package Devel::Peek;

=head1 NAME

Devel::Peek - A data debugging tool for the XS programmer

=head1 SYNOPSIS

        use Devel::Peek;
        Dump( $a );
        Dump( $a, 5 );
	mstat "Point 5";

=head1 DESCRIPTION

Devel::Peek contains functions which allows raw Perl datatypes to be
manipulated from a Perl script.  This is used by those who do XS programming
to check that the data they are sending from C to Perl looks as they think
it should look.  The trick, then, is to know what the raw datatype is
supposed to look like when it gets to Perl.  This document offers some tips
and hints to describe good and bad raw data.

It is very possible that this document will fall far short of being useful
to the casual reader.  The reader is expected to understand the material in
the first few sections of L<perlguts>.

Devel::Peek supplies a C<Dump()> function which can dump a raw Perl
datatype, and C<mstat("marker")> function to report on memory usage
(if perl is compiled with corresponding option).  The function
DeadCode() provides statistics on the data "frozen" into inactive
C<CV>.  Devel::Peek also supplies C<SvREFCNT()>, C<SvREFCNT_inc()>, and
C<SvREFCNT_dec()> which can query, increment, and decrement reference
counts on SVs.  This document will take a passive, and safe, approach
to data debugging and for that it will describe only the C<Dump()>
function.

=head1 EXAMPLES

The following examples don't attempt to show everything as that would be a
monumental task, and, frankly, we don't want this manpage to be an internals
document for Perl.  The examples do demonstrate some basics of the raw Perl
datatypes, and should suffice to get most determined people on their way.
There are no guidewires or safety nets, nor blazed trails, so be prepared to
travel alone from this point and on and, if at all possible, don't fall into
the quicksand (it's bad for business).

Oh, one final bit of advice: take L<perlguts> with you.  When you return we
expect to see it well-thumbed.

=head2 A simple scalar string

Let's begin by looking a simple scalar which is holding a string.

        use Devel::Peek 'Dump';
        $a = "hello";
        Dump $a;

The output:

        SV = PVIV(0xbc288)
          REFCNT = 1
          FLAGS = (POK,pPOK)
          IV = 0
          PV = 0xb2048 "hello"
          CUR = 5
          LEN = 6

This says C<$a> is an SV, a scalar.  The scalar is a PVIV, a string.
Its reference count is 1.  It has the C<POK> flag set, meaning its
current PV field is valid.  Because POK is set we look at the PV item
to see what is in the scalar.  If the FLAGS had been IOK we would look
at the IV item.  CUR indicates the number of characters in the PV.
LEN indicates the number of bytes requested for the PV (one more than
CUR, in this case, because LEN includes an extra byte for the
end-of-string marker).

=head2 A simple scalar number

If the scalar contains a number the raw SV will be leaner.

        use Devel::Peek 'Dump';
        $a = 42;
        Dump $a;

The output:

        SV = IV(0xbc818)
          REFCNT = 1
          FLAGS = (IOK,pIOK)
          IV = 42

This says C<$a> is an SV, a scalar.  The scalar is an IV, a number.  Its
reference count is 1.  It has the C<IOK> flag set, meaning it is currently
being evaluated as a number.  Because IOK is set we look at the IV item to
see what is in the scalar.

=head2 A simple scalar with an extra reference

If the scalar from the previous example had an extra reference:

        use Devel::Peek 'Dump';
        $a = 42;
        $b = \$a;
        Dump $a;

The output:

        SV = IV(0xbe860)
          REFCNT = 2
          FLAGS = (IOK,pIOK)
          IV = 42

Notice that this example differs from the previous example only in its
reference count.  Compare this to the next example, where we dump C<$b>
instead of C<$a>.

