Question

Is there a way to clone an existing struct with different member alignment in Visual C++?

Here is the background:

I use an 3rd-party library, which defines several structs. To fill up the structs, I pass the address of the struct instances to some functions. Unfortunately, the functions only returns unaligned buffer, so that data of some members are always wrong.

/Zp is out of choice, since it breaks the other parts of the program. I know #pragma pack modifies the alignment of the following defined struct, but I would rather avoid copying the structs into my code, for the definitions in the library might change in the future.

Sample code:

library.h:

struct am_aligned
{
  BYTE data1[10];
  ULONG data2;
};

struct untouched
{
  BYTE data1[9];
  int data2;
};

test.cpp:

#include "library.h"

// typedef alignment(1) struct am_aligned am_unaligned;

int APIENTRY wWinMain(HINSTANCE hInstance,
                     HINSTANCE hPrevInstance,
                     LPTSTR    lpCmdLine,
                     int       nCmdShow)
{
  char buffer[20] = {};

  for (int i = 0; i < sizeof(am_unaligned); i++)
  {
    buffer[i] = i;
  }

  am_aligned instance = *(am_aligned*) buffer;
  untouched useless;

  return 0;
}

am_unaligned is my custom declaration, and only effective in test.cpp. The commented line does not work of course. untouched should still has the default alignment.

instance.data2 is 0x0f0e0d0c, while 0x0d0c0b0a is desired.

Thanks for help!

Answer 1

You should use #pragma pack before including the headers of the library you use (and after, with push and pop). This way you won’t have to define the structs by yourself.

#pragma pack(push, 1)
#include <libraryheader.h>
#pragma pack(pop)

// You can now use the struct defined in the library

Indeed, the library has been compiled using a given alignment. You cannot change this, it is already hard-coded in the binary compiled library. However, you have to instruct the compiler, when you compile your files, that the structs contained in the library, are to be defined and used with another alignment.