How do I use print! with Rocket when compiling for release?

Question

I've got some route handlers with print!() statements in it. They work just fine when I compile my app for debug. However, when compiling for release, Rocket appears to be "muting" all the print statements (as well as logging functions from log crate). Why is it doing so? How do I force it to print things then building for release? I suspect it might be connected to log level, which is automatically set to critical when compiling for release, however, I'm not quite sure what level do print! statements have and how to change the logging level.

I don't remember tracing can capture `print` statement. Can you provide an example of what you get ? — Stargateur, Aug 19 '21 at 20:51
You are specifically mentioning `print!()`, which does not [`flush()`](https://doc.rust-lang.org/stable/std/io/trait.Write.html#tymethod.flush) `stdout` after writing to it, so its output usually appears to be muted. As you mention, `rocket` sets the [default log level](https://rocket.rs/v0.5-rc/guide/configuration/#overview) to `normal` in debug builds, which will produce more/any logs, which in turn cause a `flush()` and therefore your `print()`-output to be flushed as well. Use `println!()` or manually `flush()`. — user2722968, Aug 19 '21 at 20:51
@user2722968 that worked, thanks! Would you like to add this as an answer, so I could accept it? — keddad, Aug 20 '21 at 10:22

score 3 · Accepted Answer · answered Aug 20 '21 at 10:31

You are specifically mentioning print!(), which does not flush() stdout after writing to it, so its output usually appears to be muted.

As you mention, rocket sets the default log-level to normal in debug builds, which will produce more/any logs, which in turn cause a flush() and therefore your print!()-output to be flushed as well.

Use println!() or manually flush() .

score 0 · Answer 2 · answered Apr 04 '23 at 07:20

I just want to add a quick reference for How to flush after print

In Rust, the print! macro is buffered by default, which means that the output may not be immediately visible. To flush the output buffer and ensure that your print! statements are immediately visible, you can use the std::io::stdout() function and call its flush() method.

use std::io::{self, Write};

fn main() {
    print!("Hello, world!");
    io::stdout().flush().unwrap(); // flushes the output buffer
}

How do I use print! with Rocket when compiling for release?

2 Answers2