=head1 NAME

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

=head1 SYNOPSIS

	use Devel::Peek 'Dump';
	Dump( $a );
	Dump( $a, 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.  It 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 it is currently
being evaluated as a string.  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 in 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.

=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
	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.

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
	SV = IV(0xbe860)
	  REFCNT = 1
	  FLAGS = (IOK,pIOK)
	  IV = 42
	Elt No. 1
	SV = IV(0xbe818)
	  REFCNT = 1
	  FLAGS = (IOK,pIOK)
	  IV = 24

=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.

=head2 Dumping a large array or hash

The C<Dump()> function, by default, dumps up to 4 elements from an 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

=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.
