# Man Page

### Manual Section... (3) - page: atan2l

## NAME

atan2, atan2f, atan2l - arc tangent function of two variables## SYNOPSIS

Link with#include <math.h>double atan2(doubley, doublex);float atan2f(floaty, floatx);long double atan2l(long doubley, long doublex);

*-lm*.

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

**atan2f**(),
**atan2l**():
_BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600 || _ISOC99_SOURCE; or
*cc -std=c99*

## DESCRIPTION

The**atan2**() function calculates the principal value of the arc tangent of

*y/x*, using the signs of the two arguments to determine the quadrant of the result.

## RETURN VALUE

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

If
*y*
is +0 (-0) and
*x*
is less than 0, +pi (-pi) is returned.

If
*y*
is +0 (-0) and
*x*
is greater than 0, +0 (-0) is returned.

If
*y*
is less than 0 and
*x*
is +0 or -0, -pi/2 is returned.

If
*y*
is greater than 0 and
*x*
is +0 or -0, pi/2 is returned.

If either
*x*
or
*y*
is NaN, a NaN is returned.

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

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

If
*y*
is a finite value greater (less) than 0, and
*x*
is negative infinity, +pi (-pi) is returned.

If
*y*
is a finite value greater (less) than 0, and
*x*
is positive infinity, +0 (-0) is returned.

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

If
*y*
is positive infinity (negative infinity) and
*x*
is negative infinity, +3*pi/4 (-3*pi/4) is returned.

If
*y*
is positive infinity (negative infinity) and
*x*
is positive infinity, +pi/4 (-pi/4) is returned.

## ERRORS

No errors occur.## CONFORMING TO

C99, POSIX.1-2001. The variant returning*double*also conforms to SVr4, 4.3BSD, C89.

## SEE ALSO

**acos**(3),

**asin**(3),

**atan**(3),

**carg**(3),

**cos**(3),

**sin**(3),

**tan**(3)

## COLOPHON

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## Index

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Time: 15:26:42 GMT, June 11, 2010