MATH_ERROR(7) Linux Programmer's Manual MATH_ERROR(7)
SYNOPSIS
#include <<math.h>>
#include <<errno.h>>
#include <<fenv.h>>
NAME
math_error  detecting errors from mathematical functions
DESCRIPTION
On error, many of the mathematical functions declared in <math.h>
return a NaN (not a number). However, rather than looking at the
return value (which is not always possible) one can also check whether
an error was signaled. There are two signaling mechanisms: the older
one sets errno; the newer one uses the floatingpoint exception mecha
nism (the use of feclearexcept(3) and fetestexcept(3), as outlined
below) described in fenv(3).
C99 and POSIX.12001 specify a math_errhandling identifier, which is
supposed to indicate which of these two mechanisms is in use; the stan
dards require that at least one be in use, but permit both to be avail
able. Although glibc does not support this identifier, in practice it
supports both mechanisms.
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, errno is set to EDOM, and an "invalid" (FE_INVALID)
floatingpoint 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 "dividebyzero" (FE_DIVBYZERO) floatingpoint 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 func
tion 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) floatingpoint
exception is raised.
A floating result underflows if the result is too small to be repre
sented in the result type. If an underflow occurs, a mathematical
function typically returns 0.0 (C99 says a function shall return "an
implementationdefined 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) floatingpoint
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 sig
nificant 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.12001 is
not supported.
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.12001.
The gcc(1) fnomatherrno 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)
ffastmath option also enables fnomatherrno.) 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
COLOPHON
This page is part of release 3.05 of the Linux manpages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/manpages/.
Linux 20080721 MATH_ERROR(7)
