1 @node Foreign Function Interface
2 @comment node-name, next, previous, up
3 @chapter 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{lingua
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 Shared 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 provides 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} &rest
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, as guessed by the SBCL
200 internals. An optional @code{:alignment} keyword argument can be
201 specified for each field to explicitly control the alignment of a
202 field. If @var{name} is @code{nil} then the structure is anonymous.
204 If a named foreign @code{struct} specifier is passed to
205 @code{define-alien-type} or @code{with-alien}, then this defines,
206 respectively, a new global or local foreign structure type. If no
207 @var{fields} are specified, then the fields are taken
208 from the current (local or global) alien structure type definition of
212 The foreign type specifier @code{(sb-alien:union @var{name} &rest
213 @var{fields})} is similar to @code{sb-alien:struct}, but describes a
214 union type. All fields are allocated at the same offset, and the size
215 of the union is the size of the largest field. The programmer must
216 determine which field is active from context.
219 The foreign type specifier @code{(sb-alien:enum @var{name} &rest
220 @var{specs})} describes an enumeration type that maps between integer
221 values and symbols. If @var{name} is @code{nil}, then the type is
222 anonymous. Each element of the @var{specs} list is either a Lisp
223 symbol, or a list @code{(@var{symbol} @var{value})}. @var{value} is
224 an integer. If @var{value} is not supplied, then it defaults to one
225 greater than the value for the preceding spec (or to zero if it is the
229 The foreign type specifier @code{(sb-alien:signed &optional
230 @var{bits})} specifies a signed integer with the specified number of
231 @var{bits} precision. The upper limit on integer
232 precision is determined by the machine's word size. If
233 @var{bits} is not specified, the maximum size will be
237 The foreign type specifier @code{(integer &optional @var{bits})}
238 is equivalent to the corresponding type specifier using
239 @code{sb-alien:signed} instead of @code{integer}.
242 The foreign type specifier @code{(sb-alien:unsigned &optional
243 @var{bits})} is like corresponding type specifier using
244 @code{sb-alien:signed} except that the variable is treated as an
248 The foreign type specifier @code{(boolean &optional @var{bits})} is
249 similar to an enumeration type, but maps from Lisp @code{nil} and
250 @code{t} to C @code{0} and @code{1} respectively. @var{bits}
251 determines the amount of storage allocated to hold the truth value.
254 The foreign type specifier @code{single-float} describes a
255 floating-point number in IEEE single-precision format.
258 The foreign type specifier @code{double-float} describes a
259 floating-point number in IEEE double-precision format.
262 The foreign type specifier @code{(function @var{result-type} &rest
263 @var{arg-types})} describes a foreign function that takes arguments of
264 the specified @var{arg-types} and returns a result of type
265 @var{result-type}. Note that the only context where a foreign
266 @code{function} type is directly specified is in the argument to
267 @code{sb-alien:alien-funcall}. In all other contexts, foreign
268 functions are represented by foreign function pointer types: @code{(*
269 (function @dots{}))}.
272 The foreign type specifier @code{sb-alien:system-area-pointer}
273 describes a pointer which is represented in Lisp as a
274 @code{system-area-pointer} object. SBCL exports this type from
275 @code{sb-alien} because CMUCL did, but tentatively (as of the first
276 draft of this section of the manual, SBCL 0.7.6) it is deprecated,
277 since it doesn't seem to be required by user code.
280 The foreign type specifier @code{sb-alien:void} is used in function
281 types to declare that no useful value is returned. Using
282 @code{alien-funcall} to call a @code{void} foreign function will
286 @cindex External formats
287 The foreign type specifier @code{(sb-alien:c-string &key
288 external-format element-type not-null)} is similar to
289 @code{(* char)}, but is interpreted as a null-terminated string, and
290 is automatically converted into a Lisp string when accessed; or if the
291 pointer is C @code{NULL} or @code{0}, then accessing it gives Lisp
292 @code{nil} unless @code{not-null} is true, in which case a type-error
295 External format conversion is automatically done when Lisp strings are
296 passed to foreign code, or when foreign strings are passed to Lisp code.
297 If the type specifier has an explicit @code{external-format}, that
298 external format will be used. Otherwise a default external format that
299 has been determined at SBCL startup time based on the current locale
300 settings will be used. For example, when the following alien routine is
301 called, the Lisp string given as argument is converted to an
302 @code{ebcdic} octet representation.
