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

    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.

    The default action is to log the calls to STDERR. Passing in any of the
    Calls, PreCall, or PostCall options disables this default behavior (and
    passing in an TraceOutput option reenables it).

    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.

    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",

            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 the Data Capture Format entry
        elsewhere in this document), 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.

    3   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

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

    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.

            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.

            PreCall => \&sub_to_call_after_calling_the_target,

        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.

    Data Capture Options
        These options affect the data captured in the "Calls" array (see the
        The Calls ARRAY entry elsewhere in this document) 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).

    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

    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).

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
            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 the
    OPTIONS entry elsewhere in this document 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.

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 the Devel::TraceSAX
    manpage module for details.

    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

    Does not get parameters from the call stack. This will change when I get
    a chance. It will be optional, on by default, if it changes.

    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, then after
    tracin,g is disabled it will refer to the wrapper subroutine that no
    longer has something to wrap. We can't use closures to get around this
    because closures can't be prototyped.

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

        Barrie Slaymaker <>

        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.