Maybe I'm missing the idea, but I'm not sure how this idea is supposed to work without using something like alloca.

If you have:

    struct foo;

in foo.h and:

    struct foo {
        int a;
        double x;
        ...
    };

in foo.c, you won't have sizeof(struct foo) available from bar.c, so your construction won't work in bar.c. You could define a function:

    size_t sizeof_foo(void);

which just returns sizeof(struct foo) from inside foo.c, but since this size is now only known at runtime, you'll need to resort to alloca or VLAs...