# math_error - detecting errors from mathematical functions - man(7) - [phpMan] [_math_error_(7)](https://www.chedong.com/phpMan.php/man/matherror/7/markdown) Miscellaneous Information Manual [_math_error_(7)](https://www.chedong.com/phpMan.php/man/matherror/7/markdown) ## NAME math_error - detecting errors from mathematical functions ## SYNOPSIS ### #include ### #include ### #include ## DESCRIPTION When an error occurs, most library functions indicate this fact by returning a special value (e.g., -1 or NULL). Because they typically return a floating-point number, the mathematical functions declared in __ indicate an error using other mechanisms. There are two er‐ ror-reporting mechanisms: the older one sets _errno_; the newer one uses the floating-point ex‐ ception mechanism (the use of [**feclearexcept**(3)](https://www.chedong.com/phpMan.php/man/feclearexcept/3/markdown) and [**fetestexcept**(3)](https://www.chedong.com/phpMan.php/man/fetestexcept/3/markdown), as outlined below) de‐ scribed in [**fenv**(3)](https://www.chedong.com/phpMan.php/man/fenv/3/markdown). A portable program that needs to check for an error from a mathematical function should set _errno_ to zero, and make the following call feclearexcept(FE_ALL_EXCEPT); before calling a mathematical function. Upon return from the mathematical function, if _errno_ is nonzero, or the following call (see [**fenv**(3)](https://www.chedong.com/phpMan.php/man/fenv/3/markdown)) returns nonzero fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW); then an error occurred in the mathematical function. The error conditions that can occur for mathematical functions are described below. ### Domain error A _domain_ _error_ occurs when a mathematical function is supplied with an argument whose value falls outside the domain for which the function is defined (e.g., giving a negative argument to [**log**(3)](https://www.chedong.com/phpMan.php/man/log/3/markdown)). When a domain error occurs, math functions commonly return a NaN (though some functions return a different value in this case); _errno_ is set to **EDOM**, and an "invalid" (**FE_INVALID**) floating-point exception is raised. ### Pole error A _pole_ _error_ occurs when the mathematical result of a function is an exact infinity (e.g., the logarithm of 0 is negative infinity). When a pole error occurs, the function returns the (signed) value **HUGE_VAL**, **HUGE_VALF**, or **HUGE_VALL**, depending on whether the function result type is _double_, _float_, or _long_ _double_. The sign of the result is that which is mathemati‐ cally correct for the function. _errno_ is set to **ERANGE**, and a "divide-by-zero" (**FE_DI‐** **VBYZERO**) floating-point exception is raised. ### Range error A _range_ _error_ occurs when the magnitude of the function result means that it cannot be repre‐ sented in the result type of the function. The return value of the function depends on whether the range error was an overflow or an underflow. A floating result _overflows_ if the result is finite, but is too large to represented in the result type. When an overflow occurs, the function returns the value **HUGE_VAL**, **HUGE_VALF**, or **HUGE_VALL**, depending on whether the function result type is _double_, _float_, or _long_ _double_. _errno_ is set to **ERANGE**, and an "overflow" (**FE_OVERFLOW**) floating-point exception is raised. A floating result _underflows_ if the result is too small to be represented in the result type. If an underflow occurs, a mathematical function typically returns 0.0 (C99 says a function shall return "an implementation-defined value whose magnitude is no greater than the smallest normalized positive number in the specified type"). _errno_ may be set to **ERANGE**, and an "un‐ derflow" (**FE_UNDERFLOW**) floating-point exception may be raised. Some functions deliver a range error if the supplied argument value, or the correct function result, would be _subnormal_. A subnormal value is one that is nonzero, but with a magnitude that is so small that it can't be presented in normalized form (i.e., with a 1 in the most significant bit of the significand). The representation of a subnormal number will contain one or more leading zeros in the significand. ## NOTES The _math_errhandling_ identifier specified by C99 and POSIX.1 is not supported by glibc. This identifier is supposed to indicate which of the two error-notification mechanisms (_errno_, ex‐ ceptions retrievable via [**fetestexcept**(3)](https://www.chedong.com/phpMan.php/man/fetestexcept/3/markdown)) is in use. The standards require that at least one be in use, but permit both to be available. The current (glibc 2.8) situation under glibc is messy. Most (but not all) functions raise exceptions on errors. Some also set _errno_. A few functions set _errno_, but don't raise an exception. A very few functions do neither. See the individual manual pages for details. To avoid the complexities of using _errno_ and [**fetestexcept**(3)](https://www.chedong.com/phpMan.php/man/fetestexcept/3/markdown) for error checking, it is often advised that one should instead check for bad argument values before each call. For example, the following code ensures that [**log**(3)](https://www.chedong.com/phpMan.php/man/log/3/markdown)'s argument is not a NaN and is not zero (a pole error) or less than zero (a domain error): double x, r; if (isnan(x) || islessequal(x, 0)) { /* Deal with NaN / pole error / domain error */ } r = log(x); The discussion on this page does not apply to the complex mathematical functions (i.e., those declared by __), which in general are not required to return errors by C99 and POSIX.1. The [**gcc**(1)](https://www.chedong.com/phpMan.php/man/gcc/1/markdown) _-fno-math-errno_ option causes the executable to employ implementations of some mathematical functions that are faster than the standard implementations, but do not set _er‐_ _rno_ on error. (The [**gcc**(1)](https://www.chedong.com/phpMan.php/man/gcc/1/markdown) _-ffast-math_ option also enables _-fno-math-errno_.) An error can still be tested for using [**fetestexcept**(3)](https://www.chedong.com/phpMan.php/man/fetestexcept/3/markdown). ## SEE ALSO [**gcc**(1)](https://www.chedong.com/phpMan.php/man/gcc/1/markdown), [**errno**(3)](https://www.chedong.com/phpMan.php/man/errno/3/markdown), [**fenv**(3)](https://www.chedong.com/phpMan.php/man/fenv/3/markdown), [**fpclassify**(3)](https://www.chedong.com/phpMan.php/man/fpclassify/3/markdown), [**INFINITY**(3)](https://www.chedong.com/phpMan.php/man/INFINITY/3/markdown), [**isgreater**(3)](https://www.chedong.com/phpMan.php/man/isgreater/3/markdown), [**matherr**(3)](https://www.chedong.com/phpMan.php/man/matherr/3/markdown), [**nan**(3)](https://www.chedong.com/phpMan.php/man/nan/3/markdown) _info_ _libc_ Linux man-pages 6.7 2023-10-31 [_math_error_(7)](https://www.chedong.com/phpMan.php/man/matherror/7/markdown)