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Parent process unable to read messages from child

Posted by gauri (gauri), 28 February 2007

I've a process which is supposed to do the following:
1.A Parent process has to fork to create fixed number of child
processes. (This fixed number changes each time my code runs.)
2. Each child computes certain values and writes these values on to a
3. The parent should read each of these values from pipe output of all
children and store them in a global variable to be used later in the

Fork work correct, Child processes compute the correct value, however
Parent is unable to read output of all children. For e.g. when 3
children are created, one child completes execution first, and parent
is able to read it's value. The other two processes complete 20
seconds later and exit immediately one after the other. The parent is
unable to read the values of the 2nd and 3rd child.

I want the parent to be able to read the values of all the children
immediately after they exit.
Can some one help me fix the problem?

Here is my script:

use IO::Select;
use IO::File;

my $selectpipe = new IO::Select;

for ($splitnum = 1; $splitnum <= $numparts; $splitnum++) { #$numparts
changes each time.
     pipe($uncompread, $uncompwrite) or die "Cannot pipe: $!";
     my $pid = fork();
     die "fork: $!" unless defined $pid;
     if ($pid){#parent
         push (@childs, $pid);
         close $uncompwrite;
     } else { #child
         close $uncompread;

        # compute the following arrays:
        # $uncompfile[$splitnum],
        # $uncompsize[$splitnum],
        # $uncompcksum[$splitnum]
        # and store the above values in the array: $uncomp[$splitnum - 1] as follows:

         $uncomp[$splitnum - 1] = "$uncompfile[$splitnum]
$uncompsize[$splitnum] $uncompcksum[$splitnum]";
         print $uncompwrite $uncomp[$splitnum - 1];



# In the following part I'm trying to store the @uncomp() array above
in the @filedetail() array.
# The number of elements in @filedetail() array should be equal to
$numparts above.
# This is the part which causes error. I'm unable to read all elements
in the @uncomp() array
# above. After the first element in the @uncomp() array is read, the
process exits. Can
# somebody please help?

$count = 0;
  foreach my $client ($selectpipe->can_read) {
       $filedetail[$count] = <$uncompclient>;


close $uncompread;
close $uncompwrite;



I'm using Perl version 5.8.5 on OS-Red Hat Linux.


Posted by admin (Graham Ellis), 1 March 2007
Isn't your $uncompread getting reset each time around?

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