Struct flexi_logger::Logger

pub struct Logger { /* private fields */ }

The entry-point for using flexi_logger.

Logger is a builder class that allows you to

• specify your desired (initial) loglevel-specification

  • either as a String (Logger::try_with_str)
  • or by providing it in the environment (Logger::try_with_env),
  • or by combining both options (Logger::try_with_env_or_str),
  • or by building a LogSpecification programmatically (Logger::with),

• use the desired configuration methods,

• and finally start the logger with

  • Logger::start, or
  • [Logger::start_with_specfile].

Usage

See code_examples for a comprehensive list of usage possibilities.

Implementations

impl Logger

Create a Logger instance and define how to access the (initial) loglevel-specification.

pub fn with(logspec: LogSpecification) -> Self

Creates a Logger that you provide with an explicit LogSpecification.

pub fn try_with_str<S: AsRef<str>>(s: S) -> Result<Self, FlexiLoggerError>

Creates a Logger that reads the LogSpecification from a String or &str. See LogSpecification for the syntax.

Errors

FlexiLoggerError::Parse if the String uses an erroneous syntax.

pub fn try_with_env() -> Result<Self, FlexiLoggerError>

Creates a Logger that reads the LogSpecification from the environment variable RUST_LOG.

Note that if RUST_LOG is not set, nothing is logged.

Errors

FlexiLoggerError::Parse if the value of RUST_LOG is malformed.

pub fn try_with_env_or_str<S: AsRef<str>>( s: S ) -> Result<Self, FlexiLoggerError>

Creates a Logger that reads the LogSpecification from the environment variable RUST_LOG, or derives it from the given String, if RUST_LOG is not set.

Errors

FlexiLoggerError::Parse if the chosen value is malformed.

impl Logger

Simple methods for influencing the behavior of the Logger.

pub fn log_to_stderr(self) -> Self

Log is written to stderr (which is the default).

pub fn log_to_stdout(self) -> Self

Log is written to stdout.

pub fn log_to_file(self, file_spec: FileSpec) -> Self

Log is written to a file.

The default filename pattern is <program_name>_<date>_<time>.<suffix>, e.g. myprog_2015-07-08_10-44-11.log.

You can duplicate to stdout and stderr, and you can add additional writers.

pub fn log_to_writer(self, w: Box<dyn LogWriter>) -> Self

Log is written to the provided writer.

You can duplicate to stdout and stderr, and you can add additional writers.

pub fn log_to_file_and_writer( self, file_spec: FileSpec, w: Box<dyn LogWriter> ) -> Self

Log is written to a file, as with Logger::log_to_file, and to an alternative LogWriter implementation.

And you can duplicate to stdout and stderr, and you can add additional writers.

pub fn do_not_log(self) -> Self

Log is processed, including duplication, but not written to any destination.

This can be useful e.g. for running application tests with all log-levels active and still avoiding tons of log files etc. Such tests ensure that the log calls which are normally not active will not cause undesired side-effects when activated (note that the log macros may prevent arguments of inactive log-calls from being evaluated).

Or, if you want to get logs both to stdout and stderr, but nowhere else, then use this option and combine it with Logger::duplicate_to_stdout and Logger::duplicate_to_stderr.

pub fn print_message(self) -> Self

Makes the logger print an info message to stdout with the name of the logfile when a logfile is opened for writing.

pub fn duplicate_to_stderr(self, dup: Duplicate) -> Self

Makes the logger write messages with the specified minimum severity additionally to stderr.

Does not work with Logger::log_to_stdout or Logger::log_to_stderr.

pub fn duplicate_to_stdout(self, dup: Duplicate) -> Self

Makes the logger write messages with the specified minimum severity additionally to stdout.

Does not work with Logger::log_to_stdout or Logger::log_to_stderr.

pub fn format(self, format: FormatFunction) -> Self

Makes the logger use the provided format function for all messages that are written to files, stderr, stdout, or to an additional writer.

You can either choose one of the provided log-line formatters, or you create and use your own format function with the signature

fn my_format(
   write: &mut dyn std::io::Write,
   now: &mut flexi_logger::DeferredNow,
   record: &log::Record,
) -> std::io::Result<()>

By default, default_format is used for output to files and to custom writers, and AdaptiveFormat::Default is used for output to stderr and stdout. If the feature colors is switched off, default_format is used for all outputs.

pub fn format_for_files(self, format: FormatFunction) -> Self

Makes the logger use the provided format function for messages that are written to files.

