Question

If I have a requirement to create a data structure that has the following fields:

16-bit Size field

3-bit Version field

1-bit CRC field

How would I code this struct? I know the Size field would be an unsigned short type, but what about the other two fields?

Answer 1

First, unsigned short isn’t guaranteed to be only 16 bits, just at least 16 bits.

You could do this:

struct Data
{
   unsigned short size : 16;
   unsigned char version : 3;
   unsigned char crc : 1;
};

Assuming you want no padding between the fields, you’ll have to issue the appropriate instructions to your compiler. With gcc, you can decorate the structure with __attribute__((packed)):

struct Data
{
   // ...
} __attribute__((packed));

In Visual C++, you can use #pragma pack:

#pragma pack(push, 0)
struct Data
{
   // ...
};
#pragma pack(pop)