# Ruby threads and the system call

**URL:** <https://www.ruby-forum.com/t/ruby-threads-and-the-system-call/66583>\
**Category:** Ruby\
**Created:** [August 27, 2006, 10:17am UTC](https://www.ruby-forum.com/t/ruby-threads-and-the-system-call/66583 "2006-08-27T10:17:36Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![gnuvince](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/82dd89/32.png) [@gnuvince](https://www.ruby-forum.com/u/gnuvince)\
**Post date:** [August 27, 2006, 10:17am UTC](https://www.ruby-forum.com/t/ruby-threads-and-the-system-call/66583/1 "2006-08-27T10:17:36Z")

</div>

Hello !

I’ve recently had quite a fair bit of trouble with Ruby threads (on a  
Linux box). I wrote three different programs (attached) that write  
numbers from within threads:

- one with a standard puts
- one with a system “echo …”
- one with a fork do system “echo …” end

The first one behaves as expected (although the numbers are perfectly  
ordered, which looks suspicious). In the second one, ruby never manages  
to make a second system call in a thread (and finishes before the  
subprograms are terminated). The third behaves a little better, but  
crashes after, say, 3 to 4 system calls in a thread…

Is this true on other boxes ? Is it a flaw of my understanding of  
threads, or a limitation inherent to the way threads are coded in Ruby ?  
If that is so, it’s really annoying - no way to delegate heavy tasks to  
other well-written programs…

Thoughts ?

```
Vince
```

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<div class="post-metadata">

**Author:** ![gnuvince](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/82dd89/32.png) [@gnuvince](https://www.ruby-forum.com/u/gnuvince)\
**Post date:** [August 27, 2006, 1:36pm UTC](https://www.ruby-forum.com/t/ruby-threads-and-the-system-call/66583/2 "2006-08-27T13:36:21Z")

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Vincent F. wrote:

/ …

> Thoughts ?

Try this code:

* * *

#!/usr/bin/ruby -w

system “rm temp.txt” if File.exists? “temp.txt”

10.times { |i|  
x = Thread.new(i) {  
10.times { |j|  
fork {  
system “echo #{i}, #{j} \>\> temp.txt”  
}  
Process::wait  
}  
}  
x.join  
}

system “wc -l temp.txt” # should print “100 temp.txt”

* * *

According to the documentation for “fork”, “Process::wait” is needed to  
avoid the creation of zombie processes. “x.join” assures that the log  
file  
isn’t written by multiple processes in a way that corrupts it.

If you want to test simultaneous independent threads, you could write to  
a  
separate file per thread, then combine the files after all threads have  
completed as an accuracy check. But I don’t see this as anything but  
multiple writes to a single file (or console output) that creates I/O  
inconsistencies, in an otherwise consistent program.

> The third behaves a little better, but  
> crashes after, say, 3 to 4 system calls in a thread…

As to “crash”, what error messages did you see? How do you define  
“crash”?

This may be system-related. You might not have the resources required  
for  
this program, if all threads and forks run at once. Just a guess.

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<div class="post-metadata">

**Author:** ![gnuvince](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/82dd89/32.png) [@gnuvince](https://www.ruby-forum.com/u/gnuvince)\
**Post date:** [August 27, 2006, 4:56pm UTC](https://www.ruby-forum.com/t/ruby-threads-and-the-system-call/66583/3 "2006-08-27T16:56:42Z")

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On 8/27/06, Vincent F. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> - one with a fork do system “echo …” end  
> Both fork and the system call have the effect of creating a subprocess  
> which  
> then interacts with your main process through open file descriptors and  
> through signals. Both interact badly with threads. I suggest you  
> consider a  
> non-threaded design, fork off your subprocesses, and use waitpid2 to  
> distinguish the various returns. If you need to capture data generated  
> by  
> print statements in the subprocesses, then you can play painful games  
> like  
> redirecting the standard file descriptors to other file numbers, or you  
> can  
> simply make sure you only have one subprocess in flight at a time.

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<div class="post-metadata">

**Author:** ![gnuvince](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/g/82dd89/32.png) [@gnuvince](https://www.ruby-forum.com/u/gnuvince)\
**Post date:** [August 27, 2006, 8:17pm UTC](https://www.ruby-forum.com/t/ruby-threads-and-the-system-call/66583/4 "2006-08-27T20:17:00Z")

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Hello !

> x.join  
> Great thanks, I had completely forgotten to call join for the  
> threads… By the way, the version with fork is around 4 times faster on  
> my box:

ruby test\_threads\_system.rb \> /dev/null 0.04s user 0.10s system 21% cpu  
0.639 total  
ruby test\_threads\_system\_fork.rb \> /dev/null 0.00s user 0.04s system  
25% cpu 0.171 total

Cheers and thanks !!

```
Vince
```