peek.pm

source of perl for linux application,

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;
        $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;
        $a = {hello=>42};
        Dump $a;

The output:

	SV = RV(0x8177858) at 0x816a618
	  REFCNT = 1
	  FLAGS = (ROK)
	  RV = 0x814fc10
	  SV = PVHV(0x8167768) at 0x814fc10
	    REFCNT = 1
	    FLAGS = (SHAREKEYS)
	    IV = 1
	    NV = 0
	    ARRAY = 0x816c5b8  (0:7, 1:1)
	    hash quality = 100.0%
	    KEYS = 1
	    FILL = 1
	    MAX = 7
	    RITER = -1
	    EITER = 0x0
	    Elt "hello" HASH = 0xc8fd181b
	    SV = IV(0x816c030) at 0x814fcf4
	      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>>.

The "quality" of a hash is defined as the total number of comparisons needed
to access every element once, relative to the expected number needed for a
random hash. The value can go over 100%.

The total number of comparisons is equal to the sum of the squares of the
number of entries in each bucket.  For a random hash of C<<n>> keys into
C<<k>> buckets, the expected value is:

		n + n(n-1)/2k

=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;
        $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"
	  FILE = "(eval 5)"
	  DEPTH = 0
	  PADLIST = 0x1c9338

This shows that 

=over 4

=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).

=back

=head1 EXPORTS

C<Dump>, C<mstat>, C<DeadCode>, C<DumpArray>, C<DumpWithOP> and
C<DumpProg>, C<fill_mstats>, C<mstats_fillhash>, C<mstats2hash> by
default. Additionally available C<SvREFCNT>, C<SvREFCNT_inc> and
C<SvREFCNT_dec>.

=head1 BUGS

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

=head1 AUTHOR

Ilya Zakharevich	[e-mail]

Copyright (c) 1995-98 Ilya Zakharevich. All rights reserved.
This program is free software; you can redistribute it and/or
modify it under the same terms as Perl itself.

Author of this software makes no claim whatsoever about suitability,
reliability, edability, editability or usability of this product, and
should not be kept liable for any damage resulting from the use of
it. If you can use it, you are in luck, if not, I should not be kept
responsible. Keep a handy copy of your backup tape at hand.

=head1 SEE ALSO

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

=cut