305 (define-alien-routine test int (str (c-string :external-format :ebcdic-us)))
308 Lisp strings of type @code{base-string} are stored with a trailing NUL
309 termination, so no copying (either by the user or the implementation) is
310 necessary when passing them to foreign code, assuming that the
311 @code{external-format} and @code{element-type} of the @code{c-string}
312 type are compatible with the internal representation of the string. For
313 an SBCL built with Unicode support that means an @code{external-format}
314 of @code{:ascii} and an @code{element-type} of @code{base-char}. Without
315 Unicode support the @code{external-format} can also be
316 @code{:iso-8859-1}, and the @code{element-type} can also be
317 @code{character}. If the @code{external-format} or @code{element-type}
318 is not compatible, or the string is a @code{(simple-array character
319 (*))}, this data is copied by the implementation as required.
321 Assigning a Lisp string to a @code{c-string} structure field or
322 variable stores the contents of the string to the memory already
323 pointed to by that variable. When a foreign object of type @code{(*
324 char)} is assigned to a @code{c-string}, then the
325 @code{c-string} pointer is assigned to. This allows
326 @code{c-string} pointers to be initialized. For example:
329 (cl:in-package "CL-USER") ; which USEs package "SB-ALIEN"
331 (define-alien-type nil (struct foo (str c-string)))
333 (defun make-foo (str)
334 (let ((my-foo (make-alien (struct foo))))
335 (setf (slot my-foo 'str) (make-alien char (length str))
336 (slot my-foo 'str) str)
340 Storing Lisp @code{NIL} in a @code{c-string} writes C @code{NULL} to
344 @code{sb-alien} also exports translations of these C type
345 specifiers as foreign type specifiers:
350 @code{unsigned-char},
351 @code{unsigned-short},
353 @code{unsigned-long},
354 @code{float}, @code{double},
355 @code{size-t}, and @code{off-t}.
359 @node Operations On Foreign Values
360 @comment node-name, next, previous, up
361 @section Operations On Foreign Values
362 @c AKA "Alien Operations" in the CMU CL manual
364 This section describes how to read foreign values as Lisp values, how
365 to coerce foreign values to different kinds of foreign values, and how
366 to dynamically allocate and free foreign variables.
369 * Accessing Foreign Values::
370 * Coercing Foreign Values::
371 * Foreign Dynamic Allocation::
374 @node Accessing Foreign Values
375 @comment node-name, next, previous, up
376 @subsection Accessing Foreign Values
378 @defun @sbalien{deref} @var{pointer-or-array} &rest @var{indices}
380 The @code{sb-alien:deref} function returns the value pointed to by a
381 foreign pointer, or the value of a foreign array element. When
382 dereferencing a pointer, an optional single index can be specified to
383 give the equivalent of C pointer arithmetic; this index is scaled by
384 the size of the type pointed to. When dereferencing an array, the
385 number of indices must be the same as the number of dimensions in the
386 array type. @code{deref} can be set with @code{setf} to assign a new
390 @defun @sbalien{slot} @var{struct-or-union} @var{slot-name}
392 The @code{sb-alien:slot} function extracts the value of the slot named
393 @var{slot-name} from a foreign @code{struct} or @code{union}. If
394 @var{struct-or-union} is a pointer to a structure or union, then it is
395 automatically dereferenced. @code{sb-alien:slot} can be set with
396 @code{setf} to assign a new value. Note that @var{slot-name} is
397 evaluated, and need not be a compile-time constant (but only constant
398 slot accesses are efficiently compiled).
402 @subsubsection Untyped memory
404 As noted at the beginning of the chapter, the System Area Pointer
405 facilities allow untyped access to foreign memory. @acronym{SAP}s can
406 be converted to and from the usual typed foreign values using
407 @code{sap-alien} and @code{alien-sap} (described elsewhere), and also
408 to and from integers - raw machine addresses. They should thus be
409 used with caution; corrupting the Lisp heap or other memory with
410 @acronym{SAP}s is trivial.
412 @defun @sbsys{int-sap} @var{machine-address}
414 Creates a @acronym{SAP} pointing at the virtual address
415 @var{machine-address}.
418 @defun @sbsys{sap-ref-32} @var{sap} @var{offset}
420 Access the value of the memory location at @var{offset} bytes from
421 @var{sap}. This form may also be used with @code{setf} to alter the
422 memory at that location.
