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1 | # Devel::Peek - A data debugging tool for the XS programmer |
2 | # The documentation is after the __END__ |
3 | |
4 | package Devel::Peek; |
5 | |
6 | $VERSION = $VERSION = 0.95; |
7 | |
8 | require Exporter; |
9 | require DynaLoader; |
10 | |
11 | @ISA = qw(Exporter DynaLoader); |
12 | @EXPORT = qw(Dump mstat DeadCode DumpArray DumpWithOP DumpProg); |
13 | @EXPORT_OK = qw(SvREFCNT SvREFCNT_inc SvREFCNT_dec); |
14 | %EXPORT_TAGS = ('ALL' => [@EXPORT, @EXPORT_OK]); |
15 | |
16 | bootstrap Devel::Peek; |
17 | |
18 | sub DumpWithOP ($;$) { |
19 | local($Devel::Peek::dump_ops)=1; |
20 | my $depth = @_ > 1 ? $_[1] : 4 ; |
21 | Dump($_[0],$depth); |
22 | } |
23 | |
24 | 1; |
25 | __END__ |
26 | |
27 | =head1 NAME |
28 | |
29 | Devel::Peek - A data debugging tool for the XS programmer |
30 | |
31 | =head1 SYNOPSIS |
32 | |
33 | use Devel::Peek; |
34 | Dump( $a ); |
35 | Dump( $a, 5 ); |
36 | DumpArray( 5, $a, $b, ... ); |
37 | mstat "Point 5"; |
38 | |
39 | =head1 DESCRIPTION |
40 | |
41 | Devel::Peek contains functions which allows raw Perl datatypes to be |
42 | manipulated from a Perl script. This is used by those who do XS programming |
43 | to check that the data they are sending from C to Perl looks as they think |
44 | it should look. The trick, then, is to know what the raw datatype is |
45 | supposed to look like when it gets to Perl. This document offers some tips |
46 | and hints to describe good and bad raw data. |
47 | |
48 | It is very possible that this document will fall far short of being useful |
49 | to the casual reader. The reader is expected to understand the material in |
50 | the first few sections of L<perlguts>. |
51 | |
52 | Devel::Peek supplies a C<Dump()> function which can dump a raw Perl |
53 | datatype, and C<mstat("marker")> function to report on memory usage |
54 | (if perl is compiled with corresponding option). The function |
55 | DeadCode() provides statistics on the data "frozen" into inactive |
56 | C<CV>. Devel::Peek also supplies C<SvREFCNT()>, C<SvREFCNT_inc()>, and |
57 | C<SvREFCNT_dec()> which can query, increment, and decrement reference |
58 | counts on SVs. This document will take a passive, and safe, approach |
59 | to data debugging and for that it will describe only the C<Dump()> |
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60 | function. For format of output of mstats() see |
61 | L<perldebug/Using C<$ENV{PERL_DEBUG_MSTATS}>>. |
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62 | |
63 | Function C<DumpArray()> allows dumping of multiple values (useful when you |
64 | need to analize returns of functions). |
65 | |
66 | The global variable $Devel::Peek::pv_limit can be set to limit the |
67 | number of character printed in various string values. Setting it to 0 |
68 | means no limit. |
69 | |
70 | =head1 EXAMPLES |
71 | |
72 | The following examples don't attempt to show everything as that would be a |
73 | monumental task, and, frankly, we don't want this manpage to be an internals |
74 | document for Perl. The examples do demonstrate some basics of the raw Perl |
75 | datatypes, and should suffice to get most determined people on their way. |
76 | There are no guidewires or safety nets, nor blazed trails, so be prepared to |
77 | travel alone from this point and on and, if at all possible, don't fall into |
78 | the quicksand (it's bad for business). |
79 | |
80 | Oh, one final bit of advice: take L<perlguts> with you. When you return we |
81 | expect to see it well-thumbed. |
82 | |
83 | =head2 A simple scalar string |
84 | |
