The atan2() function computes the principal value of the arc tangent of y/x, using the signs of both arguments to determine the quadrant of the return value.

Upon successful completion, atan2() returns the arc tangent of y/x in the range [-pi,pi] radians. If both arguments are 0.0, 0.0 is returned and errno may be set to EDOM.

If x or y is NaN, NaN is returned.

In IEEE754 mode atan2() handles the following exceptional arguments in the spirit of ANSI/IEEE Std 754-1985.

atan2(+-0,x) returns +-0 for x > 0 or x = +0;

atan2(+-0,x) returns +-pi for x < 0 or x = -0;

atan2(y,+-0) returns pi/2 for y > 0;

atan2(y,+-0) returns -pi/2 for y < 0;

atan2(+-y,Inf) returns +-0 for finite y > 0;

atan2(+-Inf,x) returns +-pi/2 for finite x;

atan2(+-y,-Inf) returns +-pi for finite y > 0;

atan2(+-Inf,Inf) returns +-pi/4;

atan2(+-Inf,-Inf) returns +-3pi/4.

For exceptional cases, matherr(3M) tabulates the values to be returned as dictated by Standards other than XPG4.

The atan2() function may fail if:

EDOM

Both arguments are 0.0.

An application wishing to check for error situations should set errno to 0 before calling atan2(). If errno is non-zero on return, or the return value is NaN, an error has occurred.

See attributes(5) for descriptions of the following attributes:

atan(3M), isnan(3M), matherr(3M), tan(3M), attributes(5), standards(5)