Unix – In what manner, fork() system call making child process

cforkunix

Here is my code for fork() system call,

#include<stdio.h>
#include<stdlib.h>
#include<sys/types.h>
#include<sys/stat.h>
#include<unistd.h>
#include<errno.h>
int main(int argc, char *argv[])
{
 pid_t pid;
 pid=fork();
 printf("1st Fork\n");
 printf("Process ID : %d, Parent Process ID : %d\n",getpid(),getppid());
 pid=fork();
 printf("2nd Fork\n");
 printf("Process ID : %d, Parent Process ID : %d\n",getpid(),getppid());
 pid=fork();
 printf("3rd Fork\n");
 printf("Process ID : %d, Parent Process ID : %d\n",getpid(),getppid());
 return 0;
}

While running the code, i am getting output like

1st Fork
Process ID : 3393, Parent Process ID : 3392
2nd Fork
Process ID : 3394, Parent Process ID : 3393
3rd Fork
Process ID : 3395, Parent Process ID : 3394
3rd Fork
Process ID : 3394, Parent Process ID : 3393
2nd Fork
Process ID : 3393, Parent Process ID : 3392
3rd Fork
Process ID : 3397, Parent Process ID : 3393
3rd Fork
Process ID : 3393, Parent Process ID : 3392
1st Fork
Process ID : 3392, Parent Process ID : 3440
2nd Fork
Process ID : 3398, Parent Process ID : 3392
3rd Fork
Process ID : 3400, Parent Process ID : 3398
3rd Fork
Process ID : 3398, Parent Process ID : 3392
2nd Fork
Process ID : 3392, Parent Process ID : 3440
3rd Fork
Process ID : 3401, Parent Process ID : 3392
3rd Fork
Process ID : 3392, Parent Process ID : 3440

Why this fork() system call making 8 process and how?

Also i am getting 14 printf() statement executions. Why?

Best Answer

Each time you call fork it returns twice. Once in the parent, once in a new process. Then as they go on, they both fork again.

It's most likely you didn't expect the children to fork again. With each fork you usually have:

switch (fork()) {
case -1:
    /* ERROR. */
    break;
case 0:
    /* Child process. */
    break;
default:
    /* Parent. */
    break;
}

In your code it's like this:

You have one process, then it forks
You now have two processes and both fork
You now have 4 processes and all of them fork
You now have 8 processes and you're asking questions on SO

Related Solutions

Linux – How to make child process die after parent exits

Child can ask kernel to deliver SIGHUP (or other signal) when parent dies by specifying option PR_SET_PDEATHSIG in prctl() syscall like this:

prctl(PR_SET_PDEATHSIG, SIGHUP);

See man 2 prctl for details.

Edit: This is Linux-only

Unix – the purpose of fork()

fork() is how you create new processes in Unix. When you call fork, you're creating a copy of your own process that has its own address space. This allows multiple tasks to run independently of one another as though they each had the full memory of the machine to themselves.

Here are some example usages of fork:

Your shell uses fork to run the programs you invoke from the command line.
Web servers like apache use fork to create multiple server processes, each of which handles requests in its own address space. If one dies or leaks memory, others are unaffected, so it functions as a mechanism for fault tolerance.
Google Chrome uses fork to handle each page within a separate process. This will prevent client-side code on one page from bringing your whole browser down.
fork is used to spawn processes in some parallel programs (like those written using MPI). Note this is different from using threads, which don't have their own address space and exist within a process.
Scripting languages use fork indirectly to start child processes. For example, every time you use a command like subprocess.Popen in Python, you fork a child process and read its output. This enables programs to work together.

Typical usage of fork in a shell might look something like this:

int child_process_id = fork();
if (child_process_id) {
    // Fork returns a valid pid in the parent process.  Parent executes this.

    // wait for the child process to complete
    waitpid(child_process_id, ...);  // omitted extra args for brevity

    // child process finished!
} else {
    // Fork returns 0 in the child process.  Child executes this.

    // new argv array for the child process
    const char *argv[] = {"arg1", "arg2", "arg3", NULL};

    // now start executing some other program
    exec("/path/to/a/program", argv);
}

The shell spawns a child process using exec and waits for it to complete, then continues with its own execution. Note that you don't have to use fork this way. You can always spawn off lots of child processes, as a parallel program might do, and each might run a program concurrently. Basically, any time you're creating new processes in a Unix system, you're using fork(). For the Windows equivalent, take a look at CreateProcess.

If you want more examples and a longer explanation, Wikipedia has a decent summary. And here are some slides here on how processes, threads, and concurrency work in modern operating systems.

Best Answer

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Unix – the purpose of fork()

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