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 2 weeks ago by cm0002@ttrpg.network to c/linux@programming.dev
24 comments fedilink hide all child comments
 

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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[–] iloveDigit@piefed.social 20 points 2 weeks ago (5 children)

Why?

[–] entwine@programming.dev 68 points 2 weeks ago (2 children)

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-extensions flag 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. b and c will be members of struct test2, not test. 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; //OK

That 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

[–] iloveDigit@piefed.social 8 points 2 weeks ago

Nice, thank you

[–] DeltaWingDragon@sh.itjust.works 1 points 1 week ago (1 children)

What's the point of this? If you have a struct within a struct, you probably want them to nest. The -fms-extensions will un-nest them, which is not what you mean.

// If I type this:
struct test {
        int a;
        struct test2 {
                char b;
                float c;
        };
        double d;
};

struct test my_test;

// I want this to happen:
my_test.test2.b = 'x'; //assigning the members of the nested struct
my_test.test2.c = 3.141; //this will work
printf("%f", my_test.c); //this will NOT work since c is a property of test2
[–] entwine@programming.dev 1 points 1 week ago (1 children)

You can already do that in standard C like this:

struct test {
        int a;
        struct {
                char b;
                float c;
        } test2;
        double d;
};

I can't think of any particular reason why you'd want an unnamed struct inside a struct, but you definitely would want to be able to have an unnamed struct inside a union. I suspect the struct-inside-struct thing can become useful in some scenarios involving unions.

[–] DeltaWingDragon@sh.itjust.works 1 points 1 week ago (1 children)

Does that really have the same results as the example scenario I described? How would you even access the unnamed struct, since it is unnamed?

[–] entwine@programming.dev 1 points 1 week ago* (last edited 1 week ago)

The same way you did, via the name of the member: my_test.test2.b = 'x';

The unnamed struct provides the type for a member named test2. Doing it this way saves you the trouble of defining the struct externally and giving it a name. It's identical to this, except in this example you can reuse the struct definition:

struct crappyname {
    char b;
    float c;
};
struct test {
    int a;
    struct crappyname test2;
    double d;
};
[–] UnityDevice@lemmy.zip 8 points 1 week 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.

[–] deegeese@sopuli.xyz 6 points 2 weeks ago

Old languages should be able to learn new tricks.

[–] HappyFrog@lemmy.blahaj.zone 6 points 2 weeks ago

Some software require it sadly

[–] ZILtoid1991@lemmy.world 2 points 1 week ago

The "extend" phase of EEE.