NAME

Devel::Peek - A data debugging tool for the XS programmer

SYNOPSIS

use Devel::Peek 'Dump';
Dump( $a );
Dump( $a, 5 );

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. It 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.

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 '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 it is currently being evaluated as a string. 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 in 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 '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.

A simple scalar with an extra reference

If the scalar from the previous example had an extra reference:

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.

A reference to a simple scalar

This shows what a reference looks like when it references a simple scalar.

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.

A reference to an array

This shows what a reference to an array looks like.

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
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.

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
SV = IV(0xbe860)
  REFCNT = 1
  FLAGS = (IOK,pIOK)
  IV = 42
Elt No. 1
SV = IV(0xbe818)
  REFCNT = 1
  FLAGS = (IOK,pIOK)
  IV = 24

A reference to a hash

The following shows the raw form of a reference to a hash.

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.

Dumping a large array or hash

The Dump() function, by default, dumps up to 4 elements from an 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;

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

BUGS

Readers have been known to skip important parts of perlguts, causing much frustration for all.

SEE ALSO

perlguts, and perlguts, again.

cpanm CookBookA::Ex6

perl -MCPAN -e shell
install CookBookA::Ex6