Forks and Processes

Ok im trying to make this simple program in UNIX using forks. What it does is it creates two child processes, and they both should print out a string, and then when the program finishes it prints out the getpid() function. Only three lines should appear, however, my code just jumps to the second child first, then prints that, then prints the first child stuff and prints the second child stuff again. Here's my code, I have no idea what is going on.

http://sourcepost.sytes.net/sourcevi...ource_id=16947

There's a lot of code in there that I can't quite understand the logic of. Like:

Code:

            if(pid != 0)
            {
                while(pid ==0)
                {
                      //do nothing
                }

If pid isn't 0 then how will the while() loop ever do anything? I'd set it up something like this:

Code:

pid1 = fork();
if pid1 is not 0
  pid2 = fork();
else
  print a line from child1
if pid2 is 0
  print a line from child2
if pid1 is not 0 and pid2 is not 0
  wait for children to exit and then print parent's pid

Ok I did what you said but it first prints getpid, then it prints the pid1 and pid2, and they are said to both be 0. heres the code.

http://sourcepost.sytes.net/sourcevi...ource_id=16951

That's one of the side effects. Each process is scheduled independently by the kernel so you really have no way of knowing which message is going to show up first without waiting for children or something like that. If you run the program repeatedly you should see the messages come up in different orders.

The reason it's showing the pid as 0 inside the child process is because that's what it's supposed to do. This is from 'man fork':

Code:

RETURN VALUE
       On  success,  the  PID of the child process is returned in
       the parent's thread of execution, and a 0 is  returned  in
       the child's thread of execution.  On failure, a -1 will be
       returned in the parent's context, no child process will be
       created, and errno will be set appropriately.

If you want to print the pids of the child processes you'll have to call getpid() from inside those processes.

Here's the simplest implementation I can think of; each process printing its own pid:

Code:

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

int main(void)
{
  if(fork())
    fork();

  printf("I am pid %d!\n", getpid());

  return 0;
}

Should give you something like this (assuming fork() doesn't fail).

Code:

I am pid 2913!
I am pid 2914!
I am pid 2912!