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---
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source: tests/output.rs
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expression: "get_stdout(path, &[\"log\"])"
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---
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log Module log rusty-man
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DESCRIPTION
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A lightweight logging facade.
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The `log` crate provides a single logging API that abstracts over the actual logging
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implementation. Libraries can use the logging API provided by this crate, and the consumer of
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those libraries can choose the logging implementation that is most suitable for its use case.
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If no logging implementation is selected, the facade falls back to a “noop” implementation
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that ignores all log messages. The overhead in this case is very small - just an integer load,
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comparison and jump.
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A log request consists of a *target*, a *level*, and a *body*. A target is a string which
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defaults to the module path of the location of the log request, though that default may be
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overridden. Logger implementations typically use the target to filter requests based on some
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user configuration.
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# Use
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The basic use of the log crate is through the five logging macros: `error!`, `warn!`, `info!`,
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`debug!` and `trace!` where `error!` represents the highest-priority log messages and `trace!`
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the lowest. The log messages are filtered by configuring the log level to exclude messages
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with a lower priority. Each of these macros accept format strings similarly to
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[`println!`][1].
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## In libraries
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Libraries should link only to the `log` crate, and use the provided macros to log whatever
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information will be useful to downstream consumers.
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### Examples
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ⓘ
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`use log::{info, warn};
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pub fn shave_the_yak(yak: &mut Yak) {
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info!(target: "yak_events", "Commencing yak shaving for {:?}", yak);
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loop {
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match find_a_razor() {
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Ok(razor) => {
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info!("Razor located: {}", razor);
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yak.shave(razor);
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break;
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}
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Err(err) => {
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warn!("Unable to locate a razor: {}, retrying", err);
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}
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}
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}
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}`
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## In executables
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Executables should choose a logging implementation and initialize it early in the runtime of
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the program. Logging implementations will typically include a function to do this. Any log
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messages generated before the implementation is initialized will be ignored.
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The executable itself may use the `log` crate to log as well.
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### Warning
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The logging system may only be initialized once.
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# Available logging implementations
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In order to produce log output executables have to use a logger implementation compatible with
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the facade. There are many available implementations to choose from, here are some of the most
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popular ones:
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* Simple minimal loggers:
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* [env_logger][1]
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[1] https://docs.rs/env_logger/*/env_logger/
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* [simple_logger][1]
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[1] https://github.com/borntyping/rust-simple_logger
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* [simplelog][1]
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[1] https://github.com/drakulix/simplelog.rs
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* [pretty_env_logger][1]
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[1] https://docs.rs/pretty_env_logger/*/pretty_env_logger/
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* [stderrlog][1]
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[1] https://docs.rs/stderrlog/*/stderrlog/
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* [flexi_logger][1]
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[1] https://docs.rs/flexi_logger/*/flexi_logger/
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* Complex configurable frameworks:
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* [log4rs][1]
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[1] https://docs.rs/log4rs/*/log4rs/
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* [fern][1]
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[1] https://docs.rs/fern/*/fern/
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* Adaptors for other facilities:
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* [syslog][1]
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[1] https://docs.rs/syslog/*/syslog/
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* [slog-stdlog][1]
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[1] https://docs.rs/slog-stdlog/*/slog_stdlog/
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# Implementing a Logger
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Loggers implement the `Log` trait. Here’s a very basic example that simply logs all messages
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at the `Error`, `Warn` or `Info` levels to stdout:
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ⓘ
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`use log::{Record, Level, Metadata};
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struct SimpleLogger;
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impl log::Log for SimpleLogger {
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fn enabled(&self, metadata: &Metadata) -> bool {
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metadata.level() <= Level::Info
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}
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fn log(&self, record: &Record) {
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if self.enabled(record.metadata()) {
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println!("{} - {}", record.level(), record.args());
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}
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}
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fn flush(&self) {}
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}
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`
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Loggers are installed by calling the `set_logger` function. The maximum log level also needs
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to be adjusted via the `set_max_level` function. The logging facade uses this as an
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optimization to improve performance of log messages at levels that are disabled. It’s
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important to set it, as it defaults to `Off`, so no log messages will ever be captured! In the
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case of our example logger, we’ll want to set the maximum log level to `Info`, since we ignore
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any `Debug` or `Trace` level log messages. A logging implementation should provide a function
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that wraps a call to `set_logger` and `set_max_level`, handling initialization of the logger:
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ⓘ
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`use log::{SetLoggerError, LevelFilter};
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static LOGGER: SimpleLogger = SimpleLogger;
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pub fn init() -> Result<(), SetLoggerError> {
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log::set_logger(&LOGGER)
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.map(|()| log::set_max_level(LevelFilter::Info))
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}`
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Implementations that adjust their configurations at runtime should take care to adjust the
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maximum log level as well.
