@c -*-texinfo-*-
@c This is part of the GNU Guile Reference Manual.
@c Copyright (C)  1996, 1997, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2011
@c   Free Software Foundation, Inc.
@c See the file guile.texi for copying conditions.

@node Linking Guile with Libraries
@section Linking Guile with Libraries

The previous section has briefly explained how to write programs that
make use of an embedded Guile interpreter.  But sometimes, all you
want to do is make new primitive procedures and data types available
to the Scheme programmer.  Writing a new version of @code{guile} is
inconvenient in this case and it would in fact make the life of the
users of your new features needlessly hard.

For example, suppose that there is a program @code{guile-db} that is a
version of Guile with additional features for accessing a database.
People who want to write Scheme programs that use these features would
have to use @code{guile-db} instead of the usual @code{guile} program.
Now suppose that there is also a program @code{guile-gtk} that extends
Guile with access to the popular Gtk+ toolkit for graphical user
interfaces.  People who want to write GUIs in Scheme would have to use
@code{guile-gtk}.  Now, what happens when you want to write a Scheme
application that uses a GUI to let the user access a database?  You
would have to write a @emph{third} program that incorporates both the
database stuff and the GUI stuff.  This might not be easy (because
@code{guile-gtk} might be a quite obscure program, say) and taking this
example further makes it easy to see that this approach can not work in
practice.

It would have been much better if both the database features and the GUI
feature had been provided as libraries that can just be linked with
@code{guile}.  Guile makes it easy to do just this, and we encourage you
to make your extensions to Guile available as libraries whenever
possible.

You write the new primitive procedures and data types in the normal
fashion, and link them into a shared library instead of into a
stand-alone program.  The shared library can then be loaded dynamically
by Guile.

@menu
* A Sample Guile Extension::
@end menu


@node A Sample Guile Extension
@subsection A Sample Guile Extension

This section explains how to make the Bessel functions of the C library
available to Scheme.  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.

Consider the following file @file{bessel.c}.

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

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

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

This C source file needs to be compiled into a shared library.  Here is
how to do it on GNU/Linux:

@smallexample
gcc `pkg-config --cflags guile-@value{EFFECTIVE-VERSION}` \
  -shared -o libguile-bessel.so -fPIC bessel.c
@end smallexample

For creating shared libraries portably, we recommend the use of GNU
Libtool (@pxref{Top, , Introduction, libtool, GNU Libtool}).

A shared library can be loaded into a running Guile process with the
function @code{load-extension}.  In addition to the name of the
library to load, this function also expects the name of a function from
that library that will be called to initialize it.  For our example,
we are going to call the function @code{init_bessel} which will make
@code{j0_wrapper} available to Scheme programs with the name
@code{j0}.  Note that we do not specify a filename extension such as
@file{.so} when invoking @code{load-extension}.  The right extension for
the host platform will be provided automatically.

@lisp
(load-extension "libguile-bessel" "init_bessel")
(j0 2)
@result{} 0.223890779141236
@end lisp

For this to work, @code{load-extension} must be able to find
@file{libguile-bessel}, of course.  It will look in the places that
are usual for your operating system, and it will additionally look
into the directories listed in the @code{LTDL_LIBRARY_PATH}
environment variable.

To see how these Guile extensions via shared libraries relate to the
module system, @xref{Putting Extensions into Modules}.


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