asterisk/doc/modules.txt

All modules must have at least the following functions:

int load_module():

	Do what you need to do when you get started.  This function
returns 0 on success and non-zero on failure (it is not considered loaded
if it fails.

int unload_module():
	
	The module will soon be unloaded.  If any channels are using your
features, you should give them a softhangup in an effort to keep the
program from crashing.  Generally, unload_module is only called when the
usecount is 0 or less, but the user can force unloading at their
discretion, and thus a module should do its best to comply (although in
some cases there may be no way to avoid a crash).  This function should
return 0 on success and non-zero on failure (i.e. it cannot yet be
unloaded).

char *description():

	Return a description of the module's functionality.

int usecnt():
	
	Return the number of channels, etc that are using you.