Re: Managing the fork/execing and reaping of child processes

Simplify and add lightness (oh, wait, adding lightness is for aircraft design :)

#!/usr/bin/perl

# http://perlmonks.org/?node_id=1135034

use strict;
$| = 1;

my $active_readers = 0;
my $current_reader_limit = 10;
my $total = 0;

while(1)
  {
  if($active_readers < $current_reader_limit)
    {
    $active_readers++;
    ++$total;
    if(my $pid = fork) # parent
      {
      print "Spawned child $active_readers pid $pid  total $total\n";
      }
    elsif(defined $pid) # child
      {
      exec "echo Hello from child $active_readers pid \$\$"
        or die "exec failed with $!";
      }
    else # fork failed
      {
      die "fork failed with $!";
      }
    }
  elsif((my $pid = wait()) > 0)
    {
    $active_readers--;
    print "Reaped $pid, active = $active_readers\n"
    }
  }
[download]

Comment on Re: Managing the fork/execing and reaping of child processes Download Code

Replies are listed 'Best First'.
Re^2: Managing the fork/execing and reaping of child processes by Anonymous Monk on Jul 16, 2015 at 18:10 UTC
Rendered readable: #!/usr/bin/perl # http://perlmonks.org/?node_id=1135034 use strict; $\| = 1; my $active_readers = 0; my $current_reader_limit = 10; my $total = 0; while(1) { if($active_readers < $current_reader_limit) { $active_readers++; ++$total; if(my $pid = fork) { # parent print "Spawned child $active_readers pid $pid total $tota +l\n"; } elsif(defined $pid) { # child exec "echo Hello from child $active_readers pid \$\$" or die "exec failed with $!"; } else { # fork failed die "fork failed with $!"; } } elsif((my $pid = wait()) > 0) { $active_readers--; print "Reaped $pid, active = $active_readers\n" } } [download]	[reply] [d/l]
Re^3: Managing the fork/execing and reaping of child processes by Anonymous Monk on Jul 16, 2015 at 19:31 UTC
Sigh...	[reply]
Re^2: Managing the fork/execing and reaping of child processes by mr_mischief (Monsignor) on Jul 16, 2015 at 19:19 UTC
If the fork fails, you're still incrementing both `$active_readers` and `$total`. Those should probably be tracked in the parent.	[reply] [d/l] [select]
Re^3: Managing the fork/execing and reaping of child processes by Anonymous Monk on Jul 16, 2015 at 19:27 UTC
If the fork fails, the parent dies. Is that tracking enough? I figure that if the fork fails it's going to muck things up enough that the die is proper.	[reply]
Re^4: Managing the fork/execing and reaping of child processes by mr_mischief (Monsignor) on Jul 16, 2015 at 19:44 UTC
Okay, let me put it this way. If one wanted to not just die and to attempt to actually deal with not being able to fork more than X children even though Y children were requested, the example would need more changes than just removing the `die()` and reporting the error. The counts would also need to be moved. So in an example program that's going to be expanded upon and altered, it may be fruitful to track those things in the proper place from the beginning.	[reply] [d/l]
Re^5: Managing the fork/execing and reaping of child processes by Anonymous Monk on Jul 16, 2015 at 19:51 UTC
Re^2: Managing the fork/execing and reaping of child processes by Anonymous Monk on Jul 16, 2015 at 17:07 UTC
It works. That's beautifully simple. Now to debug what I actually WANTED to debug. I can't thank you enough.	[reply]


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