fpclassify, isfinite, isnormal, isnan, isinf - floating-point classification macros

#include <math.h>
int fpclassify(x);
int isfinite(x);
int isnormal(x);
int isnan(x);
int isinf(x);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

    _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L

    _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
        || _XOPEN_SOURCE
        || /* Since glibc 2.19: */ _DEFAULT_SOURCE
        || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

    _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
        || /* Since glibc 2.19: */ _DEFAULT_SOURCE
        || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

Floating point numbers can have special values, such as infinite or NaN. With the macro fpclassify(x) you can find out what type x is. The macro takes any floating-point expression as argument. The result is one of the following values:

FP_NAN

x is "Not a Number".

FP_INFINITE

x is either positive infinity or negative infinity.

FP_ZERO

x is zero.

FP_SUBNORMAL

x is too small to be represented in normalized format.

FP_NORMAL

if nothing of the above is correct then it must be a normal floating-point number.

The other macros provide a short answer to some standard questions.

isfinite(x)

returns a nonzero value if
(fpclassify(x) != FP_NAN && fpclassify(x) != FP_INFINITE)

isnormal(x)

returns a nonzero value if (fpclassify(x) == FP_NORMAL)

isnan(x)

returns a nonzero value if (fpclassify(x) == FP_NAN)

isinf(x)

returns 1 if x is positive infinity, and -1 if x is negative infinity.

finite(3), INFINITY(3), isgreater(3), signbit(3)