add pointer->scm, scm->pointer

[bpt/guile.git] / doc / ref / api-foreign.texi

@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@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.

@node Foreign Function Interface
@section Foreign Function Interface
@cindex foreign function interface
@cindex ffi

The more one hacks in Scheme, the more one realizes that there are
actually two computational worlds: one which is warm and alive, that
land of parentheses, and one cold and dead, the land of C and its ilk.

But yet we as programmers live in both worlds, and Guile itself is half
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
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 Pointers::            Accessing global variables.
* Dynamic FFI::                 Calling arbitrary C functions.
@end menu


@node Foreign Libraries
@subsection Foreign Libraries

Most modern Unices have something called @dfn{shared libraries}.  This
ordinarily means that they have the capability to share the executable
image of a library between several running programs to save memory and
disk space.  But generally, shared libraries give a lot of additional
flexibility compared to the traditional static libraries.  In fact,
calling them `dynamic' libraries is as correct as calling them `shared'.

Shared libraries really give you a lot of flexibility in addition to the
memory and disk space savings.  When you link a program against a shared
library, that library is not closely incorporated into the final
executable.  Instead, the executable of your program only contains
enough information to find the needed shared libraries when the program
is actually run.  Only then, when the program is starting, is the final
step of the linking process performed.  This means that you need not
recompile all programs when you install a new, only slightly modified
version of a shared library.  The programs will pick up the changes
automatically the next time they are run.

Now, when all the necessary machinery is there to perform part of the
linking at run-time, why not take the next step and allow the programmer
to explicitly take advantage of it from within his program?  Of course,
many operating systems that support shared libraries do just that, and
chances are that Guile will allow you to access this feature from within
your Scheme programs.  As you might have guessed already, this feature
is called @dfn{dynamic linking}.@footnote{Some people also refer to the
final linking stage at program startup as `dynamic linking', so if you
want to make yourself perfectly clear, it is probably best to use the
more technical term @dfn{dlopening}, as suggested by Gordon Matzigkeit
in his libtool documentation.}

We titled this section ``foreign libraries'' because although the name
``foreign'' doesn't leak into the API, the world of C really is foreign
to Scheme -- and that estrangement extends to components of foreign
libraries as well, as we see in future sections.

@deffn {Scheme Procedure} dynamic-link [library]
@deffnx {C Function} scm_dynamic_link (library)
Find the shared library denoted by @var{library} (a string) and link it
into the running Guile application.  When everything works out, return a
Scheme object suitable for representing the linked object file.
Otherwise an error is thrown.  How object files are searched is system
dependent.

Normally, @var{library} is just the name of some shared library file
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
loaded.
@end deffn

@deffn {Scheme Procedure} dynamic-object? obj
@deffnx {C Function} scm_dynamic_object_p (obj)
Return @code{#t} if @var{obj} is a dynamic library handle, or @code{#f}
otherwise.
@end deffn

@deffn {Scheme Procedure} dynamic-unlink dobj
@deffnx {C Function} scm_dynamic_unlink (dobj)
Unlink the indicated object file from the application.  The
argument @var{dobj} must have been obtained by a call to
@code{dynamic-link}.  After @code{dynamic-unlink} has been
called on @var{dobj}, its content is no longer accessible.
@end deffn

@smallexample
(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.

When dynamic linking is disabled or not supported on your system,
the above functions throw errors, but they are still available.


@node Foreign Functions
@subsection Foreign Functions

The most natural thing to do with a dynamic library is to grovel around
in it for a function pointer: a @dfn{foreign function}.
@code{dynamic-func} exists for that purpose.

@deffn {Scheme Procedure} dynamic-func name dobj
@deffnx {C Function} scm_dynamic_func (name, dobj)
Return a ``handle'' for the func @var{name} in the shared object referred to
by @var{dobj}. The handle can be passed to @code{dynamic-call} to
actually call the function.

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

Guile has static support for calling functions with no arguments,
@code{dynamic-call}.

@deffn {Scheme Procedure} dynamic-call func dobj
@deffnx {C Function} scm_dynamic_call (func, dobj)
Call the C function indicated by @var{func} and @var{dobj}.
The function is passed no arguments and its return value is
ignored.  When @var{function} is something returned by
@code{dynamic-func}, call that function and ignore @var{dobj}.
When @var{func} is a string , look it up in @var{dynobj}; this
is equivalent to
@smallexample
(dynamic-call (dynamic-func @var{func} @var{dobj}) #f)
@end smallexample

Interrupts are deferred while the C function is executing (with
@code{SCM_DEFER_INTS}/@code{SCM_ALLOW_INTS}).
@end deffn

@code{dynamic-call} is not very powerful. It is mostly intended to be
used for calling specially written initialization functions that will
then add new primitives to Guile. For example, we do not expect that you
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-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
function. That function would add all the new types and primitives to
the Guile interpreter that it has to offer.

(There is actually another, better option: simply to create a
@file{libX11} wrapper in Scheme via the dynamic FFI. @xref{Dynamic FFI},
for more information.)

Given some set of C extensions to Guile, the next logical step is to
integrate these glue libraries into the module system of Guile so that
you can load new primitives into a running system just as you can load
new Scheme code.

@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.

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
@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
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 C Extensions
@subsection C Extensions

The most interesting application of dynamically linked libraries is
probably to use them for providing @emph{compiled code modules} to
Scheme programs.  As much fun as programming in Scheme is, every now and
then comes the need to write some low-level C stuff to make Scheme even
more fun.

Not only can you put these new primitives into their own module (see the
previous section), you can even put them into a shared library that is
only then linked to your running Guile image when it is actually
needed.