85 | Let's begin by looking a simple scalar which is holding a string. |
86 | |
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87 | use Devel::Peek; |
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88 | $a = "hello"; |
89 | Dump $a; |
90 | |
91 | The output: |
92 | |
93 | SV = PVIV(0xbc288) |
94 | REFCNT = 1 |
95 | FLAGS = (POK,pPOK) |
96 | IV = 0 |
97 | PV = 0xb2048 "hello"\0 |
98 | CUR = 5 |
99 | LEN = 6 |
100 | |
101 | This says C<$a> is an SV, a scalar. The scalar is a PVIV, a string. |
102 | Its reference count is 1. It has the C<POK> flag set, meaning its |
103 | current PV field is valid. Because POK is set we look at the PV item |
104 | to see what is in the scalar. The \0 at the end indicate that this |
105 | PV is properly NUL-terminated. |
106 | If the FLAGS had been IOK we would look |
107 | at the IV item. CUR indicates the number of characters in the PV. |
108 | LEN indicates the number of bytes requested for the PV (one more than |
109 | CUR, in this case, because LEN includes an extra byte for the |
110 | end-of-string marker). |
111 | |
112 | =head2 A simple scalar number |
113 | |
114 | If the scalar contains a number the raw SV will be leaner. |
115 | |
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116 | use Devel::Peek; |
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117 | $a = 42; |
118 | Dump $a; |
119 | |
120 | The output: |
121 | |
122 | SV = IV(0xbc818) |
123 | REFCNT = 1 |
124 | FLAGS = (IOK,pIOK) |
125 | IV = 42 |
126 | |
127 | This says C<$a> is an SV, a scalar. The scalar is an IV, a number. Its |
128 | reference count is 1. It has the C<IOK> flag set, meaning it is currently |
129 | being evaluated as a number. Because IOK is set we look at the IV item to |
130 | see what is in the scalar. |
131 | |
132 | =head2 A simple scalar with an extra reference |
133 | |
134 | If the scalar from the previous example had an extra reference: |
135 | |
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136 | use Devel::Peek; |
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137 | $a = 42; |
138 | $b = \$a; |
139 | Dump $a; |
140 | |
141 | The output: |
142 | |
143 | SV = IV(0xbe860) |
144 | REFCNT = 2 |
145 | FLAGS = (IOK,pIOK) |
146 | IV = 42 |
147 | |
148 | Notice that this example differs from the previous example only in its |
149 | reference count. Compare this to the next example, where we dump C<$b> |
150 | instead of C<$a>. |
151 | |
152 | =head2 A reference to a simple scalar |
153 | |
154 | This shows what a reference looks like when it references a simple scalar. |
155 | |
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156 | use Devel::Peek; |
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157 | $a = 42; |
158 | $b = \$a; |
159 | Dump $b; |
160 | |
161 | The output: |
162 | |
163 | SV = RV(0xf041c) |
164 | REFCNT = 1 |
165 | FLAGS = (ROK) |
166 | RV = 0xbab08 |
167 | SV = IV(0xbe860) |
168 | REFCNT = 2 |
169 | FLAGS = (IOK,pIOK) |
170 | IV = 42 |
171 | |
172 | Starting from the top, this says C<$b> is an SV. The scalar is an RV, a |
173 | reference. It has the C<ROK> flag set, meaning it is a reference. Because |
174 | ROK is set we have an RV item rather than an IV or PV. Notice that Dump |
175 | follows the reference and shows us what C<$b> was referencing. We see the |
176 | same C<$a> that we found in the previous example. |
177 | |
178 | Note that the value of C<RV> coincides with the numbers we see when we |
179 | stringify $b. The addresses inside RV() and IV() are addresses of |
180 | C<X***> structure which holds the current state of an C<SV>. This |
181 | address may change during lifetime of an SV. |
182 | |
