liblfds.org

A very good and question, which I spent some time trying to do something about, some years ago.

The basic problem is that the queue (and all the data structures) are running in user-mode, no switching to the kernel, which is one of the ways in which they are efficient. You absolutely do -not- want to be switching back to kernel mode to wait for a next element.

Problem then is how -do- you wait?

Turns out there's very little - almost no - support for user-mode waiting.

You can call yield(), but then you're kinda busy waiting.

Or maybe the OS has support for user-mode threads (Windows calls them fibers) and then you can control when threads sleep or run; but that still leaves you with the problem of knowing when a queue element has arrived. There's no user-mode event or semaphore or what-have-you.

So right now there *is* no proper, user-mode solution. You have to go to the OS, and you have to use a kernel-based blocking mechanism, where the sender writes to the queue and then triggers the listener(s) to wake up. Which sucks.