@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
-@c Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003, 2004, 2007, 2008, 2009, 2010
-@c Free Software Foundation, Inc.
+@c Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003, 2004, 2007, 2008,
+@c 2009, 2010, 2011 Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.
-@page
@node Foreign Function Interface
@section Foreign Function Interface
@cindex foreign function interface
implemented in C. So it is that Guile's living half pays respect to its
dead counterpart, via a spectrum of interfaces to C ranging from dynamic
loading of Scheme primitives to dynamic binding of stock C library
-prodedures.
+procedures.
@menu
* Foreign Libraries:: Dynamically linking to libraries.
* Foreign Functions:: Simple calls to C procedures.
* C Extensions:: Extending Guile in C with loadable modules.
* Modules and Extensions:: Loading C extensions into modules.
-* Foreign Values:: Accessing global variables.
-* Dynamic FFI:: Fu.
+* Foreign Pointers:: Accessing global variables.
+* Dynamic FFI:: Calling arbitrary C functions.
@end menu
that will be searched for in the places where shared libraries usually
reside, such as in @file{/usr/lib} and @file{/usr/local/lib}.
+@var{library} should not contain an extension such as @code{.so}. The
+correct file name extension for the host operating system is provided
+automatically, according to libltdl's rules (@pxref{Libltdl interface,
+lt_dlopenext, @code{lt_dlopenext}, libtool, Shared Library Support for
+GNU}).
+
When @var{library} is omitted, a @dfn{global symbol handle} is returned. This
handle provides access to the symbols available to the program at run-time,
including those exported by the program itself and the shared libraries already
@end deffn
@smallexample
-(define libc-obj (dynamic-link "libc.so"))
-libc-obj
-@result{} #<dynamic-object "libc.so">
-(dynamic-args-call 'rand libc-obj '())
-@result{} 269167349
-(dynamic-unlink libc-obj)
-libc-obj
-@result{} #<dynamic-object "libc.so" (unlinked)>
+(define libgl-obj (dynamic-link "libGL"))
+libgl-obj
+@result{} #<dynamic-object "libGL">
+(dynamic-unlink libGL-obj)
+libGL-obj
+@result{} #<dynamic-object "libGL" (unlinked)>
@end smallexample
As you can see, after calling @code{dynamic-unlink} on a dynamically
linked library, it is marked as @samp{(unlinked)} and you are no longer
able to use it with @code{dynamic-call}, etc. Whether the library is
really removed from you program is system-dependent and will generally
-not happen when some other parts of your program still use it. In the
-example above, @code{libc} is almost certainly not removed from your
-program because it is badly needed by almost everything.
+not happen when some other parts of your program still use it.
When dynamic linking is disabled or not supported on your system,
the above functions throw errors, but they are still available.
will dynamically link @file{libX11} with @code{dynamic-link} and then
construct a beautiful graphical user interface just by using
@code{dynamic-call}. Instead, the usual way would be to write a special
-Guile<->X11 glue library that has intimate knowledge about both Guile
+Guile-to-X11 glue library that has intimate knowledge about both Guile
and X11 and does whatever is necessary to make them inter-operate
smoothly. This glue library could then be dynamically linked into a
vanilla Guile interpreter and activated by calling its initialization
library, and register its init function right after Guile has been
initialized.
-LIB should be a string denoting a shared library without any file type
-suffix such as ".so". The suffix is provided automatically. It
+As for @code{dynamic-link}, @var{lib} should not contain any suffix such
+as @code{.so} (@pxref{Foreign Libraries, dynamic-link}). It
should also not contain any directory components. Libraries that
implement Guile Extensions should be put into the normal locations for
shared libraries. We recommend to use the naming convention
-libguile-bla-blum for a extension related to a module `(bla blum)'.
+@file{libguile-bla-blum} for a extension related to a module @code{(bla
+blum)}.
The normal way for a extension to be used is to write a small Scheme
file that defines a module, and to load the extension into this
That's it!
-@deffn {Scheme Procedure} load-extension lib init
-@deffnx {C Function} scm_load_extension (lib, init)
-Load and initialize the extension designated by LIB and INIT.
-When there is no pre-registered function for LIB/INIT, this is
-equivalent to
-
-@lisp
-(dynamic-call INIT (dynamic-link LIB))
-@end lisp
-
-When there is a pre-registered function, that function is called
-instead.