An example will hopefully make everything clear.  Suppose we want to
make the Bessel functions of the C library available to Scheme in the
module @samp{(math bessel)}.  First we need to write the appropriate
glue code to convert the arguments and return values of the functions
from Scheme to C and back.  Additionally, we need a function that will
add them to the set of Guile primitives.  Because this is just an
example, we will only implement this for the @code{j0} function.

@smallexample
#include <math.h>
#include <libguile.h>

SCM
j0_wrapper (SCM x)
@{
  return scm_from_double (j0 (scm_to_double (x, "j0")));
@}

void
init_math_bessel ()
@{
  scm_c_define_gsubr ("j0", 1, 0, 0, j0_wrapper);
@}
@end smallexample

We can already try to bring this into action by manually calling the low
level functions for performing dynamic linking.  The C source file needs
to be compiled into a shared library.  Here is how to do it on
GNU/Linux, please refer to the @code{libtool} documentation for how to
create dynamically linkable libraries portably.

@smallexample
gcc -shared -o libbessel.so -fPIC bessel.c
@end smallexample

Now fire up Guile:

@lisp
(define bessel-lib (dynamic-link "./libbessel.so"))
(dynamic-call "init_math_bessel" bessel-lib)
(j0 2)
@result{} 0.223890779141236
@end lisp

The filename @file{./libbessel.so} should be pointing to the shared
library produced with the @code{gcc} command above, of course.  The
second line of the Guile interaction will call the
@code{init_math_bessel} function which in turn will register the C
function @code{j0_wrapper} with the Guile interpreter under the name
@code{j0}.  This function becomes immediately available and we can call
it from Scheme.

Fun, isn't it?  But we are only half way there.  This is what
@code{apropos} has to say about @code{j0}:

@smallexample
(apropos "j0")
@print{} (guile-user): j0     #<primitive-procedure j0>
@end smallexample

As you can see, @code{j0} is contained in the root module, where all
the other Guile primitives like @code{display}, etc live.  In general,
a primitive is put into whatever module is the @dfn{current module} at
the time @code{scm_c_define_gsubr} is called.

A compiled module should have a specially named @dfn{module init
function}.  Guile knows about this special name and will call that
function automatically after having linked in the shared library.  For
our example, we replace @code{init_math_bessel} with the following code in
@file{bessel.c}:

@smallexample
void
init_math_bessel (void *unused)
@{
  scm_c_define_gsubr ("j0", 1, 0, 0, j0_wrapper);
  scm_c_export ("j0", NULL);
@}

void
scm_init_math_bessel_module ()
@{
  scm_c_define_module ("math bessel", init_math_bessel, NULL);   
@}
@end smallexample

The general pattern for the name of a module init function is:
@samp{scm_init_}, followed by the name of the module where the
individual hierarchical components are concatenated with underscores,
followed by @samp{_module}.

After @file{libbessel.so} has been rebuilt, we need to place the shared
library into the right place.

Once the module has been correctly installed, it should be possible to
use it like this:

@smallexample
guile> (load-extension "./libbessel.so" "scm_init_math_bessel_module")
guile> (use-modules (math bessel))
guile> (j0 2)
0.223890779141236
guile> (apropos "j0")
@print{} (math bessel): j0      #<primitive-procedure j0>
@end smallexample

That's it!


@node Modules and Extensions
@subsection Modules and Extensions

The new primitives that you add to Guile with @code{scm_c_define_gsubr}
(@pxref{Primitive Procedures}) or with any of the other mechanisms are
placed into the module that is current when the
@code{scm_c_define_gsubr} is executed. Extensions loaded from the REPL,
for example, will be placed into the @code{(guile-user)} module, if the
REPL module was not changed.

To define C primitives within a specific module, the simplest way is:

@example
(define-module (foo bar))
(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 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
install location in the @file{foo/bar.scm} file.  This is convenient
for the user and also guarantees the intended object is read, even if
stray older or newer versions are in the loader's path.

The usual way to specify an install location is with a @code{prefix}
at the configure stage, for instance @samp{./configure prefix=/opt}
results in library files as say @file{/opt/lib/foobar-c-code.so}.
When using Autoconf (@pxref{Top, , Introduction, autoconf, The GNU
Autoconf Manual}), the library location is in a @code{libdir}
variable.  Its value is intended to be expanded by @command{make}, and
can by substituted into a source file like @file{foo.scm.in}

@example
(define-module (foo bar))
(load-extension "XXextensiondirXX/foobar-c-code" "foo_bar_init")
@end example

@noindent
with the following in a @file{Makefile}, using @command{sed}
(@pxref{Top, , Introduction, sed, SED, A Stream Editor}),

@example
foo.scm: foo.scm.in
        sed 's|XXextensiondirXX|$(libdir)/guile/@value{EFFECTIVE-VERSION}|' <foo.scm.in >foo.scm
@end example

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{extensiondir} location, and use that as required.

@example
(define-module (foo config))
(define-public foo-config-extensiondir "XXextensiondirXX"")
@end example

Such a file might have other locations too, for instance a data
directory for auxiliary files, or @code{localedir} if the module has
its own @code{gettext} message catalogue
(@pxref{Internationalization}).

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
when installing Guile modules in non-default locations.  But having
reached the Scheme code, that code should take care of hitting any of
its own private files etc.


@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.

These ``C type values'' may be constructed using the constants and
procedures from the @code{(system foreign)} module, which may be loaded
like this:

@example
(use-modules (system foreign))
@end example

@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
Return a foreign pointer to a nul-terminated copy of @var{string} in 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_locale_string}.
@end deffn

@deffn {Scheme Procedure} pointer->string pointer
Return the string representing the C nul-terminated string
pointed to by @var{pointer}.  The C string is assumed to be
in the current locale encoding.

This is the Scheme equivalent of @code{scm_from_locale_string}.
@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

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"
@c End: