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From: tchrist@convex.COM (Tom Christiansen)
Newsgroups: comp.lang.perl
Subject: Re: grabchars in perl?
Message-ID: <1991Jun05.103001.3365@convex.com>
Date: 5 Jun 91 10:30:01 GMT
References: <1991Jun5.052344.22684@newshost.anu.edu.au>
Sender: usenet@convex.com (news access account)
Reply-To: tchrist@convex.COM (Tom Christiansen)
Organization: CONVEX Software Development, Richardson, TX
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From the keyboard of rjm@vulcan.anu.edu.au (Robert J. McArthur):
:Has anyone implemented the grabchars package in perl?
:
:For those who don't know, grabchars is a PD program from comp.sources.misc
:that implements timed reads from ttys and non-CR-requiring input.  eg. you
:can  just type one letter in answer to a y/n question without needing the
:CR.

Use "system stty" or ioctl to get yourself in the moral equivalent of
cbreak mode (per FAQ #16).  Use an ioctl if you're going to be doing a lot
of these.  Use system if you're only running it once.

Then set a timer with alarm (or SYS_setitimer if you have it and want that
granularity).  If your reads don't continue after interruptions, then just
return from your interrupt routine.  If they do, you need a
setjmp/longjmp, so do the getc in an eval, and have your interrupt routine
die, effectively longjumping out of the eval.

    sub BANG { die "alarm went off"; }
    $SIG{'ALRM'} = 'BANG';
    system "stty cbreak </dev/tty >/dev/tty 2>&1";
    print "go\n";
    alarm 2; $key = eval "getc(STDIN)"; alarm 0;
    if (!defined($key)) { 
	die $@ if $@ && $@ !~ /alarm/; # not my exception?
	print "timeout!\n";
    } else {
	printf "\nkey was %s (0x%x)\n", $key, ord($key);
    } 
    system "stty cooked </dev/tty >/dev/tty 2>&1";

I believe that this code needs 4.003 to work correctly.  Again, you
poor folks (SysV, POSIX) with non-restarting reads don't need to worry 
about the eval.  I wish perl gave me more control here, but I'm at
the mercy of my C library, and getting at the SV_INTERRUPT bit is 
non-trivial.


--tom
--
Tom Christiansen		tchrist@convex.com	convex!tchrist
	    "Perl is to sed as C is to assembly language."  -me