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



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";

               use Devel::Peek ':opd=st';

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 perlguts.

       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 corresponding 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.

       Function "DumpArray()" allows dumping of multiple values (useful when you need to analyze returns of func-
       tions).

       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.

       If "use Devel::Peek" directive has a ":opd=FLAGS" argument, this switches on debugging of opcode dispatch.
       "FLAGS" should be a combination of "s", "t", and "P" (see -D flags in perlrun).  ":opd" is a shortcut for
       ":opd=st".

       Runtime debugging

       "CvGV($cv)" return one of the globs associated to a subroutine reference $cv.

       debug_flags() returns a string representation of $^D (similar to what is allowed for -D flag).  When called
       with a numeric argument, sets $^D to the corresponding value.  When called with an argument of the form
       "flags-flags", set on/off bits of $^D corresponding to letters before/after "-".  (The returned value is for
       $^D before the modification.)

       runops_debug() returns true if the current opcode dispatcher is the debugging one.  When called with an argu-
       ment, switches to debugging or non-debugging dispatcher depending on the argument (active for newly-entered
       subs/etc only).  (The returned value is for the dispatcher before the modification.)

       Memory footprint debugging

       When perl is compiled with support for memory footprint debugging (default with Perl's malloc()), Devel::Peek
       provides an access to this API.

       Use mstat() function to emit a memory state statistic to the terminal.  For more information on the format of
       output of mstat() see "Using $ENV{PERL_DEBUG_MSTATS}" in perldebguts.

       Three additional functions allow access to this statistic from Perl.  First, use "mstats_fillhash(%hash)" to
       get the information contained in the output of mstat() into %hash. The field of this hash are

         minbucket nbuckets sbrk_good sbrk_slack sbrked_remains sbrks start_slack
         topbucket topbucket_ev topbucket_odd total total_chain total_sbrk totfree

       Two additional fields "free", "used" contain array references which provide per-bucket count of free and used
       chunks.  Two other fields "mem_size", "available_size" contain array references which provide the information
       about the allocated size and usable size of chunks in each bucket.  Again, see "Using $ENV{PERL_DEBUG_MSTATS}"
       in perldebguts for details.

       Keep in mind that only the first several "odd-numbered" buckets are used, so the information on size of the
       "odd-numbered" buckets which are not used is probably meaningless.

       The information in

        mem_size available_size minbucket nbuckets

       is the property of a particular build of perl, and does not depend on the current process.  If you do not pro-
       vide the optional argument to the functions mstats_fillhash(), fill_mstats(), mstats2hash(), then the informa-
       tion in fields "mem_size", "available_size" is not updated.

       "fill_mstats($buf)" is a much cheaper call (both speedwise and memory-wise) which collects the statistic into
       $buf in machine-readable form.  At a later moment you may need to call "mstats2hash($buf, %hash)" to use this
       information to fill %hash.

       All three APIs "fill_mstats($buf)", "mstats_fillhash(%hash)", and "mstats2hash($buf, %hash)" are designed to
       allocate no memory if used the second time on the same $buf and/or %hash.

       So, if you want to collect memory info in a cycle, you may call

         $#buf = 999;
         fill_mstats($_) for @buf;
         mstats_fillhash(%report, 1);          # Static info too

         foreach (@buf) {
           # Do something...
           fill_mstats $_;                     # Collect statistic
         }
         foreach (@buf) {
           mstats2hash($_, %report);           # Preserve static info
           # Do something with %report
         }

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 perlguts 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 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).

       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 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 reference:

               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.

       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:

               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:

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

       The "quality" of a hash is defined as the total number of comparisons needed to access every element once, rel-
       ative 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 "<n"> keys into "<k"> buckets, the expected value is:

                       n + n(n-1)/2k

       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
       structure that pointer is stored in an SV and a reference to that SV is placed on the XSUB stack.  So the out-
       put 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 "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:

               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

       ?   the subroutine is not an XSUB (since "START" and "ROOT" are non-zero, and "XSUB" is zero);

       ?   that it was compiled in the package "main";

       ?   under the name "MY::top_targets";

       ?   inside a 5th eval in the program;

       ?   it is not currently executed (see "DEPTH");

       ?   it has no prototype ("PROTOTYPE" field is missing).

EXPORTS
       "Dump", "mstat", "DeadCode", "DumpArray", "DumpWithOP" and "DumpProg", "fill_mstats", "mstats_fillhash",
       "mstats2hash" by default. Additionally available "SvREFCNT", "SvREFCNT_inc" and "SvREFCNT_dec".

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

AUTHOR
       Ilya Zakharevich    ilyaATmath.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 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
       perlguts, and perlguts, again.



perl v5.8.8                       2001-09-21                    Devel::Peek(3)