обнаружение ошибок математических функций (detecting errors from mathematical functions)
Имя (Name)
math_error - detecting errors from mathematical functions
Синопсис (Synopsis)
#include <math.h>
#include <errno.h>
#include <fenv.h>
Описание (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.
Примечание (Note)
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 (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).
Смотри также (See also)
gcc(1), errno(3), fenv(3), fpclassify(3), INFINITY(3),
isgreater(3), matherr(3), nan(3)
info libc
