1 # Devel::Peek - A data debugging tool for the XS programmer
2 # The documentation is after the __END__
7 $XS_VERSION = $VERSION;
8 $VERSION = eval $VERSION;
14 @EXPORT = qw(Dump mstat DeadCode DumpArray DumpWithOP DumpProg
15 fill_mstats mstats_fillhash mstats2hash runops_debug debug_flags);
16 @EXPORT_OK = qw(SvREFCNT SvREFCNT_inc SvREFCNT_dec CvGV);
17 %EXPORT_TAGS = ('ALL' => [@EXPORT, @EXPORT_OK]);
23 my $ops_rx = qr/^:opd(=[stP]*)?\b/;
24 my @db = grep m/$ops_rx/, @_;
25 @_ = grep !m/$ops_rx/, @_;
27 die "Too many :opd options" if @db > 1;
29 my $flags = ($db[0] =~ m/$ops_rx/ and $1);
30 $flags = 'st' unless defined $flags;
32 $f |= 2 if $flags =~ /s/;
33 $f |= 8 if $flags =~ /t/;
34 $f |= 64 if $flags =~ /P/;
38 goto &Exporter::import;
41 sub DumpWithOP ($;$) {
42 local($Devel::Peek::dump_ops)=1;
43 my $depth = @_ > 1 ? $_[1] : 4 ;
47 $D_flags = 'psltocPmfrxuLHXDSTR';
49 sub debug_flags (;$) {
51 for my $i (0 .. length($D_flags)-1) {
52 $out .= substr $D_flags, $i, 1 if $^D & (1<<$i);
56 if (defined $arg and $arg =~ /\D/) {
57 die "unknown flags in debug_flags()" if $arg =~ /[^-$D_flags]/;
58 my ($on,$off) = split /-/, "$arg-";
60 $num |= (1<<index($D_flags, $_)) for split //, $on;
61 $num &= ~(1<<index($D_flags, $_)) for split //, $off;
63 $^D = $num if defined $arg;
72 Devel::Peek - A data debugging tool for the XS programmer
79 DumpArray( 5, $a, $b, ... );
82 use Devel::Peek ':opd=st';
86 Devel::Peek contains functions which allows raw Perl datatypes to be
87 manipulated from a Perl script. This is used by those who do XS programming
88 to check that the data they are sending from C to Perl looks as they think
89 it should look. The trick, then, is to know what the raw datatype is
90 supposed to look like when it gets to Perl. This document offers some tips
91 and hints to describe good and bad raw data.
93 It is very possible that this document will fall far short of being useful
94 to the casual reader. The reader is expected to understand the material in
95 the first few sections of L<perlguts>.
97 Devel::Peek supplies a C<Dump()> function which can dump a raw Perl
98 datatype, and C<mstat("marker")> function to report on memory usage
99 (if perl is compiled with corresponding option). The function
100 DeadCode() provides statistics on the data "frozen" into inactive
101 C<CV>. Devel::Peek also supplies C<SvREFCNT()>, C<SvREFCNT_inc()>, and
102 C<SvREFCNT_dec()> which can query, increment, and decrement reference
103 counts on SVs. This document will take a passive, and safe, approach
104 to data debugging and for that it will describe only the C<Dump()>
107 Function C<DumpArray()> allows dumping of multiple values (useful when you
108 need to analyze returns of functions).
110 The global variable $Devel::Peek::pv_limit can be set to limit the
111 number of character printed in various string values. Setting it to 0
114 If C<use Devel::Peek> directive has a C<:opd=FLAGS> argument,
115 this switches on debugging of opcode dispatch. C<FLAGS> should be a
116 combination of C<s>, C<t>, and C<P> (see B<-D> flags in L<perlrun>).
117 C<:opd> is a shortcut for C<:opd=st>.
119 =head2 Runtime debugging
121 C<CvGV($cv)> return one of the globs associated to a subroutine reference $cv.
123 debug_flags() returns a string representation of C<$^D> (similar to
124 what is allowed for B<-D> flag). When called with a numeric argument,
125 sets $^D to the corresponding value. When called with an argument of
126 the form C<"flags-flags">, set on/off bits of C<$^D> corresponding to
127 letters before/after C<->. (The returned value is for C<$^D> before
130 runops_debug() returns true if the current I<opcode dispatcher> is the
131 debugging one. When called with an argument, switches to debugging or
132 non-debugging dispatcher depending on the argument (active for
133 newly-entered subs/etc only). (The returned value is for the dispatcher before the modification.)
135 =head2 Memory footprint debugging
137 When perl is compiled with support for memory footprint debugging
138 (default with Perl's malloc()), Devel::Peek provides an access to this API.
