I'm working on a project that makes heavy use of multithreading; I think it's been years since I wrote code that didn't at least use std::thread or std::async to some extent.

I started programming in C for bare-metal AVR microcontrollers (no threading needed in that case), and I didn't really move into C++ until just after C++11 was already established; that is to say, I have always had access to concurrency tools that are built into the standard library.

I'm curious how threads were implemented prior to C++11; I know that 3rd-party libraries exist for this, but how did the libraries themselves handle it? I'm assuming the only option was to use calls to the OS with a lot of preprocessor macros depending on the target OS. Writing loops with a stored state would work, but not only did coroutines not exist in the STL until much later, but this wouldn't take advantage of multi-core CPUs, which were already commonplace before C++11.

There are certainly some times I take modern language features for granted. So, for the experienced programmers out there: How did it used to be done?