Devel::Peek(3) Perl Programmers Reference Guide Devel::Peek(3)NAMEDevel::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";
DESCRIPTIONDevel::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 sec
tions 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 corre
sponding 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 $Devel::Peek::pv_limit can be set to
limit the number of character printed in various string
values. Setting it to 0 means no limit.
Memory footprint debugging
When perl is compiled with support for memory footprint
debugging (default with Perl's malloc()), Devel::Peek pro
vides an access to this API.
Use mstat() function to emit a memory state statistic to
the terminal. For more information on the format of out
put of mstat() see the Using "$ENV{PERL_DEBUG_MSTATS}"
entry in the perldebug manpage.
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"
contain array references which provide the information
about the allocated size and usable size of chunks in each
bucket. Again, see the Using "$ENV{PERL_DEBUG_MSTATS}"
entry in the perldebug manpage for details.
Keep in mind that only the first several "odd-numbered"
buckets are used, so the information on size of the "odd-
numbered" buckets which are not used is probably meaning
less.
The information in
mem_size available_size minbucket nbuckets
is the property of a particular build of perl, and does
not depend on the current process. If you do not provide
the optional argument to the functions mstats_fillhash(),
fill_mstats(), mstats2hash(), then the information in
fields "mem_size", "available_size" is not updated.
"fill_mstats($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($buf)", "mstats_fill
hash(%hash)", and "mstats2hash($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
datatypes, and should suffice to get most determined peo
ple 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.
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 prop
erly NUL-terminated. If the FLAGS had been IOK we would
look at the IV item. CUR indicates the number of charac
ters 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.
A simple scalar with an extra reference
If the scalar from the previous example had an extra ref
erence:
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".
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 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:
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 $a;
The output:
SV = RV(0xf041c)
REFCNT = 1
FLAGS = (ROK)
RV = 0xb2850
SV = PVHV(0xbd448)
REFCNT = 1
FLAGS = ()
NV = 0
ARRAY = 0xbd748
KEYS = 1
FILL = 1
MAX = 7
RITER = -1
EITER = 0x0
Elt "hello" => 0xbaaf0
SV = IV(0xbe860)
REFCNT = 1
FLAGS = (IOK,pIOK)
IV = 42
This shows "$a" is a reference pointing to an SV. That SV
is a PVHV, a hash. Fields RITER and EITER are used by "the
each manpage".
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 struc
ture 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
A reference to a subroutine
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
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 "SvRE
FCNT", "SvREFCNT_inc" and "SvREFCNT_dec".
BUGS
Readers have been known to skip important parts of the
perlguts manpage, 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 redis
tribute 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.
SEE ALSO
the perlguts manpage, and the perlguts manpage, again.
2001-03-03 perl v5.6.1 Devel::Peek(3)
