Rust memory layout
Before learning about rust layout we need to know about data alignment.
Data aligment is all about aligning data on the basis of word size so that CPUs can read data more efficiently and fastly. word size is a smallest chunk of data CPUs can read at a time or in one cycle. 64 bit machine have a 8byte word size, 32 bit machine have 4byte word size. That’s mean 64-bit machine can read 8-bytes of data from memory at a time
Let’s take an example, assuming we’ve 64-bit computer
struct NotGood {
a: u8, // 1byte
b: u64, // 8byte
}Q. What will be the size of this struct?
A.
- [Noob] 9 byte
- [Pro] 16 byte
Q. How this struct is 16-byte?
A. Well as it turns out that we are using 64-bit machine, the word size is 8 byte. And by default rust’s struct will use #[repr(Rust)]. #[repr(rust)] will align struct according to the word size. So the memory layout of the struct NotaGood is 16bytes (field a is 8bytes and field b is again 8byte)