425 @defun @sbsys{sap=} @var{sap1} @var{sap2}
427 Compare @var{sap1} and @var{sap2} for equality.
430 Similarly named functions exist for accessing other sizes of word,
431 other comparisons, and other conversions. The reader is invited to
432 use @code{apropos} and @code{describe} for more details
435 (apropos "sap" :sb-sys)
439 @node Coercing Foreign Values
440 @comment node-name, next, previous, up
441 @subsection Coercing Foreign Values
443 @defmac @sbalien{addr} @var{alien-expr}
445 The @code{sb-alien:addr} macro returns a pointer to the location
446 specified by @var{alien-expr}, which must be either a foreign
447 variable, a use of @code{sb-alien:deref}, a use of
448 @code{sb-alien:slot}, or a use of @code{sb-alien:extern-alien}.
451 @defmac @sbalien{cast} @var{foreign-value} @var{new-type}
453 The @code{sb-alien:cast} macro converts @var{foreign-value} to a new
454 foreign value with the specified @var{new-type}. Both types, old and
455 new, must be foreign pointer, array or function types. Note that the
456 resulting Lisp foreign variable object is not @code{eq} to the
457 argument, but it does refer to the same foreign data bits.
460 @defmac @sbalien{sap-alien} @var{sap} @var{type}
462 The @code{sb-alien:sap-alien} macro converts @var{sap} (a system
463 area pointer) to a foreign value with the specified
464 @var{type}. @var{type} is not evaluated.
466 The @var{type} must be some foreign pointer, array, or record type.
469 @defun @sbalien{alien-sap} @var{foreign-value}
471 The @code{sb-alien:alien-sap} function returns the @acronym{SAP} which
472 points to @var{alien-value}'s data.
474 The @var{foreign-value} must be of some foreign pointer, array, or
479 @node Foreign Dynamic Allocation
480 @comment node-name, next, previous, up
481 @subsection Foreign Dynamic Allocation
483 Lisp code can call the C standard library functions @code{malloc} and
484 @code{free} to dynamically allocate and deallocate foreign variables.
485 The Lisp code shares the same allocator with foreign C code, so it's
486 OK for foreign code to call @code{free} on the result of Lisp
487 @code{sb-alien:make-alien}, or for Lisp code to call
488 @code{sb-alien:free-alien} on foreign objects allocated by C code.
490 @include macro-sb-alien-make-alien.texinfo
491 @include fun-sb-alien-make-alien-string.texinfo
492 @include fun-sb-alien-free-alien.texinfo
494 @node Foreign Variables
495 @comment node-name, next, previous, up
496 @section Foreign Variables
497 @c AKA "Alien Variables" in the CMU CL manual
499 Both local (stack allocated) and external (C global) foreign variables
503 * Local Foreign Variables::
504 * External Foreign Variables::
507 @node Local Foreign Variables
508 @comment node-name, next, previous, up
509 @subsection Local Foreign Variables
511 @defmac @sbalien{with-alien} @var{var-definitions} &body @var{body}
513 The @code{with-alien} macro establishes local foreign variables with
514 the specified alien types and names. This form is analogous to
515 defining a local variable in C: additional storage is allocated, and
516 the initial value is copied. This form is less analogous to
517 @code{LET}-allocated Lisp variables, since the variables can't be
518 captured in closures: they live only for the dynamic extent of the
519 body, and referring to them outside is a gruesome error.
521 The @var{var-definitions} argument is a list of
522 variable definitions, each of the form
524 (@var{name} @var{type} &optional @var{initial-value})
527 The names of the variables are established as symbol-macros; the
528 bindings have lexical scope, and may be assigned with @code{setq} or
531 The @code{with-alien} macro also establishes a new scope for named
532 structures and unions. Any @var{type} specified for a variable may
533 contain named structure or union types with the slots specified.
534 Within the lexical scope of the binding specifiers and body, a locally
535 defined foreign structure type @var{foo} can be referenced by its name
536 using @code{(struct @var{foo})}.
539 @node External Foreign Variables
540 @comment node-name, next, previous, up
541 @subsection External Foreign Variables
543 External foreign names are strings, and Lisp names are symbols. When
544 an external foreign value is represented using a Lisp variable, there
545 must be a way to convert from one name syntax into the other. The
546 macros @code{extern-alien}, @code{define-alien-variable} and
547 @code{define-alien-routine} use this conversion heuristic:
552 Alien names are converted to Lisp names by uppercasing and replacing
553 underscores with hyphens.
