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Portable/strictly conforming alternative to the "struct hack" (?)

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I believe I've found a way to achieve something like the well-known "struct hack". I'm curoius if this strictly conforms to C89/C90 (I'm writing a library and my goal is full, strict C89/C90 conformance.)

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  large enough so that `K * sizeof(array_base_type) >= sizeof(the_struct)`, and `N` is the number of array elements.

Then, I use the pointer that `malloc()` returned to store `the_struct`, then I use pointer arithmetic to obtain a pointer to the beginning of an array of `N` elements of the array following the struct.

One line of code is worth more than a thousand words, so here is an 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 = /* arbitrary array size here */;
    void *frame = malloc((n_hdr + n_arr) * sizeof(Value));

    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?
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Portable/strictly conforming alternative to the "struct hack" (?) Árpád Goretity <arpad.goretity@gmail.com> - 2013-09-02 04:08 -0500
  Re: Portable/strictly conforming alternative to the "struct hack" (?) Jasen Betts <jasen@xnet.co.nz> - 2013-09-06 23:24 -0500
    Re: Portable/strictly conforming alternative to the "struct hack" (?) Jasen Betts <jasen@xnet.co.nz> - 2013-09-11 17:26 -0500
      Re: Portable/strictly conforming alternative to the "struct hack" (?) Árpád Goretity <arpad.goretity@gmail.com> - 2013-10-03 13:37 -0500
        Re: Portable/strictly conforming alternative to the "struct hack" (?) Keith Thompson <kst-u@mib.org> - 2013-10-08 16:38 -0500

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