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

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

NNAAMMEE #

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

SSYYNNOOPPSSIISS #

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

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

DDEESSCCRRIIPPTTIIOONN #

 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 DDeeaaddCCooddee(())
 provides statistics on the data "frozen" into inactive "CV".  Devel::Peek
 also supplies "SvREFCNT()" which can query 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.

 All output is to STDERR.

 The "Dump()" function takes one or two arguments: something to dump, and
 an optional limit for recursion and array elements (default is 4).  The
 first argument is evaluated in rvalue scalar context, with exceptions for
 @array and %hash, which dump the array or hash itself.  So "Dump @array"
 works, as does "Dump $foo".  And "Dump pos" will call "pos" in rvalue
 context, whereas "Dump ${\pos}" will call it in lvalue context.

 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 --DD flags in perlrun).

 ":opd" is a shortcut for ":opd=st".

RRuunnttiimmee ddeebbuuggggiinngg “CvGV($cv)” return one of the globs associated to a subroutine reference $cv.

 ddeebbuugg__ffllaaggss(()) returns a string representation of $^D (similar to what is
 allowed for --DD 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.)

 rruunnooppss__ddeebbuugg(()) returns true if the current _o_p_c_o_d_e _d_i_s_p_a_t_c_h_e_r 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.)

MMeemmoorryy ffoooottpprriinntt ddeebbuuggggiinngg When perl is compiled with support for memory footprint debugging (default with Perl’s mmaalllloocc(())), Devel::Peek provides an access to this

API. #

 Use mmssttaatt(()) function to emit a memory state statistic to the terminal.
 For more information on the format of output of mmssttaatt(()) 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 mmssttaatt(()) 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 provide the optional argument to the
 functions mmssttaattss__ffiillllhhaasshh(()), ffiillll__mmssttaattss(()), mmssttaattss22hhaasshh(()), 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 _t_h_e
 _s_e_c_o_n_d _t_i_m_e 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
   }

EEXXAAMMPPLLEESS #

 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.

AA ssiimmppllee ssccaallaarr ssttrriinngg 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 #

           FLAGS = (POK,pPOK)

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).

AA ssiimmppllee ssccaallaarr nnuummbbeerr 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 #

           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.

AA ssiimmppllee ssccaallaarr wwiitthh aann eexxttrraa rreeffeerreennccee 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 #

           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.

AA rreeffeerreennccee ttoo aa ssiimmppllee ssccaallaarr 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 = 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 example.

 Note that the value of "RV" coincides with the numbers we see when we
 stringify $b. The addresses inside IIVV(()) are addresses of "X***"
 structures which hold the current state of an "SV". This address may
 change during lifetime of an SV.

AA rreeffeerreennccee ttoo aann aarrrraayy 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 #

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
 following.

         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 #

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 _a_l_l the elements in the array, only
 several first (depending on how deep it already went into the report
 tree).

AA rreeffeerreennccee ttoo aa hhaasshh The following shows the raw form of a reference to a hash.

         use Devel::Peek;
         $a = {hello=>42};
         Dump $a;

 The output:

     SV = IV(0x55cb50b50fb0) at 0x55cb50b50fc0

REFCNT = 1 #

FLAGS = (ROK) #

       RV = 0x55cb50b2b758
       SV = PVHV(0x55cb50b319c0) at 0x55cb50b2b758

REFCNT = 1 #

FLAGS = (SHAREKEYS) #

         ARRAY = 0x55cb50b941a0  (0:7, 1:1)
         hash quality = 100.0%

KEYS = 1 #

FILL = 1 #

MAX = 7 #

         Elt "hello" HASH = 0x3128ece4
         SV = IV(0x55cb50b464f8) at 0x55cb50b46508

REFCNT = 1 #

           FLAGS = (IOK,pIOK)

IV = 42 #

 This shows $a is a reference pointing to an SV.  That SV is a PVHV, a
 hash.

 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

DDuummppiinngg aa llaarrggee aarrrraayy oorr hhaasshh 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;

AA rreeffeerreennccee ttoo aann SSVV wwhhiicchh hhoollddss aa CC ppooiinntteerr 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 #

             FLAGS = (OBJECT,IOK,pIOK)

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 #

AA rreeffeerreennccee ttoo aa ssuubbrroouuttiinnee Looks like this:

         SV = IV(0x24d2dd8) at 0x24d2de8

REFCNT = 1 #

FLAGS = (TEMP,ROK) #

           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 (because "DEPTH" is 0);

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

EEXXPPOORRTTSS #

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

BBUUGGSS #

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

AAUUTTHHOORR #

 Ilya Zakharevich    ilya@math.ohio-state.edu

 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.

SSEEEE AALLSSOO #

 perlguts, and perlguts, again.

perl v5.36.3 2023-02-15 Devel::Peek(3p)