It's not, they're two separate things. Unfortunately, nobody can agree on terminology for this stuff. I try not to ever use the word 'coroutine' anymore because of this.
libco is what I call a cooperative threading library. It allocates a stack for each thread. This means that thread A can call six functions deep into helper functions, and suspend in the middle of a helper function, and then resume right back at that same point.
Thus, you program libco very similarly to how you do preemptive threading, only you don't ever need any locks, and you have to do your own scheduling (which can be as simple as "co_call and co_return", or "co_jump" in assembly terms.)
The CORO #define wrappers in the article is what I would call a very simple state machine. There is no thread, there is no state machine, there is no user-space context switching going on. You only get the call/ret style behavior, and you can't ret from a subroutine called inside of it. It's far, far less useful.
However, when you can get away with it, CORO will be about ten times faster on average. libco is optimized as much as possible with pure assembler. On ARM, it's literally three instructions. On x86, it's ten. The entirety of the burden is because modern CPUs do not enjoy having their stack pointer changed. Go to older CPUs and the overhead drops substantially, but few people are coding on a 68000 anymore. This also means that libco is not ISO C. It requires an assembly implementation or a platform that lets you poke at jmpbuf from setjmp/longjmp to work. In practice, it'll work anywhere you'd reasonably want. x86, amd64, ppc, ppc64, arm, mips, sparc, alpha; Windows, macOS, Linux, *BSD, Haiku; etc have all been tested. And even if you find something crazy obscure, you only have to write about twenty lines of code to port it.
The magic of libco is that it's about 100 times faster than preemptive threading would be for the context switching overhead. So when you actually need subroutines, and when you don't want to be swamped in locks/semaphores/critical sections, it can eliminate giant swaths of nested state machines (switch tables) by keeping all that state inside each local stack frame, transparently and out of your codebase. Deeply nested state machines are a serious pain in the ass to reason about and maintain, trust me.
Also ... both of these methods have a serious flaw (as does preemptive threading): serialization. If you want to save the state of your program in the middle of working, and restore to that position later (think save states in an emulator) ... it'll be a bit tricky with libco. But since the state is hidden by the #define, it'll be absolutely impossible with CORO, unless you redesigned it to use external state, which would make it even clunkier to use.
In conclusion: both complement each other. I feel C++20 or whatever should add CORO-style stackless state machine wrappers to the language for convenience. But it should definitely leave stack-based cooperative threading out, because that can easily be done as a library. (It can include library functions for that purpose, but it doesn't need language-level primitives.)
There's a hundred cooperative threading libraries, by the way. My aim with libco was to make a library that has four functions (co_create, co_delete, co_active, co_switch) and zero types/structs. As simple and fast as possible, supplying as many optimized backends as possible. Then let people build on top of that. The downside is most people still choose to write their own libraries from scratch, so there's a lot of wheel reinventing going on.
> It's not, they're two separate things. Unfortunately, nobody can agree on terminology for this stuff.
There is standard terminology though. Coroutines allow arbitrarily nested calls back and forth. So-called "stackful" coroutines are the only possibility in C. Other languages perform a program translation into continuation-passing style to support stackless coroutines with the same semantics.
CORO falls under a class of abstractions known as "generators". You can think of it like a degenerate coroutine limited to a single call level, but I don't think conflating the terminology like this is useful.
You got it backwards though; all the name means is the subroutine has multiple entry points for suspending and resuming execution, which can be used to implement nonpreemptive threads, generators and other abstractions.
