugger.

Documentation for inspect is accessed by typing help at the inspect prompt.

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7.11 Resolution of Name Conflicts

The ANSI standard (section requires that name conflicts in packages be resolvable in favour of any of the conflicting symbols. In the interactive debugger, this is achieved by prompting for the symbol in whose favour the conflict should be resolved; for programmatic use, the sb-ext:resolve-conflict restart should be invoked with one argument, which should be a member of the list returned by the condition accessor sb-ext:name-conflict-symbols.

Next: , Previous: Resolution of Name Conflicts, Up: Beyond the ANSI Standard

7.12 Hash Table Extensions

Hash table extensions supported by SBCL are all controlled by keyword arguments to make-hash-table.

— Function: make-hash-table [cl] &key test size rehash-size rehash-threshold hash-function weakness synchronized

Create and return a new hash table. The keywords are as follows:

Determines how keys are compared. Must a designator for one of the standard hash table tests, or a hash table test defined using sb-ext:define-hash-table-test. Additionally, when an explicit hash-function is provided (see below), any two argument equivalence predicate can be used as the test.
A hint as to how many elements will be put in this hash table.
Indicates how to expand the table when it fills up. If an integer, add space for that many elements. If a floating point number (which must be greater than 1.0), multiply the size by that amount.
Indicates how dense the table can become before forcing a rehash. Can be any positive number <=1, with density approaching zero as the threshold approaches 0. Density 1 means an average of one entry per bucket.
If nil (the default), a hash function based on the test argument is used, which then must be one of the standardized hash table test functions, or one for which a default hash function has been defined using sb-ext:define-hash-table-test. If hash-function is specified, the test argument can be any two argument predicate consistent with it. The hash-function is expected to return a non-negative fixnum hash code.
When :weakness is not nil, garbage collection may remove entries from the hash table. The value of :weakness specifies how the presence of a key or value in the hash table preserves their entries from garbage collection.

Valid values are:

:key means that the key of an entry must be live to guarantee that the entry is preserved.

:value means that the value of an entry must be live to guarantee that the entry is preserved.

:key-and-value means that both the key and the value must be live to guarantee that the entry is preserved.

:key-or-value means that either the key or the value must be live to guarantee that the entry is preserved.

nil (the default) means that entries are always preserved.

If nil (the default), the hash-table may have multiple concurrent readers, but results are undefined if a thread writes to the hash-table concurrently with another reader or writer. If t, all concurrent accesses are safe, but note that clhs 3.6 (Traversal Rules and Side Effects) remains in force. See also: sb-ext:with-locked-hash-table. This keyword argument is experimental, and may change incompatibly or be removed in the future.

— Macro: define-hash-table-test [sb-ext] name hash-function

Defines name as a new kind of hash table test for use with the :test argument to make-hash-table, and associates a default hash-function with it.

name must be a symbol naming a global two argument equivalence predicate. Afterwards both 'name and #'name can be used with :test argument. In both cases hash-table-test will return the symbol name.

hash-function must be a symbol naming a global hash function consistent with the predicate, or be a lambda form implementing one in the current lexical environment. The hash function must compute the same hash code for any two objects for which name returns true, and subsequent calls with already hashed objects must always return the same hash code.

Note: The :hash-function keyword argument to make-hash-table can be used to override the specified default hash-function.

Attempting to define name in a locked package as hash-table test causes a package lock violation.


            ;;; 1.
            ;; We want to use objects of type FOO as keys (by their
            ;; names.) EQUALP would work, but would make the names
            ;; case-insensitive -- which we don't want.
            (defstruct foo (name nil :type (or null string)))
            ;; Define an equivalence test function and a hash function.
            (defun foo-name= (f1 f2) (equal (foo-name f1) (foo-name f2)))
            (defun sxhash-foo-name (f) (sxhash (foo-name f)))
            (define-hash-table-test foo-name= sxhash-foo-name)
            ;; #'foo-name would work too.
            (defun make-foo-table () (make-hash-table :test 'foo-name=))
            ;;; 2.
            (defun == (x y) (= x y))
            (define-hash-table-test ==
              (lambda (x)
                ;; Hash codes must be consistent with test, so
                ;; not (SXHASH X), since
                ;;   (= 1 1.0)                   => T
                ;;   (= (SXHASH 1) (SXHASH 1.0)) => NIL
                ;; Note: this doesn't deal with complex numbers or
                ;; bignums too large to represent as double floats.
                (sxhash (coerce x 'double-float))))
            ;; #'== would work too
            (defun make-number-table () (make-hash-table :test '==))

— Macro: with-locked-hash-table [sb-ext] (hash-table) &body body

Limits concurrent accesses to hash-table for the duration of body. If hash-table is synchronized, body will execute with exclusive ownership of the table. If hash-table is not synchronized, body will execute with other with-locked-hash-table bodies excluded -- exclusion of hash-table accesses not surrounded by with-locked-hash-table is unspecified.

— Function: hash-table-synchronized-p [sb-ext] instance

Returns t if hash-table is synchronized.

— Function: hash-table-weakness [sb-ext] instance

Return the weakness of hash-table which is one of nil, :key, :value, :key-and-value, :key-or-value.

Next: , Previous: Hash Table Extensions, Up: Beyond the ANSI Standard

7.13 Random Number Generation

The initial value of *random-state* is the same each time SBCL is started. This makes it possible for user code to obtain repeatable pseudo random numbers using only standard-provided functionality. See seed-random-state below for an SBCL extension that allows to seed the random number generator from given data for an additional possibility to achieve this. Non-repeatable random numbers can always be obtained using (make-random-state t).

The sequence of numbers produced by repeated calls to random starting with the same random state and using the same sequence of limit arguments is guaranteed to be reproducible only in the same version of SBCL on the same platform, using the same code under the same evaluator mode and compiler optimization qualities. Just two examples of differences that may occur otherwise: calls to random can be compiled differently depending on how much is known about the limit argument at compile time, yielding different results even if called with the same argument at run time, and the results can differ depending on the machine's word size, for example for limits that are fixnums under 64-bit word size but bignums under 32-bit word size.

— Function: seed-random-state [sb-ext] &optional state

Make a random state object. The optional state argument specifies a seed for deterministic pseudo-random number generation.

As per the Common Lisp standard for make-random-state,

As a supported sbcl extension, we also support receiving as a seed an object of the following types: While we support arguments of any size and will mix the provided bits into the random state, it is probably overkill to provide more than 256 bits worth of actual information.

This particular sbcl version also accepts an argument of the following type: (simple-array (unsigned-byte 32) (*))

This particular sbcl version uses the popular MT19937 prng algorithm, and its internal state only effectively contains about 19937 bits of information.

Some notes on random floats: The standard doesn't prescribe a specific method of generating random floats. The following paragraph describes SBCL's current implementation and should be taken purely informational, that is, user code should not depend on any of its specific properties. The method used has been chosen because it is common, conceptually simple and fast.

To generate random floats, SBCL evaluates code that has an equivalent effect as

     (* limit
        (float (/ (random (expt 2 23)) (expt 2 23)) 1.0f0))

(for single-floats) and correspondingly (with 52 and 1.0d0 instead of 23 and 1.0f0) for double-floats. Note especially that this means that zero is a possible return value occurring with probability (expt 2 -23) respectively (expt 2 -52). Also note that there exist twice as many equidistant floats between 0 and 1 as are generated. For example, the largest number that (random 1.0f0) ever returns is (float (/ (1- (expt 2 23)) (expt 2 23)) 1.0f0) while (float (/ (1- (expt 2 24)) (expt 2 24)) 1.0f0) is the largest single-float less than 1. This is a side effect of the fact that the implementation uses the fastest possible conversion from bits to floats.

SBCL currently uses the Mersenne Twister as its random number generator, specifically the 32-bit version under both 32- and 64-bit word size. The seeding algorithm has been improved several times by the authors of the Mersenne Twister; SBCL uses the third version (from 2002) which is still the most recent as of June 2012. The implementation has been tested to provide output identical to the recommended C implementation.

While the Mersenne Twister generates random numbers of much better statistical quality than other widely used generators, it uses only linear operations modulo 2 and thus fails some statistical tests9. For example, the distribution of ranks of (sufficiently large) random binary matrices is much distorted compared to the theoretically expected one when the matrices are generated by the Mersenne Twister. Thus, applications that are sensitive to this aspect should use a different type of generator.

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7.14 Miscellaneous Extensions

— Function: array-storage-vector [sb-ext] array

Returns the underlying storage vector of array, which must be a non-displaced array.

In sbcl, if array is a of type (simple-array * (*)), it is its own storage vector. Multidimensional arrays, arrays with fill pointers, and adjustable arrays have an underlying storage vector with the same array-element-type as array, which this function returns.

Important note: the underlying vector is an implementation detail. Even though this function exposes it, changes in the implementation may cause this function to be removed without further warning.

— Function: delete-directory [sb-ext] pathspec &key recursive

Deletes the directory designated by pathspec (a pathname designator). Returns the truename of the directory deleted.

If recursive is false (the default), signals an error unless the directory is empty. If recursive is true, first deletes all files and subdirectories. If recursive is true and the directory contains symbolic links, the links are deleted, not the files and directories they point to.

Signals an error if pathspec designates a file or a symbolic link instead of a directory, or if the directory could not be deleted for any reason.


             (DELETE-DIRECTORY "/tmp/foo")
             (DELETE-DIRECTORY "/tmp/foo/")

delete the "foo" subdirectory of "/tmp", or signal an error if it does not exist or if is a file or a symbolic link.

— Function: get-time-of-day [sb-ext]

Return the number of seconds and microseconds since the beginning of the unix epoch (January 1st 1970.)

— Macro: wait-for [sb-ext] test-form &key timeout

Wait until test-form evaluates to true, then return its primary value. If timeout is provided, waits at most approximately timeout seconds before returning nil.

If with-deadline has been used to provide a global deadline, signals a deadline-timeout if test-form doesn't evaluate to true before the deadline.

Experimental: subject to change without prior notice.

— Function: assert-version->= [sb-ext] &rest subversions

Asserts that the current sbcl is of version equal to or greater than the version specified in the arguments. A continuable error is signaled otherwise.

The arguments specify a sequence of subversion numbers in big endian order. They are compared lexicographically with the runtime version, and versions are treated as though trailed by an unbounded number of 0s.

For example, (assert-version->= 1 1 4) asserts that the current sbcl is version 1.1.4[.0.0...] or greater, and (assert-version->= 1) that it is version 1[.0.0...] or greater.

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7.15 Stale Extensions

SBCL has inherited from CMUCL various hooks to allow the user to tweak and monitor the garbage collection process. These are somewhat stale code, and their interface might need to be cleaned up. If you have urgent need of them, look at the code in src/code/gc.lisp and bring it up on the developers' mailing list.

SBCL has various hooks inherited from CMUCL, like sb-ext:float-denormalized-p, to allow a program to take advantage of IEEE floating point arithmetic properties which aren't conveniently or efficiently expressible using the ANSI standard. These look good, and their interface looks good, but IEEE support is slightly broken due to a stupid decision to remove some support for infinities (because it wasn't in the ANSI spec and it didn't occur to me that it was in the IEEE spec). If you need this stuff, take a look at the code and bring it up on the developers' mailing list.

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7.16 Efficiency Hacks

The sb-ext:purify function causes SBCL first to collect all garbage, then to mark all uncollected objects as permanent, never again attempting to collect them as garbage. This can cause a large increase in efficiency when using a primitive garbage collector, or a more moderate increase in efficiency when using a more sophisticated garbage collector which is well suited to the program's memory usage pattern. It also allows permanent code to be frozen at fixed addresses, a precondition for using copy-on-write to share code between multiple Lisp processes. This is less important with modern generational garbage collectors, but not all SBCL platforms use such a garbage collector.

— Function: purify [sb-ext] &key root-structures environment-name

This function optimizes garbage collection by moving all currently live objects into non-collected storage. root-structures is an optional list of objects which should be copied first to maximize locality.

defstruct structures defined with the (:pure t) option are moved into read-only storage, further reducing gc cost. List and vector slots of pure structures are also moved into read-only storage.

environment-name is unused.

This function is a no-op on platforms using the generational garbage collector (x86, x86-64, ppc, arm, arm64).

The sb-ext:truly-the special form declares the type of the result of the operations, producing its argument; the declaration is not checked. In short: don't use it.

— Special Operator: truly-the [sb-ext] value-type form

Specifies that the values returned by form conform to the value-type, and causes the compiler to trust this information unconditionally.

Consequences are undefined if any result is not of the declared type -- typical symptoms including memory corruptions. Use with great care.

The sb-ext:freeze-type declaration declares that a type will never change, which can make type testing (typep, etc.) more efficient for structure types.

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8 Foreign Function Interface

This chapter describes SBCL's interface to C programs and libraries (and, since C interfaces are a sort of lingua franca of the Unix world, to other programs and libraries in general.)

Note: In the modern Lisp world, the usual term for this functionality is Foreign Function Interface, or FFI, where despite the mention of “function” in this term, FFI also refers to direct manipulation of C data structures as well as functions. The traditional CMUCL terminology is Alien Interface, and while that older terminology is no longer used much in the system documentation, it still reflected in names in the implementation, notably in the name of the SB-ALIEN package.

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8.1 Introduction to the Foreign Function Interface

Because of Lisp's emphasis on dynamic memory allocation and garbage collection, Lisp implementations use non-C-like memory representations for objects. This representation mismatch creates friction when a Lisp program must share objects with programs which expect C data. There are three common approaches to establishing communication:

SBCL, like CMUCL before it, relies primarily on the automatic conversion and direct manipulation approaches. The SB-ALIEN package provides a facility wherein foreign values of simple scalar types are automatically converted and complex types are directly manipulated in their foreign representation. Additionally the lower-level System Area Pointers (or SAPs) can be used where necessary to provide untyped access to foreign memory.

Any foreign objects that can't automatically be converted into Lisp values are represented by objects of type alien-value. Since Lisp is a dynamically typed language, even foreign objects must have a run-time type; this type information is provided by encapsulating the raw pointer to the foreign data within an alien-value object.

The type language and operations on foreign types are intentionally similar to those of the C language.

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8.2 Foreign Types

Alien types have a description language based on nested list structure. For example the C type

     struct foo {
         int a;
         struct foo *b[100];

has the corresponding SBCL FFI type

     (struct foo
       (a int)
       (b (array (* (struct foo)) 100)))

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8.2.1 Defining Foreign Types

Types may be either named or anonymous. With structure and union types, the name is part of the type specifier, allowing recursively defined types such as:

     (struct foo (a (* (struct foo))))

An anonymous structure or union type is specified by using the name nil. The with-alien macro defines a local scope which “captures” any named type definitions. Other types are not inherently named, but can be given named abbreviations using the define-alien-type macro.

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8.2.2 Foreign Types and Lisp Types

The foreign types form a subsystem of the SBCL type system. An alien type specifier provides a way to use any foreign type as a Lisp type specifier. For example,

     (typep foo '(alien (* int)))

can be used to determine whether foo is a pointer to a foreign int. alien type specifiers can be used in the same ways as ordinary Lisp type specifiers (like string.) Alien type declarations are subject to the same precise type checking as any other declaration. See Precise Type Checking.

Note that the type identifiers used in the foreign type system overlap with native Lisp type specifiers in some cases. For example, the type specifier (alien single-float) is identical to single-float, since foreign floats are automatically converted to Lisp floats. When type-of is called on an alien value that is not automatically converted to a Lisp value, then it will return an alien type specifier.

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8.2.3 Foreign Type Specifiers

Note: All foreign type names are exported from the sb-alien package. Some foreign type names are also symbols in the common-lisp package, in which case they are reexported from the sb-alien package, so that e.g. it is legal to refer to sb-alien:single-float.

These are the basic foreign type specifiers:

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8.3 Operations On Foreign Values

This section describes how to read foreign values as Lisp values, how to coerce foreign values to different kinds of foreign values, and how to dynamically allocate and free foreign variables.

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8.3.1 Accessing Foreign Values

— Function: deref [sb-alien] pointer-or-array &rest indices

The sb-alien:deref function returns the value pointed to by a foreign pointer, or the value of a foreign array element. When dereferencing a pointer, an optional single index can be specified to give the equivalent of C pointer arithmetic; this index is scaled by the size of the type pointed to. When dereferencing an array, the number of indices must be the same as the number of dimensions in the array type. deref can be set with setf to assign a new value.

— Function: slot [sb-alien] struct-or-union slot-name

The sb-alien:slot function extracts the value of the slot named slot-name from a foreign struct or union. If struct-or-union is a pointer to a structure or union, then it is automatically dereferenced. sb-alien:slot can be set with setf to assign a new value. Note that slot-name is evaluated, and need not be a compile-time constant (but only constant slot accesses are efficiently compiled). Untyped memory

As noted at the beginning of the chapter, the System Area Pointer facilities allow untyped access to foreign memory. SAPs can be converted to and from the usual typed foreign values using sap-alien and alien-sap (described elsewhere), and also to and from integers - raw machine addresses. They should thus be used with caution; corrupting the Lisp heap or other memory with SAPs is trivial.

— Function: int-sap [sb-sys] machine-address

Creates a SAP pointing at the virtual address machine-address.

— Function: sap-ref-32 [sb-sys] sap offset

Access the value of the memory location at offset bytes from sap. This form may also be used with setf to alter the memory at that location.

— Function: sap= [sb-sys] sap1 sap2

Compare sap1 and sap2 for equality.

Similarly named functions exist for accessing other sizes of word, other comparisons, and other conversions. The reader is invited to use apropos and describe for more details

     (apropos "sap" :sb-sys)

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8.3.2 Coercing Foreign Values

— Macro: addr [sb-alien] alien-expr

The sb-alien:addr macro returns a pointer to the location specified by alien-expr, which must be either a foreign variable, a use of sb-alien:deref, a use of sb-alien:slot, or a use of sb-alien:extern-alien.

— Macro: cast [sb-alien] foreign-value new-type

The sb-alien:cast macro converts foreign-value to a new foreign value with the specified new-type. Both types, old and new, must be foreign pointer, array or function types. Note that the resulting Lisp foreign variable object is not eq to the argument, but it does refer to the same foreign data bits.

— Macro: sap-alien [sb-alien] sap type

The sb-alien:sap-alien macro converts sap (a system area pointer) to a foreign value with the specified type. type is not evaluated.

The type must be some foreign pointer, array, or record type.

