fpclassify(3) Library Functions Manual fpclassify(3)

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

Math library (libm, -lm)

#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)):

fpclassify(), isfinite(), isnormal():

    _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:

x is "Not a Number".
x is either positive infinity or negative infinity.
x is zero.
x is too small to be represented in normalized format.
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.

returns a nonzero value if
(fpclassify(x) != FP_NAN && fpclassify(x) != FP_INFINITE)
returns a nonzero value if (fpclassify(x) == FP_NORMAL)
returns a nonzero value if (fpclassify(x) == FP_NAN)
returns 1 if x is positive infinity, and -1 if x is negative infinity.

For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value
fpclassify (), isfinite (), isnormal (), isnan (), isinf () Thread safety MT-Safe

C11, POSIX.1-2008.

POSIX.1-2001, C99.

In glibc 2.01 and earlier, isinf() returns a nonzero value (actually: 1) if x is positive infinity or negative infinity. (This is all that C99 requires.)

For isinf(), the standards merely say that the return value is nonzero if and only if the argument has an infinite value.

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

2024-05-02 Linux man-pages 6.9.1