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