# Threads issue

**URL:** <https://www.ruby-forum.com/t/threads-issue/108180>\
**Category:** Ruby\
**Created:** [June 29, 2007, 8:27pm UTC](https://www.ruby-forum.com/t/threads-issue/108180 "2007-06-29T20:27:49Z")\
**Posts on this page:** 6\
**Page:** 1

<div class="post-metadata">

**Author:** ![sirtyman](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/6a8cbe/32.png) [@sirtyman](https://www.ruby-forum.com/u/sirtyman)\
**Post date:** [June 29, 2007, 8:27pm UTC](https://www.ruby-forum.com/t/threads-issue/108180/1 "2007-06-29T20:27:49Z")

</div>

## Hi guys, Here is my program:

def my()  
i=0

```
while i<100
    i+=1
    puts "Into my: #{Thread.current}"
    Thread.stop if i == 40
end

```

end

myThread = Thread.new {  
puts “Into thread: #{Thread.current}”  
#Thread.stop  
my()  
}

i=0  
while i\<10  
i+=1  
puts “Main thread: #{Thread.current}”  
end

## puts “myThread is not alive” if !myThread.alive? Thread.pass #####################################here is my issue!!! myThread.run myThread.join()

I cannot understand why this program cannot run without Thread.pass  
line. Without the line following message appeard:

deadlock 0x283fef8: sleep:- - c:/Documents and Settings/Marcin/Moje  
dokumenty/P  
roximetry/Ruby/Threads2/threads2.rb:7  
deadlock 0x284c748: sleep:J(0x283fef8) (main) - c:/Documents and  
Settings/Marcin  
/Moje dokumenty/Proximetry/Ruby/Threads2/threads2.rb:27  
c:/Documents and Settings/Marcin/Moje  
dokumenty/Proximetry/Ruby/Threads2/threads  
2.rb:27:in `join’: Thread(0x284c748): deadlock (fatal)  
from c:/Documents and Settings/Marcin/Moje  
dokumenty/Proximetry/Ruby/Thr  
eads2/threads2.rb:27

I have lack of knowledge in programming threads but I supposed that  
myThread.run should starts stopped thread before myThread.join. Did I  
forget about something?

I will be appreciated for any help  
Thanks in advance.

---

<div class="post-metadata">

**Author:** ![sirtyman](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/6a8cbe/32.png) [@sirtyman](https://www.ruby-forum.com/u/sirtyman)\
**Post date:** [June 29, 2007, 9:21pm UTC](https://www.ruby-forum.com/t/threads-issue/108180/2 "2007-06-29T21:21:12Z")

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Marcin T. wrote:

> ```
> end
> 
> ```
> 
> while i\<10  
> i+=1  
> puts “Main thread: #{Thread.current}”  
> end
> 
> puts “myThread is not alive” if !myThread.alive?  
> Thread.pass #####################################here is my  
> issue!!!  
> myThread.run

Only an educated guess :  
myThread may not have stopped yet here. Strictly speaking, even  
Thread.pass can not guarantee it because the scheduler could decide to  
pass the control back to the “primary” thread before i == 40 in my().  
If you get a deadlock it probably means that Thread#run cannot be called  
on an already running Thread (which isn’t really surprising but if true  
should be mentionned in its documentation).

If it works with either Thread.pass or Thread.stop in the Thread.new  
block uncommented my guess could very well be the actual source of your  
problem.

Lionel

---

<div class="post-metadata">

**Author:** ![sirtyman](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/6a8cbe/32.png) [@sirtyman](https://www.ruby-forum.com/u/sirtyman)\
**Post date:** [June 29, 2007, 9:42pm UTC](https://www.ruby-forum.com/t/threads-issue/108180/3 "2007-06-29T21:42:24Z")

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Lionel B. wrote:

> Marcin T. wrote:
> 
> > ```
> > end
> > 
> > ```
> > 
> > while i\<10  
> > i+=1  
> > puts “Main thread: #{Thread.current}”  
> > end
> > 
> > puts “myThread is not alive” if !myThread.alive?  
> > Thread.pass #####################################here is my  
> > issue!!!  
> > myThread.run
> 
> Only an educated guess :  
> myThread may not have stopped yet here. Strictly speaking, even  
> Thread.pass can not guarantee it because the scheduler could decide to  
> pass the control back to the “primary” thread before i == 40 in my().  
> If you get a deadlock it probably means that Thread#run cannot be called  
> on an already running Thread (which isn’t really surprising but if true  
> should be mentionned in its documentation).
> 
> If it works with either Thread.pass or Thread.stop in the Thread.new  
> block uncommented my guess could very well be the actual source of your  
> problem.
> 
> Lionel

Lionel,  
It looks like the issue is in Thread#run which doesn’t guarantee  
starting myThread before Thread#join. If i.e. myThread.pass is commented  
out and any time consuming loop is included before myThread.run  
everything works fine (without Thread#pass).

---

<div class="post-metadata">

**Author:** ![sirtyman](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/6a8cbe/32.png) [@sirtyman](https://www.ruby-forum.com/u/sirtyman)\
**Post date:** [June 29, 2007, 9:57pm UTC](https://www.ruby-forum.com/t/threads-issue/108180/4 "2007-06-29T21:57:18Z")

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Marcin T. wrote:

> Lionel,  
> It looks like the issue is in Thread#run which doesn’t guarantee  
> starting myThread before Thread#join. If i.e. myThread.pass is commented  
> out and any time consuming loop

In the primary Thread I suppose?

> is included before myThread.run  
> everything works fine (without Thread#pass).

Which means you let myThread stop before running myThread.run, doesn’t  
it?

Does the rubydoc example for Thread.stop:

a = Thread.new { print “a”; Thread.stop; print “c” }  
Thread.pass  
print “b”  
a.run  
a.join

at least outputs ‘abc’ for you ?

Lionel

---

<div class="post-metadata">

**Author:** ![sirtyman](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/6a8cbe/32.png) [@sirtyman](https://www.ruby-forum.com/u/sirtyman)\
**Post date:** [June 29, 2007, 10:58pm UTC](https://www.ruby-forum.com/t/threads-issue/108180/5 "2007-06-29T22:58:50Z")

</div>

Preaty good experience, isn’t it? I know what is going on! 🙂  
Thread#run in main thread is invoked earlier than myThread is stopped.  
myThread is stopped when the main thread is fineshed (so after  
myThread.run statement). That is why Thread#join returns exception (dead  
lock) becouse myThread stops when condition in while in my() function is  
true.

Thanks for help. Now everything is clear.

---

<div class="post-metadata">

**Author:** ![sirtyman](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/6a8cbe/32.png) [@sirtyman](https://www.ruby-forum.com/u/sirtyman)\
**Post date:** [June 30, 2007, 10:42am UTC](https://www.ruby-forum.com/t/threads-issue/108180/6 "2007-06-30T10:42:08Z")

</div>

Joining a stopped thread causes a deadlock.

Consider this deadlocking code:

Thread.fork do  
Thread.stop  
end.join

On the other hand, the following code does work:

Thread.fork do  
Thread.stop  
end.run.join

To prove that Thread.stop was really executed, I ran both  
scripts with a trace:

[http://www.erikveen.dds.nl/tmp/code1.trace.png](http://www.erikveen.dds.nl/tmp/code1.trace.png)  
[http://www.erikveen.dds.nl/tmp/code2.trace.png](http://www.erikveen.dds.nl/tmp/code2.trace.png)

gegroet,  
Erik V. - [http://www.erikveen.dds.nl/](http://www.erikveen.dds.nl/)