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.

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