asterisk/doc/apps.txt

Asterisk applications register themselves with ast_application_register.
They should have a short, unique name, and an exec function which takes
as its arguments a channel and some data that might be useful for callback
stuff.  Remember to keep track of how many and which channels are using
your application so that should the module need to be unloaded
(particularly force unloaded), you will be able to ast_softhangup all the
channels.  An application should *never* call ast_hangup on the channel
that it is running on (although it could conceivably hang up other
channels that it allocates).  See app_playback.c as an example of a simple
application.