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: @code{sb-alien:char},
346 @code{sb-alien:short}, @code{sb-alien:int},
347 @code{sb-alien:long}, @code{sb-alien:unsigned-char},
348 @code{sb-alien:unsigned-short},
349 @code{sb-alien:unsigned-int},
350 @code{sb-alien:unsigned-long}, @code{sb-alien:float}, and
351 @code{sb-alien:double}.
355 @node Operations On Foreign Values
356 @comment node-name, next, previous, up
357 @section Operations On Foreign Values
358 @c AKA "Alien Operations" in the CMU CL manual
360 This section describes how to read foreign values as Lisp values, how
361 to coerce foreign values to different kinds of foreign values, and how
362 to dynamically allocate and free foreign variables.
365 * Accessing Foreign Values::
366 * Coercing Foreign Values::
367 * Foreign Dynamic Allocation::
370 @node Accessing Foreign Values
371 @comment node-name, next, previous, up
372 @subsection Accessing Foreign Values
374 @defun @sbalien{deref} @var{pointer-or-array} &rest @var{indices}
376 The @code{sb-alien:deref} function returns the value pointed to by a
377 foreign pointer, or the value of a foreign array element. When
378 dereferencing a pointer, an optional single index can be specified to
379 give the equivalent of C pointer arithmetic; this index is scaled by
380 the size of the type pointed to. When dereferencing an array, the
381 number of indices must be the same as the number of dimensions in the
382 array type. @code{deref} can be set with @code{setf} to assign a new
386 @defun @sbalien{slot} @var{struct-or-union} @var{slot-name}
388 The @code{sb-alien:slot} function extracts the value of the slot named
389 @var{slot-name} from a foreign @code{struct} or @code{union}. If
390 @var{struct-or-union} is a pointer to a structure or union, then it is
391 automatically dereferenced. @code{sb-alien:slot} can be set with
392 @code{setf} to assign a new value. Note that @var{slot-name} is
393 evaluated, and need not be a compile-time constant (but only constant
394 slot accesses are efficiently compiled).
398 @subsubsection Untyped memory
400 As noted at the beginning of the chapter, the System Area Pointer
401 facilities allow untyped access to foreign memory. @acronym{SAP}s can
402 be converted to and from the usual typed foreign values using
403 @code{sap-alien} and @code{alien-sap} (described elsewhere), and also
404 to and from integers - raw machine addresses. They should thus be
405 used with caution; corrupting the Lisp heap or other memory with
406 @acronym{SAP}s is trivial.
408 @defun @sbsys{int-sap} @var{machine-address}
410 Creates a @acronym{SAP} pointing at the virtual address
411 @var{machine-address}.
414 @defun @sbsys{sap-ref-32} @var{sap} @var{offset}
416 Access the value of the memory location at @var{offset} bytes from
417 @var{sap}. This form may also be used with @code{setf} to alter the
418 memory at that location.
421 @defun @sbsys{sap=} @var{sap1} @var{sap2}
423 Compare @var{sap1} and @var{sap2} for equality.
426 Similarly named functions exist for accessing other sizes of word,
427 other comparisons, and other conversions. The reader is invited to
428 use @code{apropos} and @code{describe} for more details
431 (apropos "sap" :sb-sys)
435 @node Coercing Foreign Values
436 @comment node-name, next, previous, up
437 @subsection Coercing Foreign Values
439 @defmac @sbalien{addr} @var{alien-expr}
441 The @code{sb-alien:addr} macro returns a pointer to the location
442 specified by @var{alien-expr}, which must be either a foreign
443 variable, a use of @code{sb-alien:deref}, a use of
444 @code{sb-alien:slot}, or a use of @code{sb-alien:extern-alien}.
447 @defmac @sbalien{cast} @var{foreign-value} @var{new-type}
449 The @code{sb-alien:cast} macro converts @var{foreign-value} to a new
450 foreign value with the specified @var{new-type}. Both types, old and
451 new, must be foreign pointer, array or function types. Note that the
452 resulting Lisp foreign variable object is not @code{eq} to the
453 argument, but it does refer to the same foreign data bits.
456 @defmac @sbalien{sap-alien} @var{sap} @var{type}
458 The @code{sb-alien:sap-alien} macro converts @var{sap} (a system
459 area pointer) to a foreign value with the specified
460 @var{type}. @var{type} is not evaluated.
