atanh, atanhf, atanhl - inverse hyperbolic tangent function

Math library (`libm`

, `-lm`

)

```
#include <math.h>
double atanh(double x);
float atanhf(float x);
long double atanhl(long double x);
```

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

atanh():

```
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| _XOPEN_SOURCE >= 500
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
```

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

These functions calculate the inverse hyperbolic tangent of
`x`

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

.

On success, these functions return the inverse hyperbolic tangent of
`x`

.

If `x`

is a NaN, a NaN is returned.

If `x`

is +0 (-0), +0 (-0) is returned.

If `x`

is +1 or -1, a pole error occurs, and the functions
return **HUGE_VAL**, **HUGE_VALF**, or
**HUGE_VALL**, respectively, with the mathematically
correct sign.

If the absolute value of `x`

is greater than 1, a domain error
occurs, and a NaN is returned.

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`

less than -1 or greater than +1 -
`errno`

is set to**EDOM**. An invalid floating-point exception (**FE_INVALID**) is raised. - Pole error:
`x`

is +1 or -1 -
`errno`

is set to**ERANGE**(but see BUGS). A divide-by-zero floating-point exception (**FE_DIVBYZERO**) is raised.

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

Interface | Attribute | Value |

Thread safety | MT-Safe |

C11, POSIX.1-2008.

C99, POSIX.1-2001.

The variant returning `double`

also conforms to SVr4,
4.3BSD.

In glibc 2.9 and earlier, when a pole error occurs, `errno`

is
set to **EDOM** instead of the POSIX-mandated
**ERANGE**. Since glibc 2.10, glibc does the right
thing.