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

isnan():


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

isinf():


_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

POSIX.1-2001, POSIX.1-2008, C99.

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

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.)

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

2022-12-15 Linux man-pages 6.02