1 @node The Foreign Function Interface
2 @comment node-name, next, previous, up
3 @chapter The Foreign Function Interface
5 This chapter describes SBCL's interface to C programs and
6 libraries (and, since C interfaces are a sort of @emph{ingua
7 franca} of the Unix world, to other programs and libraries in
11 Note: In the modern Lisp world, the usual term for this functionality
12 is Foreign Function Interface, or @acronym{FFI}, where despite the
13 mention of ``function'' in this term, @acronym{FFI} also
14 refers to direct manipulation of C data structures as well as
15 functions. The traditional CMUCL terminology is Alien Interface, and
16 while that older terminology is no longer used much in the system
17 documentation, it still reflected in names in the implementation,
18 notably in the name of the @code{SB-ALIEN} package.
22 * Introduction to the Foreign Function Interface::
24 * Operations On Foreign Values::
26 * Foreign Data Structure Examples::
27 * Loading Unix Object Files::
28 * Foreign Function Calls::
29 * Step-By-Step Example of the Foreign Function Interface::
32 @node Introduction to the Foreign Function Interface
33 @comment node-name, next, previous, up
34 @section Introduction to the Foreign Function Interface
35 @c AKA "Introduction to Aliens" in the CMU CL manual
37 Because of Lisp's emphasis on dynamic memory allocation and garbage
38 collection, Lisp implementations use non-C-like memory representations
39 for objects. This representation mismatch creates friction when a Lisp
40 program must share objects with programs which expect C data. There
41 are three common approaches to establishing communication:
45 The burden can be placed on the foreign program (and programmer) by
46 requiring the knowledge and use of the representations used internally
47 by the Lisp implementation. This can require a considerable amount of
48 ``glue'' code on the C side, and that code tends to be sensitively
49 dependent on the internal implementation details of the Lisp system.
52 The Lisp system can automatically convert objects back and forth
53 between the Lisp and foreign representations. This is convenient, but
54 translation becomes prohibitively slow when large or complex data
55 structures must be shared. This approach is supported by the SBCL
56 @acronym{FFI}, and used automatically by the when passing integers and
60 The Lisp program can directly manipulate foreign objects through the
61 use of extensions to the Lisp language.
65 SBCL, like CMUCL before it, relies primarily on the automatic
66 conversion and direct manipulation approaches. The @code{SB-ALIEN}
67 package provices a facility wherein foreign values of simple scalar
68 types are automatically converted and complex types are directly
69 manipulated in their foreign representation. Additionally the
70 lower-level System Area Pointers (or @acronym{SAP}s) can be used where
71 necessary to provide untyped access to foreign memory.
73 Any foreign objects that can't automatically be converted into Lisp
74 values are represented by objects of type @code{alien-value}. Since
75 Lisp is a dynamically typed language, even foreign objects must have a
76 run-time type; this type information is provided by encapsulating the
77 raw pointer to the foreign data within an @code{alien-value} object.
79 The type language and operations on foreign types are
80 intentionally similar to those of the C language.
83 @comment node-name, next, previous, up
84 @section Foreign Types
85 @c AKA "Alien Types" in the CMU CL manual
87 Alien types have a description language based on nested list
88 structure. For example the C type
97 has the corresponding SBCL @acronym{FFI} type
102 (b (array (* (struct foo)) 100)))
107 * Defining Foreign Types::
108 * Foreign Types and Lisp Types::
109 * Foreign Type Specifiers::
112 @node Defining Foreign Types
113 @comment node-name, next, previous, up
114 @subsection Defining Foreign Types
116 Types may be either named or anonymous. With structure and union
117 types, the name is part of the type specifier, allowing recursively
118 defined types such as:
121 (struct foo (a (* (struct foo))))
124 An anonymous structure or union type is specified by using the name
125 @code{nil}. The @code{with-alien} macro defines a local scope which
126 ``captures'' any named type definitions. Other types are not
127 inherently named, but can be given named abbreviations using the
128 @code{define-alien-type} macro.
130 @node Foreign Types and Lisp Types
131 @comment node-name, next, previous, up
132 @subsection Foreign Types and Lisp Types
134 The foreign types form a subsystem of the SBCL type system. An
135 @code{alien} type specifier provides a way to use any foreign type as a
136 Lisp type specifier. For example,
139 (typep @var{foo} '(alien (* int)))
142 can be used to determine whether @var{foo} is a pointer to a foreign
143 @code{int}. @code{alien} type specifiers can be used in the same ways
144 as ordinary Lisp type specifiers (like @code{string}.) Alien type
145 declarations are subject to the same precise type checking as any
146 other declaration. @xref{Precise Type Checking}.
148 Note that the type identifiers used in the foreign type system overlap
149 with native Lisp type specifiers in some cases. For example, the type
150 specifier @code{(alien single-float)} is identical to
151 @code{single-float}, since foreign floats are automatically converted
152 to Lisp floats. When @code{type-of} is called on an alien value that
153 is not automatically converted to a Lisp value, then it will return an
154 @code{alien} type specifier.
156 @node Foreign Type Specifiers
157 @comment node-name, next, previous, up
158 @subsection Foreign Type Specifiers
160 Note: All foreign type names are exported from the @code{sb-alien}
161 package. Some foreign type names are also symbols in
162 the @code{common-lisp} package, in which case they are
163 reexported from the @code{sb-alien} package, so that
164 e.g. it is legal to refer to @code{sb-alien:single-float}.
