# atanh - Man Page

inverse hyperbolic tangent function

## Library

Math library (*libm*, *-lm*)

## Synopsis

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

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

**atanhf**(), **atanhl**():

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

## Description

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

## Return Value

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.

## 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*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.

## Attributes

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

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

atanh(), atanhf(), atanhl() | Thread safety | MT-Safe |

## Standards

C11, POSIX.1-2008.

## History

C99, POSIX.1-2001.

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

## Bugs

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.

## See Also

## Referenced By

acosh(3), asinh(3), catanh(3), cosh(3), sinh(3), tanh(3).

The man pages atanhf(3) and atanhl(3) are aliases of atanh(3).