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   Alexander Monakov (amonakov@mastodon.gamedev.place)'s status on Friday, 24-Jan-2025 19:15:29 JST Alexander Monakov

   Let's say a C program comprises two translation units, each of which declares the following type:

   struct list {
   struct list *next;
   void *payload;
   };

   For deciding whether they are compatible, the following clause of the language standard applies:

   "there shall be a one-to-one correspondence between their members such that each pair of corresponding members are declared with compatible types;"

   Apparently the implementation may claim that such self-referential types are not compatible! 🙃

   • Embed this notice
     Wolf480pl (wolf480pl@mstdn.io)'s status on Friday, 24-Jan-2025 19:15:26 JST Wolf480pl

     in reply to

     @amonakov what I'm saying is major compilers can abuse every loophole in the spec and if they decide to do so, there's no convincing them to stop

     Haelwenn /элвэн/ :triskell: likes this.
   • Embed this notice
     Wolf480pl (wolf480pl@mstdn.io)'s status on Friday, 24-Jan-2025 19:15:27 JST Wolf480pl

     in reply to

     @amonakov depends. Is the implementation's name "clang" or "gcc"?

     Haelwenn /элвэн/ :triskell: likes this.
   • Embed this notice
     Alexander Monakov (amonakov@mastodon.gamedev.place)'s status on Friday, 24-Jan-2025 19:15:27 JST Alexander Monakov

     in reply to

     • Wolf480pl

     @wolf480pl I'm probably missing what you're hinting at, but to me the most "interesting" (but not really) question is what choice an implementation aiming to detect many instances of UB, such as TIS-interpreter, would make.

   • Embed this notice
     Alexander Monakov (amonakov@mastodon.gamedev.place)'s status on Friday, 24-Jan-2025 19:15:28 JST Alexander Monakov

     in reply to

     Now suppose the program contains not two, but three such translation units. Let's say the instances of the struct type are A, B, and C. Can the implementation choose to A be compatible with B, B with C, but C incompatible with A?

   • Embed this notice
     Rich Felker (dalias@hachyderm.io)'s status on Friday, 24-Jan-2025 23:14:35 JST Rich Felker

     in reply to

     @amonakov Not specific to your example, but yes compatibility of structs is non transitive. We intentionally make use of something like this in the musl c11 thread primitives IIRC, to reuse pthread ones:

     https://git.musl-libc.org/cgit/musl/commit/?id=b7cf71a190813590860af25b32532b6c360ac502

   • Embed this notice
     Rich Felker (dalias@hachyderm.io)'s status on Saturday, 25-Jan-2025 00:07:55 JST Rich Felker

     in reply to

     @amonakov Maybe this isn't quite the same thing, but it's related. I'll think about it more.

   • Embed this notice
     Alexander Monakov (amonakov@mastodon.gamedev.place)'s status on Saturday, 25-Jan-2025 00:07:56 JST Alexander Monakov

     in reply to

     • Rich Felker

     @dalias I do not see how this relates to [non-]transitivity. What would be the hypothetical third struct that would be compatible with exactly one of pthread_mutex_t and mtx_t?

     Generally speaking, do you have an example that illustrates non-transitivity of compatibility relation that does not involve unprototyped function type?