
ATAN2(P) POSIX Programmer's Manual ATAN2(P)
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 nonzero,
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 nonzero, then errno shall be
set to [ERANGE]. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non
zero, then the underflow floatingpoint 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 nonzero.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
atan() , feclearexcept() , fetestexcept() , isnan() , tan() , the Base Definitions volume
of IEEE Std 1003.12001, 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) 20012003 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 orig
inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 ATAN2(P) 
