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# MATH_ERRNO, MATH_ERREXCEPT, math_errhandling

From cppreference.com
< cpp‎ | numeric‎ | math

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

### [edit] 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.

### [edit] Example

Run this code

    #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

### [edit] See also

FE_ALL_EXCEPTFE_DIVBYZEROFE_INEXACTFE_INVALIDFE_OVERFLOWFE_UNDERFLOW (C++11) | floating-point exceptions (macro constant) [edit]
errno | macro which expands to POSIX-compatible thread-local error number variable (macro variable) [edit]
C documentation for math_errhandling

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