Parent process unable to read messages from child process

[Gauri]

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,
Regards,
Gauri

 

[anno4000]

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:

Unfortunately, it isn't strict-compliant. That makes actual debugging
harder, so I'll only add a few remarks.

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;
}
}

It appears that each child prints only a single line of output. That
may simplify things.

Up to here your code looks like you know what you are doing. That
ends here.

# 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){

Starting an endless loop...

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

Why aren't you using the file handle that ->can_read returned to you?
Instead, you're reading from the fixed file handle $uncompclient
that is left over from the first part. That will only work once.

In a well-scoped program $uncompclient wouldn't even be in sight.
All file handles in this part should come from $selectpipe.

$count++;
}
}

You did nothing to break the endless loop. I find it hard to believe
that the "process exits", as your comment above states.

close $uncompread;
close $uncompwrite;

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

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

If the amount of output of each child process is small, as your code
seems to indicate, a simple solution (without select) is possible.
It relies on the assumption that the kids never block on output and
*is* fragile. For a better solution, fix your original code.

use Vi::QuickFix;
use Data:umper;

my $numparts = 5;
my %pipes;
for (my $splitnum = 1; $splitnum <= $numparts; $splitnum++) {
pipe my ( $uncompread, $uncompwrite) or die "Cannot pipe: $!";
defined( my $pid = fork()) or die "fork: $!";
if ( $pid ) { # parent
close $uncompwrite;
$pipes{ $pid} = $uncompread;
} else { # child
close $uncompread;
sleep rand 5; # random execution time
print $uncompwrite "result of part $splitnum\n"; # some output
exit;
}
}

while ( ( my $pid = waitpid( -1, 0)) > 0 ) { # wait for next kid to be done
my $pipe = $pipes{ $pid}; # find its pipe
push @detail, scalar <$pipe>; # read output
}
chomp @detail; # make output more readable

print Dumper \ @detail;
__END__

Anno

 

[Gauri]

Hi,
Thanks for the quick. I tried to implement the solution you suggested
exactly as you've posted here, However, the parent process populates
the @detail array with only one pipe output. i.e. the output of the
child which exited first. Perhaps there is some delay between the
time, Child process writes to the pipe and parent reads from the
pipe.
Could someone suggest something more?

-Gauri

 

[xhoster]

Hi,
Thanks for the quick. I tried to implement the solution you suggested
exactly as you've posted here, However, the parent process populates
the @detail array with only one pipe output. i.e. the output of the
child which exited first. Perhaps there is some delay between the
time, Child process writes to the pipe and parent reads from the
pipe.

It worked fine for me (well, once I declared @detail because I had added
use strict). Post exactly what you ran and what output you got.

Could someone suggest something more?

Please don't top post.

Xho

 

[anno4000]

It worked fine for me (well, once I declared @detail because I had added
use strict).

Ah... sorry, that's two copy/paste errors. It should have had
strictures and warnings (instead of using Vi::QuickFix), and
@detail was supposed to be declared.

Post exactly what you ran and what output you got.


Please don't top post.

Hehe

Anno