MATH_ERRNO, MATH_ERREXCEPT, math_errhandling

 

C++

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Technical Specifications

 

Numerics library

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Common mathematical functions

Functions
Basic operations
abs(int)labsllabsimaxabs (C++11) abs(float)fabs divldivlldivimaxdiv (C++11) fmod	remainder (C++11) remquo (C++11) fma (C++11) fmax (C++11) fmin (C++11) fdim (C++11) nannanfnanl (C++11)(C++11)(C++11)
Exponential functions
exp exp2 (C++11) expm1 (C++11)	log log10 log1p (C++11) log2 (C++11)
Power functions
sqrt cbrt (C++11)	hypot (C++11) pow
Trigonometric and hyperbolic functions
sin cos tan asin acos atan atan2	sinh cosh tanh asinh (C++11) acosh (C++11) atanh (C++11)

Error and gamma functions
erf (C++11) erfc (C++11)	lgamma (C++11) tgamma (C++11)
Nearest integer floating point operations
ceil floor roundlroundllround (C++11)(C++11)(C++11)	trunc (C++11) nearbyint (C++11) rintlrintllrint (C++11)(C++11)(C++11)
Floating point manipulation functions
ldexp scalbnscalbln (C++11)(C++11) ilogb (C++11) logb (C++11)	frexp modf nextafternexttoward (C++11)(C++11) copysign (C++11)
Classification/Comparison
fpclassify (C++11) isfinite (C++11) isinf (C++11) isnan (C++11) isnormal (C++11) signbit (C++11)	isgreater (C++11) isgreaterequal (C++11) isless (C++11) islessequal (C++11) islessgreater (C++11) isunordered (C++11)
Macro constants
HUGE_VALFHUGE_VALHUGE_VALL (C++11)(C++11) INFINITY (C++11) NAN (C++11) math_errhandlingMATH_ERRNOMATH_ERREXCEPT (C++11)	FP_NORMALFP_SUBNORMALFP_ZEROFP_INFINITEFP_NAN (C++11)(C++11)(C++11)(C++11)(C++11)

 

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

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

See also

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)