462 The @var{type} must be some foreign pointer, array, or record type.
465 @defun @sbalien{alien-sap} @var{foreign-value}
467 The @code{sb-alien:alien-sap} function returns the @acronym{SAP} which
468 points to @var{alien-value}'s data.
470 The @var{foreign-value} must be of some foreign pointer, array, or
475 @node Foreign Dynamic Allocation
476 @comment node-name, next, previous, up
477 @subsection Foreign Dynamic Allocation
479 Lisp code can call the C standard library functions @code{malloc} and
480 @code{free} to dynamically allocate and deallocate foreign
481 variables. The Lisp code shares the same allocator with foreign C
482 code, so it's OK for foreign code to call @code{free} on the result of
483 Lisp @code{sb-alien:make-alien}, or for Lisp code to call
484 @code{sb-alien:free-alien} on foreign objects allocated by C
487 @defmac @sbalien{make-alien} @var{type} @var{size}
489 The @code{sb-alien:make-alien} macro
490 returns a dynamically allocated foreign value of the specified
491 @var{type} (which is not evaluated.) The allocated memory is not
492 initialized, and may contain arbitrary junk. If supplied,
493 @var{size} is an expression to evaluate to compute the size of the
494 allocated object. There are two major cases:
498 When @var{type} is a foreign array type, an array of that type is
499 allocated and a pointer to it is returned. Note that you must use
500 @code{deref} to change the result to an array before you can use
501 @code{deref} to read or write elements:
504 (cl:in-package "CL-USER") ; which USEs package "SB-ALIEN"
505 (defvar *foo* (make-alien (array char 10)))
506 (type-of *foo*) @result{} (alien (* (array (signed 8) 10)))
507 (setf (deref (deref foo) 0) 10) @result{} 10
510 If supplied, @var{size} is used as the first dimension for the
514 When @var{type} is any other foreign type, then an object for that
515 type is allocated, and a pointer to it is returned. So
516 @code{(make-alien int)} returns a @code{(* int)}. If @var{size} is
517 specified, then a block of that many objects is allocated, with the
518 result pointing to the first one.
524 @defun @sbalien{free-alien} @var{foreign-value}
526 The @code{sb-alien:free-alien} macro
527 frees the storage for @var{foreign-value},
528 which must have been allocated with Lisp @code{make-alien}
531 See also the @code{sb-alien:with-alien} macro, which allocates foreign
535 @node Foreign Variables
536 @comment node-name, next, previous, up
537 @section Foreign Variables
538 @c AKA "Alien Variables" in the CMU CL manual
540 Both local (stack allocated) and external (C global) foreign variables
544 * Local Foreign Variables::
545 * External Foreign Variables::
548 @node Local Foreign Variables
549 @comment node-name, next, previous, up
550 @subsection Local Foreign Variables
552 @defmac @sbalien{with-alien} @var{var-definitions} &body @var{body}
554 The @code{with-alien} macro establishes local foreign variables with
555 the specified alien types and names. This form is analogous to
556 defining a local variable in C: additional storage is allocated, and
557 the initial value is copied. This form is less analogous to
558 @code{LET}-allocated Lisp variables, since the variables can't be
559 captured in closures: they live only for the dynamic extent of the
560 body, and referring to them outside is a gruesome error.
562 The @var{var-definitions} argument is a list of
563 variable definitions, each of the form
565 (@var{name} @var{type} &optional @var{initial-value})
568 The names of the variables are established as symbol-macros; the
569 bindings have lexical scope, and may be assigned with @code{setq} or
572 The @code{with-alien} macro also establishes a new scope for named
573 structures and unions. Any @var{type} specified for a variable may
574 contain named structure or union types with the slots specified.
575 Within the lexical scope of the binding specifiers and body, a locally
576 defined foreign structure type @var{foo} can be referenced by its name
577 using @code{(struct @var{foo})}.
580 @node External Foreign Variables
581 @comment node-name, next, previous, up
582 @subsection External Foreign Variables
584 External foreign names are strings, and Lisp names are symbols. When
585 an external foreign value is represented using a Lisp variable, there
586 must be a way to convert from one name syntax into the other. The
587 macros @code{extern-alien}, @code{define-alien-variable} and
588 @code{define-alien-routine} use this conversion heuristic:
593 Alien names are converted to Lisp names by uppercasing and replacing
594 underscores with hyphens.
