acosh, acoshf, acoshl - inverse hyperbolic cosine function

```
#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
```

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

These functions calculate the inverse hyperbolic cosine of `x`

; that is the value whose hyperbolic cosine is `x`

.

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.

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.

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

Interface | Attribute | Value |

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

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

The variant returning `double`

also conforms to SVr4, 4.3BSD.

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

This page is part of release 4.15 of the Linux `man-pages`

project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.