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PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface
may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface
may not be implemented on Linux.
NAME
fma, fmaf, fmal — floating-point multiply-add
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);
DESCRIPTION
The functionality described on this reference page is aligned with the ISO C standard. Any conflict
between the requirements described here and the ISO C standard is unintentional. This volume of
POSIX.1‐2017 defers to the ISO C standard.
These functions shall compute (x * y) + z, rounded as one ternary operation: they shall 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.
An application wishing to check for error situations should set errno to zero and call
feclearexcept(FE_ALL_EXCEPT) before calling these functions. On return, if errno is non-zero or
fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an error has occurred.
RETURN VALUE
Upon successful completion, these functions shall return (x * y) + z, rounded as one ternary operation.
If the result overflows or underflows, a range error may occur. On systems that support the IEC 60559
Floating-Point option, if the result overflows a range error shall occur.
If x or y are NaN, a NaN shall be returned.
If x multiplied by y is an exact infinity and z is also an infinity but with the opposite sign, a domain
error shall occur, and either a NaN (if supported), or an implementation-defined value shall be returned.
If one of x and y is infinite, the other is zero, and z is not a NaN, a domain error shall occur, and
either a NaN (if supported), or an implementation-defined value shall be returned.
If one of x and y is infinite, the other is zero, and z is a NaN, a NaN shall be returned and a domain
error may occur.
If x*y is not 0*Inf nor Inf*0 and z is a NaN, a NaN shall be returned.
ERRORS
These functions shall fail if:
Domain Error
The value of x*y+z is invalid, or the value x*y is invalid and z is not a NaN.
If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then errno shall be
set to [EDOM]. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero,
then the invalid floating-point exception shall be raised.
Range Error The result overflows.
If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then errno shall be
set to [ERANGE]. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero,
then the overflow floating-point exception shall be raised.
These functions may fail if:
Domain Error
The value x*y is invalid and z is a NaN.
If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then errno shall be
set to [EDOM]. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero,
then the invalid floating-point exception shall be raised.
Range Error The result underflows.
If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then errno shall be
set to [ERANGE]. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero,
then the underflow floating-point exception shall be raised.
Range Error The result overflows.
If the integer expression (math_errhandling & MATH_ERRNO) is non-zero, then errno shall be
set to [ERANGE]. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero,
then the overflow floating-point exception shall be raised.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
On error, the expressions (math_errhandling & MATH_ERRNO) and (math_errhandling & MATH_ERREXCEPT) are
independent of each other, but at least one of them must be non-zero.
RATIONALE
In many cases, clever use of floating (fused) multiply-add leads to much improved code; but its
unexpected use by the compiler can undermine carefully written code. The FP_CONTRACT macro can be used to
disallow use of floating multiply-add; and the fma() function guarantees its use where desired. Many
current machines provide hardware floating multiply-add instructions; software implementation can be used
for others.
FUTURE DIRECTIONS
None.
SEE ALSO
feclearexcept(), fetestexcept()
The Base Definitions volume of POSIX.1‐2017, Section 4.20, Treatment of Error Conditions for Mathematical
Functions, <math.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original
IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document.
The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page are most likely to have been introduced
during the conversion of the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 FMA(3POSIX)