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';
22 my $ops_rx = qr/^:opd(=[stP]*)?\b/;
23 my @db = grep m/$ops_rx/, @_;
24 @_ = grep !m/$ops_rx/, @_;
26 die "Too many :opd options" if @db > 1;
28 my $flags = ($db[0] =~ m/$ops_rx/ and $1);
29 $flags = 'st' unless defined $flags;
31 $f |= 2 if $flags =~ /s/;
32 $f |= 8 if $flags =~ /t/;
33 $f |= 64 if $flags =~ /P/;
37 goto &Exporter::import;
40 sub DumpWithOP ($;$) {
41 local($Devel::Peek::dump_ops)=1;
42 my $depth = @_ > 1 ? $_[1] : 4 ;
46 $D_flags = 'psltocPmfrxuLHXDSTR';
48 sub debug_flags (;$) {
50 for my $i (0 .. length($D_flags)-1) {
51 $out .= substr $D_flags, $i, 1 if $^D & (1<<$i);
55 if (defined $arg and $arg =~ /\D/) {
56 die "unknown flags in debug_flags()" if $arg =~ /[^-$D_flags]/;
57 my ($on,$off) = split /-/, "$arg-";
59 $num |= (1<<index($D_flags, $_)) for split //, $on;
60 $num &= ~(1<<index($D_flags, $_)) for split //, $off;
62 $^D = $num if defined $arg;
71 Devel::Peek - A data debugging tool for the XS programmer
78 DumpArray( 5, $a, $b, ... );
81 use Devel::Peek ':opd=st';
85 Devel::Peek contains functions which allows raw Perl datatypes to be
86 manipulated from a Perl script. This is used by those who do XS programming
87 to check that the data they are sending from C to Perl looks as they think
88 it should look. The trick, then, is to know what the raw datatype is
89 supposed to look like when it gets to Perl. This document offers some tips
90 and hints to describe good and bad raw data.
92 It is very possible that this document will fall far short of being useful
93 to the casual reader. The reader is expected to understand the material in
94 the first few sections of L<perlguts>.
96 Devel::Peek supplies a C<Dump()> function which can dump a raw Perl
97 datatype, and C<mstat("marker")> function to report on memory usage
98 (if perl is compiled with corresponding option). The function
99 DeadCode() provides statistics on the data "frozen" into inactive
100 C<CV>. Devel::Peek also supplies C<SvREFCNT()>, C<SvREFCNT_inc()>, and
101 C<SvREFCNT_dec()> which can query, increment, and decrement reference
102 counts on SVs. This document will take a passive, and safe, approach
103 to data debugging and for that it will describe only the C<Dump()>
106 Function C<DumpArray()> allows dumping of multiple values (useful when you
107 need to analyze returns of functions).
109 The global variable $Devel::Peek::pv_limit can be set to limit the
110 number of character printed in various string values. Setting it to 0
113 If C<use Devel::Peek> directive has a C<:opd=FLAGS> argument,
114 this switches on debugging of opcode dispatch. C<FLAGS> should be a
115 combination of C<s>, C<t>, and C<P> (see B<-D> flags in L<perlrun>).
116 C<:opd> is a shortcut for C<:opd=st>.
118 =head2 Runtime debugging
120 C<CvGV($cv)> return one of the globs associated to a subroutine reference $cv.
122 debug_flags() returns a string representation of C<$^D> (similar to
123 what is allowed for B<-D> flag). When called with a numeric argument,
124 sets $^D to the corresponding value. When called with an argument of
125 the form C<"flags-flags">, set on/off bits of C<$^D> corresponding to
126 letters before/after C<->. (The returned value is for C<$^D> before
129 runops_debug() returns true if the current I<opcode dispatcher> is the
130 debugging one. When called with an argument, switches to debugging or
131 non-debugging dispatcher depending on the argument (active for
132 newly-entered subs/etc only). (The returned value is for the dispatcher before the modification.)
134 =head2 Memory footprint debugging
136 When perl is compiled with support for memory footprint debugging
137 (default with Perl's malloc()), Devel::Peek provides an access to this API.
