Re: [9fans] waitfor() call question?

[Joel “chesky” Salomon <chesky@plan9.jp>]

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

Just for fun:
	Waitmsg *
	waitfor(int pid)
	{
		Waitmsg *m;
		while(m = wait()){
			if(m->pid != 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’re asked
for—or 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 ☺)

Why is waitfor() “occasionally unavoidable” on UNIX?

--Joel