Bitfields in C with struct containing union of structs

Question

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
            {
                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. :\

Answer 1

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.