Detecting read errors whilst reading from a network drive

[Scarborough]

A simple question, on a windows based network would it be possible to open a file and then not be able to read it at a later stage due to a network failure for example. If so is there a read error. The code I am using is as follows,

 open RESTART, "x:\\myfiles\file.txt" or die;

 while(<RESTART>){
     #do something with $_
 }

Thanks for the help. UPDATE
Maybe my wording is wrong or a little misleading

 open RESTART, "x:\\myfiles\file.txt" or die;
 #Assume the file opens correctly

 while(<RESTART>){
     #do something with $_

     #My Worry is if the network collapses in this 
     #loop. Will I know about it or will it hang?
     
 }

20040414 Edit by BazB: Changed title from 'Open and reading files'

[hmerrill]

This sounds like a loaded question - why are you asking? Did you run into a problem with the code above? Are you wondering if there is more error trapping you can do to detect a read problem?

The code you gave is exactly the code I would use to open and read/process all the records in a file. Except I would put a message in my die - something like this:

   my $abs_filename = "x:\\myfiles\file.txt";
   open(RESTART, "<$abs_filename")
       or die "Can't open $abs_filename: $!"; 
   while (<RESTART>) {
       # do something with $_
   }

[crabbdean]

In general yes you'll get an error. The error will be pretty much similar to the one if you had a read error on a local drive. You should try using  or die $!; if you want the reasons for the error to be output. At least then your 'die' is a little informative.

If you want the bells and whistles version you could drop the below into your code and call it with dying($!) instead. If you don't want the bells remove the 'sound' related lines I appended with ##>> and if you don't want the error message brutally forced to the front of all your other applications remove the 'win32Util' lines that I appended with ##%%

Happy travels! :-)

## put into the top of your main code

## Show fatal error dialog
sub dying {
    my ($error) = @_;
        use Tk;
        use Win32::Sound;  ##>>
        use Win32Util;  ##%%

    
    if (!$error) {
        $error = "(none given)";
    }
    my $text;
    $text .= "\n\nError:\n\n$error";
    my $box = new MainWindow(-title => "Error",
                -bg => 'yellow',
                -bd => 4,
                -relief => 'ridge');
    $box->overrideredirect(1);

    my $label = $box->Label(
                -textvariable=> \$text,
                -bg=> 'yellow',
                )->pack(-fill=>'both',
                    -padx => 40,
                    );
    Win32::Sound::Play('SystemExclamation');  ##>>
    my $ok = $box->Button(-text => "OK",
                -command => [     sub{print STDOUT "User selected 'OK'
+ on error message\n";
                        $box->destroy;
                        exit;
                        } ]
                )->pack(-padx => 20,
                    -pady => 20
                    );

    $box->withdraw;
    $box->Popup;
    Win32Util::keep_on_top($box, 1); ##%%
    MainLoop;
}

[Anonymous Monk]

The file is already open. He's reading the file. There is a network failure and he can't read the file anymore. There should be an error in $! and $^E.

[crabbdean]

Well yes. But if the only indicator that your network just failed is the $! and $^E then I'd suggest hiring better network administrator and getting better network monitors haha! Sorry couldn't resist. :-) I just get see the chaos that goes on around my work when the network dies. haha. I was running a script once when my network went down. Besides the thought of "Oh hell, did I just do that?" it was pretty obvious why my script failed.

[PodMaster]

Sure, why not? You probably won't notice using readline, so you might try read if you care.

update: Do a test. Start reading a file from a networked drive (actually through the network) and be sure to sleep in between each read (so you don't need a big file). Then, while your program is running, disconnect your computer from the network (pull cable, whatever) and see what happens (the next readline should fail). Be sure to check $! and $^E. If that doesn't work for you, try Win32API::File for more control.

[iburrell]

You can check for errors at the bottom of the loop. The <> operator will return undef on errors which ends the loop and sets $!. The other way to find errors is to check the return status of close.

open RESTART, "x:\\myfiles\file.txt" or die;
while (<RESTART>) {

}
die "Read error: $!" if $!;
close(RESTART) or die "Close error: $!";