libco

libco is a C and C++ library from Tencent that has been used in WeChat's backend systems since 2013, running on tens of thousands of machines. It provides a way to write server-side code that handles many simultaneous tasks efficiently without the complexity of managing large numbers of threads.

The core idea is coroutines, which are units of work that can pause and resume. When one coroutine is waiting on a slow operation like a network request, instead of blocking a thread while waiting, the system can switch to running another coroutine. This makes it possible to handle far more concurrent connections than a traditional multi-thread approach would allow on the same hardware.

Libco offers two ways to use this. You can use a hook mechanism that intercepts standard network calls at the system level, which means existing code written in a straightforward sequential style can be converted to use coroutines with little or no changes to the business logic. Alternatively, you can use the explicit interfaces (co_create, co_resume, co_yield) to create and control coroutines directly with more fine-grained control.

The library also includes a shared-stack mode for cases where you need to support tens of millions of TCP connections on a single machine.

Building the library requires running make or cmake from the source directory. The README is brief and covers only the core description and build steps. Further documentation is limited.