**#include <math.h>
**

**
double atan2(double** *y***, double** *x***);
float atan2f(float**

long double atan2l(long double

These functions shall compute the principal value of the arc tangent
of *y*/ *x*, using the signs of both arguments to
determine the quadrant of the return value.

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

Upon successful completion, these functions shall return the arc tangent
of *y*/ *x* in the range [-pi,pi] radians.

If *y* is ±0 and *x* is < 0, ±pi shall be returned.

If *y* is ±0 and *x* is > 0, ±0 shall be returned.

If *y* is < 0 and *x* is ±0, -pi/2 shall be returned.

If *y* is > 0 and *x* is ±0, pi/2 shall be returned.

If *x* is 0, a pole error shall not occur.

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

If the result underflows, a range error may occur and *y*/ *x*
should be returned.

If *y* is ±0 and *x* is -0, ±pi shall be returned.

If *y* is ±0 and *x* is +0, ±0 shall be returned.

For finite values of ± *y* > 0, if *x* is -Inf, ±pi
shall be
returned.

For finite values of ± *y* > 0, if *x* is +Inf, ±0 shall
be returned.

For finite values of *x*, if *y* is ±Inf, ±pi/2 shall
be
returned.

If *y* is ±Inf and *x* is -Inf, ±3pi/4 shall be returned.

If *y* is ±Inf and *x* is +Inf, ±pi/4 shall be returned.

If both arguments are 0, a domain error shall not occur.

These functions may fail if:

- Range Error
- The result underflows.

If the integer expression (math_errhandling & MATH_ERRNO) is non-zero,
then *errno* shall be set to [ERANGE]. If the
integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero,
then the underflow floating-point exception shall be
raised.

*The following sections are informative.*

On error, the expressions (math_errhandling & MATH_ERRNO) and (math_errhandling & MATH_ERREXCEPT) are independent of each other, but at least one of them must be non-zero.

*atan*() , *feclearexcept*() , *fetestexcept*() , *isnan*()
, *tan*() , the
Base Definitions volume of IEEE Std 1003.1-2001, Section 4.18, Treatment
of Error Conditions for Mathematical Functions, *<math.h>*

- NAME
- SYNOPSIS
- DESCRIPTION
- RETURN VALUE
- ERRORS
- EXAMPLES
- APPLICATION USAGE
- RATIONALE
- FUTURE DIRECTIONS
- SEE ALSO
- COPYRIGHT

Time: 21:42:00 GMT, April 16, 2011