# Threading weirdness

**URL:** <https://www.ruby-forum.com/t/threading-weirdness/131229>\
**Category:** Ruby\
**Created:** [March 7, 2008, 1:20am UTC](https://www.ruby-forum.com/t/threading-weirdness/131229 "2008-03-07T01:20:35Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Philip\_Scott](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/2bfe46/32.png) [@Philip\_Scott](https://www.ruby-forum.com/u/Philip_Scott)\
**Post date:** [March 7, 2008, 1:20am UTC](https://www.ruby-forum.com/t/threading-weirdness/131229/1 "2008-03-07T01:20:35Z")

</div>

Hi guys,

I’d be grateful if someone could take a look at the following two test  
cases I made after noticing some strange behavior with ruby threads. I  
think the  
code speaks clearer than I could, but in essence if you create a lot  
of threads from the same block all reading from a single array, things  
go wierd. It is almost as if variables get mixed up between the  
threads. I am fairly sure this is not expected behavior… If I make  
the threads as part of a class, this doesn’t seem to happen.

The first does as you’d expect and never terminates. The second  
invariably stops after a few thousand iterations when my exception  
gets raised.

I hope I am not wasting your time and have overlooked some glaringly  
obvious details about threads, but I am banging my head on the wall  
here 🙂

Cheers,

Philip

################################ PROGRAM 1: WORKS, NEVER TERMINATES  
require ‘thread’  
require ‘digest/md5’

# A little class that takes references to two arrays in its constructor

# then starts a thread which compares random elements from the two and

# throws an exception if they are ever different.

class Tester  
def initialize (arrayA, arrayB)  
@arrayA = arrayA  
@arrayB = arrayB  
Thread.start {mainloop}  
end

def mainloop  
testThread = Thread.new do  
while true  
i = rand(9)  
if (@arrayA[i] != @arrayB[i])  
raise Exception.new(“Threading Fuckup”)  
end  
end  
end  
end  
end

a = []  
b = []

# Fill an array with 10 random strings, and mirror this

# in another identical array.

for i in (0…9)  
a[i] = Digest::MD5.hexdigest(rand(1000).to\_s).to\_s  
b[i] = a[i]  
end

print “Starting threads…\n”  
Thread.abort\_on\_exception = true

# Start 100 threads comparing the elements in the array

for x in (0…100)  
Tester.new(a,b)  
end

print “And we’re off”

# And spin forever

while true do  
end

######################################## PROGRAM 2: DOESNT WORK,  
TERMINATES  
require ‘thread’  
require ‘digest/md5’

a = []  
b = []

# Fill two identical arrays full of 10 random strings

for i in (0…9)  
a[i] = Digest::MD5.hexdigest(rand(1000).to\_s).to\_s  
b[i] = a[i]  
end

print “Starting threads\n”  
Thread.abort\_on\_exception = true

# Start a hundred threads reading items from the two arrays and

comparing them  
for x in (0…100)  
testThread = Thread.new do  
while true  
i = rand(9)  
if (a[i] != b[i])  
raise Exception.new(“Threading Broke”)  
end  
end  
end  
end

print “And we’re off”

# Spin forever (or at least until an Exception gets thrown)

while true do  
end

---

<div class="post-metadata">

**Author:** ![Philip\_Scott](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/2bfe46/32.png) [@Philip\_Scott](https://www.ruby-forum.com/u/Philip_Scott)\
**Post date:** [March 7, 2008, 1:52am UTC](https://www.ruby-forum.com/t/threading-weirdness/131229/2 "2008-03-07T01:52:48Z")

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Philip Scott wrote:

> # Fill two identical arrays full of 10 random strings
> 
> for x in (0…100)  
> print “And we’re off”
> 
> # Spin forever (or at least until an Exception gets thrown)
> 
> while true do  
> end

The variable ‘i’ is shared across all threads because it was created  
when you initialized the arrays and is still in scope when you create  
the threads.

-Justin

---

<div class="post-metadata">

**Author:** ![Philip\_Scott](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/2bfe46/32.png) [@Philip\_Scott](https://www.ruby-forum.com/u/Philip_Scott)\
**Post date:** [March 7, 2008, 2:03am UTC](https://www.ruby-forum.com/t/threading-weirdness/131229/3 "2008-03-07T02:03:21Z")

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Quoting Justin C. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid):

> The variable ‘i’ is shared across all threads because it was created  
> when you initialized the arrays and is still in scope when you create  
> the threads.

Well don’t I feel like a dolt 🙂

Thank you ever so much, you have restored my faith in Ruby 🙂

Cheers,

Philip