Log::Trace - provides a unified approach to tracing


        # The tracing targets
        use Log::Trace; # No output
        use Log::Trace 'print'; # print to STDOUT
        use Log::Trace log => '/var/log/foo.log'; # Output to log file
        use Log::Trace print => { Level => 3 };

        # Switch on/off logging with a constant
        use Log::Trace;
        import Log::Trace ('log' => LOGFILE) if TRACING;

        # Set up tracing for all packages that advertise TRACE
        use Foo;
        use Bar;
        use Log::Trace warn => { Deep => 1 };

        # Sets up tracing in all subpackages excluding Foo
        use Log::Trace warn => {Deep => 1, 'Exclude' => 'Foo'};

        # Exported functions
        TRACE("Record this...");
        TRACE({Level => 2}, "Only shown if tracing level is 2 or higher");
        TRACEF("A la printf: %d-%.2f", 1, 2.9999);
        TRACE_HERE();           # Record where we are (file, line, sub, args)
        DUMP(\@loh, \%hoh);     # Trace out via Data::Dumper
        DUMP("Title", \@loh);   # Trace out via Data::Dumper
        my $dump = DUMP(@args); # Dump is returned without being traced


A module to provide a unified approach to tracing. A script can use Log::Trace qw( < mode > ) to set the behaviour of the TRACE function.

By default, the trace functions are exported to the calling package only. You can export the trace functions to other packages with the Deep option. See "OPTIONS" for more information.

All exports are in uppercase (to minimise collisions with "real" functions).



Output a message. Where the message actually goes depends on how you imported Log::Trace (See "enabling Log::Trace"" in "Importing)

The first argument is an optional hashref of options:

        TRACE('A simple message');


        TRACE({ Level => 2.1 }, 'A message at a specified trace level');
TRACEF($format, @args)

printf() equivalent of TRACE. Also accepts an optional hashref:

        TRACEF('%d items', scalar @items);
        TRACEF({ Level => 5 }, '$%1.2d', $value);
DUMP([$message,] @args)

Serialises each of @args, optionally prepended with $message. If called in a non-void context, DUMP will return the serialised data rather than TRACE it. This is useful if you want to DUMP a datastructure at a specific tracing level.

        DUMP('colours', [qw(red green blue)]);             # outputs via TRACE
        my $dump = DUMP('colours', [qw(red green blue)]);  # output returned

TRACEs the current position on the call stack (file, line number, subroutine name, subroutine args).

        TRACE_HERE({Level => 99});

Importing/enabling Log::Trace

import($target, [$arg], [\%params])

Controls where TRACE messages go. This method is called automatically when you call 'use Log::Trace;', but you may explicitly call this method at runtime. Compare the following:

        use Log::Trace 'print';

which is the same as

        BEGIN {
                require Log::Trace;

Valid combinations of $target and arg are:

Prints trace messages to the supplied $filehandle. Defaults to STDOUT if no file handle is specified.


Prints trace messages via warn()s to STDERR.

buffer => \$buffer

Appends trace messages to a string reference.

file => $filename

Append trace messages to a file. If the file doesn't exist, it will be created.

log => $filename

This is equivalent to:

        use Log::Trace file => $filename, {Verbose => 2};
syslog => $priority

Logs trace messages to syslog via Sys::Syslog, if available.

You should consult your syslog configuration before using this option.

The default $priority is 'debug', and the ident is set to Log::Trace. You can configure the priority, but beyond that, you can implement your own syslogging via the custom trace target.

custom => \&custom_trace_sub

Trace messages are processed by a custom subroutine. E.g.

        use Log::Trace custom => \&mylogger;

        sub mylogger {
                my @messages = @_;
                foreach (@messages) {
                        # highly sensitive trace messages!

The import \%params are optional. These two statements are functionally the same:

        import Log::Trace print => {Level => undef};
        import Log::Trace 'print';

See "OPTIONS" for more information.

Note: If you use the custom tracing option, you should be careful about supplying a subroutine named TRACE.


AllSubs => BOOL

Attaches a TRACE statement to all subroutines in the package. This can be used to track the execution path of your code. It is particularly useful when used in conjunction with Deep and Everywhere options.

Note: Anonymous subroutines and AUTOLOAD are not TRACEd.

AutoImport => BOOL

By default, Log::Trace will only set up TRACE routines in modules that have already been loaded. This option overrides require() so that modules loaded after Log::Trace can automatically be set up for tracing.

