I have a client and server application.MY client is sending the username(which it gets through getlogin()) to server.I want server to save all the registered username in a file called username.On receiving the request from client,it checks the username of client in the file and if it founds it ,its gives him permission to go ahead otherwise rejects it. here is what i did:

$user = <$client_socket>;
 chop($user); #remove the \n
 print "$prefix USER = $user\n";
  unless (open (USERNAME, "username")) 
     {
       print "ERROR:Cannot open username file.\n";
       exit(1); # ERROR
      }
    while(<USERNAME>)
        {
        my @newusername = <USERNAME>;
    foreach $name (@newusername)
        {
       if($name ne $user) {
       print $client_socket "ERR\n";
       print "$prefix ERROR: user $user not permitted to use this mode
+.  Exiting.\n";
       close(USERNAME);
       exit(1);
              }
        }
        }
     #else, continue
     print $client_socket "ACK\n";  # approve the user.  Limit this in
+ the future.
     close(USERNAME);
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this isnt working as even when the file username has client name it is giving "user $user not allowed".

Your main problem is that you're checking ne

       if($name ne $user) {
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so, unless every entry in the USERNAME file holds the name in question, this will match and quit with ERR.

Also, your way of reading the file

    while(<USERNAME>)                     # reads first entry
        {
        my @newusername = <USERNAME>;     # reads remaining entries
    foreach $name (@newusername)
        ...
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skips the first entry for the comparison...

Try something like this:

...
my $found;
while (my $name = <USERNAME>) {
    if ($name eq $user) {
        $found = 1;
        last;
    }
}
unless ($found) {
    print $client_socket "ERR\n";
    ...
    exit;
}
print $client_socket "ACK\n";
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And either chomp both $user and $name, or none.  As you have it, $name has a trailing \n, while $user hasn't.

 $user = <$client_socket>;
 chomp($user); #remove the \n
 print "$prefix USER = $user\n";
    unless (open (USERNAME, "username")) 
     {
       print "ERROR:Cannot open username file.\n";
       exit(1); # ERROR
      }
    my $found;
    while(my $newusername = <USERNAME>)
        {
                chomp($newusername);
                if($newusername eq $user) 
                {
                $found = 1;
                last;
                }
                
                unless ($found) {
                print $client_socket "ERR\n";
                print "$prefix ERROR: user $user not allowed  Exiting.
+\n";
                close(USERNAME);
                exit(1);
                 }
        
    }
     #else, continue
     print $client_socket "ACK\n";  # approve the user.  
     #print "user $user is allowed.";
     close(USERNAME);
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I tried what you said as above but still it got same problem..still its not goin on else part where user is approved.

Your code isn't properly formatted, so it's not easy to see what you're trying to post. But you don't seem to be stripping the line break characters from the lines you read from the file.

Did you mean to say adding a chomp function after reading the file,like below:

$user = <$client_socket>;
 chop($user); #remove the \n
 print "$prefix USER = $user\n";
    unless (open (USERNAME, "username")) 
     {
       print "ERROR:Cannot open username file.\n";
       exit(1); # ERROR
      }
    while(<USERNAME>)
    {
        my @newusername = <USERNAME>;
        chomp($newusername);
        foreach $name (@newusername)
        {
            if($name ne $user) 
                {
                print $client_socket "ERR\n";
                print "$prefix ERROR: user $user not permitted to use 
+this mode.  Exiting.\n";
                close(USERNAME);
                exit(1);
                 }
        }
    }
     #else, continue
     print $client_socket "ACK\n";  # approve the user.
    close(USERNAME);
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unless (open (USERNAME, "username")) 
   {
     print "ERROR:Cannot open username file.\n";
     exit(1); # ERROR
    }
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Just a parenthetic note: The code quoted above is more idiomatically (and, IMHO, better) written as below, which uses both a lexical filehandle and the three-argument form of open. (See also perlopentut.)

my $filename = 'foo';
open my $filehandle, '<', $filename or die "opening '$filename': $!";
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