about fork(), a few questions

feetyouwell · 09-16-2004, 12:29 AM

I wrote a program as following in c

<all the include stuff>

for (i = 1; i<=3; i ++) {
fork();
printf("the fork number is %i", i);
}

return 0;

the results for that is:
The fork number is 1
The fork number is 2
The fork number is 3
The fork number is 3
The fork number is 2
The fork number is 1
The fork number is 2
The fork number is 2
The fork number is 3
The fork number is 3
[user@localhost c]$ The fork number is 3
The fork number is 3
The fork number is 3
The fork number is 3

why is it? I thought i was only expecting 3 results since I only fork() three times, only three child processes are created right? When i run fork() for the first time, does it create a parent process or child process? If all fork() creates is child process, when how can i find about which / where is the parent for the child processes? Is the parent process the program I am running? if so, then I should see 3 * 3 = 9 results, why there are 14 printouts? Please advise.

itsme86 · 09-16-2004, 12:41 AM

Try this:

Code:

pid_t pid;

for(i = 0;i < 3;++i)
{
  pid = fork();
  if(!pid)
  {
    printf("I'm in the child process! fork number %d\n", i);
    exit(0);
  }
  printf("I'm in the parent process! Child process created. fork number %d, pid %d\n", i, pid);
}

fork() returns twice, once in the parent and once in the child. It returns 0 to the child process and it returns the pid of the child process to the parent.

itsme86 · 09-16-2004, 12:46 AM

Maybe this helps:

Code:

itsme@dreams:~/C$ cat forktest.c
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>

int main(void)
{
  int i;
  pid_t pid;

  for(i = 0;i < 3;++i)
  {
    pid = fork();
    if(!pid)
    {
      printf("In child process. fork number %d\n", i);
      exit(0);
    }
    printf("In parent process. child created is number %d, pid %d\n", i, pid);
  }

  return 0;
}
itsme@dreams:~/C$ ./forktest
In parent process. child created is number 0, pid 12432
In child process. fork number 0
In parent process. child created is number 1, pid 12433
In child process. fork number 2
In child process. fork number 1
In parent process. child created is number 2, pid 12434
itsme@dreams:~/C$

feetyouwell · 09-16-2004, 08:18 PM

yeah that's better, but in your program, now we have 3 parents and 3 children right? so there 6 processes in total, what if I only want to create children out of the same parent, how can i do that?

itsme86 · 09-16-2004, 09:40 PM

There's only 1 parent and 3 children in my example.

feetyouwell · 09-16-2004, 09:44 PM

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

int main (void)
{
printf("Initial process \t PID %d", getpid());
printf("\t PPID %d", getppid());
printf("\t GID %d\n\n", getpgid(0));

// printf("%d\n", getpgid(pid_t(getppid())));

int i;
pid_t pid;

for (i = 0; i<3; ++i) {
pid = fork();
if (!pid) {
printf("New process \t\t PID %d", getpid());
printf("\t PPID %d", getppid());
printf("\t GID %d\n", getpgid(0));
}
}

return 0;

}

if you run the code above in your computer, the computer only create 3 children for the initial process (parent), but there are also child processes created from the child. So I am only expecting 4 processes here in total but I ended up with a lot more, why is that happening?

itsme86 · 09-17-2004, 12:25 AM

Because you're not using exit() at the end of your child process so each child loops creates children processes itself. Use exit() like I did in my example.