From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <e0e53b081fe30e1db228d82ddebb5e06@9netics.com>
To: 9fans@cse.psu.edu
Subject: Re: [9fans] trying to understand how fork/pipe a filtering program
Date: Mon,  4 Sep 2006 12:26:27 -0700
From: Skip Tavakkolian <9nut@9netics.com>
In-Reply-To: <200609031909.k83J9UV23187@demeter.cs.utwente.nl>
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> regarding Skip's comments: I'm not sure I completely understand.
> the parallelism is not vital. I just want to use an existing
> program (resample in this particular case) as filter to read
> some of my data, process it, and give me the result, after which
> I continue with my own thing. 

just this: in your example, if parent needs to write more than some
pipe-buf-limit (i think 32k) to child and/or if child produces output
larger than this limit and blocks on the write, it would be a problem.

i attached my example of using only one pipe which requires two
rendezvous points and customization of the slave proc; this could
have a deadlock when a child requires multiple reads to cause a write.

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#include <u.h>
#include <libc.h>

void
ping(int *fd)
{
	int n, i = 0;
	char buf[1024];

	while (1) {
		i++;
		memset(buf, 'a', 1024);
		rendezvous(0, 0);

		if (i > 2) break;
		if ((n = write(fd[1], buf, sizeof(buf))) != sizeof(buf))
			break;

		print("ping: wrote %d bytes\n", n);

		rendezvous(1,0);
		if ((n = read(fd[0], buf, sizeof(buf))) <= 0)
			break;

		print("ping: read %d bytes\n", n);
		write(1, buf, n);
		write(1, "\n\n", 2);
	}
	close(fd[1]);
	close(fd[0]);
}

void
pong(int *fd)
{
	int n;
	char buf[1024];

	while(1) {
		rendezvous(0, 0);
		if ((n = read(fd[0], buf, sizeof(buf))) <= 0) break;

		print("pong: read %d bytes\n", n);
		write(1, buf, n);
		write(1, "\n\n", 2);

		memset(buf, 'b', sizeof(buf));
		rendezvous(1, 0);
		if ((n = write(fd[1], buf, sizeof(buf))) != sizeof(buf)) break;

		print("pong: wrote %d bytes\n", n);
	}
	close(fd[0]);
	close(fd[1]);
}

void
pingpong(void)
{
	int fd[2], pid, kid;
	
	if (pipe(fd) < 0)
		sysfatal("pipe failed: %r");

	switch (kid = rfork(RFPROC|RFFDG)) {
	case -1:
		sysfatal("fork failed: %r");
	case 0:
		pong(fd);
		break;
	default:
		ping(fd);
		while ((pid = waitpid()) != kid && pid != -1)
			;
		break;
	}
}

void
main(int argc, char **argv)
{
	pingpong();
	exits(nil);
}
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