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Devel::TraceCalls - Track calls to subs, classes and object instances


  ## From the command line
    perl -d:TraceCalls=Subs,foo,bar

  ## Quick & dirty via use
    use Devel::TraceCalls { Package => "Foo" };

  ## Procedural
    use Devel::TraceCalls;

    trace_calls qw( foo bar Foo::bar ); ## Explicitly named subs

    trace_calls {
         Subs => [qw( foo bar Foo::bar )],

    trace_calls {
        Package => "Foo",        ## All subs in this package

    trace_calls {         ## Just these subs
        Package => "Foo",        ## Optional
        Subs    => qw( foo, bar ),

    trace_calls $object;  ## Just track this instance

    trace_calls {
        Objects => [ $obj1, $obj2 ];  ## Just track these instances

    ... time passes, sub calls happen ...

    my @calls = $t1->calls;  ## retrieve what happned

  ## Object orented
    my $t = Devel::TraceCalls->new( ...parameters... );

    undef $t;  ## disable tracing

  ## Emitting additional messages:
    use Devel::TraceCalls qw( emit_trace_message );

    emit_trace_message( "ouch!" );


ALPHA CODE ALERT. This module may change before "official" release".

Devel::TraceCalls allows subroutine calls to be tracked on a per-subroutine, per-package, per-class, or per object instance basis. This can be quite useful when trying to figure out how some poor thing is being misused in a program you don't fully understand.

Devel::TraceCalls works on subroutines and classes by installing wrapper subroutines and on objects by temporarily reblessing the objects in to specialized subclasses with "shim" methods. Such objects are reblessed back when the tracker is DESTROYed.

The default action is to log the calls to STDERR. Passing in a PreCall, or PostCall options disables this default behavior, you can reenable it by manually setting <LogTo = \*STDERR>>.

There are 4 ways to specify what to trace.

  1. By Explicit Sub Name

        trace_calls "foo", "bar";   ## trace to STDOUT.
        trace_calls {
            Subs => [ "foo", "bar" ],

    The first form enables tracking with all Capture options enabled (other than CaptureSelf which has no effect when capturing plain subs). The second allows you to control the options.

  2. By Package Name

        trace_calls {
            Package => "My::Module",
        # Multiple package names
        trace_calls {
            Package => [ "My::Module", "Another::Module" ],
        trace_calls {
            Package => "My::Module",
            Subs    => [ "foo", "bar" ],

    This allows you to provide a package prefix for subroutine names to be tracked. If no "Subs" option is provided, all subroutines in the package will be tracked.

    This does not examine @ISA like the Class and Objects (covered next) techniques do.

  3. By Class Name

        trace_calls {
            Class => "My::Class",
        trace_calls {
            Class => "My::Class",
        trace_calls {
            Class => "My::Class",
            Subs  => [ "foo", "bar" ],

    This allows tracking of method calls (or things that look like method calls) for a class and it's base classes. The $self ($_[0]) will not be captured in Args (see "Data Capture Format"), but may be captured in Self if CaptureSelf is enabled.

    Devel::TraceCalls can't differentiate between $obj-foo( ... )> and foo( $obj, ... ), which can lead to extra calls being tracked if the latter form is used. The good news is that this means that idioms like:

        $meth = $obj->can( "foo" );
        $meth->( $obj, ... ) if $meth;

    are captured.

    If a Subs parameter is provided, only the named methods will be tracked. Otherwise all subs in the class and in all parent classes are tracked.

  4. By Object Instance

        trace_calls $obj1, $obj2;
        trace_calls {
            Objects => [ $obj1, $obj2 ],
        trace_calls {
            Objects => [ $obj1, $obj2 ],
            Subs    => [ "foo", "bar" ],

    This allows tracking of method calls (or things that look like method calls) for specific instances. The $self ($_[0]) will not be captured in Args, but may be captured in Self if CaptureSelf is enabled.

    The first form (track $obj, ...) enables all capture options, including CaptureSelf.

Emitting messages if and only if Devel::TraceCalls is loaded

    use constant _tracing => defined $Devel::TraceCalls::VERSION;

    BEGIN {
        eval "use Devel::TraceCalls qw( emit_trace_message )"
            if _tracing;

    emit_trace_message( "hi!" ) if _tracing;

Using the constant _tracing allows expressions like

    emit_trace_message(...) if _tracing;

to be optimized away at compile time, resulting in little or no performance penalty.


there are several options that may be passed in the HASH ref style parameters in addition to the Package, Subs, Objects and Class settings covered above.

