1Devel::Peek(3pm) Perl Programmers Reference Guide Devel::Peek(3pm)
2
6 Devel::Peek - A data debugging tool for the XS programmer
7
9 use Devel::Peek;
10 Dump( $a );
11 Dump( $a, 5 );
12 Dump( @a );
13 Dump( %h );
14 DumpArray( 5, $a, $b, ... );
15 mstat "Point 5";
16 use Devel::Peek ':opd=st';
18
20 Devel::Peek contains functions which allows raw Perl datatypes to be
21 manipulated from a Perl script. This is used by those who do XS
22 programming to check that the data they are sending from C to Perl
23 looks as they think it should look. The trick, then, is to know what
24 the raw datatype is supposed to look like when it gets to Perl. This
25 document offers some tips and hints to describe good and bad raw data.
26 It is very possible that this document will fall far short of being
28 useful to the casual reader. The reader is expected to understand the
29 material in the first few sections of perlguts.
30 Devel::Peek supplies a Dump() function which can dump a raw Perl
32 datatype, and mstat("marker") function to report on memory usage (if
33 perl is compiled with corresponding option). The function DeadCode()
34 provides statistics on the data "frozen" into inactive "CV".
35 Devel::Peek also supplies SvREFCNT() which can query reference counts
36 on SVs. This document will take a passive, and safe, approach to data
37 debugging and for that it will describe only the Dump() function.
38 All output is to STDERR.
40 The Dump() function takes one or two arguments: something to dump, and
42 an optional limit for recursion and array elements (default is 4). The
43 first argument is evaluated in rvalue scalar context, with exceptions
44 for @array and %hash, which dump the array or hash itself. So "Dump
45 @array" works, as does "Dump $foo". And "Dump pos" will call "pos" in
46 rvalue context, whereas "Dump ${\pos}" will call it in lvalue context.
47 Function DumpArray() allows dumping of multiple values (useful when you
49 need to analyze returns of functions).
50 The global variable $Devel::Peek::pv_limit can be set to limit the
52 number of character printed in various string values. Setting it to 0
53 means no limit.
54 If "use Devel::Peek" directive has a ":opd=FLAGS" argument, this
56 switches on debugging of opcode dispatch. "FLAGS" should be a
57 combination of "s", "t", and "P" (see -D flags in perlrun).
58 ":opd" is a shortcut for ":opd=st".
60 Runtime debugging
62 CvGV($cv) return one of the globs associated to a subroutine reference
63 $cv.
64 debug_flags() returns a string representation of $^D (similar to what
66 is allowed for -D flag). When called with a numeric argument, sets $^D
67 to the corresponding value. When called with an argument of the form
68 "flags-flags", set on/off bits of $^D corresponding to letters
69 before/after "-". (The returned value is for $^D before the
70 modification.)
71 runops_debug() returns true if the current opcode dispatcher is the
73 debugging one. When called with an argument, switches to debugging or
74 non-debugging dispatcher depending on the argument (active for newly-
75 entered subs/etc only). (The returned value is for the dispatcher
76 before the modification.)
77 Memory footprint debugging
79 When perl is compiled with support for memory footprint debugging
80 (default with Perl's malloc()), Devel::Peek provides an access to this
81 API.
82 Use mstat() function to emit a memory state statistic to the terminal.
84 For more information on the format of output of mstat() see "Using
85 $ENV{PERL_DEBUG_MSTATS}" in perldebguts.
86 Three additional functions allow access to this statistic from Perl.
88 First, use mstats_fillhash(%hash) to get the information contained in
89 the output of mstat() into %hash. The field of this hash are
90 minbucket nbuckets sbrk_good sbrk_slack sbrked_remains sbrks
92 start_slack topbucket topbucket_ev topbucket_odd total total_chain
93 total_sbrk totfree
94 Two additional fields "free", "used" contain array references which
96 provide per-bucket count of free and used chunks. Two other fields
97 "mem_size", "available_size" contain array references which provide the
98 information about the allocated size and usable size of chunks in each
99 bucket. Again, see "Using $ENV{PERL_DEBUG_MSTATS}" in perldebguts for
100 details.
101 Keep in mind that only the first several "odd-numbered" buckets are
103 used, so the information on size of the "odd-numbered" buckets which
104 are not used is probably meaningless.