556 Conversely, Lisp names are converted to alien names by lowercasing and
557 replacing hyphens with underscores.
560 Both the Lisp symbol and alien string names may be separately
561 specified by using a list of the form
564 (alien-string lisp-symbol)
569 @defmac @sbalien{define-alien-variable} @var{name} @var{type}
571 The @code{define-alien-variable} macro defines @var{name} as an
572 external foreign variable of the specified foreign @code{type}.
573 @var{name} and @code{type} are not evaluated. The Lisp name of the
574 variable (see above) becomes a global alien variable. Global alien
575 variables are effectively ``global symbol macros''; a reference to the
576 variable fetches the contents of the external variable. Similarly,
577 setting the variable stores new contents -- the new contents must be
578 of the declared @code{type}. Someday, they may well be implemented
579 using the @acronym{ANSI} @code{define-symbol-macro} mechanism, but as
580 of SBCL 0.7.5, they are still implemented using an older more-or-less
581 parallel mechanism inherited from CMUCL.
583 For example, to access a C-level counter @var{foo}, one could write
586 (define-alien-variable "foo" int)
587 ;; Now it is possible to get the value of the C variable foo simply by
588 ;; referencing that Lisp variable:
595 @defun @sbalien{get-errno}
597 Since in modern C libraries, the @code{errno} ``variable'' is typically
598 no longer a variable, but some bizarre artificial construct
599 which behaves superficially like a variable within a given thread,
600 it can no longer reliably be accessed through the ordinary
601 @code{define-alien-variable} mechanism. Instead, SBCL provides
602 the operator @code{sb-alien:get-errno} to allow Lisp code to read it.
605 @defmac @sbalien{extern-alien} @var{name} @var{type}
607 The @code{extern-alien} macro returns an alien with the specified
608 @var{type} which points to an externally defined value. @var{name} is
609 not evaluated, and may be either a string or a symbol. @var{type} is
610 an unevaluated alien type specifier.
613 @node Foreign Data Structure Examples
614 @comment node-name, next, previous, up
615 @section Foreign Data Structure Examples
616 @c AKA "Alien Data Structure Example" in the CMU CL manual
618 Now that we have alien types, operations and variables, we can
619 manipulate foreign data structures. This C declaration
628 can be translated into the following alien type:
631 (define-alien-type nil
634 (b (array (* (struct foo)) 100))))
637 Once the @code{foo} alien type has been defined as above, the C
645 can be translated in this way:
648 (with-alien ((f (struct foo)))
649 (slot (deref (slot f 'b) 7) 'a)
651 ;; Do something with f...
655 Or consider this example of an external C variable and some accesses:
664 extern struct c_struct *my_struct;
667 my_struct = my_struct->n;
670 which can be manipulated in Lisp like this:
673 (define-alien-type nil
681 (define-alien-variable "my_struct" (* c-struct))
682 (incf (slot my-struct 'x))
683 (setf (slot my-struct 'a) 5)
684 (setq my-struct (slot my-struct 'n))
687 @node Loading Shared Object Files
688 @comment node-name, next, previous, up
689 @section Loading Shared Object Files
691 Foreign object files can be loaded into the running Lisp process by
692 calling @code{load-shared-object}.
694 @include fun-sb-alien-load-shared-object.texinfo
696 @include fun-sb-alien-unload-shared-object.texinfo
698 @node Foreign Function Calls
699 @comment node-name, next, previous, up
700 @section Foreign Function Calls
702 The foreign function call interface allows a Lisp program to call
703 many functions written in languages that use the C calling convention.
705 Lisp sets up various signal handling routines and other environment
706 information when it first starts up, and expects these to be in place
707 at all times. The C functions called by Lisp should not change the
708 environment, especially the signal handlers: the signal handlers
709 installed by Lisp typically have interesting flags set (e.g to request
710 machine context information, or for signal delivery on an alternate
711 stack) which the Lisp runtime relies on for correct operation.
