# log(3tcl) 1.4 log "Logging facility"

## Name

log - Procedures to log messages of libraries and applications.

## Synopsis

- package require Tcl 8
- package require log ?1.4?

- ::log::levels
- ::log::lv2longform level
- ::log::lv2color level
- ::log::lv2priority level
- ::log::lv2cmd level
- ::log::lv2channel level
- ::log::lvCompare level1 level2
- ::log::lvSuppress level {suppress 1}
- ::log::lvSuppressLE level {suppress 1}
- ::log::lvIsSuppressed level
- ::log::lvCmd level cmd
- ::log::lvCmdForall cmd
- ::log::lvChannel level chan
- ::log::lvChannelForall chan
- ::log::lvColor level color
- ::log::lvColorForall color
- ::log::log level text
- ::log::logarray level arrayvar ?pattern?
- ::log::loghex level text data
- ::log::logsubst level msg
- ::log::logMsg text
- ::log::logError text
- ::log::Puts level text

## Description

The log package provides commands that allow libraries and applications to selectively log information about their internal operation and state.

To use the package just execute

        package require log
        log::log notice "Some message"

As can be seen above, each message given to the log facility is associated with a level determining the importance of the message. The user can then select which levels to log, what commands to use for the logging of each level and the channel to write the message to. In the following example the logging of all message with level debug is deactivated.

        package require log
        log::lvSuppress debug
        log::log debug "Unseen message" ; # No output

By default all messages associated with an error-level (emergency, alert, critical, and error) are written to stderr. Messages with any other level are written to stdout. In the following example the log module is reconfigured to write debug messages to stderr too.

        package require log
        log::lvChannel debug stderr
        log::log debug "Written to stderr"

Each message level is also associated with a command to use when logging a message with that level. The behaviour above for example relies on the fact that all message levels use by default the standard command ::log::Puts to log any message. In the following example all messages of level notice are given to the non-standard command toText for logging. This disables the channel setting for such messages, assuming that toText does not use it by itself.

        package require log
        log::lvCmd notice toText
        log::log notice "Handled by \"toText\""

Another database maintained by this facility is a map from message levels to colors. The information in this database has no influence on the behaviour of the module. It is merely provided as a convenience and in anticipation of the usage of this facility in tk-based application which may want to colorize message logs.

## API

The following commands are available:

::log::levels

  Returns the names of all known levels, in alphabetical order.

::log::lv2longform level

  Converts any unique abbreviation of a level name to the full level name.

::log::lv2color level

  Converts any level name including unique abbreviations to the corresponding color.

::log::lv2priority level

  Converts any level name including unique abbreviations to the corresponding priority.

::log::lv2cmd level

  Converts any level name including unique abbreviations to the command prefix used to write messages with that level.

::log::lv2channel level

  Converts any level name including unique abbreviations to the channel used by ::log::Puts to write messages with that level.

::log::lvCompare level1 level2

  Compares two levels (including unique abbreviations) with respect to their priority. This command can be used by the -command option of lsort. The result is one of -1, 0 or 1 or an error. A result of -1 signals that level1 is of less priority than level2. 0 signals that both levels have the same priority. 1 signals that level1 has higher priority than level2.

::log::lvSuppress level {suppress 1}

  (Un)suppresses the output of messages having the specified level. Unique abbreviations for the level are allowed here too.

::log::lvSuppressLE level {suppress 1}

  (Un)suppresses the output of messages having the specified level or one of lesser priority. Unique abbreviations for the level are allowed here too.

::log::lvIsSuppressed level

  Asks the package whether the specified level is currently suppressed. Unique abbreviations of level names are allowed.

::log::lvCmd level cmd

  Defines for the specified level with which command to write the messages having this level. Unique abbreviations of level names are allowed. The command is actually a command prefix and this facility will append 2 arguments before calling it, the level of the message and the message itself, in this order.

::log::lvCmdForall cmd

  Defines for all known levels with which command to write the messages having this level. The command is actually a command prefix and this facility will append 2 arguments before calling it, the level of the message and the message itself, in this order.

::log::lvChannel level chan

  Defines for the specified level into which channel ::log::Puts (the standard command) shall write the messages having this level. Unique abbreviations of level names are allowed. The command is actually a command prefix and this facility will append 2 arguments before calling it, the level of the message and the message itself, in this order.

::log::lvChannelForall chan

  Defines for all known levels with which which channel ::log::Puts (the standard command) shall write the messages having this level. The command is actually a command prefix and this facility will append 2 arguments before calling it, the level of the message and the message itself, in this order.

::log::lvColor level color

  Defines for the specified level the color to return for it in a call to ::log::lv2color. Unique abbreviations of level names are allowed.

::log::lvColorForall color

  Defines for all known levels the color to return for it in a call to ::log::lv2color. Unique abbreviations of level names are allowed.

::log::log level text

  Log a message according to the specifications for commands, channels and suppression. In other words: The command will do nothing if the specified level is suppressed. If it is not suppressed the actual logging is delegated to the specified command. If there is no command specified for the level the message won't be logged. The standard command ::log::Puts will write the message to the channel specified for the given level. If no channel is specified for the level the message won't be logged. Unique abbreviations of level names are allowed. Errors in the actual logging command are not caught, but propagated to the caller, as they may indicate misconfigurations of the log facility or errors in the callers code itself.

::log::logarray level arrayvar ?pattern?

  Like ::log::log, but logs the contents of the specified array variable arrayvar, possibly restricted to entries matching the pattern. The pattern defaults to * (i.e. all entries) if none was specified.

::log::loghex level text data

  Like ::log::log, but assumes that data contains binary data. It converts this into a mixed hex/ascii representation before writing them to the log.

::log::logsubst level msg

  Like ::log::log, but msg may contain substitutions and variable references, which are evaluated in the caller scope first. The purpose of this command is to avoid overhead in the non-logging case, if the log message building is expensive. Any substitution errors raise an error in the command execution. The following example shows an xml text representation, which is only generated in debug mode:

          log::logsubst debug {XML of node $node is '[$node toXml]'}

::log::logMsg text

  Convenience wrapper around ::log::log. Equivalent to ::log::log info text.

::log::logError text

  Convenience wrapper around ::log::log. Equivalent to ::log::log error text.

::log::Puts level text

  The standard log command, it writes messages and their levels to user-specified channels. Assumes that the suppression checks were done by the caller. Expects full level names, abbreviations are not allowed.

## LEVELS

The package currently defines the following log levels, the level of highest importance listed first.

- emergency

- alert

- critical

- error

- warning

- notice

- info

- debug

Note that by default all messages with levels warning down to debug are suppressed. This is done intentionally, because (we believe that) in most situations debugging output is not wanted. Most people wish to have such output only when actually debugging an application.

## Bugs, Ideas, Feedback

This document, and the package it describes, will undoubtedly contain bugs and other problems. Please report such in the category log of the Tcllib Trackers. Please also report any ideas for enhancements you may have for either package and/or documentation.

When proposing code changes, please provide unified diffs, i.e the output of diff -u.

Note further that attachments are strongly preferred over inlined patches. Attachments can be made by going to the Edit form of the ticket immediately after its creation, and then using the left-most button in the secondary navigation bar.

## Keywords

log, log level, message, message level

## Category

Programming tools

## Copyright

Copyright © 2001-2009 Andreas Kupries <andreas_kupries@users.sourceforge.net>