105 The information in
107 mem_size available_size minbucket nbuckets
109 is the property of a particular build of perl, and does not depend on
111 the current process. If you do not provide the optional argument to
112 the functions mstats_fillhash(), fill_mstats(), mstats2hash(), then the
113 information in fields "mem_size", "available_size" is not updated.
114 fill_mstats($buf) is a much cheaper call (both speedwise and memory-
116 wise) which collects the statistic into $buf in machine-readable form.
117 At a later moment you may need to call "mstats2hash($buf, %hash)" to
118 use this information to fill %hash.
119 All three APIs fill_mstats($buf), mstats_fillhash(%hash), and
121 "mstats2hash($buf, %hash)" are designed to allocate no memory if used
122 the second time on the same $buf and/or %hash.
123 So, if you want to collect memory info in a cycle, you may call
125 $#buf = 999;
127 fill_mstats($_) for @buf;
128 mstats_fillhash(%report, 1); # Static info too
129 foreach (@buf) {
131 # Do something...
132 fill_mstats $_; # Collect statistic
133 }
134 foreach (@buf) {
135 mstats2hash($_, %report); # Preserve static info
136 # Do something with %report
137 }
138
140 The following examples don't attempt to show everything as that would
141 be a monumental task, and, frankly, we don't want this manpage to be an
142 internals document for Perl. The examples do demonstrate some basics
143 of the raw Perl datatypes, and should suffice to get most determined
144 people on their way. There are no guidewires or safety nets, nor
145 blazed trails, so be prepared to travel alone from this point and on
146 and, if at all possible, don't fall into the quicksand (it's bad for
147 business).
148 Oh, one final bit of advice: take perlguts with you. When you return
150 we expect to see it well-thumbed.
151 A simple scalar string
153 Let's begin by looking a simple scalar which is holding a string.
154 use Devel::Peek;
156 $a = 42; $a = "hello";
157 Dump $a;
158 The output:
160 SV = PVIV(0xbc288) at 0xbe9a8
162 REFCNT = 1
163 FLAGS = (POK,pPOK)
164 IV = 42
165 PV = 0xb2048 "hello"\0
166 CUR = 5
167 LEN = 8
168 This says $a is an SV, a scalar. The scalar type is a PVIV, which is
170 capable of holding an integer (IV) and/or a string (PV) value. The
171 scalar's head is allocated at address 0xbe9a8, while the body is at
172 0xbc288. Its reference count is 1. It has the "POK" flag set, meaning
173 its current PV field is valid. Because POK is set we look at the PV
174 item to see what is in the scalar. The \0 at the end indicate that
175 this PV is properly NUL-terminated. Note that the IV field still
176 contains its old numeric value, but because FLAGS doesn't have IOK set,
177 we must ignore the IV item. CUR indicates the number of characters in
178 the PV. LEN indicates the number of bytes allocated for the PV (at
179 least one more than CUR, because LEN includes an extra byte for the
180 end-of-string marker, then usually rounded up to some efficient
181 allocation unit).
182 A simple scalar number
184 If the scalar contains a number the raw SV will be leaner.
185 use Devel::Peek;
187 $a = 42;
188 Dump $a;
189 The output:
191 SV = IV(0xbc818) at 0xbe9a8
193 REFCNT = 1
194 FLAGS = (IOK,pIOK)
195 IV = 42
196 This says $a is an SV, a scalar. The scalar is an IV, a number. Its
198 reference count is 1. It has the "IOK" flag set, meaning it is
199 currently being evaluated as a number. Because IOK is set we look at
200 the IV item to see what is in the scalar.
201 A simple scalar with an extra reference
203 If the scalar from the previous example had an extra reference:
204 use Devel::Peek;
206 $a = 42;
207 $b = \$a;
208 Dump $a;
209 The output:
211 SV = IV(0xbe860) at 0xbe9a8
213 REFCNT = 2
214 FLAGS = (IOK,pIOK)
215 IV = 42
216 Notice that this example differs from the previous example only in its
218 reference count. Compare this to the next example, where we dump $b
219 instead of $a.
220 A reference to a simple scalar
222 This shows what a reference looks like when it references a simple
223 scalar.