712 Precise details of how this works may change without notice between
713 versions; the source, or the brain of a friendly SBCL developer, is
714 the only documentation. Users of a Lisp built with the
715 @code{:sb-thread} feature should also read the section about threads,
719 * The alien-funcall Primitive::
720 * The define-alien-routine Macro::
721 * define-alien-routine Example::
722 * Calling Lisp From C::
725 @node The alien-funcall Primitive
726 @comment node-name, next, previous, up
727 @subsection The @code{alien-funcall} Primitive
729 @defun @sbalien{alien-funcall} @var{alien-function} &rest @var{arguments}
731 The @code{alien-funcall} function is the foreign function call
732 primitive: @var{alien-function} is called with the supplied
733 @var{arguments} and its C return value is returned as a Lisp value.
734 The @var{alien-function} is an arbitrary run-time expression; to refer
735 to a constant function, use @code{extern-alien} or a value defined by
736 @code{define-alien-routine}.
738 The type of @code{alien-function} must be @code{(alien (function
739 ...))} or @code{(alien (* (function ...)))}. The function type is
740 used to determine how to call the function (as though it was declared
741 with a prototype.) The type need not be known at compile time, but
742 only known-type calls are efficiently compiled. Limitations:
747 Structure type return values are not implemented.
750 Passing of structures by value is not implemented.
756 Here is an example which allocates a @code{(struct foo)}, calls a
757 foreign function to initialize it, then returns a Lisp vector of all
758 the @code{(* (struct foo))} objects filled in by the foreign call:
761 ;; Allocate a foo on the stack.
762 (with-alien ((f (struct foo)))
763 ;; Call some C function to fill in foo fields.
764 (alien-funcall (extern-alien "mangle_foo" (function void (* foo)))
766 ;; Find how many foos to use by getting the A field.
767 (let* ((num (slot f 'a))
768 (result (make-array num)))
769 ;; Get a pointer to the array so that we don't have to keep extracting it:
770 (with-alien ((a (* (array (* (struct foo)) 100)) (addr (slot f 'b))))
771 ;; Loop over the first N elements and stash them in the result vector.
773 (setf (svref result i) (deref (deref a) i)))
778 @node The define-alien-routine Macro
779 @comment node-name, next, previous, up
780 @subsection The @code{define-alien-routine} Macro
782 @defmac @sbalien{define-alien-routine} @var{name} @var{result-type} &rest @var{arg-specifiers}
784 The @code{define-alien-routine} macro is a convenience for
785 automatically generating Lisp interfaces to simple foreign functions.
786 The primary feature is the parameter style specification, which
787 translates the C pass-by-reference idiom into additional return
790 @var{name} is usually a string external symbol, but may also be a
791 symbol Lisp name or a list of the foreign name and the Lisp name. If
792 only one name is specified, the other is automatically derived as for
793 @code{extern-alien}. @var{result-type} is the alien type of the
796 Each element of the @var{arg-specifiers} list
797 specifies an argument to the foreign function, and is
800 (aname atype &optional style)
803 @var{aname} is the symbol name of the argument to the constructed
804 function (for documentation). @var{atype} is the alien type of
805 corresponding foreign argument. The semantics of the actual call are
806 the same as for @code{alien-funcall}. @var{style} specifies how this
807 argument should be handled at call and return time, and should be one
813 @code{:in} specifies that the argument is passed by value. This is the
814 default. @code{:in} arguments have no corresponding return value from
818 @code{:copy} is similar to @code{:in}, but the argument is copied to a
819 pre-allocated object and a pointer to this object is passed to the
823 @code{:out} specifies a pass-by-reference output value. The type of
824 the argument must be a pointer to a fixed-sized object (such as an
825 integer or pointer). @code{:out} and @code{:in-out} style cannot be
826 used with pointers to arrays, records or functions. An object of the
827 correct size is allocated on the stack, and its address is passed to
828 the foreign function. When the function returns, the contents of this
829 location are returned as one of the values of the Lisp function (and
830 the location is automatically deallocated).
833 @code{:in-out} is a combination of @code{:copy} and @code{:out}. The
834 argument is copied to a pre-allocated object and a pointer to this
835 object is passed to the foreign routine. On return, the contents of
836 this location is returned as an additional value.
841 Note: Any efficiency-critical foreign interface function should be inline
842 expanded, which can be done by preceding the
843 @code{define-alien-routine} call with:
846 (declaim (inline lisp-name))
849 In addition to avoiding the Lisp call overhead, this allows
850 pointers, word-integers and floats to be passed using non-descriptor
851 representations, avoiding consing.)
