Header File 

math.h  

Category 

Math Routines 

Prototype 

double atan2(double y, double x); 

long double atan2l(long double y, long double x); 

Description 

Calculates the arc tangent of y/x. 

atan2 returns the arc tangent of y/x; it produces correct results even when the resulting angle is near pi/2 or -pi/2 (x near 0). If both x and y are set to 0, the function sets the global variable errno to EDOM, indicating a domain error. 

atan2l is the long double version; it takes long double arguments and returns a long double result. 

Return Value 

atan2 and atan2l return a value in the range -pi to pi. Error handling for these functions can be modified through the functions _matherr and _matherrl. 

Example  

#include <stdio.h>
#include <math.h>
int main(void)
{
   double result;
   double x = 90.0, y = 45.0;
   result = atan2(y, x);
   printf("The arc tangent ratio of %lf is %lf\n", (y / x), result);
   return 0;
}

Portability