# acosh man page

acosh, acoshf, acoshl — inverse hyperbolic cosine function

## Synopsis

**#include <math.h>**
**double acosh(double** *x***);**
**float acoshf(float** *x***);**
**long double acoshl(long double** *x***);**

`Link with `

*-lm*.

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):**acosh**():

_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L

|| _XOPEN_SOURCE >= 500

|| /* Since glibc 2.19: */ _DEFAULT_SOURCE

|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

**acoshf**(), **acoshl**():

_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L

|| /* Since glibc 2.19: */ _DEFAULT_SOURCE

|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

## Description

These functions calculate the inverse hyperbolic cosine of *x*; that is the value whose hyperbolic cosine is *x*.

## Return Value

On success, these functions return the inverse hyperbolic cosine of *x*.

If *x* is a NaN, a NaN is returned.

If *x* is +1, +0 is returned.

If *x* is positive infinity, positive infinity is returned.

If *x* is less than 1, a domain error occurs, and the functions return a NaN.

## Errors

See math_error(7) for information on how to determine whether an error has occurred when calling these functions.

The following errors can occur:

- Domain error:
*x*is less than 1 *errno*is set to**EDOM**. An invalid floating-point exception (**FE_INVALID**) is raised.

## Attributes

For an explanation of the terms used in this section, see attributes(7).

Interface | Attribute | Value |
---|---|---|

acosh(), acoshf(), acoshl() | Thread safety | MT-Safe |

## Conforming to

C99, POSIX.1-2001, POSIX.1-2008.

The variant returning *double* also conforms to SVr4, 4.3BSD, C89.

## See Also

## Colophon

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## Referenced By

asinh(3), atanh(3), cacosh(3), cosh(3), sinh(3), tanh(3).

acoshf(3) and acoshl(3) are aliases of acosh(3).