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Full Discussion: fork(), parent and child processes???
Top Forums Programming fork(), parent and child processes??? Post 302572993 by gabam on Saturday 12th of November 2011 09:29:59 AM
Old 11-12-2011
fork(), parent and child processes???
Hi friends,
I have a small question regarding unix system call fork, I hope you will solve my problem. Here is the small program

Code:
 
$ cat fork1.c
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
int main()
{
int pid;
int x = 0;
x = x + 1;
pid = fork();
if(pid < 0)
{
printf("Fork failed\n");
return -1;
}
if(pid == 0)
printf("I am the child, my x is %d and my PID is %d\n",x,getpid());
else
printf("I am the parent, my x is %d and my PID is %d\n",x,getpid());
return 0;
}

And here is the output
Code:
$ ./fork1.exe
I am the child, my x is 1 and my PID is 1282
I am the parent, my x is 1 and my PID is 1281

Ok, when we call fork(), the following things happen
1. The parent process passes it's memory image, including the variables
Code:
pid

and
Code:
x

, to the child process that is created.
2. fork returns zero to the child, and the child's PID, a positive integer, to the parent.
3. If fork returns a negative value, it means there was some error, and no process was created.
Ok, I am going all right till now, here comes the question that has been bothering me.
How can the single variable
Code:
pid

contain two values at the same time? zero for the child process and a positive integer for the parent process? We have two values saved in the same memory location identified by
Code:
pid

, how does that make sense. Why the values are not overwritten???
I hope you guys will help me with this mystery thing, and explain it to me.
Thanks a lot, looking forward to your nice replies!
 

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LEARN ABOUT BSD

vfork

VFORK(2)							System Calls Manual							  VFORK(2)

NAME

       vfork - spawn new process in a virtual memory efficient way

SYNOPSIS

       pid = vfork()
       int pid;

DESCRIPTION

       Vfork  can be used to create new processes without fully copying the address space of the old process, which is horrendously inefficient in
       a paged environment.  It is useful when the purpose of fork(2) would have been to create a new system context for an execve.  Vfork differs
       from  fork  in  that  the child borrows the parent's memory and thread of control until a call to execve(2) or an exit (either by a call to
       exit(2) or abnormally.)	The parent process is suspended while the child is using its resources.

       Vfork returns 0 in the child's context and (later) the pid of the child in the parent's context.

       Vfork can normally be used just like fork.  It does not work, however, to return while running in the childs  context  from  the  procedure
       that  called  vfork  since the eventual return from vfork would then return to a no longer existent stack frame.  Be careful, also, to call
       _exit rather than exit if you can't execve, since exit will flush and close standard I/O channels, and thereby mess up the parent processes
       standard I/O data structures.  (Even with fork it is wrong to call exit since buffered data would then be flushed twice.)

SEE ALSO

       fork(2), execve(2), sigvec(2), wait(2),

DIAGNOSTICS

       Same as for fork.

BUGS

       This  system  call will be eliminated when proper system sharing mechanisms are implemented.  Users should not depend on the memory sharing
       semantics of vfork as it will, in that case, be made synonymous to fork.

       To avoid a possible deadlock situation, processes that are children in the middle of a vfork are never sent  SIGTTOU  or  SIGTTIN  signals;
       rather, output or ioctls are allowed and input attempts result in an end-of-file indication.

4th Berkeley Distribution					   June 30, 1985							  VFORK(2)
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