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Why is the return code of child required by parent ?

 
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# 1  
Old 11-27-2012
Question Why is the return code of child required by parent ?
Hello everyone,
I am a complete newbie to UNIX. I am using Debian LXDE 64-bit. I have a question regarding the child and parent process communication. According to wikipedia.org and various other sources, when a child process exits it sends the SIGCHLD signal to its parent process, meaning that the child has finished executing. Then the parent process calls wait() to get the return code of the child process and once it has that it removes the entry of the child process from the process table.
Now when the child process finishes execution before the parent and if the parent fails to retrieve the return code sent by the child the entry of the of the process won't be removed from the table, this is what is known as a zombie process.

Now my questions are-:
1) How will parent process get the return code from the child process? Is it the same value that is returned by the fork() system call in the parent process?
2) Why does the parent need the return code return by the child ? Why is it so necessary to get the code?
3) Why does the parent process need to execute wait() to get the return code? I mean how does wait() or halting execution get it the return code ? And does it execute wait() on itself or on the child process??

Thank You.
# 2  
Old 11-27-2012
Quote:
Originally Posted by sreyan32
Hello everyone,
I am a complete newbie to UNIX. I am using Debian LXDE 64-bit. I have a question regarding the child and parent process communication. According to wikipedia.org and various other sources, when a child process exits it sends the SIGCHLD signal to its parent process, meaning that the child has finished executing. Then the parent process calls wait() to get the return code of the child process and once it has that it removes the entry of the child process from the process table.
Now when the child process finishes execution before the parent and if the parent fails to retrieve the return code sent by the child the entry of the of the process won't be removed from the table, this is what is known as a zombie process.

Now my questions are-:
1) How will parent process get the return code from the child process? Is it the same value that is returned by the fork() system call in the parent process?
2) Why does the parent need the return code return by the child ? Why is it so necessary to get the code?
3) Why does the parent process need to execute wait() to get the return code? I mean how does wait() or halting execution get it the return code ? And does it execute wait() on itself or on the child process??

Thank You.
A process becomes a zombie when it exits. The zombie remains in the process table until another process collects its exit status. As long as the process that started the zombie (its parent) is still running, only it can wait() for for the zombie to collect its exit status.

If a parent exits without collecting the exit status of its children (zombies or still running), a system process (usually known as init) will collect the status of those zombie children when the parent dies and will collect the status of the orphaned children that were still running when they die.

In response to your other questions:
  1. The exit status of the dead child is collected by calling wait(), waitid(), or waitpid() (all of which are defined by the POSIX standards and Single UNIX Specification) or a similar system specific system call. The return code from fork to the parent is the process ID of the child. The return code from one of the functions above will be the process ID of the child that exited or stopped. The exit status will be stored in the location pointed to by one of the arguments passed to those routines
  2. The exit status of the child tells the parent if the child exited normally (and if so what value was passed to exit() when the child terminated itself), was terminated by the receipt of a signal, or was stopped by a tracing event. (You don't need to worry about tracing events for now.)
  3. Look at the man page for the wait() system call on your system. The parent calls wait to get the status of one of its children. The return code from wait() will tell you which child's status change caused wait to return.
 
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