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6.20.3 C Extensions

The most interesting application of dynamically linked libraries is probably to use them for providing 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 ‘(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 j0 function.

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

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

init_math_bessel ()
  scm_c_define_gsubr ("j0", 1, 0, 0, j0_wrapper);

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 libtool documentation for how to create dynamically linkable libraries portably.

gcc -shared -o -fPIC bessel.c

Now fire up Guile:

(define bessel-lib (dynamic-link "./"))
(dynamic-call "init_math_bessel" bessel-lib)
(j0 2)
⇒ 0.223890779141236

The filename ./ should be pointing to the shared library produced with the gcc command above, of course. The second line of the Guile interaction will call the init_math_bessel function which in turn will register the C function j0_wrapper with the Guile interpreter under the name 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 apropos has to say about j0:

(apropos "j0")
-| (guile-user): j0     #<primitive-procedure j0>

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

A compiled module should have a specially named 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 init_math_bessel with the following code in bessel.c:

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

scm_init_math_bessel_module ()
  scm_c_define_module ("math bessel", init_math_bessel, NULL);   

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

After 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:

guile> (load-extension "./" "scm_init_math_bessel_module")
guile> (use-modules (math bessel))
guile> (j0 2)
guile> (apropos "j0")
-| (math bessel): j0      #<primitive-procedure j0>

That’s it!

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