-
-Normally, there is no pre-registered function. This option exists
-only for situations where dynamic linking is unavailable or unwanted.
-In that case, you would statically link your program with the desired
-library, and register its init function right after Guile has been
-initialized.
-
-LIB should be a string denoting a shared library without any file type
-suffix such as ".so". The suffix is provided automatically. It
-should also not contain any directory components. Libraries that
-implement Guile Extensions should be put into the normal locations for
-shared libraries. We recommend to use the naming convention
-libguile-bla-blum for a extension related to a module `(bla blum)'.
-
-The normal way for a extension to be used is to write a small Scheme
-file that defines a module, and to load the extension into this
-module. When the module is auto-loaded, the extension is loaded as
-well. For example,
-
-@lisp
-(define-module (bla blum))
-
-(load-extension "libguile-bla-blum" "bla_init_blum")
-@end lisp
-@end deffn
@node Modules and Extensions
@subsection Modules and Extensions
(load-extension "foobar-c-code" "foo_bar_init")
@end example
+@cindex extensiondir
When loaded with @code{(use-modules (foo bar))}, the
@code{load-extension} call looks for the @file{foobar-c-code.so} (etc)
-object file in the standard system locations, such as @file{/usr/lib}
-or @file{/usr/local/lib}.
+object file in Guile's @code{extensiondir}, which is usually a
+subdirectory of the @code{libdir}. For example, if your libdir is
+@file{/usr/lib}, the @code{extensiondir} for the Guile @value{EFFECTIVE-VERSION}.@var{x}
+series will be @file{/usr/lib/guile/@value{EFFECTIVE-VERSION}/}.
+
+The extension path includes the major and minor version of Guile (the
+``effective version''), because Guile guarantees compatibility within a
+given effective version. This allows you to install different versions
+of the same extension for different versions of Guile.
+
+If the extension is not found in the @code{extensiondir}, Guile will
+also search the standard system locations, such as @file{/usr/lib} or
+@file{/usr/local/lib}. It is preferable, however, to keep your extension
+out of the system library path, to prevent unintended interference with
+other dynamically-linked C libraries.
If someone installs your module to a non-standard location then the
object file won't be found. You can address this by inserting the
@example
(define-module (foo bar))
-(load-extension "XXlibdirXX/foobar-c-code" "foo_bar_init")
+(load-extension "XXextensiondirXX/foobar-c-code" "foo_bar_init")
@end example
@noindent
@example
foo.scm: foo.scm.in
- sed 's|XXlibdirXX|$(libdir)|' <foo.scm.in >foo.scm
+ sed 's|XXextensiondirXX|$(libdir)/guile/@value{EFFECTIVE-VERSION}|' <foo.scm.in >foo.scm
@end example
-The actual pattern @code{XXlibdirXX} is arbitrary, it's only something
+The actual pattern @code{XXextensiondirXX} is arbitrary, it's only something
which doesn't otherwise occur. If several modules need the value, it
can be easier to create one @file{foo/config.scm} with a define of the
-@code{libdir} location, and use that as required.
+@code{extensiondir} location, and use that as required.
@example
(define-module (foo config))
-(define-public foo-config-libdir "XXlibdirXX"")
+(define-public foo-config-extensiondir "XXextensiondirXX"")
@end example
Such a file might have other locations too, for instance a data
its own @code{gettext} message catalogue
(@pxref{Internationalization}).
-When installing multiple C code objects, it can be convenient to put
-them in a subdirectory of @code{libdir}, thus giving for example
-@code{/usr/lib/foo/some-obj.so}. If the objects are only meant to be
-used through the module, then a subdirectory keeps them out of sight.
-
It will be noted all of the above requires that the Scheme code to be
found in @code{%load-path} (@pxref{Build Config}). Presently it's
left up to the system administrator or each user to augment that path
reached the Scheme code, that code should take care of hitting any of
its own private files etc.
-Presently there's no convention for having a Guile version number in
-module C code filenames or directories. This is primarily because
-there's no established principles for two versions of Guile to be
-installed under the same prefix (eg. two both under @file{/usr}).
