A 2024 Discussion Whether To Convert The Linux Kernel From C To Modern C++
The Linux kernel is predominantly made up of C code with various hand-written Assembly plus the growing work around supporting Rust within the Linux kernel. While it's not clear yet if there's sufficient weight to make it a reality, a Linux kernel mailing list discussion has been restarted over potentially seeing the Linux kernel C code converted to C++ in the future.
Back on 1 April 2018 was a set of 45 patches by Red Hat engineer David Howells to begin converting the kernel to C++. This would allow the mainline kernel to make use of inline template functions, inline overloaded functions, class inheritance, and other features not currently supported by the Linux kernel with its C code. A bit hard to make serious discussions that day and ultimately the patches resided on the Linux kernel mailing list for six years without much discussion.
But yesterday longtime Linux developer H. Peter Anvin responded to that kernel mailing list thread. Anvin wrote a lengthy LKML post with his reasoning around why C++ for the Linux kernel may finally be the right time:
"Andrew Pinski recently made aware of this thread. I realize it was released on April 1, 2018, and either was a joke or might have been taken as one. However, I think there is validity to it, and I'm going to try to motivate my opinion here.
Both C and C++ has had a lot of development since 1999, and C++ has in fact, in my personal opinion, finally "grown up" to be a better C for the kind of embedded programming that an OS kernel epitomizes. I'm saying that as the author of a very large number of macro and inline assembly hacks in the kernel.
What really makes me say that is that a lot of things we have recently asked for gcc-specific extensions are in fact relatively easy to implement in standard C++ and, in many cases, allows for infrastructure improvement *without* global code changes (see below.)
C++14 is in my option the "minimum" version that has reasonable metaprogramming support has most of it without the type hell of earlier versions (C++11 had most of it, but C++14 fills in some key missing pieces).
However C++20 is really the main game changer in my opinion; although earlier versions could play a lot of SFINAE hacks they also gave absolutely useless barf as error messages. C++20 adds concepts, which makes it possible to actually get reasonable errors."
For those that may then raise the matter of "rewrite the C code in Rust!", Anvin proactively added in his message:
"Now, "why not Rust"? First of all, Rust uses a different (often, in my opinion, gratuitously so) syntax, and not only would all the kernel developers need to become intimately familiar to the level of getting the same kind of "feel" as we have for C, but converting C code to Rust isn't something that can be done piecemeal, whereas with some cleanups the existing C code can be compiled as C++.
However, I find that I disagree with some of David's conclusions; in fact I believe David is unnecessarily *pessimistic* at least given modern C++.
Note that no one in their sane mind would expect to use all the features of C++. Just like we have "kernel C" (currently a subset of C11 with a relatively large set of allowed compiler-specific extensions) we would have "kernel C++", which I would suggest to be a strictly defined subset of C++20 combined with a similar set of compiler extensions.) I realize C++20 compiler support is still very new for obvious reasons, so at least some of this is forward looking."
Jiri Slaby of SUSE Lans has come out in support of this C++ initiative for the Linux kernel. David Howells of Red Hat who originally posted the kernel patches has also chimed back in favoring this discussion.
We'll see where this LKML discussion leads and if there is finally enough traction in 2024+ for supporting modern C++ code -- or at least some defined C++14~20 subset -- within the Linux kernel. Linus Torvalds in the past has been passionately against C++ but we'll see if the tides have finally turned if he's more satisfied with recent C++ standards or if he remains defiant in keeping the Linux kernel in C.
It was only in 2022 meanwhile that the Linux kernel began moving from C89 to C11. Especially if there is consensus to permit a subset of C++14/C++20 programming in the kernel, it may still be some time before it's adopted to allow for broader compiler support to roll out before raising the base compiler requirements and even if receiving the miraculous endorsement of Torvalds it's not a decision to be taken lightly.
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