Devel::Peek
Devel::Peek(3) Perl Programmers Reference Guide Devel::Peek(3)
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 datatypes to be
manipulated from a Perl script. This is used by those who do XS pro-
gramming 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 "SvRE-
FCNT_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 $Devel::Peek::pv_limit can be set to limit the num-
ber of character printed in various string values. Setting it to 0
means no limit.
If "use Devel::Peek" directive has a ":opd=FLAGS" argument, this
switches on debugging of opcode dispatch. "FLAGS" should be a combina-
tion of "s", "t", and "P" (see -D flags in perlrun). ":opd" is a
shortcut for ":opd=st".
Runtime debugging
"CvGV($cv)" return one of the globs associated to a subroutine refer-
ence $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 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 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 mem-
ory-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 = "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 cur-
rently 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)
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 follow-
ing.
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(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 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 = 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
o the subroutine is not an XSUB (since "START" and "ROOT" are
non-zero, and "XSUB" is zero);
o that it was compiled in the package "main";
o under the name "MY::top_targets";
o inside a 5th eval in the program;
o it is not currently executed (see "DEPTH");
o it has no prototype ("PROTOTYPE" field is missing).
EXPORTS
"Dump", "mstat", "DeadCode", "DumpArray", "DumpWithOP" and "DumpProg",
"fill_mstats", "mstats_fillhash", "mstats2hash" by default. Addition-
ally 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 pro-
gram 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.
perl v5.8.6 2001-09-21 Devel::Peek(3)