    LogTo => \*FOO,
    LogTo => \@array,
    LogTo => undef,

Setting this to a filehandle causes tracing messages to be emitted to that filehandle. This is set to STDERR by default if no PreCall or PostCall intercepts are given. It may be set to undef to suppress tracing if you need to.

Setting this to an ARRAY reference allows call data to be captured, see below for more details.


This is not supported yet, the API will be changing.

But, it allows you some small control over how the parameters list gets traced when LogTo points to a filehandle.


Setting this causes the call stack to be logged.

    PreCall => \&sub_to_call_before_calling_the_target,

A reference to a subroutine to call before calling the target sub. This will be passed a reference to the data captured before the call and a reference to the options passed in when defining the trace point (this does not contain the Package, Subs, Objects and Class settings.

The parameters are:

    ( $trace_point, $captured_data, $params )
    PreCall => \&sub_to_call_after_calling_the_target,

    ( $trace_point, $captured_data, $params )

A reference to a subroutine to call after calling the target sub. This will be passed a reference to the data captured before and after the call and a reference to the options passed in when defining the trace point (this does not contain the Package, Subs, Objects and Class settings.

The parameters are:

    ( $trace_point, $captured_data, $params )


    Wrapper => \&sub_to_delegate_the_target_call_to,

A reference to a subroutine that will be called instead of calling the target sub. The parameters are:

    ( $code_ref, $trace_point, $captured_data, $params )
Data Capture Options

These options affect the data captured in the Calls array (see "The Calls ARRAY") and passed to the PreCall and PostCall handlers.

Options may be added to the hash refs passed to trace_calls. Here are the options and their default values (all defaults chosen to minimize overhead):

    CaptureStack       => 0,
    CaptureCallTimes   => 0,
    CaptureReturnTimes => 0,
    CaptureSelf        => 0,
    CaptureArgs  => 0,
    CaptureResult      => 0,

    CaptureAll         => 0,  ## Shorthand for setting all of the others

Is CaptureStack is true, the

    StackCaptureDepth => 1_000_000,

option controls the maximum number of stack frames that will be captured. Set this to "1" to capture just a single stack frame (equiv. to caller 0).

Captured Data Format

The LogTo option can be used to log all data to an array instead of to a filehandle by passing it an array reference:

    LogTo => \@data,

When passing in an array to capture call data (by using the Calls option), the elements will look like:

        Name       => "SubName",
        Self       => "$obj",
        CallTime   => $seconds,  ## A float if Time::HiRes installed
        ReturnTime => $seconds,  ## A float if Time::HiRes installed
        TraceDepth => $count,    ## How deeply nested the trace is.
        WantArray  => $wantarray_result,
        Result     => [ "c" ],   ## Dumped with Data::Dumper, if need be
        Exception  => "$@",
        Args       => [
            "foo",               ## A scalar was passed
            "{ a => 'b' }",      ## A HASH (dumped with Data::Dumper)
        Stack      => [
            [ ... ],             ## Results of caller(0).
            ....                 ## More frames if requested

NOTE: Many of these fields are optional and off by default. See the "OPTIONS" section for details. Tracing (via the LogTo parameter) enables several Capture options regardless of the passed-in settings.

Result is an array of 0 or more elements. It will always be empty if the sub was called in void context ( WantArray => undef ).

Note that Self, Args and Result are converted to strings to avoid keeping references that might prevent things from being destroyed in a timely manner. Data::Dumper is used for Args and Result, plain stringification is used for Self.

    Devel::TraceCalls::hide_package $pkg;

Tells Deve::TraceCalls to ignore stack frames with caller eq $pkg. The caller's package is used by default. This is useful when overloading require().

    Devel::TraceCalls::unhide_package $pkg;

Undoes the last hide_package. These calls nest, so


leaves the caller's package hidden.

Showing skipped traces

Sometimes it's nice to see what you're missing. This can be helpful if you want to be sure that all the methods of a class are being logged for all instance, for instance.

Set the environment variable SHOWSKIPPED to "yes" or calling show_skipped_trace_points to enable or disable this.

To enable:

    Devel::TraceCalls::set_show_skipped_trace_points( 1 );

To disable:

    Devel::TraceCalls::set_show_skipped_trace_points( 0 );

Calling the subroutine overrides the environment variable.

