structure alignment issues?

[Regan Heath]

On Wed, 31 May 2006 20:31:42 -0400, akcom <CppCoder at gmail.com> wrote:
> akcom wrote:
>> Translating the following from windows:
>>  --- C Code:
>>  #define IMAGE_SIZEOF_SHORT_NAME              8
>>  typedef struct _IMAGE_SECTION_HEADER {
>>     BYTE    Name[IMAGE_SIZEOF_SHORT_NAME];
>>     union {
>>             DWORD   PhysicalAddress;
>>             DWORD   VirtualSize;
>>     } Misc;
>>     DWORD   VirtualAddress;
>>     DWORD   SizeOfRawData;
>>     DWORD   PointerToRawData;
>>     DWORD   PointerToRelocations;
>>     DWORD   PointerToLinenumbers;
>>     WORD    NumberOfRelocations;
>>     WORD    NumberOfLinenumbers;
>>     DWORD   Characteristics;
>> } IMAGE_SECTION_HEADER, *PIMAGE_SECTION_HEADER;
>>  #define IMAGE_SIZEOF_SECTION_HEADER          40
>>  --- D Code:
>>  const uint IMAGE_SIZEOF_SHORT_NAME = 8;
>> const uint IMAGE_SIZEOF_SECTION_HEADER = 40;
>>  align struct ImageSectionHeader
>> {
>>     BYTE    Name[IMAGE_SIZEOF_SHORT_NAME];
>>     union Misc
>>     {
>>             DWORD   PhysicalAddress;
>>             DWORD   VirtualSize;
>>     }
>>     DWORD   VirtualAddress;
>>     DWORD   SizeOfRawData;
>>     DWORD   PointerToRawData;
>>     DWORD   PointerToRelocations;
>>     DWORD   PointerToLinenumbers;
>>     WORD    NumberOfRelocations;
>>     WORD    NumberOfLinenumbers;
>>     DWORD   Characteristics;
>> }
>> alias ImageSectionHeader IMAGE_SECTION_HEADER;
>> alias ImageSectionHeader *PIMAGE_SECTION_HEADER;
>>  the problem is that ImageSectionHeader.sizeof == 36
>> Any ideas as to why?
>
> edit:
> it appears as if when the union is replaced by a single DWORD,  
> everything works as expected.  Should this be filed as a bug?

No. The D struct you give above declares a type called "Misc" which is a  
union containing 2 DWORDS, but, you don't actually declare a struct member  
of that type. In short, your struct does not contain the union at all,  
only it's type declaration.

You have 2 options:

struct ImageSectionHeader
{
   ..
   union {
     DWORD   PhysicalAddress;
     DWORD   VirtualSize;
   }
   ..
}

this is an anonymous union, of 2 DWORDS, or:

struct ImageSectionHeader
{
   ..
   union NAMED_BOB {  //creates type called NAMED_BOB
     DWORD   PhysicalAddress;
     DWORD   VirtualSize;
   }
   NAMED_BOB Misc;    //includes a NAMED_BOB in the struct, called 'Misc'
   ..
}

both of these give a sizeof 40 for me.

Regan