How to check if child process is still alive?

how do you check if your child process from fork still alive or not? In perl I can send kill(0, $cid), I think. But Process.kill(0, child_id) always returns 1.

To be honest, I couldn't get the perl code to work either. It always returns 1 too.

I can do `ps -p #{child_id}` but I wonder if Process.kill should work too.

-andre

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how do you check if your child process from fork still alive or not? In perl I can send kill(0, $cid), I think. But Process.kill(0, child_id) always returns 1.

To be honest, I couldn't get the perl code to work either. It always returns 1 too.

I can do `ps -p #{child_id}` but I wonder if Process.kill should work too.

-andre

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On Thu, 1 Mar 2007, Andreas S wrote:
     #
     # returns true if pid is running, false otherwise
     #
       def alive pid
#--{{{
         pid = Integer("#{ pid }")
         begin
           Process::kill 0, pid
           true
         rescue Errno::ESRCH
           false
         end
#--}}}
       end
       alias alive? alive

it should always return 1. it raises if the pid is dead or you do not have
permission to signal the process.

-a
--
be kind whenever possible... it is always possible.
- the dalai lama

Using Process.kill(0, pid) will return 1 as long as the process exists. In
your situation, I'm guessing that the child process has terminated
but continues to exist as a zombie process. You have to call Process.wait
to reap the child process in order for the child process to disappear entirely.

pid = fork { sleep(10) }

puts Process.kill(0, pid) # 1
sleep 15
puts Process.kill(0, pid) # 1, process is a zombie at this point
Process.wait
puts Process.kill(0, pid) # exception, process doesn't exist

Gary Wright

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On Feb 28, 2007, at 5:17 PM, Andreas S wrote:

how do you check if your child process from fork still alive or not? In perl I can send kill(0, $cid), I think. But Process.kill(0, child_id) always returns 1.

To be honest, I couldn't get the perl code to work either. It always returns 1 too.

I can do `ps -p #{child_id}` but I wonder if Process.kill should work too.

You have to call Process.wait to reap the child process in order for the child process to disappear entirely.

Gary Wright

I don't want to use Process.wait because it will block. I have a queue that I want to refill with the next job in line when a slot is available. I'm not gaining much from forking jobs like this but my main purpose is to emulate LSF queue on local machine, in case its down or something. By doing so, the rest of the code still sees a queue.

I tried Ara's suggestion, but it doesn't seem to throw exception.

Thanks for such prompt response and I love how Ara, even for merely showing example, gave
  alias alive? alive
:slight_smile:

-andre

def alive pid
  pid = Integer("#{ pid }")
  begin
    Process::kill 0, pid
    true
  rescue Errno::ESRCH
    false
  end
end

c = fork do
   sleep 5
   print "Child exits"
   exit
end

while alive c
  print '.'; STDOUT.flush
  sleep 1
end

ruby test.rb

......Child exits.........

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Andreas S wrote:

You have to call Process.wait to reap the child process in order for the child process to disappear entirely.

I don't want to use Process.wait because it will block.

Then you should use Process.detach(child_pid)

Daniel

Then you should use Process.detach(child_pid)

Daniel

Hey, that does the trick. Thanks a lot Daniel, and to Gary and Ara.

-andre

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