Showing the call stack

To show the call stack in the log at each trace point, set the environment variable SHOWSTACK to "yes" or calling show_stack to enable or disable this.

To enable:

    Devel::TraceCalls::set_show_stack( 1 );

To disable:

    Devel::TraceCalls::set_show_stack( 0 );

Calling the subroutine overrides the environment variable.


The object oriented interface provides for more flexible than the other APIs. A tracer will remove all of it's trace points when it is deleted and you can add (and someday, remove) trace points from a running tracer.

Someday you'll also be able to enable and disable tracers.

    my $t = Devel::TraceCalls->new(
        ... any params you might pass to trace_calls...
        ...any params you might pass to trace_calls...

Add trace points to an existing tracer. Trace points for subs that already have trace points will be ignored (we can add an option to enable this; send me a patch or contact me if need be).

Using in other Devel:: modules

The main advantage of the Devel:: namespace is that the perl -d:Foo ... syntax is pretty handy. Other modules which use this might want to be in the Devel:: namespace. The only trick is avoiding calling Devel::TraceCalls' import() routine when you do this (unless you want to for some reason).

To do this, you can either carefully avoid placing Devel::TraceCalls in your Devel::* module's @ISA hierarchy or make sure that your module's import() method is called instead of Devel::TraceCalls'. If you do this, you'll need to have a sub DB::DB defined, because Devel::TraceCalls' wont be. See the source and the Devel::TraceSAX module for details.

A Word on Devel::TraceCall Overhead


Devel::TraceCall is a debugging aid and is designed to provide a lot of detail and flexibility. This comes at a price, namely overhead.

One of the side effects of this overhead is that Devel::TraceCall is useless as a profiling tool, since a function that calls a number of other functions, all of them being traced, will see all of the overhead of Devel::TraceCall in its elapsed time. This could be worked around, but it is outside the scope of this module, see Devel::DProf for profiling needs.


  • Wrap AUTOLOAD and automatically enable tracing on subs handled by and created by AUTOLOADing.

  • Wrapper subs.

  • Does not get parameters from the call stack. It will be optional, on by default.

  • Flesh out and debug the -d:TraceCalls=... feature.

  • Add testing for PreCall and PostCall features.

  • Migrate the CORE::GLOBAL::require feature from Devel::TraceSAX so that run-time require statements can result in classes being traced.

  • Enable wildcards, probably by passing qr/.../ refs, in class, package and sub names.

  • Migrate the namespace walking feature from Devel::TraceSAX, so that the above wildcards can be used to specify categories of classes and packages to trace.

  • Optional logging of returned values.


There are several minor limitations.

Exports a subroutine by default. Do a use Devel::TraceCalls (); to suppress that.

If perl's optimized away constant functions, well, there is no call to trace.

Because a wrapper subroutine gets installed in place of the original subroutine, anything that has cached a reference (with code like $foo = \&foo or $foo = Bar->can( "foo" )) will bypass the tracing.

If a subroutine reference is taken while tracing is enabled and then used after tracing is disabled, it will refer to the wrapper subroutine that no longer has something to wrap. Devel::TraceCalls does not pass these through in that case, but it could.

The import based use Devel::TraceCalls { ... } feature relies on a CHECK subroutine, which is not present on older perls. See perlmod for details.

Doesn't warn if you point it at an empty class, or if you pass no subs. This is because you might be passing in a possibly empty list. Check the return value's subs method to count up how many overrides occured.


See Devel::TraceMethods and Aspect::Trace for similar functionality.

Merlyn also suggested using Class::Prototyped to implement the instance subclassing, but it seems too simple to do without incurring a prerequisite module.

A miscellany of tricky modules like Sub::Versive, Hook::LexWrap, and Sub::Uplevel.


Devel::DProf for profiling, Devel::TraceSAX for an example of a client module.


    Barrie Slaymaker <>

    Maintainer from version 0.04 is
    Cosimo Streppone <>


    Copyright (c) 2002 Barrie Slaymaker, All Rights Reserved.

You may use this module under the terms of the Artistic License or the GPL, any version.

3 POD Errors

The following errors were encountered while parsing the POD:

Around line 160:

You have '=item 3' instead of the expected '=item 4'

Around line 418:

'=item' outside of any '=over'

Around line 586:

You forgot a '=back' before '=head1'