Comment by apaprocki

You can play tricks if you’re willing to compromise on the ABI:

    typedef struct foo_ foo;
    enum { FOO_SIZE = 64 };
    foo *foo_init(void *p, size_t sz);
    void foo_destroy(foo *p);
    #define FOO_ALLOCA() \
      foo_init(alloca(FOO_SIZE), FOO_SIZE)

Implementation (size checks, etc. elided):

    struct foo_ {
        uint32_t magic;
        uint32_t val;
    };
    
    foo *foo_init(void *p, size_t sz) {
        foo *f = (foo *)p;
        f->magic = 1234;
        f->val = 0;
        return f;
    }

Caller:

    foo *f = FOO_ALLOCA();
    // Can’t see inside
    // APIs validate magic