— Function: alien-sap [sb-alien] foreign-value

The sb-alien:alien-sap function returns the SAP which points to alien-value's data.

The foreign-value must be of some foreign pointer, array, or record type.

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8.3.3 Foreign Dynamic Allocation

Lisp code can call the C standard library functions malloc and free to dynamically allocate and deallocate foreign variables. The Lisp code shares the same allocator with foreign C code, so it's OK for foreign code to call free on the result of Lisp sb-alien:make-alien, or for Lisp code to call sb-alien:free-alien on foreign objects allocated by C code.

— Macro: make-alien [sb-alien] type &optional size

Allocate an alien of type type in foreign heap, and return an alien pointer to it. The allocated memory is not initialized, and may contain garbage. The memory is allocated using malloc(3), so it can be passed to foreign functions which use free(3), or released using free-alien.

For alien stack allocation, see macro with-alien.

The type argument is not evaluated. If size is supplied, how it is interpreted depends on type:

            (defvar *foo* (make-alien (array char 10)))
            (type-of *foo*)                 ; => (alien (* (array (signed 8) 10)))
            (setf (deref (deref foo) 0) 10) ; => 10
            (make-alien char 12)            ; => (alien (* (signed 8)))

— Function: make-alien-string [sb-alien] string &rest rest &key start end external-format null-terminate

Copy part of string delimited by start and end into freshly allocated foreign memory, freeable using free(3) or free-alien. Returns the allocated string as a (* char) alien, and the number of bytes allocated as secondary value.

The string is encoded using external-format. If null-terminate is true (the default), the alien string is terminated by an additional null byte.

— Function: free-alien [sb-alien] alien

Dispose of the storage pointed to by alien. The alien must have been allocated by make-alien, make-alien-string or malloc(3).

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8.4 Foreign Variables

Both local (stack allocated) and external (C global) foreign variables are supported.

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8.4.1 Local Foreign Variables

— Macro: with-alien [sb-alien] var-definitions &body body

The with-alien macro establishes local foreign variables with the specified alien types and names. This form is analogous to defining a local variable in C: additional storage is allocated, and the initial value is copied. This form is less analogous to LET-allocated Lisp variables, since the variables can't be captured in closures: they live only for the dynamic extent of the body, and referring to them outside is a gruesome error.

The var-definitions argument is a list of variable definitions, each of the form

          (name type &optional initial-value)

The names of the variables are established as symbol-macros; the bindings have lexical scope, and may be assigned with setq or setf.

The with-alien macro also establishes a new scope for named structures and unions. Any type specified for a variable may contain named structure or union types with the slots specified. Within the lexical scope of the binding specifiers and body, a locally defined foreign structure type foo can be referenced by its name using (struct foo).

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8.4.2 External Foreign Variables

External foreign names are strings, and Lisp names are symbols. When an external foreign value is represented using a Lisp variable, there must be a way to convert from one name syntax into the other. The macros extern-alien, define-alien-variable and define-alien-routine use this conversion heuristic:

— Macro: define-alien-variable [sb-alien] name type

The define-alien-variable macro defines name as an external foreign variable of the specified foreign type. name and type are not evaluated. The Lisp name of the variable (see above) becomes a global alien variable. Global alien variables are effectively “global symbol macros”; a reference to the variable fetches the contents of the external variable. Similarly, setting the variable stores new contents – the new contents must be of the declared type. Someday, they may well be implemented using the ANSI define-symbol-macro mechanism, but as of SBCL 0.7.5, they are still implemented using an older more-or-less parallel mechanism inherited from CMUCL.

For example, to access a C-level counter foo, one could write

          (define-alien-variable "foo" int)
          ;; Now it is possible to get the value of the C variable foo simply by
          ;; referencing that Lisp variable:
          (print foo)
          (setf foo 14)
          (incf foo)
— Function: get-errno [sb-alien]

Since in modern C libraries, the errno “variable” is typically no longer a variable, but some bizarre artificial construct which behaves superficially like a variable within a given thread, it can no longer reliably be accessed through the ordinary define-alien-variable mechanism. Instead, SBCL provides the operator sb-alien:get-errno to allow Lisp code to read it.

— Macro: extern-alien [sb-alien] name type

The extern-alien macro returns an alien with the specified type which points to an externally defined value. name is not evaluated, and may be either a string or a symbol. type is an unevaluated alien type specifier.

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8.5 Foreign Data Structure Examples

Now that we have alien types, operations and variables, we can manipulate foreign data structures. This C declaration

     struct foo {
         int a;
         struct foo *b[100];

can be translated into the following alien type:

     (define-alien-type nil
       (struct foo
         (a int)
         (b (array (* (struct foo)) 100))))

Once the foo alien type has been defined as above, the C expression

     struct foo f;

can be translated in this way:

     (with-alien ((f (struct foo)))
       (slot (deref (slot f 'b) 7) 'a)
       ;; Do something with f...

Or consider this example of an external C variable and some accesses:

     struct c_struct {
             short x, y;
             char a, b;
             int z;
             c_struct *n;
     extern struct c_struct *my_struct;
     my_struct->a = 5;
     my_struct = my_struct->n;

which can be manipulated in Lisp like this:

     (define-alien-type nil
       (struct c-struct
               (x short)
               (y short)
               (a char)
               (b char)
               (z int)
               (n (* c-struct))))
     (define-alien-variable "my_struct" (* c-struct))
     (incf (slot my-struct 'x))
     (setf (slot my-struct 'a) 5)
     (setq my-struct (slot my-struct 'n))

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8.6 Loading Shared Object Files

Foreign object files can be loaded into the running Lisp process by calling load-shared-object.

— Function: load-shared-object [sb-alien] pathname &key dont-save

Load a shared library / dynamic shared object file / similar foreign container specified by designated pathname, such as a .so on an elf platform.

Locating the shared object follows standard rules of the platform, consult the manual page for dlopen(3) for details. Typically paths specified by environment variables such as LD_LIBRARY_PATH are searched if the pathname has no directory, but on some systems (eg. Mac os X) search may happen even if pathname is absolute. (On Windows LoadLibrary is used instead of dlopen(3).)

On non-Windows platforms calling load-shared-object again with a pathname equal to the designated pathname of a previous call will replace the old definitions; if a symbol was previously referenced through the object and is not present in the reloaded version an error will be signalled. Reloading may not work as expected if user or library-code has called dlopen(3) on the same shared object.

load-shared-object interacts with sb-ext:save-lisp-and-die:

1. If dont-save is true (default is NIL), the shared object will be dropped when save-lisp-and-die is called -- otherwise shared objects are reloaded automatically when a saved core starts up. Specifying dont-save can be useful when the location of the shared object on startup is uncertain.

2. On most platforms references in compiled code to foreign symbols in shared objects (such as those generated by DEFINE-ALIEN-ROUTINE) remain valid across save-lisp-and-die. On those platforms where this is not supported, a warning will be signalled when the core is saved -- this is orthogonal from dont-save.

— Function: unload-shared-object [sb-alien] pathname

Unloads the shared object loaded earlier using the designated pathname with load-shared-object, to the degree supported on the platform.


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8.7 Foreign Function Calls

The foreign function call interface allows a Lisp program to call many functions written in languages that use the C calling convention.

Lisp sets up various signal handling routines and other environment information when it first starts up, and expects these to be in place at all times. The C functions called by Lisp should not change the environment, especially the signal handlers: the signal handlers installed by Lisp typically have interesting flags set (e.g to request machine context information, or for signal delivery on an alternate stack) which the Lisp runtime relies on for correct operation. Precise details of how this works may change without notice between versions; the source, or the brain of a friendly SBCL developer, is the only documentation. Users of a Lisp built with the :sb-thread feature should also read the section about threads, Threading.

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8.7.1 The alien-funcall Primitive

— Function: alien-funcall [sb-alien] alien-function &rest arguments

The alien-funcall function is the foreign function call primitive: alien-function is called with the supplied arguments and its C return value is returned as a Lisp value. The alien-function is an arbitrary run-time expression; to refer to a constant function, use extern-alien or a value defined by define-alien-routine.

The type of alien-function must be (alien (function ...)) or (alien (* (function ...))). The function type is used to determine how to call the function (as though it was declared with a prototype.) The type need not be known at compile time, but only known-type calls are efficiently compiled. Limitations:

Here is an example which allocates a (struct foo), calls a foreign function to initialize it, then returns a Lisp vector of all the (* (struct foo)) objects filled in by the foreign call:

     ;; Allocate a foo on the stack.
     (with-alien ((f (struct foo)))
       ;; Call some C function to fill in foo fields.
       (alien-funcall (extern-alien "mangle_foo" (function void (* foo)))
                      (addr f))
       ;; Find how many foos to use by getting the A field.
       (let* ((num (slot f 'a))
              (result (make-array num)))
         ;; Get a pointer to the array so that we don't have to keep extracting it:
         (with-alien ((a (* (array (* (struct foo)) 100)) (addr (slot f 'b))))
           ;; Loop over the first N elements and stash them in the result vector.
           (dotimes (i num)
             (setf (svref result i) (deref (deref a) i)))
           ;; Voila.

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8.7.2 The define-alien-routine Macro

— Macro: define-alien-routine [sb-alien] name result-type &rest arg-specifiers

The define-alien-routine macro is a convenience for automatically generating Lisp interfaces to simple foreign functions. The primary feature is the parameter style specification, which translates the C pass-by-reference idiom into additional return values.

name is usually a string external symbol, but may also be a symbol Lisp name or a list of the foreign name and the Lisp name. If only one name is specified, the other is automatically derived as for extern-alien. result-type is the alien type of the return value.

Each element of the arg-specifiers list specifies an argument to the foreign function, and is of the form

          (aname atype &amp;optional style)

aname is the symbol name of the argument to the constructed function (for documentation). atype is the alien type of corresponding foreign argument. The semantics of the actual call are the same as for alien-funcall. style specifies how this argument should be handled at call and return time, and should be one of the following:

Note: Any efficiency-critical foreign interface function should be inline expanded, which can be done by preceding the define-alien-routine call with:
          (declaim (inline lisp-name))

In addition to avoiding the Lisp call overhead, this allows pointers, word-integers and floats to be passed using non-descriptor representations, avoiding consing.)

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8.7.3 define-alien-routine Example

Consider the C function cfoo with the following calling convention:

     cfoo (str, a, i)
         char *str;
         char *a; /* update */
         int *i; /* out */
       /* body of cfoo(...) */

This can be described by the following call to define-alien-routine:

     (define-alien-routine "cfoo" void
       (str c-string)
       (a char :in-out)
       (i int :out))

The Lisp function cfoo will have two arguments (str and a) and two return values (a and i).

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8.7.4 Calling Lisp From C

Calling Lisp functions from C is sometimes possible, but is extremely hackish and poorly supported as of SBCL 0.7.5. See funcall0 ... funcall3 in the runtime system. The arguments must be valid SBCL object descriptors (so that e.g. fixnums must be left-shifted by 2.) As of SBCL 0.7.5, the format of object descriptors is documented only by the source code and, in parts, by the old CMUCL INTERNALS documentation.

Note that the garbage collector moves objects, and won't be able to fix up any references in C variables. There are three mechanisms for coping with this:

  1. The sb-ext:purify moves all live Lisp data into static or read-only areas such that it will never be moved (or freed) again in the life of the Lisp session
  2. sb-sys:with-pinned-objects is a macro which arranges for some set of objects to be pinned in memory for the dynamic extent of its body forms. On ports which use the generational garbage collector (as of SBCL 0.8.3, only the x86) this has a page granularity - i.e. the entire 4k page or pages containing the objects will be locked down. On other ports it is implemented by turning off GC for the duration (so could be said to have a whole-world granularity).
  3. Disable GC, using the without-gcing macro.

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8.8 Step-By-Step Example of the Foreign Function Interface

This section presents a complete example of an interface to a somewhat complicated C function.

Suppose you have the following C function which you want to be able to call from Lisp in the file test.c

     struct c_struct
       int x;
       char *s;
     struct c_struct *c_function (i, s, r, a)
         int i;
         char *s;
         struct c_struct *r;
         int a[10];
       int j;
       struct c_struct *r2;
       printf("i = %d\n", i);
       printf("s = %s\n", s);
       printf("r->x = %d\n", r->x);
       printf("r->s = %s\n", r->s);
       for (j = 0; j < 10; j++) printf("a[%d] = %d.\n", j, a[j]);
       r2 = (struct c_struct *) malloc (sizeof(struct c_struct));
       r2->x = i + 5;
       r2->s = "a C string";

It is possible to call this C function from Lisp using the file test.lisp containing

     (cl:defpackage "TEST-C-CALL" (:use "CL" "SB-ALIEN" "SB-C-CALL"))
     (cl:in-package "TEST-C-CALL")
     ;;; Define the record C-STRUCT in Lisp.
     (define-alien-type nil
         (struct c-struct
                 (x int)
                 (s c-string)))
     ;;; Define the Lisp function interface to the C routine.  It returns a
     ;;; pointer to a record of type C-STRUCT.  It accepts four parameters:
     ;;; I, an int; S, a pointer to a string; R, a pointer to a C-STRUCT
     ;;; record; and A, a pointer to the array of 10 ints.
     ;;; The INLINE declaration eliminates some efficiency notes about heap
     ;;; allocation of alien values.
     (declaim (inline c-function))
     (define-alien-routine c-function
         (* (struct c-struct))
       (i int)
       (s c-string)
       (r (* (struct c-struct)))
       (a (array int 10)))
     ;;; a function which sets up the parameters to the C function and
     ;;; actually calls it
     (defun call-cfun ()
       (with-alien ((ar (array int 10))
                    (c-struct (struct c-struct)))
         (dotimes (i 10)                     ; Fill array.
           (setf (deref ar i) i))
         (setf (slot c-struct 'x) 20)
         (setf (slot c-struct 's) "a Lisp string")
         (with-alien ((res (* (struct c-struct))
                           (c-function 5 "another Lisp string" (addr c-struct) ar)))
           (format t "~&amp;back from C function~%")
               (values (slot res 'x)
                       (slot res 's))
             ;; Deallocate result. (after we are done referring to it:
             ;; "Pillage, *then* burn.")
             (free-alien res)))))

To execute the above example, it is necessary to compile the C routine, e.g.: ‘cc -c test.c && ld -shared -o test.o’ (In order to enable incremental loading with some linkers, you may need to say ‘cc -G 0 -c test.c’)

Once the C code has been compiled, you can start up Lisp and load it in: ‘sbcl’. Lisp should start up with its normal prompt.

Within Lisp, compile the Lisp file. (This step can be done separately. You don't have to recompile every time.) ‘(compile-file "test.lisp")

Within Lisp, load the foreign object file to define the necessary symbols: ‘(load-shared-object "")’.

Now you can load the compiled Lisp (“fasl”) file into Lisp: ‘(load "test.fasl")’ And once the Lisp file is loaded, you can call the Lisp routine that sets up the parameters and calls the C function: ‘(test-c-call::call-cfun)

The C routine should print the following information to standard output:

     i = 5
     s = another Lisp string
     r->x = 20
     r->s = a Lisp string
     a[0] = 0.
     a[1] = 1.
     a[2] = 2.
     a[3] = 3.
     a[4] = 4.
     a[5] = 5.
     a[6] = 6.
     a[7] = 7.
     a[8] = 8.
     a[9] = 9.

After return from the C function, the Lisp wrapper function should print the following output:

     back from C function

And upon return from the Lisp wrapper function, before the next prompt is printed, the Lisp read-eval-print loop should print the following return values:

     "a C string"

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9 Pathnames

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9.1 Lisp Pathnames

There are many aspects of ANSI Common Lisp's pathname support which are implementation-defined and so need documentation.

9.1.1 Home Directory Specifiers

SBCL accepts the keyword :home and a list of the form (:home "username") as a directory component immediately following :absolute.

:home is represented in namestrings by ~/ and (:home "username" by ~username/ at the start of the namestring. Tilde-characters elsewhere in namestrings represent themselves.

Home directory specifiers are resolved to home directory of the current or specified user by native-namestring, which is used by the implementation to translate pathnames before passing them on to operating system specific routines.

Using (:home "user") form on Windows signals an error.

9.1.2 The SYS Logical Pathname Host

The logical pathname host named by "SYS" exists in SBCL. Its logical-pathname-translations may be set by the site or the user applicable to point to the locations of the system's sources; in particular, the core system's source files match the logical pathname "SYS:SRC;**;*.*.*", and the contributed modules' source files match "SYS:CONTRIB;**;*.*.*".

— Function: set-sbcl-source-location [sb-ext] pathname

Initialize the sys logical host based on pathname, which should be the top-level directory of the sbcl sources. This will replace any existing translations for "SYS:SRC;", "SYS:CONTRIB;", and "SYS:OUTPUT;". Other "SYS:" translations are preserved.

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9.2 Native Filenames

In some circumstances, what is wanted is a Lisp pathname object which corresponds to a string produced by the Operating System. In this case, some of the default parsing rules are inappropriate: most filesystems do not have a native understanding of wild pathnames; such functionality is often provided by shells above the OS, often in mutually-incompatible ways.

To allow the user to deal with this, the following functions are provided: parse-native-namestring and native-pathname return the closest equivalent Lisp pathname to a given string (appropriate for the Operating System), while native-namestring converts a non-wild pathname designator to the equivalent native namestring, if possible. Some Lisp pathname concepts (such as the :back directory component) have no direct equivalents in most Operating Systems; the behaviour of native-namestring is unspecified if an inappropriate pathname designator is passed to it. Additionally, note that conversion from pathname to native filename and back to pathname should not be expected to preserve equivalence under equal.

— Function: parse-native-namestring [sb-ext] thing &optional host defaults &key start end junk-allowed as-directory

Convert thing into a pathname, using the native conventions appropriate for the pathname host host, or if not specified the host of defaults. If thing is a string, the parse is bounded by start and end, and error behaviour is controlled by junk-allowed, as with parse-namestring. For file systems whose native conventions allow directories to be indicated as files, if as-directory is true, return a pathname denoting thing as a directory.

— Function: native-pathname [sb-ext] pathspec

Convert pathspec (a pathname designator) into a pathname, assuming the operating system native pathname conventions.

— Function: native-namestring [sb-ext] pathname &key as-file

Construct the full native (name)string form of pathname. For file systems whose native conventions allow directories to be indicated as files, if as-file is true and the name, type, and version components of pathname are all nil or :unspecific, construct a string that names the directory according to the file system's syntax for files.

