How is the size of a struct with Bit Fields determined/measured?

Question

#include 

typedef struct size
{
        unsigned int a:1;
        unsigned int b:31;
        unsigned int c:1;
} mystruct;

int main()
{
        myst         


        

Answer 1

Alignment

The compiler is rounding the size of the structure to 32 bits, the size of each object it may try to reference to 32 bits, and at the same time it is preserving the order of your bit fields.

So if you have a 32-bit item in the middle and 1-bit items on each side, that's 3 32-bit words to allocate and so: 12 bytes.

For the other two cases, it's just a question of how few 32-bit objects your bitfield sequence can be packed into, while still preserving field order.