Bitfields in C with struct containing union of structs

Hm... why is it that, when I print sizeof(struct MyStruct), it outputs 3 (instead of 2) for this code?

#pragma pack(push, 1)
    struct MyStruct
    {
        unsigned char a : 6;
        union
        {
            struct
            {
            开发者_运维知识库    unsigned int b : 9;
            };
        };
    };
#pragma pack(pop)

In case it matters, I'm running MinGW GCC 4.5.0 on Windows 7 x64, but honestly, the result is weird enough for me that I don't think the compiler and the OS matter too much here. :\

You can't have the field starting at an address that is not byte aligned. You're expecting:

6 bits + 9 bits -> 15 bits -> 2 bytes

but what you're getting is:

6 bits -> 1 byte
9 bits -> 2 bytes
total ->  3 bytes

The data is being stored as:

| 1 byte | 2 byte |3 byte | 
 aaaaaaXX bbbbbbbb bXXXXX  

when you were expecting:

| 1 byte | 2 byte |
 aaaaaabb bbbbbbbX  

edit: To clarify based on the comments below:

The union (and the containing struct) must be byte aligned. It doesn't matter that the contents are only 9 bits, the union/struct itself is a full 16 bits. Notice that you cannot do the following:

struct MyStruct
{
    unsigned char a : 6;
    union
    {
        struct
        {
            unsigned int b : 9;
        } c:9;
    } d:9;
};

As C won't let you specify the entire struct's bit-size.

Adding to the answer given by @nss -- my apologies, this would've been a comment if comments weren't so limited on formatting:

#include <stdlib.h>

struct Test {
  unsigned short x : 6;
  unsigned short y : 1;
  unsigned short z;
};

int main( int argc, char *argv[] ) {
  printf( "sizeof( Test ) = %d\n", sizeof( struct Test ) );

  return 0;
}

It prints '4' for the size. I tested with gcc, g++, and Sun Studio's CC and cc.

Not that I recommend doing what you're attempting to do, but you could probably do what you're attempting to do with a union. I've seen (but not written myself) code that looked like this:

struct Test {
  unsigned short x1 : 6;
  unsigned short x2 : 3;
                    : 1; // unused
  unsigned short x3 : 4;
  // ...
};

I might have the syntax slightly wrong there ... but I don't think so.

Point being: create two separate structs (or a struct and a union) with the layout you were going for, then insert some dummy members where they should overlap, and union those together.