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 );
DumpArray( 5, $a, $b, ... );
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.
Function C<DumpArray()> allows dumping of multiple values (useful when you
need to analize returns of functions).
The global variable $Devel::Peek::pv_limit can be set to limit the
number of character printed in various string values. Setting it to 0
means no limit.
=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 $a;
The output:
SV = PVIV(0xbc288)
REFCNT = 1
FLAGS = (POK,pPOK)
IV = 0
PV = 0xb2048 "hello"\0
CUR = 5
LEN = 6
This says C<$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. The \0 at the end indicate that this
PV is properly NUL-terminated.
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 $a;
The output:
SV = RV(0xf041c)
REFCNT = 1
FLAGS = (ROK)
RV = 0xb2850
SV = PVHV(0xbd448)
REFCNT = 1
FLAGS = ()
NV = 0
ARRAY = 0xbd748
KEYS = 1
FILL = 1
MAX = 7
RITER = -1
EITER = 0x0
Elt "hello" => 0xbaaf0
SV = IV(0xbe860)
REFCNT = 1
FLAGS = (IOK,pIOK)
IV = 42
This shows C<$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 DumpArray DumpWithOP DumpProg
);
# Other items we are prepared to export if requested
@EXPORT_OK = qw(
Dump SvREFCNT SvREFCNT_inc SvREFCNT_dec mstat DeadCode DumpArray DumpProg DumpWithOP
);
%EXPORT_TAGS = ('ALL' => \@EXPORT_OK);
$VERSION = $VERSION = '0.96';
bootstrap Devel::Peek;
sub DumpWithOP ($;$) {
local($Devel::Peek::dump_ops)=1;
my $depth = @_ > 1 ? $_[1] : 4 ;
Dump($_[0]);
}
# Preloaded methods go here.
# Autoload methods go after __END__, and are processed by the autosplit program.
1;
__END__