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

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

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

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

       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 "DumpArray()" allows dumping of multiple values (useful when
       you need to analyze 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.

       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

       runops_debug() returns true if the current opcode dispatcher is the
       debugging one.  When called with an argument, 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

       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

       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 provide the optional argument to
       the functions mstats_fillhash(), fill_mstats(), mstats2hash(), then the
       information 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

       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

       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 = 42; $a = "hello";
	       Dump $a;

       The output:

	       SV = PVIV(0xbc288) at 0xbe9a8
		 REFCNT = 1
		 IV = 42
		 PV = 0xb2048 "hello"\0
		 CUR = 5
		 LEN = 8

       This says $a is an SV, a scalar.	 The scalar type is a PVIV, which is
       capable of holding an integer (IV) and/or a string (PV) value. The
       scalar's head is allocated at address 0xbe9a8, while the body is at
       0xbc288.	 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.  Note that the IV field still
       contains its old numeric value, but because FLAGS doesn't have IOK set,
       we must ignore the IV item.  CUR indicates the number of characters in
       the PV.	LEN indicates the number of bytes allocated for the PV (at
       least one more than CUR, because LEN includes an extra byte for the
       end-of-string marker, then usually rounded up to some efficient
       allocation unit).

   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) at 0xbe9a8
		 REFCNT = 1
		 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) at 0xbe9a8
		 REFCNT = 2
		 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

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

       The output:

	       SV = IV(0xf041c) at 0xbe9a0
		 REFCNT = 1
		 FLAGS = (ROK)
		 RV = 0xbab08
		 SV = IV(0xbe860) at 0xbe9a8
		   REFCNT = 2
		   FLAGS = (IOK,pIOK)
		   IV = 42

       Starting from the top, this says $b is an SV.  The scalar is an IV,
       which is capable of holding an integer or reference value.  It has the
       "ROK" flag set, meaning it is a reference (rather than an integer or
       string).	 Notice that Dump follows the reference and shows us what $b
       was referencing.	 We see the same $a that we found in the previous

       Note that the value of "RV" coincides with the numbers we see when we
       stringify $b. The addresses inside IV() are addresses of "X***"
       structures which hold 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 = IV(0xc85998) at 0xc859a8
		 REFCNT = 1
		 FLAGS = (ROK)
		 RV = 0xc70de8
		 SV = PVAV(0xc71e10) at 0xc70de8
		   REFCNT = 1
		   FLAGS = ()
		   ARRAY = 0xc7e820
		   FILL = 0
		   MAX = 0
		   ARYLEN = 0x0
		   FLAGS = (REAL)
		   Elt No. 0
		   SV = IV(0xc70f88) at 0xc70f98
		     REFCNT = 1
		     FLAGS = (IOK,pIOK)
		     IV = 42

       This says $a is a reference (ROK), which points to another SV which is
       a PVAV, an array.  The array has one element, element zero, which is
       another SV. The field "FILL" above indicates the last element in the
       array, similar to "$#$a".

       If $a pointed to an array of two elements then we would see the

	       use Devel::Peek 'Dump';
	       $a = [42,24];
	       Dump $a;

       The output:

	       SV = IV(0x158c998) at 0x158c9a8
		 REFCNT = 1
		 FLAGS = (ROK)
		 RV = 0x1577de8
		 SV = PVAV(0x1578e10) at 0x1577de8
		   REFCNT = 1
		   FLAGS = ()
		   ARRAY = 0x1585820
		   FILL = 1
		   MAX = 1
		   ARYLEN = 0x0
		   FLAGS = (REAL)
		   Elt No. 0
		   SV = IV(0x1577f88) at 0x1577f98
		     REFCNT = 1
		     FLAGS = (IOK,pIOK)
		     IV = 42
		   Elt No. 1
		   SV = IV(0x158be88) at 0x158be98
		     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

   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 = IV(0x8177858) at 0x816a618
		 REFCNT = 1
		 FLAGS = (ROK)
		 RV = 0x814fc10
		 SV = PVHV(0x8167768) at 0x814fc10
		   REFCNT = 1
		   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" in perlfunc".

       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 "<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 output from an XSUB which uses something like the T_PTROBJ map
       might look something like this:

	       SV = IV(0xf381c) at 0xc859a8
		 REFCNT = 1
		 FLAGS = (ROK)
		 RV = 0xb8ad8
		 SV = PVMG(0xbb3c8) at 0xc859a0
		   REFCNT = 1
		   IV = 729160
		   NV = 0
		   PV = 0
		   STASH = 0xc1d10	 "CookBookB::Opaque"

       This shows that we have an SV which is a reference, which 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 = IV(0xf381c) at 0xc859a8
		 REFCNT = 1
		 FLAGS = (ROK)
		 RV = 0xb8ad8
		 SV = PVMG(0xbb3c8) at 0xc859a0
		   REFCNT = 1
		   FLAGS = (IOK,pIOK)
		   IV = 729160
		   NV = 0
		   PV = 0

   A reference to a subroutine
       Looks like this:

	       SV = IV(0x24d2dd8) at 0x24d2de8
		 REFCNT = 1
		 RV = 0x24e79d8
		 SV = PVCV(0x24e5798) at 0x24e79d8
		   REFCNT = 2
		   FLAGS = ()
		   COMP_STASH = 0x22c9c50      "main"
		   START = 0x22eed60 ===> 0
		   ROOT = 0x22ee490
		   GVGV::GV = 0x22de9d8	       "MY" :: "top_targets"
		   FILE = "(eval 5)"
		   DEPTH = 0
		   FLAGS = 0x0
		   OUTSIDE_SEQ = 93
		   PADLIST = 0x22e9ed8
		   PADNAME = 0x22e9ec0(0x22eed00) PAD = 0x22e9ea8(0x22eecd0)
		   OUTSIDE = 0x22c9fb0 (MAIN)

       This shows that

       ·   the subroutine is not an XSUB (since "START" and "ROOT" are non-
	   zero, and "XSUB" is not listed, and is thus null);

       ·   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).

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

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

       Ilya Zakharevich

       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.

       perlguts, and perlguts, again.

perl v5.18.2			  2014-01-06		    Devel::Peek(3perl)

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