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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 (consult 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 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 Defi-
       nitions 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(3P)