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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.

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>