856 @node define-alien-routine Example
857 @comment node-name, next, previous, up
858 @subsection @code{define-alien-routine} Example
860 Consider the C function @code{cfoo} with the following calling
867 char *a; /* update */
870 /* body of cfoo(...) */
874 This can be described by the following call to
875 @code{define-alien-routine}:
878 (define-alien-routine "cfoo" void
884 The Lisp function @code{cfoo} will have two arguments (@var{str} and
885 @var{a}) and two return values (@var{a} and @var{i}).
887 @node Calling Lisp From C
888 @comment node-name, next, previous, up
889 @subsection Calling Lisp From C
891 Calling Lisp functions from C is sometimes possible, but is extremely
892 hackish and poorly supported as of SBCL 0.7.5. See @code{funcall0}
893 @dots{} @code{funcall3} in the runtime system. The arguments must be
894 valid SBCL object descriptors (so that e.g. fixnums must be
895 left-shifted by 2.) As of SBCL 0.7.5, the format of object descriptors
896 is documented only by the source code and, in parts, by the old CMUCL
897 @file{INTERNALS} documentation.
899 Note that the garbage collector moves objects, and won't be
900 able to fix up any references in C variables. There are three
901 mechanisms for coping with this:
905 The @code{sb-ext:purify} moves all live Lisp
906 data into static or read-only areas such that it will never be moved
907 (or freed) again in the life of the Lisp session
910 @code{sb-sys:with-pinned-objects} is a macro which arranges for some
911 set of objects to be pinned in memory for the dynamic extent of its
912 body forms. On ports which use the generational garbage collector (as
913 of SBCL 0.8.3, only the x86) this has a page granularity - i.e. the
914 entire 4k page or pages containing the objects will be locked down. On
915 other ports it is implemented by turning off GC for the duration (so
916 could be said to have a whole-world granularity).
919 Disable GC, using the @code{without-gcing} macro.
922 @c <!-- FIXME: This is a "changebar" section from the CMU CL manual.
923 @c I (WHN 2002-07-14) am not very familiar with this content, so
924 @c I'm not immediately prepared to try to update it for SBCL, and
925 @c I'm not feeling masochistic enough to work to encourage this
926 @c kind of low-level hack anyway. However, I acknowledge that callbacks
927 @c are sometimes really really necessary, so I include the original
928 @c text in case someone is hard-core enough to benefit from it. If
929 @c anyone brings the information up to date for SBCL, it belong
930 @c either in the main manual or on a CLiki SBCL Internals page.
931 @c LaTeX \subsection{Accessing Lisp Arrays}
933 @c LaTeX Due to the way \cmucl{} manages memory, the amount of memory that can
934 @c LaTeX be dynamically allocated by \code{malloc} or \funref{make-alien} is
935 @c LaTeX limited\footnote{\cmucl{} mmaps a large piece of memory for it's own
936 @c LaTeX use and this memory is typically about 8 MB above the start of the C
937 @c LaTeX heap. Thus, only about 8 MB of memory can be dynamically
938 @c LaTeX allocated.}.
940 @c Empirically determined to be considerably >8Mb on this x86 linux
941 @c machine, but I don't know what the actual values are - dan 2003.09.01
943 @c Note that this technique is used in SB-GROVEL in the SBCL contrib
946 @c LaTeX To overcome this limitation, it is possible to access the content of
947 @c LaTeX Lisp arrays which are limited only by the amount of physical memory
948 @c LaTeX and swap space available. However, this technique is only useful if
949 @c LaTeX the foreign function takes pointers to memory instead of allocating
950 @c LaTeX memory for itself. In latter case, you will have to modify the
951 @c LaTeX foreign functions.
