### 1.21 `fma`, `fmaf`–floating multiply add

Synopsis

         #include <math.h>
         double fma(double x, double y, double z);
         float fmaf(float x, float y, float z);

Description
The `fma` functions compute (x * y) + z, rounded as one ternary operation: they compute the value (as if) to infinite precision and round once to the result format, according to the rounding mode characterized by the value of FLT_ROUNDS. That is, they are supposed to do this: see below.

Returns
The `fma` functions return (x * y) + z, rounded as one ternary operation.

Bugs
This implementation does not provide the function that it should, purely returning "(x * y) + z;" with no attempt at all to provide the simulated infinite precision intermediates which are required. DO NOT USE THEM.

If double has enough more precision than float, then `fmaf` should provide the expected numeric results, as it does use double for the calculation. But since this is not the case for all platforms, this manual cannot determine if it is so for your case.

Portability
ANSI C, POSIX.