166 These are the basic foreign type specifiers:
170 The foreign type specifier @code{(* @var{foo})} describes a pointer to
171 an object of type @var{foo}. A pointed-to type @var{foo} of @code{t}
172 indicates a pointer to anything, similar to @code{void *} in
173 ANSI C. A null alien pointer can be detected with the
174 @code{sb-alien:null-alien} function.
177 The foreign type specifier @code{(array @var{foo} &optional
178 dimensions)} describes array of the specified @code{dimensions},
179 holding elements of type @var{foo}. Note that (unlike in C) @code{(*
180 @var{foo})} and @code{(array @var{foo})} are considered to be
181 different types when type checking is done. If equivalence of pointer
182 and array types is desired, it may be explicitly coerced using
183 @code{sb-alien:cast}.
185 Arrays are accessed using @code{sb-alien:deref}, passing the indices
186 as additional arguments. Elements are stored in column-major order
187 (as in C), so the first dimension determines only the size of the
188 memory block, and not the layout of the higher dimensions. An array
189 whose first dimension is variable may be specified by using @code{nil}
190 as the first dimension. Fixed-size arrays can be allocated as array
191 elements, structure slots or @code{sb-alien:with-alien}
192 variables. Dynamic arrays can only be allocated using
193 @code{sb-alien:make-alien}.
196 The foreign type specifier @code{(sb-alien:struct @var{name} &rest
197 @var{fields})} describes a structure type with the specified
198 @var{name} and @var{fields}. Fields are allocated at the same offsets
199 used by the implementation's C compiler. If @var{name} is @code{nil}
200 then the structure is anonymous.
202 If a named foreign @code{struct} specifier is passed to
203 @code{define-alien-type} or @code{with-alien}, then this defines,
204 respectively, a new global or local foreign structure type. If no
205 @var{fields} are specified, then the fields are taken
206 from the current (local or global) alien structure type definition of
210 The foreign type specifier @code{(sb-alien:union @var{name} &rest
211 @var{fields})} is similar to @code{sb-alien:struct}, but describes a
212 union type. All fields are allocated at the same offset, and the size
213 of the union is the size of the largest field. The programmer must
214 determine which field is active from context.
217 The foreign type specifier @code{(sb-alien:enum @var{name} &rest
218 @var{specs})} describes an enumeration type that maps between integer
219 values and keywords. If @var{name} is @code{nil}, then the type is
220 anonymous. Each element of the @var{specs} list is either a Lisp
221 keyword, or a list @code{(@var{keyword} @var{value})}. @var{value} is
222 an integer. If @var{value} is not supplied, then it defaults to one
223 greater than the value for the preceding spec (or to zero if it is the
227 The foreign type specifier @code{(sb-alien:signed &optional
228 @var{bits})} specifies a signed integer with the specified number of
229 @var{bits} precision. The upper limit on integer
230 precision is determined by the machine's word size. If
231 @var{bits} is not specified, the maximum size will be
235 The foreign type specifier @code{(integer &optional @var{bits})}
236 is equivalent to the corresponding type specifier using
237 @code{sb-alien:signed} instead of @code{integer}.
240 The foreign type specifier @code{(sb-alien:unsigned &optional
241 @var{bits})} is like corresponding type specifier using
242 @code{sb-alien:signed} except that the variable is treated as an
246 The foreign type specifier @code{(boolean &optional @var{bits})} is
247 similar to an enumeration type, but maps from Lisp @code{nil} and
248 @code{t} to C @code{0} and @code{1} respectively. @var{bits}
249 determines the amount of storage allocated to hold the truth value.
252 The foreign type specifier @code{single-float} describes a
253 floating-point number in IEEE single-precision format.
256 The foreign type specifier @code{double-float} describes a
257 floating-point number in IEEE double-precision format.
260 The foreign type specifier @code{(function @var{result-type} &rest
261 @var{arg-types})} describes a foreign function that takes arguments of
262 the specified @var{arg-types} and returns a result of type
263 @var{result-type}. Note that the only context where a foreign
264 @code{function} type is directly specified is in the argument to
265 @code{sb-alien:alien-funcall}. In all other contexts, foreign
266 functions are represented by foreign function pointer types: @code{(*
267 (function @dots{}))}.
270 The foreign type specifier @code{sb-alien:system-area-pointer}
271 describes a pointer which is represented in Lisp as a
272 @code{system-area-pointer} object. SBCL exports this type from
273 @code{sb-alien} because CMUCL did, but tentatively (as of the first
274 draft of this section of the manual, SBCL 0.7.6) it is deprecated,
275 since it doesn't seem to be required by user code.
278 The foreign type specifier @code{sb-alien:void} is used in function
279 types to declare that no useful value is returned. Using
280 @code{alien-funcall} to call a @code{void} foreign function will
284 The foreign type specifier @code{sb-alien:c-string} is similar to
285 @code{(* char)}, but is interpreted as a null-terminated string, and
286 is automatically converted into a Lisp string when accessed; or if the
287 pointer is C @code{NULL} or @code{0}, then accessing it gives Lisp
288 @code{nil}. Lisp strings are stored with a trailing NUL
289 termination, so no copying (either by the user or the implementation)
290 is necessary when passing them to foreign code.
