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# ATAN

Section: Linux Programmer's Manual (3)
Updated: 2017-09-15

## 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);

```

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).
 Interface Attribute Value atan(), atanf(), atanl() Thread safety MT-Safe

## CONFORMING TO

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

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

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

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
ATTRIBUTES
CONFORMING TO