These functions are declared in the main Allegro header file:
#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. 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.