---
name: "copysign"
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/copysign"
---

# copysign(3)

copy sign of a number

## NAME

copysign, copysignf, copysignl - copy sign of a number

## LIBRARY

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

## SYNOPSIS

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

**double copysign(double** *x***, double** *y***);** **float copysignf(float** *x***, float** *y***);** **long double copysignl(long double** *x***, long double** *y***);**

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

**copysign**(), **copysignf**(), **copysignl**():

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

## DESCRIPTION

These functions return a value whose absolute value matches that of *x*, but whose sign bit matches that of *y*.

For example, *copysign(42.0, -1.0)* and *copysign(-42.0, -1.0)* both return -42.0.

## RETURN VALUE

On success, these functions return a value whose magnitude is taken from *x* and whose sign is taken from *y*.

If *x* is a NaN, a NaN with the sign bit of *y* is returned.

## ERRORS

No errors occur.

## ATTRIBUTES

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

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **copysign**(), **copysignf**(), **copysignl**() | Thread safety | MT-Safe |

## VERSIONS

On architectures where the floating-point formats are not IEEE 754 compliant, these functions may treat a negative zero as positive.

## STANDARDS

C11, POSIX.1-2008.

This function is defined in IEC 559 (and the appendix with recommended functions in IEEE 754/IEEE 854).

## HISTORY

C99, POSIX.1-2001, 4.3BSD.

## SEE ALSO

[**signbit**(3)](/signbit/3.md)