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# Use with `std`
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`set_logger` requires you to provide a `&'static Log`, which can be hard to obtain if your
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logger depends on some runtime configuration. The `set_boxed_logger` function is available
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with the `std` Cargo feature. It is identical to `set_logger` except that it takes a
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`Box<Log>` rather than a `&'static Log`:
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ⓘ
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`pub fn init() -> Result<(), SetLoggerError> {
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log::set_boxed_logger(Box::new(SimpleLogger))
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.map(|()| log::set_max_level(LevelFilter::Info))
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}`
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# Compile time filters
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Log levels can be statically disabled at compile time via Cargo features. Log invocations at
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disabled levels will be skipped and will not even be present in the resulting binary. This
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level is configured separately for release and debug builds. The features are:
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* `max_level_off`
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* `max_level_error`
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* `max_level_warn`
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* `max_level_info`
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* `max_level_debug`
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* `max_level_trace`
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* `release_max_level_off`
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* `release_max_level_error`
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* `release_max_level_warn`
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* `release_max_level_info`
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* `release_max_level_debug`
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* `release_max_level_trace`
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These features control the value of the `STATIC_MAX_LEVEL` constant. The logging macros check
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this value before logging a message. By default, no levels are disabled.
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Libraries should avoid using the max level features because they’re global and can’t be
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changed once they’re set.
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For example, a crate can disable trace level logs in debug builds and trace, debug, and info
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level logs in release builds with the following configuration:
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`[dependencies]
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log = { version = "0.4", features = ["max_level_debug", "release_max_level_warn"] }`
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# Crate Feature Flags
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The following crate feature flags are available in addition to the filters. They are
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configured in your `Cargo.toml`.
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* `std` allows use of `std` crate instead of the default `core`. Enables using `std::error`
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and `set_boxed_logger` functionality.
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* `serde` enables support for serialization and deserialization of `Level` and `LevelFilter`.
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`[dependencies]
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log = { version = "0.4", features = ["std", "serde"] }`
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# Version compatibility
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The 0.3 and 0.4 versions of the `log` crate are almost entirely compatible. Log messages made
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using `log` 0.3 will forward transparently to a logger implementation using `log` 0.4. Log
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messages made using `log` 0.4 will forward to a logger implementation using `log` 0.3, but the
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module path and file name information associated with the message will unfortunately be lost.
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[1] https://doc.rust-lang.org/stable/std/macro.println.html
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MACROS
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debug
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Logs a message at the debug level.
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error
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Logs a message at the error level.
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info
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Logs a message at the info level.
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log
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The standard logging macro.
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log_enabled
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Determines if a message logged at the specified level in that module will be logged.
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trace
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Logs a message at the trace level.
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warn
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Logs a message at the warn level.
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STRUCTS
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Metadata
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Metadata about a log message.
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MetadataBuilder
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Builder for `Metadata`.
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ParseLevelError
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The type returned by [`from_str`][1] when the string doesn’t match any of the log
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levels.
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[1] https://doc.rust-lang.org/std/str/trait.FromStr.html#tymethod.from_str
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Record
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The “payload” of a log message.
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RecordBuilder
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Builder for `Record`.
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SetLoggerError
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The type returned by `set_logger` if `set_logger` has already been called.
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ENUMS
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Level
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An enum representing the available verbosity levels of the logger.
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LevelFilter
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An enum representing the available verbosity level filters of the logger.
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CONSTANTS
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STATIC_MAX_LEVEL
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The statically resolved maximum log level.
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TRAITS
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Log
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A trait encapsulating the operations required of a logger.
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FUNCTIONS
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logger
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Returns a reference to the logger.
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max_level
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Returns the current maximum log level.
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set_boxed_logger
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Sets the global logger to a `Box<Log>`.
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set_logger
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Sets the global logger to a `&'static Log`.
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set_logger_racy⚠
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A thread-unsafe version of `set_logger`.
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set_max_level
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Sets the global maximum log level.
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