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 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.
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 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.
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
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.
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 4.07 of the Linux man-pages project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
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