Because some file systems permit the names of directories to be expressed in multiple ways, it is occasionally necessary to parse a native file name “as a directory name” or to produce a native file name that names a directory “as a file”. For these cases, parse-native-namestring accepts the keyword argument as-directory to force a filename to parse as a directory, and native-namestring accepts the keyword argument as-file to force a pathname to unparse as a file. For example,

     ; On Unix, the directory "/tmp/" can be denoted by "/tmp/" or "/tmp".
     ; Under the default rules for native filenames, these parse and
     ; unparse differently.
     (defvar *p*)
     (setf *p* (parse-native-namestring "/tmp/")) ⇒ #P"/tmp/"
     (pathname-name *p*) ⇒ NIL
     (pathname-directory *p*) ⇒ (:ABSOLUTE "tmp")
     (native-namestring *p*) ⇒ "/tmp/"
     (setf *p* (parse-native-namestring "/tmp")) ⇒ #P"/tmp"
     (pathname-name *p*) ⇒ "tmp"
     (pathname-directory *p*) ⇒ (:ABSOLUTE)
     (native-namestring *p*) ⇒ "/tmp"
     ; A non-NIL AS-DIRECTORY argument to PARSE-NATIVE-NAMESTRING forces
     ; both the second string to parse the way the first does.
     (setf *p* (parse-native-namestring "/tmp"
                                        nil *default-pathname-defaults*
                                        :as-directory t)) ⇒ #P"/tmp/"
     (pathname-name *p*) ⇒ NIL
     (pathname-directory *p*) ⇒ (:ABSOLUTE "tmp")
     ; A non-NIL AS-FILE argument to NATIVE-NAMESTRING forces the pathname
     ; parsed from the first string to unparse as the second string.
     (setf *p* (parse-native-namestring "/tmp/")) ⇒ #P"/tmp/"
     (native-namestring *p* :as-file t) ⇒ "/tmp"

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10 Streams

Streams which read or write Lisp character data from or to the outside world – files, sockets or other external entities – require the specification of a conversion between the external, binary data and the Lisp characters. In ANSI Common Lisp, this is done by specifying the :external-format argument when the stream is created. The major information required is an encoding, specified by a keyword naming that encoding; however, it is also possible to specify refinements to that encoding as additional options to the external format designator.

In addition, SBCL supports various extensions of ANSI Common Lisp streams:

Bivalent Streams
A type of stream that can read and write both character and (unsigned-byte 8) values.
Gray Streams
User-overloadable CLOS classes whose instances can be used as Lisp streams (e.g. passed as the first argument to format).
Simple Streams
The bundled contrib module sb-simple-streams implements a subset of the Franz Allegro simple-streams proposal.

Next: , Up: Streams

10.1 External Formats

The encodings supported by SBCL as external formats are named by keyword. Each encoding has a canonical name, which will be encoding returned by stream-external-format, and a number of aliases for convenience, as shown in the table below:

:US-ASCII, :ANSI_X3.4-1968, :ISO-646, :ISO-646-US, :|646|
:|cp1250|, :WINDOWS-1250, :|windows-1250|
:|cp1251|, :WINDOWS-1251, :|windows-1251|
:|cp1252|, :WINDOWS-1252, :|windows-1252|
:|cp1253|, :WINDOWS-1253, :|windows-1253|
:|cp1255|, :WINDOWS-1255, :|windows-1255|
:|cp1256|, :WINDOWS-1256, :|windows-1256|
:|cp1257|, :WINDOWS-1257, :|windows-1257|
:|cp1258|, :WINDOWS-1258, :|windows-1258|
:CP037, :|cp037|, :IBM-037, :IBM037
:EUCJP, :|eucJP|
:|iso-8859-10|, :LATIN-6, :|latin-6|
:|iso-8859-13|, :LATIN-7, :|latin-7|
:|iso-8859-14|, :LATIN-8, :|latin-8|
:|iso-8859-2|, :LATIN-2, :|latin-2|
:|iso-8859-3|, :LATIN-3, :|latin-3|
:|iso-8859-4|, :LATIN-4, :|latin-4|
:|iso-8859-9|, :LATIN-5, :|latin-5|
:LATIN1, :ISO-8859-1, :ISO8859-1
:LATIN9, :ISO-8859-15, :ISO8859-15
:|mac-roman|, :|MacRoman|, :MAC, :|mac|, :MACINTOSH, :|macintosh|
:SJIS, :|Shift_JIS|, :CP932

In situations where an external file format designator is required, such as the :external-format argument in calls to open or with-open-file, users may supply the name of an encoding to denote the external format which is applying that encoding to Lisp characters.

In addition to the basic encoding for an external format, options controlling various special cases may be passed, by using a list (whose first element must be an encoding name and whose rest is a plist) as an external file format designator. At present, the only supported key in the plist is :replacement, where the corresponding value is a string designator which is used as a replacement when an encoding or decoding error happens, handling those errors without user intervention; for example:

     (with-open-file (i pathname :external-format '(:utf-8 :replacement #\?))
       (read-line i))

will read the first line of pathname, replacing any invalid utf-8 sequences with question marks.

Next: , Previous: External Formats, Up: Streams

10.2 Bivalent Streams

A bivalent stream can be used to read and write both character and (unsigned-byte 8) values. A bivalent stream is created by calling open with the argument :element-type :default. On such a stream, both binary and character data can be read and written with the usual input and output functions.

Streams are not created bivalent by default for performance reasons. Bivalent streams are incompatible with fast-read-char, an internal optimization in SBCL's stream machinery that bulk-converts octets to characters and implements a fast path through read-char.

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10.3 Gray Streams

The Gray Streams interface is a widely supported extension that provides for definition of CLOS-extensible stream classes. Gray stream classes are implemented by adding methods to generic functions analogous to Common Lisp's standard I/O functions. Instances of Gray stream classes may be used with any I/O operation where a non-Gray stream can, provided that all required methods have been implemented suitably.

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10.3.1 Gray Streams classes

The defined Gray Stream classes are these:

— Class: fundamental-stream [sb-gray]

Class precedence list: fundamental-stream, standard-object, stream, t

Base class for all Gray streams.

— Class: fundamental-input-stream [sb-gray]

Class precedence list: fundamental-input-stream, fundamental-stream, standard-object, stream, t

Superclass of all Gray input streams.

The function input-stream-p will return true of any generalized instance of fundamental-input-stream.

— Class: fundamental-output-stream [sb-gray]

Class precedence list: fundamental-output-stream, fundamental-stream, standard-object, stream, t

Superclass of all Gray output streams.

The function output-stream-p will return true of any generalized instance of fundamental-output-stream.

— Class: fundamental-binary-stream [sb-gray]

Class precedence list: fundamental-binary-stream, fundamental-stream, standard-object, stream, t

Superclass of all Gray streams whose element-type is a subtype of unsigned-byte or signed-byte.

Note that instantiable subclasses of fundamental-binary-stream should provide (or inherit) an applicable method for the generic function stream-element-type.

— Class: fundamental-character-stream [sb-gray]

Class precedence list: fundamental-character-stream, fundamental-stream, standard-object, stream, t

Superclass of all Gray streams whose element-type is a subtype of character.

— Class: fundamental-binary-input-stream [sb-gray]

Class precedence list: fundamental-binary-input-stream, fundamental-input-stream, fundamental-binary-stream, fundamental-stream, standard-object, stream, t

Superclass of all Gray input streams whose element-type is a subtype of unsigned-byte or signed-byte.

— Class: fundamental-binary-output-stream [sb-gray]

Class precedence list: fundamental-binary-output-stream, fundamental-output-stream, fundamental-binary-stream, fundamental-stream, standard-object, stream, t

Superclass of all Gray output streams whose element-type is a subtype of unsigned-byte or signed-byte.

— Class: fundamental-character-input-stream [sb-gray]

Class precedence list: fundamental-character-input-stream, fundamental-input-stream, fundamental-character-stream, fundamental-stream, standard-object, stream, t

Superclass of all Gray input streams whose element-type is a subtype of character.

— Class: fundamental-character-output-stream [sb-gray]

Class precedence list: fundamental-character-output-stream, fundamental-output-stream, fundamental-character-stream, fundamental-stream, standard-object, stream, t

Superclass of all Gray output streams whose element-type is a subtype of character.

Next: , Previous: Gray Streams classes, Up: Gray Streams

10.3.2 Methods common to all streams

These generic functions can be specialized on any generalized instance of fundamental-stream.

— Generic Function: stream-element-type [cl] stream

Return a type specifier for the kind of object returned by the stream. The class fundamental-character-stream provides a default method which returns character.

— Generic Function: close [cl] stream &key abort

Close the given stream. No more I/O may be performed, but inquiries may still be made. If :abort is true, an attempt is made to clean up the side effects of having created the stream.

— Generic Function: stream-file-position [sb-gray] stream &optional position-spec

Used by file-position. Returns or changes the current position within stream.

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10.3.3 Input stream methods

These generic functions may be specialized on any generalized instance of fundamental-input-stream.

— Generic Function: stream-clear-input [sb-gray] stream

This is like cl:clear-input, but for Gray streams, returning nil. The default method does nothing.

— Generic Function: stream-read-sequence [sb-gray] stream seq &optional start end

This is like cl:read-sequence, but for Gray streams.

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10.3.4 Character input stream methods

These generic functions are used to implement subclasses of fundamental-input-stream:

— Generic Function: stream-peek-char [sb-gray] stream

This is used to implement peek-char; this corresponds to peek-type of nil. It returns either a character or :eof. The default method calls stream-read-char and stream-unread-char.

— Generic Function: stream-read-char-no-hang [sb-gray] stream

This is used to implement read-char-no-hang. It returns either a character, or nil if no input is currently available, or :eof if end-of-file is reached. The default method provided by fundamental-character-input-stream simply calls stream-read-char; this is sufficient for file streams, but interactive streams should define their own method.

— Generic Function: stream-read-char [sb-gray] stream

Read one character from the stream. Return either a character object, or the symbol :eof if the stream is at end-of-file. Every subclass of fundamental-character-input-stream must define a method for this function.

— Generic Function: stream-read-line [sb-gray] stream

This is used by read-line. A string is returned as the first value. The second value is true if the string was terminated by end-of-file instead of the end of a line. The default method uses repeated calls to stream-read-char.

— Generic Function: stream-listen [sb-gray] stream

This is used by listen. It returns true or false. The default method uses stream-read-char-no-hang and stream-unread-char. Most streams should define their own method since it will usually be trivial and will always be more efficient than the default method.

— Generic Function: stream-unread-char [sb-gray] stream character

Undo the last call to stream-read-char, as in unread-char. Return nil. Every subclass of fundamental-character-input-stream must define a method for this function.

Next: , Previous: Character input stream methods, Up: Gray Streams

10.3.5 Output stream methods

These generic functions are used to implement subclasses of fundamental-output-stream:

— Generic Function: stream-clear-output [sb-gray] stream

This is like cl:clear-output, but for Gray streams: clear the given output stream. The default method does nothing.

— Generic Function: stream-finish-output [sb-gray] stream

Attempts to ensure that all output sent to the Stream has reached its destination, and only then returns false. Implements finish-output. The default method does nothing.

— Generic Function: stream-force-output [sb-gray] stream

Attempts to force any buffered output to be sent. Implements force-output. The default method does nothing.

— Generic Function: stream-write-sequence [sb-gray] stream seq &optional start end

This is like cl:write-sequence, but for Gray streams.

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10.3.6 Character output stream methods

These generic functions are used to implement subclasses of fundamental-character-output-stream:

— Generic Function: stream-advance-to-column [sb-gray] stream column

Write enough blank space so that the next character will be written at the specified column. Returns true if the operation is successful, or nil if it is not supported for this stream. This is intended for use by by pprint and format ~T. The default method uses stream-line-column and repeated calls to stream-write-char with a #space character; it returns nil if stream-line-column returns nil.

— Generic Function: stream-fresh-line [sb-gray] stream

Outputs a new line to the Stream if it is not positioned at the beginning of a line. Returns t if it output a new line, nil otherwise. Used by fresh-line. The default method uses stream-start-line-p and stream-terpri.

— Generic Function: stream-line-column [sb-gray] stream

Return the column number where the next character will be written, or nil if that is not meaningful for this stream. The first column on a line is numbered 0. This function is used in the implementation of pprint and the format ~T directive. For every character output stream class that is defined, a method must be defined for this function, although it is permissible for it to always return nil.

— Generic Function: stream-line-length [sb-gray] stream

Return the stream line length or nil.

— Generic Function: stream-start-line-p [sb-gray] stream

Is stream known to be positioned at the beginning of a line? It is permissible for an implementation to always return nil. This is used in the implementation of fresh-line. Note that while a value of 0 from stream-line-column also indicates the beginning of a line, there are cases where stream-start-line-p can be meaningfully implemented although stream-line-column can't be. For example, for a window using variable-width characters, the column number isn't very meaningful, but the beginning of the line does have a clear meaning. The default method for stream-start-line-p on class fundamental-character-output-stream uses stream-line-column, so if that is defined to return nil, then a method should be provided for either stream-start-line-p or stream-fresh-line.

— Generic Function: stream-terpri [sb-gray] stream

Writes an end of line, as for terpri. Returns nil. The default method does (stream-write-char stream #NEWLINE).

— Generic Function: stream-write-char [sb-gray] stream character

Write character to stream and return character. Every subclass of fundamental-character-output-stream must have a method defined for this function.

— Generic Function: stream-write-string [sb-gray] stream string &optional start end

This is used by write-string. It writes the string to the stream, optionally delimited by start and end, which default to 0 and nil. The string argument is returned. The default method provided by fundamental-character-output-stream uses repeated calls to stream-write-char.

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10.3.7 Binary stream methods

The following generic functions are available for subclasses of fundamental-binary-stream:

— Generic Function: stream-read-byte [sb-gray] stream

Used by read-byte; returns either an integer, or the symbol :eof if the stream is at end-of-file.

— Generic Function: stream-write-byte [sb-gray] stream integer

Implements write-byte; writes the integer to the stream and returns the integer as the result.

Previous: Binary stream methods, Up: Gray Streams

10.3.8 Gray Streams examples

Below are two classes of stream that can be conveniently defined as wrappers for Common Lisp streams. These are meant to serve as examples of minimal implementations of the protocols that must be followed when defining Gray streams. Realistic uses of the Gray Streams API would implement the various methods that can do I/O in batches, such as stream-read-line, stream-write-string, stream-read-sequence, and stream-write-sequence.

Next: , Up: Gray Streams examples Character counting input stream

It is occasionally handy for programs that process input files to count the number of characters and lines seen so far, and the number of characters seen on the current line, so that useful messages may be reported in case of parsing errors, etc. Here is a character input stream class that keeps track of these counts. Note that all character input streams must implement stream-read-char and stream-unread-char.

     (defclass wrapped-stream (fundamental-stream)
       ((stream :initarg :stream :reader stream-of)))
     (defmethod stream-element-type ((stream wrapped-stream))
       (stream-element-type (stream-of stream)))
     (defmethod close ((stream wrapped-stream) &key abort)
       (close (stream-of stream) :abort abort))
     (defclass wrapped-character-input-stream
         (wrapped-stream fundamental-character-input-stream)
     (defmethod stream-read-char ((stream wrapped-character-input-stream))
       (read-char (stream-of stream) nil :eof))
     (defmethod stream-unread-char ((stream wrapped-character-input-stream)
       (unread-char char (stream-of stream)))
     (defclass counting-character-input-stream
       ((char-count :initform 1 :accessor char-count-of)
        (line-count :initform 1 :accessor line-count-of)
        (col-count :initform 1 :accessor col-count-of)
        (prev-col-count :initform 1 :accessor prev-col-count-of)))
     (defmethod stream-read-char ((stream counting-character-input-stream))
       (with-accessors ((inner-stream stream-of) (chars char-count-of)
                        (lines line-count-of) (cols col-count-of)
                        (prev prev-col-count-of)) stream
           (let ((char (call-next-method)))
             (cond ((eql char :eof)
                   ((char= char #\Newline)
                    (incf lines)
                    (incf chars)
                    (setf prev cols)
                    (setf cols 1)
                    (incf chars)
                    (incf cols)
     (defmethod stream-unread-char ((stream counting-character-input-stream)
       (with-accessors ((inner-stream stream-of) (chars char-count-of)
                        (lines line-count-of) (cols col-count-of)
                        (prev prev-col-count-of)) stream
           (cond ((char= char #\Newline)
                  (decf lines)
                  (decf chars)
                  (setf cols prev))
                  (decf chars)
                  (decf cols)

The default methods for stream-read-char-no-hang, stream-peek-char, stream-listen, stream-clear-input, stream-read-line, and stream-read-sequence should be sufficient (though the last two will probably be slower than methods that forwarded directly).

Here's a sample use of this class:

     (with-input-from-string (input "1 2
      3 :foo  ")
       (let ((counted-stream (make-instance 'counting-character-input-stream
                              :stream input)))
         (loop for thing = (read counted-stream) while thing
            unless (numberp thing) do
              (error "Non-number ~S (line ~D, column ~D)" thing
                     (line-count-of counted-stream)
                     (- (col-count-of counted-stream)
                        (length (format nil "~S" thing))))
            do (print thing))))
     Non-number :FOO (line 2, column 5)
       [Condition of type SIMPLE-ERROR]

Previous: Character counting input stream, Up: Gray Streams examples Output prefixing character stream

One use for a wrapped output stream might be to prefix each line of text with a timestamp, e.g., for a logging stream. Here's a simple stream that does this, though without any fancy line-wrapping. Note that all character output stream classes must implement stream-write-char and stream-line-column.

