Re: [Libusb-devel] pthreads on 64bit Windows

[Orin E. <ori...@gm...>]

On Tue, Jan 26, 2010 at 8:35 PM, Michael Plante <mic...@gm...>wrote:

> Orin Eman wrote:
> >> See the pack pragma.  Not a bad idea to force the alignment in this case
> as the
> >> docs were insistent on it.
>
> Does pack apply to things not in structs/classes/unions?  I ran some simple
> test code, and it only seemed to matter when I enclose the three data items
> below in a struct.  Un-struct version at the end, with output.
>
>
> >> I've seen some header files leave the packing something other than the
> natural
> >> alignment resulting in strange alignment warnings from the compiler.
>
> Not surprising.
>
>
>
>
> ======
> #include <stdio.h>
>
> typedef unsigned __int64 HANDLE;
>
> #pragma pack(1)
>
> //struct {
> HANDLE x;
> unsigned __int32 y;
> HANDLE z;
> //} w;
> #define X /*w.*/x
> #define Y /*w.*/y
> #define Z /*w.*/z
>
> int main() {
>  printf("%p,%p,%p\n", &X, &Y, &Z);
>
>  return 0;
> }
> =======
>
> w/o struct, it prints 00409AA0,00409AA8,00409AB0
>
> w/ struct, it prints 00409AA0,00409AA8,00409AAC
>
> Clearly, it's aligned without the struct, but unaligned with the struct.
>  It
> doesn't hurt to pack(push,8), but it may also not matter, regardless of the
> headers.  Obviously this could be compiler-dependent, and this may not even
> be a good test, so perhaps we could just dump it in a struct anyway and
> pack
> it, to be safe.  What do you think?
>


Yes, along with the /Zp compiler flag, #pragma pack affects structures
only.  Everything else should have natural alignment so it's probably only
an issue if the variable is in a structure.

Orin.