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 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. For format of output of mstats() see "Using $ENV{PERL_DEBUG_MSTATS}" in perldebug.
Dump()
mstat("marker")
CV
SvREFCNT()
SvREFCNT_inc()
SvREFCNT_dec()
$ENV{PERL_DEBUG_MSTATS}
Function DumpArray() allows dumping of multiple values (useful when you need to analize returns of functions).
DumpArray()
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 perlguts 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).
$a
POK
If the scalar contains a number the raw SV will be leaner.
use Devel::Peek; $a = 42; Dump $a;
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.
IOK
If the scalar from the previous example had an extra reference:
use Devel::Peek; $a = 42; $b = \$a; Dump $a;
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.
$b
This shows what a reference looks like when it references a simple scalar.
use Devel::Peek; $a = 42; $b = \$a; Dump $b;
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.
ROK
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.
RV
X***
SV
This shows what a reference to an array looks like.
use Devel::Peek; $a = [42]; Dump $a;
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.
FILL
$#$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;
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).
Dump
The following shows the raw form of a reference to a hash.
use Devel::Peek; $a = {hello=>42}; Dump $a;
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.
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.
OBJECT
STASH
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" FILEGV = 0x1fab74 "_<(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);
START
ROOT
XSUB
that it was compiled in the package main;
main
under the name MY::top_targets;
MY::top_targets
inside a 5th eval in the program;
it is not currently executed (see DEPTH);
DEPTH
it has no prototype (PROTOTYPE field is missing).
PROTOTYPE
Dump, mstat, DeadCode, DumpArray, DumpWithOP and DumpProg by default. Additionally available SvREFCNT, SvREFCNT_inc and SvREFCNT_dec.
mstat
DeadCode
DumpArray
DumpWithOP
DumpProg
SvREFCNT
SvREFCNT_inc
SvREFCNT_dec
Readers have been known to skip important parts of perlguts, 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.
perlguts, and perlguts, again.
1 POD Error
The following errors were encountered while parsing the POD:
You forgot a '=back' before '=head1'
To install lib, copy and paste the appropriate command in to your terminal.
cpanm
cpanm lib
CPAN shell
perl -MCPAN -e shell install lib
For more information on module installation, please visit the detailed CPAN module installation guide.