Devel::Peek(3pm) Perl Programmers Reference Guide Devel::Peek(3pm)
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 evaluted 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(0x8177858) at 0x816a618
REFCNT = 1
FLAGS = (ROK)
RV = 0x814fc10
SV = PVHV(0x8167768) at 0x814fc10
REFCNT = 1
FLAGS = (SHAREKEYS)
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" in perlfunc".
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
o the subroutine is not an XSUB (since "START" and "ROOT" are non-
zero, and "XSUB" is not listed, and is thus null);
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 (because "DEPTH" is 0);
o 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 AT math.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.
perl v5.26.3 2018-03-23 Devel::Peek(3pm)
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