http://www.perlmonks.org?node_id=529812

spurperl has asked for the wisdom of the Perl Monks concerning the following question:

Dear monks,

I'm trying to make a Perl script talk in a bi-directional manner with a C++ application using a socket. For this, I want to implement non-blocking socket read. Now, there is the nice IO::select which has the can_read() function. However, can_read() doesn't actually return how much I can read, only that I can. So, I've been reading the Cookbook and the Advanced Perl book, and my options are: (1) use multi-threading and (2) using fcntl() to make sysread() on the socket non-blocking. As you probably know, the nonblocking fcntl() (option 2) doesn't work on Windows, which apparently doesn't support this mode of work with files. I know I can go to option (1), but I would prefer not to - because my script is really quite simple. So, I came up with the following approach (pseudocode): 

sub try_read(socket)

buf = ""

loop
  if (IO::Select->can_read socket)
    buf .= sysread(socket, 1);
  else
    exit loop

return buf

[download]
This works, because if can_read says that there's data to read, there's at least one byte there, so I sysread with length 1 and concatenate the byte. When the data ends, the loop ends. Quite, simple.

Problem is, it looks inefficient. If I receive a packet of several KB, this loop will happily run thousands of times, each time calling can_read() and sysread(). However, I don't see any other option for robust non-blocking read on Windows, without using threads.

Any ideas / suggestions / enlightments ?

Thanks in advance

Replies are listed 'Best First'.
Re: Non-blocking socket read on Windows
by salva (Canon) on Feb 13, 2006 at 14:41 UTC
    On Windows, you can use this ioctl call to make the socket non blocking: 
    ioctl($socket, 0x8004667e, $nonblocking);

    [download]

    update: $nonblocking = 1 => makes the socket non blocking, $nonblocking = 0 => makes the socket blocking.

[reply]
[d/l]
[select]
      This solution works admirably !

      After setting the ioctl() command you're suggesting on my socket, I'm reading from it using sysread(), which behaves in a non-blocking way, returning the amount of bytes it has actually read. This overall works much faster than my looping approach, especially for longer messages.

      Although this is probably off-topic, it's curious how the "magic value" of 0x8004667E is created. The "macro" for non-blocking IO in ioctl() is FIONBIO, and its calculation is done thus: 

      IOC_IN = 0x80000000h; 
FIONBIO = IOC_IN + 4 * 65536 + ORD ('f') * 256 + 126;

      [download]
      I wonder what is the rationale behind this ? Can someone explain how these things work ? Is it portable to just plunge the magic value in my code ?
[reply]
[d/l]
Re: Non-blocking socket read on Windows
by BrowserUk (Patriarch) on Feb 13, 2006 at 13:36 UTC

    You might take a look at the Win32::Internet module. In particular, the ControlReceiveTimeout [value], ControlSendTimeout [value], DataReceiveTimeout [value] & DataSendTimeout [value] functions.

    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    Lingua non convalesco, consenesco et abolesco. -- Rule 1 has a caveat! -- Who broke the cabal?
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.
[reply]
[d/l]
[select]
Re: Non-blocking socket read on Windows
by zentara (Archbishop) on Feb 13, 2006 at 13:33 UTC
    The only mention of this, that I've seen is in "perldoc -q filehandle" , scroll to section " How can I tell whether there's a character waiting on a filehandle?" It describes how to read the amount of bytes to be read on a filehandle, but it is for unix. But maybe its worth a try on Windows, or maybe the ActiveState documentation has some extra docs on how to do this. Their docs are better than the perldocs, for windows.
    I'm not really a human, but I play one on earth. flash japh
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Re: Non-blocking socket read on Windows
by eyepopslikeamosquito (Archbishop) on Feb 13, 2006 at 18:26 UTC
[reply]
Non-blocking socket read on Windows (WORKS!)
by wilsond (Scribe) on Jan 12, 2009 at 13:10 UTC

    In case anyone reads this these days, setting "Blocking => 0" works in Win32 now. I found this out after reading complaints that it doesn't and not even trying it myself. It works for me. Here's some sample code of what I'm doing with it:

    use IO::Socket::INET;
my $server = IO::Socket::INET->new(PeerAddr => '127.0.0.1',
                                   PeerPort => 12345,
                                   Proto    => 'tcp',
                                   Blocking => 0);

  my $buf;
  while (sysread($server, $buf, 1024))
  {
    $servdata .= $buf;
    print STDERR qq(READ.\n);
  }
  print STDERR qq(END READING FROM SERVER...\n);

    [download]

    I'm doing more with it, of course, but that's the key parts. I hope it works for everyone else out there. I'm curious to hear if it doesn't.

    BTW, I'm using this in ActivePerl 5.10 and PerlTray 7.3.0

[reply]
[d/l]
      Ok, I am trying to use Net:SSH or Net::SFTP which 'supposedly' user IO::Socket under the covers..
      but all my debugging info in IO::Socket (including syntax errors, croak, warn and print are never seen.) print()s in Net::SSH::Perl work fine.
      This leads me to believe that the underlying code doesn't handle blocking() correctly as my application hangs on Windows, but works fine on Linux (fedora 10).
      I have an application where I need to code the SFTP code and can't use an external pgm to do it.
      what am I doing wrong?
      How do you debug the libraries you 'use'?
      thanks (activestate perl 5.10.1005 just installed, windows XP 32 bit)
      Sam
[reply]

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