953 @c LaTeX This technique takes advantage of the fact that \cmucl{} has
954 @c LaTeX specialized array types (\pxlref{specialized-array-types}) that match
955 @c LaTeX a typical C array. For example, a \code{(simple-array double-float
956 @c LaTeX (100))} is stored in memory in essentially the same way as the C
957 @c LaTeX array \code{double x[100]} would be. The following function allows us
958 @c LaTeX to get the physical address of such a Lisp array:
959 @c LaTeX \begin{example}
960 @c LaTeX (defun array-data-address (array)
961 @c LaTeX "Return the physical address of where the actual data of an array is
964 @c LaTeX ARRAY must be a specialized array type in CMU Lisp. This means ARRAY
965 @c LaTeX must be an array of one of the following types:
967 @c LaTeX double-float
968 @c LaTeX single-float
969 @c LaTeX (unsigned-byte 32)
970 @c LaTeX (unsigned-byte 16)
971 @c LaTeX (unsigned-byte 8)
972 @c LaTeX (signed-byte 32)
973 @c LaTeX (signed-byte 16)
974 @c LaTeX (signed-byte 8)
976 @c LaTeX (declare (type (or #+signed-array (array (signed-byte 8))
977 @c LaTeX #+signed-array (array (signed-byte 16))
978 @c LaTeX #+signed-array (array (signed-byte 32))
979 @c LaTeX (array (unsigned-byte 8))
980 @c LaTeX (array (unsigned-byte 16))
981 @c LaTeX (array (unsigned-byte 32))
982 @c LaTeX (array single-float)
983 @c LaTeX (array double-float))
985 @c LaTeX (optimize (speed 3) (safety 0))
986 @c LaTeX (ext:optimize-interface (safety 3)))
987 @c LaTeX ;; with-array-data will get us to the actual data. However, because
988 @c LaTeX ;; the array could have been displaced, we need to know where the
989 @c LaTeX ;; data starts.
990 @c LaTeX (lisp::with-array-data ((data array)
993 @c LaTeX (declare (ignore end))
994 @c LaTeX ;; DATA is a specialized simple-array. Memory is laid out like this:
996 @c LaTeX ;; byte offset Value
997 @c LaTeX ;; 0 type code (should be 70 for double-float vector)
998 @c LaTeX ;; 4 4 * number of elements in vector
999 @c LaTeX ;; 8 1st element of vector
1002 @c LaTeX (let ((addr (+ 8 (logandc1 7 (kernel:get-lisp-obj-address data))))
1003 @c LaTeX (type-size (let ((type (array-element-type data)))
1004 @c LaTeX (cond ((or (equal type '(signed-byte 8))
1005 @c LaTeX (equal type '(unsigned-byte 8)))
1007 @c LaTeX ((or (equal type '(signed-byte 16))
1008 @c LaTeX (equal type '(unsigned-byte 16)))
1010 @c LaTeX ((or (equal type '(signed-byte 32))
1011 @c LaTeX (equal type '(unsigned-byte 32)))
1013 @c LaTeX ((equal type 'single-float)
1015 @c LaTeX ((equal type 'double-float)
1018 @c LaTeX (error "Unknown specialized array element type"))))))
1019 @c LaTeX (declare (type (unsigned-byte 32) addr)
1020 @c LaTeX (optimize (speed 3) (safety 0) (ext:inhibit-warnings 3)))
1021 @c LaTeX (system:int-sap (the (unsigned-byte 32)
1022 @c LaTeX (+ addr (* type-size start)))))))
1023 @c LaTeX \end{example}
1025 @c LaTeX Assume we have the C function below that we wish to use:
1026 @c LaTeX \begin{example}
1027 @c LaTeX double dotprod(double* x, double* y, int n)
1030 @c LaTeX double sum = 0;
1032 @c LaTeX for (k = 0; k < n; ++k) \{
1033 @c LaTeX sum += x[k] * y[k];
1036 @c LaTeX \end{example}
1037 @c LaTeX The following example generates two large arrays in Lisp, and calls the C
1038 @c LaTeX function to do the desired computation. This would not have been
1039 @c LaTeX possible using \code{malloc} or \code{make-alien} since we need about
1040 @c LaTeX 16 MB of memory to hold the two arrays.
1041 @c LaTeX \begin{example}
1042 @c LaTeX (define-alien-routine "dotprod" double
1043 @c LaTeX (x (* double-float) :in)
1044 @c LaTeX (y (* double-float) :in)
1045 @c LaTeX (n int :in))
1047 @c LaTeX (let ((x (make-array 1000000 :element-type 'double-float))
1048 @c LaTeX (y (make-array 1000000 :element-type 'double-float)))
1049 @c LaTeX ;; Initialize X and Y somehow
1050 @c LaTeX (let ((x-addr (system:int-sap (array-data-address x)))
1051 @c LaTeX (y-addr (system:int-sap (array-data-address y))))
1052 @c LaTeX (dotprod x-addr y-addr 1000000)))
1053 @c LaTeX \end{example}
1054 @c LaTeX In this example, it may be useful to wrap the inner \code{let}
1055 @c LaTeX expression in an \code{unwind-protect} that first turns off garbage
1056 @c LaTeX collection and then turns garbage collection on afterwards. This will
1057 @c LaTeX prevent garbage collection from moving \code{x} and \code{y} after we
1058 @c LaTeX have obtained the (now erroneous) addresses but before the call to
1059 @c LaTeX \code{dotprod} is made.
