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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 SUNOS

preap

preap(1)							   User Commands							  preap(1)

NAME

       preap - force a defunct process to be reaped by its parent

SYNOPSIS

       preap [-F] pid...

DESCRIPTION

       A  defunct  (or	zombie)  process  is one whose exit status has yet to be reaped by its parent. The exit status is reaped via the wait(3C),
       waitid(2), or waitpid(3C) system call. In the normal course of system operation, zombies may occur, but are typically short-lived. This may
       happen  if a parent exits without having reaped the exit status of some or all of its children. In that case, those children are reparented
       to PID 1. See init(1M), which periodically reaps such processes.

       An irresponsible parent process may not exit for a very long time and thus leave zombies on the system. Since the operating system destroys
       nearly all components of a process before it becomes defunct, such defunct processes do not normally impact system operation. However, they
       do consume a small amount of system memory.

       preap forces the parent of the process specified by pid to waitid(3C) for pid, if pid represents a defunct process.

       preap will attempt to prevent the administrator from unwisely reaping a child process which might soon be reaped by the parent, if:

	 o  The process is a child of init(1M).

	 o  The parent process is stopped and might wait on the child when it is again allowed to run.

	 o  The process has been defunct for less than one minute.

OPTIONS

       The following option is supported:

       -F	Forces the parent to reap the child, overriding safety checks.

OPERANDS

       The following operand is supported:

       pid	Process ID list.

EXIT STATUS

       The following exit values are returned by preap, which prints the exit status of each target process reaped:

       0	Successfully operation.

       non-zero Failure, such as no such process, permission denied, or invalid option.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Availability		     |SUNWesu (32-bit)		   |
       +-----------------------------+-----------------------------+
       |			     |SUNWesxu (64-bit) 	   |
       +-----------------------------+-----------------------------+

SEE ALSO

       proc(1), init(1M), waitid(2), wait(3C), waitpid(3C), proc(4), attributes(5)

WARNINGS

       preap should be applied sparingly and only in situations in which the administrator or developer has confirmed that defunct processes  will
       not be reaped by the parent process. Otherwise, applying preap may damage the parent process in unpredictable ways.

SunOS 5.10							    26 Mar 2001 							  preap(1)
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