As Dragos mentioned, there are examples in FS source tree how to do this, mod_avmd in particular.
Others are mod_cv, mod_stress, mod_soundtouch, mod_oreka, mod_vmd, mod_vmd, mod_ladspa, mod_snapshot, mod_fsk, mod_video_filter, mod_spandsp.
You may need to switch_channel_set_private/switch_channel_remove_private.
Instead of removing/re-adding the bug you can add it just once with switch_core_media_bug_add, and remove it when FS session ends.
You can then supply the state describing your app's session as the 5th argument to a call:
my_session_t *my_session = alloc_my_session_dynamically_or_use_global_variable_just_about_anything_but_local_stack(not_active) __cannot_fail__;
switch_core_media_bug_add(session, "my-function", NULL, my_callback, my_session, 0, my_flags, &my_bug)
In that state you can put whatever you need, including whether your app on given FS session is active at the moment,
and based on that you can drive your code inside my_callback.
static switch_bool_t my_callback(switch_media_bug_t * bug, void *user_data, switch_abc_type_t type)
{
my_session = (my_session_t *) user_data;
//
switch (type) {
case SWITCH_ABC_TYPE_INIT:
// if it has just been started
my_session->active = 1;
// maybe do something more if needed when your session is started (when you would (re)add bug otherwise)
break;
case SWITCH_ABC_TYPE_CLOSE:
my_session->active = 0;
// maybe do something more if needed when your session is closed (when you would remove bug otherwise)
break;
case SWITCH_ABC_TYPE_READ_REPLACE:
// processing frame of data if session is active
break;
case SWITCH_ABC_TYPE_WRITE_REPLACE:
// processing frame of data if session is active
break;
default:
// other types
why();
}
// ...
}
Callback will be called every time there is a reason to call it (considering also the flags you've supplied).
Good luck!