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_TRUE_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


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.


#include <cerrno>
#include <cfenv>
#include <cmath>
#include <cstring>
#include <iostream>
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';
    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:

log(0) = -inf
errno = ERANGE (Numerical result out of range)
FE_DIVBYZERO (pole error) reported

See also

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

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