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Re:Async.h – asynchronous, stackless subroutines in C by anfilt on Saturday September 21, 2019 @04:18AM (#21034137)
Looks like it's just using Duff's device like you would use to implement a coroutine.
Re:Async.h – asynchronous, stackless subroutines in C by kevsim on Saturday September 21, 2019 @08:34AM (#21034964)
Yep, first line from the README:

> this is a header-only async/await implementation for C based on Duff's device