     (defclass wrapped-stream (fundamental-stream)
       ((stream :initarg :stream :reader stream-of)))
     (defmethod stream-element-type ((stream wrapped-stream))
       (stream-element-type (stream-of stream)))
     (defmethod close ((stream wrapped-stream) &key abort)
       (close (stream-of stream) :abort abort))
     (defclass wrapped-character-output-stream
         (wrapped-stream fundamental-character-output-stream)
       ((col-index :initform 0 :accessor col-index-of)))
     (defmethod stream-line-column ((stream wrapped-character-output-stream))
       (col-index-of stream))
     (defmethod stream-write-char ((stream wrapped-character-output-stream)
       (with-accessors ((inner-stream stream-of) (cols col-index-of)) stream
         (write-char char inner-stream)
         (if (char= char #\Newline)
             (setf cols 0)
             (incf cols))))
     (defclass prefixed-character-output-stream
       ((prefix :initarg :prefix :reader prefix-of)))
     (defgeneric write-prefix (prefix stream)
       (:method ((prefix string) stream) (write-string prefix stream))
       (:method ((prefix function) stream) (funcall prefix stream)))
     (defmethod stream-write-char ((stream prefixed-character-output-stream)
       (with-accessors ((inner-stream stream-of) (cols col-index-of)
                        (prefix prefix-of)) stream
         (when (zerop cols)
           (write-prefix prefix inner-stream))

As with the example input stream, this implements only the minimal protocol. A production implementation should also provide methods for at least stream-write-line, stream-write-sequence.

And here's a sample use of this class:

     (flet ((format-timestamp (stream)
              (apply #'format stream "[~2@*~2,' D:~1@*~2,'0D:~0@*~2,'0D] "
                     (multiple-value-list (get-decoded-time)))))
       (let ((output (make-instance 'prefixed-character-output-stream
                                    :stream *standard-output*
                                    :prefix #'format-timestamp)))
         (loop for string in '("abc" "def" "ghi") do
              (write-line string output)
              (sleep 1))))
     [ 0:30:05] abc
     [ 0:30:06] def
     [ 0:30:07] ghi

Previous: Gray Streams, Up: Streams

10.4 Simple Streams

Simple streams are an extensible streams protocol that avoids some problems with Gray streams.

Documentation about simple streams is available at:

The implementation should be considered Alpha-quality; the basic framework is there, but many classes are just stubs at the moment.


Known differences to the ACL behaviour:

Next: , Previous: Streams, Up: Top

11 Package Locks

None of the following sections apply to SBCL built without package locking support.

warning: The interface described here is experimental: incompatible changes in future SBCL releases are possible, even expected: the concept of “implementation packages” and the associated operators may be renamed; more operations (such as naming restarts or catch tags) may be added to the list of operations violating package locks.

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11.1 Package Lock Concepts

Next: , Up: Package Lock Concepts

11.1.1 Package Locking Overview

Package locks protect against unintentional modifications of a package: they provide similar protection to user packages as is mandated to common-lisp package by the ANSI specification. They are not, and should not be used as, a security measure.

Newly created packages are by default unlocked (see the :lock option to defpackage).

The package common-lisp and SBCL internal implementation packages are locked by default, including sb-ext.

It may be beneficial to lock common-lisp-user as well, to ensure that various libraries don't pollute it without asking, but this is not currently done by default.

Next: , Previous: Package Lock Overview, Up: Package Lock Concepts

11.1.2 Implementation Packages

Each package has a list of associated implementation packages. A locked package, and the symbols whose home package it is, can be modified without violating package locks only when *package* is bound to one of the implementation packages of the locked package.

Unless explicitly altered by defpackage, sb-ext:add-implementation-package, or sb-ext:remove-implementation-package each package is its own (only) implementation package.

Next: , Previous: Implementation Packages, Up: Package Lock Concepts

11.1.3 Package Lock Violations Lexical Bindings and Declarations

Lexical bindings or declarations that violate package locks cause a compile-time warning, and a runtime program-error when the form that violates package locks would be executed.

A complete listing of operators affect by this is: let, let*, flet, labels, macrolet, and symbol-macrolet, declare.

Package locks affecting both lexical bindings and declarations can be disabled locally with sb-ext:disable-package-locks declaration, and re-enabled with sb-ext:enable-package-locks declaration.


     (in-package :locked)
     (defun foo () ...)
     (defmacro with-foo (&body body)
       `(locally (declare (disable-package-locks locked:foo))
          (flet ((foo () ...))
            (declare (enable-package-locks locked:foo)) ; re-enable for body
            ,@body))) Other Operations

If an non-lexical operation violates a package lock, a continuable error that is of a subtype of sb-ext:package-lock-violation (subtype of package-error) is signalled when the operation is attempted.

Additional restarts may be established for continuable package lock violations for interactive use.

The actual type of the error depends on circumstances that caused the violation: operations on packages signal errors of type sb-ext:package-locked-error, and operations on symbols signal errors of type sb-ext:symbol-package-locked-error.

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11.1.4 Package Locks in Compiled Code Interned Symbols

If file-compiled code contains interned symbols, then loading that code into an image without the said symbols will not cause a package lock violation, even if the packages in question are locked. Other Limitations on Compiled Code

With the exception of interned symbols, behaviour is unspecified if package locks affecting compiled code are not the same during loading of the code or execution.

Specifically, code compiled with packages unlocked may or may not fail to signal package-lock-violations even if the packages are locked at runtime, and code compiled with packages locked may or may not signal spurious package-lock-violations at runtime even if the packages are unlocked.

In practice all this means that package-locks have a negligible performance penalty in compiled code as long as they are not violated.

Previous: Package Locks in Compiled Code, Up: Package Lock Concepts

11.1.5 Operations Violating Package Locks Operations on Packages

The following actions cause a package lock violation if the package operated on is locked, and *package* is not an implementation package of that package, and the action would cause a change in the state of the package (so e.g. exporting already external symbols is never a violation). Package lock violations caused by these operations signal errors of type sb-ext:package-locked-error.

  1. Shadowing a symbol in a package.
  2. Importing a symbol to a package.
  3. Uninterning a symbol from a package.
  4. Exporting a symbol from a package.
  5. Unexporting a symbol from a package.
  6. Changing the packages used by a package.
  7. Renaming a package.
  8. Deleting a package.
  9. Adding a new package local nickname to a package.
  10. Removing an existing package local nickname to a package. Operations on Symbols

Following actions cause a package lock violation if the home package of the symbol operated on is locked, and *package* is not an implementation package of that package. Package lock violations caused by these action signal errors of type sb-ext:symbol-package-locked-error.

These actions cause only one package lock violation per lexically apparent violated package.


     ;;; Packages FOO and BAR are locked.
     ;;; Two lexically apparent violated packages: exactly two
     ;;; package-locked-errors will be signalled.
     (defclass foo:point ()
       ((x :accessor bar:x)
        (y :accessor bar:y)))
  1. Binding or altering its value lexically or dynamically, or establishing it as a symbol-macro.


  2. Defining, undefining, or binding it, or its setf name as a function.


  3. Defining, undefining, or binding it as a macro or compiler macro.


  4. Defining it as a type specifier or structure.
  5. Defining it as a declaration with a declaration proclamation.
  6. Declaring or proclaiming it special.
  7. Declaring or proclaiming its type or ftype.


  8. Defining a setf expander for it.
  9. Defining it as a method combination type.
  10. Using it as the class-name argument to setf of find-class.
  11. Defining it as a hash table test using sb-ext:define-hash-table-test.

Previous: Package Lock Concepts, Up: Package Locks

11.2 Package Lock Dictionary

— Declaration: disable-package-locks [sb-ext]

Syntax: (sb-ext:disable-package-locks symbol*)

Disables package locks affecting the named symbols during compilation in the lexical scope of the declaration. Disabling locks on symbols whose home package is unlocked, or disabling an already disabled lock, has no effect.

— Declaration: enable-package-locks [sb-ext]

Syntax: (sb-ext:enable-package-locks symbol*)

Re-enables package locks affecting the named symbols during compilation in the lexical scope of the declaration. Enabling locks that were not first disabled with sb-ext:disable-package-locks declaration, or enabling locks that are already enabled has no effect.

— Condition: package-lock-violation [sb-ext]

Class precedence list: package-lock-violation, package-error, error, serious-condition, condition, t

Subtype of cl:package-error. A subtype of this error is signalled when a package-lock is violated.

— Condition: package-locked-error [sb-ext]

Class precedence list: package-locked-error, package-lock-violation, package-error, error, serious-condition, condition, t

Subtype of sb-ext:package-lock-violation. An error of this type is signalled when an operation on a package violates a package lock.

— Condition: symbol-package-locked-error [sb-ext]

Class precedence list: symbol-package-locked-error, package-lock-violation, package-error, error, serious-condition, condition, t

Subtype of sb-ext:package-lock-violation. An error of this type is signalled when an operation on a symbol violates a package lock. The symbol that caused the violation is accessed by the function sb-ext:package-locked-error-symbol.

— Function: package-locked-error-symbol [sb-ext] symbol-package-locked-error

Returns the symbol that caused the symbol-package-locked-error condition.

— Function: package-locked-p [sb-ext] package

Returns t when package is locked, nil otherwise. Signals an error if package doesn't designate a valid package.

— Function: lock-package [sb-ext] package

Locks package and returns t. Has no effect if package was already locked. Signals an error if package is not a valid package designator

— Function: unlock-package [sb-ext] package

Unlocks package and returns t. Has no effect if package was already unlocked. Signals an error if package is not a valid package designator.

— Function: package-implemented-by-list [sb-ext] package

Returns a list containing the implementation packages of package. Signals an error if package is not a valid package designator.

— Function: package-implements-list [sb-ext] package

Returns the packages that package is an implementation package of. Signals an error if package is not a valid package designator.

— Function: add-implementation-package [sb-ext] packages-to-add &optional package

Adds packages-to-add as implementation packages of package. Signals an error if package or any of the packages-to-add is not a valid package designator.

— Function: remove-implementation-package [sb-ext] packages-to-remove &optional package

Removes packages-to-remove from the implementation packages of package. Signals an error if package or any of the packages-to-remove is not a valid package designator.

— Macro: without-package-locks [sb-ext] &body body

Ignores all runtime package lock violations during the execution of body. Body can begin with declarations.

— Macro: with-unlocked-packages [sb-ext] (&rest packages) &body forms

Unlocks packages for the dynamic scope of the body. Signals an error if any of packages is not a valid package designator.

— Macro: defpackage [cl] name [[option]]* ⇒ package

Options are extended to include the following:


          (defpackage "FOO" (:export "BAR") (:lock t) (:implement))
          (defpackage "FOO-INT" (:use "FOO") (:implement "FOO" "FOO-INT"))
          ;;; is equivalent to
          (defpackage "FOO") (:export "BAR"))
          (lock-package "FOO")
          (remove-implementation-package "FOO" "FOO")
          (defpackage "FOO-INT" (:use "BAR"))
          (add-implementation-package "FOO-INT" "FOO")

Next: , Previous: Package Locks, Up: Top

12 Threading

SBCL supports a fairly low-level threading interface that maps onto the host operating system's concept of threads or lightweight processes. This means that threads may take advantage of hardware multiprocessing on machines that have more than one CPU, but it does not allow Lisp control of the scheduler. This is found in the SB-THREAD package.

Threads are part of the default build on x86[-64]/ARM64 Linux and Windows.

They are also supported on: x86[-64] Darwin (Mac OS X), x86[-64] FreeBSD, x86 SunOS (Solaris), PPC Linux, ARM64 Linux. On these platforms threads must be explicitly enabled at build-time, see INSTALL for directions.

Next: , Up: Threading

12.1 Threading basics

     (make-thread (lambda () (write-line "Hello, world")))

12.1.1 Thread Objects

— Structure: thread [sb-thread]

Class precedence list: thread, structure-object, t

Thread type. Do not rely on threads being structs as it may change in future versions.

— Variable: *current-thread* [sb-thread]

Bound in each thread to the thread itself.

— Function: list-all-threads [sb-thread]

Return a list of the live threads. Note that the return value is potentially stale even before the function returns, as new threads may be created and old ones may exit at any time.

— Function: thread-alive-p [sb-thread] thread

Return t if thread is still alive. Note that the return value is potentially stale even before the function returns, as the thread may exit at any time.

— Function: thread-name [sb-thread] instance

Name of the thread. Can be assigned to using setf. Thread names can be arbitrary printable objects, and need not be unique.

— Function: main-thread-p [sb-thread] &optional thread

True if thread, defaulting to current thread, is the main thread of the process.

— Function: main-thread [sb-thread]

Returns the main thread of the process.

12.1.2 Making, Returning From, Joining, and Yielding Threads

— Function: make-thread [sb-thread] function &key name arguments ephemeral

Create a new thread of name that runs function with the argument list designator provided (defaults to no argument). Thread exits when the function returns. The return values of function are kept around and can be retrieved by join-thread.

Invoking the initial abort restart established by make-thread terminates the thread.

See also: return-from-thread, abort-thread.

— Macro: return-from-thread [sb-thread] values-form &key allow-exit

Unwinds from and terminates the current thread, with values from values-form as the results visible to join-thread.

If current thread is the main thread of the process (see MAIN-THREAD-P), signals an error unless allow-exit is true, as terminating the main thread would terminate the entire process. If allow-exit is true, returning from the main thread is equivalent to calling sb-ext:exit with :code 0 and :abort nil.

See also: abort-thread and sb-ext:exit.

— Function: abort-thread [sb-thread] &key allow-exit

Unwinds from and terminates the current thread abnormally, causing join-thread on current thread to signal an error unless a default-value is provided.

If current thread is the main thread of the process (see MAIN-THREAD-P), signals an error unless allow-exit is true, as terminating the main thread would terminate the entire process. If allow-exit is true, aborting the main thread is equivalent to calling sb-ext:exit code 1 and :abort nil.

Invoking the initial abort restart established by make-thread is equivalent to calling abort-thread in other than main threads. However, whereas abort restart may be rebound, abort-thread always unwinds the entire thread. (Behaviour of the initial abort restart for main thread depends on the :toplevel argument to sb-ext:save-lisp-and-die.)

See also: return-from-thread and sb-ext:exit.

— Function: join-thread [sb-thread] thread &key default timeout

Suspend current thread until thread exits. Return the result values of the thread function.

If thread does not exit within timeout seconds and default is supplied, return two values: 1) default 2) :timeout. If default is not supplied, signal a join-thread-error with join-thread-problem equal to :timeout.

If thread does not exit normally (i.e. aborted) and default is supplied, return two values: 1) default 2) :abort. If default is not supplied, signal a join-thread-error with join-thread-problem equal to :abort.

If thread is the current thread, signal a join-thread-error with join-thread-problem equal to :self-join.

Trying to join the main thread causes join-thread to block until timeout occurs or the process exits: when the main thread exits, the entire process exits.

note: Return convention in case of a timeout is experimental and subject to change.

— Function: thread-yield [sb-thread]

Yield the processor to other threads.

12.1.3 Asynchronous Operations

— Function: interrupt-thread [sb-thread] thread function

Interrupt thread and make it run function.

The interrupt is asynchronous, and can occur anywhere with the exception of sections protected using sb-sys:without-interrupts.

function is called with interrupts disabled, under sb-sys:allow-with-interrupts. Since functions such as grab-mutex may try to enable interrupts internally, in most cases function should either enter sb-sys:with-interrupts to allow nested interrupts, or sb-sys:without-interrupts to prevent them completely.

When a thread receives multiple interrupts, they are executed in the order they were sent -- first in, first out.

This means that a great degree of care is required to use interrupt-thread safely and sanely in a production environment. The general recommendation is to limit uses of interrupt-thread for interactive debugging, banning it entirely from production environments -- it is simply exceedingly hard to use correctly.

With those caveats in mind, what you need to know when using it:

Taken together, these two restrict the "safe" things to do using interrupt-thread to a fairly minimal set. One useful one -- exclusively for interactive development use is using it to force entry to debugger to inspect the state of a thread:
            (interrupt-thread thread #'break)

Short version: be careful out there.

— Function: terminate-thread [sb-thread] thread

Terminate the thread identified by thread, by interrupting it and causing it to call sb-ext:abort-thread with :allow-exit t.

The unwind caused by terminate-thread is asynchronous, meaning that eg. thread executing

            (let (foo)
                      (setf foo (get-foo))
                      (work-on-foo foo))
                 (when foo
                   ;; An interrupt occurring inside the cleanup clause
                   ;; will cause cleanups from the current UNWIND-PROTECT
                   ;; to be dropped.
                   (release-foo foo))))

might miss calling release-foo despite get-foo having returned true if the interrupt occurs inside the cleanup clause, eg. during execution of release-foo.

Thus, in order to write an asynch unwind safe unwind-protect you need to use without-interrupts:

            (let (foo)
                      (setf foo (sb-sys:allow-with-interrupts
                        (work-on-foo foo)))
                 (when foo
                   (release-foo foo)))))

Since most libraries using unwind-protect do not do this, you should never assume that unknown code can safely be terminated using terminate-thread.

12.1.4 Miscellaneous Operations

— Function: symbol-value-in-thread [sb-thread] symbol thread &optional errorp

Return the local value of symbol in thread, and a secondary value of t on success.

If the value cannot be retrieved (because the thread has exited or because it has no local binding for NAME) and errorp is true signals an error of type symbol-value-in-thread-error; if errorp is false returns a primary value of nil, and a secondary value of nil.

Can also be used with setf to change the thread-local value of symbol.

symbol-value-in-thread is primarily intended as a debugging tool, and not as a mechanism for inter-thread communication.

12.1.5 Error Conditions

— Condition: thread-error [sb-thread]

Class precedence list: thread-error, error, serious-condition, condition, t

Conditions of type thread-error are signalled when thread operations fail. The offending thread is initialized by the :thread initialization argument and read by the function thread-error-thread.

— Function: thread-error-thread [sb-thread] condition

Return the offending thread that the thread-error pertains to.

— Condition: interrupt-thread-error [sb-thread]

Class precedence list: interrupt-thread-error, thread-error, error, serious-condition, condition, t

Signalled when interrupting a thread fails because the thread has already exited. The offending thread can be accessed using thread-error-thread.

— Condition: join-thread-error [sb-thread]

Class precedence list: join-thread-error, thread-error, error, serious-condition, condition, t

Signalled when joining a thread fails due to abnormal exit of the thread to be joined. The offending thread can be accessed using thread-error-thread.

Next: , Previous: Threading basics, Up: Threading

12.2 Special Variables

The interaction of special variables with multiple threads is mostly as one would expect, with behaviour very similar to other implementations.

The last point means that

     (defparameter *x* 0)
     (let ((*x* 1))
       (sb-thread:make-thread (lambda () (print *x*))))

prints 0 and not 1 as of 0.9.6.

Next: , Previous: Special Variables, Up: Threading

12.3 Atomic Operations

Following atomic operations are particularly useful for implementing lockless algorithms.

— Macro: atomic-decf [sb-ext] place &optional diff

Atomically decrements place by diff, and returns the value of place before the decrement.

place must access one of the following:

Macroexpansion is performed on place before expanding atomic-decf.

