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 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.

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 .