in reply to Cannot print child process command during fork

It's hard to see why you get 18 vs 24 when the code you provided only creates one child. Could you provide some runnable code?

By the way, 

      system("/usr/bin/rsh $proc $cmd");
      exit(0);

[download]

is an expensive way of doing 

      exec("/usr/bin/rsh $proc $cmd");
      exit(0);  # In case exec fails.

[download]

Using system will needlessly fork a second time.

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Re^2: Cannot print child process command during fork
by whatwhat (Novice) on Aug 02, 2007 at 07:39 UTC
    I solved my problem, all I needed to do was move the print statement before the fork. It prints out all the commands now. I'll try the exec function, and make sure it works.

    I only included the forking subroutine in the previous post. For the sake of completeness here is a condensed (runnable) version of my code:

    #!/usr/bin/perl
use IO::File;
use POSIX ":sys_wait_h";

open(JUNKD,">test_rsh-commands.txt");
# Phase 1: Setup phase to spawn jobs
&spawn_jobs(1,handle_child); 
for($i=1;$i<=24;$i++) # BEGIN: loop 
  {
  $proc = "sp" . "$i"; # remote node to run command on
  $cmd = "date"; # simple test command
  
  # Phase 2: Spawn the jobs
  &spawn_jobs(2,$proc,$cmd,1,2,3);
  } # END: loop
# Phase 3: Wait for the jobs to finish
&spawn_jobs(3,26);
close JUNKD;

## BEGIN: Spwan children jobs on slave nodes ##
sub spawn_jobs
  {
  my @a=@_;
  my $phase,$i,$proc,$nt,$mc,$um,$sleep,$sub;

  $phase = $a[0];
  if($phase==1) { $sub = $a[1] }
  elsif($phase==2) { ($proc,$cmd,$nt,$mc,$um) = @a[1..5] }
  elsif($phase==3) { $sleep = $a[1] }
  
  if($phase==1) # Setup phase
    {
    # set up child signal handler
    $SIG{'CHLD'} = \&$sub;
    $|++;
    %fhlist;
    %fhlist2;
    %fhlist3;
    }
  elsif($phase==2) # Spawn the jobs phase
    {       
    # Create an anonymous file handle
    print JUNKD "/usr/bin/rsh $proc $cmd\n";
    $pid = fork();
    if($pid < 0 or not defined $pid)
      {
      print LOG "$#-> Can't fork! Bad kernel!";
      close LOG;
      die "$#-> Can't fork! Bad kernel!";
      }
    elsif($pid == 0)
      {
      # child process 
#      system("/usr/bin/rsh $proc $cmd");
# I'm commmenting out the above line, since not everyone
# has 24 remote nodes to run on.
#      system("$cmd");
      exec("$cmd");
      exit(0);
      }
    else
      {
      # Parent process, toss child file handle into the hash and move 
+on with
      # our lives.
      $fhlist{"$pid"} = $nt;
      $fhlist2{"$pid"} = $mc;
      $fhlist3{"$pid"} = $um;
      }
    }
  elsif($phase==3) # Wait till the children are done phase
    {
    while(1)   
      {
      @kl = keys(%fhlist);
      if($#kl >= 0)  
        {
        # mo' to do...
        sleep($sleep);
        }
      else
        {
        last;
        }
      }
    }
  }
### END: Spwan children jobs on slave nodes ##

sub handle_child 
  {
  # This gets called when a child dies... maybe more than one
  # died at the same time, so it's best to do this in a loop
  my $temp, $mcopy, $umbr, $nbias, $nmat;
  while(($dead_kid = waitpid(-1, WNOHANG)) > 0) 
    {
    $temp = $fhlist{"$dead_kid"}; # get the file descriptor back
    $mcopy = $fhlist2{"$dead_kid"};
    $umbr = $fhlist3{"$dead_kid"};
    delete($fhlist{"$dead_kid"});
    delete($fhlist2{"$dead_kid"});
    delete($fhlist3{"$dead_kid"});
    }
  }

    [download]
    I supplied a simple command (date), but in my full code I am issuing commands to run other programs on the remote node.

    Thanks for everyone's input!

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[d/l]

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