Decrementing is done using modular arithmetic, which is well-defined over two different domains:

diff defaults to 1.

experimental: Interface subject to change.

— Macro: atomic-incf [sb-ext] place &optional diff

Atomically increments place by diff, and returns the value of place before the increment.

place must access one of the following:

Macroexpansion is performed on place before expanding atomic-incf.

Incrementing is done using modular arithmetic, which is well-defined over two different domains:

diff defaults to 1.

experimental: Interface subject to change.

— Macro: atomic-pop [sb-ext] place

Like pop, but atomic. place may be read multiple times before the operation completes -- the write does not occur until such time that no other thread modified place between the read and the write.

Works on all CASable places.

— Macro: atomic-push [sb-ext] obj place

Like push, but atomic. place may be read multiple times before the operation completes -- the write does not occur until such time that no other thread modified place between the read and the write.

Works on all CASable places.

— Macro: atomic-update [sb-ext] place update-fn &rest arguments

Updates place atomically to the value returned by calling function designated by update-fn with arguments and the previous value of place.

place may be read and update-fn evaluated and called multiple times before the update succeeds: atomicity in this context means that the value of place did not change between the time it was read, and the time it was replaced with the computed value.

place can be any place supported by sb-ext:compare-and-swap.


            ;;; Conses T to the head of FOO-LIST.
            (defstruct foo list)
            (defvar *foo* (make-foo))
            (atomic-update (foo-list *foo*) #'cons t)
            (let ((x (cons :count 0)))
               (mapc #'sb-thread:join-thread
                     (loop repeat 1000
                           collect (sb-thread:make-thread
                                    (lambda ()
                                      (loop repeat 1000
                                            do (atomic-update (cdr x) #'1+)
                                               (sleep 0.00001))))))
               ;; Guaranteed to be (:COUNT . 1000000) -- if you replace
               ;; atomic update with (INCF (CDR X)) above, the result becomes
               ;; unpredictable.

— Macro: compare-and-swap [sb-ext] place old new

Atomically stores new in place if old matches the current value of place. Two values are considered to match if they are eq. Returns the previous value of place: if the returned value is eq to old, the swap was carried out.

place must be an CAS-able place. Built-in CAS-able places are accessor forms whose car is one of the following:

car, cdr, first, rest, svref, symbol-plist, symbol-value, svref, slot-value sb-mop:standard-instance-access, sb-mop:funcallable-standard-instance-access,

or the name of a defstruct created accessor for a slot whose declared type is either fixnum or t. Results are unspecified if the slot has a declared type other than fixnum or t.

In case of slot-value, if the slot is unbound, slot-unbound is called unless old is eq to sb-pcl:+slot-unbound+ in which case sb-pcl:+slot-unbound+ is returned and new is assigned to the slot. Additionally, the results are unspecified if there is an applicable method on either sb-mop:slot-value-using-class, (setf sb-mop:slot-value-using-class), or sb-mop:slot-boundp-using-class.

Additionally, the place can be a anything for which a CAS-expansion has been specified using defcas, define-cas-expander, or for which a CAS-function has been defined. (See sb-ext:cas for more information.)

CAS Protocol

Our compare-and-swap is user-extensible using a protocol similar to setf, allowing users to add CAS support to new places via e.g. defcas.

At the same time, new atomic operations can be built on top of CAS using get-cas-expansion. See atomic-update, atomic-push, and atomic-pop for examples of how to do this.

— Macro: cas [sb-ext] place old new

Synonym for compare-and-swap.

Additionally defun, defgeneric, defmethod, flet, and labels can be also used to define CAS-functions analogously to SETF-functions:

            (defvar *foo* nil)
            (defun (cas foo) (old new)
              (cas (symbol-value '*foo*) old new))

First argument of a cas function is the expected old value, and the second argument of is the new value. Note that the system provides no automatic atomicity for cas functions, nor can it verify that they are atomic: it is up to the implementor of a cas function to ensure its atomicity.

experimental: Interface subject to change.

— Macro: define-cas-expander [sb-ext] accessor lambda-list &body body

Analogous to define-setf-expander. Defines a CAS-expansion for accessor. body must return six values as specified in get-cas-expansion.

Note that the system provides no automatic atomicity for cas expansion, nor can it verify that they are atomic: it is up to the implementor of a cas expansion to ensure its atomicity.

experimental: Interface subject to change.

— Macro: defcas [sb-ext] accessor lambda-list function &optional docstring

Analogous to short-form defsetf. Defines function as responsible for compare-and-swap on places accessed using accessor. lambda-list must correspond to the lambda-list of the accessor.

Note that the system provides no automatic atomicity for cas expansions resulting from defcas, nor can it verify that they are atomic: it is up to the user of defcas to ensure that the function specified is atomic.

experimental: Interface subject to change.

— Function: get-cas-expansion [sb-ext] place &optional environment

Analogous to get-setf-expansion. Returns the following six values:

            (get-cas-expansion '(car x))
            ; => (#:CONS871), (X), #:OLD872, #:NEW873,
            ;    (SB-KERNEL:%COMPARE-AND-SWAP-CAR #:CONS871 #:OLD872 :NEW873).
            ;    (CAR #:CONS871)
            (defmacro my-atomic-incf (place &optional (delta 1) &environment env)
              (multiple-value-bind (vars vals old new cas-form read-form)
                  (get-cas-expansion place env)
               (let ((delta-value (gensym "DELTA")))
                 `(let* (,@(mapcar 'list vars vals)
                         (,old ,read-form)
                         (,delta-value ,delta)
                         (,new (+ ,old ,delta-value)))
                    (loop until (eq ,old (setf ,old ,cas-form))
                          do (setf ,new (+ ,old ,delta-value)))

experimental: Interface subject to change.

Next: , Previous: Atomic Operations, Up: Threading

12.4 Mutex Support

Mutexes are used for controlling access to a shared resource. One thread is allowed to hold the mutex, others which attempt to take it will be made to wait until it's free. Threads are woken in the order that they go to sleep.

     (defpackage :demo (:use "CL" "SB-THREAD" "SB-EXT"))
     (in-package :demo)
     (defvar *a-mutex* (make-mutex :name "my lock"))
     (defun thread-fn ()
       (format t "Thread ~A running ~%" *current-thread*)
       (with-mutex (*a-mutex*)
         (format t "Thread ~A got the lock~%" *current-thread*)
         (sleep (random 5)))
       (format t "Thread ~A dropped lock, dying now~%" *current-thread*))
     (make-thread #'thread-fn)
     (make-thread #'thread-fn)

— Structure: mutex [sb-thread]

Class precedence list: mutex, structure-object, t

Mutex type.

— Macro: with-mutex [sb-thread] (mutex &key wait-p timeout value) &body body

Acquire mutex for the dynamic scope of body. If wait-p is true (the default), and the mutex is not immediately available, sleep until it is available.

If timeout is given, it specifies a relative timeout, in seconds, on how long the system should try to acquire the lock in the contested case.

If the mutex isn't acquired successfully due to either wait-p or timeout, the body is not executed, and with-mutex returns nil.

Otherwise body is executed with the mutex held by current thread, and with-mutex returns the values of body.

Historically with-mutex also accepted a value argument, which when provided was used as the new owner of the mutex instead of the current thread. This is no longer supported: if value is provided, it must be either nil or the current thread.

— Macro: with-recursive-lock [sb-thread] (mutex &key wait-p timeout) &body body

Acquire mutex for the dynamic scope of body.

If wait-p is true (the default), and the mutex is not immediately available or held by the current thread, sleep until it is available.

If timeout is given, it specifies a relative timeout, in seconds, on how long the system should try to acquire the lock in the contested case.

If the mutex isn't acquired successfully due to either wait-p or timeout, the body is not executed, and with-recursive-lock returns nil.

Otherwise body is executed with the mutex held by current thread, and with-recursive-lock returns the values of body.

Unlike with-mutex, which signals an error on attempt to re-acquire an already held mutex, with-recursive-lock allows recursive lock attempts to succeed.

— Function: make-mutex [sb-thread] &key name

Create a mutex.

— Function: mutex-name [sb-thread] instance

The name of the mutex. Setfable.

— Function: mutex-owner [sb-thread] mutex

Current owner of the mutex, nil if the mutex is free. Naturally, this is racy by design (another thread may acquire the mutex after this function returns), it is intended for informative purposes. For testing whether the current thread is holding a mutex see holding-mutex-p.

— Function: mutex-value [sb-thread] mutex

Current owner of the mutex, nil if the mutex is free. May return a stale value, use mutex-owner instead.

— Function: grab-mutex [sb-thread] mutex &key waitp timeout

Acquire mutex for the current thread. If waitp is true (the default) and the mutex is not immediately available, sleep until it is available.

If timeout is given, it specifies a relative timeout, in seconds, on how long grab-mutex should try to acquire the lock in the contested case.

If grab-mutex returns t, the lock acquisition was successful. In case of waitp being nil, or an expired timeout, grab-mutex may also return nil which denotes that grab-mutex did -not- acquire the lock.


— Function: release-mutex [sb-thread] mutex &key if-not-owner

Release mutex by setting it to nil. Wake up threads waiting for this mutex.

release-mutex is not interrupt safe: interrupts should be disabled around calls to it.

If the current thread is not the owner of the mutex then it silently returns without doing anything (if if-not-owner is :PUNT), signals a warning (if if-not-owner is :WARN), or releases the mutex anyway (if if-not-owner is :FORCE).

Next: , Previous: Mutex Support, Up: Threading

12.5 Semaphores

Semaphores are among other things useful for keeping track of a countable resource, e.g. messages in a queue, and sleep when the resource is exhausted.

— Structure: semaphore [sb-thread]

Class precedence list: semaphore, structure-object, t

Semaphore type. The fact that a semaphore is a structure-object should be considered an implementation detail, and may change in the future.

— Function: make-semaphore [sb-thread] &key name count

Create a semaphore with the supplied count and name.

— Function: signal-semaphore [sb-thread] semaphore &optional n

Increment the count of semaphore by n. If there are threads waiting on this semaphore, then n of them is woken up.

— Function: wait-on-semaphore [sb-thread] semaphore &key n timeout notification

Decrement the count of semaphore by n if the count would not be negative.

Else blocks until the semaphore can be decremented. Returns the new count of semaphore on success.

If timeout is given, it is the maximum number of seconds to wait. If the count cannot be decremented in that time, returns nil without decrementing the count.

If notification is given, it must be a semaphore-notification object whose semaphore-notification-status is nil. If wait-on-semaphore succeeds and decrements the count, the status is set to t.

— Function: try-semaphore [sb-thread] semaphore &optional n notification

Try to decrement the count of semaphore by n. If the count were to become negative, punt and return nil, otherwise return the new count of semaphore.

If notification is given it must be a semaphore notification object with semaphore-notification-status of nil. If the count is decremented, the status is set to t.

— Function: semaphore-count [sb-thread] instance

Returns the current count of the semaphore instance.

— Function: semaphore-name [sb-thread] instance

The name of the semaphore instance. Setfable.

— Structure: semaphore-notification [sb-thread]

Class precedence list: semaphore-notification, structure-object, t

Semaphore notification object. Can be passed to wait-on-semaphore and try-semaphore as the :notification argument. Consequences are undefined if multiple threads are using the same notification object in parallel.

— Function: make-semaphore-notification [sb-thread]

Constructor for semaphore-notification objects. semaphore-notification-status is initially nil.

— Function: semaphore-notification-status [sb-thread] semaphore-notification

Returns t if a wait-on-semaphore or try-semaphore using semaphore-notification has succeeded since the notification object was created or cleared.

— Function: clear-semaphore-notification [sb-thread] semaphore-notification

Resets the semaphore-notification object for use with another call to wait-on-semaphore or try-semaphore.

Next: , Previous: Semaphores, Up: Threading

12.6 Waitqueue/condition variables

These are based on the POSIX condition variable design, hence the annoyingly CL-conflicting name. For use when you want to check a condition and sleep until it's true. For example: you have a shared queue, a writer process checking “queue is empty” and one or more readers that need to know when “queue is not empty”. It sounds simple, but is astonishingly easy to deadlock if another process runs when you weren't expecting it to.

There are three components:

Important stuff to be aware of:

     (defvar *buffer-queue* (make-waitqueue))
     (defvar *buffer-lock* (make-mutex :name "buffer lock"))
     (defvar *buffer* (list nil))
     (defun reader ()
       (with-mutex (*buffer-lock*)
          (condition-wait *buffer-queue* *buffer-lock*)
           (unless *buffer* (return))
           (let ((head (car *buffer*)))
             (setf *buffer* (cdr *buffer*))
             (format t "reader ~A woke, read ~A~%"
                     *current-thread* head))))))
     (defun writer ()
        (sleep (random 5))
        (with-mutex (*buffer-lock*)
          (let ((el (intern
                     (string (code-char
                              (+ (char-code #\A) (random 26)))))))
            (setf *buffer* (cons el *buffer*)))
          (condition-notify *buffer-queue*))))
     (make-thread #'writer)
     (make-thread #'reader)
     (make-thread #'reader)

— Structure: waitqueue [sb-thread]

Class precedence list: waitqueue, structure-object, t

Waitqueue type.

— Function: make-waitqueue [sb-thread] &key name

Create a waitqueue.

— Function: waitqueue-name [sb-thread] instance

The name of the waitqueue. Setfable.

— Function: condition-wait [sb-thread] queue mutex &key timeout

Atomically release mutex and start waiting on queue until another thread wakes us up using either condition-notify or condition-broadcast on queue, at which point we re-acquire mutex and return t.

Spurious wakeups are possible.

If timeout is given, it is the maximum number of seconds to wait, including both waiting for the wakeup and the time to re-acquire mutex. When neither a wakeup nor a re-acquisition occurs within the given time, returns nil without re-acquiring mutex.

If condition-wait unwinds, it may do so with or without mutex being held.

Important: Since condition-wait may return without condition-notify or condition-broadcast having occurred, the correct way to write code that uses condition-wait is to loop around the call, checking the associated data:

            (defvar *data* nil)
            (defvar *queue* (make-waitqueue))
            (defvar *lock* (make-mutex))
            ;; Consumer
            (defun pop-data (&optional timeout)
              (with-mutex (*lock*)
                (loop until *data*
                      do (or (condition-wait *queue* *lock* :timeout timeout)
                             ;; Lock not held, must unwind without touching *data*.
                             (return-from pop-data nil)))
                (pop *data*)))
            ;; Producer
            (defun push-data (data)
              (with-mutex (*lock*)
                (push data *data*)
                (condition-notify *queue*)))

— Function: condition-notify [sb-thread] queue &optional n

Notify n threads waiting on queue.

important: The same mutex that is used in the corresponding condition-wait must be held by this thread during this call.

— Function: condition-broadcast [sb-thread] queue

Notify all threads waiting on queue.

important: The same mutex that is used in the corresponding condition-wait must be held by this thread during this call.

Next: , Previous: Waitqueue/condition variables, Up: Threading

12.7 Barriers

These are based on the Linux kernel barrier design, which is in turn based on the Alpha CPU memory model. They are presently implemented for x86, x86-64, PPC, and ARM64 systems, and behave as compiler barriers on all other CPUs.

In addition to explicit use of the sb-thread:barrier macro, the following functions and macros also serve as :memory barriers:

— Macro: barrier [sb-thread] (kind) &body forms

Insert a barrier in the code stream, preventing some sort of reordering.

kind should be one of:

Prevent the compiler from reordering memory access across the barrier.
Prevent the cpu from reordering any memory access across the barrier.
Prevent the cpu from reordering any read access across the barrier.
Prevent the cpu from reordering any write access across the barrier.
Prevent the cpu from reordering dependent memory reads across the barrier (requiring reads before the barrier to complete before any reads after the barrier that depend on them). This is a weaker form of the :read barrier.

forms is an implicit progn, evaluated before the barrier. barrier returns the values of the last form in forms.

The file "memory-barriers.txt" in the Linux kernel documentation is highly recommended reading for anyone programming at this level.

Next: , Previous: Barriers, Up: Threading

12.8 Sessions/Debugging

If the user has multiple views onto the same Lisp image (for example, using multiple terminals, or a windowing system, or network access) they are typically set up as multiple sessions such that each view has its own collection of foreground/background/stopped threads. A thread which wishes to create a new session can use sb-thread:with-new-session to remove itself from the current session (which it shares with its parent and siblings) and create a fresh one. # See also sb-thread:make-listener-thread.

Within a single session, threads arbitrate between themselves for the user's attention. A thread may be in one of three notional states: foreground, background, or stopped. When a background process attempts to print a repl prompt or to enter the debugger, it will stop and print a message saying that it has stopped. The user at his leisure may switch to that thread to find out what it needs. If a background thread enters the debugger, selecting any restart will put it back into the background before it resumes. Arbitration for the input stream is managed by calls to sb-thread:get-foreground (which may block) and sb-thread:release-foreground.

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12.9 Foreign threads

Direct calls to pthread_create (instead of MAKE-THREAD) create threads that SBCL is not aware of, these are called foreign threads. Currently, it is not possible to run Lisp code in such threads. This means that the Lisp side signal handlers cannot work. The best solution is to start foreign threads with signals blocked, but since third party libraries may create threads, it is not always feasible to do so. As a workaround, upon receiving a signal in a foreign thread, SBCL changes the thread's sigmask to block all signals that it wants to handle and resends the signal to the current process which should land in a thread that does not block it, that is, a Lisp thread.

The resignalling trick cannot work for synchronously triggered signals (SIGSEGV and co), take care not to trigger any. Resignalling for synchronously triggered signals in foreign threads is subject to --lose-on-corruption, see Runtime Options.

Previous: Foreign threads, Up: Threading

12.10 Implementation (Linux x86/x86-64)

Threading is implemented using pthreads and some Linux specific bits like futexes.

On x86 the per-thread local bindings for special variables is achieved using the %fs segment register to point to a per-thread storage area. This may cause interesting results if you link to foreign code that expects threading or creates new threads, and the thread library in question uses %fs in an incompatible way. On x86-64 the r12 register has a similar role.

Queues require the sys_futex() system call to be available: this is the reason for the NPTL requirement. We test at runtime that this system call exists.

