atan

#include <math.h>

double atan (
  double x);   /* number to calculate arc tangent of */

The atan function calculates the arc tangent of the floating-point number x.

The atan function returns the arc tangent of x.

For x ∈ (-∞, ∞), atan(x) → (-π/2, π/2).
For x = NaN, atan(x) → NaN.
For x = ±INF, atan(x) → ±π/2.

acos, asin, atan2

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

void tst_atan (void) {
  float x;
  float y;

  for (x = -10.0; x <= 10.0; x += 0.1) {
    y = atan (x);
    printf ("ATAN(%f) = %f\n", x, y);
  }
}