292 Assigning a Lisp string to a @code{c-string} structure field or
293 variable stores the contents of the string to the memory already
294 pointed to by that variable. When a foreign object of type @code{(*
295 char)} is assigned to a @code{c-string}, then the
296 @code{c-string} pointer is assigned to. This allows
297 @code{c-string} pointers to be initialized. For example:
300 (cl:in-package "CL-USER") ; which USEs package "SB-ALIEN"
302 (define-alien-type nil (struct foo (str c-string)))
304 (defun make-foo (str)
305 (let ((my-foo (make-alien (struct foo))))
306 (setf (slot my-foo 'str) (make-alien char (length str))
307 (slot my-foo 'str) str)
311 Storing Lisp @code{NIL} in a @code{c-string} writes C @code{NULL} to
315 @code{sb-alien} also exports translations of these C type
316 specifiers as foreign type specifiers: @code{sb-alien:char},
317 @code{sb-alien:short}, @code{sb-alien:int},
318 @code{sb-alien:long}, @code{sb-alien:unsigned-char},
319 @code{sb-alien:unsigned-short},
320 @code{sb-alien:unsigned-int},
321 @code{sb-alien:unsigned-long}, @code{sb-alien:float}, and
322 @code{sb-alien:double}.
326 @node Operations On Foreign Values
327 @comment node-name, next, previous, up
328 @section Operations On Foreign Values
329 @c AKA "Alien Operations" in the CMU CL manual
331 This section describes how to read foreign values as Lisp values, how
332 to coerce foreign values to different kinds of foreign values, and how
333 to dynamically allocate and free foreign variables.
336 * Accessing Foreign Values::
337 * Coercing Foreign Values::
338 * Foreign Dynamic Allocation::
341 @node Accessing Foreign Values
342 @comment node-name, next, previous, up
343 @subsection Accessing Foreign Values
345 @defun sb-alien:deref @var{pointer-or-array} &rest @var{indices}
348 The @code{sb-alien:deref} function returns the value pointed to by a
349 foreign pointer, or the value of a foreign array element. When
350 dereferencing a pointer, an optional single index can be specified to
351 give the equivalent of C pointer arithmetic; this index is scaled by
352 the size of the type pointed to. When dereferencing an array, the
353 number of indices must be the same as the number of dimensions in the
354 array type. @code{deref} can be set with @code{setf} to assign a new
358 @defun sb-alien:slot @var{struct-or-union} &rest @var{slot-names}
361 The @code{sb-alien:slot} function extracts the value of the slot named
362 @var{slot-name} from a foreign @code{struct} or @code{union}. If
363 @var{struct-or-union} is a pointer to a structure or union, then it is
364 automatically dereferenced. @code{sb-alien:slot} can be set with
365 @code{setf} to assign a new value. Note that @var{slot-name} is
366 evaluated, and need not be a compile-time constant (but only constant
367 slot accesses are efficiently compiled).
371 @subsubsection Untyped memory
373 As noted at the beginning of the chapter, the System Area Pointer
374 facilities allow untyped access to foreign memory. @acronym{SAP}s can
375 be converted to and from the usual typed foreign values using
376 @code{sap-alien} and @code{alien-sap} (described elsewhere), and also
377 to and from integers - raw machine addresses. They should thus be
378 used with caution; corrupting the Lisp heap or other memory with
379 @acronym{SAP}s is trivial.
381 @defun sb-sys:int-sap @var{machine-address}
384 Creates a @acronym{SAP} pointing at the virtual address
385 @var{machine-address}.
388 @defun sb-sys:sap-ref-32 @var{sap} @var{offset}
391 Access the value of the memory location at @var{offset} bytes from
392 @var{sap}. This form may also be used with @code{setf} to alter the
393 memory at that location.
396 @defun sb-sys:sap= @var{sap1} @var{sap2}
399 Compare @var{sap1} and @var{sap2} for equality.
402 Similarly named functions exist for accessing other sizes of word,
403 other comparisons, and other conversions. The reader is invited to
404 use @code{apropos} and @code{describe} for more details
407 (apropos "sap" :sb-sys)
411 @node Coercing Foreign Values
412 @comment node-name, next, previous, up
413 @subsection Coercing Foreign Values
415 @defun sb-alien:addr @var{alien-expr}
418 The @code{sb-alien:addr} macro returns a pointer to the location
419 specified by @var{alien-expr}, which must be either a foreign
420 variable, a use of @code{sb-alien:deref}, a use of
421 @code{sb-alien:slot}, or a use of @code{sb-alien:extern-alien}.
424 @defun sb-alien:cast @var{foreign-value} @var{new-type}
427 The @code{sb-alien:cast} macro converts @var{foreign-value} to a new
428 foreign value with the specified @var{new-type}. Both types, old and
429 new, must be foreign pointer, array or function types. Note that the
430 resulting Lisp foreign variable object is not @code{eq} to the
431 argument, but it does refer to the same foreign data bits.
434 @defun sb-alien:sap-alien @var{sap} @var{type}
437 The @code{sb-alien:sap-alien} function converts @var{sap} (a system
438 area pointer) to a foreign value with the specified
439 @var{type}. @var{type} is not evaluated. </para>
441 The @var{type} must be some foreign pointer, array, or record type.
444 @defun sb-alien:alien-sap @var{foreign-value} @var{type}
447 The @code{sb-alien:alien-sap} function returns the @acronym{SAP} which
448 points to @var{alien-value}'s data.