597 Conversely, Lisp names are converted to alien names by lowercasing and
598 replacing hyphens with underscores.
601 Both the Lisp symbol and alien string names may be separately
602 specified by using a list of the form
605 (alien-string lisp-symbol)
610 @defmac @sbalien{define-alien-variable} @var{name} @var{type}
612 The @code{define-alien-variable} macro defines @var{name} as an
613 external foreign variable of the specified foreign @code{type}.
614 @var{name} and @code{type} are not evaluated. The Lisp name of the
615 variable (see above) becomes a global alien variable. Global alien
616 variables are effectively ``global symbol macros''; a reference to the
617 variable fetches the contents of the external variable. Similarly,
618 setting the variable stores new contents -- the new contents must be
619 of the declared @code{type}. Someday, they may well be implemented
620 using the @acronym{ANSI} @code{define-symbol-macro} mechanism, but as
621 of SBCL 0.7.5, they are still implemented using an older more-or-less
622 parallel mechanism inherited from CMUCL.
624 For example, to access a C-level counter @var{foo}, one could write
627 (define-alien-variable "foo" int)
628 ;; Now it is possible to get the value of the C variable foo simply by
629 ;; referencing that Lisp variable:
636 @defun @sbalien{get-errno}
638 Since in modern C libraries, the @code{errno} ``variable'' is typically
639 no longer a variable, but some bizarre artificial construct
640 which behaves superficially like a variable within a given thread,
641 it can no longer reliably be accessed through the ordinary
642 @code{define-alien-variable} mechanism. Instead, SBCL provides
643 the operator @code{sb-alien:get-errno} to allow Lisp code to read it.
646 @defmac @sbalien{extern-alien} @var{name} @var{type}
648 The @code{extern-alien} macro returns an alien with the specified
649 @var{type} which points to an externally defined value. @var{name} is
650 not evaluated, and may be either a string or a symbol. @var{type} is
651 an unevaluated alien type specifier.
654 @node Foreign Data Structure Examples
655 @comment node-name, next, previous, up
656 @section Foreign Data Structure Examples
657 @c AKA "Alien Data Structure Example" in the CMU CL manual
659 Now that we have alien types, operations and variables, we can
660 manipulate foreign data structures. This C declaration
669 can be translated into the following alien type:
672 (define-alien-type nil
675 (b (array (* (struct foo)) 100))))
678 Once the @code{foo} alien type has been defined as above, the C
686 can be translated in this way:
689 (with-alien ((f (struct foo)))
690 (slot (deref (slot f 'b) 7) 'a)
692 ;; Do something with f...
696 Or consider this example of an external C variable and some accesses:
705 extern struct c_struct *my_struct;
708 my_struct = my_struct->n;
711 which can be manipulated in Lisp like this:
714 (define-alien-type nil
722 (define-alien-variable "my_struct" (* c-struct))
723 (incf (slot my-struct 'x))
724 (setf (slot my-struct 'a) 5)
725 (setq my-struct (slot my-struct 'n))
728 @node Loading Shared Object Files
729 @comment node-name, next, previous, up
730 @section Loading Shared Object Files
732 Foreign object files can be loaded into the running Lisp process by
733 calling @code{load-shared-object}.
735 @include fun-sb-alien-load-shared-object.texinfo
737 @include fun-sb-alien-unload-shared-object.texinfo
739 @node Foreign Function Calls
740 @comment node-name, next, previous, up
741 @section Foreign Function Calls
743 The foreign function call interface allows a Lisp program to call
744 many functions written in languages that use the C calling convention.
746 Lisp sets up various signal handling routines and other environment
747 information when it first starts up, and expects these to be in place
748 at all times. The C functions called by Lisp should not change the
749 environment, especially the signal handlers: the signal handlers
750 installed by Lisp typically have interesting flags set (e.g to request
751 machine context information, or for signal delivery on an alternate
752 stack) which the Lisp runtime relies on for correct operation.
753 Precise details of how this works may change without notice between
754 versions; the source, or the brain of a friendly SBCL developer, is
755 the only documentation. Users of a Lisp built with the
756 @code{:sb-thread} feature should also read the section about threads,
760 * The alien-funcall Primitive::
761 * The define-alien-routine Macro::
762 * define-alien-routine Example::
763 * Calling Lisp From C::
766 @node The alien-funcall Primitive
767 @comment node-name, next, previous, up
768 @subsection The @code{alien-funcall} Primitive
770 @defun @sbalien{alien-funcall} @var{alien-function} &rest @var{arguments}
772 The @code{alien-funcall} function is the foreign function call
773 primitive: @var{alien-function} is called with the supplied
774 @var{arguments} and its C return value is returned as a Lisp value.
