Question

I am reading a book about Operating Systems and am fairly new to C programming as well (tends to complicate things) but am curious as to why I cannot print the incremented i value when every child is created via the fork(). Does it lie with the child processes or something else? Thanks in advance.

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

int main ()
{
    pid_t pid;
    int i = 1;

    pid = fork();
    pid = fork();
    pid = fork();
    pid = fork();

    if (pid == 0) {
        printf("got child %d\n", i);
        i++;
    }
}

Answer 1

You can’t print an incremented value because i is not being printed after being incremented. The processes spun off by fork() don’t share resources, they each get an independent copy of i. So you’ll see:

got child 1

over and over. Try this tweak and you’ll see that each of the child processes are getting in to the conditional with the same value:

int main () {
  pid_t pid;
  int i = 1;
  pid = fork();
  pid = fork();
  pid = fork();
  pid = fork();
  if (pid == 0) {
     printf("the child %d, got %d\n", getpid(), i);
     i++;
  }
}