[−][src]Crate log
A lightweight logging facade.
The log
crate 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
implementation 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
The basic use of the log crate is through the five logging macros: error!
,
warn!
, info!
, debug!
and trace!
where error!
represents the highest-priority log messages
and trace!
the lowest. The log messages are filtered by configuring
the log level to exclude messages with a lower priority.
Each of these macros accept format strings similarly to println!
.
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
use log::{info, warn}; pub fn shave_the_yak(yak: &mut 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 implementation and initialize it early in the runtime of the program. Logging implementations will typically include a function to do this. Any log messages generated before the implementation 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.
Available logging implementations
In order to produce log output executables have to use a logger implementation compatible with the facade. There are many available implementations to choose from, here are some of the most popular ones:
- Simple minimal loggers:
- Complex configurable frameworks:
- Adaptors for other facilities:
Implementing a Logger
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:
use log::{Record, Level, Metadata}; struct SimpleLogger; impl log::Log for SimpleLogger { fn enabled(&self, metadata: &Metadata) -> bool { metadata.level() <= Level::Info } fn log(&self, record: &Record) { if self.enabled(record.metadata()) { println!("{} - {}", record.level(), record.args()); } } fn flush(&self) {} }
Loggers are installed by calling the set_logger
function. The maximum
log level also needs to be adjusted via the set_max_level
function. The
logging facade uses this as an optimization to improve performance of log
messages at levels that are disabled. It's important to set it, as it
defaults to Off
, so no log messages will ever be captured!
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 implementation should
provide a function that wraps a call to set_logger
and
set_max_level
, handling initialization of the logger:
use log::{SetLoggerError, LevelFilter}; static LOGGER: SimpleLogger = SimpleLogger; pub fn init() -> Result<(), SetLoggerError> { log::set_logger(&LOGGER) .map(|()| log::set_max_level(LevelFilter::Info)) }
Implementations that adjust their configurations at runtime should take care to adjust the maximum log level as well.
Use with std
set_logger
requires you to provide a &'static Log
, which can be hard to
obtain if your logger depends on some runtime configuration. The
set_boxed_logger
function is available with the std
Cargo feature. It is
identical to set_logger
except that it takes a Box<Log>
rather than a
&'static Log
:
pub fn init() -> Result<(), SetLoggerError> { log::set_boxed_logger(Box::new(SimpleLogger)) .map(|()| log::set_max_level(LevelFilter::Info)) }
Compile time filters
Log levels can be statically disabled at compile time via Cargo features. Log invocations at disabled levels will be skipped and will not even be present in the resulting binary. This level is configured separately for release and debug builds. The features are:
max_level_off
max_level_error
max_level_warn
max_level_info
max_level_debug
max_level_trace
release_max_level_off
release_max_level_error
release_max_level_warn
release_max_level_info
release_max_level_debug
release_max_level_trace
These features control the value of the STATIC_MAX_LEVEL
constant. The logging macros check
this value before logging a message. By default, no levels are disabled.
Libraries should avoid using the max level features because they're global and can't be changed once they're set.
For example, a crate can disable trace level logs in debug builds and trace, debug, and info level logs in release builds with the following configuration:
[dependencies]
log = { version = "0.4", features = ["max_level_debug", "release_max_level_warn"] }
Crate Feature Flags
The following crate feature flags are available in addition to the filters. They are
configured in your Cargo.toml
.
std
allows use ofstd
crate instead of the defaultcore
. Enables usingstd::error
andset_boxed_logger
functionality.serde
enables support for serialization and deserialization ofLevel
andLevelFilter
.
[dependencies]
log = { version = "0.4", features = ["std", "serde"] }
Version compatibility
The 0.3 and 0.4 versions of the log
crate are almost entirely compatible. Log messages
made using log
0.3 will forward transparently to a logger implementation using log
0.4. Log
messages made using log
0.4 will forward to a logger implementation using log
0.3, but the
module path and file name information associated with the message will unfortunately be lost.
Modules
kv | UNSTABLE: Structured key-value pairs. |
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
Metadata | Metadata about a log message. |
MetadataBuilder | Builder for |
ParseLevelError | The type returned by |
Record | The "payload" of a log message. |
RecordBuilder | Builder for |
SetLoggerError | The type returned by |
Enums
Level | An enum representing the available verbosity levels of the logger. |
LevelFilter | An enum representing the available verbosity level filters of the logger. |
Constants
STATIC_MAX_LEVEL | The statically resolved maximum log level. |
Traits
Log | A trait encapsulating the operations required of a logger. |
Functions
logger | Returns a reference to the logger. |
max_level | Returns the current maximum log level. |
set_boxed_logger | Sets the global logger to a |
set_logger | Sets the global logger to a |
set_logger_racy⚠ | A thread-unsafe version of |
set_max_level | Sets the global maximum log level. |