Thread + fork warning

B

Bedo Sandor

# ruby -e 'a = Thread.new { fork {} }; a.join'
-e:1: warning: fork terminates thread at -e:1
# ruby -v
ruby 1.8.0 (2003-10-05) [i386-freebsd4]

is this intentional?


Hello!

I don't know, but I have the same warning, if I make a
fork{} in the main thread, and there is one or more
other running thread.

% vi thread.rb
1.: #!/usr/bin/ruby -w
2.:
3.: def forkAndWaitInThread
4.: Kernel.fork {
5.: Kernel.exec("sleep 5")
6.: }
7.: Thread.new {
8.: Process.wait
9.: }
10.: end
11.:
12.: 2.times { forkAndWaitInThread }
13.:
14.: Thread.list.each { |t| t.join unless t == Thread.current }
15.:

% chmod +x thread.rb ; ./thread.rb
./thread.rb:4: warning: fork terminates thread at ./thread.rb:8

Seems to work as I expected, but warnings make me misgiving.
Could somebody explain me the reasons? Am I doing wrong
if I leave out of consideration this warning? What is
the correct/usual method to execute processes in background?

% ruby -v
ruby 1.8.0 (2003-10-05) [i386-linux]
...from Debian/sid package
 
Y

Yukihiro Matsumoto

Hi,

In message "Re: Thread + fork warning"

|% chmod +x thread.rb ; ./thread.rb
|./thread.rb:4: warning: fork terminates thread at ./thread.rb:8
|
|Seems to work as I expected, but warnings make me misgiving.
|Could somebody explain me the reasons? Am I doing wrong
|if I leave out of consideration this warning? What is
|the correct/usual method to execute processes in background?

When you fork, all other threads are terminated in the child process.
The simplest solution is not mixing fork and threads, for example, use
system instead of fork+exec.

matz.
 
B

Bedo Sandor

Hi,

In message "Re: Thread + fork warning"

|% chmod +x thread.rb ; ./thread.rb
|./thread.rb:4: warning: fork terminates thread at ./thread.rb:8
|

When you fork, all other threads are terminated in the child process.

But only in the child process!? This is exactly what I
want! In my program the main process forks&execs other
processes, and in the main process creates a thread to
wait/waitpid the forked process, and log the time and
the pid when done. I don't need thread in the Ruby
interpreter in the child processes, they only execs an
external command. I think the correct behavior would
be in a threaded Ruby program that fork does not harm
threads in the main process, but starts an other
interpreter without threads running only with the
commands in the block passed to the Kernel.fork.

The simplest solution is not mixing fork and threads, for example, use
system instead of fork+exec.

I have two problems with system():

- I can't log the termination of the spawned
process.

- Don't know if this is a correct way to start
background processes: (I need background processes,
so I have to continue the main Ruby program!)

def startInBg(unixCommand)
system(unixCommand.to_s + "&")
end
...
startInBg 'zcat /var/log/messages*.gz | logClassifier'

It works, and there's no warnin, but what
do You think about +"&" ?
 
Y

Yukihiro Matsumoto

Hi,

In message "Re: Thread + fork warning"

|- I can't log the termination of the spawned
| process.

How do you want to watch process? If you want to store logs into a
file, you can use shell redirect such as ">" etc.

If you want to get logs as stream output, you can use popen3 library.

|- Don't know if this is a correct way to start
| background processes: (I need background processes,
| so I have to continue the main Ruby program!)
|
| def startInBg(unixCommand)
| system(unixCommand.to_s + "&")
| end
| ...
| startInBg 'zcat /var/log/messages*.gz | logClassifier'
|
| It works, and there's no warnin, but what
| do You think about +"&" ?

+"&" should work OK; you are working on Unix.

matz.
 

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments. After that, you can post your question and our members will help you out.

Ask a Question

Similar Threads


Members online

No members online now.

Forum statistics

Threads
473,744
Messages
2,569,483
Members
44,903
Latest member
orderPeak8CBDGummies

Latest Threads

Top