TIL RUST #9: stdout vs stderr, Decoding the Panic, and Stack vs Heap

stdout vs stderr

At first glance, println! and eprintln! seem identical. They both output text to the terminal. But separating them unlocks powerful CLI behaviors.

• stdout (println!) is for your program’s actual output.
• stderr (eprintln!) is for errors and logs.

The cool part is redirection. If I run cargo run > output.txt, my normal output saves to the file, but errors still pop up on my screen.

Also, be careful with > versus >>. A single > overwrites the file every time you run it. If you want to keep a log of previous runs, use >> to append the new output to the end of the file instead.

Decoding the Panic

When my code crashed with a panic, I learned that running RUST_BACKTRACE=1 reveals the “stack trace”.

cargo run with RUST_BACKTRACE=1

If you use full backtrace, you’ll even see hex addresses (like 0x1049...). These point to the Text Segment where your binary’s instructions live, which is separate from the stack and heap.

cargo run with RUST_BACKTRACE=full

The Stack vs. Heap

Speaking of memory, I learned a fascinating detail about layout: the Stack and Heap grow towards each other.

• The Stack starts at high memory addresses and grows downward.
• The Heap starts low and grows upward.

They do this to maximize the available free space in the middle without colliding with each other.

Using println! without import

I noticed I never have to import println!, but I do have to use std::io for input. That’s because print commands are macros built into the language, available globally. Input functions like stdin() live in the std::io module, so you either call them as std::io::stdin() or bring them into scope with use.