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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 dif-
       fer (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  feclearex-
       cept(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 indepen-
       dent 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, Stan-
       dard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base  Specifica-
       tions  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 Stan-
       dard,  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)