• ALLEGRO_PI
• al_run_main

ALLEGRO_PI

#define ALLEGRO_PI        3.14159265358979323846

C99 compilers have no predefined value like M_PI for the constant π, but you can use this one instead.

al_run_main

int al_run_main(int argc, char **argv, int (*user_main)(int, char **))

This function is useful in cases where you don't have a main() function but want to run Allegro (mostly useful in a wrapper library). Under Windows and Linux this is no problem because you simply can call al_install_system. But some other system (like OSX) don't allow calling al_install_system in the main thread. al_run_main will know what to do in that case.

The passed argc and argv will simply be passed on to user_main and the return value of user_main will be returned.