al_fixed al_fixsub(al_fixed x, al_fixed y);
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.
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. */
Returns the clamped result of subtracting y from x, setting Allegro's errno to ERANGE if there was an overflow.