#define al_init() (al_install_system(ALLEGRO_VERSION_INT, atexit))
Like al_install_system(3alleg5), but automatically passes in the version and uses the atexit function visible in the current binary.
Note: It is typically wrong to call al_init anywhere except the final game binary. In particular, do not call it inside a shared library unless you know what you're doing. In those cases, it is better to call al_install_system either with a NULL atexit_ptr, or with a pointer to atexit provided by the user of this shared library.