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RustPrint

Rust output macros: print!, eprintln!, write!, and format!

Rust’s formatting macros use the same format-string language, but they send the result to different destinations. Use format! for a new String, print! or println! for stdout, eprint! or eprintln! for stderr, and write! or writeln! for an existing writer.

If you know the destination and whether you want a newline but do not remember the macro, use the Rust Output Macro Chooser.

Macro Destination Newline Return
format! new String no String
print! / println! stdout no / yes ()
eprint! / eprintln! stderr no / yes ()
write! / writeln! fmt::Write or io::Write no / yes fmt::Result or io::Result
format_args! deferred formatting arguments no fmt::Arguments

stdout and stderr

Section titled “stdout and stderr”

Use stdout for a program’s primary output. Use stderr for errors and progress that should remain separate from piped data.

EXAMPLE: CI-CHECKEDCOMPILED · examples/format-output/src/reference_examples.rs#output-streams
println!("result={}", 42);
eprintln!("warning: retrying");

print! does not append a newline. Because stdout is frequently line-buffered, call flush() when text must be visible immediately.

EXAMPLE: CI-CHECKEDCOMPILED · examples/format-output/src/reference_examples.rs#output-flush
use std::io::{self, Write};

print!("Continue? [y/N] ");
io::stdout().flush().unwrap();

See flush stdout in Rust for the std::io::Write import, prompt pattern, and the common “no method named flush” error.

Lock repeated output

Section titled “Lock repeated output”

The standard documentation notes that print! locks stdout on every call. For a hot loop, lock once and write through the handle.

EXAMPLE: CI-CHECKEDCOMPILED · examples/format-output/src/reference_examples.rs#output-lock
use std::io::{self, Write};

let stdout = io::stdout();
let mut output = stdout.lock();
for value in 0..3 {
    writeln!(output, "{value}").unwrap();
}

Write without an intermediate String

Section titled “Write without an intermediate String”

write! targets either std::fmt::Write for Unicode strings or std::io::Write for byte-oriented output.

EXAMPLE: CI-CHECKEDTEST ASSERTED · examples/format-output/src/reference_examples.rs#output-string
use std::fmt::Write;

let mut output = String::new();
write!(&mut output, "{:#06x}", 42).unwrap();
assert_eq!(output, "0x002a");

Choose the destination

Section titled “Choose the destination”