Updated: 2021/Apr/14

ATAN2(3) Library Functions Manual ATAN2(3)NAMEatan2,atan2f,atan2l- arc tangent function of two variablesLIBRARYMath Library (libm, -lm)SYNOPSIS#include<math.h>doubleatan2(double y, double x); floatatan2f(float y, float x); long doubleatan2l(long double y, long double x);DESCRIPTIONTheatan2(),atan2f(), andatan2l() functions 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.RETURNVALUESTheatan2() function, if successful, returns the arc tangent of y/x in the range [-pi, +pi] radians. If both x and y are zero, the global variable errno is set to EDOM. On the VAX:atan2(y, x) :=atan(y/x) if x > 0, sign(y)*(pi -atan(|y/x|)) if x < 0, 0 if x = y = 0, or sign(y)*pi/2 if x = 0 y.NOTESThe functionatan2() defines "if x > 0,"atan2(0, 0) = 0 on a VAX despite that previouslyatan2(0, 0) may have generated an error message. The reasons for assigning a value toatan2(0, 0) are these: 1. Programs that test arguments to avoid computingatan2(0, 0) must be indifferent to its value. Programs that require it to be invalid are vulnerable to diverse reactions to that invalidity on diverse computer systems. 2. Theatan2() function is used mostly to convert from rectangular (x,y) to polar (r,theta) coordinates that must satisfy x = r*cos theta and y = r*sin theta. These equations are satisfied when (x=0,y=0) is mapped to (r=0,theta=0) on a VAX. In general, conversions to polar coordinates should be computed thus: r := hypot(x,y); ... := sqrt(x*x+y*y) theta := atan2(y,x). 3. The foregoing formulas need not be altered to cope in a reasonable way with signed zeros and infinities on a machine that conforms to IEEE 754; the versions of hypot(3) andatan2() provided for such a machine are designed to handle all cases. That is whyatan2(+-0, -0) = +-pi for instance. In general the formulas above are equivalent to these: r := sqrt(x*x+y*y); if r = 0 then x := copysign(1,x);SEEALSOacos(3), asin(3), atan(3), cos(3), cosh(3), math(3), sin(3), sinh(3), tan(3), tanh(3)STANDARDSTheatan2() function conforms to ISO/IEC 9899:1999 ("ISO C99"). NetBSD 9.99 January 29, 2013 NetBSD 9.99