450 The @var{foreign-value} must be of some foreign pointer, array, or
455 @node Foreign Dynamic Allocation
456 @comment node-name, next, previous, up
457 @subsection Foreign Dynamic Allocation
459 Lisp code can call the C standard library functions @code{malloc} and
460 @code{free} to dynamically allocate and deallocate foreign
461 variables. The Lisp code shares the same allocator with foreign C
462 code, so it's OK for foreign code to call @code{free} on the result of
463 Lisp @code{sb-alien:make-alien}, or for Lisp code to call
464 @code{sb-alien:free-alien} on foreign objects allocated by C
467 @defmac sb-alien:make-alien @var{type} @var{size}
470 The @code{sb-alien:make-alien} macro
471 returns a dynamically allocated foreign value of the specified
472 @var{type} (which is not evaluated.) The allocated memory is not
473 initialized, and may contain arbitrary junk. If supplied,
474 @var{size} is an expression to evaluate to compute the size of the
475 allocated object. There are two major cases:
479 When @var{type} is a foreign array type, an array of that type is
480 allocated and a pointer to it is returned. Note that you must use
481 @code{deref} to change the result to an array before you can use
482 @code{deref} to read or write elements:
485 (cl:in-package "CL-USER") ; which USEs package "SB-ALIEN"
486 (defvar *foo* (make-alien (array char 10)))
487 (type-of *foo*) @result{} (alien (* (array (signed 8) 10)))
488 (setf (deref (deref foo) 0) 10) @result{} 10
491 If supplied, @var{size} is used as the first dimension for the
495 When @var{type} is any other foreign type, then an object for that
496 type is allocated, and a pointer to it is returned. So
497 @code{(make-alien int)} returns a @code{(* int)}. If @var{size} is
498 specified, then a block of that many objects is allocated, with the
499 result pointing to the first one.
505 @defun sb-alien:free-alien @var{foreign-value}
508 The @code{sb-alien:free-alien} function
509 frees the storage for @var{foreign-value},
510 which must have been allocated with Lisp @code{make-alien}
513 See also the @code{sb-alien:with-alien} macro, which allocates foreign
517 @node Foreign Variables
518 @comment node-name, next, previous, up
519 @section Foreign Variables
520 @c AKA "Alien Variables" in the CMU CL manual
522 Both local (stack allocated) and external (C global) foreign variables
526 * Local Foreign Variables::
527 * External Foreign Variables::
530 @node Local Foreign Variables
531 @comment node-name, next, previous, up
532 @subsection Local Foreign Variables
534 @defmac sb-alien:with-alien @var{var-definitions} &body @var{body}
537 The @code{with-alien} macro establishes local foreign variables with
538 the specified alien types and names. This form is analogous to
539 defining a local variable in C: additional storage is allocated, and
540 the initial value is copied. This form is less analogous to
541 @code{LET}-allocated Lisp variables, since the variables can't be
542 captured in closures: they live only for the dynamic extent of the
543 body, and referring to them outside is a gruesome error.
545 The @var{var-definitions} argument is a list of
546 variable definitions, each of the form
548 (@var{name} @var{type} &optional @var{initial-value})
551 The names of the variables are established as symbol-macros; the
552 bindings have lexical scope, and may be assigned with @code{setq} or
555 The @code{with-alien} macro also establishes a new scope for named
556 structures and unions. Any @var{type} specified for a variable may
557 contain named structure or union types with the slots specified.
558 Within the lexical scope of the binding specifiers and body, a locally
559 defined foreign structure type @var{foo} can be referenced by its name
560 using @code{(struct @var{foo})}.
563 @node External Foreign Variables
564 @comment node-name, next, previous, up
565 @subsection External Foreign Variables
567 External foreign names are strings, and Lisp names are symbols. When
568 an external foreign value is represented using a Lisp variable, there
569 must be a way to convert from one name syntax into the other. The
570 macros @code{extern-alien}, @code{define-alien-variable} and
571 @code{define-alien-routine} use this conversion heuristic:
576 Alien names are converted to Lisp names by uppercasing and replacing
577 underscores with hyphens.
580 Conversely, Lisp names are converted to alien names by lowercasing and
581 replacing hyphens with underscores.
584 Both the Lisp symbol and alien string names may be separately
585 specified by using a list of the form
588 (alien-string lisp-symbol)
593 @defmac sb-alien:define-alien-variable @var{name} @var{type}
594 @findex define-alien-variable
596 The @code{define-alien-variable} macro defines @var{name} as an
597 external foreign variable of the specified foreign @code{type}.
598 @var{name} and @code{type} are not evaluated. The Lisp name of the
599 variable (see above) becomes a global alien variable. Global alien
600 variables are effectively ``global symbol macros''; a reference to the
601 variable fetches the contents of the external variable. Similarly,
602 setting the variable stores new contents -- the new contents must be
603 of the declared @code{type}. Someday, they may well be implemented
604 using the @acronym{ANSI} @code{define-symbol-macro} mechanism, but as
605 of SBCL 0.7.5, they are still implemented using an older more-or-less
606 parallel mechanism inherited from CMUCL.
608 For example, to access a C-level counter @var{foo}, one could write
611 (define-alien-variable "foo" int)
612 ;; Now it is possible to get the value of the C variable foo simply by
613 ;; referencing that Lisp variable:
620 @defun sb-alien:get-errno
623 Since in modern C libraries, the @code{errno} ``variable'' is typically
624 no longer a variable, but some bizarre artificial construct
625 which behaves superficially like a variable within a given thread,
626 it can no longer reliably be accessed through the ordinary
627 @code{define-alien-variable} mechanism. Instead, SBCL provides
628 the operator @code{sb-alien:get-errno} to allow Lisp code to read it.