224 use Devel::Peek;
226 $a = 42;
227 $b = \$a;
228 Dump $b;
229 The output:
231 SV = IV(0xf041c) at 0xbe9a0
233 REFCNT = 1
234 FLAGS = (ROK)
235 RV = 0xbab08
236 SV = IV(0xbe860) at 0xbe9a8
237 REFCNT = 2
238 FLAGS = (IOK,pIOK)
239 IV = 42
240 Starting from the top, this says $b is an SV. The scalar is an IV,
242 which is capable of holding an integer or reference value. It has the
243 "ROK" flag set, meaning it is a reference (rather than an integer or
244 string). Notice that Dump follows the reference and shows us what $b
245 was referencing. We see the same $a that we found in the previous
246 example.
247 Note that the value of "RV" coincides with the numbers we see when we
249 stringify $b. The addresses inside IV() are addresses of "X***"
250 structures which hold the current state of an "SV". This address may
251 change during lifetime of an SV.
252 A reference to an array
254 This shows what a reference to an array looks like.
255 use Devel::Peek;
257 $a = [42];
258 Dump $a;
259 The output:
261 SV = IV(0xc85998) at 0xc859a8
263 REFCNT = 1
264 FLAGS = (ROK)
265 RV = 0xc70de8
266 SV = PVAV(0xc71e10) at 0xc70de8
267 REFCNT = 1
268 FLAGS = ()
269 ARRAY = 0xc7e820
270 FILL = 0
271 MAX = 0
272 FLAGS = (REAL)
273 Elt No. 0
274 SV = IV(0xc70f88) at 0xc70f98
275 REFCNT = 1
276 FLAGS = (IOK,pIOK)
277 IV = 42
278 This says $a is a reference (ROK), which points to another SV which is
280 a PVAV, an array. The array has one element, element zero, which is
281 another SV. The field "FILL" above indicates the last element in the
282 array, similar to "$#$a".
283 If $a pointed to an array of two elements then we would see the
285 following.
286 use Devel::Peek 'Dump';
288 $a = [42,24];
289 Dump $a;
290 The output:
292 SV = IV(0x158c998) at 0x158c9a8
294 REFCNT = 1
295 FLAGS = (ROK)
296 RV = 0x1577de8
297 SV = PVAV(0x1578e10) at 0x1577de8
298 REFCNT = 1
299 FLAGS = ()
300 ARRAY = 0x1585820
301 FILL = 1
302 MAX = 1
303 FLAGS = (REAL)
304 Elt No. 0
305 SV = IV(0x1577f88) at 0x1577f98
306 REFCNT = 1
307 FLAGS = (IOK,pIOK)
308 IV = 42
309 Elt No. 1
310 SV = IV(0x158be88) at 0x158be98
311 REFCNT = 1
312 FLAGS = (IOK,pIOK)
313 IV = 24
314 Note that "Dump" will not report all the elements in the array, only
316 several first (depending on how deep it already went into the report
317 tree).
318 A reference to a hash
320 The following shows the raw form of a reference to a hash.
321 use Devel::Peek;
323 $a = {hello=>42};
324 Dump $a;
325 The output:
327 SV = IV(0x55cb50b50fb0) at 0x55cb50b50fc0
329 REFCNT = 1
330 FLAGS = (ROK)
331 RV = 0x55cb50b2b758
332 SV = PVHV(0x55cb50b319c0) at 0x55cb50b2b758
333 REFCNT = 1
334 FLAGS = (SHAREKEYS)
335 ARRAY = 0x55cb50b941a0 (0:7, 1:1)
336 hash quality = 100.0%
337 KEYS = 1
338 FILL = 1
339 MAX = 7
340 Elt "hello" HASH = 0x3128ece4
341 SV = IV(0x55cb50b464f8) at 0x55cb50b46508
342 REFCNT = 1
343 FLAGS = (IOK,pIOK)
344 IV = 42
345 This shows $a is a reference pointing to an SV. That SV is a PVHV, a
347 hash.
348 The "quality" of a hash is defined as the total number of comparisons
350 needed to access every element once, relative to the expected number
351 needed for a random hash. The value can go over 100%.
352 The total number of comparisons is equal to the sum of the squares of
354 the number of entries in each bucket. For a random hash of "<n"> keys
355 into "<k"> buckets, the expected value is:
356 n + n(n-1)/2k
358 Dumping a large array or hash
360 The Dump() function, by default, dumps up to 4 elements from a toplevel
361 array or hash. This number can be increased by supplying a second
362 argument to the function.
