c - Conforming variant of the old "struct hack" (?)

Question

I believe I've found a way to achieve something like the well-known "struct hack" in portable C89. I'm curious if this really strictly conforms to C89.

The main idea is: I allocate memory large enough to hold an initial struct and the elements of the array. The exact size is (K + N) * sizeof(array_base_type), where K is chosen so that K * sizeof(array_base_type) >= sizeof(the_struct) and N is the number of array elements.

First, I dereference the pointer that malloc() returned to store the_struct, then I use pointer arithmetic to obtain a pointer to the beginning of the array following the struct.

One line of code is worth more than a thousand words, so here is a minimal implementation:

typedef struct Header {
    size_t length;
    /* other members follow */
} Header;

typedef struct Value {
    int type;
    union {
        int intval;
        double fltval;
    } v;
} Value;

/* round up to nearest multiple of sizeof(Value) so that a Header struct fits in */
size_t n_hdr = (sizeof(Header) + sizeof(Value) - 1) / sizeof(Value);

size_t n_arr = 42; /* arbitrary array size here */
void *frame = malloc((n_hdr + n_arr) * sizeof(Value));

if (!frame)
    return NULL;

Header *hdr = frame;
Value *stack_bottom = (Value *)frame + n_hdr;

My main concern is that the last two assignments (using frame as both a pointer to Header and a pointer to Value) may violate the strict aliasing rule. I do not, however, dereference hdr as a pointer to Value - it's only pointer arithmetic that is performed on frame in order to access the first element of the value array, so I don't effectively access the same object using pointers of different types.

So, is this approach any better than the classic struct hack (which has been officially deemed UB), or is it UB too?

Answer 1

The "obvious" (well... not exactly obvious, but it's what comes to my mind anyway :-) ) way to cause this to break is to use a vectorizing compiler that somehow decides it's OK to load, say, 64 Headers into a vector register from the 42-rounded-up-to-64+ area at hdr which comes from malloc which always allocates enough to vectorize. Storing the vector register back to memory might overwrite one of the Values.

I think this vectorizing compiler could point to the standard (well, if a compiler has fingers...) and claim conformance.

In practice, though, I'd expect this code to work. If you come across a vectorizing compiler, add even more space (do the rounding up with a machine-dependent macro that can insert a minimum) and charge on. :-)