# fma

## 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.
    


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