MATH_ERRNO, MATH_ERREXCEPT, math_errhandling
Description
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 |
Declarations
#define MATH_ERRNO
#define MATH_ERREXCEPT
#define math_errhandling /*implementation defined*/
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.
Examples
#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";
}
MATH_ERRNO is set
MATH_ERREXCEPT is set
log(0) = -inf
errno = ERANGE (Numerical result out of range)
FE_DIVBYZERO (pole error) reported