1 @node Beyond the ANSI Standard
2 @comment node-name, next, previous, up
3 @chapter Beyond the ANSI Standard
5 SBCL is derived from CMUCL, which implements many extensions to the
6 ANSI standard. SBCL doesn't support as many extensions as CMUCL, but
7 it still has quite a few. @xref{Contributed Modules}.
10 * Garbage Collection::
11 * Metaobject Protocol::
13 * Customization Hooks for Users::
14 * Tools To Help Developers::
15 * Resolution of Name Conflicts::
20 @node Garbage Collection
21 @comment node-name, next, previous, up
22 @section Garbage Collection
24 SBCL provides additional garbage collection functionality not
25 specified by ANSI. Weak pointers allow references to objects to be
26 maintained without keeping them from being garbage collected, and
27 ``finalization'' hooks are available to cause code to be executed when
28 an object has been garbage collected. Additionally users can specify
29 their own cleanup actions to be executed with garbage collection.
31 @include fun-sb-ext-finalize.texinfo
32 @include fun-sb-ext-cancel-finalization.texinfo
33 @include fun-sb-ext-make-weak-pointer.texinfo
34 @include fun-sb-ext-weak-pointer-value.texinfo
35 @include var-sb-ext-star-after-gc-hooks-star.texinfo
37 @node Metaobject Protocol
38 @comment node-name, next, previous, up
39 @section Metaobject Protocol
41 SBCL supports a metaobject protocol which is intended to be compatible
42 with AMOP; present exceptions to this (as distinct from current bugs)
48 @findex compute-effective-method
49 @findex sb-mop:compute-effective-method
50 @code{compute-effective-method} only returns one value, not two.
52 There is no record of what the second return value was meant to
53 indicate, and apparently no clients for it.
56 @findex ensure-generic-function
57 @findex generic-function-declarations
58 @findex sb-mop:generic-function-declarations
59 the arguments @code{:declare} and @code{:declarations} to
60 @code{ensure-generic-function} are both accepted, with the leftmost
61 argument defining the declarations to be stored and returned by
62 @code{generic-function-declarations}.
64 Where AMOP specifies @code{:declarations} as the keyword argument to
65 @code{ensure-generic-function}, the Common Lisp standard specifies
66 @code{:declare}. Portable code should use @code{:declare}.
69 @findex validate-superclass
70 @findex finalize-inheritance
71 @findex sb-mop:validate-superclass
72 @findex sb-mop:finalize-inheritance
73 @tindex standard-class
74 @tindex funcallable-standard-class
75 @tindex sb-mop:funcallable-standard-class
77 @findex sb-mop:class-prototype
78 @findex class-prototype
79 although SBCL obeys the requirement in AMOP for
80 @code{validate-superclass} for @code{standard-class} and
81 @code{funcallable-standard-class} to be compatible metaclasses, we
82 impose an additional requirement at class finalization time: a class
83 of metaclass @code{funcallable-standard-class} must have
84 @code{function} in its superclasses, and a class of metaclass
85 @code{standard-class} must not.
90 At class finalization, a class prototype which is accessible by a
91 standard mop function @code{sb-mop:class-prototype}. The user can
92 then ask whether this object is a @code{function} or not in several
93 different ways: whether it is a function according to @code{typep};
94 whether its @code{class-of} is @code{subtypep} @code{function}, or
95 whether @code{function} appears in the superclasses of the class. The
96 additional consistency requirement comes from the desire to make all
97 of these answers the same.
99 The following class definitions are bad, and will lead to errors
100 either immediately or if an instance is created:
102 (defclass bad-object (funcallable-standard-object)
104 (:metaclass standard-class))
107 (defclass bad-funcallable-object (standard-object)
109 (:metaclass funcallable-standard-class))
111 The following definition is acceptable:
114 ((slot :initarg slot)))
115 (defclass funcallable-object (funcallable-standard-object mixin)
117 (:metaclass funcallable-standard-class))
119 and leads to a class whose instances are funcallable and have one slot.
123 @node Support For Unix
124 @comment node-name, next, previous, up
125 @section Support For Unix
127 The UNIX command line can be read from the variable
128 @code{sb-ext:*posix-argv*}. The UNIX environment can be queried with
129 the @code{sb-ext:posix-getenv} function.