363 use Devel::Peek;
365 $a = [10,11,12,13,14];
366 Dump $a;
367 Notice that Dump() prints only elements 10 through 13 in the above
369 code. The following code will print all of the elements.
370 use Devel::Peek 'Dump';
372 $a = [10,11,12,13,14];
373 Dump $a, 5;
374 A reference to an SV which holds a C pointer
376 This is what you really need to know as an XS programmer, of course.
377 When an XSUB returns a pointer to a C structure that pointer is stored
378 in an SV and a reference to that SV is placed on the XSUB stack. So
379 the output from an XSUB which uses something like the T_PTROBJ map
380 might look something like this:
381 SV = IV(0xf381c) at 0xc859a8
383 REFCNT = 1
384 FLAGS = (ROK)
385 RV = 0xb8ad8
386 SV = PVMG(0xbb3c8) at 0xc859a0
387 REFCNT = 1
388 FLAGS = (OBJECT,IOK,pIOK)
389 IV = 729160
390 NV = 0
391 PV = 0
392 STASH = 0xc1d10 "CookBookB::Opaque"
393 This shows that we have an SV which is a reference, which points at
395 another SV. In this case that second SV is a PVMG, a blessed scalar.
396 Because it is blessed it has the "OBJECT" flag set. Note that an SV
397 which holds a C pointer also has the "IOK" flag set. The "STASH" is
398 set to the package name which this SV was blessed into.
399 The output from an XSUB which uses something like the T_PTRREF map,
401 which doesn't bless the object, might look something like this:
402 SV = IV(0xf381c) at 0xc859a8
404 REFCNT = 1
405 FLAGS = (ROK)
406 RV = 0xb8ad8
407 SV = PVMG(0xbb3c8) at 0xc859a0
408 REFCNT = 1
409 FLAGS = (IOK,pIOK)
410 IV = 729160
411 NV = 0
412 PV = 0
413 A reference to a subroutine
415 Looks like this:
416 SV = IV(0x24d2dd8) at 0x24d2de8
418 REFCNT = 1
419 FLAGS = (TEMP,ROK)
420 RV = 0x24e79d8
421 SV = PVCV(0x24e5798) at 0x24e79d8
422 REFCNT = 2
423 FLAGS = ()
424 COMP_STASH = 0x22c9c50 "main"
425 START = 0x22eed60 ===> 0
426 ROOT = 0x22ee490
427 GVGV::GV = 0x22de9d8 "MY" :: "top_targets"
428 FILE = "(eval 5)"
429 DEPTH = 0
430 FLAGS = 0x0
431 OUTSIDE_SEQ = 93
432 PADLIST = 0x22e9ed8
433 PADNAME = 0x22e9ec0(0x22eed00) PAD = 0x22e9ea8(0x22eecd0)
434 OUTSIDE = 0x22c9fb0 (MAIN)
435 This shows that
437 • the subroutine is not an XSUB (since "START" and "ROOT" are non-
439 zero, and "XSUB" is not listed, and is thus null);
440 • that it was compiled in the package "main";
442 • under the name "MY::top_targets";
444 • inside a 5th eval in the program;
446 • it is not currently executed (because "DEPTH" is 0);
448 • it has no prototype ("PROTOTYPE" field is missing).
450
452 "Dump", "mstat", "DeadCode", "DumpArray", "DumpWithOP" and "DumpProg",
453 "fill_mstats", "mstats_fillhash", "mstats2hash" by default.
454 Additionally available "SvREFCNT", "SvREFCNT_inc" and "SvREFCNT_dec".
455
457 Readers have been known to skip important parts of perlguts, causing
458 much frustration for all.
459
461 Ilya Zakharevich ilya@math.ohio-state.edu
462 Copyright (c) 1995-98 Ilya Zakharevich. All rights reserved. This
464 program is free software; you can redistribute it and/or modify it
465 under the same terms as Perl itself.
466 Author of this software makes no claim whatsoever about suitability,
468 reliability, edability, editability or usability of this product, and
469 should not be kept liable for any damage resulting from the use of it.
470 If you can use it, you are in luck, if not, I should not be kept
471 responsible. Keep a handy copy of your backup tape at hand.
472
474 perlguts, and perlguts, again.
475perl v5.38.2 2023-11-30 Devel::Peek(3pm)