@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2012, 2014
@c   Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@node Function Snarfing
@section Function Snarfing

When writing C code for use with Guile, you typically define a set of
C functions, and then make some of them visible to the Scheme world by
calling @code{scm_c_define_gsubr} or related functions.  If you have
many functions to publish, it can sometimes be annoying to keep the
list of calls to @code{scm_c_define_gsubr} in sync with the list of
function definitions.

Guile provides the @code{guile-snarf} program to manage this problem.
Using this tool, you can keep all the information needed to define the
function alongside the function definition itself; @code{guile-snarf}
will extract this information from your source code, and automatically
generate a file of calls to @code{scm_c_define_gsubr} which you can
@code{#include} into an initialization function.

The snarfing mechanism works for many kind of initialization actions,
not just for collecting calls to @code{scm_c_define_gsubr}.  For a
full list of what can be done, @xref{Snarfing Macros}.

@cindex guile-snarf invocation
@cindex guile-snarf example

The @code{guile-snarf} program is invoked like this:

@smallexample
guile-snarf [-o @var{outfile}] [@var{cpp-args} ...]
@end smallexample

This command will extract initialization actions to @var{outfile}.
When no @var{outfile} has been specified or when @var{outfile} is
@code{-}, standard output will be used.  The C preprocessor is called
with @var{cpp-args} (which usually include an input file) and the
output is filtered to extract the initialization actions.

If there are errors during processing, @var{outfile} is deleted and the
program exits with non-zero status.

During snarfing, the pre-processor macro @code{SCM_MAGIC_SNARFER} is
defined.  You could use this to avoid including snarfer output files
that don't yet exist by writing code like this:

@smallexample
#ifndef SCM_MAGIC_SNARFER
#include "foo.x"
#endif
@end smallexample

Here is how you might define the Scheme function @code{clear-image},
implemented by the C function @code{clear_image}:

@example
@group
#include <libguile.h>

SCM_DEFINE (clear_image, "clear-image", 1, 0, 0,
            (SCM image),
            "Clear the image.")
@{
  /* C code to clear the image in @code{image}... */
@}

void
init_image_type ()
@{
#include "image-type.x"
@}
@end group
@end example

The @code{SCM_DEFINE} declaration says that the C function
@code{clear_image} implements a Scheme function called
@code{clear-image}, which takes one required argument (of type
@code{SCM} and named @code{image}), no optional arguments, and no rest
argument.  The string @code{"Clear the image."} provides a short help
text for the function, it is called a @dfn{docstring}.

@code{SCM_DEFINE} macro also defines a static array of characters
initialized to the Scheme name of the function.  In this case,
@code{s_clear_image} is set to the C string, "clear-image".  You might
want to use this symbol when generating error messages.

Assuming the text above lives in a file named @file{image-type.c}, you
will need to execute the following command to prepare this file for
compilation:

@example
guile-snarf -o image-type.x image-type.c
@end example

This scans @file{image-type.c} for @code{SCM_DEFINE}
declarations, and writes to @file{image-type.x} the output:

@example
scm_c_define_gsubr ("clear-image", 1, 0, 0, (SCM (*)() ) clear_image);
@end example

When compiled normally, @code{SCM_DEFINE} is a macro which expands to
the function header for @code{clear_image}.

Note that the output file name matches the @code{#include} from the
input file.  Also, you still need to provide all the same information
you would if you were using @code{scm_c_define_gsubr} yourself, but you
can place the information near the function definition itself, so it is
less likely to become incorrect or out-of-date.

If you have many files that @code{guile-snarf} must process, you should
consider using a fragment like the following in your Makefile:

@example
snarfcppopts = $(DEFS) $(INCLUDES) $(CPPFLAGS) $(CFLAGS)
.SUFFIXES: .x
.c.x:
	guile-snarf -o $@@ $< $(snarfcppopts)
@end example

This tells make to run @code{guile-snarf} to produce each needed
@file{.x} file from the corresponding @file{.c} file.

The program @code{guile-snarf} passes its command-line arguments
directly to the C preprocessor, which it uses to extract the
information it needs from the source code. this means you can pass
normal compilation flags to @code{guile-snarf} to define preprocessor
symbols, add header file directories, and so on.