-Assuming upward compatibility is maintained then this should be
-unnecessary, and if compatibility is not maintained then it's highly
-likely a package will need to be revisited anyway.
-The present suggestion is that modules should assume when they're
-installed under a particular @code{prefix} that there's a single
-version of Guile there, and the @code{guile-config} at build time has
-the necessary information about it. C code or Scheme code might adapt
-itself accordingly (allowing for features not available in an older
-version for instance).
+@node Foreign Pointers
+@subsection Foreign Pointers
+
+The previous sections have shown how Guile can be extended at runtime by
+loading compiled C extensions. This approach is all well and good, but
+wouldn't it be nice if we didn't have to write any C at all? This
+section takes up the problem of accessing C values from Scheme, and the
+next discusses C functions.
+
+@menu
+* Foreign Types:: Expressing C types in Scheme.
+* Foreign Variables:: Pointers to C symbols.
+* Void Pointers and Byte Access:: Pointers into the ether.
+* Foreign Structs:: Packing and unpacking structs.
+@end menu
+
+@node Foreign Types
+@subsubsection Foreign Types
+
+The first impedance mismatch that one sees between C and Scheme is that
+in C, the storage locations (variables) are typed, but in Scheme types
+are associated with values, not variables. @xref{Values and Variables}.
+So when describing a C function or a C structure so that it can be
+accessed from Scheme, the data types of the parameters or fields must be
+passed explicitly.
-@node Foreign Values
-@subsection Foreign Values
+These ``C type values'' may be constructed using the constants and
+procedures from the @code{(system foreign)} module, which may be loaded
+like this:
-@deffn {Scheme Procedure} dynamic-pointer name type dobj [len]
-@deffnx {C Function} scm_dynamic_pointer (name, type, dobj, len)
-Return a ``handle'' for the pointer @var{name} in the shared object referred to
-by @var{dobj}. The handle aliases a C value, and is declared to be of type
-@var{type}. Valid types are defined in the @code{(system foreign)} module.
+@example
+(use-modules (system foreign))
+@end example
-This facility works by asking the dynamic linker for the address of a symbol,
-then assuming that it aliases a value of a given type. Obviously, the user must
-be very careful to ensure that the value actually is of the declared type, or
-bad things will happen.
+@code{(system foreign)} exports a number of values expressing the basic
+C types:
+
+@defvr {Scheme Variable} int8
+@defvrx {Scheme Variable} uint8
+@defvrx {Scheme Variable} uint16
+@defvrx {Scheme Variable} int16
+@defvrx {Scheme Variable} uint32
+@defvrx {Scheme Variable} int32
+@defvrx {Scheme Variable} uint64
+@defvrx {Scheme Variable} int64
+@defvrx {Scheme Variable} float
+@defvrx {Scheme Variable} double
+These values represent the C numeric types of the specified sizes and
+signednesses.
+@end defvr
+
+In addition there are some convenience bindings for indicating types of
+platform-dependent size:
+
+@defvr {Scheme Variable} int
+@defvrx {Scheme Variable} unsigned-int
+@defvrx {Scheme Variable} long
+@defvrx {Scheme Variable} unsigned-long
+@defvrx {Scheme Variable} size_t
+Values exported by the @code{(system foreign)} module, representing C
+numeric types. For example, @code{long} may be @code{equal?} to
+@code{int64} on a 64-bit platform.
+@end defvr
+
+@defvr {Scheme Variable} void
+The @code{void} type. It can be used as the first argument to
+@code{pointer->procedure} to wrap a C function that returns nothing.
+@end defvr
+
+In addition, the symbol @code{*} is used by convention to denote pointer
+types. Procedures detailed in the following sections, such as
+@code{pointer->procedure}, accept it as a type descriptor.
+
+@node Foreign Variables
+@subsubsection Foreign Variables
+
+Pointers to variables in the current address space may be looked up
+dynamically using @code{dynamic-pointer}.
+
+@deffn {Scheme Procedure} dynamic-pointer name dobj
+@deffnx {C Function} scm_dynamic_pointer (name, dobj)
+Return a ``wrapped pointer'' for the symbol @var{name} in the shared
+object referred to by @var{dobj}. The returned pointer points to a C
+object.
Regardless whether your C compiler prepends an underscore @samp{_} to the global
names in a program, you should @strong{not} include this underscore in
@var{name} since it will be added automatically when necessary.
@end deffn
+For example, currently Guile has a variable, @code{scm_numptob}, as part
+of its API. It is declared as a C @code{long}. So, to create a handle
+pointing to that foreign value, we do:
+
+@example
+(use-modules (system foreign))
+(define numptob (dynamic-pointer "scm_numptob" (dynamic-link)))
+numptob
+@result{} #<pointer 0x7fb35b1b4688>
+@end example
+
+(The next section discusses ways to dereference pointers.)
+
+A value returned by @code{dynamic-pointer} is a Scheme wrapper for a C
+pointer.
+
+@deffn {Scheme Procedure} pointer-address pointer
+@deffnx {C Function} scm_pointer_address pointer
+Return the numerical value of @var{pointer}.
+
+@example
+(pointer-address numptob)
+@result{} 139984413364296 ; YMMV
+@end example
+@end deffn
+
+@deffn {Scheme Procedure} make-pointer address [finalizer]
+Return a foreign pointer object pointing to @var{address}. If
+@var{finalizer} is passed, it should be a pointer to a one-argument C
+function that will be called when the pointer object becomes
+unreachable.
+@end deffn
+
+@deffn {Scheme Procedure} pointer? obj
+Return @code{#t} if @var{obj} is a pointer object, @code{#f} otherwise.
+@end deffn
+
+@defvr {Scheme Variable} %null-pointer
+A foreign pointer whose value is 0.
+@end defvr
+
+@deffn {Scheme Procedure} null-pointer? pointer
+Return @code{#t} if @var{pointer} is the null pointer, @code{#f} otherwise.
+@end deffn
+
+For the purpose of passing SCM values directly to foreign functions, and
+allowing them to return SCM values, Guile also supports some unsafe
+casting operators.
+
+@deffn {Scheme Procedure} scm->pointer scm
+Return a foreign pointer object with the @code{object-address}
+of @var{scm}.
+@end deffn
+
+@deffn {Scheme Procedure} pointer->scm pointer
+Unsafely cast @var{pointer} to a Scheme object.
+Cross your fingers!
+@end deffn
+
+
+@node Void Pointers and Byte Access
+@subsubsection Void Pointers and Byte Access
+
+Wrapped pointers are untyped, so they are essentially equivalent to C
+@code{void} pointers. As in C, the memory region pointed to by a
+pointer can be accessed at the byte level. This is achieved using
+@emph{bytevectors} (@pxref{Bytevectors}). The @code{(rnrs bytevector)}
+module contains procedures that can be used to convert byte sequences to
+Scheme objects such as strings, floating point numbers, or integers.
+
+@deffn {Scheme Procedure} pointer->bytevector pointer len [offset [uvec_type]]
+@deffnx {C Function} scm_foreign_to_bytevector pointer len offset uvec_type
+Return a bytevector aliasing the @var{len} bytes pointed to by
+@var{pointer}.
+
+The user may specify an alternate default interpretation for
+the memory by passing the @var{uvec_type} argument, to indicate
+that the memory is an array of elements of that type.
+@var{uvec_type} should be something that
+@code{uniform-vector-element-type} would return, like @code{f32}
+or @code{s16}.
+
+When @var{offset} is passed, it specifies the offset in bytes relative
+to @var{pointer} of the memory region aliased by the returned
+bytevector.
+
+Mutating the returned bytevector mutates the memory pointed to by
+@var{pointer}, so buckle your seatbelts.
+@end deffn
+
+@deffn {Scheme Procedure} bytevector->pointer bv [offset]
+@deffnx {C Function} scm_bytevector_to_pointer bv offset
+Return a pointer pointer aliasing the memory pointed to by @var{bv} or
+@var{offset} bytes after @var{bv} when @var{offset} is passed.
+@end deffn
+
+In addition to these primitives, convenience procedures are available:
+
+@deffn {Scheme Procedure} dereference-pointer pointer
+Assuming @var{pointer} points to a memory region that holds a pointer,
+return this pointer.
+@end deffn
+
+@deffn {Scheme Procedure} string->pointer string [encoding]
+Return a foreign pointer to a nul-terminated copy of @var{string} in the
+given @var{encoding}, defaulting to the current locale encoding. The C
+string is freed when the returned foreign pointer becomes unreachable.
+
+This is the Scheme equivalent of @code{scm_to_stringn}.
+@end deffn
+
+@deffn {Scheme Procedure} pointer->string pointer [length] [encoding]
+Return the string representing the C string pointed to by @var{pointer}.
+If @var{length} is omitted or @code{-1}, the string is assumed to be
+nul-terminated. Otherwise @var{length} is the number of bytes in memory
+pointed to by @var{pointer}. The C string is assumed to be in the given
+@var{encoding}, defaulting to the current locale encoding.
+
+This is the Scheme equivalent of @code{scm_from_stringn}.
+@end deffn
+
+@cindex wrapped pointer types
+Most object-oriented C libraries use pointers to specific data
+structures to identify objects. It is useful in such cases to reify the
+different pointer types as disjoint Scheme types. The
+@code{define-wrapped-pointer-type} macro simplifies this.
+
+@deffn {Scheme Syntax} define-wrapped-pointer-type type-name pred wrap unwrap print
+Define helper procedures to wrap pointer objects into Scheme objects
+with a disjoint type. Specifically, this macro defines:
+
+@itemize
+@item @var{pred}, a predicate for the new Scheme type;
+@item @var{wrap}, a procedure that takes a pointer object and returns an
+object that satisfies @var{pred};
+@item @var{unwrap}, which does the reverse.
+@end itemize
+
+@var{wrap} preserves pointer identity, for two pointer objects @var{p1}
+and @var{p2} that are @code{equal?}, @code{(eq? (@var{wrap} @var{p1})
+(@var{wrap} @var{p2})) @result{} #t}.
+
+Finally, @var{print} should name a user-defined procedure to print such
+objects. The procedure is passed the wrapped object and a port to write
+to.
+
+For example, assume we are wrapping a C library that defines a type,
+@code{bottle_t}, and functions that can be passed @code{bottle_t *}
+pointers to manipulate them. We could write:
+
+@example
+(define-wrapped-pointer-type bottle
+ bottle?
+ wrap-bottle unwrap-bottle
+ (lambda (b p)
+ (format p "#<bottle of ~a ~x>"
+ (bottle-contents b)
+ (pointer-address (unwrap-foo b)))))
+
+(define grab-bottle
+ ;; Wrapper for `bottle_t *grab (void)'.
+ (let ((grab (pointer->procedure '*
+ (dynamic-func "grab_bottle" libbottle)
+ '())))
+ (lambda ()
+ "Return a new bottle."
+ (wrap-bottle (grab)))))
+
+(define bottle-contents
+ ;; Wrapper for `const char *bottle_contents (bottle_t *)'.
+ (let ((contents (pointer->procedure '*
+ (dynamic-func "bottle_contents"
+ libbottle)
+ '(*))))
+ (lambda (b)
+ "Return the contents of B."
+ (pointer->string (contents (unwrap-bottle b))))))
+
+(write (grab-bottle))
+@result{} #<bottle of Ch@^ateau Haut-Brion 803d36>
+@end example
+
+In this example, @code{grab-bottle} is guaranteed to return a genuine
+@code{bottle} object satisfying @code{bottle?}. Likewise,
+@code{bottle-contents} errors out when its argument is not a genuine
+@code{bottle} object.
+@end deffn
+
+Going back to the @code{scm_numptob} example above, here is how we can
+read its value as a C @code{long} integer:
+
+@example
+(use-modules (rnrs bytevectors))
+
+(bytevector-uint-ref (pointer->bytevector numptob (sizeof long))
+ 0 (native-endianness)
+ (sizeof long))
+@result{} 8
+@end example
+
+If we wanted to corrupt Guile's internal state, we could set
+@code{scm_numptob} to another value; but we shouldn't, because that
+variable is not meant to be set. Indeed this point applies more widely:
+the C API is a dangerous place to be. Not only might setting a value
+crash your program, simply accessing the data pointed to by a dangling
+pointer or similar can prove equally disastrous.
+
+@node Foreign Structs
+@subsubsection Foreign Structs
+
+Finally, one last note on foreign values before moving on to actually
+calling foreign functions. Sometimes you need to deal with C structs,
+which requires interpreting each element of the struct according to the
+its type, offset, and alignment. Guile has some primitives to support
+this.
+
+@deffn {Scheme Procedure} sizeof type
+@deffnx {C Function} scm_sizeof type
+Return the size of @var{type}, in bytes.
+
+@var{type} should be a valid C type, like @code{int}.
+Alternately @var{type} may be the symbol @code{*}, in which
+case the size of a pointer is returned. @var{type} may
+also be a list of types, in which case the size of a
+@code{struct} with ABI-conventional packing is returned.
+@end deffn
+
+@deffn {Scheme Procedure} alignof type
+@deffnx {C Function} scm_alignof type
+Return the alignment of @var{type}, in bytes.
+
+@var{type} should be a valid C type, like @code{int}.
+Alternately @var{type} may be the symbol @code{*}, in which
+case the alignment of a pointer is returned. @var{type} may
+also be a list of types, in which case the alignment of a
+@code{struct} with ABI-conventional packing is returned.
+@end deffn
+
+Guile also provides some convenience methods to pack and unpack foreign
+pointers wrapping C structs.
+
+@deffn {Scheme Procedure} make-c-struct types vals
+Create a foreign pointer to a C struct containing @var{vals} with types
+@code{types}.
+
+@var{vals} and @code{types} should be lists of the same length.
+@end deffn
+
+@deffn {Scheme Procedure} parse-c-struct foreign types
+Parse a foreign pointer to a C struct, returning a list of values.
+
+@code{types} should be a list of C types.
+@end deffn
+
+For example, to create and parse the equivalent of a @code{struct @{
+int64_t a; uint8_t b; @}}:
+
+@example
+(parse-c-struct (make-c-struct (list int64 uint8)
+ (list 300 43))
+ (list int64 uint8))
+@result{} (300 43)
+@end example
+
+As yet, Guile only has convenience routines to support
+conventionally-packed structs. But given the @code{bytevector->foreign}
+and @code{foreign->bytevector} routines, one can create and parse
+tightly packed structs and unions by hand. See the code for
+@code{(system foreign)} for details.
+
@node Dynamic FFI
@subsection Dynamic FFI
-TBD
+Of course, the land of C is not all nouns and no verbs: there are
+functions too, and Guile allows you to call them.
+
+@deffn {Scheme Procedure} pointer->procedure return_type func_ptr arg_types
+@deffnx {C Procedure} scm_pointer_to_procedure return_type func_ptr arg_types
+Make a foreign function.
+
+Given the foreign void pointer @var{func_ptr}, its argument and
+return types @var{arg_types} and @var{return_type}, return a
+procedure that will pass arguments to the foreign function
+and return appropriate values.
+
+@var{arg_types} should be a list of foreign types.
+@code{return_type} should be a foreign type. @xref{Foreign Types}, for
+more information on foreign types.
+@end deffn
+
+Here is a better definition of @code{(math bessel)}:
+
+@example
+(define-module (math bessel)
+ #:use-module (system foreign)
+ #:export (j0))
+
+(define libm (dynamic-link "libm"))
+
+(define j0
+ (pointer->procedure double
+ (dynamic-func "j0" libm)
+ (list double)))
+@end example
+
+That's it! No C at all.
+
+Numeric arguments and return values from foreign functions are
+represented as Scheme values. For example, @code{j0} in the above
+example takes a Scheme number as its argument, and returns a Scheme
+number.
+
+Pointers may be passed to and returned from foreign functions as well.
+In that case the type of the argument or return value should be the
+symbol @code{*}, indicating a pointer. For example, the following
+code makes @code{memcpy} available to Scheme:
+
+@example
+(define memcpy
+ (let ((this (dynamic-link)))
+ (pointer->procedure '*
+ (dynamic-func "memcpy" this)
+ (list '* '* size_t))))
+@end example
+
+To invoke @code{memcpy}, one must pass it foreign pointers:
+
+@example
+(use-modules (rnrs bytevectors))
+
+(define src-bits
+ (u8-list->bytevector '(0 1 2 3 4 5 6 7)))
+(define src
+ (bytevector->pointer src-bits))
+(define dest
+ (bytevector->pointer (make-bytevector 16 0)))
+
+(memcpy dest src (bytevector-length src-bits))
+
+(bytevector->u8-list (pointer->bytevector dest 16))
+@result{} (0 1 2 3 4 5 6 7 0 0 0 0 0 0 0 0)
+@end example
+
+One may also pass structs as values, passing structs as foreign
+pointers. @xref{Foreign Structs}, for more information on how to express
+struct types and struct values.
+
+``Out'' arguments are passed as foreign pointers. The memory pointed to
+by the foreign pointer is mutated in place.
+
+@example
+;; struct timeval @{
+;; time_t tv_sec; /* seconds */
+;; suseconds_t tv_usec; /* microseconds */
+;; @};
+;; assuming fields are of type "long"
+
+(define gettimeofday
+ (let ((f (pointer->procedure
+ int
+ (dynamic-func "gettimeofday" (dynamic-link))
+ (list '* '*)))
+ (tv-type (list long long)))
+ (lambda ()
+ (let* ((timeval (make-c-struct tv-type (list 0 0)))
+ (ret (f timeval %null-pointer)))
+ (if (zero? ret)
+ (apply values (parse-c-struct timeval tv-type))
+ (error "gettimeofday returned an error" ret))))))
+
+(gettimeofday)
+@result{} 1270587589
+@result{} 499553
+@end example
+
+As you can see, this interface to foreign functions is at a very low,
+somewhat dangerous level@footnote{A contribution to Guile in the form of
+a high-level FFI would be most welcome.}.
+
+@cindex callbacks
+The FFI can also work in the opposite direction: making Scheme
+procedures callable from C. This makes it possible to use Scheme
+procedures as ``callbacks'' expected by C function.
+
+@deffn {Scheme Procedure} procedure->pointer return-type proc arg-types
+@deffnx {C Function} scm_procedure_to_pointer (return_type, proc, arg_types)
+Return a pointer to a C function of type @var{return-type}
+taking arguments of types @var{arg-types} (a list) and
+behaving as a proxy to procedure @var{proc}. Thus
+@var{proc}'s arity, supported argument types, and return
+type should match @var{return-type} and @var{arg-types}.
+@end deffn
+
+As an example, here's how the C library's @code{qsort} array sorting
+function can be made accessible to Scheme (@pxref{Array Sort Function,
+@code{qsort},, libc, The GNU C Library Reference Manual}):
+
+@example
+(define qsort!
+ (let ((qsort (pointer->procedure void
+ (dynamic-func "qsort"
+ (dynamic-link))
+ (list '* size_t size_t '*))))
+ (lambda (bv compare)
+ ;; Sort bytevector BV in-place according to comparison
+ ;; procedure COMPARE.
+ (let ((ptr (procedure->pointer int
+ (lambda (x y)
+ ;; X and Y are pointers so,
+ ;; for convenience, dereference
+ ;; them before calling COMPARE.
+ (compare (dereference-uint8* x)
+ (dereference-uint8* y)))
+ (list '* '*))))
+ (qsort (bytevector->pointer bv)
+ (bytevector-length bv) 1 ;; we're sorting bytes
+ ptr)))))
+
+(define (dereference-uint8* ptr)
+ ;; Helper function: dereference the byte pointed to by PTR.
+ (let ((b (pointer->bytevector ptr 1)))
+ (bytevector-u8-ref b 0)))
+
+(define bv
+ ;; An unsorted array of bytes.
+ (u8-list->bytevector '(7 1 127 3 5 4 77 2 9 0)))
+
+;; Sort BV.
+(qsort! bv (lambda (x y) (- x y)))
+
+;; Let's see what the sorted array looks like:
+(bytevector->u8-list bv)
+@result{} (0 1 2 3 4 5 7 9 77 127)
+@end example
+
+And voil@`a!
+
+Note that @code{procedure->pointer} is not supported (and not defined)
+on a few exotic architectures. Thus, user code may need to check
+@code{(defined? 'procedure->pointer)}. Nevertheless, it is available on
+many architectures, including (as of libffi 3.0.9) x86, ia64, SPARC,
+PowerPC, ARM, and MIPS, to name a few.
@c Local Variables:
@c TeX-master: "guile.texi"
HCoop / bpt / guile.git / blobdiff