info guile-tut

4.4.4 What learn1 is doing

If you compare learn1 to learn0, you will find that learn1 uses a new Guile construct: the function gh_new_procedure(), and its siblings:

  /* now try to define some new builtins, coded in C, so that they are
     available in Scheme. */
  gh_new_procedure1_0("c-factorial", c_factorial);
  gh_new_procedure1_0("c-sin", c_sin);
  gh_new_procedure1_0("v-t", vector_test);

It is clear that gh_new_procedure() adds a new builtin routine written in C which can be invoked from Scheme. We can now revise our checklist for programming with libguile, so it includes adding callbacks.

#include <guile/gh.h>
You need to invoke the initialization routine gh_enter(). This starts up a Scheme interpreter, handling many details.
Your main() function should be almost empty: the real main program goes in a separate function main_prog() which is passed to gh_enter(). This rather arcane convention is due to the way Guile's garbage collector works: the whole program has to run in the dynamic context of gh_enter().
You pass strings to the Scheme interpreter with the gh_eval_str() routine.
[new] You can now define new builtin Scheme functions; i.e. define new builtin Scheme functions, with the gh_new_procedure() routine.
You pass strings to the Scheme interpreter with the gh_eval_str() routine.
You link your program with -lguile.

I breezed by the issue of how to write your C routines that are registered to be called from Scheme. This is non-trivial, and is discussed at length in the Guile Programmer's Manual.