2 @c This is part of the GNU Guile Reference Manual.
3 @c Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2011
4 @c Free Software Foundation, Inc.
5 @c See the file guile.texi for copying conditions.
7 @node Linking Guile with Libraries
8 @section Linking Guile with Libraries
10 The previous section has briefly explained how to write programs that
11 make use of an embedded Guile interpreter. But sometimes, all you
12 want to do is make new primitive procedures and data types available
13 to the Scheme programmer. Writing a new version of @code{guile} is
14 inconvenient in this case and it would in fact make the life of the
15 users of your new features needlessly hard.
17 For example, suppose that there is a program @code{guile-db} that is a
18 version of Guile with additional features for accessing a database.
19 People who want to write Scheme programs that use these features would
20 have to use @code{guile-db} instead of the usual @code{guile} program.
21 Now suppose that there is also a program @code{guile-gtk} that extends
22 Guile with access to the popular Gtk+ toolkit for graphical user
23 interfaces. People who want to write GUIs in Scheme would have to use
24 @code{guile-gtk}. Now, what happens when you want to write a Scheme
25 application that uses a GUI to let the user access a database? You
26 would have to write a @emph{third} program that incorporates both the
27 database stuff and the GUI stuff. This might not be easy (because
28 @code{guile-gtk} might be a quite obscure program, say) and taking this
29 example further makes it easy to see that this approach can not work in
32 It would have been much better if both the database features and the GUI
33 feature had been provided as libraries that can just be linked with
34 @code{guile}. Guile makes it easy to do just this, and we encourage you
35 to make your extensions to Guile available as libraries whenever
38 You write the new primitive procedures and data types in the normal
39 fashion, and link them into a shared library instead of into a
40 stand-alone program. The shared library can then be loaded dynamically
44 * A Sample Guile Extension::
48 @node A Sample Guile Extension
49 @subsection A Sample Guile Extension
51 This section explains how to make the Bessel functions of the C library
52 available to Scheme. First we need to write the appropriate glue code
53 to convert the arguments and return values of the functions from Scheme
54 to C and back. Additionally, we need a function that will add them to
55 the set of Guile primitives. Because this is just an example, we will
56 only implement this for the @code{j0} function.
58 Consider the following file @file{bessel.c}.
67 return scm_from_double (j0 (scm_to_double (x)));
73 scm_c_define_gsubr ("j0", 1, 0, 0, j0_wrapper);
77 This C source file needs to be compiled into a shared library. Here is
78 how to do it on GNU/Linux:
81 gcc `pkg-config --cflags guile-@value{EFFECTIVE-VERSION}` \
82 -shared -o libguile-bessel.so -fPIC bessel.c
85 For creating shared libraries portably, we recommend the use of GNU
86 Libtool (@pxref{Top, , Introduction, libtool, GNU Libtool}).
88 A shared library can be loaded into a running Guile process with the
89 function @code{load-extension}. In addition to the name of the
90 library to load, this function also expects the name of a function from
91 that library that will be called to initialize it. For our example,
92 we are going to call the function @code{init_bessel} which will make
93 @code{j0_wrapper} available to Scheme programs with the name
94 @code{j0}. Note that we do not specify a filename extension such as
95 @file{.so} when invoking @code{load-extension}. The right extension for
96 the host platform will be provided automatically.
99 (load-extension "libguile-bessel" "init_bessel")
101 @result{} 0.223890779141236
104 For this to work, @code{load-extension} must be able to find
105 @file{libguile-bessel}, of course. It will look in the places that
106 are usual for your operating system, and it will additionally look
107 into the directories listed in the @code{LTDL_LIBRARY_PATH}
108 environment variable.
110 To see how these Guile extensions via shared libraries relate to the
111 module system, @xref{Putting Extensions into Modules}.
115 @c TeX-master: "guile.texi"