fma(), fmaf(), fmal()

Multiply two floating-point numbers and then add a third number

Synopsis:

#include <math.h>

double fma( double x,
            double y,
            double z );

float fmaf( float x,
            float y,
            float z );

long double fmal( long double x,
                  long double y,
                  long double z );

Arguments:

x, y: The numbers that you want to multiply together.
z: The number you want to add to the product.

Library:

libm

Use the -l m option to qcc to link against this library.

Description:

The fma(), fmaf(), and fmal() (“fused multiply-add”) functions calculate (x * y) + z, rounded as one ternary operation. That is, they compute the value to infinite precision and then round the result once, according to the current rounding mode as specified by FLT_ROUNDS.

To check for error situations, use feclearexcept() and fetestexcept(). For example:

Call feclearexcept(FE_ALL_EXCEPT) before calling fma(), fmaf(), or fmal().
On return, if fetestexcept(FE_ALL_EXCEPT) is nonzero, then an error has occurred.

Returns:

(x * y) + z, rounded as one ternary operation.

If:	These functions return:	Errors:
The result overflows	Inf	FE_OVERFLOW
The result underflows	The correct result, after rounding	FE_UNDERFLOW
`x` or `y` is NaN	NaN	—
`x` * `y` is an exact infinity, and `z` is also an infinity but with the opposite sign	NaN	FE_INVALID
One of `x` and `y` is infinite, the other is zero, and `z` isn't a NaN	NaN	FE_INVALID
One of `x` and `y` is infinite, the other is zero, and `z` is NaN	NaN	FE_INVALID
`x` * `y` isn't 0.0 * Inf or Inf * 0.0, and `z` is NaN	NaN	—

These functions raise FE_INEXACT if the FPU reports that the result can't be exactly represented as a floating-point number.

Classification:

ANSI, POSIX 1003.1

Safety:
Cancellation point	No
Interrupt handler	No
Signal handler	No
Thread	Yes