# log(n) 1.0.1 log "Logging facility"

## NAME

log - Procedures to log messages of libraries and applications.

## SYNOPSIS

package require Tcl 8
package require log ?1.0.1?

::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::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::lvSupress 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::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

## KEYWORDS

log, log level, message level, message