Why fork() use the same variable but different value?

Here's the code:

#include <stdio.h>                                                                                                                                                            
#include <unistd.h>

void f(int&);
void g(int&);

int main(int argc, char **argv)
{
    printf("--beginning of program\n");

    int counter = 0;
    pid_t pid = fork();
    if (pid == 0) {
        f(counter);
        printf("child process: %d, %p", counter, &counter);
    } else if (pid>0) {
        g(counter);
        for (int i=0; i<5; ++i) {
            sleep(1);
            printf("parent process: %d, %p\n", counter, &counter);
        }
    }
    printf("--end of program--\n");
    return 0;
}

void f(int& counter) {
    counter = 1;
    printf("in f: %d, %p-\n", counter, &counter);
}
void g(int& counter){
} 

and here's the result:

--beginning of program
in f: 1, 0x7ffc9b01c6a4-
child process: 1, 0x7ffc9b01c6a4--end of program--
parent process: 0, 0x7ffc9b01c6a4
parent process: 0, 0x7ffc9b01c6a4
parent process: 0, 0x7ffc9b01c6a4
parent process: 0, 0x7ffc9b01c6a4
parent process: 0, 0x7ffc9b01c6a4
--end of program--

Clearly in child process it's the same parameter with the same address, but the value is different.

Why is it happening?