#include <math.h>
/* or */
#include <stdlib.h>

int abs (
  int val);     /* number to take absolute value of */

The abs function determines the absolute value of the integer val.

The abs function returns the absolute value of val.

fabs, labs

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

void tst_abs (void) {
  int x;
  int y;

  x = -42;
  y = abs (x);

  printf ("ABS(%d) = %d\n", x, y);
}