775 The @var{alien-function} is an arbitrary run-time expression; to refer
776 to a constant function, use @code{extern-alien} or a value defined by
777 @code{define-alien-routine}.
779 The type of @code{alien-function} must be @code{(alien (function
780 ...))} or @code{(alien (* (function ...)))}. The function type is
781 used to determine how to call the function (as though it was declared
782 with a prototype.) The type need not be known at compile time, but
783 only known-type calls are efficiently compiled. Limitations:
788 Structure type return values are not implemented.
791 Passing of structures by value is not implemented.
797 Here is an example which allocates a @code{(struct foo)}, calls a
798 foreign function to initialize it, then returns a Lisp vector of all
799 the @code{(* (struct foo))} objects filled in by the foreign call:
802 ;; Allocate a foo on the stack.
803 (with-alien ((f (struct foo)))
804 ;; Call some C function to fill in foo fields.
805 (alien-funcall (extern-alien "mangle_foo" (function void (* foo)))
807 ;; Find how many foos to use by getting the A field.
808 (let* ((num (slot f 'a))
809 (result (make-array num)))
810 ;; Get a pointer to the array so that we don't have to keep extracting it:
811 (with-alien ((a (* (array (* (struct foo)) 100)) (addr (slot f 'b))))
812 ;; Loop over the first N elements and stash them in the result vector.
814 (setf (svref result i) (deref (deref a) i)))
819 @node The define-alien-routine Macro
820 @comment node-name, next, previous, up
821 @subsection The @code{define-alien-routine} Macro
823 @defmac @sbalien{define-alien-routine} @var{name} @var{result-type} &rest @var{arg-specifiers}
825 The @code{define-alien-routine} macro is a convenience for
826 automatically generating Lisp interfaces to simple foreign functions.
827 The primary feature is the parameter style specification, which
828 translates the C pass-by-reference idiom into additional return
831 @var{name} is usually a string external symbol, but may also be a
832 symbol Lisp name or a list of the foreign name and the Lisp name. If
833 only one name is specified, the other is automatically derived as for
834 @code{extern-alien}. @var{result-type} is the alien type of the
837 Each element of the @var{arg-specifiers} list
838 specifies an argument to the foreign function, and is
841 (aname atype &optional style)
844 @var{aname} is the symbol name of the argument to the constructed
845 function (for documentation). @var{atype} is the alien type of
846 corresponding foreign argument. The semantics of the actual call are
847 the same as for @code{alien-funcall}. @var{style} specifies how this
848 argument should be handled at call and return time, and should be one
854 @code{:in} specifies that the argument is passed by value. This is the
855 default. @code{:in} arguments have no corresponding return value from
859 @code{:copy} is similar to @code{:in}, but the argument is copied to a
860 pre-allocated object and a pointer to this object is passed to the
864 @code{:out} specifies a pass-by-reference output value. The type of
865 the argument must be a pointer to a fixed-sized object (such as an
866 integer or pointer). @code{:out} and @code{:in-out} style cannot be
867 used with pointers to arrays, records or functions. An object of the
868 correct size is allocated on the stack, and its address is passed to
869 the foreign function. When the function returns, the contents of this
870 location are returned as one of the values of the Lisp function (and
871 the location is automatically deallocated).
874 @code{:in-out} is a combination of @code{:copy} and @code{:out}. The
875 argument is copied to a pre-allocated object and a pointer to this
876 object is passed to the foreign routine. On return, the contents of
877 this location is returned as an additional value.
882 Note: Any efficiency-critical foreign interface function should be inline
883 expanded, which can be done by preceding the
884 @code{define-alien-routine} call with:
887 (declaim (inline lisp-name))
890 In addition to avoiding the Lisp call overhead, this allows
891 pointers, word-integers and floats to be passed using non-descriptor
892 representations, avoiding consing.)
