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NAME
Devel::Peek - A data debugging tool for the XS programmer
SYNOPSIS
use Devel::Peek;
Dump( $a );
Dump( $a, 5 );
Dump( @a );
Dump( %h );
DumpArray( 5, $a, $b, ... );
mstat "Point 5";
use Devel::Peek ':opd=st';
DESCRIPTION
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() 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 $Devel::Peek::pv_limit can be set to limit the number of character
printed in various string values. Setting it to 0 means no limit.
If "use Devel::Peek" directive has a ":opd=FLAGS" argument, this switches on debugging of
opcode dispatch. "FLAGS" should be a combination of "s", "t", and "P" (see -D flags in
perlrun).
":opd" is a shortcut for ":opd=st".
Runtime debugging
CvGV($cv) return one of the globs associated to a subroutine reference $cv.
debug_flags() returns a string representation of $^D (similar 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 modification.)
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 modification.)
Memory footprint debugging
When perl is compiled with support for memory footprint debugging (default with Perl's
malloc()), Devel::Peek provides 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 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 $ENV{PERL_DEBUG_MSTATS}" in perldebguts for
details.
Keep in mind that only the first several "odd-numbered" buckets are used, so the
information on size of the "odd-numbered" buckets which are not used is probably
meaningless.
The information in
mem_size available_size minbucket nbuckets
is the property of a particular build of perl, and does not depend on the current process.
If you do not provide the optional argument to the functions mstats_fillhash(),
fill_mstats(), mstats2hash(), then the information in fields "mem_size", "available_size"
is not updated.
fill_mstats($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 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.
A simple scalar string
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).
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) 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.
A simple scalar with an extra reference
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.
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 = 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 IV() are addresses of "X***" structures which hold 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 = 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 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 = 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
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 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
A reference to a subroutine
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).
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.
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
perlguts, and perlguts, again.