Enumerate contents about concurrentqueue HOT 3 CLOSED

There is a blocking version of the queue (added not too long ago) that will make your job easier -- it handles all the blocking on dequeue using a lightweight semaphore (fast when the queue still has items, and doesn't spin-wait forever when it's empty). It doesn't have any blocking enqueue operations, though (that may be an interesting feature to add, though -- I'll have to think about it more).

As for enumerating the contents, I'm afraid that's far more tricky. Even under the assumption that all other threads are stopped, it's entirely possible that the memory effects of operations done on those threads are not yet propagated to the CPU core on which the GC thread is running, which means items might be "missed" even if they're actually logically in the queue. It depends on how the other threads are paused -- there would have to be a memory barrier just before pausing that release all changes to the GC thread, and the GC thread would need a corresponding memory barrier after that point to acquire all those changes.

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The world stop operation I use has a flag which is checked under a mutex which acts as a full barrier. It's a cooperative stop. Which is rather unfortunate.

However I have to back off my original need for blocking enqueue and dequeue. I have a queue already that does that (and allows enumerating all elements). I also have monitors. I'm using blocking condition variables/semaphores. Unfortunately that leads to a deadlock so I'm right now having to change all blocking operations to used timed waits that loop around and also check the world stop flag.

When a thread stops, it does a setjump() to save all the registers on the stack and then it saves its stack to a control object so the GC can scan all the thread stacks as roots.

With heavy trickery on some OS better can be done but its hard to provide a portable GC whilst still using C/C++ and OS native operations (Posix/Linux, OSX, Windows at least). OTOH any language without a GC is screwed.

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I'm continuing to maintain the queue but at this point will not be adding any new features.

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