Question: what causes stdout to flush after `main`

Lucretiel · August 17, 2020, 1:52am

Currently, when I write this:

fn main() {
    // No newline
    print!("Hello, World");
    std::thread::sleep(1);
}

I see Hello World printed to stdout after the sleep, when the program finishes. This would imply that there is some mechanism by which the BufWriter is flushed (presumably it's dropped). Does anyone know what that mechanism is or where it happens? Normally, globals aren't dropped at all, even after the program returns (as they are in C++). I wasn't seeing anything obvious in stdio.rs that would cause this to happen in the stdout cast.

matklad · August 17, 2020, 5:39am

github.com

rust-lang/rust/blob/ac48e62db85e6db4bbe026490381ab205f4a614d/library/std/src/io/lazy.rs#L47


        Some((*ptr).clone())
    }
}

// Must only be called with `lock` held
unsafe fn init(&'static self, init: fn() -> Arc<T>) -> Arc<T> {
    // If we successfully register an at exit handler, then we cache the
    // `Arc` allocation in our own internal box (it will get deallocated by
    // the at exit handler). Otherwise we just return the freshly allocated
    // `Arc`.
    let registered = sys_common::at_exit(move || {
        let ptr = {
            let _guard = self.lock.lock();
            self.ptr.replace(done())
        };
        drop(Box::from_raw(ptr))
    });
    // This could reentrantly call `init` again, which is a problem
    // because our `lock` allows reentrancy!
    // That's why `get` is unsafe and requires the caller to ensure no reentrancy happens.
    let ret = init();

Found by looking at

github.com

rust-lang/rust/blob/5f88bea878767abf2e4305bbb60116b4b57f6eb3/library/std/src/rt.rs#L23


    issue = "none"
)]
#![doc(hidden)]

// Re-export some of our utilities which are expected by other crates.
pub use crate::panicking::{begin_panic, begin_panic_fmt, panic_count};

// To reduce the generated code of the new `lang_start`, this function is doing
// the real work.
#[cfg(not(test))]
fn lang_start_internal(
    main: &(dyn Fn() -> i32 + Sync + crate::panic::RefUnwindSafe),
    argc: isize,
    argv: *const *const u8,
) -> isize {
    use crate::panic;
    use crate::sys;
    use crate::sys_common;
    use crate::sys_common::thread_info;
    use crate::thread::Thread;

Lucretiel · August 17, 2020, 4:39pm

Excellent, thank you very much!

system · November 15, 2020, 4:40pm

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Question: what causes stdout to flush after `main`

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