Parent process unable to read messages from child process

[himagauri]

Hello,

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
pipe.
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
process.


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;
$selectpipe->add($uncompread);
} else { #child
close $uncompread;
$uncompwrite->autoflush(1);


# 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];


exit;
}



}


# 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;
while(1){
foreach my $client ($selectpipe->can_read) {
$filedetail[$count] = <$uncompclient>;
$count++;
}


}


close $uncompread;
close $uncompwrite;

foreach(@childs){
waitpid($_,0);



}


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

-Thanks,

 

[PROFESSORSPARKIE]

From some old thoughts.

Without my writing a lot of code and if you are not writing for very high concurrent level transaction rate where going to disk can be a problem, I remember doing something like that a long time ago. I start each child with a unique name of something like “child1" concatenated with the dynamic time-date of the start of the child. Pass it the time-date as a parameter and the child can then read/create a unique communication file to contain any thing you want to pass in either direction.

Example:
Parent creates a file & starts the child using the same name.time-date. The parent or child can read/write to the file as desired. Setup a folder to hold these files and have the parent remove the file at completion. Locks should not be a problem on most jobs if the parent waits for the child to complete because each communication file is uniquely named.

You can set up a time triggered task to run every once in a to clear out old files in case some bad coding or ab-end of any parents before they clean up the folder/dir.

Again this is not fancy or high performance but can work. You can if allowed use a RAM-drive to hold the dir/folder to improve performance.

 

[travs69]

2 questions on this

$count = 0; 
while(1){ 
foreach my $client ($selectpipe->can_read) { 
$filedetail[$count] = <$uncompclient>; 
$count++; 
} 
}

Where does $uncompclient come from? I can't find this anywhere else in your code. Did you mean that to just be $client?

Isn't this going to loop forever? I don't see anyway for it to ever exit.