From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <e05cb1dde77f831e8cc594d5b40591fd@plan9.jp>
To: 9fans@cse.psu.edu
Subject: Re: [9fans] waitfor() call question?
From: Joel “chesky” Salomon <chesky@plan9.jp>
Date: Thu, 12 Oct 2006 21:27:22 -0400
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> There was never a waitfor on Plan 9.
> There is a waitpid, or perhaps (as you suggest)
> you are thinking of the one I added for use on
> Unix, where it is occasionally unavoidable.

Nemo=E2=80=99s book (http://lsub.org/who/nemo/9.intro.pdf, page 91) gives=
 code
for waiting for a particular descendant process if that=E2=80=99s what=E2=
=80=99s
wanted.  Actually, he uses it for the immediate child process, which
makes me wonder=E2=80=94after the child process has exited, can the paren=
t
wait() for a grandchild?

Just for fun:
	Waitmsg *
	waitfor(int pid)
	{
		Waitmsg *m;
		while(m =3D wait()){
			if(m->pid !=3D pid)
				free(m);
			else
				return m;
		}
		return nil;
	}
To better mimic lunix semantics, a real implementation probably ought
to remember all the other pids and return them when they=E2=80=99re asked
for=E2=80=94or rather would directly try to read process information from
/proc.  (Reading the FreeBSD man page for wait(2)
(http://tinyurl.com/yy6tp2) is an educational experience in why Plan 9
is necessary =E2=98=BA)

Why is waitfor() =E2=80=9Coccasionally unavoidable=E2=80=9D on UNIX?

--Joel