... it gives me 24, and I am confused. What are the additional 7 bytes?
These are padding bytes inserted by the compiler. Data structure padding is implementation dependent.
From Wikipedia, Data structure alignment:
Data alignment means putting the data at a memory offset equal to some multiple of the word size, which increases the system's performance due to the way the CPU handles memory. To align the data, it may be necessary to insert some meaningless bytes between the end of the last data structure and the start of the next, which is data structure padding.
