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Re: open2() in Windows

by BrowserUk (Pope)
on Jul 23, 2007 at 08:35 UTC ( #628192=note: print w/replies, xml ) Need Help??


in reply to open2() in Windows

You are connecting to both stdin & stdout, but never processing stdout. Consequently, if the child process produces any output, it will eventually fill the pipe that you are not reading and the OS will block the child from writing anymore until you do something in the parent to reduce the content of the pipe.

The number of iterations it will run will depend upon the amount of output produced by the child. Ie. How quickly it fills the pipe buffer. Eg. In the following which just echos the 'some command's it reads, the iteration count gets to 83 before it blocks.

#! perl -slw use strict; use IPC::Open2; $|++; my $pid = open2 \*READ, \*WRITE, 'perl -wne "$|++; chomp; print"'; my $read = ''; for ( 1 .. 1e4 ) { Win32::Sleep( 10 ); printf "\r$_\tGot'$read'\t"; print WRITE 'some command'; } __END__ c:\test>junk Name "main::READ" used only once: possible typo at c:\test\junk.pl lin +e 7. 83 Got'' Terminating on signal SIGINT(2)

83 * 13 = 1079, suggesting roughly a 1k buffer.

Reduce the output to a single char:

#! perl -slw use strict; use IPC::Open2; $|++; my $pid = open2 \*READ, \*WRITE, 'perl -wne "$|++; chomp; print 1"'; my $read = ''; for ( 1 .. 1e4 ) { Win32::Sleep( 10 ); printf "\r$_\tGot'$read'\t"; print WRITE 'some command'; } __END__ c:\test>junk Name "main::READ" used only once: possible typo at c:\test\junk.pl lin +e 7. 553 Got'' Terminating on signal SIGINT(2)

This time the iteration count gets to 553, which means that the size of the buffer is a little more complicated.

But if you process the output from the child within the parent's loop:

#! perl -slw use strict; use IPC::Open2; $|++; my $pid = open2 \*READ, \*WRITE, 'perl -wne "$|++; chomp; print 1"'; my $read = ''; for ( 1 .. 1e6 ) { # Win32::Sleep( 10 ); printf "\r$_\tGot'$read'\t"; print WRITE 'some command'; sysread( READ, $read, 1024 ); } __END__ c:\test>junk 1000000 Got'1'

Now the loop happily runs for as long as you care to leave it running. The OS is simply protecting you from yourself, by preventing you from consuming all your memory by filling a pipe with output that you are never processing.

Note: I used the non-blocking sysread to process the output as the child process is never producing newlines, so <READ> or read would block.


Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

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Re^2: open2() in Windows
by bhaveshbp (Initiate) on Jul 23, 2007 at 16:07 UTC
    Sorry I forgot to mention in my previous post that inside the parent loop it is processing the output from the child process. But not with the sysread() function. We are doing it like this:
    print WRITE $cmd while (<READ>) { if (/blah/) { do something } else { do something else } }
      But if I do it this way:
      $|++; open2(READ,WRITE,program); for (1..6000) { print WRITE $cmd; while (<READ>) { if (/blah/) { do something; } else { do another thing; } } }
      This should flush the pipes everytime right because im setting $| to something greater than zero? Thus, the pipes should never be full?

        If your code is as you show it in this post, then I don't see how you are ever getting to a loop count greater than 1?

        The problem is, while( <READ>) { ... } doesn't terminate until it gets EOF, but as the child process isn't closing the file, it never will. So you will issue one command, process it's output, and then block in the inner while loop. You'll never get back to issuie the second command.


        Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
        "Science is about questioning the status quo. Questioning authority".
        In the absence of evidence, opinion is indistinguishable from prejudice.

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