Date: Thu, 25 Apr 2002 23:37:59 +0200 From: Paul Everlund <tdv94ped@cs.umu.se> To: Bsd Neophyte <bsdneophyte@yahoo.com> Cc: freebsd-questions@FreeBSD.ORG Subject: Re: what are parent and child processes all about? Message-ID: <3CC87737.9E146891@cs.umu.se> References: <20020425201553.43809.qmail@web20106.mail.yahoo.com>
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Bsd Neophyte wrote:
>
> I don't follow what these are...
>
> this is what my text says...
>
> "When a process is started, a duplicate of that process is created. This
> new process is called the child and the process that created it is called
> the parent. The child process then replaces the copy for the code the
> parent process created with the code the child process is supposted to
> execute."
>
> The last sentance confuses me the most. Actually the entire thing
> confsues me. Why is a child process started anyways? Why does the parent
> process spawn a child process in the first place?
You can create a child process by fork (see man 2 fork). The process that
creates it is called the parent. If you create a server, it works as this
(maybe somewhat simplified):
1. Execute the command that starts the server, as httpd.
2. Instantly after it is started, it maybe does some things, then it forks
a child. This child process contains a copy of file descriptors, environ-
ment variables and so on, and even the executable code. Its a copy of the
parent in other words, but it has its own process id.
3. The parent then kills itself, but the child keeps on running in the back-
ground handling requests or other things.
You get almost(?) the same thing by starting a program with &.
> And what's all this code copy stuff all about?
The code is copied, as stated before, but the child can execute its "own"
code by a simple if (something like this if I remember it correct):
.
.
pid = fork();
if(pid == 0) // This means it's the child process.
{
do some stuff... maybe in a loop of some kind...
}
exit(0); // If here the parent/child exits.
I guess it's copied as the child then can use the parent process things,
and do not have to open files and so on once more.
> The next paragraph states...
>
> "While the command is executing, the shell waits until the child process
> has completed. After is completes, the parent process terminates the
> child process, and a prompt is displated, ready for a new command"
The parent can wait for the child to finish, but it can also, as I wrote
before, exit and let the child continue to run in the background.
> Why does this happen? I guess it goes back to my confusion about why the
> child process is created in the first place.
A nice way of doing simultaneous stuff, or making a server.
Best regards,
Paul
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