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0 pointspjc5015h ago0 comments

> You can roll stackful coroutines in C++ (or C) with 50-ish lines of Assembly

I'm not normally keen to "well actually" people with the C standard, but .. if you're writing in assembly, you're not writing in C. And the obvious consequence is that it stops being portable. Minicoro only supports three architectures. Granted, those are the three most popular ones, but other architectures exist.

(just double checked and it doesn't do Windows/ARM, for example. Not that I'm expecting Microsoft to ship full conformance for C++23 any time soon, but they have at least some of it)

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giancarlostoro11h ago

> Not that I'm expecting Microsoft to ship full conformance for C++23 any time soon,

They are actively working on it for their VS2026 C++ compiler. I think since 2017 or so they've kept up with C++ standards reasonably? I'm not a heavy C++ guy, so maybe I'm wrong, but my understanding is they match the standards.

manwe15014h ago

Boost has stackful coroutines. They also used to be in posix (makecontext).

audidude10h ago

> I'm not normally keen to "well actually" people with the C standard, but .. if you're writing in assembly, you're not writing in C.

These days on Linux/BSD/Solaris/macOS you can use makecontext()/swapcontext() from ucontext.h and it will turn out roughly the same performance on important architectures as what everyone used to do with custom assembly. And you already have fiber functions as part of the Windows API to trampoline.

I had to support a number of architectures in libdex for Debian. This is GNOME code of course, which isn't everyone's cup of C. (It also supports BSDs/Linux/macOS/Solaris/Windows).

* https://packages.debian.org/sid/libdex-1-1

* https://gitlab.gnome.org/GNOME/libdex

gpderetta9h ago

Unfortunately swap context requires saving and restoring the signal mask, which, at least on Linux, requires a syscall so it is going to be at least a hundred times slower than an hand rolled implementation.

Also, although not likely to be removed anytime soon from existing systems, POSIX has declared the context API obsolescent a while ago (it might actually no longer be part of the standard).

bonzini3h ago

Stackful coroutines also can't be used to "send" a coroutine to a worker thread, because the compiler might save the address of a thread local variable across the thread switch (happened in QEMU).

cyberax5h ago

Signal mask? What century are we in?

It can be safely ignored for the vast majority of apps. If you're using multithreading (quite likely if you're doing coroutines), then signals are not a good fit anyway.

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blacklion12h ago

There is no "Linux/ARM[64]". But there are "Raspberry Pi" and "RISC-V". I don't know such OSes, to be honest :-)

This support table is complete mess. And saying "most platforms are supported" is too optimistic or even cocky.

fluoridation14h ago

I think what they meant is that that what it takes to add coroutines support to a C/++ program. Adding it to, say, Java or C# is much more involved.

ndiddy11h ago

Looking at the repo, it falls back to Windows fibers on Windows/ARM. If you'd like a coroutine with more backends, I'm a fan of libco: https://github.com/higan-emu/libco/ which has assembly backends for x86, amd64, ppc, ppc-64, arm, and arm64 (and falls back to setjmp on POSIX platforms and fibers on Windows). Obviously the real solution would be for the C or C++ committees to add stackful coroutines to the standard, but unless that happens I would rather give up support for hppa or alpha or 8-bit AVR or whatever than not be able to use stackful corountines.

gpderetta8h ago

A proposal to add stackfull coroutines has been around forever and gets updated at every single mailing. Unfortunately the authors don't really have backing from any major company.

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