NAME | SYNOPSIS | DESCRIPTION | NOTES | SEE ALSO | COLOPHON
MATH_ERROR(7) Linux Programmer's Manual MATH_ERROR(7)
math_error - detecting errors from mathematical functions
#include <math.h>
#include <errno.h>
#include <fenv.h>
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 non-zero, or the
following call (see fenv(3)) returns non-zero
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.
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.
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.
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 "overflow"
(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
non-zero, 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.
The math_errhandling identifier specified by C99 and POSIX.1-2001 is not
supported by glibc. This identifier is supposed to indicate which of the two
error-notification mechanisms (errno, exceptions retrievable via
fettestexcept(3)) is in use. The standards require that at least one be in
use, but permit both to be available. The current (version 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.1-2001.
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).
gcc(1), errno(3), fenv(3), fpclassify(3), INFINITY(3), isgreater(3),
matherr(3), nan(3)
info libc
This page is part of release 3.08 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2008-08-11 MATH_ERROR(7)