# ATAN

NAME

SYNOPSIS

DESCRIPTION

RETURN VALUE

ERRORS

ATTRIBUTES

CONFORMING TO

SEE ALSO

COLOPHON

## NAME

atan, atanf,
atanl - arc tangent function

## SYNOPSIS

**#include
<math.h>**

**double
atan(double** *x***); **

float atanf(float *x***); **

long double atanl( long double *x***);**

Link with
*-lm*.

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

**atanf**(),
**atanl**():

_ISOC99_SOURCE ||
_POSIX_C_SOURCE >= 200112L

|| /* Since glibc 2.19: */ _DEFAULT_SOURCE

|| /* Glibc versions <= 2.19: */ _BSD_SOURCE ||
_SVID_SOURCE

## DESCRIPTION

These functions
calculate the principal value of the arc tangent of
*x*; that is the value whose tangent is *x*.

## RETURN VALUE

On success,
these functions return the principal value of the arc
tangent of *x* in radians; the return value is in the
range [-pi/2, pi/2].

If *x* is
a NaN, a NaN is returned.

If *x* is
+0 (-0), +0 (-0) is returned.

If *x* is
positive infinity (negative infinity), +pi/2 (-pi/2) is
returned.

## ERRORS

No errors
occur.

## ATTRIBUTES

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

## CONFORMING TO

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

The variant
returning *double* also conforms to SVr4, 4.3BSD,
C89.

## SEE ALSO

**acos**(3),
**asin**(3), **atan2**(3), **carg**(3),
**catan**(3), **cos**(3), **sin**(3),
**tan**(3)

## COLOPHON

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