631 @defmac sb-alien:extern-alien @var{name} @var{type}
634 The @code{extern-alien} macro returns an alien with the specified
635 @var{type} which points to an externally defined value. @var{name} is
636 not evaluated, and may be either a string or a symbol. @var{type} is
637 an unevaluated alien type specifier.
640 @node Foreign Data Structure Examples
641 @comment node-name, next, previous, up
642 @section Foreign Data Structure Examples
643 @c AKA "Alien Data Structure Example" in the CMU CL manual
645 Now that we have alien types, operations and variables, we can
646 manipulate foreign data structures. This C declaration
655 can be translated into the following alien type:
658 (define-alien-type nil
661 (b (array (* (struct foo)) 100))))
664 Once the @code{foo} alien type has been defined as above, the C
672 can be translated in this way:
675 (with-alien ((f (struct foo)))
676 (slot (deref (slot f 'b) 7) 'a)
678 ;; Do something with f...
682 Or consider this example of an external C variable and some accesses:
691 extern struct c_struct *my_struct;
694 my_struct = my_struct->n;
697 which can be manipulated in Lisp like this:
700 (define-alien-type nil
708 (define-alien-variable "my_struct" (* c-struct))
709 (incf (slot my-struct 'x))
710 (setf (slot my-struct 'a) 5)
711 (setq my-struct (slot my-struct 'n))
714 @node Loading Unix Object Files
715 @comment node-name, next, previous, up
716 @section Loading Unix Object Files
718 Foreign object files can be loaded into the running Lisp process by
719 calling the functions @code{load-foreign} or @code{load-1-foreign}.
721 The @code{sb-alien:load-1-foreign} function is the more primitive of
722 the two operations. It loads a single object file into the currently
723 running Lisp. The external symbols defining routines and variables are
724 made available for future external references (e.g. by
725 @code{extern-alien}). Forward references to foreign symbols aren't
726 supported: @code{load-1-foreign} must be run before any of the defined
727 symbols are referenced.
729 @code{sb-alien:load-foreign} is built in terms of
730 @code{load-1-foreign} and some other machinery like
731 @code{sb-ext:run-program}. It accepts a list of files and libraries,
732 and runs the linker on the files and libraries, creating an absolute
733 Unix object file which is then processed by @code{load-1-foreign}.
736 Note: As of SBCL 0.7.5, all foreign code (code loaded with
737 @code{load-1-function} or @code{load-function}) is lost when a Lisp
738 core is saved with @code{sb-ext:save-lisp-and-die}, and no attempt is
739 made to restore it when the core is loaded. Historically this has been
740 an annoyance both for SBCL users and for CMUCL users. It's hard to
741 solve this problem completely cleanly, but some generally-reliable
742 partial solution might be useful. Once someone in either camp gets
743 sufficiently annoyed to create it, SBCL is likely to adopt some
744 mechanism for automatically restoring foreign code when a saved core
749 @node Foreign Function Calls
750 @comment node-name, next, previous, up
751 @section Foreign Function Calls
753 The foreign function call interface allows a Lisp program to call
754 many functions written in languages that use the C calling convention.
756 Lisp sets up various signal handling routines and other environment
757 information when it first starts up, and expects these to be in place
758 at all times. The C functions called by Lisp should not change the
759 environment, especially the signal handlers: the signal handlers
760 installed by Lisp typically have interesting flags set (e.g to request
761 machine context information, or for signal delivery on an alternate
762 stack) which the Lisp runtime relies on for correct operation.
763 Precise details of how this works may change without notice between
764 versions; the source, or the brain of a friendly SBCL developer, is
765 the only documentation. Users of a Lisp built with the
766 @code{:sb-thread} feature should also read the section about threads,
770 * The alien-funcall Primitive::
771 * The define-alien-routine Macro::
772 * define-alien-routine Example::
773 * Calling Lisp From C::
776 @node The alien-funcall Primitive
777 @comment node-name, next, previous, up
778 @subsection The @code{alien-funcall} Primitive
780 @defun sb-alien:alien-funcall @var{alien-function} &rest @var{arguments}
781 @findex alien-funcall
783 The @code{alien-funcall} function is the foreign function call
784 primitive: @var{alien-function} is called with the supplied
785 @var{arguments} and its C return value is returned as a Lisp value.
786 The @var{alien-function} is an arbitrary run-time expression; to refer
787 to a constant function, use @code{extern-alien} or a value defined by
788 @code{define-alien-routine}.
790 The type of @code{alien-function} must be @code{(alien (function
791 ...))} or @code{(alien (* (function ...)))}. The function type is
792 used to determine how to call the function (as though it was declared
793 with a prototype.) The type need not be known at compile time, but
794 only known-type calls are efficiently compiled. Limitations:
799 Structure type return values are not implemented.
802 Passing of structures by value is not implemented.
808 Here is an example which allocates a @code{(struct foo)}, calls a
809 foreign function to initialize it, then returns a Lisp vector of all
810 the @code{(* (struct foo))} objects filled in by the foreign call:
813 ;; Allocate a foo on the stack.
