1 # Devel::Peek - A data debugging tool for the XS programmer
2 # The documentation is after the __END__
6 # Underscore to allow older Perls to access older version from CPAN
13 @EXPORT = qw(Dump mstat DeadCode DumpArray DumpWithOP DumpProg);
14 @EXPORT_OK = qw(SvREFCNT SvREFCNT_inc SvREFCNT_dec CvGV);
15 %EXPORT_TAGS = ('ALL' => [@EXPORT, @EXPORT_OK]);
17 XSLoader::load 'Devel::Peek';
19 sub DumpWithOP ($;$) {
20 local($Devel::Peek::dump_ops)=1;
21 my $depth = @_ > 1 ? $_[1] : 4 ;
30 Devel::Peek - A data debugging tool for the XS programmer
37 DumpArray( 5, $a, $b, ... );
42 Devel::Peek contains functions which allows raw Perl datatypes to be
43 manipulated from a Perl script. This is used by those who do XS programming
44 to check that the data they are sending from C to Perl looks as they think
45 it should look. The trick, then, is to know what the raw datatype is
46 supposed to look like when it gets to Perl. This document offers some tips
47 and hints to describe good and bad raw data.
49 It is very possible that this document will fall far short of being useful
50 to the casual reader. The reader is expected to understand the material in
51 the first few sections of L<perlguts>.
53 Devel::Peek supplies a C<Dump()> function which can dump a raw Perl
54 datatype, and C<mstat("marker")> function to report on memory usage
55 (if perl is compiled with corresponding option). The function
56 DeadCode() provides statistics on the data "frozen" into inactive
57 C<CV>. Devel::Peek also supplies C<SvREFCNT()>, C<SvREFCNT_inc()>, and
58 C<SvREFCNT_dec()> which can query, increment, and decrement reference
59 counts on SVs. This document will take a passive, and safe, approach
60 to data debugging and for that it will describe only the C<Dump()>
61 function. For format of output of mstats() see
62 L<perldebug/Using C<$ENV{PERL_DEBUG_MSTATS}>>.
64 Function C<DumpArray()> allows dumping of multiple values (useful when you
65 need to analize returns of functions).
67 The global variable $Devel::Peek::pv_limit can be set to limit the
68 number of character printed in various string values. Setting it to 0
73 The following examples don't attempt to show everything as that would be a
74 monumental task, and, frankly, we don't want this manpage to be an internals
75 document for Perl. The examples do demonstrate some basics of the raw Perl
76 datatypes, and should suffice to get most determined people on their way.
77 There are no guidewires or safety nets, nor blazed trails, so be prepared to
78 travel alone from this point and on and, if at all possible, don't fall into
79 the quicksand (it's bad for business).
81 Oh, one final bit of advice: take L<perlguts> with you. When you return we
82 expect to see it well-thumbed.
84 =head2 A simple scalar string
86 Let's begin by looking a simple scalar which is holding a string.
98 PV = 0xb2048 "hello"\0
102 This says C<$a> is an SV, a scalar. The scalar is a PVIV, a string.
103 Its reference count is 1. It has the C<POK> flag set, meaning its
104 current PV field is valid. Because POK is set we look at the PV item
105 to see what is in the scalar. The \0 at the end indicate that this
106 PV is properly NUL-terminated.
107 If the FLAGS had been IOK we would look
108 at the IV item. CUR indicates the number of characters in the PV.
109 LEN indicates the number of bytes requested for the PV (one more than
110 CUR, in this case, because LEN includes an extra byte for the
111 end-of-string marker).
113 =head2 A simple scalar number
115 If the scalar contains a number the raw SV will be leaner.
128 This says C<$a> is an SV, a scalar. The scalar is an IV, a number. Its
129 reference count is 1. It has the C<IOK> flag set, meaning it is currently
130 being evaluated as a number. Because IOK is set we look at the IV item to
131 see what is in the scalar.
