Two patches queued into the Linux kernel’s build system development tree, kbuild-next, would enable the -fms-extensions compiler argument everywhere for allowing GCC and LLVM/Clang to use the Microsoft C Extensions when compiling the Linux kernel. Being in kbuild-next these patches will likely be submitted for the Linux 6.19 kernel merge window next month but remains to be seen if there will be any last minute objections to this change.
The -fms-extensions compiler option honored by the GNU Compiler Collection and LLVM/Clang allow enabling some non-standard C/C++ constructs used within Microsoft header files and honored by the the Microsoft Visual C/C++ compiler. For Linux kernel development purposes, enabling the Microsoft C Extensions would allow including a tagged struct or union anonymously in another struct/union.
Pedant thought: The thumbnail shows one biting everywhere but the bullet.

it’s time we nibbled the casing and come to terms with bite the bullet, it’s just a bad metaphor.
In Danish and Norwegian the equivalent idiom is to “Swallow a camel” which sounds much funnier xD
This can only be to allow compilation of some MS headers during the kernel build, so what headers are they and why are they in the kernel?
No? The article literally explains that it’s for something else?
Would this cause consequences in terms of the project’s independence? Or are these extensions able to be distributed freely?
From what I can see, the GNU Compiler Collection supports this flag, so you can still build it with 100% free software.
Basically, it’s just behavior that doesn’t align with the C standard, but was introduced by MS. Then, GCC added a compiler flag which makes it behave like that, so that you can build code that requires that behavior.
It doesn’t seem to actually be dependent on MS, rather it’s named after them because it emulates the way their compiler works. I hope no Linux maintainers would entertain the idea of making it dependent on a non-free compiler.
int $ = 3;
Compiled with msvc back in the day for example, could be stuff like that. But IDK.
behavior that doesn’t align with the C standard … but was introduced by MS
Yep, that tracks. I’m still pissed off about microsoft’s non-standard implementation of HTTP 1.1 from however long ago it was that I had to conditionally work around it on the server side. They believe standards don’t apply to them and it seems like they’re right.
Ah, that makes sense. Thanks for the explanation! :)
The correct answer. It’s just using an extension Microsoft happens to have made, and everything still works fine without it.
Why?
The “extend” phase of EEE.
Once in a while, it turns out that enabling -fms-extensions could allow some slightly prettier code. But every time it has come up, the code that had to be used instead has been deemed “not too awful” and not worth introducing another compiler flag for.
That’s probably true for each individual case, but then it’s somewhat of a chicken/egg situation.
If we just “bite the bullet” as Linus says and enable it once and for all, it is available whenever a use case turns up, and no individual case has to justify it.
A lore.kernel.org search provides these examples:
- https://lore.kernel.org/lkml/200706301813.58435.agruen@suse.de/
- https://lore.kernel.org/lkml/20180419152817.GD25406@bombadil.infradead.org/
- https://lore.kernel.org/lkml/170622208395.21664.2510213291504081000@noble.neil.brown.name/
- https://lore.kernel.org/lkml/87h6475w9q.fsf@prevas.dk/
- https://lore.kernel.org/lkml/CAHk-=wjeZwww6Zswn6F_iZTpUihTSNKYppLqj36iQDDhfntuEw@mail.gmail.com/
Undoubtedly, there are more places in the code where this could also be used but where -fms-extensions just didn’t come up in any discussion.
Basically the extensions are useful sometimes. Note that they have nothing to do with Microsoft other than being invented by them.
I’m sitting around doing IT shit waiting things to download/backup/install/etc and have nothing better to do, so here’s an AI-free explanation with code samples:
It’s basically just a code style thing. Standard C allows you to declare unnamed structs/unions within other structs/unions. They must be unnamed, so it’d look like this:
struct test { int a; struct { char b; float c; }; double d; };Which is fine, but the
-fms-extensionsflag enables you to do the same thing with named structs. For example:struct test { int a; struct test2 { char b; float c; }; double d; };without
-fms-extensions, the above will compile, but won’t do what you might assume.bandcwill be members of structtest2, nottest. So something like this won’t compile:struct test my_test; my_test.b = 1; // error: ‘struct test’ has no member named ‘b’But with the flag, not only does it work, it also lets you do some convenient things like this:
struct test2 { char b; float c; }; struct test { int a; struct test2; double d; }; //... struct test my_test; my_test.b = 1; //OKThat is, you can reuse an existing struct definition, which gives you a nice little tool to organize your code.
Source: https://gcc.gnu.org/onlinedocs/gcc/Unnamed-Fields.html
If this is so convenient, why wasn’t it made a part of a newer C standard?
Nice, thank you
Some software require it sadly
Old languages should be able to learn new tricks.