=head2 A reference to a simple scalar

This shows what a reference looks like when it references a simple scalar.

        use Devel::Peek 'Dump';
        $a = 42;
        $b = \$a;
        Dump $b;

The output:

        SV = RV(0xf041c)
          REFCNT = 1
          FLAGS = (ROK)
          RV = 0xbab08
        SV = IV(0xbe860)
          REFCNT = 2
          FLAGS = (IOK,pIOK)
          IV = 42

Starting from the top, this says C<$b> is an SV.  The scalar is an RV, a
reference.  It has the C<ROK> flag set, meaning it is a reference.  Because
ROK is set we have an RV item rather than an IV or PV.  Notice that Dump
follows the reference and shows us what C<$b> was referencing.  We see the
same C<$a> that we found in the previous example.

Note that the value of C<RV> coincides with the numbers we see when we
stringify $b. The addresses inside RV() and IV() are addresses of
C<X***> structure which holds the current state of an C<SV>. This
address may change during lifetime of an SV.

=head2 A reference to an array

This shows what a reference to an array looks like.

        use Devel::Peek 'Dump';
        $a = [42];
        Dump $a;

The output:

        SV = RV(0xf041c)
          REFCNT = 1
          FLAGS = (ROK)
          RV = 0xb2850
        SV = PVAV(0xbd448)
          REFCNT = 1
          FLAGS = ()
          IV = 0
          NV = 0
          ARRAY = 0xb2048
          ALLOC = 0xb2048
          FILL = 0
          MAX = 0
          ARYLEN = 0x0
          FLAGS = (REAL)
        Elt No. 0 0xb5658
        SV = IV(0xbe860)
          REFCNT = 1
          FLAGS = (IOK,pIOK)
          IV = 42

This says C<$a> is an SV and that it is an RV.  That RV points to
another SV which is a PVAV, an array.  The array has one element,
element zero, which is another SV. The field C<FILL> above indicates
the last element in the array, similar to C<$#$a>.

If C<$a> pointed to an array of two elements then we would see the
following.

        use Devel::Peek 'Dump';
        $a = [42,24];
        Dump $a;

The output:

        SV = RV(0xf041c)
          REFCNT = 1
          FLAGS = (ROK)
          RV = 0xb2850
        SV = PVAV(0xbd448)
          REFCNT = 1
          FLAGS = ()
          IV = 0
          NV = 0
          ARRAY = 0xb2048
          ALLOC = 0xb2048
          FILL = 0
          MAX = 0
          ARYLEN = 0x0
          FLAGS = (REAL)
        Elt No. 0  0xb5658
        SV = IV(0xbe860)
          REFCNT = 1
          FLAGS = (IOK,pIOK)
          IV = 42
        Elt No. 1  0xb5680
        SV = IV(0xbe818)
          REFCNT = 1
          FLAGS = (IOK,pIOK)
          IV = 24

Note that C<Dump> will not report I<all> the elements in the array,
only several first (depending on how deep it already went into the
report tree).

=head2 A reference to a hash

The following shows the raw form of a reference to a hash.

        use Devel::Peek 'Dump';
        $a = {hello=>42};
        Dump $a;

The output:

        SV = RV(0xf041c)
          REFCNT = 1
          FLAGS = (ROK)
          RV = 0xb2850
        SV = PVHV(0xbd448)
          REFCNT = 1
          FLAGS = ()
          IV = 1
          NV = 0
          ARRAY = 0xbd748
          KEYS = 1
          FILL = 1
          MAX = 7
          RITER = -1
          EITER = 0x0
        Elt "hello" => 0xbaaf0
        SV = IV(0xbe860)
          REFCNT = 1
          FLAGS = (IOK,pIOK)
          IV = 42

This shows C<$a> is a reference pointing to an SV.  That SV is a PVHV, a
hash. Fields RITER and EITER are used by C<L<each>>.

