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Devel::Peek - A data debugging tool for the XS programmer |
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";
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 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 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. For format of output of mstats() see
Using $ENV{PERL_DEBUG_MSTATS} 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.
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 the perlguts manpage with you. When you return we expect to see it well-thumbed.
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).
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.
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.
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.
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).
The following shows the raw form of a reference to a hash.
use Devel::Peek;
$a = {hello=>42};
Dump $a;
The output:
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 each.
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;
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 = 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
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
START and ROOT are
non-zero, and XSUB is zero);
main;
MY::top_targets;
DEPTH);
PROTOTYPE field is missing).
Dump, mstat, DeadCode, DumpArray, DumpWithOP and
DumpProg by default. Additionally available SvREFCNT,
SvREFCNT_inc and SvREFCNT_dec.
Readers have been known to skip important parts of the perlguts manpage, causing much frustration for all.
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.
the perlguts manpage, and the perlguts manpage, again.
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Devel::Peek - A data debugging tool for the XS programmer |