abs (isn't a function but a macro) is used for calculating the absolute value of a number.
#include <stdio.h>
#include <math.h>
int main()
{
int n, result;
printf("Enter an integer to calculate its absolute value\n");
scanf("%d", &n);
result = abs(n);
printf("Absolute value of %d = %d\n", n, result);
return 0;