183 | =head2 A reference to an array |
184 | |
185 | This shows what a reference to an array looks like. |
186 | |
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187 | use Devel::Peek; |
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188 | $a = [42]; |
189 | Dump $a; |
190 | |
191 | The output: |
192 | |
193 | SV = RV(0xf041c) |
194 | REFCNT = 1 |
195 | FLAGS = (ROK) |
196 | RV = 0xb2850 |
197 | SV = PVAV(0xbd448) |
198 | REFCNT = 1 |
199 | FLAGS = () |
200 | IV = 0 |
201 | NV = 0 |
202 | ARRAY = 0xb2048 |
203 | ALLOC = 0xb2048 |
204 | FILL = 0 |
205 | MAX = 0 |
206 | ARYLEN = 0x0 |
207 | FLAGS = (REAL) |
208 | Elt No. 0 0xb5658 |
209 | SV = IV(0xbe860) |
210 | REFCNT = 1 |
211 | FLAGS = (IOK,pIOK) |
212 | IV = 42 |
213 | |
214 | This says C<$a> is an SV and that it is an RV. That RV points to |
215 | another SV which is a PVAV, an array. The array has one element, |
216 | element zero, which is another SV. The field C<FILL> above indicates |
217 | the last element in the array, similar to C<$#$a>. |
218 | |
219 | If C<$a> pointed to an array of two elements then we would see the |
220 | following. |
221 | |
222 | use Devel::Peek 'Dump'; |
223 | $a = [42,24]; |
224 | Dump $a; |
225 | |
226 | The output: |
227 | |
228 | SV = RV(0xf041c) |
229 | REFCNT = 1 |
230 | FLAGS = (ROK) |
231 | RV = 0xb2850 |
232 | SV = PVAV(0xbd448) |
233 | REFCNT = 1 |
234 | FLAGS = () |
235 | IV = 0 |
236 | NV = 0 |
237 | ARRAY = 0xb2048 |
238 | ALLOC = 0xb2048 |
239 | FILL = 0 |
240 | MAX = 0 |
241 | ARYLEN = 0x0 |
242 | FLAGS = (REAL) |
243 | Elt No. 0 0xb5658 |
244 | SV = IV(0xbe860) |
245 | REFCNT = 1 |
246 | FLAGS = (IOK,pIOK) |
247 | IV = 42 |
248 | Elt No. 1 0xb5680 |
249 | SV = IV(0xbe818) |
250 | REFCNT = 1 |
251 | FLAGS = (IOK,pIOK) |
252 | IV = 24 |
253 | |
254 | Note that C<Dump> will not report I<all> the elements in the array, |
255 | only several first (depending on how deep it already went into the |
256 | report tree). |
257 | |
258 | =head2 A reference to a hash |
259 | |
260 | The following shows the raw form of a reference to a hash. |
261 | |
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262 | use Devel::Peek; |
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263 | $a = {hello=>42}; |
264 | Dump $a; |
265 | |
266 | The output: |
267 | |
268 | SV = RV(0xf041c) |
269 | REFCNT = 1 |
270 | FLAGS = (ROK) |
271 | RV = 0xb2850 |
272 | SV = PVHV(0xbd448) |
273 | REFCNT = 1 |
274 | FLAGS = () |
275 | NV = 0 |
276 | ARRAY = 0xbd748 |
277 | KEYS = 1 |
278 | FILL = 1 |
279 | MAX = 7 |
280 | RITER = -1 |
281 | EITER = 0x0 |
282 | Elt "hello" => 0xbaaf0 |
283 | SV = IV(0xbe860) |
284 | REFCNT = 1 |
285 | FLAGS = (IOK,pIOK) |
286 | IV = 42 |
287 | |
288 | This shows C<$a> is a reference pointing to an SV. That SV is a PVHV, a |
289 | hash. Fields RITER and EITER are used by C<L<each>>. |
290 | |
291 | =head2 Dumping a large array or hash |
292 | |
293 | The C<Dump()> function, by default, dumps up to 4 elements from a |
294 | toplevel array or hash. This number can be increased by supplying a |
295 | second argument to the function. |
296 | |
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297 | use Devel::Peek; |
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298 | $a = [10,11,12,13,14]; |
299 | Dump $a; |
300 | |
301 | Notice that C<Dump()> prints only elements 10 through 13 in the above code. |
302 | The following code will print all of the elements. |
303 | |
304 | use Devel::Peek 'Dump'; |
305 | $a = [10,11,12,13,14]; |
306 | Dump $a, 5; |
307 | |
308 | =head2 A reference to an SV which holds a C pointer |
309 | |
310 | This is what you really need to know as an XS programmer, of course. When |
