Extra bytes when declaring a member of a struct as uint32_t

Question

I have a problem when using the uint32_t type from the stdint.h library. If I run the following code (on Ubuntu linux 11.10 x86_64, g++ version 4.6.1):

#include          


        

Answer 1

It's because of alignment. On your platform uint32_t needs to be 4 byte aligned. In order to achieve that dest_addr_64_h has to have two bytes of padding right in front of it because the position right after the two uint8_t members is a multiple of 2 but not 4.

You can use the macro offsetof() to figure out exactly where members are placed within a struct, to see that this is true.

You can either try to make the compiler pack the members tighter together, or you can rearrange the members so that padding isn't needed.