139 Use mstat() function to emit a memory state statistic to the terminal.
140 For more information on the format of output of mstat() see
141 L<perldebguts/Using C<$ENV{PERL_DEBUG_MSTATS}>>.
143 Three additional functions allow access to this statistic from Perl.
144 First, use C<mstats_fillhash(%hash)> to get the information contained
145 in the output of mstat() into %hash. The field of this hash are
147 minbucket nbuckets sbrk_good sbrk_slack sbrked_remains sbrks start_slack
148 topbucket topbucket_ev topbucket_odd total total_chain total_sbrk totfree
150 Two additional fields C<free>, C<used> contain array references which
151 provide per-bucket count of free and used chunks. Two other fields
152 C<mem_size>, C<available_size> contain array references which provide
153 the information about the allocated size and usable size of chunks in
154 each bucket. Again, see L<perldebguts/Using C<$ENV{PERL_DEBUG_MSTATS}>>
157 Keep in mind that only the first several "odd-numbered" buckets are
158 used, so the information on size of the "odd-numbered" buckets which are
159 not used is probably meaningless.
163 mem_size available_size minbucket nbuckets
165 is the property of a particular build of perl, and does not depend on
166 the current process. If you do not provide the optional argument to
167 the functions mstats_fillhash(), fill_mstats(), mstats2hash(), then
168 the information in fields C<mem_size>, C<available_size> is not
171 C<fill_mstats($buf)> is a much cheaper call (both speedwise and
172 memory-wise) which collects the statistic into $buf in
173 machine-readable form. At a later moment you may need to call
174 C<mstats2hash($buf, %hash)> to use this information to fill %hash.
176 All three APIs C<fill_mstats($buf)>, C<mstats_fillhash(%hash)>, and
177 C<mstats2hash($buf, %hash)> are designed to allocate no memory if used
178 I<the second time> on the same $buf and/or %hash.
180 So, if you want to collect memory info in a cycle, you may call
183 fill_mstats($_) for @buf;
184 mstats_fillhash(%report, 1); # Static info too
188 fill_mstats $_; # Collect statistic
191 mstats2hash($_, %report); # Preserve static info
192 # Do something with %report
197 The following examples don't attempt to show everything as that would be a
198 monumental task, and, frankly, we don't want this manpage to be an internals
199 document for Perl. The examples do demonstrate some basics of the raw Perl
200 datatypes, and should suffice to get most determined people on their way.
201 There are no guidewires or safety nets, nor blazed trails, so be prepared to
202 travel alone from this point and on and, if at all possible, don't fall into
203 the quicksand (it's bad for business).
205 Oh, one final bit of advice: take L<perlguts> with you. When you return we
206 expect to see it well-thumbed.
208 =head2 A simple scalar string
210 Let's begin by looking a simple scalar which is holding a string.
222 PV = 0xb2048 "hello"\0
226 This says C<$a> is an SV, a scalar. The scalar is a PVIV, a string.
227 Its reference count is 1. It has the C<POK> flag set, meaning its
228 current PV field is valid. Because POK is set we look at the PV item
229 to see what is in the scalar. The \0 at the end indicate that this
230 PV is properly NUL-terminated.
231 If the FLAGS had been IOK we would look
232 at the IV item. CUR indicates the number of characters in the PV.
233 LEN indicates the number of bytes requested for the PV (one more than
234 CUR, in this case, because LEN includes an extra byte for the
235 end-of-string marker).
237 =head2 A simple scalar number
239 If the scalar contains a number the raw SV will be leaner.
252 This says C<$a> is an SV, a scalar. The scalar is an IV, a number. Its
253 reference count is 1. It has the C<IOK> flag set, meaning it is currently
254 being evaluated as a number. Because IOK is set we look at the IV item to
255 see what is in the scalar.
257 =head2 A simple scalar with an extra reference
259 If the scalar from the previous example had an extra reference:
273 Notice that this example differs from the previous example only in its
274 reference count. Compare this to the next example, where we dump C<$b>
277 =head2 A reference to a simple scalar
279 This shows what a reference looks like when it references a simple scalar.
297 Starting from the top, this says C<$b> is an SV. The scalar is an RV, a
298 reference. It has the C<ROK> flag set, meaning it is a reference. Because
299 ROK is set we have an RV item rather than an IV or PV. Notice that Dump
300 follows the reference and shows us what C<$b> was referencing. We see the
301 same C<$a> that we found in the previous example.