140 Use mstat() function to emit a memory state statistic to the terminal.
141 For more information on the format of output of mstat() see
142 L<perldebguts/Using $ENV{PERL_DEBUG_MSTATS}>.
144 Three additional functions allow access to this statistic from Perl.
145 First, use C<mstats_fillhash(%hash)> to get the information contained
146 in the output of mstat() into %hash. The field of this hash are
148 minbucket nbuckets sbrk_good sbrk_slack sbrked_remains sbrks start_slack
149 topbucket topbucket_ev topbucket_odd total total_chain total_sbrk totfree
151 Two additional fields C<free>, C<used> contain array references which
152 provide per-bucket count of free and used chunks. Two other fields
153 C<mem_size>, C<available_size> contain array references which provide
154 the information about the allocated size and usable size of chunks in
155 each bucket. Again, see L<perldebguts/Using $ENV{PERL_DEBUG_MSTATS}>
159 Keep in mind that only the first several "odd-numbered" buckets are
160 used, so the information on size of the "odd-numbered" buckets which are
161 not used is probably meaningless.
165 mem_size available_size minbucket nbuckets
167 is the property of a particular build of perl, and does not depend on
168 the current process. If you do not provide the optional argument to
169 the functions mstats_fillhash(), fill_mstats(), mstats2hash(), then
170 the information in fields C<mem_size>, C<available_size> is not
173 C<fill_mstats($buf)> is a much cheaper call (both speedwise and
174 memory-wise) which collects the statistic into $buf in
175 machine-readable form. At a later moment you may need to call
176 C<mstats2hash($buf, %hash)> to use this information to fill %hash.
178 All three APIs C<fill_mstats($buf)>, C<mstats_fillhash(%hash)>, and
179 C<mstats2hash($buf, %hash)> are designed to allocate no memory if used
180 I<the second time> on the same $buf and/or %hash.
182 So, if you want to collect memory info in a cycle, you may call
185 fill_mstats($_) for @buf;
186 mstats_fillhash(%report, 1); # Static info too
190 fill_mstats $_; # Collect statistic
193 mstats2hash($_, %report); # Preserve static info
194 # Do something with %report
199 The following examples don't attempt to show everything as that would be a
200 monumental task, and, frankly, we don't want this manpage to be an internals
201 document for Perl. The examples do demonstrate some basics of the raw Perl
202 datatypes, and should suffice to get most determined people on their way.
203 There are no guidewires or safety nets, nor blazed trails, so be prepared to
204 travel alone from this point and on and, if at all possible, don't fall into
205 the quicksand (it's bad for business).
207 Oh, one final bit of advice: take L<perlguts> with you. When you return we
208 expect to see it well-thumbed.
210 =head2 A simple scalar string
212 Let's begin by looking a simple scalar which is holding a string.
215 $a = 42; $a = "hello";
220 SV = PVIV(0xbc288) at 0xbe9a8
224 PV = 0xb2048 "hello"\0
228 This says C<$a> is an SV, a scalar. The scalar type is a PVIV, which is
229 capable of holding an integer (IV) and/or a string (PV) value. The scalar's
230 head is allocated at address 0xbe9a8, while the body is at 0xbc288.
231 Its reference count is 1. It has the C<POK> flag set, meaning its
232 current PV field is valid. Because POK is set we look at the PV item
233 to see what is in the scalar. The \0 at the end indicate that this
234 PV is properly NUL-terminated.
235 Note that the IV field still contains its old numeric value, but because
236 FLAGS doesn't have IOK set, we must ignore the IV item.
237 CUR indicates the number of characters in the PV. LEN indicates the
238 number of bytes allocated for the PV (at least one more than CUR, because
239 LEN includes an extra byte for the end-of-string marker, then usually
240 rounded up to some efficient allocation unit).
242 =head2 A simple scalar number
244 If the scalar contains a number the raw SV will be leaner.
252 SV = IV(0xbc818) at 0xbe9a8
257 This says C<$a> is an SV, a scalar. The scalar is an IV, a number. Its
258 reference count is 1. It has the C<IOK> flag set, meaning it is currently
259 being evaluated as a number. Because IOK is set we look at the IV item to
260 see what is in the scalar.
262 =head2 A simple scalar with an extra reference
264 If the scalar from the previous example had an extra reference:
273 SV = IV(0xbe860) at 0xbe9a8
278 Notice that this example differs from the previous example only in its
279 reference count. Compare this to the next example, where we dump C<$b>
282 =head2 A reference to a simple scalar
284 This shows what a reference looks like when it references a simple scalar.
293 SV = IV(0xf041c) at 0xbe9a0
297 SV = IV(0xbe860) at 0xbe9a8
302 Starting from the top, this says C<$b> is an SV. The scalar is an IV,
303 which is capable of holding an integer or reference value.
304 It has the C<ROK> flag set, meaning it is a reference (rather than an
305 integer or string). Notice that Dump
306 follows the reference and shows us what C<$b> was referencing. We see the
307 same C<$a> that we found in the previous example.
309 Note that the value of C<RV> coincides with the numbers we see when we
310 stringify $b. The addresses inside IV() are addresses of
311 C<X***> structures which hold the current state of an C<SV>. This
312 address may change during lifetime of an SV.
