Forks & Shared Memory

Question

If a process and its fork have different copies of the data, then why is their pointer the same?

In the example below, if count was shared between the parent and child processes, we would see count: 2. However, count is not shared. But then, why does &count return the same value in both the parent and child process?

Output:

count: 1 0x7fff5a617510
count: 1 0x7fff5a617510

Program:

#include <stdio.h>
#include <pthread.h>
#include <sys/types.h>
#include <unistd.h>

int main() {
    pid_t pid;
    int count = 0;

    pid = fork();

    count++;
    printf("count: %d %p \n", count, &count);

    return 0;
}

It's virtual addressing. The address your process sees doesn't correspond directly to physical addresses. — Daniel Fischer, Jun 14 '13 at 15:40
Hmm. The output for me is: `error: 'new_count' undeclared (first use in this function)`. :) — pilcrow, Jun 14 '13 at 15:41
This question has already been answered here! http://stackoverflow.com/questions/7253659/why-the-address-of-variable-of-child-process-and-parent-process-is-same — Whelchel, Jun 14 '13 at 15:44
@Tatterdemalion Thanks, I could not find that. Should I delete this question? — Luqmaan, Jun 14 '13 at 15:45
@luqmaan That shouldn't be necessary, a duplicate question may make it easier for the next person to find the answer. — Daniel Fischer, Jun 14 '13 at 15:48

score 3 · Answer 1 · answered Jun 14 '13 at 16:22

As noted in comments, this is because of virtual addressing. AFAIK there is no way to see the physical address, as this is handled by the kernel and the MMU.

Also, note than even if count was shared between the two process (using shared memory; see man shmget for example), there would be a race condition.

Forks & Shared Memory

1 Answers1