fpclassify man page

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);

Link with -lm.

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

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



   || /* Since glibc 2.19: */ _DEFAULT_SOURCE
   || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE


   || /* Since glibc 2.19: */ _DEFAULT_SOURCE
   || /* Glibc versions <= 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

Conforming to

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.)

See Also

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


This page is part of release 4.16 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/.

Referenced By

finite(3), INFINITY(3), isgreater(3), math_error(7), nan(3).

The man pages isfinite(3), isinf(3), isnan(3) and isnormal(3) are aliases of fpclassify(3).

2017-09-15 Linux Programmer's Manual