# Section (3) fpclassify

## Name

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

## Synopsis

`#include <math.h>`
 ```int fpclassify(``` x`)`;

 ```int isfinite(``` x`)`;

 ```int isnormal(``` x`)`;

 ```int isnan(``` x`)`;

 ```int isinf(``` x`)`; Note
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 versions <= 2.19:
*/ `_BSD_SOURCE` || `_SVID_SOURCE`
`isinf`():
`_ISOC99_SOURCE` || `_POSIX_C_SOURCE` >= 200112L || /* Since glibc 2.19:
*/ `_DEFAULT_SOURCE` || /* Glibc versions <= 2.19:
*/ `_BSD_SOURCE` || `_SVID_SOURCE`
Note Link with `−lm`.

## DESCRIPTION

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.

## ATTRIBUTES

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.

## NOTES

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.)

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