MATH_ERRNO, MATH_ERREXCEPT, math_errhandling

Defined in header `<cmath>`
#define MATH_ERRNO 1		(since C++11)
#define MATH_ERREXCEPT 2		(since C++11)
#define math_errhandling /implementation defined/		(since C++11)

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 `<cfenv>`.
`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`	`std::acos(2)`
Pole error	the mathematical result of the function is exactly infinite or undefined	`ERANGE`	`FE_DIVBYZERO`	`std::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`	`std::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)	`std::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. std::rint vs std::nearbyint).

Before C++11, 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 <iostream>
#include <cfenv>
#include <cmath>
#include <cerrno>
#include <cstring>
#pragma STDC FENV_ACCESS ON
int main()
{
    std::cout << "MATH_ERRNO is "
              << (math_errhandling & MATH_ERRNO ? "set" : "not set") << '\n'
              << "MATH_ERREXCEPT is "
              << (math_errhandling & MATH_ERREXCEPT ? "set" : "not set") << '\n';
    std::feclearexcept(FE_ALL_EXCEPT);
    errno = 0;
    std::cout <<  "log(0) = " << std::log(0) << '\n';
    if(errno == ERANGE)
            std::cout << "errno = ERANGE (" << std::strerror(errno) << ")\n";
    if(std::fetestexcept(FE_DIVBYZERO))
        std::cout << "FE_DIVBYZERO (pole error) reported\n";
}

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

FE_ALL_EXCEPTFE_DIVBYZEROFE_INEXACTFE_INVALIDFE_OVERFLOWFE_UNDERFLOW (C++11)	floating-point exceptions (macro constant)
errno	macro which expands to POSIX-compatible thread-local error number variable (macro variable)

MATH_ERRNO, MATH_ERREXCEPT, math_errhandling

Notes

Example

See also