ALLEGRO_GET_EVENT_TYPE

#define ALLEGRO_GET_EVENT_TYPE(a, b, c, d) AL_ID(a, b, c, d) Introduced in 5.0.0

Make an event type identifier, which is a 32-bit integer. Usually, but not necessarily, this will be made from four 8-bit character codes, for example:

#define MY_EVENT_TYPE   ALLEGRO_GET_EVENT_TYPE('M','I','N','E')

IDs less than 1024 are reserved for Allegro or its addons. Don't use anything lower than ALLEGRO_GET_EVENT_TYPE(0, 0, 4, 0).

You should try to make your IDs unique so they don't clash with any 3rd party code you may be using. Be creative. Numbering from 1024 is not creative.

If you need multiple identifiers, you could define them like this:

#define BASE_EVENT   ALLEGRO_GET_EVENT_TYPE('M','I','N','E')
#define BARK_EVENT   (BASE_EVENT + 0)
#define MEOW_EVENT   (BASE_EVENT + 1)
#define SQUAWK_EVENT (BASE_EVENT + 2)

/* Alternatively */
enum {
   BARK_EVENT = ALLEGRO_GET_EVENT_TYPE('M','I','N','E'),
   MEOW_EVENT,
   SQUAWK_EVENT
};

Examples: ex_native_filechooser, ex_user_events

Most helpful discussions:

Manual

Event system and events

ALLEGRO_GET_EVENT_TYPE