From owner-freebsd-hackers  Mon Feb 26 18:40:40 2001
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To: Marc W <mwlist@lanfear.com>
Cc: freebsd-hackers@freebsd.org
Subject: Re: Why does a named pipe (FIFO) give me my data twice ??? 
In-Reply-To: Message from Marc W <mwlist@lanfear.com> 
   of "Mon, 26 Feb 2001 12:58:36 PST." <200102262058.MAA39013@akira.lanfear.com> 
Date: Mon, 26 Feb 2001 18:40:36 -0800
From: Dima Dorfman <dima@unixfreak.org>
Message-Id: <20010227024036.46EB03E09@bazooka.unixfreak.org>
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If the first program calls open(2) before the second one calls
close(2) the former will not block because there's already a writer on
the pipe.  A possible workaround would be to unlink and recreate the
fifo in program one, like so:

	for (;;)
	{
		int fifo;
		mkfifo(fifo_path, fifo_mode);
		fifo = open(fifo_path, O_RDONLY);
		read(fifo, ...);
		printf(...);
		close(fifo);
		unlink(fifo_path);
	}
		
As for why this actually happens, I don't know.  None of my Stevens
books explain this.

Hope this helps

					Dima Dorfman
					dima@unixfreak.org
> 
> hello!
> 
>     I've got a program that creates a named pipe, and then spawns a
> thread
> which sits in a loop:
> 
>     // error checking snipped.
>     //
>     while (1) {
>         int fifo = open(fifoPath, O_RDONLY);  // this blocks
>         fprintf(stderr, "somebody opened the other end!\n");
>         read(fifo, buf, sizeof(buf));
>         fprintf(stderr, "got the following data: %s\n", buf);
>         close(fifo);
>     }
> 
>     i then have another instance of the same program do the following:
> 
>     fifo = open(fifoPath, O_WRONLY);
>     write(fifo, buf, strlen(buf);
> 
> 
>     now, the problem is that the first process succeeds suddenly for
> the
> open, reads the data, closes the fifo, and IMMEDIATELY SUCCEEDS THE
> open(2)
> again, reading in the same data.
>  After that, all is as expected.  
> 
>     Note that this doesn't happen ALL the time -- only about 80% of the
> time.
> 
>     Any idea why this would happen?

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