1064 @node Step-By-Step Example of the Foreign Function Interface
1065 @comment node-name, next, previous, up
1066 @section Step-By-Step Example of the Foreign Function Interface
1068 This section presents a complete example of an interface to a somewhat
1069 complicated C function.
1071 Suppose you have the following C function which you want to be able to
1072 call from Lisp in the file @file{test.c}
1081 struct c_struct *c_function (i, s, r, a)
1088 struct c_struct *r2;
1090 printf("i = %d\n", i);
1091 printf("s = %s\n", s);
1092 printf("r->x = %d\n", r->x);
1093 printf("r->s = %s\n", r->s);
1094 for (j = 0; j < 10; j++) printf("a[%d] = %d.\n", j, a[j]);
1095 r2 = (struct c_struct *) malloc (sizeof(struct c_struct));
1097 r2->s = "a C string";
1102 It is possible to call this C function from Lisp using the file
1103 @file{test.lisp} containing
1106 (cl:defpackage "TEST-C-CALL" (:use "CL" "SB-ALIEN" "SB-C-CALL"))
1107 (cl:in-package "TEST-C-CALL")
1109 ;;; Define the record C-STRUCT in Lisp.
1110 (define-alien-type nil
1115 ;;; Define the Lisp function interface to the C routine. It returns a
1116 ;;; pointer to a record of type C-STRUCT. It accepts four parameters:
1117 ;;; I, an int; S, a pointer to a string; R, a pointer to a C-STRUCT
1118 ;;; record; and A, a pointer to the array of 10 ints.
1120 ;;; The INLINE declaration eliminates some efficiency notes about heap
1121 ;;; allocation of alien values.
1122 (declaim (inline c-function))
1123 (define-alien-routine c-function
1124 (* (struct c-struct))
1127 (r (* (struct c-struct)))
1130 ;;; a function which sets up the parameters to the C function and
1131 ;;; actually calls it
1133 (with-alien ((ar (array int 10))
1134 (c-struct (struct c-struct)))
1135 (dotimes (i 10) ; Fill array.
1136 (setf (deref ar i) i))
1137 (setf (slot c-struct 'x) 20)
1138 (setf (slot c-struct 's) "a Lisp string")
1140 (with-alien ((res (* (struct c-struct))
1141 (c-function 5 "another Lisp string" (addr c-struct) ar)))
1142 (format t "~&back from C function~%")
1143 (multiple-value-prog1
1144 (values (slot res 'x)
1147 ;; Deallocate result. (after we are done referring to it:
1148 ;; "Pillage, *then* burn.")
1149 (free-alien res)))))
1152 To execute the above example, it is necessary to compile the C
1153 routine, e.g.: @samp{cc -c test.c && ld -shared -o test.so test.o} (In
1154 order to enable incremental loading with some linkers, you may need to
1155 say @samp{cc -G 0 -c test.c})
1157 Once the C code has been compiled, you can start up Lisp and load it in:
1158 @samp{sbcl}. Lisp should start up with its normal prompt.
1160 Within Lisp, compile the Lisp file. (This step can be done
1161 separately. You don't have to recompile every time.)
1162 @samp{(compile-file "test.lisp")}
1164 Within Lisp, load the foreign object file to define the necessary
1165 symbols: @samp{(load-shared-object "test.so")}.
1167 Now you can load the compiled Lisp (``fasl'') file into Lisp:
1168 @samp{(load "test.fasl")}
1169 And once the Lisp file is loaded, you can call the
1170 Lisp routine that sets up the parameters and calls the C
1172 @samp{(test-c-call::call-cfun)}
1174 The C routine should print the following information to standard output:
1178 s = another Lisp string
1180 r->s = a Lisp string
1193 After return from the C function,
1194 the Lisp wrapper function should print the following output:
1197 back from C function
1200 And upon return from the Lisp wrapper function,
1201 before the next prompt is printed, the
1202 Lisp read-eval-print loop should print the following return values: