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04c692a8 DR |
2 | =encoding utf8 |
3 | ||
4 | =for comment | |
5 | Consistent formatting of this file is achieved with: | |
6 | perl ./Porting/podtidy pod/perlhacktips.pod | |
7 | ||
8 | =head1 NAME | |
9 | ||
10 | perlhacktips - Tips for Perl core C code hacking | |
11 | ||
12 | =head1 DESCRIPTION | |
13 | ||
14 | This document will help you learn the best way to go about hacking on | |
15 | the Perl core C code. It covers common problems, debugging, profiling, | |
16 | and more. | |
17 | ||
18 | If you haven't read L<perlhack> and L<perlhacktut> yet, you might want | |
19 | to do that first. | |
20 | ||
21 | =head1 COMMON PROBLEMS | |
22 | ||
23 | Perl source plays by ANSI C89 rules: no C99 (or C++) extensions. In | |
24 | some cases we have to take pre-ANSI requirements into consideration. | |
25 | You don't care about some particular platform having broken Perl? I | |
26 | hear there is still a strong demand for J2EE programmers. | |
27 | ||
28 | =head2 Perl environment problems | |
29 | ||
30 | =over 4 | |
31 | ||
32 | =item * | |
33 | ||
34 | Not compiling with threading | |
35 | ||
36 | Compiling with threading (-Duseithreads) completely rewrites the | |
37 | function prototypes of Perl. You better try your changes with that. | |
38 | Related to this is the difference between "Perl_-less" and "Perl_-ly" | |
39 | APIs, for example: | |
40 | ||
41 | Perl_sv_setiv(aTHX_ ...); | |
42 | sv_setiv(...); | |
43 | ||
44 | The first one explicitly passes in the context, which is needed for | |
45 | e.g. threaded builds. The second one does that implicitly; do not get | |
46 | them mixed. If you are not passing in a aTHX_, you will need to do a | |
47 | dTHX (or a dVAR) as the first thing in the function. | |
48 | ||
49 | See L<perlguts/"How multiple interpreters and concurrency are | |
50 | supported"> for further discussion about context. | |
51 | ||
52 | =item * | |
53 | ||
54 | Not compiling with -DDEBUGGING | |
55 | ||
56 | The DEBUGGING define exposes more code to the compiler, therefore more | |
57 | ways for things to go wrong. You should try it. | |
58 | ||
59 | =item * | |
60 | ||
61 | Introducing (non-read-only) globals | |
62 | ||
63 | Do not introduce any modifiable globals, truly global or file static. | |
64 | They are bad form and complicate multithreading and other forms of | |
65 | concurrency. The right way is to introduce them as new interpreter | |
66 | variables, see F<intrpvar.h> (at the very end for binary | |
67 | compatibility). | |
68 | ||
69 | Introducing read-only (const) globals is okay, as long as you verify | |
70 | with e.g. C<nm libperl.a|egrep -v ' [TURtr] '> (if your C<nm> has | |
71 | BSD-style output) that the data you added really is read-only. (If it | |
72 | is, it shouldn't show up in the output of that command.) | |
73 | ||
74 | If you want to have static strings, make them constant: | |
75 | ||
76 | static const char etc[] = "..."; | |
77 | ||
78 | If you want to have arrays of constant strings, note carefully the | |
79 | right combination of C<const>s: | |
80 | ||
81 | static const char * const yippee[] = | |
82 | {"hi", "ho", "silver"}; | |
83 | ||
84 | There is a way to completely hide any modifiable globals (they are all | |
85 | moved to heap), the compilation setting | |
86 | C<-DPERL_GLOBAL_STRUCT_PRIVATE>. It is not normally used, but can be | |
87 | used for testing, read more about it in L<perlguts/"Background and | |
88 | PERL_IMPLICIT_CONTEXT">. | |
89 | ||
90 | =item * | |
91 | ||
92 | Not exporting your new function | |
93 | ||
94 | Some platforms (Win32, AIX, VMS, OS/2, to name a few) require any | |
95 | function that is part of the public API (the shared Perl library) to be | |
96 | explicitly marked as exported. See the discussion about F<embed.pl> in | |
97 | L<perlguts>. | |
98 | ||
99 | =item * | |
100 | ||
101 | Exporting your new function | |
102 | ||
103 | The new shiny result of either genuine new functionality or your | |
104 | arduous refactoring is now ready and correctly exported. So what could | |
105 | possibly go wrong? | |
106 | ||
107 | Maybe simply that your function did not need to be exported in the | |
108 | first place. Perl has a long and not so glorious history of exporting | |
109 | functions that it should not have. | |
110 | ||
111 | If the function is used only inside one source code file, make it | |
112 | static. See the discussion about F<embed.pl> in L<perlguts>. | |
113 | ||
114 | If the function is used across several files, but intended only for | |
115 | Perl's internal use (and this should be the common case), do not export | |
116 | it to the public API. See the discussion about F<embed.pl> in | |
117 | L<perlguts>. | |
118 | ||
119 | =back | |
120 | ||
121 | =head2 Portability problems | |
122 | ||
123 | The following are common causes of compilation and/or execution | |
124 | failures, not common to Perl as such. The C FAQ is good bedtime | |
125 | reading. Please test your changes with as many C compilers and | |
126 | platforms as possible; we will, anyway, and it's nice to save oneself | |
127 | from public embarrassment. | |
128 | ||
129 | If using gcc, you can add the C<-std=c89> option which will hopefully | |
130 | catch most of these unportabilities. (However it might also catch | |
131 | incompatibilities in your system's header files.) | |
132 | ||
133 | Use the Configure C<-Dgccansipedantic> flag to enable the gcc C<-ansi | |
134 | -pedantic> flags which enforce stricter ANSI rules. | |
135 | ||
136 | If using the C<gcc -Wall> note that not all the possible warnings (like | |
137 | C<-Wunitialized>) are given unless you also compile with C<-O>. | |
138 | ||
139 | Note that if using gcc, starting from Perl 5.9.5 the Perl core source | |
140 | code files (the ones at the top level of the source code distribution, | |
141 | but not e.g. the extensions under ext/) are automatically compiled with | |
142 | as many as possible of the C<-std=c89>, C<-ansi>, C<-pedantic>, and a | |
143 | selection of C<-W> flags (see cflags.SH). | |
144 | ||
145 | Also study L<perlport> carefully to avoid any bad assumptions about the | |
146 | operating system, filesystems, and so forth. | |
147 | ||
148 | You may once in a while try a "make microperl" to see whether we can | |
149 | still compile Perl with just the bare minimum of interfaces. (See | |
150 | README.micro.) | |
151 | ||
152 | Do not assume an operating system indicates a certain compiler. | |
153 | ||
154 | =over 4 | |
155 | ||
156 | =item * | |
157 | ||
158 | Casting pointers to integers or casting integers to pointers | |
159 | ||
160 | void castaway(U8* p) | |
161 | { | |
162 | IV i = p; | |
163 | ||
164 | or | |
165 | ||
166 | void castaway(U8* p) | |
167 | { | |
168 | IV i = (IV)p; | |
169 | ||
170 | Both are bad, and broken, and unportable. Use the PTR2IV() macro that | |
171 | does it right. (Likewise, there are PTR2UV(), PTR2NV(), INT2PTR(), and | |
172 | NUM2PTR().) | |
173 | ||
174 | =item * | |
175 | ||
176 | Casting between data function pointers and data pointers | |
177 | ||
178 | Technically speaking casting between function pointers and data | |
179 | pointers is unportable and undefined, but practically speaking it seems | |
180 | to work, but you should use the FPTR2DPTR() and DPTR2FPTR() macros. | |
181 | Sometimes you can also play games with unions. | |
182 | ||
183 | =item * | |
184 | ||
185 | Assuming sizeof(int) == sizeof(long) | |
186 | ||
187 | There are platforms where longs are 64 bits, and platforms where ints | |
188 | are 64 bits, and while we are out to shock you, even platforms where | |
189 | shorts are 64 bits. This is all legal according to the C standard. (In | |
190 | other words, "long long" is not a portable way to specify 64 bits, and | |
191 | "long long" is not even guaranteed to be any wider than "long".) | |
192 | ||
193 | Instead, use the definitions IV, UV, IVSIZE, I32SIZE, and so forth. | |
194 | Avoid things like I32 because they are B<not> guaranteed to be | |
195 | I<exactly> 32 bits, they are I<at least> 32 bits, nor are they | |
196 | guaranteed to be B<int> or B<long>. If you really explicitly need | |
197 | 64-bit variables, use I64 and U64, but only if guarded by HAS_QUAD. | |
198 | ||
199 | =item * | |
200 | ||
201 | Assuming one can dereference any type of pointer for any type of data | |
202 | ||
203 | char *p = ...; | |
204 | long pony = *p; /* BAD */ | |
205 | ||
206 | Many platforms, quite rightly so, will give you a core dump instead of | |
768312ab | 207 | a pony if the p happens not to be correctly aligned. |
04c692a8 DR |
208 | |
209 | =item * | |
210 | ||
211 | Lvalue casts | |
212 | ||
213 | (int)*p = ...; /* BAD */ | |
214 | ||
215 | Simply not portable. Get your lvalue to be of the right type, or maybe | |
216 | use temporary variables, or dirty tricks with unions. | |
217 | ||
218 | =item * | |
219 | ||
220 | Assume B<anything> about structs (especially the ones you don't | |
221 | control, like the ones coming from the system headers) | |
222 | ||
223 | =over 8 | |
224 | ||
225 | =item * | |
226 | ||
227 | That a certain field exists in a struct | |
228 | ||
229 | =item * | |
230 | ||
231 | That no other fields exist besides the ones you know of | |
232 | ||
233 | =item * | |
234 | ||
235 | That a field is of certain signedness, sizeof, or type | |
236 | ||
237 | =item * | |
238 | ||
239 | That the fields are in a certain order | |
240 | ||
241 | =over 8 | |
242 | ||
243 | =item * | |
244 | ||
245 | While C guarantees the ordering specified in the struct definition, | |
246 | between different platforms the definitions might differ | |
247 | ||
248 | =back | |
249 | ||
250 | =item * | |
251 | ||
252 | That the sizeof(struct) or the alignments are the same everywhere | |
253 | ||
254 | =over 8 | |
255 | ||
256 | =item * | |
257 | ||
258 | There might be padding bytes between the fields to align the fields - | |
259 | the bytes can be anything | |
260 | ||
261 | =item * | |
262 | ||
263 | Structs are required to be aligned to the maximum alignment required by | |
264 | the fields - which for native types is for usually equivalent to | |
265 | sizeof() of the field | |
266 | ||
267 | =back | |
268 | ||
269 | =back | |
270 | ||
271 | =item * | |
272 | ||
273 | Assuming the character set is ASCIIish | |
274 | ||
275 | Perl can compile and run under EBCDIC platforms. See L<perlebcdic>. | |
276 | This is transparent for the most part, but because the character sets | |
277 | differ, you shouldn't use numeric (decimal, octal, nor hex) constants | |
278 | to refer to characters. You can safely say 'A', but not 0x41. You can | |
279 | safely say '\n', but not \012. If a character doesn't have a trivial | |
eb6d698b KW |
280 | input form, you should add it to the list in |
281 | F<regen/unicode_constants.pl>, and have Perl create #defines for you, | |
282 | based on the current platform. | |
04c692a8 DR |
283 | |
284 | Also, the range 'A' - 'Z' in ASCII is an unbroken sequence of 26 upper | |
285 | case alphabetic characters. That is not true in EBCDIC. Nor for 'a' to | |
286 | 'z'. But '0' - '9' is an unbroken range in both systems. Don't assume | |
287 | anything about other ranges. | |
288 | ||
289 | Many of the comments in the existing code ignore the possibility of | |
290 | EBCDIC, and may be wrong therefore, even if the code works. This is | |
291 | actually a tribute to the successful transparent insertion of being | |
292 | able to handle EBCDIC without having to change pre-existing code. | |
293 | ||
294 | UTF-8 and UTF-EBCDIC are two different encodings used to represent | |
295 | Unicode code points as sequences of bytes. Macros with the same names | |
296 | (but different definitions) in C<utf8.h> and C<utfebcdic.h> are used to | |
297 | allow the calling code to think that there is only one such encoding. | |
298 | This is almost always referred to as C<utf8>, but it means the EBCDIC | |
299 | version as well. Again, comments in the code may well be wrong even if | |
300 | the code itself is right. For example, the concept of C<invariant | |
301 | characters> differs between ASCII and EBCDIC. On ASCII platforms, only | |
302 | characters that do not have the high-order bit set (i.e. whose ordinals | |
303 | are strict ASCII, 0 - 127) are invariant, and the documentation and | |
304 | comments in the code may assume that, often referring to something | |
305 | like, say, C<hibit>. The situation differs and is not so simple on | |
306 | EBCDIC machines, but as long as the code itself uses the | |
307 | C<NATIVE_IS_INVARIANT()> macro appropriately, it works, even if the | |
308 | comments are wrong. | |
309 | ||
310 | =item * | |
311 | ||
312 | Assuming the character set is just ASCII | |
313 | ||
314 | ASCII is a 7 bit encoding, but bytes have 8 bits in them. The 128 extra | |
315 | characters have different meanings depending on the locale. Absent a | |
316 | locale, currently these extra characters are generally considered to be | |
317 | unassigned, and this has presented some problems. This is being changed | |
318 | starting in 5.12 so that these characters will be considered to be | |
319 | Latin-1 (ISO-8859-1). | |
320 | ||
321 | =item * | |
322 | ||
323 | Mixing #define and #ifdef | |
324 | ||
325 | #define BURGLE(x) ... \ | |
326 | #ifdef BURGLE_OLD_STYLE /* BAD */ | |
327 | ... do it the old way ... \ | |
328 | #else | |
329 | ... do it the new way ... \ | |
330 | #endif | |
331 | ||
332 | You cannot portably "stack" cpp directives. For example in the above | |
333 | you need two separate BURGLE() #defines, one for each #ifdef branch. | |
334 | ||
335 | =item * | |
336 | ||
337 | Adding non-comment stuff after #endif or #else | |
338 | ||
339 | #ifdef SNOSH | |
340 | ... | |
341 | #else !SNOSH /* BAD */ | |
342 | ... | |
343 | #endif SNOSH /* BAD */ | |
344 | ||
345 | The #endif and #else cannot portably have anything non-comment after | |
346 | them. If you want to document what is going (which is a good idea | |
347 | especially if the branches are long), use (C) comments: | |
348 | ||
349 | #ifdef SNOSH | |
350 | ... | |
351 | #else /* !SNOSH */ | |
352 | ... | |
353 | #endif /* SNOSH */ | |
354 | ||
355 | The gcc option C<-Wendif-labels> warns about the bad variant (by | |
356 | default on starting from Perl 5.9.4). | |
357 | ||
358 | =item * | |
359 | ||
360 | Having a comma after the last element of an enum list | |
361 | ||
362 | enum color { | |
363 | CERULEAN, | |
364 | CHARTREUSE, | |
365 | CINNABAR, /* BAD */ | |
366 | }; | |
367 | ||
368 | is not portable. Leave out the last comma. | |
369 | ||
370 | Also note that whether enums are implicitly morphable to ints varies | |
371 | between compilers, you might need to (int). | |
372 | ||
373 | =item * | |
374 | ||
375 | Using //-comments | |
376 | ||
377 | // This function bamfoodles the zorklator. /* BAD */ | |
378 | ||
379 | That is C99 or C++. Perl is C89. Using the //-comments is silently | |
380 | allowed by many C compilers but cranking up the ANSI C89 strictness | |
381 | (which we like to do) causes the compilation to fail. | |
382 | ||
383 | =item * | |
384 | ||
385 | Mixing declarations and code | |
386 | ||
387 | void zorklator() | |
388 | { | |
389 | int n = 3; | |
390 | set_zorkmids(n); /* BAD */ | |
391 | int q = 4; | |
392 | ||
393 | That is C99 or C++. Some C compilers allow that, but you shouldn't. | |
394 | ||
395 | The gcc option C<-Wdeclaration-after-statements> scans for such | |
396 | problems (by default on starting from Perl 5.9.4). | |
397 | ||
398 | =item * | |
399 | ||
400 | Introducing variables inside for() | |
401 | ||
402 | for(int i = ...; ...; ...) { /* BAD */ | |
403 | ||
404 | That is C99 or C++. While it would indeed be awfully nice to have that | |
405 | also in C89, to limit the scope of the loop variable, alas, we cannot. | |
406 | ||
407 | =item * | |
408 | ||
409 | Mixing signed char pointers with unsigned char pointers | |
410 | ||
411 | int foo(char *s) { ... } | |
412 | ... | |
413 | unsigned char *t = ...; /* Or U8* t = ... */ | |
414 | foo(t); /* BAD */ | |
415 | ||
416 | While this is legal practice, it is certainly dubious, and downright | |
417 | fatal in at least one platform: for example VMS cc considers this a | |
418 | fatal error. One cause for people often making this mistake is that a | |
419 | "naked char" and therefore dereferencing a "naked char pointer" have an | |
420 | undefined signedness: it depends on the compiler and the flags of the | |
421 | compiler and the underlying platform whether the result is signed or | |
422 | unsigned. For this very same reason using a 'char' as an array index is | |
423 | bad. | |
424 | ||
425 | =item * | |
426 | ||
427 | Macros that have string constants and their arguments as substrings of | |
428 | the string constants | |
429 | ||
430 | #define FOO(n) printf("number = %d\n", n) /* BAD */ | |
431 | FOO(10); | |
432 | ||
433 | Pre-ANSI semantics for that was equivalent to | |
434 | ||
435 | printf("10umber = %d\10"); | |
436 | ||
437 | which is probably not what you were expecting. Unfortunately at least | |
438 | one reasonably common and modern C compiler does "real backward | |
439 | compatibility" here, in AIX that is what still happens even though the | |
440 | rest of the AIX compiler is very happily C89. | |
441 | ||
442 | =item * | |
443 | ||
444 | Using printf formats for non-basic C types | |
445 | ||
446 | IV i = ...; | |
447 | printf("i = %d\n", i); /* BAD */ | |
448 | ||
449 | While this might by accident work in some platform (where IV happens to | |
450 | be an C<int>), in general it cannot. IV might be something larger. Even | |
451 | worse the situation is with more specific types (defined by Perl's | |
452 | configuration step in F<config.h>): | |
453 | ||
454 | Uid_t who = ...; | |
455 | printf("who = %d\n", who); /* BAD */ | |
456 | ||
457 | The problem here is that Uid_t might be not only not C<int>-wide but it | |
458 | might also be unsigned, in which case large uids would be printed as | |
459 | negative values. | |
460 | ||
461 | There is no simple solution to this because of printf()'s limited | |
462 | intelligence, but for many types the right format is available as with | |
463 | either 'f' or '_f' suffix, for example: | |
464 | ||
465 | IVdf /* IV in decimal */ | |
466 | UVxf /* UV is hexadecimal */ | |
467 | ||
468 | printf("i = %"IVdf"\n", i); /* The IVdf is a string constant. */ | |
469 | ||
470 | Uid_t_f /* Uid_t in decimal */ | |
471 | ||
472 | printf("who = %"Uid_t_f"\n", who); | |
473 | ||
474 | Or you can try casting to a "wide enough" type: | |
475 | ||
476 | printf("i = %"IVdf"\n", (IV)something_very_small_and_signed); | |
477 | ||
478 | Also remember that the C<%p> format really does require a void pointer: | |
479 | ||
480 | U8* p = ...; | |
481 | printf("p = %p\n", (void*)p); | |
482 | ||
483 | The gcc option C<-Wformat> scans for such problems. | |
484 | ||
485 | =item * | |
486 | ||
487 | Blindly using variadic macros | |
488 | ||
489 | gcc has had them for a while with its own syntax, and C99 brought them | |
490 | with a standardized syntax. Don't use the former, and use the latter | |
491 | only if the HAS_C99_VARIADIC_MACROS is defined. | |
492 | ||
493 | =item * | |
494 | ||
495 | Blindly passing va_list | |
496 | ||
497 | Not all platforms support passing va_list to further varargs (stdarg) | |
498 | functions. The right thing to do is to copy the va_list using the | |
499 | Perl_va_copy() if the NEED_VA_COPY is defined. | |
500 | ||
501 | =item * | |
502 | ||
503 | Using gcc statement expressions | |
504 | ||
505 | val = ({...;...;...}); /* BAD */ | |
506 | ||
507 | While a nice extension, it's not portable. The Perl code does | |
508 | admittedly use them if available to gain some extra speed (essentially | |
509 | as a funky form of inlining), but you shouldn't. | |
510 | ||
511 | =item * | |
512 | ||
513 | Binding together several statements in a macro | |
514 | ||
515 | Use the macros STMT_START and STMT_END. | |
516 | ||
517 | STMT_START { | |
518 | ... | |
519 | } STMT_END | |
520 | ||
521 | =item * | |
522 | ||
523 | Testing for operating systems or versions when should be testing for | |
524 | features | |
525 | ||
526 | #ifdef __FOONIX__ /* BAD */ | |
527 | foo = quux(); | |
528 | #endif | |
529 | ||
530 | Unless you know with 100% certainty that quux() is only ever available | |
531 | for the "Foonix" operating system B<and> that is available B<and> | |
532 | correctly working for B<all> past, present, B<and> future versions of | |
533 | "Foonix", the above is very wrong. This is more correct (though still | |
534 | not perfect, because the below is a compile-time check): | |
535 | ||
536 | #ifdef HAS_QUUX | |
537 | foo = quux(); | |
538 | #endif | |
539 | ||
540 | How does the HAS_QUUX become defined where it needs to be? Well, if | |
541 | Foonix happens to be Unixy enough to be able to run the Configure | |
542 | script, and Configure has been taught about detecting and testing | |
543 | quux(), the HAS_QUUX will be correctly defined. In other platforms, the | |
544 | corresponding configuration step will hopefully do the same. | |
545 | ||
546 | In a pinch, if you cannot wait for Configure to be educated, or if you | |
547 | have a good hunch of where quux() might be available, you can | |
548 | temporarily try the following: | |
549 | ||
550 | #if (defined(__FOONIX__) || defined(__BARNIX__)) | |
551 | # define HAS_QUUX | |
552 | #endif | |
553 | ||
554 | ... | |
555 | ||
556 | #ifdef HAS_QUUX | |
557 | foo = quux(); | |
558 | #endif | |
559 | ||
560 | But in any case, try to keep the features and operating systems | |
561 | separate. | |
562 | ||
563 | =back | |
564 | ||
565 | =head2 Problematic System Interfaces | |
566 | ||
567 | =over 4 | |
568 | ||
569 | =item * | |
570 | ||
571 | malloc(0), realloc(0), calloc(0, 0) are non-portable. To be portable | |
572 | allocate at least one byte. (In general you should rarely need to work | |
573 | at this low level, but instead use the various malloc wrappers.) | |
574 | ||
575 | =item * | |
576 | ||
577 | snprintf() - the return type is unportable. Use my_snprintf() instead. | |
578 | ||
579 | =back | |
580 | ||
581 | =head2 Security problems | |
582 | ||
583 | Last but not least, here are various tips for safer coding. | |
584 | ||
585 | =over 4 | |
586 | ||
587 | =item * | |
588 | ||
589 | Do not use gets() | |
590 | ||
591 | Or we will publicly ridicule you. Seriously. | |
592 | ||
593 | =item * | |
594 | ||
595 | Do not use strcpy() or strcat() or strncpy() or strncat() | |
596 | ||
597 | Use my_strlcpy() and my_strlcat() instead: they either use the native | |
598 | implementation, or Perl's own implementation (borrowed from the public | |
599 | domain implementation of INN). | |
600 | ||
601 | =item * | |
602 | ||
603 | Do not use sprintf() or vsprintf() | |
604 | ||
605 | If you really want just plain byte strings, use my_snprintf() and | |
606 | my_vsnprintf() instead, which will try to use snprintf() and | |
607 | vsnprintf() if those safer APIs are available. If you want something | |
608 | fancier than a plain byte string, use SVs and Perl_sv_catpvf(). | |
609 | ||
610 | =back | |
611 | ||
612 | =head1 DEBUGGING | |
613 | ||
614 | You can compile a special debugging version of Perl, which allows you | |
615 | to use the C<-D> option of Perl to tell more about what Perl is doing. | |
616 | But sometimes there is no alternative than to dive in with a debugger, | |
617 | either to see the stack trace of a core dump (very useful in a bug | |
618 | report), or trying to figure out what went wrong before the core dump | |
619 | happened, or how did we end up having wrong or unexpected results. | |
620 | ||
621 | =head2 Poking at Perl | |
622 | ||
623 | To really poke around with Perl, you'll probably want to build Perl for | |
624 | debugging, like this: | |
625 | ||
626 | ./Configure -d -D optimize=-g | |
627 | make | |
628 | ||
629 | C<-g> is a flag to the C compiler to have it produce debugging | |
630 | information which will allow us to step through a running program, and | |
631 | to see in which C function we are at (without the debugging information | |
632 | we might see only the numerical addresses of the functions, which is | |
633 | not very helpful). | |
634 | ||
635 | F<Configure> will also turn on the C<DEBUGGING> compilation symbol | |
636 | which enables all the internal debugging code in Perl. There are a | |
637 | whole bunch of things you can debug with this: L<perlrun> lists them | |
638 | all, and the best way to find out about them is to play about with | |
639 | them. The most useful options are probably | |
640 | ||
641 | l Context (loop) stack processing | |
642 | t Trace execution | |
643 | o Method and overloading resolution | |
644 | c String/numeric conversions | |
645 | ||
646 | Some of the functionality of the debugging code can be achieved using | |
647 | XS modules. | |
648 | ||
649 | -Dr => use re 'debug' | |
650 | -Dx => use O 'Debug' | |
651 | ||
652 | =head2 Using a source-level debugger | |
653 | ||
654 | If the debugging output of C<-D> doesn't help you, it's time to step | |
655 | through perl's execution with a source-level debugger. | |
656 | ||
657 | =over 3 | |
658 | ||
659 | =item * | |
660 | ||
661 | We'll use C<gdb> for our examples here; the principles will apply to | |
662 | any debugger (many vendors call their debugger C<dbx>), but check the | |
663 | manual of the one you're using. | |
664 | ||
665 | =back | |
666 | ||
667 | To fire up the debugger, type | |
668 | ||
669 | gdb ./perl | |
670 | ||
671 | Or if you have a core dump: | |
672 | ||
673 | gdb ./perl core | |
674 | ||
675 | You'll want to do that in your Perl source tree so the debugger can | |
676 | read the source code. You should see the copyright message, followed by | |
677 | the prompt. | |
678 | ||
679 | (gdb) | |
680 | ||
681 | C<help> will get you into the documentation, but here are the most | |
682 | useful commands: | |
683 | ||
684 | =over 3 | |
685 | ||
686 | =item * run [args] | |
687 | ||
688 | Run the program with the given arguments. | |
689 | ||
690 | =item * break function_name | |
691 | ||
692 | =item * break source.c:xxx | |
693 | ||
694 | Tells the debugger that we'll want to pause execution when we reach | |
695 | either the named function (but see L<perlguts/Internal Functions>!) or | |
696 | the given line in the named source file. | |
697 | ||
698 | =item * step | |
699 | ||
700 | Steps through the program a line at a time. | |
701 | ||
702 | =item * next | |
703 | ||
704 | Steps through the program a line at a time, without descending into | |
705 | functions. | |
706 | ||
707 | =item * continue | |
708 | ||
709 | Run until the next breakpoint. | |
710 | ||
711 | =item * finish | |
712 | ||
713 | Run until the end of the current function, then stop again. | |
714 | ||
715 | =item * 'enter' | |
716 | ||
717 | Just pressing Enter will do the most recent operation again - it's a | |
718 | blessing when stepping through miles of source code. | |
719 | ||
720 | =item * print | |
721 | ||
722 | Execute the given C code and print its results. B<WARNING>: Perl makes | |
723 | heavy use of macros, and F<gdb> does not necessarily support macros | |
724 | (see later L</"gdb macro support">). You'll have to substitute them | |
725 | yourself, or to invoke cpp on the source code files (see L</"The .i | |
726 | Targets">) So, for instance, you can't say | |
727 | ||
728 | print SvPV_nolen(sv) | |
729 | ||
730 | but you have to say | |
731 | ||
732 | print Perl_sv_2pv_nolen(sv) | |
733 | ||
734 | =back | |
735 | ||
736 | You may find it helpful to have a "macro dictionary", which you can | |
737 | produce by saying C<cpp -dM perl.c | sort>. Even then, F<cpp> won't | |
738 | recursively apply those macros for you. | |
739 | ||
740 | =head2 gdb macro support | |
741 | ||
742 | Recent versions of F<gdb> have fairly good macro support, but in order | |
743 | to use it you'll need to compile perl with macro definitions included | |
744 | in the debugging information. Using F<gcc> version 3.1, this means | |
745 | configuring with C<-Doptimize=-g3>. Other compilers might use a | |
746 | different switch (if they support debugging macros at all). | |
