Non-buffered read from program

[arc_of_descent]

Hi,

I'm stuck with this problem. I searched the docs and this site, but most of them deal with non-buffered output.

What I'm looking for is a way to read the output from a command which initially shows some output, and then blocks due to a network issue

This is the command when run on a Linux shell:

$ whois x.x.x.x
[Querying whois.server.net]

The above command then hangs, most probably as that particular server is not available. Whats important is that this command does indicate which server it is trying.

I'm running the above command in a Perl script, and trying to capture the initial 2-3 lines, irrespective of whether the command blocks or not.

None of the following work:

open FH, "whois x.x.x.x 2>&1 |";
my $ofh = select FH;
$|=1;
while (<FH>) {
    print;
}
close FH;

open FH, "whois x.x.x.x 2>&1 |";
sysread FH, $output, 4192;
close FH;

Any help would be much appreciated. Thanks!

[derby]

open FH, "whois x.x.x.x 2>&1 |";
my $ofh = select FH;
$|=1;
while (<FH>) {
    print;
}
close FH;

Turn on warnings and you'll see exactly what the problem is with this piece of code.

[rinceWind]

Check out IO::Select, which does what you want.

[arc_of_descent]

IO::Select seems like a good choice, but unfortunately it does not work. i.e. if the whois server is unavailable, I get no output.

open my $WHOIS, "jwhois x.x.x.x 2>&1 |";
my $s = IO::Select->new($WHOIS);

while (my @ready = $s->can_read) {
    for my $fh (@ready) {
        if ($fh == $WHOIS) {
            my $buf = <$fh>;
            if ($buf) {
                print $buf;
            }
            else {
               close($fh);
            }
        }
    }
}

Note that this works great, if the server sends some output and then closes the connection, but not for servers which send some output and then hang.

[PreferredUserName]

Fundamentally, the problem is that when the "whois" program (or most any other program) prints to its standard output using the standard C libraries, it does so in a heavily buffered fashion *unless* it's printing to a terminal, in which case its output is buffered line-by-line.

Since his standard output is connected to your standard input, and not to a terminal, it is heavily buffered.

Mucking around with $| will never help, since it's the other program's buffering, not your own, that's causing the problem.

The solution is to trick it into thinking it's talking to a terminal by using Expect.pm or Expect::Simple.

[arc_of_descent]

Yes. Expect did solve this problem for me. Especially the raw_pty method.

[ph713]

Or if you don't want to use Expect when you're not planning on using all of its complexity, you could do something like: http://www.perlmonks.org/?node_id=393312 to put a pesudo-tty inbetween you and the command.

[myuji]

Try this code:

open FH, "whois x.x.x.x 2>&1 |";
my $ofh =select(FH);
$|=1;
select($ofh); #changed
while (<FH>) {
    print;
}
close FH;

On my environment ,this works good. Hope this helps,

[arc_of_descent]

Doesn't seem to work on mine. IO::Handle too does not help.

use strict; use warnings;
use IO::Handle;

open my $WHOIS, "jwhois x.x.x.x 2>&1 |";
my $io = IO::Handle->new;
$io->fdopen($WHOIS, "r");
$io->blocking(0);
print $io->getline;
$io->close;

I don't get any output, if the server is unavailable.

[myuji]

Do you mean to use $io->autoflush(1) ? Try this code:

use strict; use warnings;
use IO::Handle;

open my $WHOIS, "whois x.x.x.x 2>&1 |";
my $io = IO::Handle->new;
$io->fdopen($WHOIS, "r");
$io->autoflush(1);
while(my $line = $io->getline){
    print $line;
}
$io->close;

This is working fine on my environment.

[Celada]

Both sysread and <FH> will return immediately if some data (for <FH>, a whole line) were collected from the pipe. There is no need to turn on autoflush ($|) for the FH filehandle. If you are not getting the data it is because whois is probably not sending it. Probably it is using stdio, and since its output is a pipe, it is buffered. If that's the case, then there's not much you can do about it. None of the versions of whois that I have produce this status message so I cannot test whether this is so, but you can verify whether yours is outputting any data with truss or strace:

truss -t write whois x.x.x.x 2>&1 | cat

[holli]

I have no idea about the whois command, but there are a couple of modules that seem to be relevant, most likely Net::Whois.

[arc_of_descent]

Its not particularly about the whois command, but on how to obtain program output which blocks. The whois command is a perfect example for this.

[Anonymous Monk]

perhaps the backticks will give you a way. Then you can use something like this $myvar = `whois x.x.x.x` ; Then you can manipulate/search/test the output from the command, through $myvar. G

[zentara]

If you read "perldoc -f select", it allows you to put a timeout on the select statement, maybe that is what you are after?

Or maybe you want to regex the output, and if it is empty, leave the loop.