docs: Clarify `std::io::Stdin::read_to_string` behavior

[ayyEve]

Hi all,
I'm getting a weird stdin blocking issue. when reading from stdin, the console appears to freeze, nothing except what i type is written to the console. however, on windows, when the app is terminated the console buffer is "flushed", and all missing output is written, this doesnt seem to happen on linux (again, through ssh).
I originally encountered this issue on windows, but i was able to replicate it on linux with an (slightly) older toolchain (through ssh)
I will test beta, nightly, and a much older toolchain soon.

I tried this code:

use std::io::Read;
fn main() -> std::io::Result<()> {
    println!("type your input");
    
    let mut buffer = String::new();
    std::io::stdin().read_to_string(&mut buffer)?;

    println!("got input {}", buffer);
    Ok(())
}

I expected to see this happen: "type your input" logged, the message i typed, and after hitting enter, "got input {X}" being logged, and the program closing

Instead, this happened: *"type out input" is logged, my message appears as im writing, but hitting enter does not input the message, almost like it inserts a newline instead. after this, the program must be terminated with ctrl+c *
linux:

windows:

Meta

rustc --version --verbose:

(linux)
rustc 1.52.1 (9bc8c42bb 2021-05-09)
binary: rustc
commit-hash: 9bc8c42bb2f19e745a63f3445f1ac248fb015e53
commit-date: 2021-05-09
host: x86_64-unknown-linux-gnu
release: 1.52.1
LLVM version: 12.0.0

(windows)
rustc 1.53.0 (53cb7b09b 2021-06-17)
binary: rustc
commit-hash: 53cb7b09b00cbea8754ffb78e7e3cb521cb8af4b
commit-date: 2021-06-17
host: x86_64-pc-windows-msvc
release: 1.53.0
LLVM version: 12.0.1

Backtrace

N/A, the code does not crash

lmk if i forgot anything and i'll update it asap

[AngelOnFira]

I tested this in Docker on Linux with Rust 1.33 from Feb 2019, and got the same result.

root@359301ed4de1:/app# rustup show
Default host: x86_64-unknown-linux-gnu
rustup home:  /usr/local/rustup

installed toolchains
--------------------

1.33-x86_64-unknown-linux-gnu
1.53.0-x86_64-unknown-linux-gnu (default)

active toolchain
----------------

1.33-x86_64-unknown-linux-gnu (overridden by '/app/rust-toolchain')
rustc 1.33.0 (2aa4c46cf 2019-02-28)

[fee1-dead]

This might be expected behavior on linux that is platform-specific, but I am not sure though. read_to_string would read from standard input until EOF is reached (you can input EOF by pressing Ctrl+D on the start of a new line). I don't know about windows though.

[jonas-schievink]

This works correctly on Linux/Unix. stdin only closes when pressing Ctrl+D and read_to_string will block until the stream is closed. Not sure about the Windows behavior though, that looks odd.

[ayyEve]

Hi all, so it appears this is working as expected (at least on linux). however, I personally think the docs should explain this a bit better.
The example for stdin() is the read_to_string, however i am used to stdin being used for user input, not piped input. This example only works for piped input, however it doesnt specify this, so a noob (like me) can easily have issues.
It would be nice if there was also a read_line example for stdin() which explains the differences between it and read_to_string.

[inquisitivecrystal]

@rustbot label A-io T-libs

[ben0x539]

Is this fixed by #87820?

[ayyEve]

yep, that resolves this issue