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From: poage@sunny.ucdavis.edu (Tom Poage)
Newsgroups: comp.lang.perl
Subject: Re: open pipe in and out
Keywords: perl pipe
Message-ID: <481@sunny.ucdavis.edu>
Date: 11 Jul 90 16:38:14 GMT
References: <250@carssdf.UUCP>
Reply-To: poage@sunny.ucdavis.edu (Tom Poage)
Organization: UCDMC Clinical Engineering, Sacto., CA
Lines: 55


Here's some clips of code I've experimented with to perform multiple
(sorta ad-hoc) database queries from another program using a single
invocation of SQL.  The query language is written to the SQL process
and the results are read back from the same process.  I thought it'd be
efficient to start SQL only once since it is a BIG program with lots of
internal initialization.

BTW, I used a named pipe instead of piping stdin directly to SQL
because of the vendor's (Un*fy) stupid implementation; this SQL doesn't
know if it's getting input from a pipe or a tty, hence writes prompts,
etc., to stdout along with query results--this makes for ugly output.
The only way around it is to dupe SQL into thinking it's getting query
input from a file (Un*fy, do you read this group?).

Unfortunately, this example needs select(), so is not portable to all
UNIX domains.  In the end, I just used temporary files instead of pipes.

Lots deleted ....

$NAMED_PIPE = "/var/tmp/PIPE.$$";
if (system("/usr/etc/mknod $NAMED_PIPE p")) { die "mknod(named-pipe): $!\n"; }

...

# The pipe open fails (hangs) if the following construct is used.
# This happens on a Sun 3/150 (4.0.3)--anyone else?
#if (!open(SQLOUT, "SQL $NAMED_PIPE |")) { die "open(SQL): $!\n"; }
open(SQLOUT, "-|") || exec 'SQL', $NAMED_PIPE;
if (!open(SQLSCRIPT, ">$NAMED_PIPE")) { die "open(named-pipe): $!\n"; }

...

vec($rin, fileno(SQLOUT), 1) = 1;
vec($win, fileno(SQLSCRIPT), 1) = 1;

loop: for (;;) {
	$n = select($rout=$rin, $wout=$win, undef, undef);
	die "select error: $!\n" if ($n < 0);
	if ($wout eq $win) {
		# write some stuff to the process.
	}
	if ($rout eq $rin) {
		last loop unless ($_ = <SQLOUT>);
		# Do something with what you've just read.
	}
}

close(SQLOUT);
unlink($NAMED_PIPE);
exit(0);
-- 
Tom Poage, Clinical Engineering
Universiy of California, Davis, Medical Center, Sacramento, CA
poage@sunny.ucdavis.edu  {...,ucbvax,uunet}!ucdavis!sunny!poage