Creating a module system (dynamic loading) in C

How would one go about loading compiled C code at run time, and then calling functions within it? Not like simply calling exec().

EDIT: The the program loading the module is in C.

dlopen is the way to go. Here are a few examples:

Loading a plugin with dlopen:

#include <dlfcn.h>
...
int
main (const int argc, const char *argv[])
{

  char *plugin_name;
  char file_name[80];
  void *plugin;
  ...
  plugin = dlopen(file_name, RTLD_NOW);
  if (!plugin)
  {
     fatal("Cannot load %s: %s", plugin_name, dlerror ());
  }

Compiling the above:

% cc  -ldl -o program program.o 

Then, assuming this API for the plugins:

/* The functions we will find in the plugin */
typedef void (*init_f) ();
init_f init;
typedef int (*query_f) ();
query_f query;

Finding the address of init() in the plugin:

init = dlsym(plugin, "init");
result = dlerror();
if (result)
{
   fatal("Cannot find init in %s: %s", plugin_name, result);
}
init();

With the other function, query(), which returns a value:

query = dlsym (plugin, "query");
result = dlerror();
if (result)
{
    fatal("Cannot find query in %s: %s", plugin_name, result);
}
printf("Result of plugin %s is %d\n", plugin_name, query ());

You can retrieve the complete example on line.

In Linux/UNIX you can use the POSIX dlopen / dlsym / dlerror / dlclose functions to dynamically open shared libraries and access the symbols (including functions) they provide, see the man page for details.