double atan2(double , double );

calculates arctangent from 2 numbers

compile in linux with: gcc atan2.c -o atan2 -lm -Wall

#include <stdio.h> /* including standard library */
//#include <windows.h> /* uncomment this for Windows */
 
#include <math.h> /* including math library */
 
int main ( void ){
    double a=1.2, b=-2.6;
    printf("atan2 from %f and %f is %f\n", a, b, atan2(a, b) * 90 / 3.17);
    return 0;
}

  output:
  cd@najai:~$ ./atan2
  atan2 from 1.200000 and -2.600000 is 76.916921