Macro println

macro_rules! println {
    () => { ... };
    ($($arg:tt)*) => { ... };
}

Prints to the standard output, with a newline.

On all platforms, the newline is the LINE FEED character (\n/U+000A) alone (no additional CARRIAGE RETURN (\r/U+000D)).

This macro uses the same syntax as format!, but writes to the standard output instead. See std::fmt for more information.

The println! macro will lock the standard output on each call. If you call println! within a hot loop, this behavior may be the bottleneck of the loop. To avoid this, lock stdout with io::stdout().lock():

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

let mut lock = stdout().lock();
writeln!(lock, "hello world").unwrap();

Use println! only for the primary output of your program. Use eprintln! instead to print error and progress messages.

See the formatting documentation in std::fmt for details of the macro argument syntax.

Panics

Panics if writing to io::stdout fails.

Writing to non-blocking stdout can cause an error, which will lead this macro to panic.

Examples

println!(); // prints just a newline
println!("hello there!");
println!("format {} arguments", "some");
let local_variable = "some";
println!("format {local_variable} arguments");