Why does Rust not have a return value in the main function, and how to return a value anyway?

Question

In Rust the main function is defined like this:

fn main() {  } 

This function does not allow for a return value though. Why would a language not allow for a return value and is there a way to return something anyway? Would I be able to safely use the C exit(int) function, or will this cause leaks and whatnot?

Answer 1

As of Rust 1.26, main can return a Result:

use std::fs::File;  fn main() -> Result<(), std::io::Error> {     let f = File::open("bar.txt")?;      Ok(()) } 

The returned error code in this case is 1 in case of an error. With File::open("bar.txt").expect("file not found"); instead, an error value of 101 is returned (at least on my machine).

Also, if you want to return a more generic error, use:

use std::error::Error; ...  fn main() -> Result<(), Box<dyn Error>> {    ... } 

Answer 2

std::process::exit(code: i32) is the way to exit with a code.

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Rust does it this way so that there is a consistent explicit interface for returning a value from a program, wherever it is set from. If main starts a series of tasks then any of these can set the return value, even if main has exited.

Rust does have a way to write a main function that returns a value, however it is normally abstracted within stdlib. See the documentation on writing an executable without stdlib for details.