atan2(3M)

NAME

     atan2 - arc tangent function

SYNOPSIS

     cc [ flag ... ] file ... -lm [ library ... ]
     #include <math.h>

     double atan2(double y, double x);

DESCRIPTION

     The atan2() function computes 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.

RETURN VALUES

     Upon successful completion, atan2() returns the arc  tangent
     of  y/x in the range [-pi,pi] radians. If both arguments are
     0.0, 0.0 is returned and errno may be set to EDOM.

     If x or y is NaN, NaN is returned.

     In IEEE754 mode atan2() handles  the  following  exceptional
     arguments in the spirit of ANSI/IEEE Std 754-1985.

               atan2(_0,x) returns _0 for x > 0 or x = +0;

               atan2(_0,x) returns _pi for x < 0 or x = -0;

               atan2(y,_0) returns pi/2 for y > 0;

               atan2(y,_0) returns -pi/2 for y < 0;

               atan2(_y,Inf) returns _0 for finite y > 0;

               atan2(_Inf,x) returns _pi/2 for finite x;

               atan2(_y,-Inf) returns _pi for finite y > 0;

               atan2(_Inf,Inf) returns _pi/4;

               atan2(_Inf,-Inf) returns _3pi/4.

     For exceptional cases, matherr(3M) tabulates the  values  to
     be returned as dictated by Standards other than XPG4.

ERRORS

     The atan2() function may fail if:

     EDOM  Both arguments are 0.0.

USAGE

     An application wishing to check for error situations  should
     set  errno to 0 before calling atan2(). If errno is non-zero
     on return,  or  the  return  value  is  NaN,  an  error  has
     occurred.

ATTRIBUTES

     See attributes(5) for descriptions of the  following  attri-
     butes:

     ____________________________________________________________
    |       ATTRIBUTE TYPE        |       ATTRIBUTE VALUE       |
    |_____________________________|_____________________________|
    | MT-Level                    | MT-Safe                     |
    |_____________________________|_____________________________|

SEE ALSO

     atan(3M), isnan(3M),  matherr(3M),  tan(3M),  attributes(5),
     standards(5)