Jump to content
  • Advertisement
Sign in to follow this  
Anand Baumunk

std::Queue and multithreading

This topic is 2139 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

If you intended to correct an error in the post then please contact us.

Recommended Posts

Hello,
I have a problem with the multithreading in c++, since I use it the first time. I have a std::queue, which will have added stuff at the front at random times by thread 1. Thread 2 then reads in the objects at the back of the queue and pops them out.
But since the queue is critical memory, the programm will spit out memoryerrors from time to time.

How can I make sure that the queue is not accessed by thread 2, when thread 1 needs to write. What I found on the www wasn't really helpful, so I hope you can help me out here!

Thanks a bunch.

Share this post


Link to post
Share on other sites
Advertisement
You could try using concurrent_queue if it's available on your compiler: http://msdn.microsoft.com/en-us/library/ee355358.aspx

Edit:
Ffs... Full Editor button doesn't work and forum doesn't parse link correctly -.- Edited by Zaoshi Kaba

Share this post


Link to post
Share on other sites
Normally seeing if you don't have access to specialised structures like the concurrent_queue Zaoshi mentioned, you would have to enforce a mutual exclusion on your data so it can accessed safely.

An often-used solution for this is, as Alvaro mentioned, a lock.
Be very careful when using synchronization mechanisms like these though, careless usage of these mechanisms may lead to some really nasty issues (deadlock anyone?) which will guarantee some massive headaches.

Share this post


Link to post
Share on other sites
The easy solution is to use a lock or someone else's thread safe queue as has already been suggested. If you're just trying to solve a problem, this is definitely the way to go.

If you're interested in really digging into concurrent programming for the purpose of learning, you may want to investigate writing your own lockless queue where exactly one thread can write and exactly one thread can read. It doesn't require a ton of code to do something simple and it's well worth the learning experience. The need to pass data continuously from thread A to thread B in this way is extremely common. An easy implementation is to have a "read pointer" and a "write pointer" that wraps in a circular buffer - just be careful of order of operations and make the read pointer and write pointer volatile. The "pointer" can be an index into the static-sized circular buffer.

Using a queue like this is great for passing data to be processed. It can also proxy a function call to another thread by using the "command" pattern. You can even pass exceptions this way. Edited by aclysma

Share this post


Link to post
Share on other sites

The easy solution is to use a lock or someone else's thread safe queue as has already been suggested. If you're just trying to solve a problem, this is definitely the way to go.

If you're interested in really digging into concurrent programming for the purpose of learning, you may want to investigate writing your own lockless queue where exactly one thread can write and exactly one thread can read. It doesn't require a ton of code to do something simple and it's well worth the learning experience. The need to pass data continuously from thread A to thread B in this way is extremely common. An easy implementation is to have a "read pointer" and a "write pointer" that wraps in a circular buffer - just be careful of order of operations and make the read pointer and write pointer volatile. The "pointer" can be an index into the static-sized circular buffer.

Using a queue like this is great for passing data to be processed. It can also proxy a function call to another thread by using the "command" pattern. You can even pass exceptions this way.



Lockless programming is a black art. I strongly suggest not messing with it until you are highly proficient with concurrent systems and understand them deeply. Moreover, it requires a very rich comprehension of how the OS and even hardware work to do lockless correctly.

IMHO, suggesting lockless programming to someone who is not already very experienced with concurrency is utterly bad advice.

Share this post


Link to post
Share on other sites
I recently saw an implementation of a lock free ringbuffer which would fit your problem, unfortunately it is written in Go, but it is a very small amount of code so if you are interested you could take a look:

https://github.com/textnode/gringo/blob/master/gringo.go

Share this post


Link to post
Share on other sites
I am not sure why you recommend a lock free ringbuffer, it will use a busy wait, won't it? A busy wait is seldom a good idea.

The Go example is short and nice, and will always yield the CPU to another process, if there is one that wants to execute. But if there isn't, it will busy wait.

Share this post


Link to post
Share on other sites
Sign in to follow this  

  • Advertisement
×

Important Information

By using GameDev.net, you agree to our community Guidelines, Terms of Use, and Privacy Policy.

We are the game development community.

Whether you are an indie, hobbyist, AAA developer, or just trying to learn, GameDev.net is the place for you to learn, share, and connect with the games industry. Learn more About Us or sign up!

Sign me up!