---
name: "cos"
section: "3"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["linux", "freebsd"]
url: "https://new.carta.tech/cos"
---

# cos(3)

cosine functions

## NAME

**cos**, **cosf**, **cosl** – cosine functions

## LIBRARY

Math Library (libm, -lm)

## SYNOPSIS

**#include \<math.h\>**

*double*\

**cos**(*double x*);

*float*\

**cosf**(*float x*);

*long* *double*\

**cosl**(*long double x*);

## DESCRIPTION

The **cos**(), **cosf**(), and **cosl**() functions compute the cosine of *x* (measured in radians). A large magnitude argument may yield a result with little or no significance. For a discussion of error due to roundoff, see [math(3)](/math/3.md).

## RETURN VALUES

The **cos**(), **cosf**(), and **cosl**() functions return the cosine value.

## SEE ALSO

[acos(3)](/acos/3.md), [asin(3)](/asin/3.md), [atan(3)](/atan/3.md), [atan2(3)](/atan2/3.md), [ccos(3)](/ccos/3.md), [cosh(3)](/cosh/3.md), [math(3)](/math/3.md), [sin(3)](/sin/3.md), [sinh(3)](/sinh/3.md), [tan(3)](/tan/3.md), [tanh(3)](/tanh/3.md)

## STANDARDS

These functions conform to ISO/IEC 9899:1999 (“ISO C99”).

## HISTORY

The **cos**() function first appeared in Version 1 AT&T UNIX.