Why is the size of this struct 12 and not 10? [duplicate]

Answer 1

Note That bool size on x64 is 2 bytes

Answer 2

It is because of structure packing. Compiler would make the size of structure a multiple of 4 bytes. If you do not want this to happen and see size as 10 bytes in your case, use __attribute__((packed))

struct ID3v2_frame{
char id[4];
unsigned int size;
bool flags[2];
}__attribute__((packed));

Note that this might affect the performance of the program. See this SO question:

Can __attribute__((packed)) affect the performance of a program?

Answer 3

You may find your answer from here or here.

Please check What packing? default packing (8)

Answer 4

My guess would be the terminator character in the flags and id array.

Answer 5

Please check in your main program:

sizeof(bool) //should return 2 my guess
sizeof(char) //should return 1
sizeof(unsigned int) //should return 4

Every compiler can have different sizes for data types.