897 @node define-alien-routine Example
898 @comment node-name, next, previous, up
899 @subsection @code{define-alien-routine} Example
901 Consider the C function @code{cfoo} with the following calling
908 char *a; /* update */
911 /* body of cfoo(...) */
915 This can be described by the following call to
916 @code{define-alien-routine}:
919 (define-alien-routine "cfoo" void
925 The Lisp function @code{cfoo} will have two arguments (@var{str} and
926 @var{a}) and two return values (@var{a} and @var{i}).
928 @node Calling Lisp From C
929 @comment node-name, next, previous, up
930 @subsection Calling Lisp From C
932 Calling Lisp functions from C is sometimes possible, but is extremely
933 hackish and poorly supported as of SBCL 0.7.5. See @code{funcall0}
934 @dots{} @code{funcall3} in the runtime system. The arguments must be
935 valid SBCL object descriptors (so that e.g. fixnums must be
936 left-shifted by 2.) As of SBCL 0.7.5, the format of object descriptors
937 is documented only by the source code and, in parts, by the old CMUCL
938 @file{INTERNALS} documentation.
940 Note that the garbage collector moves objects, and won't be
941 able to fix up any references in C variables. There are three
942 mechanisms for coping with this:
946 The @code{sb-ext:purify} moves all live Lisp
947 data into static or read-only areas such that it will never be moved
948 (or freed) again in the life of the Lisp session
951 @code{sb-sys:with-pinned-objects} is a macro which arranges for some
952 set of objects to be pinned in memory for the dynamic extent of its
953 body forms. On ports which use the generational garbage collector (as
954 of SBCL 0.8.3, only the x86) this has a page granularity - i.e. the
955 entire 4k page or pages containing the objects will be locked down. On
956 other ports it is implemented by turning off GC for the duration (so
957 could be said to have a whole-world granularity).
960 Disable GC, using the @code{without-gcing} macro.
963 @c <!-- FIXME: This is a "changebar" section from the CMU CL manual.
964 @c I (WHN 2002-07-14) am not very familiar with this content, so
965 @c I'm not immediately prepared to try to update it for SBCL, and
966 @c I'm not feeling masochistic enough to work to encourage this
967 @c kind of low-level hack anyway. However, I acknowledge that callbacks
968 @c are sometimes really really necessary, so I include the original
969 @c text in case someone is hard-core enough to benefit from it. If
970 @c anyone brings the information up to date for SBCL, it belong
971 @c either in the main manual or on a CLiki SBCL Internals page.
972 @c LaTeX \subsection{Accessing Lisp Arrays}
974 @c LaTeX Due to the way \cmucl{} manages memory, the amount of memory that can
975 @c LaTeX be dynamically allocated by \code{malloc} or \funref{make-alien} is
976 @c LaTeX limited\footnote{\cmucl{} mmaps a large piece of memory for it's own
977 @c LaTeX use and this memory is typically about 8 MB above the start of the C
978 @c LaTeX heap. Thus, only about 8 MB of memory can be dynamically
979 @c LaTeX allocated.}.
981 @c Empirically determined to be considerably >8Mb on this x86 linux
982 @c machine, but I don't know what the actual values are - dan 2003.09.01
984 @c Note that this technique is used in SB-GROVEL in the SBCL contrib
987 @c LaTeX To overcome this limitation, it is possible to access the content of
988 @c LaTeX Lisp arrays which are limited only by the amount of physical memory
989 @c LaTeX and swap space available. However, this technique is only useful if
990 @c LaTeX the foreign function takes pointers to memory instead of allocating
991 @c LaTeX memory for itself. In latter case, you will have to modify the
992 @c LaTeX foreign functions.