=head2 Dumping a large array or hash

The C<Dump()> function, by default, dumps up to 4 elements from a
toplevel array or hash.  This number can be increased by supplying a
second argument to the function.

        use Devel::Peek 'Dump';
        $a = [10,11,12,13,14];
        Dump $a;

Notice that C<Dump()> prints only elements 10 through 13 in the above code.
The following code will print all of the elements.

        use Devel::Peek 'Dump';
        $a = [10,11,12,13,14];
        Dump $a, 5;

=head2 A reference to an SV which holds a C pointer

This is what you really need to know as an XS programmer, of course.  When
an XSUB returns a pointer to a C structure that pointer is stored in an SV
and a reference to that SV is placed on the XSUB stack.  So the output from
an XSUB which uses something like the T_PTROBJ map might look something like
this:

        SV = RV(0xf381c)
          REFCNT = 1
          FLAGS = (ROK)
          RV = 0xb8ad8
        SV = PVMG(0xbb3c8)
          REFCNT = 1
          FLAGS = (OBJECT,IOK,pIOK)
          IV = 729160
          NV = 0
          PV = 0
          STASH = 0xc1d10       "CookBookB::Opaque"

This shows that we have an SV which is an RV.  That RV points at another
SV.  In this case that second SV is a PVMG, a blessed scalar.  Because it is
blessed it has the C<OBJECT> flag set.  Note that an SV which holds a C
pointer also has the C<IOK> flag set.  The C<STASH> is set to the package
name which this SV was blessed into.

The output from an XSUB which uses something like the T_PTRREF map, which
doesn't bless the object, might look something like this:

        SV = RV(0xf381c)
          REFCNT = 1
          FLAGS = (ROK)
          RV = 0xb8ad8
        SV = PVMG(0xbb3c8)
          REFCNT = 1
          FLAGS = (IOK,pIOK)
          IV = 729160
          NV = 0
          PV = 0

=head2 A reference to a subroutine

Looks like this:

	SV = RV(0x798ec)
	  REFCNT = 1
	  FLAGS = (TEMP,ROK)
	  RV = 0x1d453c
	SV = PVCV(0x1c768c)
	  REFCNT = 2
	  FLAGS = ()
	  IV = 0
	  NV = 0
	  COMP_STASH = 0x31068  "main"
	  START = 0xb20e0
	  ROOT = 0xbece0
	  XSUB = 0x0
	  XSUBANY = 0
	  GVGV::GV = 0x1d44e8   "MY" :: "top_targets"
	  FILEGV = 0x1fab74     "_<(eval 5)"
	  DEPTH = 0
	  PADLIST = 0x1c9338

This shows that 

=over

=item

the subroutine is not an XSUB (since C<START> and C<ROOT> are
non-zero, and C<XSUB> is zero);

=item

that it was compiled in the package C<main>;

=item

under the name C<MY::top_targets>; 

=item

inside a 5th eval in the program;

=item

it is not currently executed (see C<DEPTH>);


=item

it has no prototype (C<PROTOTYPE> field is missing).

=over

=head1 EXPORTS

C<Peek>, C<mstats>, C<DeadCode> by default. Additionally available C<SvREFCNT>,
C<SvREFCNT_inc>, C<SvREFCNT_dec>.

=head1 BUGS

Readers have been known to skip important parts of L<perlguts>, causing much
frustration for all.

=head1 SEE ALSO

L<perlguts>, and L<perlguts>, again.

=cut

require Exporter;
require DynaLoader;

@ISA = qw(Exporter DynaLoader);
# Items to export into callers namespace by default. Note: do not export
# names by default without a very good reason. Use EXPORT_OK instead.
# Do not simply export all your public functions/methods/constants.
@EXPORT = qw(
	Dump mstat DeadCode
);
# Other items we are prepared to export if requested
@EXPORT_OK = qw(
		Dump SvREFCNT SvREFCNT_inc SvREFCNT_dec mstat DeadCode
);
%EXPORT_TAGS = ('ALL' => \@EXPORT_OK);

$VERSION = $VERSION = 0.81;

bootstrap Devel::Peek;

# Preloaded methods go here.

# Autoload methods go after __END__, and are processed by the autosplit program.

1;
__END__
