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 fill_mstats mstats_fillhash mstats2hash runops_debug debug_flags);
15 @EXPORT_OK = qw(SvREFCNT SvREFCNT_inc SvREFCNT_dec CvGV);
16 %EXPORT_TAGS = ('ALL' => [@EXPORT, @EXPORT_OK]);
18 XSLoader::load 'Devel::Peek';
20 sub DumpWithOP ($;$) {
21 local($Devel::Peek::dump_ops)=1;
22 my $depth = @_ > 1 ? $_[1] : 4 ;
26 $D_flags = 'psltocPmfrxuLHXDSTR';
28 sub debug_flags (;$) {
30 for my $i (0 .. length($D_flags)-1) {
31 $out .= substr $D_flags, $i, 1 if $^D & (1<<$i);
35 if (defined $arg and $arg =~ /\D/) {
36 die "unknown flags in debug_flags()" if $arg =~ /[^-$D_flags]/;
37 my ($on,$off) = split /-/, "$arg-";
39 $num |= (1<<index($D_flags, $_)) for split //, $on;
40 $num &= ~(1<<index($D_flags, $_)) for split //, $off;
42 $^D = $num if defined $arg;
51 Devel::Peek - A data debugging tool for the XS programmer
58 DumpArray( 5, $a, $b, ... );
63 Devel::Peek contains functions which allows raw Perl datatypes to be
64 manipulated from a Perl script. This is used by those who do XS programming
65 to check that the data they are sending from C to Perl looks as they think
66 it should look. The trick, then, is to know what the raw datatype is
67 supposed to look like when it gets to Perl. This document offers some tips
68 and hints to describe good and bad raw data.
70 It is very possible that this document will fall far short of being useful
71 to the casual reader. The reader is expected to understand the material in
72 the first few sections of L<perlguts>.
74 Devel::Peek supplies a C<Dump()> function which can dump a raw Perl
75 datatype, and C<mstat("marker")> function to report on memory usage
76 (if perl is compiled with corresponding option). The function
77 DeadCode() provides statistics on the data "frozen" into inactive
78 C<CV>. Devel::Peek also supplies C<SvREFCNT()>, C<SvREFCNT_inc()>, and
79 C<SvREFCNT_dec()> which can query, increment, and decrement reference
80 counts on SVs. This document will take a passive, and safe, approach
81 to data debugging and for that it will describe only the C<Dump()>
84 Function C<DumpArray()> allows dumping of multiple values (useful when you
85 need to analyze returns of functions).
87 The global variable $Devel::Peek::pv_limit can be set to limit the
88 number of character printed in various string values. Setting it to 0
91 =head2 Runtime debugging
93 C<CvGV($cv)> return one of the globs associated to a subroutine reference $cv.
95 debug_flags() returns a string representation of C<$^D> (similar to
96 what is allowed for B<-D> flag). When called with a numeric argument,
97 sets $^D to the corresponding value. When called with an argument of
98 the form C<"flags-flags">, set on/off bits of C<$^D> corresponding to
99 letters before/after C<->. (The returned value is for C<$^D> before
102 runops_debug() returns true if the current I<opcode dispatcher> is the
103 debugging one. When called with an argument, switches to debugging or
104 non-debugging dispatcher depending on the argument (active for
105 newly-entered subs/etc only). (The returned value is for the dispatcher before the modification.)
107 =head2 Memory footprint debugging
109 When perl is compiled with support for memory footprint debugging
110 (default with Perl's malloc()), Devel::Peek provides an access to this API.