Regarding the default, see Logger::format.

pub fn format_for_stderr(self, format_function: FormatFunction) -> Self

Makes the logger use the provided format function for messages that are written to stderr.

Regarding the default, see Logger::format.

pub fn adaptive_format_for_stderr(self, adaptive_format: AdaptiveFormat) -> Self

Makes the logger use the specified format for messages that are written to stderr. Coloring is used if stderr is a tty.

Regarding the default, see Logger::format.

pub fn format_for_stdout(self, format_function: FormatFunction) -> Self

Makes the logger use the provided format function to format messages that are written to stdout.

Regarding the default, see Logger::format.

pub fn adaptive_format_for_stdout(self, adaptive_format: AdaptiveFormat) -> Self

Makes the logger use the specified format for messages that are written to stdout. Coloring is used if stdout is a tty.

Regarding the default, see Logger::format.

pub fn format_for_writer(self, format: FormatFunction) -> Self

Allows specifying a format function for an additional writer. Note that it is up to the implementation of the additional writer whether it evaluates this setting or not.

Regarding the default, see Logger::format.

pub fn set_palette(self, palette: String) -> Self

Sets the color palette for function style, which is used in the provided coloring format functions.

The palette given here overrides the default palette.

The palette is specified in form of a String that contains a semicolon-separated list of numbers (0..=255) and/or dashes (´-´). The first five values denote the fixed color that is used for coloring error, warn, info, debug, and trace messages.

The String "196;208;-;7;8" describes the default palette, where color 196 is used for error messages, and so on. The - means that no coloring is done, i.e., with "-;-;-;-;-" all coloring is switched off.

Prefixing a number with ‘b’ makes the output being written in bold. The String "b1;3;2;4;6" e.g. describes the palette used by env_logger.

The palette can further be overridden at runtime by setting the environment variable FLEXI_LOGGER_PALETTE to a palette String. This allows adapting the used text colors to differently colored terminal backgrounds.

For your convenience, if you want to specify your own palette, you can produce a colored list with all 255 colors with cargo run --example colors.

pub fn rotate(self, criterion: Criterion, naming: Naming, cleanup: Cleanup) -> Self

Prevent indefinite growth of the log file by applying file rotation and a clean-up strategy for older log files.

By default, the log file is fixed while your program is running and will grow indefinitely. With this option being used, when the log file reaches the specified criterion, the file will be closed and a new file will be opened.

Note that also the filename pattern changes:

• by default, no timestamp is added to the filename if rotation is used
• the logs are always written to a file with infix _rCURRENT
• when the rotation criterion is fulfilled, it is closed and renamed to a file with another infix (see Naming), and then the logging continues again to the (fresh) file with infix _rCURRENT.

Example:

After some logging with your program my_prog and rotation with Naming::Numbers, you will find files like

my_prog_r00000.log
my_prog_r00001.log
my_prog_r00002.log
my_prog_rCURRENT.log

Parameters

criterion defines when the log file should be rotated, based on its size or age. See Criterion for details.

naming defines the naming convention for the rotated log files. See Naming for details.

cleanup defines the strategy for dealing with older files. See Cleanup for details.

pub fn cleanup_in_background_thread(self, use_background_thread: bool) -> Self

When Logger::rotate is used with some Cleanup variant other than Cleanup::Never, then this method can be used to define if the cleanup activities (finding files, deleting files, evtl compressing files) are delegated to a background thread (which is the default, to minimize the blocking impact to your application caused by IO operations), or whether they are done synchronously in the current log-call.

If you call this method with use_background_thread = false, the cleanup is done synchronously.

pub fn filter(self, filter: Box<dyn LogLineFilter + Send + Sync>) -> Self

Apply the provided filter before really writing log lines.

See the documentation of module filter for a usage example.

pub fn append(self) -> Self

Makes the logger append to the specified output file, if it exists already; by default, the file would be truncated.

