basic chat system on perl under linux - linux

Question

Im trying to write some basic chat system just to learn perl. Im trying to get the chatlog into a 1 file and print new message if it's appears in the chatlog.dat file, So i've wrote a function that does almost the same thing, but I have got some problems and don't know how to solve them.
So now I have 2 problems!
I could not understand how to keep checkFile function always active (like multiprocession) to continuously check for new messages
This problem occurs when I'm trying to write a new message that will be appended into the chatlog. The Interpreter waits for my input on the line my $newMessage = <STDIN>;, but, what if someone writes a new message? it will not be shown until he press enter... how to void that?
my ($sec,$min,$hour) = localtime();
while(1){
my $userMessage = <STDIN>;
last if $userMessage eq "::quit";
`echo "($hour:$min:$sec): $userMessage" >>chatlog.dat`;
}
sub checkFile{
my $lastMessage = "";
my $newMessage = "";
while (1) {
my $context = `cat chatlog.dat`;
split(/\n/, $context);
$newMessage = $_[$#_];
if ($newMessage ne $lastMessage) {
print $newMessage;
$lastMessage = $newMessage;
}
}
}

Answer 1

First:
don't use echo within a perl script. It's nasty to shell escape when you've got perfectly good IO routines.
using cat to read files is about as nasty as using 'echo'.
reading <STDIN> like that will be a blocking call - which means your script will pause.
but that's not as bad as it sounds, because otherwise you're running a 'busy wait' loop which'll repeatedy cat the file. This is a very bad idea.
You're assuming writing a file like that is an atomic operation, when it's not. You'll hit problems with doing that too.
What I would suggest you do it look at IO::Handle and also consider using flock to ensure you've got the file locked for IO. You may also wish to consider File::Tail instead.
I would actually suggest though, you want to consider a different mode of IPC - as 'file swapping' is quite inefficient. If you really want to use the filesystem for your IO, you might want to consider using a FIFO pipe - have each 'client' open it's own, and have a server reading and coalescing them.
Either way though - you'll either need to use IO::Select or perhaps multithreading, just to swap back and forth between reading and writing. http://perldoc.perl.org/IO/Select.html

Answer 2

Answering my own question
sub checkFile{
my $lastMessage = "";
my $newMessage = "";
my $userName = $_[0];
while (1) {
my $context = `cat chatlog.dat`;
split(/\n/, $context);
$newMessage = $_[$#_];
if ($newMessage ne $lastMessage) {
$newMessage =~ /^\(.+\):\((.+)\) (.+$)/;
if ($1 ne $userName) { print "[$1]: $2";}
$lastMessage = $newMessage;
}
}
}
my $userName = "Rocker";
my ($sec,$min,$hour) = localtime();
my $thr = threads -> create ( \&checkFile, $userName ); #Starting a thread to continuously check for the file update
while (1) {
my $userMessage = <STDIN>; #STDIN will not interfere file checking
last if $userMessage eq "::quit";
`echo "($hour:$min:$sec):($userName) $userMessage" >>chatlog.dat` if $userMessage =~ /\S+/;
}
$thr -> join();