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ATAN2(P)		   POSIX Programmer's Manual		      ATAN2(P)

NAME
       atan2, atan2f, atan2l - arc tangent functions

SYNOPSIS
       #include <math.h>

       double atan2(double y, double x);
       float atan2f(float y, float x);
       long double atan2l(long double y, long double x);

DESCRIPTION
       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.

RETURN VALUE
       Upon  successful	 completion, these functions shall return the arc tan‐
       gent 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.

ERRORS
       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.

EXAMPLES
       None.

APPLICATION USAGE
       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.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       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>

COPYRIGHT
       Portions of this text are reprinted and reproduced in  electronic  form
       from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX),	The  Open  Group  Base
       Specifications  Issue  6,  Copyright  (C) 2001-2003 by the Institute of
       Electrical and Electronics Engineers, Inc and The Open  Group.  In  the
       event of any discrepancy between this version and the original IEEE and
       The Open Group Standard, the original IEEE and The Open Group  Standard
       is  the	referee document. The original Standard can be obtained online
       at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group		     2003			      ATAN2(P)
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