libco is a cooperative multithreading library written in C89.
Although cooperative multithreading is limited to a single CPU core, it scales substantially better than preemptive multithreading.
For applications that need 100,000 or more context switches per second, the kernel overhead involved in preemptive multithreading can end up becoming the bottleneck in the application. libco can easily scale to 10,000,000 or more context switches per second.
Ideal use cases include servers (HTTP, RDBMS) and emulators (CPU cores, etc.)
It currently includes backends for:
libco v18 (source code)
libco is released under the ISC license.