[SOLVED] FORK question

elico · 05-06-2012, 02:55 PM

hi all

What is the point to create a child process ?
why not create a new process ?

Elico

zer0signal · 05-06-2012, 03:09 PM

Lots of reasons, having the Parent PID as the controlling process.. IE: Daemon.. Server/Client model, PPID manages incoming connections, and creates a child process to manage that new connections, to free up the parent to allow new connections.

rknichols · 05-06-2012, 03:29 PM

But the main reason is that fork() is the only mechanism for spawning a new process.

elico · 05-06-2012, 03:33 PM

But isnt the child code a copy of father code ?
So they are meant to do the same task , am I right ?

Elico

rknichols · 05-06-2012, 09:08 PM

That's right, but the most common sequence is for the child process to follow a different branch in the code, set up a few things (mainly closing and opening files, etc.), and then call exec(), which completely replaces the parent's image with something new. Aside from process number 1 (init), that's how every process in the system begins life.

elico · 05-07-2012, 02:22 AM

Thanks

Can any one show a code example for generating two or more independent processes w/o children
and another code sample for a father process and a child process .

By the way can a father process have two or more children ?

Thanks
Elico

Satyaveer Arya · 05-07-2012, 06:59 AM

Quote:

What is the point to create a child process ?

checkout this discussion, http://www.linuxforums.org/forum/pro...d-process.html

Quote:

can a father process have two or more children ?

Yes, father process can create two or more child processes.

Satyaveer Arya · 05-07-2012, 07:06 AM

Quote:

code sample for a father process and a child process

You can checkout on this link - http://stackoverflow.com/questions/7...rent-processes.

rknichols · 05-07-2012, 08:33 AM

Quote:

Originally Posted by elico

Can any one show a code example for generating two or more independent processes w/o children
and another code sample for a father process and a child process .

Sure. Here is very rudimentary code to create an independent process by breaking the inheritance chain:

Code:

pid1 = fork();
if(pid1)  waitpid(pid1, NULL, 0);    /* Parent waits for 1st child */
else {        /* Child process continues here */
        if(fork())  exit(0);         /* 1st child forks again and exits */
        /* The orphaned 2nd child process continues here */
             .
             .
             .
}

That orphaned child process gets adopted by the init process (PID 1). Note that the waitpid() call in the original parent is important to keep that short-lived 1st child from hanging around as a zombie. Note: Besides error checking, this code also omits a lot of other things you would want to do to create a truly independent process (setpgrp(), setting up the standard file descriptors, breaking the association with the controlling tty, ...).

The usual parent<->child arrangement is simpler. Just don't do the second fork(), and the parent has the option of (a) waiting for the child immediately, (b) continuing processing and doing the wait() later, or (c) continuing processing and exiting without doing a wait(). For case c, the child process then gets adopted by init, just as in the first example.

Code:

pid1 = fork();
if(pid1 == 0) {      /* Child process continues here */
     .
     .
     .
}

elico · 05-07-2012, 01:30 PM

Thanks

Elico