Logging macros

Logging is done using the error!, warn!, info!, debug! and trace! macros.

There is also the println! macro that always prints the content.

Each macro logs at the logging level indicated by its name. The syntax of these macros is roughly the same as the println!-macro.

fn main() {
extern crate defmt;
let len = 80u8;
// -> INFO:  message arrived (length=80)
defmt::info!("message arrived (length={})", len);

struct Message;
impl Message { fn header(&self) -> u8 { 0 } }
let message = Message;
// -> DEBUG: Header { source: 2, destination: 3, sequence: 16 }
defmt::debug!("{:?}", message.header());

The Format trait

Unlike core::fmt which has several formatting traits (Debug, Display), defmt has a single formatting trait called Format. The {} formatting parameter indicates that the Format trait will be used, meaning the argument must implement the Format trait.

fn main() {
extern crate defmt;
let x = 0;
defmt::trace!("{}", x);
//                  ^ must implement the `Format` trait

Type and display hints

The defmt grammar is similar to core::fmt, but not the same. The syntax of a formatting parameter is shown below:


Type hint

The Type hint always starts with a =. For one it enables the framework to further compress the logs resulting in higher throughput. Secondly it also typechecks the supplied value to fit the specified type and possibly coerces the value into the specified type.

The type hint can be a primitive or one, of, the special types. Read on to learn more about the type hints.

Display hint

The Display hint, always starts with a : and specifies the printing on the host side. Read more about it here.

Positional parameter

The pos parameter lets you specify the position of the value to format (see "Positional parameters").