994 @c LaTeX This technique takes advantage of the fact that \cmucl{} has
995 @c LaTeX specialized array types (\pxlref{specialized-array-types}) that match
996 @c LaTeX a typical C array. For example, a \code{(simple-array double-float
997 @c LaTeX (100))} is stored in memory in essentially the same way as the C
998 @c LaTeX array \code{double x[100]} would be. The following function allows us
999 @c LaTeX to get the physical address of such a Lisp array:
1000 @c LaTeX \begin{example}
1001 @c LaTeX (defun array-data-address (array)
1002 @c LaTeX "Return the physical address of where the actual data of an array is
1005 @c LaTeX ARRAY must be a specialized array type in CMU Lisp. This means ARRAY
1006 @c LaTeX must be an array of one of the following types:
1008 @c LaTeX double-float
1009 @c LaTeX single-float
1010 @c LaTeX (unsigned-byte 32)
1011 @c LaTeX (unsigned-byte 16)
1012 @c LaTeX (unsigned-byte 8)
1013 @c LaTeX (signed-byte 32)
1014 @c LaTeX (signed-byte 16)
1015 @c LaTeX (signed-byte 8)
1017 @c LaTeX (declare (type (or #+signed-array (array (signed-byte 8))
1018 @c LaTeX #+signed-array (array (signed-byte 16))
1019 @c LaTeX #+signed-array (array (signed-byte 32))
1020 @c LaTeX (array (unsigned-byte 8))
1021 @c LaTeX (array (unsigned-byte 16))
1022 @c LaTeX (array (unsigned-byte 32))
1023 @c LaTeX (array single-float)
1024 @c LaTeX (array double-float))
1026 @c LaTeX (optimize (speed 3) (safety 0))
1027 @c LaTeX (ext:optimize-interface (safety 3)))
1028 @c LaTeX ;; with-array-data will get us to the actual data. However, because
1029 @c LaTeX ;; the array could have been displaced, we need to know where the
1030 @c LaTeX ;; data starts.
1031 @c LaTeX (lisp::with-array-data ((data array)
1034 @c LaTeX (declare (ignore end))
1035 @c LaTeX ;; DATA is a specialized simple-array. Memory is laid out like this:
1037 @c LaTeX ;; byte offset Value
1038 @c LaTeX ;; 0 type code (should be 70 for double-float vector)
1039 @c LaTeX ;; 4 4 * number of elements in vector
1040 @c LaTeX ;; 8 1st element of vector
1043 @c LaTeX (let ((addr (+ 8 (logandc1 7 (kernel:get-lisp-obj-address data))))
1044 @c LaTeX (type-size (let ((type (array-element-type data)))
1045 @c LaTeX (cond ((or (equal type '(signed-byte 8))
1046 @c LaTeX (equal type '(unsigned-byte 8)))
1048 @c LaTeX ((or (equal type '(signed-byte 16))
1049 @c LaTeX (equal type '(unsigned-byte 16)))
1051 @c LaTeX ((or (equal type '(signed-byte 32))
1052 @c LaTeX (equal type '(unsigned-byte 32)))
1054 @c LaTeX ((equal type 'single-float)
1056 @c LaTeX ((equal type 'double-float)
1059 @c LaTeX (error "Unknown specialized array element type"))))))
1060 @c LaTeX (declare (type (unsigned-byte 32) addr)
1061 @c LaTeX (optimize (speed 3) (safety 0) (ext:inhibit-warnings 3)))
1062 @c LaTeX (system:int-sap (the (unsigned-byte 32)
1063 @c LaTeX (+ addr (* type-size start)))))))
1064 @c LaTeX \end{example}
1066 @c LaTeX Assume we have the C function below that we wish to use:
1067 @c LaTeX \begin{example}
1068 @c LaTeX double dotprod(double* x, double* y, int n)
1071 @c LaTeX double sum = 0;
1073 @c LaTeX for (k = 0; k < n; ++k) \{
1074 @c LaTeX sum += x[k] * y[k];
1077 @c LaTeX \end{example}
1078 @c LaTeX The following example generates two large arrays in Lisp, and calls the C
1079 @c LaTeX function to do the desired computation. This would not have been
1080 @c LaTeX possible using \code{malloc} or \code{make-alien} since we need about
1081 @c LaTeX 16 MB of memory to hold the two arrays.
1082 @c LaTeX \begin{example}
1083 @c LaTeX (define-alien-routine "dotprod" double
1084 @c LaTeX (x (* double-float) :in)
1085 @c LaTeX (y (* double-float) :in)
1086 @c LaTeX (n int :in))
1088 @c LaTeX (let ((x (make-array 1000000 :element-type 'double-float))
1089 @c LaTeX (y (make-array 1000000 :element-type 'double-float)))
1090 @c LaTeX ;; Initialize X and Y somehow
1091 @c LaTeX (let ((x-addr (system:int-sap (array-data-address x)))
1092 @c LaTeX (y-addr (system:int-sap (array-data-address y))))
1093 @c LaTeX (dotprod x-addr y-addr 1000000)))
1094 @c LaTeX \end{example}
1095 @c LaTeX In this example, it may be useful to wrap the inner \code{let}
1096 @c LaTeX expression in an \code{unwind-protect} that first turns off garbage
1097 @c LaTeX collection and then turns garbage collection on afterwards. This will
1098 @c LaTeX prevent garbage collection from moving \code{x} and \code{y} after we
1099 @c LaTeX have obtained the (now erroneous) addresses but before the call to
1100 @c LaTeX \code{dotprod} is made.
