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Comment by amiga386

5 months ago

I get that it's defined that way, but I'd really like to know why.

I can see the value in saying that struct x* isn't compatible with struct y*, because they could have different alignment or packing rules. But struct x* and void*, which is already special-cased to allow assignment without a cast? Why aren't these considered compatible in function pointer parameter definitions?

Is there any work involved in casting void* to struct* (on any architecture) that a plain function pointer would miss out?

2 comments

amiga386

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teo_zero  5 months ago

Yes, some restrictions seem arbitrary. Just like why these two types are not compatible:

  struct a {int data;};
  struct b {int data;};

I know, I know, changing it would break existing code, etc.

  • amiga386  5 months ago

    You jest, but a central feature of C is that void* is a universal pointer, and can be used as the basis of polymorphism and context passing. That's why you can do things like:

        void qsort(void *base, size_t nmemb, size_t size,
               int (*compar)(const void *, const void *));

    It would be nice to write:

        int compare_foo(const struct foo *a, const struct foo *b) {
        ...
    }

    But instead we have to write this to avoid being declared "undefined behaviour":

        int compare_foo(const void *a, const void *b) {
        const struct foo *real_a = (struct foo *)a;
        const struct foo *real_b = (struct foo *)b;
        ...
    }

    So what is the upside of having this rule, given the same thing is going to happen anyway, just more verbose?