Note: This is an experimental feature. See the ENVIRONMENT NOTES for information about behaviour under different versions of perl.

This option has no effect on perl < 5.6

Deep => BOOL

Attaches Log::Trace to all packages (that define a TRACE function). Any TRACEF, DUMP and TRACE_HERE routines will also be overridden in these packages.

Dumper => Data::Serializer backend

Specify a serialiser to be used for DUMPing data structures.

This should either be a string naming a Data::Serializer backend (e.g. "YAML") or a hashref of parameters which will be passed to Data::Serializer, e.g.

                serializer => 'XML::Dumper',
                options => {
                        dtd => 'path/to/my.dtd'

Note that the raw_serialise() method of Data::Serializer is used. See Data::Serializer for more information.

If you do not have Data::Serializer installed, leave this option undefined to use the Data::Dumper natively.

Default: undef (use standalone Data::Dumper)

Everywhere => BOOL

When used in conjunction with the Deep option, it will override the standard behaviour of only enabling tracing in packages that define TRACE stubs.

Default: false


Exclude a module or list of modules from tracing.


Specifies which trace levels to display.

If no Level is defined, all TRACE statements will be output.

If the value is numeric, only TRACEs that are at the specified level or below will be output.

If the value is a list of numbers, only TRACEs that match the specified levels are output.

The level may also be a code reference which is passed the package name and the TRACE level. It mst return a true value if the TRACE is to be output.

Default: undef

Match => REGEX

Exports trace functions to packages that match the supplied regular expression. Can be used in conjunction with Exclude. You can also use Match as an exclusion method if you give it a negative look-ahead.

For example:

        Match => qr/^(?!Acme::)/  # will exclude every module beginning with Acme::


        Match => qr/^Acme::/      # does the reverse

Default: '.' # everything

Verbose => 0|1|2

You can use this option to prepend extra information to each trace message. The levels represent increasing levels of verbosity:

        0: the default*, don't add anything
        1: adds subroutine name and line number to the trace output
        2: As [1], plus a filename and timestamp (in ISO 8601 : 2000 format)

This setting has no effect on the custom or log targets.

* the log target uses 'Verbose' level 2


The AutoImport feature overrides CORE::require() which requires perl 5.6, but you may see unexpected errors if you aren't using at least perl 5.8. The AutoImport option has no effect on perl < 5.6.

In mod_perl or other persistent interpreter environments, different applications could trample on each other's TRACE routines if they use Deep (or Everywhere) option. For example application A could route all the trace output from Package::Foo into "appA.log" and then application B could import Log::Trace over the top, re-routing all the trace output from Package::Foo to "appB.log" for evermore. One way around this is to ensure you always import Log::Trace on every run in a persistent environment from all your applications that use the Deep option. We may provide some more tools to work around this in a later version of Log::Trace.

Log::Trace has not been tested in a multi-threaded application.


        Time::HiRes      (used if available)
        Data::Dumper     (used if available - necessary for meaningful DUMP output)
        Data::Serializer (optional - to customise DUMP output)
        Sys::Syslog      (loaded on demand)



Log::TraceMessages is similar in design and purpose to Log::Trace. However, it only offers a subset of this module's functionality. Most notably, it doesn't offer a mechanism to control the tracing output of an entire application - tracing must be enabled on a module-by-module basis. Log::Trace also offers control over the output with the trace levels and supports more output targets.


Log::Agent offers a procedural interface to logging. It strikes a good balance between configurability and ease of use. It differs to Log::Trace in a number of ways. Log::Agent has a concept of channels and priorities, while Log::Trace only offers levels. Log::Trace also supports tracing code execution path and the Deep import option. Log::Trace trades a certain amount of configurability for increased ease-of use.


A feature rich perl port of the popular log4j library for Java. It is object-oriented and comprised of more than 30 modules. It has an impressive feature set, but some people may be frightened of its complexity. In contrast, to use Log::Trace you need only remember up to 4 simple functions and a handful of configuration options.


Log::Trace::Manual - A guide to using Log::Trace


$Revision: 1.70 $ on $Date: 2005/11/01 11:32:59 $ by $Author: colinr $


John Alden and Simon Flack with some additions by Piers Kent and Wayne Myers <cpan _at_ bbc _dot_ co _dot_ uk>


(c) BBC 2005. This program is free software; you can redistribute it and/or modify it under the GNU GPL.

See the file COPYING in this distribution, or