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4.2.1 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 j0 function.

Consider the following file bessel.c.

     #include <math.h>
     #include <libguile.h>
     j0_wrapper (SCM x)
       return scm_make_real (j0 (scm_num2dbl (x, "j0")));
     init_bessel ()
       scm_c_define_gsubr ("j0", 1, 0, 0, j0_wrapper);

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

     gcc -shared -o libguile-bessel.so -fPIC bessel.c

For creating shared libraries portably, we recommend the use of GNU Libtool (see Introduction).

A shared library can be loaded into a running Guile process with the function 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 init_bessel which will make j0_wrapper available to Scheme programs with the name j0. Note that we do not specify a filename extension such as .so when invoking load-extension. The right extension for the host platform will be provided automatically.

     (load-extension "libguile-bessel" "init_bessel")
     (j0 2)
     ⇒ 0.223890779141236

For this to work, load-extension must be able to find 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 LTDL_LIBRARY_PATH environment variable.

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