int install_allegro(int system_id, int *errno_ptr, int (*atexit_ptr)());
The available system ID codes will vary from one platform to another, but you will almost always want to pass SYSTEM_AUTODETECT. Alternatively, SYSTEM_NONE installs a stripped down version of Allegro that won't even try to touch your hardware or do anything platform specific: this can be useful for situations where you only want to manipulate memory bitmaps, such as the text mode datafile tools or the Windows GDI interfacing functions.
The `errno_ptr' and `atexit_ptr' parameters should point to the errno variable and atexit function from your libc: these are required because when Allegro is linked as a DLL, it doesn't have direct access to your local libc data. `atexit_ptr' may be NULL, in which case it is your responsibility to call allegro_exit() manually. Example:
install_allegro(SYSTEM_AUTODETECT, &errno, atexit);