112 Use mstat() function to emit a memory state statistic to the terminal.
113 For more information on the format of output of mstat() see
114 L<perldebug/Using C<$ENV{PERL_DEBUG_MSTATS}>>.
116 Three additional functions allow access to this statistic from Perl.
117 First, use C<mstats_fillhash(%hash)> to get the information contained
118 in the output of mstat() into %hash. The field of this hash are
120 minbucket nbuckets sbrk_good sbrk_slack sbrked_remains sbrks start_slack
121 topbucket topbucket_ev topbucket_odd total total_chain total_sbrk totfree
123 Two additional fields C<free>, C<used> contain array references which
124 provide per-bucket count of free and used chunks. Two other fields
125 C<mem_size>, C<available_size> contain array references which provide
126 the information about the allocated size and usable size of chunks in
127 each bucket. Again, see L<perldebug/Using C<$ENV{PERL_DEBUG_MSTATS}>>
130 Keep in mind that only the first several "odd-numbered" buckets are
131 used, so the information on size of the "odd-numbered" buckets which are
132 not used is probably meaningless.
136 mem_size available_size minbucket nbuckets
138 is the property of a particular build of perl, and does not depend on
139 the current process. If you do not provide the optional argument to
140 the functions mstats_fillhash(), fill_mstats(), mstats2hash(), then
141 the information in fields C<mem_size>, C<available_size> is not
144 C<fill_mstats($buf)> is a much cheaper call (both speedwise and
145 memory-wise) which collects the statistic into $buf in
146 machine-readable form. At a later moment you may need to call
147 C<mstats2hash($buf, %hash)> to use this information to fill %hash.
149 All three APIs C<fill_mstats($buf)>, C<mstats_fillhash(%hash)>, and
150 C<mstats2hash($buf, %hash)> are designed to allocate no memory if used
151 I<the second time> on the same $buf and/or %hash.
153 So, if you want to collect memory info in a cycle, you may call
156 fill_mstats($_) for @buf;
157 mstats_fillhash(%report, 1); # Static info too
161 fill_mstats $_; # Collect statistic
164 mstats2hash($_, %report); # Preserve static info
165 # Do something with %report
170 The following examples don't attempt to show everything as that would be a
171 monumental task, and, frankly, we don't want this manpage to be an internals
172 document for Perl. The examples do demonstrate some basics of the raw Perl
173 datatypes, and should suffice to get most determined people on their way.
174 There are no guidewires or safety nets, nor blazed trails, so be prepared to
175 travel alone from this point and on and, if at all possible, don't fall into
176 the quicksand (it's bad for business).
178 Oh, one final bit of advice: take L<perlguts> with you. When you return we
179 expect to see it well-thumbed.
181 =head2 A simple scalar string
183 Let's begin by looking a simple scalar which is holding a string.
195 PV = 0xb2048 "hello"\0
199 This says C<$a> is an SV, a scalar. The scalar is a PVIV, a string.
200 Its reference count is 1. It has the C<POK> flag set, meaning its
201 current PV field is valid. Because POK is set we look at the PV item
202 to see what is in the scalar. The \0 at the end indicate that this
203 PV is properly NUL-terminated.
204 If the FLAGS had been IOK we would look
205 at the IV item. CUR indicates the number of characters in the PV.
206 LEN indicates the number of bytes requested for the PV (one more than
207 CUR, in this case, because LEN includes an extra byte for the
208 end-of-string marker).
210 =head2 A simple scalar number
212 If the scalar contains a number the raw SV will be leaner.
225 This says C<$a> is an SV, a scalar. The scalar is an IV, a number. Its
226 reference count is 1. It has the C<IOK> flag set, meaning it is currently
227 being evaluated as a number. Because IOK is set we look at the IV item to
228 see what is in the scalar.
230 =head2 A simple scalar with an extra reference
232 If the scalar from the previous example had an extra reference:
246 Notice that this example differs from the previous example only in its
247 reference count. Compare this to the next example, where we dump C<$b>
250 =head2 A reference to a simple scalar
252 This shows what a reference looks like when it references a simple scalar.
270 Starting from the top, this says C<$b> is an SV. The scalar is an RV, a
271 reference. It has the C<ROK> flag set, meaning it is a reference. Because
272 ROK is set we have an RV item rather than an IV or PV. Notice that Dump
273 follows the reference and shows us what C<$b> was referencing. We see the
274 same C<$a> that we found in the previous example.
