Child process starts after parent process

Question

I have a simple code to test the fork() function.

    #include<stdio.h>
    #include<unistd.h>
    #define MAX_COUNT 10
    void main(void)
    {
        pid_t pid;
        int i;
        fork();
        pid = getpid();
        for(i = 1; i <= MAX_COUNT; i++)
        {
            printf("PID = %d, i = %d\n", pid, i);
        }
    }

It didn't work as I expected.

My expectation is: parent's result and child's result appear alternately. Can someone explain this and teach me how to fix it? Thanks!

    PID = 3663, i = 1
    PID = 3663, i = 2
    PID = 3663, i = 3
    PID = 3663, i = 4
    PID = 3663, i = 5
    PID = 3663, i = 6
    PID = 3663, i = 7
    PID = 3663, i = 8
    PID = 3663, i = 9
    PID = 3663, i = 10
    PID = 3664, i = 1
    PID = 3664, i = 2
    PID = 3664, i = 3
    PID = 3664, i = 4
    PID = 3664, i = 5
    PID = 3664, i = 6
    PID = 3664, i = 7
    PID = 3664, i = 8
    PID = 3664, i = 9
    PID = 3664, i = 10

Answer 1

The explanation is simple. Scheduling of processes is up to the kernel. If this is a single core processor then in this run it decided to suspend the child execution and allow the parent process to run first. The parent ran for a few loops before being suspended in favor of the child, and so forth.
In a multiprocessor system both process can run in tandem but the console device will alternate the output depending on timing of interrupts.

There is no assurance a different run will result in the same output. There is also no assurance that different kernel version won't do something else.

If you want the processes to alternate between loops now is the time to learn interprocess communication.

Answer 2

The main process isn't giving up control immediately, and the for loop executes so fast that it is done before the second process gets scheduled. When I execute your code I get non-sequential prints, but the behavior is system dependent.

It sometimes helps to throw sleep() commands around when you're playing with fork(). Try this code...

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

#define MAX_COUNT 10
int main(void)
{
  pid_t pid;
  int i;
  fork();

  pid = getpid();

  srand(pid); // Make sure each process has a different seed

  for(i = 1; i <= MAX_COUNT; i++)
  {
    printf("PID = %d, i = %d\n", pid, i);

    // Sleep 1-3 seconds.
    unsigned int sleep_seconds = rand() % 3 + 1;
    sleep(sleep_seconds);
  }
}