#include <stdio.h>

void addThree(int *x) *dereferences address
{
    *x = *x + 3;
}

int main()
{
    int x = 5;
    addThree(&x); //send memory address of x to addThree()
    printf("%d\n", x);

    return 0;
}

in C++ you could do this:

void addThree(int &x)
{
    x = x + 3;
}

int main()
{
    int x = 5;
    addThree(x);
    printf("%d\n", x);

    return 0;
}