276 Note that the value of C<RV> coincides with the numbers we see when we
277 stringify $b. The addresses inside RV() and IV() are addresses of
278 C<X***> structure which holds the current state of an C<SV>. This
279 address may change during lifetime of an SV.
281 =head2 A reference to an array
283 This shows what a reference to an array looks like.
312 This says C<$a> is an SV and that it is an RV. That RV points to
313 another SV which is a PVAV, an array. The array has one element,
314 element zero, which is another SV. The field C<FILL> above indicates
315 the last element in the array, similar to C<$#$a>.
317 If C<$a> pointed to an array of two elements then we would see the
320 use Devel::Peek 'Dump';
352 Note that C<Dump> will not report I<all> the elements in the array,
353 only several first (depending on how deep it already went into the
356 =head2 A reference to a hash
358 The following shows the raw form of a reference to a hash.
380 Elt "hello" => 0xbaaf0
386 This shows C<$a> is a reference pointing to an SV. That SV is a PVHV, a
387 hash. Fields RITER and EITER are used by C<L<each>>.
389 =head2 Dumping a large array or hash
391 The C<Dump()> function, by default, dumps up to 4 elements from a
392 toplevel array or hash. This number can be increased by supplying a
393 second argument to the function.
396 $a = [10,11,12,13,14];
399 Notice that C<Dump()> prints only elements 10 through 13 in the above code.
400 The following code will print all of the elements.
402 use Devel::Peek 'Dump';
403 $a = [10,11,12,13,14];
406 =head2 A reference to an SV which holds a C pointer
408 This is what you really need to know as an XS programmer, of course. When
409 an XSUB returns a pointer to a C structure that pointer is stored in an SV
410 and a reference to that SV is placed on the XSUB stack. So the output from
411 an XSUB which uses something like the T_PTROBJ map might look something like
420 FLAGS = (OBJECT,IOK,pIOK)
424 STASH = 0xc1d10 "CookBookB::Opaque"
426 This shows that we have an SV which is an RV. That RV points at another
427 SV. In this case that second SV is a PVMG, a blessed scalar. Because it is
428 blessed it has the C<OBJECT> flag set. Note that an SV which holds a C
429 pointer also has the C<IOK> flag set. The C<STASH> is set to the package
430 name which this SV was blessed into.
432 The output from an XSUB which uses something like the T_PTRREF map, which
433 doesn't bless the object, might look something like this:
446 =head2 A reference to a subroutine
459 COMP_STASH = 0x31068 "main"
464 GVGV::GV = 0x1d44e8 "MY" :: "top_targets"
475 the subroutine is not an XSUB (since C<START> and C<ROOT> are
476 non-zero, and C<XSUB> is zero);
480 that it was compiled in the package C<main>;
484 under the name C<MY::top_targets>;
488 inside a 5th eval in the program;
492 it is not currently executed (see C<DEPTH>);
496 it has no prototype (C<PROTOTYPE> field is missing).
502 C<Dump>, C<mstat>, C<DeadCode>, C<DumpArray>, C<DumpWithOP> and
503 C<DumpProg>, C<fill_mstats>, C<mstats_fillhash>, C<mstats2hash> by
504 default. Additionally available C<SvREFCNT>, C<SvREFCNT_inc> and
509 Readers have been known to skip important parts of L<perlguts>, causing much
514 Ilya Zakharevich ilya@math.ohio-state.edu
516 Copyright (c) 1995-98 Ilya Zakharevich. All rights reserved.
517 This program is free software; you can redistribute it and/or
518 modify it under the same terms as Perl itself.
520 Author of this software makes no claim whatsoever about suitability,
521 reliability, edability, editability or usability of this product, and
522 should not be kept liable for any damage resulting from the use of
523 it. If you can use it, you are in luck, if not, I should not be kept
524 responsible. Keep a handy copy of your backup tape at hand.
528 L<perlguts>, and L<perlguts>, again.