use Devel::Peek;
Dump( $a );
Dump( $a, 5 );
DumpArray( 5, $a, $b, ... );
mstat "Point 5";
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.
Function DumpArray() allows dumping of multiple values (useful when you need to analize returns
of functions).
Oh, one final bit of advice: take perlguts with you. When you return we expect to see it well-thumbed.
use Devel::Peek 'Dump';
$a = "hello";
Dump $a;
The output:
SV = PVIV(0xbc288)
REFCNT = 1
FLAGS = (POK,pPOK)
IV = 0
PV = 0xb2048 "hello"
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. 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).
use Devel::Peek 'Dump';
$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.
use Devel::Peek 'Dump';
$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.
use Devel::Peek 'Dump';
$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.
use Devel::Peek 'Dump';
$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).
use Devel::Peek 'Dump';
$a = {hello=>42};
Dump $a;
The output:
SV = RV(0xf041c)
REFCNT = 1
FLAGS = (ROK)
RV = 0xb2850
SV = PVHV(0xbd448)
REFCNT = 1
FLAGS = ()
IV = 1
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.
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 'Dump';
$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;
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
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
START and ROOT are non-zero, and XSUB is zero);
main;
MY::top_targets;
DEPTH);
PROTOTYPE field is missing).
Peek, mstats, DeadCode by default. Additionally available SvREFCNT,
SvREFCNT_inc, SvREFCNT_dec.