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# Technical Reference: Base Operating System and Extensions, Volume 1

## atan, atanf, or atanl Subroutine

### Purpose

Computes the arc tangent.

### Syntax

#include <math.h>
float atanf (x)
float x;
long double atanl (x)
long double x;
double atan (x)
double x;

### Description

The atanf, atanl, and atan subroutines compute the principal value of the arc tangent of the x parameter.

An application wishing to check for error situations should set the errno global variable to zero and call feclearexcept(FE_ALL_EXCEPT) before calling these functions. On return,
if errno is nonzero or fetestexcept(FE_INVALID
| FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is nonzero, an error has
occurred.

### Parameters

x |
Specifies the value to be computed. |

### Return Values

Upon successful completion, the atanf, atanl, and atan subroutines return the arc tangent
of x in the range [-pi /2, pi/2] radians.

If x is NaN, a NaN is returned.

If x is 0, x is returned.

If x is ±Inf, ±x/2 is returned.

If x is subnormal, a range error may occur and x is returned.

### Related Information

The atan2f, atan2l, or atan2 Subroutine and atanh, atanhf, or atanhl Subroutine.

math.h in AIX 5L Version 5.2 Files Reference.

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