this post was submitted on 09 Nov 2025
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The Linux Kernel Looks To "Bite The Bullet" In Enabling Microsoft C Extensions (www.phoronix.com)
submitted 3 days ago by cm0002@ttrpg.network to c/linux@programming.dev
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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.

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[–] UnityDevice@lemmy.zip 8 points 2 days ago

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:

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.