Garbage collection is done with the existing Conservative Generational GC. Allocation is done in small (typically 8k) regions: each thread has its own region so this involves no stopping. However, when a region fills, a lock must be obtained while another is allocated, and when a collection is required, all processes are stopped. This is achieved by sending them signals, which may make for interesting behaviour if they are interrupted in system calls. The streams interface is believed to handle the required system call restarting correctly, but this may be a consideration when making other blocking calls e.g. from foreign library code.

Large amounts of the SBCL library have not been inspected for thread-safety. Some of the obviously unsafe areas have large locks around them, so compilation and fasl loading, for example, cannot be parallelized. Work is ongoing in this area.

A new thread by default is created in the same POSIX process group and session as the thread it was created by. This has an impact on keyboard interrupt handling: pressing your terminal's intr key (typically Control-C) will interrupt all processes in the foreground process group, including Lisp threads that SBCL considers to be notionally `background'. This is undesirable, so background threads are set to ignore the SIGINT signal.

sb-thread:make-listener-thread in addition to creating a new Lisp session makes a new POSIX session, so that pressing Control-C in one window will not interrupt another listener - this has been found to be embarrassing.

Next: , Previous: Threading, Up: Top

13 Timers

SBCL supports a system-wide event scheduler implemented on top of setitimer that also works with threads but does not require a separate scheduler thread.

The following example schedules a timer that writes “Hello, word” after two seconds.

     (schedule-timer (make-timer (lambda ()
                                   (write-line "Hello, world")

It should be noted that writing timer functions requires special care, as the dynamic environment in which they run is unpredictable: dynamic variable bindings, locks held, etc, all depend on whatever code was running when the timer fired. The following example should serve as a cautionary tale:

     (defvar *foo* nil)
     (defun show-foo ()
       (format t "~&foo=~S~%" *foo*)
       (force-output t))
     (defun demo ()
       (schedule-timer (make-timer #'show-foo) 0.5)
       (schedule-timer (make-timer #'show-foo) 1.5)
       (let ((*foo* t))
         (sleep 1.0))
       (let ((*foo* :surprise!))
         (sleep 2.0)))

13.1 Timer Dictionary

— Structure: timer [sb-ext]

Class precedence list: timer, structure-object, t

Timer type. Do not rely on timers being structs as it may change in future versions.

— Function: make-timer [sb-ext] function &key name thread

Create a timer that runs function when triggered.

If a thread is supplied, function is run in that thread. If thread is t, a new thread is created for function each time the timer is triggered. If thread is nil, function is run in an unspecified thread.

When thread is not t, interrupt-thread is used to run function and the ordering guarantees of interrupt-thread apply. In that case, function runs with interrupts disabled but with-interrupts is allowed.

— Function: timer-name [sb-ext] timer

Return the name of timer.

— Function: timer-scheduled-p [sb-ext] timer &key delta

See if timer will still need to be triggered after delta seconds from now. For timers with a repeat interval it returns true.

— Function: schedule-timer [sb-ext] timer time &key repeat-interval absolute-p catch-up

Schedule timer to be triggered at time. If absolute-p then time is universal time, but non-integral values are also allowed, else time is measured as the number of seconds from the current time.

If repeat-interval is given, timer is automatically rescheduled upon expiry.

If repeat-interval is non-NIL, the Boolean catch-up controls whether timer will "catch up" by repeatedly calling its function without delay in case calls are missed because of a clock discontinuity such as a suspend and resume cycle of the computer. The default is nil, i.e. do not catch up.

— Function: unschedule-timer [sb-ext] timer

Cancel timer. Once this function returns it is guaranteed that timer shall not be triggered again and there are no unfinished triggers.

— Function: list-all-timers [sb-ext]

Return a list of all timers in the system.

Next: , Previous: Timers, Up: Top

14 Networking

The sb-bsd-sockets module provides a thinly disguised BSD socket API for SBCL. Ideas have been stolen from the BSD socket API for C and Graham Barr's IO::Socket classes for Perl.

Sockets are represented as CLOS objects, and the API naming conventions attempt to balance between the BSD names and good lisp style.

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14.1 Sockets Overview

Most of the functions are modelled on the BSD socket API. BSD sockets are widely supported, portably (“portable” by Unix standards, at least) available on a variety of systems, and documented. There are some differences in approach where we have taken advantage of some of the more useful features of Common Lisp - briefly:

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14.2 General Sockets

— Class: socket [sb-bsd-sockets]

Class precedence list: socket, standard-object, t


Common superclass of all sockets, not meant to be directly instantiated.

— Generic Function: socket-bind [sb-bsd-sockets] socket &rest address

Bind socket to address, which may vary according to socket family. For the inet family, pass address and port as two arguments; for file address family sockets, pass the filename string. See also bind(2)

— Generic Function: socket-accept [sb-bsd-sockets] socket

Perform the accept(2) call, returning a newly-created connected socket and the peer address as multiple values

— Generic Function: socket-connect [sb-bsd-sockets] socket &rest address

Perform the connect(2) call to connect socket to a remote peer. No useful return value.

— Generic Function: socket-peername [sb-bsd-sockets] socket

Return SOCKET's peer; depending on the address family this may return multiple values

— Generic Function: socket-name [sb-bsd-sockets] socket

Return the address (as vector of bytes) and port that socket is bound to, as multiple values.

— Generic Function: socket-receive [sb-bsd-sockets] socket buffer length &key oob peek waitall dontwait element-type

Read length octets from socket into buffer (or a freshly-consed buffer if NIL), using recvfrom(2). If length is nil, the length of buffer is used, so at least one of these two arguments must be non-NIL. If buffer is supplied, it had better be of an element type one octet wide. Returns the buffer, its length, and the address of the peer that sent it, as multiple values. On datagram sockets, sets MSG_TRUNC so that the actual packet length is returned even if the buffer was too small.

— Generic Function: socket-send [sb-bsd-sockets] socket buffer length &key address external-format oob eor dontroute dontwait nosignal confirm more

Send length octets from buffer into socket, using sendto(2). If buffer is a string, it will converted to octets according to external-format. If length is nil, the length of the octet buffer is used. The format of address depends on the socket type (for example for inet domain sockets it would be a list of an ip address and a port). If no socket address is provided, send(2) will be called instead. Returns the number of octets written.

— Generic Function: socket-listen [sb-bsd-sockets] socket backlog

Mark socket as willing to accept incoming connections. The integer backlog defines the maximum length that the queue of pending connections may grow to before new connection attempts are refused. See also listen(2)

— Generic Function: socket-open-p [sb-bsd-sockets] socket

Return true if socket is open; otherwise, return false.

— Generic Function: socket-close [sb-bsd-sockets] socket &key abort

Close socket, unless it was already closed.

If socket-make-stream has been called, calls close using abort on that stream. Otherwise closes the socket file descriptor using close(2).

— Generic Function: socket-shutdown [sb-bsd-sockets] socket &key direction

Indicate that no communication in direction will be performed on socket.

direction has to be one of :input, :output or :io.

After a shutdown, no input and/or output of the indicated direction can be performed on socket.

— Generic Function: socket-make-stream [sb-bsd-sockets] socket &key input output element-type external-format buffering timeout auto-close serve-events

Find or create a stream that can be used for io on socket (which must be connected). Specify whether the stream is for input, output, or both (it is an error to specify neither).

element-type and external-format are as per open.

timeout specifies a read timeout for the stream.

— Method: socket-make-stream [sb-bsd-sockets] (socket socket) &key input output (element-type (quote character)) (buffering full) (external-format default) timeout auto-close serve-events

Default method for socket objects.

element-type defaults to character, to construct a bivalent stream, capable of both binary and character io use :default.

Acceptable values for buffering are :full, :line and :none, default is :full, ie. output is buffered till it is explicitly flushed using close or finish-output. (force-output forces some output to be flushed: to ensure all buffered output is flused use finish-output.)

Streams have no timeout by default. If one is provided, it is the number of seconds the system will at most wait for input to appear on the socket stream when trying to read from it.

If auto-close is true, the underlying os socket is automatically closed after the stream and the socket have been garbage collected. Default is false.

If serve-events is true, blocking io on the socket will dispatch to the recursive event loop. Default is false.

The stream for socket will be cached, and a second invocation of this method will return the same stream. This may lead to oddities if this function is invoked with inconsistent arguments (e.g., one might request an input stream and get an output stream in response).

— Function: socket-error [sb-bsd-sockets] where &optional errno

— Generic Function: non-blocking-mode [sb-bsd-sockets] socket

Is socket in non-blocking mode?

Next: , Previous: General Sockets, Up: Networking

14.3 Socket Options

A subset of socket options are supported, using a fairly general framework which should make it simple to add more as required - see SYS:CONTRIB;SB-BSD-SOCKETS:SOCKOPT.LISP for details. The name mapping from C is fairly straightforward: SO_RCVLOWAT becomes sockopt-receive-low-water and (setf sockopt-receive-low-water).

— Function: sockopt-reuse-address [sb-bsd-sockets] socket

Return the value of the so-reuseaddr socket option for socket. This can also be updated with setf.

— Function: sockopt-keep-alive [sb-bsd-sockets] socket

Return the value of the so-keepalive socket option for socket. This can also be updated with setf.

— Function: sockopt-oob-inline [sb-bsd-sockets] socket

Return the value of the so-oobinline socket option for socket. This can also be updated with setf.

— Function: sockopt-bsd-compatible [sb-bsd-sockets] socket

Return the value of the so-bsdcompat socket option for socket. This can also be updated with setf. Available only on Linux.

— Function: sockopt-pass-credentials [sb-bsd-sockets] socket

Return the value of the so-passcred socket option for socket. This can also be updated with setf. Available only on Linux.

— Function: sockopt-debug [sb-bsd-sockets] socket

Return the value of the so-debug socket option for socket. This can also be updated with setf.

— Function: sockopt-dont-route [sb-bsd-sockets] socket

Return the value of the so-dontroute socket option for socket. This can also be updated with setf.

— Function: sockopt-broadcast [sb-bsd-sockets] socket

Return the value of the so-broadcast socket option for socket. This can also be updated with setf.

— Function: sockopt-tcp-nodelay [sb-bsd-sockets] socket

Return the value of the tcp-nodelay socket option for socket. This can also be updated with setf.

Next: , Previous: Socket Options, Up: Networking

14.4 INET Domain Sockets

The TCP and UDP sockets that you know and love. Some representation issues:

— Class: inet-socket [sb-bsd-sockets]

Class precedence list: inet-socket, socket, standard-object, t

Class representing tcp and udp over IPv4 sockets.


           (make-instance 'sb-bsd-sockets:inet-socket :type :stream :protocol :tcp)
           (make-instance 'sb-bsd-sockets:inet-socket :type :datagram :protocol :udp)

— Class: inet6-socket [sb-bsd-sockets]

Class precedence list: inet6-socket, socket, standard-object, t

Class representing tcp and udp over IPv6 sockets.


           (make-instance 'sb-bsd-sockets:inet6-socket :type :stream :protocol :tcp)
           (make-instance 'sb-bsd-sockets:inet6-socket :type :datagram :protocol :udp)

— Function: make-inet-address [sb-bsd-sockets] dotted-quads

Return a vector of octets given a string dotted-quads in the format "". Signals an error if the string is malformed.

— Function: make-inet6-address [sb-bsd-sockets] colon-separated-integers

Return a vector of octets given a string representation of an IPv6 address colon-separated-integers. Signal an error if the string is malformed.

— Function: get-protocol-by-name [sb-bsd-sockets] name

Given a protocol name, return the protocol number, the protocol name, and a list of protocol aliases

Next: , Previous: INET Domain Sockets, Up: Networking

14.5 Local (Unix) Domain Sockets

Local domain (AF_LOCAL) sockets are also known as Unix-domain sockets, but were renamed by POSIX presumably on the basis that they may be available on other systems too.

A local socket address is a string, which is used to create a node in the local filesystem. This means of course that they cannot be used across a network.

— Class: local-socket [sb-bsd-sockets]

Class precedence list: local-socket, socket, standard-object, t

Class representing local domain (AF_LOCAL) sockets, also known as unix-domain sockets.

Previous: Local (Unix) Domain Sockets, Up: Networking

14.6 Name Service

Presently name service is implemented by calling out to the getaddrinfo(3) and gethostinfo(3), or to gethostbyname(3) gethostbyaddr(3) on platforms where the preferred functions are not available. The exact details of the name resolving process (for example the choice of whether DNS or a hosts file is used for lookup) are platform dependent.

— Class: host-ent [sb-bsd-sockets]

Class precedence list: host-ent, standard-object, t


This class represents the results of an address lookup.

— Function: get-host-by-name [sb-bsd-sockets] node

Returns a host-ent instance for node or signals a name-service-error.

Another host-ent instance containing zero, one or more IPv6 addresses may be returned as a second return value.

node may also be an ip address in dotted quad notation or some other weird stuff - see getaddrinfo(3) for the details.

— Function: get-host-by-address [sb-bsd-sockets] address

Returns a host-ent instance for address, which should be a vector of (integer 0 255) with 4 elements in case of an IPv4 address and 16 elements in case of an IPv6 address, or signals a name-service-error. See gethostbyaddr(3) for details.

— Generic Function: host-ent-address [sb-bsd-sockets] host-ent

Return some valid address for host-ent.

Next: , Previous: Networking, Up: Top

15 Profiling

SBCL includes both a deterministic profiler, that can collect statistics on individual functions, and a more “modern” statistical profiler.

Inlined functions do not appear in the results reported by either.

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15.1 Deterministic Profiler

The package sb-profile provides a classic, per-function-call profiler.

note: When profiling code executed by multiple threads in parallel, the consing attributed to each function is inaccurate.

— Macro: profile [sb-profile] &rest names

profile Name*

If no names are supplied, return the list of profiled functions.

If names are supplied, wrap profiling code around the named functions. As in trace, the names are not evaluated. A symbol names a function. A string names all the functions named by symbols in the named package. If a function is already profiled, then unprofile and reprofile (useful to notice function redefinition.) If a name is undefined, then we give a warning and ignore it. See also unprofile, report and reset.

— Macro: unprofile [sb-profile] &rest names

Unwrap any profiling code around the named functions, or if no names are given, unprofile all profiled functions. A symbol names a function. A string names all the functions named by symbols in the named package. names defaults to the list of names of all currently profiled functions.

— Function: report [sb-profile] &key limit print-no-call-list

Report results from profiling. The results are approximately adjusted for profiling overhead. The compensation may be rather inaccurate when bignums are involved in runtime calculation, as in a very-long-running Lisp process.

If limit is set to an integer, only the top limit results are reported. If print-no-call-list is t (the default) then a list of uncalled profiled functions are listed.

— Function: reset [sb-profile]

Reset the counters for all profiled functions.

Previous: Deterministic Profiler, Up: Profiling

15.2 Statistical Profiler

The sb-sprof module, loadable by

     (require :sb-sprof)

provides an alternate profiler which works by taking samples of the program execution at regular intervals, instead of instrumenting functions like sb-profile:profile does. You might find sb-sprof more useful than the deterministic profiler when profiling functions in the common-lisp-package, SBCL internals, or code where the instrumenting overhead is excessive.

Additionally sb-sprof includes a limited deterministic profiler which can be used for reporting the amounts of calls to some functions during

15.2.1 Example Usage

     (in-package :cl-user)
     (require :sb-sprof)
     (declaim (optimize speed))
     (defun cpu-test-inner (a i)
       (logxor a
               (* i 5)
               (+ a i)))
     (defun cpu-test (n)
       (let ((a 0))
         (dotimes (i (expt 2 n) a)
           (setf a (cpu-test-inner a i)))))
     ;;;; CPU profiling
     ;;; Take up to 1000 samples of running (CPU-TEST 26), and give a flat
     ;;; table report at the end. Profiling will end one the body has been
     ;;; evaluated once, whether or not 1000 samples have been taken.
     (sb-sprof:with-profiling (:max-samples 1000
                               :report :flat
                               :loop nil)
       (cpu-test 26))
     ;;; Record call counts for functions defined on symbols in the CL-USER
     ;;; package.
     (sb-sprof:profile-call-counts "CL-USER")
     ;;; Take 1000 samples of running (CPU-TEST 24), and give a flat
     ;;; table report at the end. The body will be re-evaluated in a loop
     ;;; until 1000 samples have been taken. A sample count will be printed
     ;;; after each iteration.
     (sb-sprof:with-profiling (:max-samples 1000
                               :report :flat
                               :loop t
                               :show-progress t)
       (cpu-test 24))
     ;;;; Allocation profiling
     (defun foo (&rest args)
       (mapcar (lambda (x) (float x 1d0)) args))
     (defun bar (n)
       (declare (fixnum n))
       (apply #'foo (loop repeat n collect n)))
     (sb-sprof:with-profiling (:max-samples 10000
                               :mode :alloc
                               :report :flat)
       (bar 1000))

15.2.2 Output

The flat report format will show a table of all functions that the profiler encountered on the call stack during sampling, ordered by the number of samples taken while executing that function.

                Self        Total        Cumul
       Nr  Count     %  Count     %  Count     %    Calls  Function
        1     69  24.4     97  34.3     69  24.4 67108864  CPU-TEST-INNER
        2     64  22.6     64  22.6    133  47.0        -  SB-VM::GENERIC-+
        3     39  13.8    256  90.5    172  60.8        1  CPU-TEST
        4     31  11.0     31  11.0    203  71.7        -  SB-KERNEL:TWO-ARG-XOR

For each function, the table will show three absolute and relative sample counts. The Self column shows samples taken while directly executing that function. The Total column shows samples taken while executing that function or functions called from it (sampled to a platform-specific depth). The Cumul column shows the sum of all Self columns up to and including that line in the table.

Additionally the Calls column will record the amount of calls that were made to the function during the profiling run. This value will only be reported for functions that have been explicitly marked for call counting with profile-call-counts.

The profiler also hooks into the disassembler such that instructions which have been sampled are annotated with their relative frequency of sampling. This information is not stored across different sampling runs.

     ;      6CF:       702E             JO L4              ; 6/242 samples
     ;      6D1:       D1E3             SHL EBX, 1
     ;      6D3:       702A             JO L4
     ;      6D5: L2:   F6C303           TEST BL, 3         ; 2/242 samples
     ;      6D8:       756D             JNE L8
     ;      6DA:       8BC3             MOV EAX, EBX       ; 5/242 samples
     ;      6DC: L3:   83F900           CMP ECX, 0         ; 4/242 samples

15.2.3 Platform support

This module is known not to work consistently on the Alpha platform, for technical reasons related to the implementation of a machine language idiom for marking sections of code to be treated as atomic by the garbage collector; However, it should work on other platforms, and the deficiency on the Alpha will eventually be rectified.