131 @include fun-sb-ext-posix-getenv.texinfo
134 @node Customization Hooks for Users
135 @comment node-name, next, previous, up
136 @section Customization Hooks for Users
138 The toplevel repl prompt may be customized, and the function
139 that reads user input may be replaced completely.
140 @c <!-- FIXME but I don't currently remember how -->
142 The behaviour of @code{require} when called with only one argument is
143 implementation-defined. In SBCL, @code{require} behaves in the
146 @include fun-common-lisp-require.texinfo
147 @include var-sb-ext-star-module-provider-functions-star.texinfo
149 Although SBCL does not provide a resident editor, the @code{ed}
150 function can be customized to hook into user-provided editing
151 mechanisms as follows:
153 @include fun-common-lisp-ed.texinfo
154 @include var-sb-ext-star-ed-functions-star.texinfo
156 @node Tools To Help Developers
157 @comment node-name, next, previous, up
158 @section Tools To Help Developers
160 SBCL provides a profiler and other extensions to the ANSI @code{trace}
161 facility. For more information, see @ref{Macro common-lisp:trace}.
163 The debugger supports a number of options. Its documentation is
164 accessed by typing @kbd{help} at the debugger prompt. @xref{Debugger}.
166 Documentation for @code{inspect} is accessed by typing @kbd{help} at
167 the @code{inspect} prompt.
169 @node Resolution of Name Conflicts
170 @section Resolution of Name Conflicts
172 The ANSI standard (section 11.1.1.2.5) requires that name conflicts in
173 packages be resolvable in favour of any of the conflicting symbols. In
174 the interactive debugger, this is achieved by prompting for the symbol
175 in whose favour the conflict should be resolved; for programmatic use,
176 the @code{sb-ext:resolve-conflict} restart should be invoked with one
177 argument, which should be a member of the list returned by the condition
178 accessor @code{sb-ext:name-conflict-symbols}.
180 @node Stale Extensions
181 @comment node-name, next, previous, up
182 @section Stale Extensions
184 SBCL has inherited from CMUCL various hooks to allow the user to
185 tweak and monitor the garbage collection process. These are somewhat
186 stale code, and their interface might need to be cleaned up. If you
187 have urgent need of them, look at the code in @file{src/code/gc.lisp}
188 and bring it up on the developers' mailing list.
190 SBCL has various hooks inherited from CMUCL, like
191 @code{sb-ext:float-denormalized-p}, to allow a program to take
192 advantage of IEEE floating point arithmetic properties which aren't
193 conveniently or efficiently expressible using the ANSI standard. These
194 look good, and their interface looks good, but IEEE support is
195 slightly broken due to a stupid decision to remove some support for
196 infinities (because it wasn't in the ANSI spec and it didn't occur to
197 me that it was in the IEEE spec). If you need this stuff, take a look
198 at the code and bring it up on the developers' mailing
202 @node Efficiency Hacks
203 @comment node-name, next, previous, up
204 @section Efficiency Hacks
206 The @code{sb-ext:purify} function causes SBCL first to collect all
207 garbage, then to mark all uncollected objects as permanent, never again
208 attempting to collect them as garbage. This can cause a large increase
209 in efficiency when using a primitive garbage collector, or a more
210 moderate increase in efficiency when using a more sophisticated garbage
211 collector which is well suited to the program's memory usage pattern. It
212 also allows permanent code to be frozen at fixed addresses, a
213 precondition for using copy-on-write to share code between multiple Lisp
214 processes. This is less important with modern generational garbage
215 collectors, but not all SBCL platforms use such a garbage collector.
217 @include fun-sb-ext-purify.texinfo
219 The @code{sb-ext:truly-the} special form declares the type of the
220 result of the operations, producing its argument; the declaration is
221 not checked. In short: don't use it.
223 @include special-operator-sb-ext-truly-the.texinfo
225 The @code{sb-ext:freeze-type} declaration declares that a
226 type will never change, which can make type testing
227 (@code{typep}, etc.) more efficient for structure types.
229 The @code{sb-ext:constant-function} declaration specifies
230 that a function will always return the same value for the same
231 arguments, which may allow the compiler to optimize calls
232 to it. This is appropriate for functions like @code{sqrt}, but
233 is @emph{not} appropriate for functions like @code{aref},
234 which can change their return values when the underlying data are
236 @c <!-- FIXME: This declaration does not seem to be supported in the
237 @c current compiler. -->