814 (with-alien ((f (struct foo)))
815 ;; Call some C function to fill in foo fields.
816 (alien-funcall (extern-alien "mangle_foo" (function void (* foo)))
818 ;; Find how many foos to use by getting the A field.
819 (let* ((num (slot f 'a))
820 (result (make-array num)))
821 ;; Get a pointer to the array so that we don't have to keep extracting it:
822 (with-alien ((a (* (array (* (struct foo)) 100)) (addr (slot f 'b))))
823 ;; Loop over the first N elements and stash them in the result vector.
825 (setf (svref result i) (deref (deref a) i)))
830 @node The define-alien-routine Macro
831 @comment node-name, next, previous, up
832 @subsection The @code{define-alien-routine} Macro
834 @defmac sb-alien:define-alien-routine @var{name} @var{result-type} &rest @var{arg-specifiers}
835 @findex define-alien-routine
837 The @code{define-alien-routine} macro is a convenience for
838 automatically generating Lisp interfaces to simple foreign functions.
839 The primary feature is the parameter style specification, which
840 translates the C pass-by-reference idiom into additional return
843 @var{name} is usually a string external symbol, but may also be a
844 symbol Lisp name or a list of the foreign name and the Lisp name. If
845 only one name is specified, the other is automatically derived as for
846 @code{extern-alien}. @var{result-type} is the alien type of the
849 Each element of the @var{arg-specifiers} list
850 specifies an argument to the foreign function, and is
853 (aname atype &optional style)
856 @var{aname} is the symbol name of the argument to the constructed
857 function (for documentation). @var{atype} is the alien type of
858 corresponding foreign argument. The semantics of the actual call are
859 the same as for @code{alien-funcall}. @var{style} specifies how this
860 argument should be handled at call and return time, and should be one
866 @code{:in} specifies that the argument is passed by value. This is the
867 default. @code{:in} arguments have no corresponding return value from
871 @code{:copy} is similar to @code{:in}, but the argument is copied to a
872 pre-allocated object and a pointer to this object is passed to the
876 @code{:out} specifies a pass-by-reference output value. The type of
877 the argument must be a pointer to a fixed-sized object (such as an
878 integer or pointer). @code{:out} and @code{:in-out} style cannot be
879 used with pointers to arrays, records or functions. An object of the
880 correct size is allocated on the stack, and its address is passed to
881 the foreign function. When the function returns, the contents of this
882 location are returned as one of the values of the Lisp function (and
883 the location is automatically deallocated).
886 @code{:in-out} is a combination of @code{:copy} and @code{:out}. The
887 argument is copied to a pre-allocated object and a pointer to this
888 object is passed to the foreign routine. On return, the contents of
889 this location is returned as an additional value.
894 Note: Any efficiency-critical foreign interface function should be inline
895 expanded, which can be done by preceding the
896 @code{define-alien-routine} call with:
899 (declaim (inline lisp-name))
902 In addition to avoiding the Lisp call overhead, this allows
903 pointers, word-integers and floats to be passed using non-descriptor
904 representations, avoiding consing.)
909 @node define-alien-routine Example
910 @comment node-name, next, previous, up
911 @subsection @code{define-alien-routine} Example
913 Consider the C function @code{cfoo} with the following calling
920 char *a; /* update */
923 /* body of cfoo(...) */
927 This can be described by the following call to
928 @code{define-alien-routine}:
931 (define-alien-routine "cfoo" void
937 The Lisp function @code{cfoo} will have two arguments (@var{str} and
938 @var{a}) and two return values (@var{a} and @var{i}).
940 @node Calling Lisp From C
941 @comment node-name, next, previous, up
942 @subsection Calling Lisp From C
944 Calling Lisp functions from C is sometimes possible, but is extremely
945 hackish and poorly supported as of SBCL 0.7.5. See @code{funcall0}
946 @dots{} @code{funcall3} in the runtime system. The arguments must be
947 valid SBCL object descriptors (so that e.g. fixnums must be
948 left-shifted by 2.) As of SBCL 0.7.5, the format of object descriptors
949 is documented only by the source code and, in parts, by the old CMUCL
950 @file{INTERNALS} documentation.
952 Note that the garbage collector moves objects, and won't be
953 able to fix up any references in C variables. There are three
954 mechanisms for coping with this:
958 The @code{sb-ext:purify} moves all live Lisp
959 data into static or read-only areas such that it will never be moved
960 (or freed) again in the life of the Lisp session
963 @code{sb-sys:with-pinned-objects} is a macro which arranges for some
964 set of objects to be pinned in memory for the dynamic extent of its
965 body forms. On ports which use the generational garbage collector (as
966 of SBCL 0.8.3, only the x86) this has a page granularity - i.e. the
967 entire 4k page or pages containing the objects will be locked down. On
968 other ports it is implemented by turning off GC for the duration (so
969 could be said to have a whole-world granularity).
972 Disable GC, using the @code{without-gcing} macro or @code{gc-off}
976 @c <!-- FIXME: This is a "changebar" section from the CMU CL manual.
977 @c I (WHN 2002-07-14) am not very familiar with this content, so
978 @c I'm not immediately prepared to try to update it for SBCL, and
979 @c I'm not feeling masochistic enough to work to encourage this
980 @c kind of low-level hack anyway. However, I acknowledge that callbacks
981 @c are sometimes really really necessary, so I include the original
982 @c text in case someone is hard-core enough to benefit from it. If
983 @c anyone brings the information up to date for SBCL, it belong
984 @c either in the main manual or on a CLiki SBCL Internals page.