Allocation profiling is only supported on SBCL builds that use the generational garbage collector. Tracking of call stacks at a depth of more than two levels is only supported on x86 and x86-64.

15.2.4 Macros

— Macro: with-profiling [sb-sprof] (&key sample-interval alloc-interval max-samples reset mode loop max-depth show-progress threads report) &body body

Evaluate body with statistical profiling turned on. If loop is true, loop around the body until a sufficient number of samples has been collected. Returns the values from the last evaluation of body.

In multithreaded operation, only the thread in which with-profiling was evaluated will be profiled by default. If you want to profile multiple threads, invoke the profiler with start-profiling.

The following keyword args are recognized:

:sample-interval <n>
Take a sample every <n> seconds. Default is *sample-interval*.
:alloc-interval <n>
Take a sample every time <n> allocation regions (approximately 8kB) have been allocated since the last sample. Default is *alloc-interval*.
:mode <mode>
If :cpu, run the profiler in cpu profiling mode. If :alloc, run the profiler in allocation profiling mode. If :time, run the profiler in wallclock profiling mode.
:max-samples <max>
Repeat evaluating body until <max> samples are taken. Default is *max-samples*.
:max-depth <max>
Maximum call stack depth that the profiler should consider. Only has an effect on x86 and x86-64.
:report <type>
If specified, call report with :type <type> at the end.
:reset <bool>
It true, call reset at the beginning.
:threads <list-form>
Form that evaluates to the list threads to profile, or :all to indicate that all threads should be profiled. Defaults to the current thread. (Note: start-profiling defaults to all threads.)

:threads has no effect on call-counting at the moment.

On some platforms (eg. Darwin) the signals used by the profiler are not properly delivered to threads in proportion to their cpu usage when doing :cpu profiling. If you see empty call graphs, or are obviously missing several samples from certain threads, you may be falling afoul of this. In this case using :mode :time is likely to work better.

:loop <bool>
If false (the default), evaluate body only once. If true repeatedly evaluate body.

— Macro: with-sampling [sb-sprof] (&optional on) &body body

Evaluate body with statistical sampling turned on or off.

15.2.5 Functions

— Function: report [sb-sprof] &key type max min-percent call-graph sort-by sort-order stream show-progress

Report statistical profiling results. The following keyword args are recognized:

:type <type>
Specifies the type of report to generate. If :flat, show flat report, if :graph show a call graph and a flat report. If nil, don't print out a report.
:stream <stream>
Specify a stream to print the report on. Default is *standard-output*.
:max <max>
Don't show more than <max> entries in the flat report.
:min-percent <min-percent>
Don't show functions taking less than <min-percent> of the total time in the flat report.
:sort-by <column>
If :samples, sort flat report by number of samples taken. If :cumulative-samples, sort flat report by cumulative number of samples taken (shows how much time each function spent on stack.) Default is *report-sort-by*.
:sort-order <order>
If :descending, sort flat report in descending order. If :ascending, sort flat report in ascending order. Default is *report-sort-order*.
:show-progress <bool>
If true, print progress messages while generating the call graph.
:call-graph <graph>
Print a report from <graph> instead of the latest profiling results.

Value of this function is a call-graph object representing the resulting call-graph, or nil if there are no samples (eg. right after calling reset.)

Profiling is stopped before the call graph is generated.

— Function: reset [sb-sprof]

Reset the profiler.

— Function: start-profiling [sb-sprof] &key max-samples mode sample-interval alloc-interval max-depth threads sampling

Start profiling statistically in the current thread if not already profiling. The following keyword args are recognized:

:sample-interval <n>
Take a sample every <n> seconds. Default is *sample-interval*.
:alloc-interval <n>
Take a sample every time <n> allocation regions (approximately 8kB) have been allocated since the last sample. Default is *alloc-interval*.
:mode <mode>
If :cpu, run the profiler in cpu profiling mode. If :alloc, run the profiler in allocation profiling mode. If :time, run the profiler in wallclock profiling mode.
:max-samples <max>
Maximum number of samples. Default is *max-samples*.
:max-depth <max>
Maximum call stack depth that the profiler should consider. Only has an effect on x86 and x86-64.
:threads <list>
List threads to profile, or :all to indicate that all threads should be profiled. Defaults to :all. (Note: with-profiling defaults to the current thread.)

:threads has no effect on call-counting at the moment.

On some platforms (eg. Darwin) the signals used by the profiler are not properly delivered to threads in proportion to their cpu usage when doing :cpu profiling. If you see empty call graphs, or are obviously missing several samples from certain threads, you may be falling afoul of this.

:sampling <bool>
If true, the default, start sampling right away. If false, start-sampling can be used to turn sampling on.

— Function: stop-profiling [sb-sprof]

Stop profiling if profiling.

— Function: profile-call-counts [sb-sprof] &rest names

Mark the functions named by names as being subject to call counting during statistical profiling. If a string is used as a name, it will be interpreted as a package name. In this case call counting will be done for all functions with names like x or (setf x), where x is a symbol with the package as its home package.

— Function: unprofile-call-counts [sb-sprof]

Clear all call counting information. Call counting will be done for no functions during statistical profiling.

15.2.6 Variables

— Variable: *max-samples* [sb-sprof]

Default number of traces taken. This variable is somewhat misnamed: each trace may actually consist of an arbitrary number of samples, depending on the depth of the call stack.

— Variable: *sample-interval* [sb-sprof]

Default number of seconds between samples.

15.2.7 Credits

sb-sprof is an SBCL port, with enhancements, of Gerd Moellmann's statistical profiler for CMUCL.

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16 Contributed Modules

SBCL comes with a number of modules that are not part of the core system. These are loaded via (require :modulename) (see Customization Hooks for Users). This section contains documentation (or pointers to documentation) for some of the contributed modules.

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16.1 sb-aclrepl

The sb-aclrepl module offers an Allegro CL-style Read-Eval-Print Loop for SBCL, with integrated inspector. Adding a debugger interface is planned.

16.1.1 Usage

To start sb-aclrepl as your read-eval-print loop, put the form

     (require 'sb-aclrepl)

in your ~/.sbclrc initialization file.

16.1.2 Example Initialization

Here's a longer example of a ~/.sbclrc file that shows off some of the features of sb-aclrepl:

     (ignore-errors (require 'sb-aclrepl))
     (when (find-package 'sb-aclrepl)
       (push :aclrepl cl:*features*))
       (setq sb-aclrepl:*max-history* 100)
       (setf (sb-aclrepl:alias "asdc")
            #'(lambda (sys) (asdf:operate 'asdf:compile-op sys)))
       (sb-aclrepl:alias "l" (sys) (asdf:operate 'asdf:load-op sys))
       (sb-aclrepl:alias "t" (sys) (asdf:operate 'asdf:test-op sys))
       ;; The 1 below means that two characaters ("up") are required
       (sb-aclrepl:alias ("up" 1 "Use package") (package) (use-package package))
       ;; The 0 below means only the first letter ("r") is required,
       ;; such as ":r base64"
       (sb-aclrepl:alias ("require" 0 "Require module") (sys) (require sys))
       (setq cl:*features* (delete :aclrepl cl:*features*)))

Questions, comments, or bug reports should be sent to Kevin Rosenberg (

16.1.3 Credits

Allegro CL is a registered trademark of Franz Inc.

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16.2 sb-concurrency

Additional data structures, synchronization primitives and tools for concurrent programming. Similiar to Java's java.util.concurrent package.

16.2.1 Queue

sb-concurrency:queue is a lock-free, thread-safe FIFO queue datatype.

The implementation is based on An Optimistic Approach to Lock-Free FIFO Queues by Edya Ladan-Mozes and Nir Shavit.

Before SBCL 1.0.38, this implementation resided in its own contrib (see sb-queue) which is still provided for backwards-compatibility but which has since been deprecated.

— Structure: queue [sb-concurrency]

Class precedence list: queue, structure-object, t

Lock-free thread safe fifo queue.

Use enqueue to add objects to the queue, and dequeue to remove them.

— Function: dequeue [sb-concurrency] queue

Retrieves the oldest value in queue and returns it as the primary value, and t as secondary value. If the queue is empty, returns nil as both primary and secondary value.

— Function: enqueue [sb-concurrency] value queue

Adds value to the end of queue. Returns value.

— Function: list-queue-contents [sb-concurrency] queue

Returns the contents of queue as a list without removing them from the queue. Mainly useful for manual examination of queue state, as the list may be out of date by the time it is returned, and concurrent dequeue operations may in the worse case force the queue-traversal to be restarted several times.

— Function: make-queue [sb-concurrency] &key name initial-contents

Returns a new queue with name and contents of the initial-contents sequence enqueued.

— Function: queue-count [sb-concurrency] queue

Returns the number of objects in queue. Mainly useful for manual examination of queue state, and in print-object methods: inefficient as it must walk the entire queue.

— Function: queue-empty-p [sb-concurrency] queue

Returns t if queue is empty, nil otherwise.

— Function: queue-name [sb-concurrency] instance

Name of a queue. Can be assigned to using setf. Queue names can be arbitrary printable objects, and need not be unique.

— Function: queuep [sb-concurrency] object

Returns true if argument is a queue, nil otherwise.

16.2.2 Mailbox (lock-free)

sb-concurrency:mailbox is a lock-free message queue where one or multiple ends can send messages to one or multiple receivers. The difference to queues is that the receiving end may block until a message arrives.

Built on top of the queue implementation.

— Structure: mailbox [sb-concurrency]

Class precedence list: mailbox, structure-object, t

Mailbox aka message queue.

send-message adds a message to the mailbox, receive-message waits till a message becomes available, whereas receive-message-no-hang is a non-blocking variant, and receive-pending-messages empties the entire mailbox in one go.

Messages can be arbitrary objects

— Function: list-mailbox-messages [sb-concurrency] mailbox

Returns a fresh list containing all the messages in the mailbox. Does not remove messages from the mailbox.

— Function: mailbox-count [sb-concurrency] mailbox

Returns the number of messages currently in the mailbox.

— Function: mailbox-empty-p [sb-concurrency] mailbox

Returns true if mailbox is currently empty, nil otherwise.

— Function: mailbox-name [sb-concurrency] instance

Name of a mailbox. SETFable.

— Function: mailboxp [sb-concurrency] object

Returns true if argument is a mailbox, nil otherwise.

— Function: make-mailbox [sb-concurrency] &key name initial-contents

Returns a new mailbox with messages in initial-contents enqueued.

— Function: receive-message [sb-concurrency] mailbox &key timeout

Removes the oldest message from mailbox and returns it as the primary value, and a secondary value of t. If mailbox is empty waits until a message arrives.

If timeout is provided, and no message arrives within the specified interval, returns primary and secondary value of nil.

— Function: receive-message-no-hang [sb-concurrency] mailbox

The non-blocking variant of receive-message. Returns two values, the message removed from mailbox, and a flag specifying whether a message could be received.

— Function: receive-pending-messages [sb-concurrency] mailbox &optional n

Removes and returns all (or at most N) currently pending messages from mailbox, or returns nil if no messages are pending.

Note: Concurrent threads may be snarfing messages during the run of this function, so even though x,y appear right next to each other in the result, does not necessarily mean that y was the message sent right after x.

— Function: send-message [sb-concurrency] mailbox message

Adds a message to mailbox. Message can be any object.

16.2.3 Gates

sb-concurrency:gate is a synchronization object suitable for when multiple threads must wait for a single event before proceeding.

— Structure: gate [sb-concurrency]

Class precedence list: gate, structure-object, t

gate type. Gates are synchronization constructs suitable for making multiple threads wait for single event before proceeding.

Use wait-on-gate to wait for a gate to open, open-gate to open one, and close-gate to close an open gate. gate-open-p can be used to test the state of a gate without blocking.

— Function: close-gate [sb-concurrency] gate

Closes gate. Returns t if the gate was previously open, and nil if the gate was already closed.

— Function: gate-name [sb-concurrency] instance

Name of a gate. SETFable.

— Function: gate-open-p [sb-concurrency] gate

Returns true if gate is open.

— Function: gatep [sb-concurrency] object

Returns true if the argument is a gate.

— Function: make-gate [sb-concurrency] &key name open

Makes a new gate. Gate will be initially open if open is true, and closed if open is nil (the default.) name, if provided, is the name of the gate, used when printing the gate.

— Function: open-gate [sb-concurrency] gate

Opens gate. Returns t if the gate was previously closed, and nil if the gate was already open.

— Function: wait-on-gate [sb-concurrency] gate &key timeout

Waits for gate to open, or timeout seconds to pass. Returns t if the gate was opened in time, and nil otherwise.

16.2.4 Frlocks, aka Fast Read Locks

— Structure: frlock [sb-concurrency]

Class precedence list: frlock, structure-object, t

FRlock, aka Fast Read Lock.

Fast Read Locks allow multiple readers and one potential writer to operate in parallel while providing for consistency for readers and mutual exclusion for writers.

Readers gain entry to protected regions without waiting, but need to retry if a writer operated inside the region while they were reading. This makes frlocks very efficient when readers are much more common than writers.

FRlocks are not suitable when it is not safe at all for readers and writers to operate on the same data in parallel: they provide consistency, not exclusion between readers and writers. Hence using an frlock to eg. protect an sbcl hash-table is unsafe. If multiple readers operating in parallel with a writer would be safe but inconsistent without a lock, frlocks are suitable.

The recommended interface to use is frlock-read and frlock-write, but those needing it can also use a lower-level interface.


            ;; Values returned by FOO are always consistent so that
            ;; the third value is the sum of the two first ones.
            (let ((a 0)
                  (b 0)
                  (c 0)
                  (lk (make-frlock)))
              (defun foo ()
                 (frlock-read (lk) a b c))
              (defun bar (x y)
                 (frlock-write (lk)
                   (setf a x
                         b y
                         c (+ x y)))))

— Macro: frlock-read [sb-concurrency] (frlock) &body value-forms

Evaluates value-forms under frlock till it obtains a consistent set, and returns that as multiple values.

— Macro: frlock-write [sb-concurrency] (frlock &key wait-p timeout) &body body

Executes body while holding frlock for writing.

— Function: make-frlock [sb-concurrency] &key name

Returns a new frlock with name.

— Function: frlock-name [sb-concurrency] instance

Name of an frlock. SETFable.

— Function: frlock-read-begin [sb-concurrency] frlock

Start a read sequence on frlock. Returns a read-token and an epoch to be validated later.

Using frlock-read instead is recommended.

— Function: frlock-read-end [sb-concurrency] frlock

Ends a read sequence on frlock. Returns a token and an epoch. If the token and epoch are eql to the read-token and epoch returned by frlock-read-begin, the values read under the frlock are consistent and can be used: if the values differ, the values are inconsistent and the read must be restated.

Using frlock-read instead is recommended.


            (multiple-value-bind (t0 e0) (frlock-read-begin *fr*)
              (let ((a (get-a))
                    (b (get-b)))
                (multiple-value-bind (t1 e1) (frlock-read-end *fr*)
                  (if (and (eql t0 t1) (eql e0 e1))
                      (list :a a :b b)

— Function: grab-frlock-write-lock [sb-concurrency] frlock &key wait-p timeout

Acquires frlock for writing, invalidating existing and future read-tokens for the duration. Returns t on success, and nil if the lock wasn't acquired due to eg. a timeout. Using frlock-write instead is recommended.

— Function: release-frlock-write-lock [sb-concurrency] frlock

Releases frlock after writing, allowing valid read-tokens to be acquired again. Signals an error if the current thread doesn't hold frlock for writing. Using frlock-write instead is recommended.

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16.3 sb-cover

The sb-cover module provides a code coverage tool for SBCL. The tool has support for expression coverage, and for some branch coverage. Coverage reports are only generated for code compiled using compile-file with the value of the sb-cover:store-coverage-data optimization quality set to 3.

As of SBCL 1.0.6 sb-cover is still experimental, and the interfaces documented here might change in later versions.

16.3.1 Example Usage

     ;;; Load SB-COVER
     (require :sb-cover)
     ;;; Turn on generation of code coverage instrumentation in the compiler
     (declaim (optimize sb-cover:store-coverage-data))
     ;;; Load some code, ensuring that it's recompiled with the new optimization
     ;;; policy.
     (asdf:oos 'asdf:load-op :cl-ppcre-test :force t)
     ;;; Run the test suite.
     ;;; Produce a coverage report
     (sb-cover:report "/tmp/report/")
     ;;; Turn off instrumentation
     (declaim (optimize (sb-cover:store-coverage-data 0)))

16.3.2 Functions

— Function: report [sb-cover] directory &key form-mode external-format

Print a code coverage report of all instrumented files into directory. If directory does not exist, it will be created. The main report will be printed to the file cover-index.html. The external format of the source files can be specified with the external-format parameter.

If the keyword argument form-mode has the value :car, the annotations in the coverage report will be placed on the CARs of any cons-forms, while if it has the value :whole the whole form will be annotated (the default). The former mode shows explicitly which forms were instrumented, while the latter mode is generally easier to read.

— Function: reset-coverage [sb-cover]

Reset all coverage data back to the `Not executed` state.

— Function: clear-coverage [sb-cover]

Clear all files from the coverage database. The files will be re-entered into the database when the fasl files (produced by compiling store-coverage-data optimization policy set to 3) are loaded again into the image.

— Function: save-coverage [sb-cover]

Returns an opaque representation of the current code coverage state. The only operation that may be done on the state is passing it to restore-coverage. The representation is guaranteed to be readably printable. A representation that has been printed and read back will work identically in restore-coverage.

— Function: save-coverage-in-file [sb-cover] pathname

Call save-coverage and write the results of that operation into the file designated by pathname.

— Function: restore-coverage [sb-cover] coverage-state

Restore the code coverage data back to an earlier state produced by save-coverage.

— Function: restore-coverage-from-file [sb-cover] pathname

read the contents of the file designated by pathname and pass the result to restore-coverage.

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16.4 sb-grovel

The sb-grovel module helps in generation of foreign function interfaces. It aids in extracting constants' values from the C compiler and in generating SB-ALIEN structure and union types, see Defining Foreign Types.

The ASDF( component type GROVEL-CONSTANTS-FILE has its PERFORM operation defined to write out a C source file, compile it, and run it. The output from this program is Lisp, which is then itself compiled and loaded.

sb-grovel is used in a few contributed modules, and it is currently compatible only to SBCL. However, if you want to use it, here are a few directions.

