---
name: "nan"
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/nan"
---

# nan(3)

return 'Not a Number'

## NAME

nan, nanf, nanl - return 'Not a Number'

## LIBRARY

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

## SYNOPSIS

```
#include <math.h>
```

**double nan(const char \****tagp***);** **float nanf(const char \****tagp***);** **long double nanl(const char \****tagp***);**

Feature Test Macro Requirements for glibc (see [**feature\_test\_macros**(7)](/feature_test_macros/7.md)):

**nan**(), **nanf**(), **nanl**():

```
    _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
```

## DESCRIPTION

These functions return a representation (determined by *tagp*) of a quiet NaN. If the implementation does not support quiet NaNs, these functions return zero.

The call *nan("char-sequence")* is equivalent to:

```
strtod("NAN(char-sequence)", NULL);
```

Similarly, calls to **nanf**() and **nanl**() are equivalent to analogous calls to [**strtof**(3)](/strtof/3.md) and [**strtold**(3)](/strtold/3.md).

The argument *tagp* is used in an unspecified manner. On IEEE 754 systems, there are many representations of NaN, and *tagp* selects one. On other systems it may do nothing.

## ATTRIBUTES

For an explanation of the terms used in this section, see [**attributes**(7)](/attributes/7.md).

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **nan**(), **nanf**(), **nanl**() | Thread safety | MT-Safe locale |

## STANDARDS

C11, POSIX.1-2008.

See also IEC 559 and the appendix with recommended functions in IEEE 754/IEEE 854.

## HISTORY

glibc 2.1. C99, POSIX.1-2001.

## SEE ALSO

[**isnan**(3)](/isnan/3.md), [**strtod**(3)](/strtod/3.md), [**math\_error**(7)](/math_error/7.md)