985 @c LaTeX \subsection{Accessing Lisp Arrays}
987 @c LaTeX Due to the way \cmucl{} manages memory, the amount of memory that can
988 @c LaTeX be dynamically allocated by \code{malloc} or \funref{make-alien} is
989 @c LaTeX limited\footnote{\cmucl{} mmaps a large piece of memory for it's own
990 @c LaTeX use and this memory is typically about 8 MB above the start of the C
991 @c LaTeX heap. Thus, only about 8 MB of memory can be dynamically
992 @c LaTeX allocated.}.
994 @c Empirically determined to be considerably >8Mb on this x86 linux
995 @c machine, but I don't know what the actual values are - dan 2003.09.01
997 @c Note that this technique is used in SB-GROVEL in the SBCL contrib
1000 @c LaTeX To overcome this limitation, it is possible to access the content of
1001 @c LaTeX Lisp arrays which are limited only by the amount of physical memory
1002 @c LaTeX and swap space available. However, this technique is only useful if
1003 @c LaTeX the foreign function takes pointers to memory instead of allocating
1004 @c LaTeX memory for itself. In latter case, you will have to modify the
1005 @c LaTeX foreign functions.
1007 @c LaTeX This technique takes advantage of the fact that \cmucl{} has
1008 @c LaTeX specialized array types (\pxlref{specialized-array-types}) that match
1009 @c LaTeX a typical C array. For example, a \code{(simple-array double-float
1010 @c LaTeX (100))} is stored in memory in essentially the same way as the C
1011 @c LaTeX array \code{double x[100]} would be. The following function allows us
1012 @c LaTeX to get the physical address of such a Lisp array:
1013 @c LaTeX \begin{example}
1014 @c LaTeX (defun array-data-address (array)
1015 @c LaTeX "Return the physical address of where the actual data of an array is
1018 @c LaTeX ARRAY must be a specialized array type in CMU Lisp. This means ARRAY
1019 @c LaTeX must be an array of one of the following types:
1021 @c LaTeX double-float
1022 @c LaTeX single-float
1023 @c LaTeX (unsigned-byte 32)
1024 @c LaTeX (unsigned-byte 16)
1025 @c LaTeX (unsigned-byte 8)
1026 @c LaTeX (signed-byte 32)
1027 @c LaTeX (signed-byte 16)
1028 @c LaTeX (signed-byte 8)
1030 @c LaTeX (declare (type (or #+signed-array (array (signed-byte 8))
1031 @c LaTeX #+signed-array (array (signed-byte 16))
1032 @c LaTeX #+signed-array (array (signed-byte 32))
1033 @c LaTeX (array (unsigned-byte 8))
1034 @c LaTeX (array (unsigned-byte 16))
1035 @c LaTeX (array (unsigned-byte 32))
1036 @c LaTeX (array single-float)
1037 @c LaTeX (array double-float))
1039 @c LaTeX (optimize (speed 3) (safety 0))
1040 @c LaTeX (ext:optimize-interface (safety 3)))
1041 @c LaTeX ;; with-array-data will get us to the actual data. However, because
1042 @c LaTeX ;; the array could have been displaced, we need to know where the
1043 @c LaTeX ;; data starts.
1044 @c LaTeX (lisp::with-array-data ((data array)
1047 @c LaTeX (declare (ignore end))
1048 @c LaTeX ;; DATA is a specialized simple-array. Memory is laid out like this:
1050 @c LaTeX ;; byte offset Value
1051 @c LaTeX ;; 0 type code (should be 70 for double-float vector)
1052 @c LaTeX ;; 4 4 * number of elements in vector
1053 @c LaTeX ;; 8 1st element of vector
1056 @c LaTeX (let ((addr (+ 8 (logandc1 7 (kernel:get-lisp-obj-address data))))
1057 @c LaTeX (type-size (let ((type (array-element-type data)))
1058 @c LaTeX (cond ((or (equal type '(signed-byte 8))
1059 @c LaTeX (equal type '(unsigned-byte 8)))
1061 @c LaTeX ((or (equal type '(signed-byte 16))
1062 @c LaTeX (equal type '(unsigned-byte 16)))
1064 @c LaTeX ((or (equal type '(signed-byte 32))
1065 @c LaTeX (equal type '(unsigned-byte 32)))
1067 @c LaTeX ((equal type 'single-float)
1069 @c LaTeX ((equal type 'double-float)
1072 @c LaTeX (error "Unknown specialized array element type"))))))
1073 @c LaTeX (declare (type (unsigned-byte 32) addr)
1074 @c LaTeX (optimize (speed 3) (safety 0) (ext:inhibit-warnings 3)))
1075 @c LaTeX (system:int-sap (the (unsigned-byte 32)
1076 @c LaTeX (+ addr (* type-size start)))))))
1077 @c LaTeX \end{example}
1079 @c LaTeX Assume we have the C function below that we wish to use:
1080 @c LaTeX \begin{example}
1081 @c LaTeX double dotprod(double* x, double* y, int n)
1084 @c LaTeX double sum = 0;
1086 @c LaTeX for (k = 0; k < n; ++k) \{
1087 @c LaTeX sum += x[k] * y[k];
1090 @c LaTeX \end{example}
1091 @c LaTeX The following example generates two large arrays in Lisp, and calls the C
1092 @c LaTeX function to do the desired computation. This would not have been
1093 @c LaTeX possible using \code{malloc} or \code{make-alien} since we need about
1094 @c LaTeX 16 MB of memory to hold the two arrays.