311 | an XSUB returns a pointer to a C structure that pointer is stored in an SV |
312 | and a reference to that SV is placed on the XSUB stack. So the output from |
313 | an XSUB which uses something like the T_PTROBJ map might look something like |
314 | this: |
315 | |
316 | SV = RV(0xf381c) |
317 | REFCNT = 1 |
318 | FLAGS = (ROK) |
319 | RV = 0xb8ad8 |
320 | SV = PVMG(0xbb3c8) |
321 | REFCNT = 1 |
322 | FLAGS = (OBJECT,IOK,pIOK) |
323 | IV = 729160 |
324 | NV = 0 |
325 | PV = 0 |
326 | STASH = 0xc1d10 "CookBookB::Opaque" |
327 | |
328 | This shows that we have an SV which is an RV. That RV points at another |
329 | SV. In this case that second SV is a PVMG, a blessed scalar. Because it is |
330 | blessed it has the C<OBJECT> flag set. Note that an SV which holds a C |
331 | pointer also has the C<IOK> flag set. The C<STASH> is set to the package |
332 | name which this SV was blessed into. |
333 | |
334 | The output from an XSUB which uses something like the T_PTRREF map, which |
335 | doesn't bless the object, might look something like this: |
336 | |
337 | SV = RV(0xf381c) |
338 | REFCNT = 1 |
339 | FLAGS = (ROK) |
340 | RV = 0xb8ad8 |
341 | SV = PVMG(0xbb3c8) |
342 | REFCNT = 1 |
343 | FLAGS = (IOK,pIOK) |
344 | IV = 729160 |
345 | NV = 0 |
346 | PV = 0 |
347 | |
348 | =head2 A reference to a subroutine |
349 | |
350 | Looks like this: |
351 | |
352 | SV = RV(0x798ec) |
353 | REFCNT = 1 |
354 | FLAGS = (TEMP,ROK) |
355 | RV = 0x1d453c |
356 | SV = PVCV(0x1c768c) |
357 | REFCNT = 2 |
358 | FLAGS = () |
359 | IV = 0 |
360 | NV = 0 |
361 | COMP_STASH = 0x31068 "main" |
362 | START = 0xb20e0 |
363 | ROOT = 0xbece0 |
364 | XSUB = 0x0 |
365 | XSUBANY = 0 |
366 | GVGV::GV = 0x1d44e8 "MY" :: "top_targets" |
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367 | DEPTH = 0 |
368 | PADLIST = 0x1c9338 |
369 | |
370 | This shows that |
371 | |
372 | =over |
373 | |
374 | =item |
375 | |
376 | the subroutine is not an XSUB (since C<START> and C<ROOT> are |
377 | non-zero, and C<XSUB> is zero); |
378 | |
379 | =item |
380 | |
381 | that it was compiled in the package C<main>; |
382 | |
383 | =item |
384 | |
385 | under the name C<MY::top_targets>; |
386 | |
387 | =item |
388 | |
389 | inside a 5th eval in the program; |
390 | |
391 | =item |
392 | |
393 | it is not currently executed (see C<DEPTH>); |
394 | |
395 | =item |
396 | |
397 | it has no prototype (C<PROTOTYPE> field is missing). |
398 | |
399 | =over |
400 | |
401 | =head1 EXPORTS |
402 | |
403 | C<Dump>, C<mstat>, C<DeadCode>, C<DumpArray>, C<DumpWithOP> and |
404 | C<DumpProg> by default. Additionally available C<SvREFCNT>, |
405 | C<SvREFCNT_inc> and C<SvREFCNT_dec>. |
406 | |
407 | =head1 BUGS |
408 | |
409 | Readers have been known to skip important parts of L<perlguts>, causing much |
410 | frustration for all. |
411 | |
412 | =head1 AUTHOR |
413 | |
414 | Ilya Zakharevich ilya@math.ohio-state.edu |
415 | |
416 | Copyright (c) 1995-98 Ilya Zakharevich. All rights reserved. |
417 | This program is free software; you can redistribute it and/or |
418 | modify it under the same terms as Perl itself. |
419 | |
420 | Author of this software makes no claim whatsoever about suitability, |
421 | reliability, edability, editability or usability of this product, and |
422 | should not be kept liable for any damage resulting from the use of |
423 | it. If you can use it, you are in luck, if not, I should not be kept |
424 | responsible. Keep a handy copy of your backup tape at hand. |
425 | |
426 | =head1 SEE ALSO |
427 | |
428 | L<perlguts>, and L<perlguts>, again. |
429 | |
430 | =cut |