Crate log

A lightweight logging facade.

A logging facade provides a single logging API that abstracts over the actual logging implementation. Libraries can use the logging API provided by this crate, and the consumer of those libraries can choose the logging framework that is most suitable for its use case.

If no logging implementation is selected, the facade falls back to a "noop" implementation that ignores all log messages. The overhead in this case is very small - just an integer load, comparison and jump.

A log request consists of a target, a level, and a body. A target is a string which defaults to the module path of the location of the log request, though that default may be overridden. Logger implementations typically use the target to filter requests based on some user configuration.

Use

In libraries

Libraries should link only to the log crate, and use the provided macros to log whatever information will be useful to downstream consumers.

Examples

#[macro_use]
extern crate log;

pub fn shave_the_yak(yak: &Yak) {
    info!(target: "yak_events", "Commencing yak shaving for {:?}", yak);

    loop {
        match find_a_razor() {
            Ok(razor) => {
                info!("Razor located: {}", razor);
                yak.shave(razor);
                break;
            }
            Err(err) => {
                warn!("Unable to locate a razor: {}, retrying", err);
            }
        }
    }
}

In executables

Executables should choose a logging framework and initialize it early in the runtime of the program. Logging frameworks will typically include a function to do this. Any log messages generated before the framework is initialized will be ignored.

The executable itself may use the log crate to log as well.

Warning

The logging system may only be initialized once.

Examples

#[macro_use]
extern crate log;
extern crate my_logger;

fn main() {
    my_logger::init();

    info!("starting up");

    // ...
}

Logger implementations

Loggers implement the Log trait. Here's a very basic example that simply logs all messages at the Error, Warn or Info levels to stdout:

extern crate log;

use log::{LogRecord, LogLevel, LogMetadata};

struct SimpleLogger;

impl log::Log for SimpleLogger {
    fn enabled(&self, metadata: &LogMetadata) -> bool {
        metadata.level() <= LogLevel::Info
    }

    fn log(&self, record: &LogRecord) {
        if self.enabled(record.metadata()) {
            println!("{} - {}", record.level(), record.args());
        }
    }
}

Loggers are installed by calling the set_logger function. It takes a closure which is provided a MaxLogLevel token and returns a Log trait object. The MaxLogLevel token controls the global maximum log level. The logging facade uses this as an optimization to improve performance of log messages at levels that are disabled. In the case of our example logger, we'll want to set the maximum log level to Info, since we ignore any Debug or Trace level log messages. A logging framework should provide a function that wraps a call to set_logger, handling initialization of the logger:

pub fn init() -> Result<(), SetLoggerError> {
    log::set_logger(|max_log_level| {
        max_log_level.set(LogLevelFilter::Info);
        Box::new(SimpleLogger)
    })
}

Use with no_std

To use the log crate without depending on libstd, you need to specify default-features = false when specifying the dependency in Cargo.toml. This makes no difference to libraries using log since the logging API remains the same. However executables will need to use the set_logger_raw function to initialize a logger and the shutdown_logger_raw function to shut down the global logger before exiting:

pub fn init() -> Result<(), SetLoggerError> {
    unsafe {
        log::set_logger_raw(|max_log_level| {
            static LOGGER: SimpleLogger = SimpleLogger;
            max_log_level.set(LogLevelFilter::Info);
            &SimpleLogger
        })
    }
}
pub fn shutdown() -> Result<(), ShutdownLoggerError> {
    log::shutdown_logger_raw().map(|logger| {
        let logger = unsafe { &*(logger as *const SimpleLogger) };
        logger.flush();
    })
}

Macros

debug	Logs a message at the debug level.
error	Logs a message at the error level.
info	Logs a message at the info level.
log	The standard logging macro.
log_enabled	Determines if a message logged at the specified level in that module will be logged.
trace	Logs a message at the trace level.
warn	Logs a message at the warn level.

Structs

LogLocation	The location of a log message.
LogMetadata	Metadata about a log message.
LogRecord	The "payload" of a log message.
MaxLogLevelFilter	A token providing read and write access to the global maximum log level filter.
SetLoggerError	The type returned by set_logger if set_logger has already been called.
ShutdownLoggerError	The type returned by shutdown_logger_raw if shutdown_logger_raw has already been called or if set_logger_raw has not been called yet.

Enums

LogLevel	An enum representing the available verbosity levels of the logging framework
LogLevelFilter	An enum representing the available verbosity level filters of the logging framework.

Traits

Log	A trait encapsulating the operations required of a logger

Functions

max_log_level	Returns the current maximum log level.
set_logger	Sets the global logger.
set_logger_raw⚠	Sets the global logger from a raw pointer.
shutdown_logger	Shuts down the global logger.
shutdown_logger_raw	Shuts down the global logger.