This option only has an effect if logs are written to files, but it will hardly make an effect if FileSpec::suppress_timestamp is not used.

pub fn use_utc(self) -> Self

Makes the logger use UTC timestamps rather than local timestamps.

pub fn create_symlink<P: Into<PathBuf>>(self, symlink: P) -> Self

The specified path will be used on unix systems to create a symbolic link to the current log file.

This option has no effect on filesystems where symlinks are not supported, and it only has an effect if logs are written to files.

Example

You can use the symbolic link to follow the log output with tail, even if the log files are rotated.

Assuming you use create_symlink("link_to_log_file"), then use:

tail --follow=name --max-unchanged-stats=1 --retry link_to_log_file

pub fn add_writer<S: Into<String>>( self, target_name: S, writer: Box<dyn LogWriter> ) -> Self

Registers a LogWriter implementation under the given target name.

The target name must not start with an underscore. See module writers for more details.

pub fn write_mode(self, write_mode: WriteMode) -> Self

Sets the write mode for the logger.

See WriteMode for more (important!) details.

pub fn use_windows_line_ending(self) -> Self

Use Windows line endings, rather than just \n.

pub fn error_channel(self, error_channel: ErrorChannel) -> Self

Define the output channel for flexi_logger’s own error messages.

These are only written if flexi_logger cannot do what it is supposed to do, so under normal circumstances no single message shuld appear.

By default these error messages are printed to stderr.

impl Logger

Alternative set of methods to control the behavior of the Logger. Use these methods when you want to control the settings flexibly, e.g. with commandline arguments via docopts or clap.

pub fn o_print_message(self, print_message: bool) -> Self

With true, makes the logger print an info message to stdout, each time when a new file is used for log-output.

pub fn o_rotate( self, rotate_config: Option<(Criterion, Naming, Cleanup)> ) -> Self

By default, and with None, the log file will grow indefinitely. If a rotate_config is set, when the log file reaches or exceeds the specified size, the file will be closed and a new file will be opened. Also the filename pattern changes: instead of the timestamp, a serial number is included into the filename.

The size is given in bytes, e.g. o_rotate_over_size(Some(1_000)) will rotate files once they reach a size of 1 kB.

The cleanup strategy allows delimiting the used space on disk.

pub fn o_append(self, append: bool) -> Self

If append is set to true, makes the logger append to the specified output file, if it exists. By default, or with false, the file would be truncated.

This option only has an effect if logs are written to files, and it will hardly make an effect if suppress_timestamp() is not used.

pub fn o_create_symlink<P: Into<PathBuf>>(self, symlink: Option<P>) -> Self

If a String is specified, it will be used on unix systems to create in the current folder a symbolic link with this name to the current log file.

This option only has an effect on unix systems and if logs are written to files.

impl Logger

Finally, start logging, optionally with a spec-file.

pub fn start(self) -> Result<LoggerHandle, FlexiLoggerError>

Consumes the Logger object and initializes flexi_logger.

Keep the LoggerHandle alive up to the very end of your program! Dropping the LoggerHandle flushes and shuts down FileLogWriters and other LogWriters, and then may prevent further logging! This should happen immediately before the program terminates, but not earlier.

Dropping the LoggerHandle is uncritical only with Logger::log_to_stdout or Logger::log_to_stderr.

The LoggerHandle also allows updating the log specification programmatically, e.g. to intensify logging for (buggy) parts of a (test) program, etc.

Example

use flexi_logger::{Logger,WriteMode, FileSpec};
fn main() -> Result<(), Box<dyn std::error::Error>> {
    let _logger = Logger::try_with_str("info")?
        .log_to_file(FileSpec::default())
        .write_mode(WriteMode::BufferAndFlush)
        .start()?;

    // ... do all your work and join back all threads whose logs you want to see ...

    Ok(())
}

Errors

Several variants of FlexiLoggerError can occur.

pub fn build(self) -> Result<(Box<dyn Log>, LoggerHandle), FlexiLoggerError>

Builds a boxed logger and a LoggerHandle for it, but does not initialize the global logger.

The returned boxed logger implements the Log trait and can be installed manually or nested within another logger.

Keep the LoggerHandle alive up to the very end of your program! See Logger::start for more details.

Errors

Several variants of FlexiLoggerError can occur.