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From: J Wunsch <j@uriah.heep.sax.de>
Message-Id: <199504170703.JAA04291@uriah.heep.sax.de>
Subject: rewindstdin
To: freebsd-hackers@FreeBSD.org (FreeBSD hackers),
        andreas@knobel.gun.de (Andreas Klemm)
Date: Mon, 17 Apr 1995 09:03:52 +0200 (MET DST)
Reply-To: joerg_wunsch@uriah.heep.sax.de (Joerg Wunsch)
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I think the following should demonstrate a possible trick.  It's
something like a ``limited tee'', in that it reads a first buffer that
is supposed to be sufficient for file(1) (yes, it's heuristic, you
never know how many bytes file(1) will need :), forks a subprocess
just feeding those bytes to file(1) (with its FD 1 dup'ed to FD 2, so
the message will be distinguishable from regular data on FD 1), and
continues to act like cat(1) in the parent process.

j@uriah 153% cat mbox.j | ./foo | dd of=/dev/null
standard input:              mail text
358+1 records in
358+1 records out
183770 bytes transferred in 1 secs (183770 bytes/sec)
j@uriah 154% cat /kernel | ./foo | dd of=/dev/null
standard input:              demand paged executable
1750+1 records in
1750+1 records out
896446 bytes transferred in 1 secs (896446 bytes/sec)

I wrote it in Perl since this was easier for experimenting, but i
think the idea is clear enough and as easy to implement in C, too.

#!/usr/bin/perl

read(STDIN, $buf, 4096) || die "Can't even read a single byte from stdin\n";

if(fork) {	# XXX should handle -1 as "cannot fork"
    $SIG{'PIPE'} = 'exit';	# exit cleanly if file(1) closes the pipe
    open(STDOUT, ">&STDERR");	# a dup(2) looks a bit strange in perl, eh' :)
    open(FILE, "|file -");
    print(FILE $buf);
    exit(0);
} else {
    do {
	print $buf;
    } while(read(STDIN, $buf, 4096));
}

exit(0);

sub exit {exit(0);}


-- 
cheers, J"org

joerg_wunsch@uriah.heep.sax.de -- http://www.sax.de/~joerg/
Never trust an operating system you don't have sources for. ;-)