Parent process unable to read messages from child process

Discussion in 'Perl Misc' started by Gauri, Feb 28, 2007.

  1. Gauri

    Gauri Guest

    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
    Gauri, Feb 28, 2007
    #1
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  2. Gauri

    Guest

    "Gauri" <> wrote:
    > 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.


    First of all, you should be using strict and warnings. If you did
    so, it probably would have helped you identify the problem.

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


    Since $client is what can_read returned, you should be reading from
    $client, not $uncompclient, which seems to be used no where else and
    so probably undefined. You also need to remove the handle from $selectpipe
    once it reaches eof. And you probably shouldn't mix <> and read on the
    same handles you are using Select on.

    And since your code doesn't do anything meaningful until all the children
    are done anyway, there is really no reason to use IO::Select at all. Just
    keep a list of the file handles and do a series of blocking reads on them:

    my @filedetail = map {<$_>} @list_of_child_handles;

    Xho

    --
    -------------------- http://NewsReader.Com/ --------------------
    Usenet Newsgroup Service $9.95/Month 30GB
    , Feb 28, 2007
    #2
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  3. Gauri

    Gauri Guest

    Hi,
    I appreciate the quick response.

    Apologies for the typo--
    The statement in the main code is:
    foreach $uncompclient($selectpipe->can_read){


    my @filedetail = map {<$_>} @list_of_child_handles;
    Could you please explain series of blocking read in details with
    respect to my code?

    -Thanks a lot.

    -Regards,
    Gauri



    On Feb 28, 2:09 pm, wrote:
    > "Gauri" <> wrote:
    > > 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.

    >
    > First of all, you should be using strict and warnings. If you did
    > so, it probably would have helped you identify the problem.
    >
    > > $count = 0;
    > > while(1){
    > > foreach my $client ($selectpipe->can_read) {
    > > $filedetail[$count] = <$uncompclient>;
    > > $count++;
    > > }
    > > }

    >
    > Since $client is what can_read returned, you should be reading from
    > $client, not $uncompclient, which seems to be used no where else and
    > so probably undefined. You also need to remove the handle from $selectpipe
    > once it reaches eof. And you probably shouldn't mix <> and read on the
    > same handles you are using Select on.
    >
    > And since your code doesn't do anything meaningful until all the children
    > are done anyway, there is really no reason to use IO::Select at all. Just
    > keep a list of the file handles and do a series of blocking reads on them:
    >
    > my @filedetail = map {<$_>} @list_of_child_handles;
    >
    > Xho
    >
    > --
    > --------------------http://NewsReader.Com/--------------------
    > Usenet Newsgroup Service $9.95/Month 30GB- Hide quoted text -
    >
    > - Show quoted text -
    Gauri, Feb 28, 2007
    #3
  4. Gauri

    Guest

    "Gauri" <> wrote:
    > Hi,
    > I appreciate the quick response.
    >
    > Apologies for the typo--
    > The statement in the main code is:
    > foreach $uncompclient($selectpipe->can_read){
    >
    > my @filedetail = map {<$_>} @list_of_child_handles;
    > Could you please explain series of blocking read in details with
    > respect to my code?


    The reason for using a select is so that you can do something useful
    while waiting for data to become ready for reading. But you don't
    have anything useful to do. So don't use select. In your spawning code,
    instead of adding the handle to the IO::Select object just push it onto
    @list_of_child_handles.

    Xho

    --
    -------------------- http://NewsReader.Com/ --------------------
    Usenet Newsgroup Service $9.95/Month 30GB
    , Mar 1, 2007
    #4
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