


Devel::Peek(3p)  Perl Programmers Reference Guide Devel::Peek(3p)


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

SYNOPSIS
               use Devel::Peek;
               Dump( $a );
               Dump( $a, 5 );
               DumpArray( 5, $a, $b, ... );
               mstat "Point 5";


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 sec-
       tions of the perlguts manpage.

       Devel::Peek supplies a `Dump()' function which can dump a
       raw Perl datatype, and `mstat("marker")' function to
       report on memory usage (if perl is compiled with corre-
       sponding option).  The function DeadCode() provides
       statistics on the data "frozen" into inactive `CV'.
       Devel::Peek also supplies `SvREFCNT()', `SvREFCNT_inc()',
       and `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 `Dump()' function.  For
       format of output of mstats() see the Using
       `$ENV{PERL_DEBUG_MSTATS}' entry in the perldebug manpage.

       Function `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.

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 peo-
       ple on their way.  There are no guidewires or safety nets,
       nor blazed trails, so be prepared to travel alone from



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Devel::Peek(3p)  Perl Programmers Reference Guide Devel::Peek(3p)


       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 the perlguts manpage
       with you.  When you return we expect to see it well-
       thumbed.

       A simple scalar string

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

               use Devel::Peek;
               $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 `$a' is an SV, a scalar.  The scalar is a PVIV,
       a string.  Its reference count is 1.  It has the `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 prop-
       erly NUL-terminated.  If the FLAGS had been IOK we would
       look at the IV item.  CUR indicates the number of charac-
       ters 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).

       A simple scalar number

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

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

       The output:

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

       This says `$a' is an SV, a scalar.  The scalar is an IV, a



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Devel::Peek(3p)  Perl Programmers Reference Guide Devel::Peek(3p)


       number.  Its reference count is 1.  It has the `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.

       A simple scalar with an extra reference

       If the scalar from the previous example had an extra ref-
       erence:

               use Devel::Peek;
               $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 `$b' instead of `$a'.

       A reference to a simple scalar

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

               use Devel::Peek;
               $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 `$b' is an SV.  The
       scalar is an RV, a reference.  It has the `ROK' flag set,
       meaning it is a reference.  Because ROK is set we have an
       RV item rather than an IV or PV.  Notice that Dump follows
       the reference and shows us what `$b' was referencing.  We
       see the same `$a' that we found in the previous example.




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Devel::Peek(3p)  Perl Programmers Reference Guide Devel::Peek(3p)


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

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

       If `$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:






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Devel::Peek(3p)  Perl Programmers Reference Guide Devel::Peek(3p)


               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 `Dump' will not report all the elements in the
       array, only several first (depending on how deep it
       already went into the report tree).

       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:


















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Devel::Peek(3p)  Perl Programmers Reference Guide Devel::Peek(3p)


               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 `$a' is a reference pointing to an SV.  That SV
       is a PVHV, a hash. Fields RITER and EITER are used by `the
       each manpage'.

       Dumping a large array or hash

       The `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 `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;


       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 struc-
       ture 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:






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Devel::Peek(3p)  Perl Programmers Reference Guide Devel::Peek(3p)


               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
       `OBJECT' flag set.  Note that an SV which holds a C
       pointer also has the `IOK' flag set.  The `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


       A reference to a subroutine

       Looks like this:



















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Devel::Peek(3p)  Perl Programmers Reference Guide Devel::Peek(3p)


               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

       o   the subroutine is not an XSUB (since `START' and
           `ROOT' are non-zero, and `XSUB' is zero);

       o   that it was compiled in the package `main';

       o   under the name `MY::top_targets';

       o   inside a 5th eval in the program;

       o   it is not currently executed (see `DEPTH');

       o   it has no prototype (`PROTOTYPE' field is missing).

EXPORTS
       `Dump', `mstat', `DeadCode', `DumpArray', `DumpWithOP' and
       `DumpProg' by default. Additionally available `SvREFCNT',
       `SvREFCNT_inc' and `SvREFCNT_dec'.

BUGS
       Readers have been known to skip important parts of the
       perlguts manpage, causing much frustration for all.

AUTHOR
       Ilya Zakharevich    ilya@math.ohio-state.edu

       Copyright (c) 1995-98 Ilya Zakharevich. All rights
       reserved.  This program is free software; you can redis-
       tribute 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



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       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.

SEE ALSO
       the perlguts manpage, and the perlguts manpage, again.


















































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