ATAN2(3P)                                           POSIX Programmer's Manual                                          ATAN2(3P)



PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (con-
       sult the corresponding Linux manual page for details of Linux behavior), or the  interface  may  not  be  implemented  on
       Linux.

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 deter-
       mine 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  inte-
       ger  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, Copy-
       right (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.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                     ATAN2(3P)