Slim Reader/Writer Locks are neato

I’m 18 years late, but Slim Reader/Writer Locks have a fantastic interface: pointer-sized (“slim”), zero-initialized, and non-allocating. Lacking cleanup, they compose naturally with arena allocation. Sounds like a futex? That’s because they’re built on futexes introduced at the same time. They’re also complemented by condition variables with the same desirable properties. My only quibble is that slim locks could easily have been 32-bit objects, but it hardly matters. This article, while treating Win32 as a foreign interface, discusses a paper-thin C++ wrapper interface around lock and condition variables, in my own style.

If you’d like to see/try a complete, working demonstration before diving into the details: demo.cpp. We’re going to build this from the ground up, so let’s establish a few primitive integer definitions:

Think of uz as like uintptr_t. This implementation will support both 32-bit and 64-bit targets, and we’ll need it as the basis for locks and condition variables:

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