Question

In rpc.h, the GUID structure is declared as follows:

typedef struct _GUID  {      DWORD Data1;      WORD Data2;      WORD Data3;      BYTE Data[8]; } GUID; 

I understand Data1, Data2, and Data3. They define the first, second, and third sets of hex digits when writing out a GUID (XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXX).

What I never understood was why the last 2 groups were declared together in the same byte array. Wouldn’t this have made more sense (and been easier to code against)?

typedef struct _GUID  {      DWORD Data1;      WORD Data2;      WORD Data3;      WORD Data4;      BYTE Data5[6];  } GUID; 

Anyone know why it is declared this way?

Answer 1

It’s because a GUID is a special case of a UUID. For information on what all the fields mean, you can look at RFC 4122.