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.
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
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
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.
SEE ALSOatan(), 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>
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)