Data structure alignment is the way data is arranged and accessed in computer memory. It consists of two separate but related issues: data alignment and data structure padding. When a modern computer reads from or writes to a memory address, it will do this in word sized chunks (e.g. 4 byte chunks on a 32-bit system).

https://stackoverflow.com/questions/11770451/what-is-the-meaning-of-attribute-packed-aligned4