# atanh - Man Page

inverse hyperbolic tangent function

## Library

Math library (libm, -lm)

## Synopsis

```#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```

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).

InterfaceAttributeValue

## 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.