16.4.1 Using sb-grovel in your own ASDF system

  1. Create a Lisp package for the foreign constants/functions to go into.
  2. Make your system depend on the 'sb-grovel system.
  3. Create a grovel-constants data file - for an example, see example-constants.lisp in the contrib/sb-grovel/ directory in the SBCL source distribution.
  4. Add it as a component in your system. e.g.
              (eval-when (:compile-toplevel :load-toplevel :execute)
                (require :sb-grovel))
              (defpackage :example-package.system
                          (:use :cl :asdf :sb-grovel :sb-alien))
              (in-package :example-package.system)
              (defsystem example-system
                  :depends-on (sb-grovel)
                  ((:module "sbcl"
                            ((:file "defpackage")
                             (grovel-constants-file "example-constants"
                                                    :package :example-package)))))

    Make sure to specify the package you chose in step 1

  5. Build stuff.

16.4.2 Contents of a grovel-constants-file

The grovel-constants-file, typically named constants.lisp, comprises lisp expressions describing the foreign things that you want to grovel for. A constants.lisp file contains two sections:

There are two types of things that sb-grovel can sensibly extract from the C compiler: constant integers and structure layouts. It is also possible to define foreign functions in the constants.lisp file, but these definitions don't use any information from the C program; they expand directly to sb-alien:define-alien-routine (see The define-alien-routine Macro) forms.

Here's how to use the grovel clauses:

16.4.3 Programming with sb-grovel's structure types

Let us assume that you have a grovelled structure definition:

      (:structure mystruct ("struct my_structure"
                            (integer myint "int" "st_int")
                            (c-string mystring "char[]" "st_str")))

What can you do with it? Here's a short interface document: Traps and Pitfalls

Basically, you can treat functions and data structure definitions that sb-grovel spits out as if they were alien routines and types. This has a few implications that might not be immediately obvious (especially if you have programmed in a previous version of sb-grovel that didn't use alien types):

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16.5 sb-md5

The sb-md5 module implements the RFC1321 MD5 Message Digest Algorithm. [FIXME cite]

— Function: md5sum-file [sb-md5] pathname

Calculate the MD5 message-digest of the file designated by pathname.

— Function: md5sum-sequence [sb-md5] sequence &key start end

Calculate the MD5 message-digest of data bounded by start and end in sequence , which must be a vector with element-type (UNSIGNED-BYTE 8).

— Function: md5sum-stream [sb-md5] stream

Calculate an MD5 message-digest of the contents of stream, whose element-type has to be (unsigned-byte 8).

— Function: md5sum-string [sb-md5] string &key external-format start end

Calculate the MD5 message-digest of the binary representation of string (as octets) in external-format. The boundaries start and end refer to character positions in the string, not to octets in the resulting binary representation.

16.5.1 Credits

The implementation for CMUCL was largely done by Pierre Mai, with help from members of the cmucl-help mailing list. Since CMUCL and SBCL are similar in many respects, it was not too difficult to extend the low-level implementation optimizations for CMUCL to SBCL. Following this, SBCL's compiler was extended to implement efficient compilation of modular arithmetic (see Modular arithmetic), which enabled the implementation to be expressed in portable arithmetical terms, apart from the use of rotate-byte for bitwise rotation.

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16.6 sb-posix

Sb-posix is the supported interface for calling out to the operating system.10

The scope of this interface is “operating system calls on a typical Unixlike platform”. This is section 2 of the Unix manual, plus section 3 calls that are (a) typically found in libc, but (b) not part of the C standard. For example, we intend to provide support for opendir() and readdir(), but not for printf(). That said, if your favourite system call is not included yet, you are encouraged to submit a patch to the SBCL mailing list.

Some facilities are omitted where they offer absolutely no additional use over some portable function, or would be actively dangerous to the consistency of Lisp. Not all functions are available on all platforms.

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16.6.1 Lisp names for C names

All symbols are in the SB-POSIX package. This package contains a Lisp function for each supported Unix system call or function, a variable or constant for each supported Unix constant, an object type for each supported Unix structure type, and a slot name for each supported Unix structure member. A symbol name is derived from the C binding's name, by (a) uppercasing, then (b) removing leading underscores (#\_) then replacing remaining underscore characters with the hyphen (#\-). The requirement to uppercase is so that in a standard upcasing reader the user may write sb-posix:creat instead of sb-posix:|creat| as would otherise be required.

No other changes to “Lispify” symbol names are made, so creat() becomes CREAT, not CREATE.

The user is encouraged not to (USE-PACKAGE :SB-POSIX) but instead to use the SB-POSIX: prefix on all references, as some of the symbols symbols contained in the SB-POSIX package have the same name as CL symbols (OPEN, CLOSE, SIGNAL etc).

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16.6.2 Types

Generally, marshalling between Lisp and C data types is done using SBCL's FFI. See Foreign Function Interface.

Some functions accept objects such as filenames or file descriptors. In the C binding to POSIX these are represented as strings and small integers respectively. For the Lisp programmer's convenience we introduce designators such that CL pathnames or open streams can be passed to these functions. For example, rename accepts both pathnames and strings as its arguments.

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— Type: file-descriptor [sb-posix]

A fixnum designating a native file descriptor.

sb-sys:make-fd-stream can be used to construct a file-stream associated with a native file descriptor.

Note that mixing I/O operations on a file-stream with operations directly on its descriptor may produce unexpected results if the stream is buffered.

— Type: file-descriptor-designator [sb-posix]

Designator for a file-descriptor: either a fixnum designating itself, or a file-stream designating the underlying file-descriptor.

— Function: file-descriptor [sb-posix] file-descriptor

Converts file-descriptor-designator into a file-descriptor.

Previous: File-descriptors, Up: Types Filenames

— Type: filename [sb-posix]

A string designating a filename in native namestring syntax.

Note that native namestring syntax is distinct from Lisp namestring syntax:

            (pathname "/foo*/bar")

is a wild pathname with a pattern-matching directory component. sb-ext:parse-native-namestring may be used to construct Lisp pathnames that denote posix filenames as understood by system calls, and sb-ext:native-namestring can be used to coerce them into strings in the native namestring syntax.

Note also that posix filename syntax does not distinguish the names of files from the names of directories: in order to parse the name of a directory in posix filename syntax into a pathname my-defaults for which

            (merge-pathnames (make-pathname :name "FOO" :case :common)

returns a pathname that denotes a file in the directory, supply a true :as-directory argument to sb-ext:parse-native-namestring. Likewise, to supply the name of a directory to a posix function in non-directory syntax, supply a true :as-file argument to sb-ext:native-namestring.

— Type: filename-designator [sb-posix]

Designator for a filename: a string designating itself, or a designator for a pathname designating the corresponding native namestring.

— Function: filename [sb-posix] filename

Converts filename-designator into a filename.

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16.6.3 Function Parameters

The calling convention is modelled after that of CMUCL's UNIX package: in particular, it's like the C interface except that:

  1. Length arguments are omitted or optional where the sensible value is obvious. For example, read would be defined this way:
              (read fd buffer &optional (length (length buffer))) => bytes-read
  2. Where C simulates “out” parameters using pointers (for instance, in pipe() or socketpair()) these may be optional or omitted in the Lisp interface: if not provided, appropriate objects will be allocated and returned (using multiple return values if necessary).
  3. Some functions accept objects such as filenames or file descriptors. Wherever these are specified as such in the C bindings, the Lisp interface accepts designators for them as specified in the 'Types' section above.
  4. A few functions have been included in sb-posix that do not correspond exactly with their C counterparts. These are described in See Functions with idiosyncratic bindings.

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16.6.4 Function Return Values

The return value is usually the same as for the C binding, except in error cases: where the C function is defined as returning some sentinel value and setting errno on error, we instead signal an error of type SYSCALL-ERROR. The actual error value (errno) is stored in this condition and can be accessed with SYSCALL-ERRNO.

We do not automatically translate the returned value into “Lispy” objects – for example, SB-POSIX:OPEN returns a small integer, not a stream. Exception: boolean-returning functions (or, more commonly, macros) do not return a C integer, but instead a Lisp boolean.

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16.6.5 Lisp objects and C structures

Sb-posix provides various Lisp object types to stand in for C structures in the POSIX library. Lisp bindings to C functions that accept, manipulate, or return C structures accept, manipulate, or return instances of these Lisp types instead of instances of alien types.

The names of the Lisp types are chosen according to the general rules described above. For example Lisp objects of type STAT stand in for C structures of type struct stat.

Accessors are provided for each standard field in the structure. These are named structure-name-field-name where the two components are chosen according to the general name conversion rules, with the exception that in cases where all fields in a given structure have a common prefix, that prefix is omitted. For example, stat.st_dev in C becomes STAT-DEV in Lisp.

Because sb-posix might not support all semi-standard or implementation-dependent members of all structure types on your system (patches welcome), here is an enumeration of all supported Lisp objects corresponding to supported POSIX structures, and the supported slots for those structures.

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16.6.6 Functions with idiosyncratic bindings

A few functions in sb-posix don't correspond directly to their C counterparts.

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16.7 sb-queue

Since SBCL 1.0.38, the sb-queue module has been merged into the sb-concurrency module (see sb-concurrency.)

Previous: sb-queue, Up: Contributed Modules

16.8 sb-rotate-byte

The sb-rotate-byte module offers an interface to bitwise rotation, with an efficient implementation for operations which can be performed directly using the platform's arithmetic routines. It implements the specification at

Bitwise rotation is a component of various cryptographic or hashing algorithms: MD5, SHA-1, etc.; often these algorithms are specified on 32-bit rings. [FIXME cite cite cite].

— Function: rotate-byte [sb-rotate-byte] count bytespec integer

Rotates a field of bits within integer; specifically, returns an integer that contains the bits of integer rotated count times leftwards within the byte specified by bytespec, and elsewhere contains the bits of integer.

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17 Deprecation

In order to support evolution of interfaces in SBCL as well as in user code, SBCL allows declaring functions, variables and types as deprecated. Users of deprecated things are notified by means of warnings while the deprecated thing in question is still available.

This chapter documents the interfaces for being notified when using deprecated thing and declaring things as deprecated, the deprecation process used for SBCL interfaces, and lists legacy interfaces in various stages of deprecation.

Deprecation in this context should not be confused with those things the ANSI Common Lisp standard calls deprecated: the entirety of ANSI CL is supported by SBCL, and none of those interfaces are subject to censure.

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17.1 Why Deprecate?

While generally speaking we try to keep SBCL changes as backwards compatible as feasible, there are situations when existing interfaces are deprecated:

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17.2 The Deprecation Pipeline

SBCL uses a deprecation pipeline with multiple stages: as time goes by, deprecated things move from earlier stages of deprecation to later stages before finally being removed. The intention is making users aware of necessary changes early but allowing a migration to new interfaces at a reasonable pace.

Deprecation proceeds in three stages, each lasting approximately a year. In some cases it might move slower or faster, but one year per stage is what we aim at in general. During each stage warnings (and errors) of increasing severity are signaled, which note that the interface is deprecated, and point users towards any replacements when applicable.

  1. Early Deprecation

    During early deprecation the interface is kept in working condition. However, when a thing in this deprecation stage is used, an early-deprecation-warning [sb-ext], which is a style-warning [cl], is signaled at compile-time.

    The internals may change at this stage: typically because the interface is re-implemented on top of its successor. While we try to keep things as backwards-compatible as feasible (taking maintenance costs into account), sometimes semantics change slightly.

    For example, when the spinlock API was deprecated, spinlock objects ceased to exist, and the whole spinlock API became a synonym for the mutex API – so code using the spinlock API continued working, but silently switched to mutexes instead. However, if someone relied on

    (typep lock 'spinlock)

    returning NIL for a mutexes, trouble could ensue.

  2. Late Deprecation

    During late deprecation the interface remains as it was during early deprecation, but the compile-time warning is upgraded: when a thing in this deprecation stage is used, a late-deprecation-warning [sb-ext], which is a full warning [cl], is signaled at compile-time.

  3. Final Deprecation

    During final deprecation the symbols still exist. However, when a thing in this deprecation stage is used, a final-deprecation-warning [sb-ext], which is a full warning [cl], is signaled at compile-time and an error [cl] is signaled at run-time.

  4. After Final Deprecation

    The interface is deleted entirely.

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17.3 Deprecation Conditions

deprecation-condition [sb-ext] is the superclass of all deprecation-related warning and error conditions. All common slots and readers are defined in this condition class.

— Condition: deprecation-condition [sb-ext]

Class precedence list: deprecation-condition, condition, t

Superclass for deprecation-related error and warning conditions.

— Condition: early-deprecation-warning [sb-ext]

Class precedence list: early-deprecation-warning, style-warning, warning, deprecation-condition, condition, t

This warning is signaled when the use of a variable, function, type, etc. in :early deprecation is detected at compile-time. The use will work at run-time with no warning or error.

— Condition: late-deprecation-warning [sb-ext]

Class precedence list: late-deprecation-warning, warning, deprecation-condition, condition, t

This warning is signaled when the use of a variable, function, type, etc. in :late deprecation is detected at compile-time. The use will work at run-time with no warning or error.

— Condition: final-deprecation-warning [sb-ext]

Class precedence list: final-deprecation-warning, warning, deprecation-condition, condition, t

This warning is signaled when the use of a variable, function, type, etc. in :final deprecation is detected at compile-time. An error will be signaled at run-time.

— Condition: deprecation-error [sb-ext]

Class precedence list: deprecation-error, error, serious-condition, deprecation-condition, condition, t

This error is signaled at run-time when an attempt is made to use a thing that is in :final deprecation, i.e. call a function or access a variable.

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17.4 Introspecting Deprecation Information

The deprecation status of functions and variables can be inspected using the sb-cltl2:function-information and sb-cltl2:variable-information functions provided by the sb-cltl2 contributed module.

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17.5 Deprecation Declaration

The sb-ext:deprecated declaration can be used to declare objects in various namespaces11 as deprecated.

— Declaration: deprecated [sb-ext]


          sb-ext:deprecated stage since {object-clause}*
          stage ::= {:early | :late | :final}
          since ::= {version | (software version)}
          object-clause ::= (namespace name [:replacement replacement])
          namespace ::= {cl:variable | cl:function | cl:type}

were name is the name of the deprecated thing, version and software are strings describing the version in which the thing has been deprecated and replacement is a name or a list of names designating things that should be used instead of the deprecated thing.

Currently the following namespaces are supported:

Declare functions, compiler-macros or macros as deprecated.
note: When declaring a function to be in :final deprecation, there should be no actual definition of the function as the declaration emits a stub function that signals a deprecation-error [sb-ext] at run-time when called.

Declare special and global variables, constants and symbol-macros as deprecated.
note: When declaring a variable to be in :final deprecation, there should be no actual definition of the variable as the declaration emits a symbol-macro that signals a sb-ext:deprecation-error at run-time when accessed.

Declare named types (i.e. defined via deftype), standard classes, structure classes and condition classes as deprecated.

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17.6 Deprecation Examples

Marking functions as deprecated:

     (defun foo ())
     (defun bar ())
     (declaim (deprecated :early ("my-system" "1.2.3")
                          (function foo :replacement bar)))
     ;; Remember: do not define the actual function or variable in case of
     ;; :final deprecation:
     (declaim (deprecated :final ("my-system" "1.2.3")
                          (function fez :replacement whoop)))

Attempting to use the deprecated functions:

     (defun baz ()
     | STYLE-WARNING: The function CL-USER::FOO has been deprecated...
     => BAZ
     => NIL ; no error
     (defun danger ()
     | WARNING: The function CL-USER::FEZ has been deprecated...
     => DANGER
     |- ERROR: The function CL-USER::FEZ has been deprecated...

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17.7 Deprecated Interfaces in SBCL

This sections lists legacy interfaces in various stages of deprecation.

17.7.1 List of Deprecated Interfaces Early Deprecation Late Deprecation Final Deprecation

No interfaces are currently in final deprecation.

17.7.2 Historical Interfaces

The following is a partial list of interfaces present in historical versions of SBCL, which have since then been deleted.

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Appendix A Concept Index

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Appendix B Function Index

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Appendix C Variable Index

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Appendix D Type Index

Previous: Type Index, Up: Top


This manual is maintained in Texinfo, and automatically translated into other forms (e.g. HTML or pdf). If you're reading this manual in one of these non-Texinfo translated forms, that's fine, but if you want to modify this manual, you are strongly advised to seek out a Texinfo version and modify that instead of modifying a translated version. Even better might be to seek out the Texinfo version (maintained at the time of this writing as part of the SBCL project at and submit a patch.


[1] Historically, the ILISP package at provided similar functionality, but it does not support modern SBCL versions.

[2] Actually, this declaration is unnecessary in SBCL, since it already knows that position returns a non-negative fixnum or nil.

[3] A deprecated extension sb-ext:inhibit-warnings is still supported, but liable to go away at any time.

[4] Since the location of an interrupt or hardware error will always be an unknown location, non-argument variable values will never be available in the interrupted frame. See Unknown Locations and Interrupts.

[5] The variable bindings are actually created using the Lisp symbol-macrolet special form.

[6] A motivation, rationale and additional examples for the design of this extension can be found in the paper Rhodes, Christophe (2007): User-extensible sequences in Common Lisp available for download at

[7] In SBCL versions prior to 1.0.13, sb-ext:run-program searched for executables in a manner somewhat incompatible with other languages. As of this version, SBCL uses the system library routine execvp(3), and no longer contains the function, find-executable-in-search-path, which implemented the old search. Users who need this function may find it in run-program.lisp versions 1.67 and earlier in SBCL's CVS repository here However, we caution such users that this search routine finds executables that system library routines do not.

[8] Please note that the codepoint U+1F5CF (PAGE) introduced in Unicode 7.0 is named UNICODE_PAGE, since the name “Page” is required to be assigned to form-feed (U+0C) by the ANSI standard.

[9] See chapter 7 "Testing widely used RNGs" in TestU01: A C Library for Empirical Testing of Random Number Generators by Pierre L'Ecuyer and Richard Simard, ACM Transactions on Mathematical Software, Vol. 33, article 22, 2007.

[10] The functionality contained in the package SB-UNIX is for SBCL internal use only; its contents are likely to change from version to version.

[11] See “namespace” entry in the glossary of the Common Lisp Hyperspec.