pub struct Logger { /* private fields */ }
Expand description

The entry-point for using flexi_logger.

Logger is a builder class that allows you to

Usage

See code_examples for a comprehensive list of usage possibilities.

Implementations

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

Creates a Logger that you provide with an explicit LogSpecification.

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.

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.

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.

Simple methods for influencing the behavior of the Logger.

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

Log is written to stdout.

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.

Log is written to the provided writer.

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

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.

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.

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

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.

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.

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.

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

Regarding the default, see Logger::format.

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

Regarding the default, see Logger::format.

Available on crate feature atty only.

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.

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

Regarding the default, see Logger::format.

Available on crate feature atty only.

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.

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.

Available on crate feature colors only.

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.

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.

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.

Apply the provided filter before really writing log lines.

See the documentation of module filter for a usage example.

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.

Makes the logger use UTC timestamps rather than local timestamps.

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

Registers a LogWriter implementation under the given target name.

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

Sets the write mode for the logger.

See WriteMode for more (important!) details.

Use Windows line endings, rather than just \n.

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.

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.

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

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.

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.

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.

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

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.

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.

Available on crate feature specfile_without_notification only.

Consumes the Logger object and initializes flexi_logger in a way that subsequently the log specification can be updated, while the program is running, by editing a file.

Uses the spec that was given to the factory method (Logger::with etc) as initial spec and then tries to read the logspec from a file.

If the file does not exist, flexi_logger creates the file and fills it with the initial spec (and in the respective file format, of course).

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

Feature dependency

The implementation of this configuration method uses some additional crates that you might not want to depend on with your program if you don’t use this functionality. For that reason the method is only available if you activate the specfile feature. See the usage section on crates.io for details.

Usage

A logger initialization like

use flexi_logger::Logger;
Logger::try_with_str("info")
    .unwrap()
    // more logger configuration
    .start_with_specfile("logspecification.toml");

will create the file logspecification.toml (if it does not yet exist) with this content:

## Optional: Default log level
global_level = 'info'
## Optional: specify a regular expression to suppress all messages that don't match
#global_pattern = 'foo'

## Specific log levels per module are optionally defined in this section
[modules]
#'mod1' = 'warn'
#'mod2' = 'debug'
#'mod2::mod3' = 'trace'

You can subsequently edit and modify the file according to your needs, while the program is running, and it will immediately take your changes into account.

Currently only toml-files are supported, the file suffix thus must be .toml.

The initial spec remains valid if the file cannot be read.

If you update the specfile subsequently while the program is running, flexi_logger re-reads it automatically and adapts its behavior according to the new content. If the file cannot be read anymore, e.g. because the format is not correct, the previous logspec remains active. If the file is corrected subsequently, the log spec update will work again.

Errors

Several variants of FlexiLoggerError can occur.

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

See also Logger::start and Logger::start_with_specfile. for the properties of the returned logger.

Errors

Several variants of FlexiLoggerError can occur.

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