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.
122 the requirement that ``No portable class @math{C_p} may inherit, by
123 virtue of being a direct or indirect subclass of a specified class, any
124 slot for which the name is a symbol accessible in the
125 @code{common-lisp-user} package or exported by any package defined in
126 the ANSI Common Lisp standard.'' is interpreted to mean that the
127 standardized classes themselves should not have slots named by external
128 symbols of public packages.
130 The rationale behind the restriction is likely to be similar to the ANSI
131 Common Lisp restriction on defining functions, variables and types named
132 by symbols in the Common Lisp package: preventing two independent pieces
133 of software from colliding with each other.
137 @node Support For Unix
138 @comment node-name, next, previous, up
139 @section Support For Unix
141 The UNIX command line can be read from the variable
142 @code{sb-ext:*posix-argv*}. The UNIX environment can be queried with
143 the @code{sb-ext:posix-getenv} function.
145 @include fun-sb-ext-posix-getenv.texinfo
148 @node Customization Hooks for Users
149 @comment node-name, next, previous, up
150 @section Customization Hooks for Users
152 The toplevel repl prompt may be customized, and the function
153 that reads user input may be replaced completely.
154 @c <!-- FIXME but I don't currently remember how -->
156 The behaviour of @code{require} when called with only one argument is
157 implementation-defined. In SBCL, @code{require} behaves in the
160 @include fun-common-lisp-require.texinfo
161 @include var-sb-ext-star-module-provider-functions-star.texinfo
163 Although SBCL does not provide a resident editor, the @code{ed}
164 function can be customized to hook into user-provided editing
165 mechanisms as follows:
167 @include fun-common-lisp-ed.texinfo
168 @include var-sb-ext-star-ed-functions-star.texinfo
170 @node Tools To Help Developers
171 @comment node-name, next, previous, up
172 @section Tools To Help Developers
174 SBCL provides a profiler and other extensions to the ANSI @code{trace}
175 facility. For more information, see @ref{Macro common-lisp:trace}.
177 The debugger supports a number of options. Its documentation is
178 accessed by typing @kbd{help} at the debugger prompt. @xref{Debugger}.
180 Documentation for @code{inspect} is accessed by typing @kbd{help} at
181 the @code{inspect} prompt.
183 @node Resolution of Name Conflicts
184 @section Resolution of Name Conflicts
186 The ANSI standard (section 11.1.1.2.5) requires that name conflicts in
187 packages be resolvable in favour of any of the conflicting symbols. In
188 the interactive debugger, this is achieved by prompting for the symbol
189 in whose favour the conflict should be resolved; for programmatic use,
190 the @code{sb-ext:resolve-conflict} restart should be invoked with one
191 argument, which should be a member of the list returned by the condition
192 accessor @code{sb-ext:name-conflict-symbols}.
194 @node Stale Extensions
195 @comment node-name, next, previous, up
196 @section Stale Extensions
198 SBCL has inherited from CMUCL various hooks to allow the user to
199 tweak and monitor the garbage collection process. These are somewhat
200 stale code, and their interface might need to be cleaned up. If you
201 have urgent need of them, look at the code in @file{src/code/gc.lisp}
202 and bring it up on the developers' mailing list.
204 SBCL has various hooks inherited from CMUCL, like
205 @code{sb-ext:float-denormalized-p}, to allow a program to take
206 advantage of IEEE floating point arithmetic properties which aren't
207 conveniently or efficiently expressible using the ANSI standard. These
208 look good, and their interface looks good, but IEEE support is
209 slightly broken due to a stupid decision to remove some support for
210 infinities (because it wasn't in the ANSI spec and it didn't occur to
211 me that it was in the IEEE spec). If you need this stuff, take a look
212 at the code and bring it up on the developers' mailing
216 @node Efficiency Hacks
217 @comment node-name, next, previous, up
218 @section Efficiency Hacks
220 The @code{sb-ext:purify} function causes SBCL first to collect all
221 garbage, then to mark all uncollected objects as permanent, never again
222 attempting to collect them as garbage. This can cause a large increase
223 in efficiency when using a primitive garbage collector, or a more
224 moderate increase in efficiency when using a more sophisticated garbage
225 collector which is well suited to the program's memory usage pattern. It
226 also allows permanent code to be frozen at fixed addresses, a
227 precondition for using copy-on-write to share code between multiple Lisp
228 processes. This is less important with modern generational garbage
229 collectors, but not all SBCL platforms use such a garbage collector.
231 @include fun-sb-ext-purify.texinfo
233 The @code{sb-ext:truly-the} special form declares the type of the
234 result of the operations, producing its argument; the declaration is
235 not checked. In short: don't use it.
237 @include special-operator-sb-ext-truly-the.texinfo
239 The @code{sb-ext:freeze-type} declaration declares that a
240 type will never change, which can make type testing
241 (@code{typep}, etc.) more efficient for structure types.
243 The @code{sb-ext:constant-function} declaration specifies
244 that a function will always return the same value for the same
245 arguments, which may allow the compiler to optimize calls
246 to it. This is appropriate for functions like @code{sqrt}, but
247 is @emph{not} appropriate for functions like @code{aref},
248 which can change their return values when the underlying data are
250 @c <!-- FIXME: This declaration does not seem to be supported in the
251 @c current compiler. -->