about fork

eshwar_ind · 02-10-2004, 01:30 AM

Hi!
i have a doubt regarding fork. fork returns 2 values right. How come it return 2 values? ( C function can return only one value). Does it get executed again in the context of child or just it places a value in childs memory?
please reply me
bye,
Eshwar.

hakuin · 02-10-2004, 01:50 AM

fork creates a child process and the parent process remains running.

The child is a copy of the parent, and is created at the point of the fork call. Therefore the parent and the child both return to the same point in the program (of their own images), but fork returns 0 to the child process and the PID of the child to the parent (unless there is an error).

Knowing that the child will have 0 returned to it, you can now setup the code so that the parent takes one path of execution and the child another.

eshwar_ind · 02-10-2004, 01:57 AM

Thanks, can you please tell me how the fork code looks at the end of its execution? ( two returns at single shot).
please reply me.

jtshaw · 02-10-2004, 09:16 AM

fork is a system call, the code for it can be found in the kernel.

I believe it can be found in arch/<yourarch>/process.c

schmack · 02-10-2004, 03:57 PM

Here's an example:

<code>

int runChild()
{
int pid = fork();
if (pid == 0)
{
/* i am the child, run a program */
execl("/bin/cp", "file1", "file2", 0);
/* should not return... if it does, call _exit() */
_exit(1);
}
printf("started process ... pid is %d\n", pid);
/* you could do a waitpid() if you want to get the return status */
return pid;
}

</code>

eshwar_ind · 02-11-2004, 03:38 AM

thanks jtshaw & schmack. I am looking for the fork code jtshaw you gave me that address.
thanks