133 =head2 A simple scalar with an extra reference
135 If the scalar from the previous example had an extra reference:
149 Notice that this example differs from the previous example only in its
150 reference count. Compare this to the next example, where we dump C<$b>
153 =head2 A reference to a simple scalar
155 This shows what a reference looks like when it references a simple scalar.
173 Starting from the top, this says C<$b> is an SV. The scalar is an RV, a
174 reference. It has the C<ROK> flag set, meaning it is a reference. Because
175 ROK is set we have an RV item rather than an IV or PV. Notice that Dump
176 follows the reference and shows us what C<$b> was referencing. We see the
177 same C<$a> that we found in the previous example.
179 Note that the value of C<RV> coincides with the numbers we see when we
180 stringify $b. The addresses inside RV() and IV() are addresses of
181 C<X***> structure which holds the current state of an C<SV>. This
182 address may change during lifetime of an SV.
184 =head2 A reference to an array
186 This shows what a reference to an array looks like.
215 This says C<$a> is an SV and that it is an RV. That RV points to
216 another SV which is a PVAV, an array. The array has one element,
217 element zero, which is another SV. The field C<FILL> above indicates
218 the last element in the array, similar to C<$#$a>.
220 If C<$a> pointed to an array of two elements then we would see the
223 use Devel::Peek 'Dump';
255 Note that C<Dump> will not report I<all> the elements in the array,
256 only several first (depending on how deep it already went into the
259 =head2 A reference to a hash
261 The following shows the raw form of a reference to a hash.
283 Elt "hello" => 0xbaaf0
289 This shows C<$a> is a reference pointing to an SV. That SV is a PVHV, a
290 hash. Fields RITER and EITER are used by C<L<each>>.
292 =head2 Dumping a large array or hash
294 The C<Dump()> function, by default, dumps up to 4 elements from a
295 toplevel array or hash. This number can be increased by supplying a
296 second argument to the function.
299 $a = [10,11,12,13,14];
302 Notice that C<Dump()> prints only elements 10 through 13 in the above code.
303 The following code will print all of the elements.
305 use Devel::Peek 'Dump';
306 $a = [10,11,12,13,14];
309 =head2 A reference to an SV which holds a C pointer
311 This is what you really need to know as an XS programmer, of course. When
312 an XSUB returns a pointer to a C structure that pointer is stored in an SV
313 and a reference to that SV is placed on the XSUB stack. So the output from
314 an XSUB which uses something like the T_PTROBJ map might look something like
323 FLAGS = (OBJECT,IOK,pIOK)
327 STASH = 0xc1d10 "CookBookB::Opaque"
329 This shows that we have an SV which is an RV. That RV points at another
330 SV. In this case that second SV is a PVMG, a blessed scalar. Because it is
331 blessed it has the C<OBJECT> flag set. Note that an SV which holds a C
332 pointer also has the C<IOK> flag set. The C<STASH> is set to the package
333 name which this SV was blessed into.
335 The output from an XSUB which uses something like the T_PTRREF map, which
336 doesn't bless the object, might look something like this:
349 =head2 A reference to a subroutine
362 COMP_STASH = 0x31068 "main"
367 GVGV::GV = 0x1d44e8 "MY" :: "top_targets"
378 the subroutine is not an XSUB (since C<START> and C<ROOT> are
379 non-zero, and C<XSUB> is zero);
383 that it was compiled in the package C<main>;
387 under the name C<MY::top_targets>;
391 inside a 5th eval in the program;
395 it is not currently executed (see C<DEPTH>);
399 it has no prototype (C<PROTOTYPE> field is missing).
405 C<Dump>, C<mstat>, C<DeadCode>, C<DumpArray>, C<DumpWithOP> and
406 C<DumpProg> by default. Additionally available C<SvREFCNT>,
407 C<SvREFCNT_inc> and C<SvREFCNT_dec>.
411 Readers have been known to skip important parts of L<perlguts>, causing much
416 Ilya Zakharevich ilya@math.ohio-state.edu
418 Copyright (c) 1995-98 Ilya Zakharevich. All rights reserved.
419 This program is free software; you can redistribute it and/or
420 modify it under the same terms as Perl itself.
422 Author of this software makes no claim whatsoever about suitability,
423 reliability, edability, editability or usability of this product, and
424 should not be kept liable for any damage resulting from the use of
425 it. If you can use it, you are in luck, if not, I should not be kept
426 responsible. Keep a handy copy of your backup tape at hand.
430 L<perlguts>, and L<perlguts>, again.