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

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On 2013-09-02, Árpád Goretity <arpad.goretity@gmail.com> wrote:
> 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;

strange....

>     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);

that n_hdr expression looks kind of dodgy, basically it evaluates to 1, 
anyone who goes to such lengths to caclulate 1 is unlikley to have
discovered something special. After that  I stopped trying to understand 
your code

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

Look at modern C compilers ...
this has been perfectly valid, well-behaved C since the 1999 standard:

struct clump {
    int      thing ;
    sometype data[];
	}

-- ⚂⚃ 100% natural

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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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