atan2

#include <math.h>

float atan2 (
  float y,    /* denominator for arc tangent */
  float x);   /* numerator for arc tangent */

The atan2 function calculates the arc tangent of the floating-point ratio y / x. This function uses the signs of both x and y to determine the quadrant of the return value. The floating-point value returned by atan2 is a number in the range -π to π.

The atan2 function returns the arc tangent of y / x.

For y = 0 and x = 0, atan2 → NaN.
For y = 0 and x > 0, atan2 → 0.
For y = 0 and x < 0, atan2 → 2π.

acos, asin, atan

#include <math.h>
#include <stdio.h> /* for printf */

void tst_atan2 (void) {
  float x;
  float y;
  float z;

  x = -1.0;

  for (y = -10.0; y < 10.0; y += 0.1) {
    z = atan2 (y,x);
    printf ("ATAN2(%f/%f) = %f\n", y, x, z);
  }

/* z approaches -pi as y goes from -10 to 0 */
/* z approaches +pi as y goes from +10 to 0 */
}