al_fixsub - Allegro 5 API

       Although  fixed  point numbers can be subtracted with the normal - integer operator, that doesn’t provide
       any protection against overflow.  If overflow is a problem, you should use this function instead.  It  is
       slower  than using integer operators, but if an overflow occurs it will set Allegro’s errno and clamp the
       result, rather than just letting it wrap.

       Example:

              al_fixed result;

              /* This will put 4965 into `result'. */
              result = al_fixsub(al_itofix(5000), al_itofix(35));

              /* Sets errno and puts -32768 into `result'. */
              result = al_fixsub(al_itofix(-31000), al_itofix(3000));
              assert(!al_get_errno()); /* This will fail. */

       al_fixsub - Allegro 5 API

       Returns the clamped result of subtracting y from x, setting Allegro’s errno to ERANGE  if  there  was  an
       overflow.

al_fixadd(3alleg5), al_fixmul(3alleg5), al_fixdiv(3alleg5), al_get_errno(3alleg5).

Allegro reference manual                                                                      al_fixsub(3alleg5)

              #include <allegro5/allegro.h>

              al_fixed al_fixsub(al_fixed x, al_fixed y);