netcat and my perl program

humandynamo · 01-30-2009, 06:13 AM

I am currently having some struggles with the the processing of the <STDIN> from my executable.

netcat -v -v IPaddress port -e ./myprogram.pl

Within "myprogram.pl" I do the following to get the user input:
$sLine = "";
$sLine = <>;
print = "Received: $sLine\n\r";

I would like to keep "myprogram.pl" the way it is. Although it was originally built as a server it is a module that should operate the same whether being connected to or making the connection.

I am hoping to keep this model because the actual server portion is being run by xinetd which waits for the connection and then runs "myprogram.pl"

theNbomr · 01-30-2009, 09:19 AM

You haven't stated what problem you are observing. However, with respect to your statement 'a module that should operate the same whether being connected to or making the connection', it sounds like you are trying use your code as both a server and a client, but none of the code you show functions as a client. More detail is required here.
BTW, please post source code in [CODE] tags to retain clarity.
--- rod.

humandynamo · 01-30-2009, 11:55 PM

I apologize it was a major oversight to leave out the actual problem and I thank you for taking the time to even respond to an incomplete post.

My problem is that I have a program which is supposed to take some input from the user and then respond/manipulate the user input in some way. When it is in server mode it operates as expected. I wait for user input and then I print the user input back to ensure I have received the data correctly. However when I initiate a connection using netcat does not wait for user input; the program instead falls through and I get an error stating that the value $sLine in uninitialized.

The actual code does not function as a server but is executed by xinetd when a tcp connection is made to it on a certain port. I was attempting to do the same thing with netcat only this time I sought to initiate the connection.

theNbomr · 02-01-2009, 10:30 AM

Okay, I think I understand what you are doing. To rephrase a bit, your perl program reads standard input and echoes to standard output. When standard input is a terminal/console, the program works as expected, but when the standard input is redirected from the output of netcat, it fails to wait for input.
My version of netcat was built without the '-e' option, so I cannot test directly with it. I tried using the bash commandline to redirect the standard output of netcat into your program, and as expected, this worked. The difference is that in your instance, netcat is the parent of your perl program. I am hypothesizing that this is causing your program to read its standard input before the parent process has been able to receive from the network, and this is causing the read from stdin to return undefined. To test this hypothesis, try this:

Code:

#! /usr/bin/perl -w
use strict;
    my $sLine = "";
    while( ! defined ( $inp ) ){
        $sLine = <>;
    }
    print "Received: $sLine\n";

Let us know how that changes things.
--- rod.