Static Memory

Step by step

1. 01Before anything runsStatics are baked into the compiled program, so they exist before main starts. Each &str static is a pointer and a length to text in read-only memory
2. 02A static inside a functionGREETING is declared inside main, but that only limits where its name can be used. It is still a static, so it was set up with the program, not when main reaches that line
3. 03main's frame is pushedmain has no locals of its own, so its frame only reserves a return slot
4. 04Print GREETINGprintln! is drawn as a function call here (it is really a macro). It borrows GREETING, which is itself a pointer and a length aimed at the static text, so the text is never copied and nothing is allocated
5. 05Print GREETING_GLOBALThe same happens for the global static: println! borrows GREETING_GLOBAL and follows it to a different spot in static memory
6. 06main's return valuemain returns the unit type (), written into its return slot. The println! frames are long gone
7. 07Program endsmain's frame is popped, but static memory is never freed while the program runs. It disappears only when the whole process exits

Key takeaways

• Statics exist before main and last until the process exits
• A static inside a function is still a static; only its name is scoped
• A &'static str is a pointer and a length into read-only memory
• Passing a &str copies the pointer, never the text

Resources

• The Rust Reference: Static itemsThe exact rules for statics, including static mut
• Rust by Example: 'staticThe two meanings of 'static, with examples
• std::println!What println! really expands to

Research notes

• A const is inlined wherever it is used and has no fixed address, while a static has exactly one address for the whole program
• Reading or writing a static mut needs unsafe, because any thread could be changing it at the same time
• As a trait bound, T: 'static means the type holds no borrows shorter than 'static, not that the value lives forever