303 Note that the value of C<RV> coincides with the numbers we see when we
304 stringify $b. The addresses inside RV() and IV() are addresses of
305 C<X***> structure which holds the current state of an C<SV>. This
306 address may change during lifetime of an SV.
308 =head2 A reference to an array
310 This shows what a reference to an array looks like.
339 This says C<$a> is an SV and that it is an RV. That RV points to
340 another SV which is a PVAV, an array. The array has one element,
341 element zero, which is another SV. The field C<FILL> above indicates
342 the last element in the array, similar to C<$#$a>.
344 If C<$a> pointed to an array of two elements then we would see the
347 use Devel::Peek 'Dump';
379 Note that C<Dump> will not report I<all> the elements in the array,
380 only several first (depending on how deep it already went into the
383 =head2 A reference to a hash
385 The following shows the raw form of a reference to a hash.
407 Elt "hello" => 0xbaaf0
413 This shows C<$a> is a reference pointing to an SV. That SV is a PVHV, a
414 hash. Fields RITER and EITER are used by C<L<each>>.
416 =head2 Dumping a large array or hash
418 The C<Dump()> function, by default, dumps up to 4 elements from a
419 toplevel array or hash. This number can be increased by supplying a
420 second argument to the function.
423 $a = [10,11,12,13,14];
426 Notice that C<Dump()> prints only elements 10 through 13 in the above code.
427 The following code will print all of the elements.
429 use Devel::Peek 'Dump';
430 $a = [10,11,12,13,14];
433 =head2 A reference to an SV which holds a C pointer
435 This is what you really need to know as an XS programmer, of course. When
436 an XSUB returns a pointer to a C structure that pointer is stored in an SV
437 and a reference to that SV is placed on the XSUB stack. So the output from
438 an XSUB which uses something like the T_PTROBJ map might look something like
447 FLAGS = (OBJECT,IOK,pIOK)
451 STASH = 0xc1d10 "CookBookB::Opaque"
453 This shows that we have an SV which is an RV. That RV points at another
454 SV. In this case that second SV is a PVMG, a blessed scalar. Because it is
455 blessed it has the C<OBJECT> flag set. Note that an SV which holds a C
456 pointer also has the C<IOK> flag set. The C<STASH> is set to the package
457 name which this SV was blessed into.
459 The output from an XSUB which uses something like the T_PTRREF map, which
460 doesn't bless the object, might look something like this:
473 =head2 A reference to a subroutine
486 COMP_STASH = 0x31068 "main"
491 GVGV::GV = 0x1d44e8 "MY" :: "top_targets"
502 the subroutine is not an XSUB (since C<START> and C<ROOT> are
503 non-zero, and C<XSUB> is zero);
507 that it was compiled in the package C<main>;
511 under the name C<MY::top_targets>;
515 inside a 5th eval in the program;
519 it is not currently executed (see C<DEPTH>);
523 it has no prototype (C<PROTOTYPE> field is missing).
529 C<Dump>, C<mstat>, C<DeadCode>, C<DumpArray>, C<DumpWithOP> and
530 C<DumpProg>, C<fill_mstats>, C<mstats_fillhash>, C<mstats2hash> by
531 default. Additionally available C<SvREFCNT>, C<SvREFCNT_inc> and
536 Readers have been known to skip important parts of L<perlguts>, causing much
541 Ilya Zakharevich ilya@math.ohio-state.edu
543 Copyright (c) 1995-98 Ilya Zakharevich. All rights reserved.
544 This program is free software; you can redistribute it and/or
545 modify it under the same terms as Perl itself.
547 Author of this software makes no claim whatsoever about suitability,
548 reliability, edability, editability or usability of this product, and
549 should not be kept liable for any damage resulting from the use of
550 it. If you can use it, you are in luck, if not, I should not be kept
551 responsible. Keep a handy copy of your backup tape at hand.
555 L<perlguts>, and L<perlguts>, again.