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 |
[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
#include <cerrno>#include <cfenv>#include <cmath>#include <cstring>#include <iostream>// #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
| floating-point exceptions (macro constant) | |
| macro which expands to POSIX-compatible thread-local error number variable (macro variable) | |
C documentation for math_errhandling | |
