Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!elroy.jpl.nasa.gov!jpl-devvax!lwall
From: lwall@jpl-devvax.jpl.nasa.gov (Larry Wall)
Newsgroups: comp.lang.perl
Subject: Re: Reading from /dev/tty in forked process.
Message-ID: <1991May25.045321.2826@jpl-devvax.jpl.nasa.gov>
Date: 25 May 91 04:53:21 GMT
References: <1991May24.163027.12281@amc.com>
Reply-To: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Distribution: na
Organization: Jet Propulsion Laboratory, Pasadena, CA
Lines: 63

In article <1991May24.163027.12281@amc.com> stuart@anabol.amc.com () writes:
: Warning: I'm new to perl.
: 
: My question is how do you get perl to stop and read from /dev/tty when
: you have a forked process sub function that is taking its' input from
: STDIN.
: 
: For example, say under sh I decide write a "Pager" function that prints
: a screen full of lines then prompts the user to hit return to get to
: the next screen of lines.
: ...
: Thinking along these lines I tried this perl program:
: 
: #!/usr/local/bin/perl 
: 
: sub pager {
: 	local($pagesize,$linecount) = (24,1);
: 	open(TTYIN,"</dev/tty") || die "can't open /dev/tty: $!";
: 	while (<>) {
: 		if ( $linecount >= $pagesize ) {
: 			print  "Hit return to continue: ";
: 			$ans = <TTYIN>;
: 			print "\n" ;
: 			$linecount = 1;
: 		}
: 		print ; 
: 		$linecount++ ;
: 	}
: 	close(TTYIN);
: }
: 
: 
: $pid = open (OUT,"|-");
: if (!$pid) {
: 	&pager;
: 	exit;
: } else {
: 	open(INPUT,"data") || die "Cannot open data ";
: 	while(<INPUT>) {
: 		print OUT $_;
: 	}
: }
: 
: This prints out the "Hit return to continue" messages but it doesn't
: wait for the return, it keeps on going.

Oddly enough, it stops for me.  It only needs two things to work right
here.  First, $| = 1 so that the prompt comes out before it stops, and
a close OUT after the final loop so that the input process waits for
the pager process to finish.

As to why yours isn't stopping, I couldn't say.  P'raps your stdio thinks
TTYIN is always at EOF for some reason.  Try sysread(TTYIN, $ans, 1) and
see if it helps.

: Most likely my problem is I'm not thinking in perl yet, but I'm
: tryin'.

Looks to me like you're doin' a durn good job of thinking in Perl.

(At least, for someone from the north end of the lake...)    ;-)

Larry