1105 @node Step-By-Step Example of the Foreign Function Interface
1106 @comment node-name, next, previous, up
1107 @section Step-By-Step Example of the Foreign Function Interface
1109 This section presents a complete example of an interface to a somewhat
1110 complicated C function.
1112 Suppose you have the following C function which you want to be able to
1113 call from Lisp in the file @file{test.c}
1122 struct c_struct *c_function (i, s, r, a)
1129 struct c_struct *r2;
1131 printf("i = %d\n", i);
1132 printf("s = %s\n", s);
1133 printf("r->x = %d\n", r->x);
1134 printf("r->s = %s\n", r->s);
1135 for (j = 0; j < 10; j++) printf("a[%d] = %d.\n", j, a[j]);
1136 r2 = (struct c_struct *) malloc (sizeof(struct c_struct));
1138 r2->s = "a C string";
1143 It is possible to call this C function from Lisp using the file
1144 @file{test.lisp} containing
1147 (cl:defpackage "TEST-C-CALL" (:use "CL" "SB-ALIEN" "SB-C-CALL"))
1148 (cl:in-package "TEST-C-CALL")
1150 ;;; Define the record C-STRUCT in Lisp.
1151 (define-alien-type nil
1156 ;;; Define the Lisp function interface to the C routine. It returns a
1157 ;;; pointer to a record of type C-STRUCT. It accepts four parameters:
1158 ;;; I, an int; S, a pointer to a string; R, a pointer to a C-STRUCT
1159 ;;; record; and A, a pointer to the array of 10 ints.
1161 ;;; The INLINE declaration eliminates some efficiency notes about heap
1162 ;;; allocation of alien values.
1163 (declaim (inline c-function))
1164 (define-alien-routine c-function
1165 (* (struct c-struct))
1168 (r (* (struct c-struct)))
1171 ;;; a function which sets up the parameters to the C function and
1172 ;;; actually calls it
1174 (with-alien ((ar (array int 10))
1175 (c-struct (struct c-struct)))
1176 (dotimes (i 10) ; Fill array.
1177 (setf (deref ar i) i))
1178 (setf (slot c-struct 'x) 20)
1179 (setf (slot c-struct 's) "a Lisp string")
1181 (with-alien ((res (* (struct c-struct))
1182 (c-function 5 "another Lisp string" (addr c-struct) ar)))
1183 (format t "~&back from C function~%")
1184 (multiple-value-prog1
1185 (values (slot res 'x)
1188 ;; Deallocate result. (after we are done referring to it:
1189 ;; "Pillage, *then* burn.")
1190 (free-alien res)))))
1193 To execute the above example, it is necessary to compile the C
1194 routine, e.g.: @samp{cc -c test.c && ld -shared -o test.so test.o} (In
1195 order to enable incremental loading with some linkers, you may need to
1196 say @samp{cc -G 0 -c test.c})
1198 Once the C code has been compiled, you can start up Lisp and load it in:
1199 @samp{sbcl}. Lisp should start up with its normal prompt.
1201 Within Lisp, compile the Lisp file. (This step can be done
1202 separately. You don't have to recompile every time.)
1203 @samp{(compile-file "test.lisp")}
1205 Within Lisp, load the foreign object file to define the necessary
1206 symbols: @samp{(load-shared-object "test.so")}.
1208 Now you can load the compiled Lisp (``fasl'') file into Lisp:
1209 @samp{(load "test.fasl")}
1210 And once the Lisp file is loaded, you can call the
1211 Lisp routine that sets up the parameters and calls the C
1213 @samp{(test-c-call::call-cfun)}
1215 The C routine should print the following information to standard output:
1219 s = another Lisp string
1221 r->s = a Lisp string
1234 After return from the C function,
1235 the Lisp wrapper function should print the following output:
1238 back from C function
1241 And upon return from the Lisp wrapper function,
1242 before the next prompt is printed, the
1243 Lisp read-eval-print loop should print the following return values: