opendarwin man page for fixsub

fixsub(3alleg4) 						  Allegro manual						   fixsub(3alleg4)

NAME
       fixsub - Safe function to subtract fixed point numbers clamping underflow. Allegro game programming library.

SYNOPSIS
       #include <allegro.h>

       fixed fixsub(fixed x, fixed y);

DESCRIPTION
       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 `errno' and clamp the result, rather than just letting it wrap. Example:

	  fixed result;
	  /* This will put 4965 into `result'. */
	  result = fixsub(itofix(5000), itofix(35));
	  /* Sets `errno' and puts -32768 into `result'. */
	  result = fixsub(itofix(-31000), itofix(3000));
	  ASSERT(!errno); /* This will fail. */

RETURN VALUE
       Returns the clamped result of subtracting `y' from `x', setting `errno' to ERANGE if there was an overflow.

SEE ALSO
       fixadd(3alleg4), fixmul(3alleg4), fixdiv(3alleg4)

Allegro 							   version 4.4.2						   fixsub(3alleg4)