MATH_ERRNO, MATH_ERREXCEPT, math_errhandling

From cppreference.com
< clrm; | numericlrm; | math
Common mathematical functions
Functions
Basic operations
(C99)
(C99)
(C99)
(C99)
(C99)
(C99)(C99)(C99)
Exponential functions
(C99)
(C99)
(C99)
(C99)
Power functions
(C99)
(C99)
Trigonometric and hyperbolic functions
(C99)
(C99)
(C99)
Error and gamma functions
(C99)
(C99)
(C99)
(C99)
Nearest integer floating point operations
(C99)(C99)(C99)
(C99)
(C99)(C99)(C99)
Floating point manipulation functions
(C99)(C99)
(C99)
(C99)
Classification
(C99)
(C99)
(C99)
Types
(C99)(C99)
Macro constants
math_errhandlingMATH_ERRNOMATH_ERRNOEXCEPT
(C99)(C99)(C99)
Defined in header <math.h>
#define MATH_ERRNO 1
(since C99)
#define MATH_ERREXCEPT 2
(since C99)
#define math_errhandling /*implementation defined*/
(since C99)

The macro constant math_errhandling expands to an expression of type int that is either equal to MATH_ERRNO, or equal to MATH_ERREXCEPT, or equal to their bitwise OR (MATH_ERRNO | MATH_ERREXCEPT).

The value of math_errhandling indicates the type of error handling that is performed by the floating-point operators and functions:

Constant Explanation
MATH_ERREXCEPT indicates that floating-point exceptions are used: at least FE_DIVBYZERO, FE_INVALID, and FE_OVERFLOW are defined in <fenv.h>.
MATH_ERRNO indicates that floating-point operations use the variable errno to report errors.

If the implementation supports IEEE floating-point arithmetic (IEC 60559), math_errhandling & MATH_ERREXCEPT is required to be non-zero.

The following floating-point error conditions are recognized:

Condition Explanation errno floating-point exception Example
Domain error the argument is outside the range in which the operation is mathematically defined (the description of each function lists the required domain errors) EDOM FE_INVALID acos(2)
Pole error the mathematical result of the function is exactly infinite or undefined ERANGE FE_DIVBYZERO log(0.0), 1.0/0.0
Range error due to overflow the mathematical result is finite, but becomes infinite after rounding, or becomes the largest representable finite value after rounding down ERANGE FE_OVERFLOW pow(DBL_MAX,2)
Range error due to underflow the result is non-zero, but becomes zero after rounding, or becomes subnormal with a loss of precision ERANGE or unchanged (implementation-defined) FE_UNDERFLOW or nothing (implementation-defined) DBL_MIN/2
Inexact result the result has to be rounded to fit in the destination type unchanged FE_INEXACT or nothing (unspecified) sqrt(2), 1.0/10.0

Notes

Whether FE_INEXACT is raised by the mathematical library functions is unspecified in general, but may be explicitly specified in the description of the function (e.g. rint vs nearbyint).

Before C99, floating-point exceptions were not specified, EDOM was required for any domain error, ERANGE was required for overflows and implementation-defined for underflows.

Example

#include <stdio.h>
#include <fenv.h>
#include <math.h>
#include <errno.h>
#pragma STDC FENV_ACCESS ON
int main(void)
{
    printf("MATH_ERRNO is%s\n", math_errhandling & MATH_ERRNO ? "set" : "not set");
    printf("MATH_ERREXCEPT is%s\n",
           math_errhandling & MATH_ERREXCEPT ? "set" : "not set");
    feclearexcept(FE_ALL_EXCEPT);
    errno = 0;
    printf("log(0) =%f\n", log(0));
    if(errno == ERANGE)
        perror("errno == ERANGE");
    if(fetestexcept(FE_DIVBYZERO))
        puts("FE_DIVBYZERO (pole error) reported");
}

Possible output:

MATH_ERRNO is set
MATH_ERREXCEPT is set
log(0) = -inf
errno = ERANGE: Numerical result out of range
FE_DIVBYZERO (pole error) reported

References

  • C11 standard (ISO/IEC 9899:2011):
  • 7.12/9 MATH_ERRNO, MATH_ERREXCEPT, math_errhandling (p: 233)
  • F.10/4 MATH_ERREXCEPT, math_errhandling (p: 517)
  • C99 standard (ISO/IEC 9899:1999):
  • 7.12/9 MATH_ERRNO, MATH_ERREXCEPT, math_errhandling (p: 214)
  • F.9/4 MATH_ERREXCEPT, math_errhandling> (p: 454)

See also

floating-point exceptions
(macro constant)
macro which expands to POSIX-compatible thread-local error number variable
(macro variable)