math_error(7) | Miscellaneous Information Manual | math_error(7) |

# NAME

math_error - detecting errors from mathematical functions

# SYNOPSIS

#include <math.h>#include <errno.h>#include <fenv.h>

# 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
*<math.h>* indicate an error using other mechanisms. There are
two error-reporting mechanisms: the older one sets *errno*; the newer
one uses the floating-point exception mechanism (the use of
feclearexcept(3) and fetestexcept(3), as outlined below)
described in fenv(3).

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)) 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)).
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 mathematically correct
for the function. *errno* is set to **ERANGE**, and a
"divide-by-zero" (**FE_DIVBYZERO**) floating-point exception is
raised.

## Range error

A *range error* occurs when the magnitude of the function
result means that it cannot be represented 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 "underflow" (**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*, exceptions
retrievable via fetestexcept(3)) 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) 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)'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 *<complex.h>*),
which in general are not required to return errors by C99 and POSIX.1.

The gcc(1) *-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 *errno* on
error. (The gcc(1) *-ffast-math* option also enables
*-fno-math-errno*.) An error can still be tested for using
fetestexcept(3).

# SEE ALSO

gcc(1), errno(3), fenv(3), fpclassify(3), INFINITY(3), isgreater(3), matherr(3), nan(3)

*info libc*

2022-12-04 | Linux man-pages 6.02 |