math_error

MATH_ERROR(7)               Linux Programmer's 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 (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).

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 5.08 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/.



Linux                              2017-09-15                      MATH_ERROR(7)