---
name: "isnan"
section: "3"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux", "freebsd"]
url: "https://new.carta.tech/fpclassify"
---

# isnan(3)

floating-point classification macros

Part of [fpclassify(3)](/fpclassify/3.md).

## NAME

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

## LIBRARY

Math library (*libm*, *-lm*)

## SYNOPSIS

```
#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)](/feature_test_macros/7.md)):

**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
```

## 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)](/attributes/7.md).

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

## STANDARDS

C11, POSIX.1-2008.

## HISTORY

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.)

## NOTES

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

## SEE ALSO

[**finite**(3)](/finite/3.md), [**INFINITY**(3)](/INFINITY/3.md), [**isgreater**(3)](/isgreater/3.md), [**signbit**(3)](/signbit/3.md)