ATAN2(P) POSIX Programmer's Manual ATAN2(P)
atan2, atan2f, atan2l - arc tangent functions
double atan2(double y, double x);
float atan2f(float y, float x);
long double atan2l(long double y, long double x);
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.
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.
These functions may fail if:
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.
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.
atan() , feclearexcept() , fetestexcept() , isnan() , tan() , the Base Definitions volume
of IEEE Std 1003.1-2001, Section 4.18, Treatment of Error Conditions for Mathematical
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 orig-
inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 ATAN2(P)