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Re: Pipes (for interprocess communication)

by robin (Chaplain)
on Dec 09, 2005 at 18:13 UTC ( #515616=note: print w/ replies, xml ) Need Help??


in reply to Pipes (for interprocess communication)

Here is a simple example:

pipe(my $reader, my $writer); # Create a pipe if (my $pid = fork()) { # In parent: read one line from the pipe and print it print "Message from child: ".<$reader>; } else { # In child: print a line of text to the pipe print $writer ("Hello, parent!\n"); }


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Re^2: Pipes (for interprocess communication)
by penguin_asylum (Sexton) on Dec 09, 2005 at 18:46 UTC
    Thanks

    And the parent could also write to $writer with the child reading from $reader, right?
      Right.

      Update: I should have been more specific. Unless you really know what you're doing, you should choose a direction (i.e. parent -> child or child -> parent) and use the pipe only in that direction. If you want to communicate both ways, just make two pipes.

Re^2: Pipes (for interprocess communication)
by penguin_asylum (Sexton) on Dec 09, 2005 at 19:36 UTC
    Sorry, one more question.
    Why is it that "print ''.<$reader>" works, but not just "print <$reader>" ?

      .<$reader> forces scalar context while <$reader> uses the context supplied by the print, which is array context. In array context, the <> operator sucks in the whole rest of the file.

      Why does this not work? Because it is going to wait indefinately the the end of the file to come. On pipes, the end of file comes when the other end of the pipe is closed. In robin's example, the pipe is never closed.

      Mostly for this reason, the usual paradigm is this:

      1. Create the pipe
      2. fork
      3. in the parent, close the reader and use only the writer
      4. in the child, close the writer and use only the reader.
      or, of course,
      1. Create the pipe
      2. fork
      3. in the parent, close the writer and use only the reader
      4. in the child, close the reader and use only the writer.

      Trying to use both the reader and the writer end of the same pipe in the same process leads to deadlock. See the documentation of things like IPC::Open2 for how this can occur with a longer chain of pipes too.

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