Filtering

defmt supports 5 different logging levels listed below from lowest severity to highest severity:

TRACE
DEBUG
INFO
WARN
ERROR

The logging macros trace!, debug!, info!, warn! and error! match these logging levels.

By default, only ERROR level messages are emitted. All other logging levels are disabled.

Note that defmt::println! statements cannot be filtered and are always included in the output.

`DEFMT_LOG`

if you are already familiar with env_logger and RUST_LOG, defmt's filtering mechanism works very similarly

To change which logging levels are enabled use the DEFMT_LOG environment variable.

$ export DEFMT_LOG=warn
$ cargo build --bin app

$ # OR if using probe-run as the cargo runner you can use
$ DEFMT_LOG=warn cargo run --bin app

DEFMT_LOG accepts the following logging levels: error, warn, info, debug, trace. Enabling a logging level also enables higher severity logging levels. For example,

#![allow(unused)]
fn main() {
extern crate defmt;

defmt::trace!("trace");
defmt::debug!("debug");
defmt::info!("info");
defmt::warn!("warn");
defmt::error!("error");
}

$ DEFMT_LOG=warn cargo run --bin all-logging-levels
WARN  warn
ERROR error

$ DEFMT_LOG=trace cargo run --bin all-logging-levels
TRACE trace
DEBUG debug
INFO  info
WARN  warn
ERROR error

A different logging level filter can be applied to different modules using logging directives. A logging directive has the syntax krate::module::path=level. DEFMT_LOG can contain a list of comma separated logging directives.

#![allow(unused)]
fn main() {
extern crate defmt;
// crate-name = app

mod important {
    pub fn function() {
        defmt::debug!("important debug");
        defmt::info!("important info");
        defmt::warn!("important warn");
        defmt::error!("important error");
    }
}

mod noisy {
    pub fn function() {
        defmt::warn!("noisy warn");
        defmt::error!("noisy error");
        inner::function();
    }

    mod inner {
        pub fn function() {
            defmt::warn!("inner warn");
            defmt::error!("inner error");
        }
    }
}

important::function();
noisy::function();
}

$ DEFMT_LOG=app::important=info,app::noisy=error cargo run --bin app
INFO  important info
WARN  important warn
ERROR important error
ERROR noisy error
ERROR inner error

Note that the app::noisy=error directive also applies to the internal module app::noisy::inner.

Hyphens

Crate names can have hyphens (-) in Cargo metadata, and file paths, but when they appear in logging directives all hyphens must be converted into underscores (_).

Packages vs crates

Do not confuse Cargo package names with crate names. A Cargo package can contain more than one crate. By default, the main crate has the same name as the package but this can be overridden in Cargo.toml (e.g. in the [lib] and [[bin]] sections).

$ cargo new --lib my-package

$ tree my-package
my-package # package-name = my-package
├── Cargo.toml
└── src
   ├── bin
   │  └── my-binary.rs # crate-name = my_binary
   └── lib.rs          # crate-name = my_package

Overrides

Logging directives that appear later in the list override preceding instances.

#![allow(unused)]
fn main() {
extern crate defmt;
// crate-name = app
pub fn function() {
    defmt::trace!("root trace");
}

mod inner {
    pub fn function() {
        defmt::trace!("inner trace");
        defmt::error!("inner error");
    }
}

function();
inner::function();
}

$ DEFMT_LOG=trace,app::inner=error cargo run --bin app
TRACE root trace
ERROR inner error

This is equivalent to saying: app::inner emits ERROR level log messages and everything else emits TRACE level log messages.

Disabling logs

The "pseudo" logging level off can be used to disable logs globally, per crate or per module.

$ # globally disable logs
$ DEFMT_LOG=off cargo run --bin app

$ # disable logs from the `noisy` crate (dependency)
$ DEFMT_LOG=trace,noisy=off cargo run --bin app

$ # disable logs from the `noisy` module
$ DEFMT_LOG=trace,app::noisy=off cargo run --bin app

Recompilation

It should be noted that DEFMT_LOG is a compile-time mechanism. Changing the contents of DEFMT_LOG will cause all crates that depend on defmt to be recompiled.

Default logging level for a crate

At the moment it's not possible to set a default logging level, other than ERROR, for a crate.

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