314 =head2 A reference to an array
316 This shows what a reference to an array looks like.
324 SV = IV(0xc85998) at 0xc859a8
328 SV = PVAV(0xc71e10) at 0xc70de8
337 SV = IV(0xc70f88) at 0xc70f98
342 This says C<$a> is a reference (ROK), which points to
343 another SV which is a PVAV, an array. The array has one element,
344 element zero, which is another SV. The field C<FILL> above indicates
345 the last element in the array, similar to C<$#$a>.
347 If C<$a> pointed to an array of two elements then we would see the
350 use Devel::Peek 'Dump';
356 SV = IV(0x158c998) at 0x158c9a8
360 SV = PVAV(0x1578e10) at 0x1577de8
369 SV = IV(0x1577f88) at 0x1577f98
374 SV = IV(0x158be88) at 0x158be98
379 Note that C<Dump> will not report I<all> the elements in the array,
380 only several first (depending on how deep it already went into the
383 =head2 A reference to a hash
385 The following shows the raw form of a reference to a hash.
393 SV = IV(0x8177858) at 0x816a618
397 SV = PVHV(0x8167768) at 0x814fc10
400 ARRAY = 0x816c5b8 (0:7, 1:1)
401 hash quality = 100.0%
407 Elt "hello" HASH = 0xc8fd181b
408 SV = IV(0x816c030) at 0x814fcf4
413 This shows C<$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<perlfunc/each>>.
416 The "quality" of a hash is defined as the total number of comparisons needed
417 to access every element once, relative to the expected number needed for a
418 random hash. The value can go over 100%.
420 The total number of comparisons is equal to the sum of the squares of the
421 number of entries in each bucket. For a random hash of C<<n>> keys into
422 C<<k>> buckets, the expected value is:
426 =head2 Dumping a large array or hash
428 The C<Dump()> function, by default, dumps up to 4 elements from a
429 toplevel array or hash. This number can be increased by supplying a
430 second argument to the function.
433 $a = [10,11,12,13,14];
436 Notice that C<Dump()> prints only elements 10 through 13 in the above code.
437 The following code will print all of the elements.
439 use Devel::Peek 'Dump';
440 $a = [10,11,12,13,14];
443 =head2 A reference to an SV which holds a C pointer
445 This is what you really need to know as an XS programmer, of course. When
446 an XSUB returns a pointer to a C structure that pointer is stored in an SV
447 and a reference to that SV is placed on the XSUB stack. So the output from
448 an XSUB which uses something like the T_PTROBJ map might look something like
451 SV = IV(0xf381c) at 0xc859a8
455 SV = PVMG(0xbb3c8) at 0xc859a0
457 FLAGS = (OBJECT,IOK,pIOK)
461 STASH = 0xc1d10 "CookBookB::Opaque"
463 This shows that we have an SV which is a reference, which points at another
464 SV. In this case that second SV is a PVMG, a blessed scalar. Because it is
465 blessed it has the C<OBJECT> flag set. Note that an SV which holds a C
466 pointer also has the C<IOK> flag set. The C<STASH> is set to the package
467 name which this SV was blessed into.
469 The output from an XSUB which uses something like the T_PTRREF map, which
470 doesn't bless the object, might look something like this:
472 SV = IV(0xf381c) at 0xc859a8
476 SV = PVMG(0xbb3c8) at 0xc859a0
483 =head2 A reference to a subroutine
487 SV = IV(0x24d2dd8) at 0x24d2de8
491 SV = PVCV(0x24e5798) at 0x24e79d8
494 COMP_STASH = 0x22c9c50 "main"
495 START = 0x22eed60 ===> 0
497 GVGV::GV = 0x22de9d8 "MY" :: "top_targets"
503 PADNAME = 0x22e9ec0(0x22eed00) PAD = 0x22e9ea8(0x22eecd0)
504 OUTSIDE = 0x22c9fb0 (MAIN)
513 the subroutine is not an XSUB (since C<START> and C<ROOT> are
514 non-zero, and C<XSUB> is not listed, and is thus null);
518 that it was compiled in the package C<main>;
522 under the name C<MY::top_targets>;
526 inside a 5th eval in the program;
530 it is not currently executed (see C<DEPTH>);
534 it has no prototype (C<PROTOTYPE> field is missing).
540 C<Dump>, C<mstat>, C<DeadCode>, C<DumpArray>, C<DumpWithOP> and
541 C<DumpProg>, C<fill_mstats>, C<mstats_fillhash>, C<mstats2hash> by
542 default. Additionally available C<SvREFCNT>, C<SvREFCNT_inc> and
547 Readers have been known to skip important parts of L<perlguts>, causing much
552 Ilya Zakharevich ilya@math.ohio-state.edu
554 Copyright (c) 1995-98 Ilya Zakharevich. All rights reserved.
555 This program is free software; you can redistribute it and/or
556 modify it under the same terms as Perl itself.
558 Author of this software makes no claim whatsoever about suitability,
559 reliability, edability, editability or usability of this product, and
560 should not be kept liable for any damage resulting from the use of
561 it. If you can use it, you are in luck, if not, I should not be kept
562 responsible. Keep a handy copy of your backup tape at hand.
566 L<perlguts>, and L<perlguts>, again.