MirOS Manual: Devel::Peek(3p)


ext::Devel::Peek:PerlkProgrammers Referext::Devel::Peek::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";

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

DESCRIPTION

     Devel::Peek contains functions which allows raw Perl data-
     types 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 sup-
     posed 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 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 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".

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     Runtime debugging

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

     debug_flags() returns a string representation of $^D (simi-
     lar 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 modifica-
     tion.)

     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 modifi-
     cation.)

     Memory footprint debugging

     When perl is compiled with support for memory footprint
     debugging (default with Perl's malloc()), Devel::Peek pro-
     vides 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 refer-
     ences which provide per-bucket count of free and used
     chunks.  Two other fields "mem_size", "available_size" con-
     tain 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 mean-
     ingless.

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     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
       }

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 data-
     types, 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.

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     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.

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     A simple scalar with an extra reference

     If the scalar from the previous example had an extra refer-
     ence:

             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 exam-
     ple, 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

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     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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             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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             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, 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 ran-
     dom 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.

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             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 some-
     thing 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:

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             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    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.

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     Author of this software makes no claim whatsoever about sui-
     tability, reliability, edability, editability or usability
     of this product, and should not be kept liable for any dam-
     age 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.

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