start

The tracking issue for this feature is: #29633

Allows you to mark a function as the entry point of the executable, which is necessary in #![no_std] environments.

The function marked #[start] is passed the command line parameters in the same format as the C main function (aside from the integer types being used). It has to be non-generic and have the following signature:

let _:
fn(isize, *const *const u8) -> isize
;

This feature should not be confused with the start lang item which is defined by the std crate and is written #[lang = "start"].

Usage together with the std crate

#[start] can be used in combination with the std crate, in which case the normal main function (which would get called from the std crate) won't be used as an entry point. The initialization code in std will be skipped this way.

Example:

#![allow(unused)]
#![feature(start)]

fn main() {
#[start]
fn start(_argc: isize, _argv: *const *const u8) -> isize {
    0
}
}

Unwinding the stack past the #[start] function is currently considered Undefined Behavior (for any unwinding implementation):

#![feature(start)]

#[start]
fn start(_argc: isize, _argv: *const *const u8) -> isize {
    std::panic::catch_unwind(|| {
        panic!(); // panic safely gets caught or safely aborts execution
    });

    panic!(); // UB!

    0
}