1095 @c LaTeX \begin{example}
1096 @c LaTeX (define-alien-routine "dotprod" double
1097 @c LaTeX (x (* double-float) :in)
1098 @c LaTeX (y (* double-float) :in)
1099 @c LaTeX (n int :in))
1101 @c LaTeX (let ((x (make-array 1000000 :element-type 'double-float))
1102 @c LaTeX (y (make-array 1000000 :element-type 'double-float)))
1103 @c LaTeX ;; Initialize X and Y somehow
1104 @c LaTeX (let ((x-addr (system:int-sap (array-data-address x)))
1105 @c LaTeX (y-addr (system:int-sap (array-data-address y))))
1106 @c LaTeX (dotprod x-addr y-addr 1000000)))
1107 @c LaTeX \end{example}
1108 @c LaTeX In this example, it may be useful to wrap the inner \code{let}
1109 @c LaTeX expression in an \code{unwind-protect} that first turns off garbage
1110 @c LaTeX collection and then turns garbage collection on afterwards. This will
1111 @c LaTeX prevent garbage collection from moving \code{x} and \code{y} after we
1112 @c LaTeX have obtained the (now erroneous) addresses but before the call to
1113 @c LaTeX \code{dotprod} is made.
1118 @node Step-By-Step Example of the Foreign Function Interface
1119 @comment node-name, next, previous, up
1120 @section Step-By-Step Example of the Foreign Function Interface
1122 This section presents a complete example of an interface to a somewhat
1123 complicated C function.
1125 Suppose you have the following C function which you want to be able to
1126 call from Lisp in the file @file{test.c}
1135 struct c_struct *c_function (i, s, r, a)
1142 struct c_struct *r2;
1144 printf("i = %d\n", i);
1145 printf("s = %s\n", s);
1146 printf("r->x = %d\n", r->x);
1147 printf("r->s = %s\n", r->s);
1148 for (j = 0; j < 10; j++) printf("a[%d] = %d.\n", j, a[j]);
1149 r2 = (struct c_struct *) malloc (sizeof(struct c_struct));
1151 r2->s = "a C string";
1156 It is possible to call this C function from Lisp using the file
1157 @file{test.lisp} containing
1160 (cl:defpackage "TEST-C-CALL" (:use "CL" "SB-ALIEN" "SB-C-CALL"))
1161 (cl:in-package "TEST-C-CALL")
1163 ;;; Define the record C-STRUCT in Lisp.
1164 (define-alien-type nil
1169 ;;; Define the Lisp function interface to the C routine. It returns a
1170 ;;; pointer to a record of type C-STRUCT. It accepts four parameters:
1171 ;;; I, an int; S, a pointer to a string; R, a pointer to a C-STRUCT
1172 ;;; record; and A, a pointer to the array of 10 ints.
1174 ;;; The INLINE declaration eliminates some efficiency notes about heap
1175 ;;; allocation of alien values.
1176 (declaim (inline c-function))
1177 (define-alien-routine c-function
1178 (* (struct c-struct))
1181 (r (* (struct c-struct)))
1184 ;;; a function which sets up the parameters to the C function and
1185 ;;; actually calls it
1187 (with-alien ((ar (array int 10))
1188 (c-struct (struct c-struct)))
1189 (dotimes (i 10) ; Fill array.
1190 (setf (deref ar i) i))
1191 (setf (slot c-struct 'x) 20)
1192 (setf (slot c-struct 's) "a Lisp string")
1194 (with-alien ((res (* (struct c-struct))
1195 (c-function 5 "another Lisp string" (addr c-struct) ar)))
1196 (format t "~&back from C function~%")
1197 (multiple-value-prog1
1198 (values (slot res 'x)
1201 ;; Deallocate result. (after we are done referring to it:
1202 ;; "Pillage, *then* burn.")
1203 (free-alien res)))))
1206 To execute the above example, it is necessary to compile the C
1207 routine, e.g.: @samp{cc -c test.c} (In order to enable incremental
1208 loading with some linkers, you may need to say @samp{cc -G 0 -c
1211 Once the C code has been compiled, you can start up Lisp and load it
1212 in: @samp{sbcl} Lisp should start up with its normal prompt.
1214 Within Lisp, compile the Lisp file. (This step can be done
1215 separately. You don't have to recompile every time.)
1216 @samp{(compile-file "test.lisp")}
1218 Within Lisp, load the foreign object file to define the necessary
1219 symbols: @samp{(load-foreign "test.o")}. This must be done before
1220 loading any code that refers to these symbols.
1222 Now you can load the compiled Lisp (``fasl'') file into Lisp:
1223 @samp{(load "test.fasl")}
1224 And once the Lisp file is loaded, you can call the
1225 Lisp routine that sets up the parameters and calls the C
1227 @samp{(test-c-call::call-cfun)}
1229 The C routine should print the following information to standard output:
1233 s = another Lisp string
1235 r->s = a Lisp string
1248 After return from the C function,
1249 the Lisp wrapper function should print the following output:
1252 back from C function
1255 And upon return from the Lisp wrapper function,
1256 before the next prompt is printed, the
1257 Lisp read-eval-print loop should print the following return values: