SBCL 2.1.1.debian User Manual

Table of Contents

sbcl

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.

1 Getting Support and Reporting Bugs

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:

    https://lists.sourceforge.net/lists/listinfo/sbcl-help

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

    http://sourceforge.net/mailarchive/forum.php?forum_name=sbcl-help

or

    http://news.gmane.org/gmane.lisp.steel-bank.general

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

    http://www.catb.org/~esr/faqs/smart-questions.html.

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).

1.3 Reporting Bugs

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

    https://bugs.launchpad.net/sbcl

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 sbcl-bugs@lists.sourceforge.net 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
   (DEFSTRUCT (FOO (:PRINT-OBJECT (LAMBDA (X Y)
                                    (LET ((*PRINT-LENGTH* 4))
                                      (PRINT X Y)))))
     X Y)
then at the command line type
   (MAKE-FOO)
the program loops endlessly instead of printing the object.

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

    http://www.chiark.greenend.org.uk/~sgtatham/bugs.html.

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 enabled (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, ...

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.

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.

2.1.1 Exceptions

• prog2 returns the primary value of its second form, as specified in the Arguments and Values section of the specification for that operator, not that of its first form, as specified in the Description.
• The string type is considered to be the union of all types (array c (size)) for all non-nil subtypes c of character, excluding arrays specialized to the empty type.

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 (asdf)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.

Timeouts and Deadlines

SBCL allows restricting the execution time of individual operations or parts of a computation using :timeout arguments to certain blocking operations, synchronous timeouts and asynchronous timeouts. The latter two affect operations without explicit timeout support (such as standard functions and macros). See Timeouts and Deadlines.

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.

Profiling

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.

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.

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.

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)
         (call-next-method))))

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.

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.

2.3.5 Style Warnings

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

• multiple defuns of the same symbol in different units;
• special variables not named in the conventional *foo* style, and lexical variables unconventionally named in the *foo* style

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.)

2.4 Development Tools

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 http://www.common-lisp.net/project/slime/.

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.

See: http://www.lispworks.com/reference/HyperSpec/index.html

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.

2.5 More SBCL Information

2.5.1 SBCL Homepage

The SBCL website at http://www.sbcl.org/ 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.

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.

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/.

COPYING

Licence and copyright summary.

CREDITS

Authorship information on various parts of SBCL.

INSTALL

Covers installing SBCL from both source and binary distributions on your system, and also has some installation related troubleshooting information.

NEWS

Summarizes changes between various SBCL versions.

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: http://sbcl-internals.cliki.net/.

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.

2.6 More Common Lisp Information

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. http://www.lisp.org and http://www.cliki.net contain numerous pointers places in the net where lispers talks shop.

2.6.2 Third-party Libraries

For a wealth of information about free Common Lisp libraries and tools we recommend checking out CLiki: http://www.cliki.net/.

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: http://www.gigamonkeys.com/book/.

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 http://www.paulgraham.com/onlisp.html.

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 http://mop.lisp.se/www.alu.org/mop/.

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:

• The system expects to be loaded into a fixed-at-compile-time location in virtual memory, and also expects the location of all of its heap storage to be specified at compile time.
• The system overcommits memory, allocating large amounts of address space from the system (often more than the amount of virtual memory available) and then failing if ends up using too much of the allocated storage.
• The system is implemented as a C program which is responsible for supplying low-level services and loading a Lisp .core file.

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

• SBCL has removed many CMUCL extensions, (e.g. IP networking, remote procedure call, Unix system interface, and X11 interface) from the core system. Most of these are available as contributed modules (distributed with SBCL) or third-party modules instead.
• SBCL has deleted or deprecated some nonstandard features and code complexity which helped efficiency at the price of maintainability. For example, the SBCL compiler no longer implements memory pooling internally (and so is simpler and more maintainable, but generates more garbage and runs more slowly), and various block-compilation efficiency-increasing extensions to the language have been deleted or are no longer used in the implementation of SBCL itself.

3 Starting and Stopping

3.1 Starting SBCL

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 0.8.13.60, an implementation of ANSI Common Lisp.
More information about SBCL is available at <http://www.sbcl.org/>.

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)

4
* (exit)
$

See also Command Line Options and Stopping SBCL.

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.

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!

3.2 Stopping SBCL

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 if 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.

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.

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:

:toplevel

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.

:executable

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.

:save-runtime-options

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.

:purify

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.

:root-structures

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.

:environment-name

This has no purpose; it is accepted only for legacy compatibility.

:compression

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).

:application-type

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.

*default-pathname-defaults*

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:

• It corrupts the current Lisp image enough that the current process needs to be killed afterwards. This can be worked around by forking another process that saves the core.
• There is absolutely no binary compatibility of core images between different runtime support programs. Even runtimes built from the same sources at different times are treated as incompatible for this purpose.

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.

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.

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.

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.

--noinform

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-ldb

Disable the low-level debugger. Only effective if SBCL is compiled with LDB.

--lose-on-corruption

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.

--merge-core-pages

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. By default only compressed cores trigger hinting.

--no-merge-core-pages

Ensures that no sharing hint is provided to the operating system.

--help

Print some basic information about SBCL, then exit.

--version

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.

3.3.2 Toplevel Options

--sysinit filename

Load filename instead of the default system initialization file (see Initialization Files.)

--no-sysinit

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.)

--no-userinit

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.

--noprint

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.

--disable-debugger

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.

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.

System Initialization File

Defaults to $SBCL_HOME/sbclrc, or if that doesn’t exist to /etc/sbclrc. Can be overridden with the command line option --sysinit or --no-sysinit (see Toplevel Options).

The system initialization file is intended for system administrators and software packagers to configure locations of installed third party modules, etc.

User Initialization File

Defaults to $HOME/.sbclrc. Can be overridden with the command line option --userinit or --no-userinit (see Toplevel Options).

The user initialization file is intended for personal customizations, such as loading certain modules at startup, defining convenience functions to use in the REPL, handling automatic recompilation of FASLs (see FASL Format), etc.

Neither initialization file is required.

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.

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.

4.1 Diagnostic Messages

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
    (locally
      (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.

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.

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.

4.1.3.1 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:

• ‘file:’ and ‘in:’ mark the file and definition, respectively.
• The original source is an indented form with no prefix.
• Each line of the processing path is prefixed with ‘-->’
• The actual source form is indented like the original source, but is marked by a preceding ‘==>’ line.
• The explanation is prefixed with the diagnostic severity, which can be ‘caught ERROR:’, ‘caught WARNING:’, ‘caught STYLE-WARNING:’, or ‘note:’.

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.

4.1.3.2 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))
    a))

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.

4.1.3.3 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*))
; --> DO BLOCK LET TAGBODY RETURN-FROM
; ==>
;   (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.

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.

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).

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.

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.

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:

• Proclaimed constraints on argument and result types of a function are supposed to be checked by the function. If the function type is proclaimed before function definition, type checks are inserted by the compiler, but the standard allows the reversed order, in which case the compiler will trust the declaration.
• The compiler cannot check types of an unknown number of values; if the number of generated values is unknown, but the number of consumed is known, only consumed values are checked.

  For example,

  (defun foo (x)
    (the integer (bar x)))
  

  causes the following compiler diagnostic to be emitted:

  ; note: type assertion too complex to check:
  ;  (VALUES INTEGER &REST T).
  

  A partial workaround is instead write:

  (defun foo (x)
    (the (values integer &optional) (bar x)))
  

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

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 max

Assign a minimum value to an optimization quality. quality is the name of the optimization quality to restrict, min (defaulting to zero) is the minimum allowed value, and max (defaults to 3) is the maximum.

Returns the alist describing the current policy restrictions.

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

Otherwise, if min is zero or max is 3 or neither are given, any existing restrictions of quality are removed.

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

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.

Examples:

  ;; 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"))

4.4 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))
; --> LET COND IF COND IF COND IF
; ==>
;   (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.

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))
;       (WHEN (EQ (CAR CURRENT) E) (RETURN CURRENT)))
;
; caught ERROR:
;   (in macroexpansion of (DO # #))
;   (hint: For more precise location, try *BREAK-ON-SIGNALS*.)
;   DO step variable is not a symbol: (ATOM CURRENT)

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.

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:

• Attempts to redefine standard functions may be frustrated, since the function may never be called. Although it is technically illegal to redefine standard functions, users sometimes want to implicitly redefine these functions when they are debugging using the trace macro. Special-casing of standard functions can be inhibited using the notinline declaration, but even then some phases of analysis such as type inferencing are applied by the compiler.
• The compiler can have multiple alternate implementations of standard functions that implement different trade-offs of speed, space and safety. This selection is based on the compiler policy, Compiler Policy.

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.

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.

4.7 Advanced Compiler Use and Efficiency Hints

For more advanced usages of the compiler, please see the chapter of the same name in the CMUCL manual. Many aspects of the compiler have stayed exactly the same, and there is a much more detailed explanation of the compiler’s behavior and how to maximally optimize code in their manual. In particular, while SBCL no longer supports byte-code compilation, it does support CMUCL’s block compilation facility allowing whole program optimization and increased use of the local call convention.

Unlike CMUCL, SBCL is able to open-code forward-referenced type tests while block compiling. This helps for mutually referential defstructs in particular.

5 Debugger

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

5.1 Debugger Entry

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)
0]

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.

5.1.2 Debugger Invocation

The debugger is invoked when:

• error is called, and the condition it signals is not handled.
• break is called, or signal is called with a condition that matches the current *break-on-signals*.
• the debugger is explicitly entered with the invoke-debugger function.

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.

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.

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:

• variables (see Variable Access), which are the values being operated on.
• arguments to the call (which are really just particularly interesting variables).
• a current source location (see Source Location Printing), which is the place in the program where the function was running when it stopped to call another function, or because of an interrupt or error.

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.

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.

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 (LAMBDA lambda-list).

5.3.3.1 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 or block compiled functions, an additional external frame may appear before the frame representing the first call to the recursive function or entry to the compiled block. 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.

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.

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:

• There is inadequate debug information due to the value of the debug optimization quality. See Debugger Policy Control.
• The debugger was entered because of an interrupt such as C-c.
• A hardware error such as “‘bus error’” occurred in code that was compiled unsafely due to the value of the safety optimization quality.

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, see 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.

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.

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:

• The value of the debug optimization quality may have omitted debug information needed to determine whether the variable is available. Unless a variable is an argument, its value will only be available when debug is at least 2.
• The compiler did lifetime analysis and determined that the value was no longer needed, even though its scope had not been exited. Lifetime analysis is inhibited when the debug optimization quality is 3.
• The variable’s name is an uninterned symbol (gensym). To save space, the compiler only dumps debug information about uninterned variables when the debug optimization quality is 3.
• The frame’s location is unknown (see Unknown Locations and Interrupts) because the debugger was entered due to an interrupt or unexpected hardware error. Under these conditions the values of arguments will be available, but might be incorrect. This is the exception mentioned above.
• The variable (or the code referencing it) was optimized out of existence. Variables with no reads are always optimized away. The degree to which the compiler deletes variables will depend on the value of the compilation-speed optimization quality, but most source-level optimizations are done under all compilation policies.
• The variable is never set and its definition looks like

  (LET ((var1 var2))
     ...)
  

  In this case, var1 is substituted with var2.

• The variable is never set and is referenced exactly once. In this case, the reference is substituted with the variable initial value.

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.

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.

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 ()
  '(myfun))

(defun foo ()
  (mymac)
  ...)

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

; File: /usr/me/mystuff.lisp

(MYMAC)

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 ()
  (#:***HERE***
   (MYMAC))
  ...)

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:

• No top-level forms before the top-level form containing the source have been added or deleted, and
• The top-level form containing the source has not been modified much. (More precisely, none of the list forms beginning before the source form have been added or deleted.)

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.

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:

• The block start location may be the same as the true location.
• The block start location will never be later in the program’s flow of control than the true location.
• No conditional control structures (such as if, cond, or) will intervene between the block start and the true location (but note that some conditionals present in the original source could be optimized away.) Function calls do not end basic blocks.
• The head of a loop will be the start of a block.
• The programming language concept of “block structure” and the Lisp block special form are totally unrelated to the compiler’s basic block.

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.)

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:

0

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.

1

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.

2

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.

3

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.

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 a global function. If the function is redefined in the debugger before the frame is restarted, the new function will be used.

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*.

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 default, tracing is done via encapsulation (redefining the function name) rather than by modifying the function. :default is not the default, but 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 also use sb-debug:arg. In forms which are evaluated after the function call, (sb-debug:arg n) returns the N-th value returned by the function.

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

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.

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.

6 Efficiency

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.

Example:

(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)
    old))

;; Slow: not a specializer
(defmethod quux ((foo foo) new)
  (let* ((temp foo)
         (old (slot-value temp 'bar)))
    (setf (slot-value temp 'bar) new)
    old))

;; Slow: assignment to FOO
(defmethod quux ((foo foo) new)
  (let ((old (slot-value foo 'bar)))
    (setf (slot-value foo 'bar) new)
    (setf foo new)
    old))

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.

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 1.0.48.21 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

• &rest lists, when these are declared dynamic-extent.
• cons, list, list*, and vector when the result is bound to a variable declared dynamic-extent.
• simple forms of make-array, whose result is bound to a variable declared dynamic-extent: stack allocation is possible only if the resulting array is known to be both simple and one-dimensional, and has a constant :element-type.

  Note: stack space is limited, so allocation of a large vector may cause stack overflow. For this reason potentially large vectors, which might circumvent stack overflow detection, are stack allocated only in zero safety policies.

• closures defined with flet or labels, with a bound dynamic-extent declaration. Blocks and tags are also allocated on the heap, unless all non-local control transfers to them are compiled with zero safety.
• user-defined structures when the structure constructor defined using defstruct has been declared inline and the result of the call to the constructor is bound to a variable declared dynamic-extent.

  Note: structures with “raw” slots can currently be stack-allocated only on x86 and x86-64. A “raw” slot is one whose declared type is a subtype of exactly one of: double-float, single-float, (complex double-float), (complex single-float), or sb-ext:word; but as an exception to the preceding, any subtype of fixnum is not stored as raw despite also being a subtype of sb-ext:word.

• all of the above when they appear as initial parts of another stack-allocated object.

Examples:

;;; 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

• Automatic detection of the common idiom of calling quantifiers with a closure, even when the closure is not declared dynamic-extent.

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 32-bit CPUs and 64 on 64-bit CPUs. While it is possible to support smaller widths as well, currently this is not implemented.

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.

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

• Advanced Compiler Use and Efficiency Hints
• Advanced Compiler Introduction
• More About Types in Python
• Type Inference
• Source Optimization
• Tail Recursion
• Local Call
• Block Compilation
• Inline Expansion
• Object Representation
• Numbers
• General Efficiency Hints
• Efficiency Notes

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

• The CMUCL manual doesn’t seem to state it explicitly, but Python has a mental block about type inference when assignment is involved. Python is very aggressive and clever about inferring the types of values bound with let, let*, inline function call, and so forth. However, it’s much more passive and dumb about inferring the types of values assigned with setq, setf, and friends. It would be nice to fix this, but in the meantime don’t expect that just because it’s very smart about types in most respects it will be smart about types involved in assignments. (This doesn’t affect its ability to benefit from explicit type declarations involving the assigned variables, only its ability to get by without explicit type declarations.)
• Since the time the CMUCL manual was written, CMUCL (and thus SBCL) has gotten a generational garbage collector. This means that there are some efficiency implications of various patterns of memory usage which aren’t discussed in the CMUCL manual. (Some new material should be written about this.)
• SBCL has some important known efficiency problems. Perhaps the most important are
  • - The garbage collector is not particularly efficient, at least on platforms without the generational collector (as of SBCL 0.8.9, all except x86).
  • - Various aspects of the PCL implementation of CLOS are more inefficient than necessary.

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

• (reduce #'f x) where the type of x is known at compile time
• various bit vector operations, e.g. (position 0 some-bit-vector)
• specialized sequence idioms, e.g. (remove item list :count 1)
• cases where local compilation policy does not require excessive type checking, e.g. (locally (declare (safety 1)) (assoc item list)) (which currently performs safe endp checking internal to assoc).

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).

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.

7.1 Reader Extensions

7.1.1 Extended Package Prefix Syntax

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:

package-name::form-with-interning-into-package

Example:

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

*package* is not rebound during the course of reading a form with extended package prefix syntax; if foo::bar would cause a read-time package lock violation, so does foo::(bar).

7.1.2 Symbol Name Normalization

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.

7.1.3 Decimal Syntax for Rationals

SBCL supports a decimal syntax for rationals, modelled after the standard syntax for floating-point numbers. If a number with floating-point syntax has an exponent marker of r or R (rather than one of the standard exponent markers), it is read as the rational with the exact value of the decimal number expressed as a float.

In addition, setting or binding the value of *read-default-float-format* to rational around a call to read or read-from-string has the effect that floating-point numbers without exponent markers are read as rational numbers, as if there had been an explicit r or R marker.

Floating point numbers of all types are printed with an exponent marker while the value of *read-default-float-format* is rational; however, rational numbers are printed in their standard syntax, irrespective of the value of *read-default-float-format*.

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

• :local-nicknames (local-nickname actual-package-name)*

  The package has the specified local nicknames for the corresponding actual packages.

Example:

(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.

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.

Example:

  (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.

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.

Examples:

  ;;; GOOD, assuming RELEASE-HANDLE is re-entrant.
  (let* ((handle (get-handle))
         (object (make-object handle)))
   (finalize object (lambda () (release-handle handle)))
   object)

  ;;; 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

  (progn
    (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 all finalizations 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.