Thursday, April 2, 2015

Semaphores are Surprisingly Versatile

Jeff Preshing (via Jean-Francois Roy):

I used to think semaphores were strange and old-fashioned. They were invented by Edsger Dijkstra back in the early 1960s, before anyone had done much multithreaded programming, or much programming at all, for that matter. I knew that a semaphore could keep track of available units of a resource, or function as a clunky kind of mutex, but that seemed to be about it.

My opinion changed once I realized that, using only semaphores and atomic operations, it’s possible to implement all of the following primitives[…]

Not only that, but these implementations share some desirable properties. They’re lightweight, in the sense that some operations happen entirely in userspace, and they can (optionally) spin for a short period before sleeping in the kernel.


With all of these applications, semaphores are more general-purpose than I originally thought – and this wasn’t even a complete list. So why are semaphores absent from the standard C++11 library? For the same reason they’re absent from Boost: a preference for mutexes and condition variables. From the library maintainers’ point of view, conventional semaphore techniques are just too error prone.

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