fma(), fmaf(), fmal()

QNX SDP8.0C Library ReferenceAPIDeveloper

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
The general-purpose math library.
libm-sve
A library that optimizes the code for ARMv8.2 chips that have Scalable Vector Extension hardware.

Your system requirements will determine how you should work with these libraries:

  • If you want only selected processes to run with the SVE version, you can include both libraries in your OS image and use the -l m or -l m-sve option to qcc to link explicitly against the appropriate one.
  • If you want all processes to use the SVE version, include libm-sve.so in your OS image and set up a symbolic link from libm.so to libm-sve.so. Use the -l m option to qcc to link against the library.
Note:
Compile your program with the -fno-builtin option to prevent the compiler from using a built-in version of the function.

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:

C11, POSIX 1003.1

Safety:
Cancellation pointNo
Signal handlerYes
ThreadYes
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