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This manual page is part of the POSIX Programmer's Manual. The Linux implementation of
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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 .