747 | ||
748 | =head2 Dumping Perl Data Structures | |
749 | ||
750 | One way to get around this macro hell is to use the dumping functions | |
751 | in F<dump.c>; these work a little like an internal | |
752 | L<Devel::Peek|Devel::Peek>, but they also cover OPs and other | |
753 | structures that you can't get at from Perl. Let's take an example. | |
754 | We'll use the C<$a = $b + $c> we used before, but give it a bit of | |
755 | context: C<$b = "6XXXX"; $c = 2.3;>. Where's a good place to stop and | |
756 | poke around? | |
757 | ||
758 | What about C<pp_add>, the function we examined earlier to implement the | |
759 | C<+> operator: | |
760 | ||
761 | (gdb) break Perl_pp_add | |
762 | Breakpoint 1 at 0x46249f: file pp_hot.c, line 309. | |
763 | ||
764 | Notice we use C<Perl_pp_add> and not C<pp_add> - see | |
765 | L<perlguts/Internal Functions>. With the breakpoint in place, we can | |
766 | run our program: | |
767 | ||
768 | (gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c' | |
769 | ||
770 | Lots of junk will go past as gdb reads in the relevant source files and | |
771 | libraries, and then: | |
772 | ||
773 | Breakpoint 1, Perl_pp_add () at pp_hot.c:309 | |
774 | 309 dSP; dATARGET; tryAMAGICbin(add,opASSIGN); | |
775 | (gdb) step | |
776 | 311 dPOPTOPnnrl_ul; | |
777 | (gdb) | |
778 | ||
779 | We looked at this bit of code before, and we said that | |
780 | C<dPOPTOPnnrl_ul> arranges for two C<NV>s to be placed into C<left> and | |
781 | C<right> - let's slightly expand it: | |
782 | ||
783 | #define dPOPTOPnnrl_ul NV right = POPn; \ | |
784 | SV *leftsv = TOPs; \ | |
785 | NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0 | |
786 | ||
787 | C<POPn> takes the SV from the top of the stack and obtains its NV | |
788 | either directly (if C<SvNOK> is set) or by calling the C<sv_2nv> | |
789 | function. C<TOPs> takes the next SV from the top of the stack - yes, | |
790 | C<POPn> uses C<TOPs> - but doesn't remove it. We then use C<SvNV> to | |
791 | get the NV from C<leftsv> in the same way as before - yes, C<POPn> uses | |
792 | C<SvNV>. | |
793 | ||
794 | Since we don't have an NV for C<$b>, we'll have to use C<sv_2nv> to | |
795 | convert it. If we step again, we'll find ourselves there: | |
796 | ||
797 | Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669 | |
798 | 1669 if (!sv) | |
799 | (gdb) | |
800 | ||
801 | We can now use C<Perl_sv_dump> to investigate the SV: | |
802 | ||
803 | SV = PV(0xa057cc0) at 0xa0675d0 | |
804 | REFCNT = 1 | |
805 | FLAGS = (POK,pPOK) | |
806 | PV = 0xa06a510 "6XXXX"\0 | |
807 | CUR = 5 | |
808 | LEN = 6 | |
809 | $1 = void | |
810 | ||
811 | We know we're going to get C<6> from this, so let's finish the | |
812 | subroutine: | |
813 | ||
814 | (gdb) finish | |
815 | Run till exit from #0 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671 | |
816 | 0x462669 in Perl_pp_add () at pp_hot.c:311 | |
817 | 311 dPOPTOPnnrl_ul; | |
818 | ||
819 | We can also dump out this op: the current op is always stored in | |
820 | C<PL_op>, and we can dump it with C<Perl_op_dump>. This'll give us | |
821 | similar output to L<B::Debug|B::Debug>. | |
822 | ||
823 | { | |
824 | 13 TYPE = add ===> 14 | |
825 | TARG = 1 | |
826 | FLAGS = (SCALAR,KIDS) | |
827 | { | |
828 | TYPE = null ===> (12) | |
829 | (was rv2sv) | |
830 | FLAGS = (SCALAR,KIDS) | |
831 | { | |
832 | 11 TYPE = gvsv ===> 12 | |
833 | FLAGS = (SCALAR) | |
834 | GV = main::b | |
835 | } | |
836 | } | |
837 | ||
838 | # finish this later # | |
839 | ||
840 | =head1 SOURCE CODE STATIC ANALYSIS | |
841 | ||
842 | Various tools exist for analysing C source code B<statically>, as | |
843 | opposed to B<dynamically>, that is, without executing the code. It is | |
844 | possible to detect resource leaks, undefined behaviour, type | |
845 | mismatches, portability problems, code paths that would cause illegal | |
846 | memory accesses, and other similar problems by just parsing the C code | |
847 | and looking at the resulting graph, what does it tell about the | |
848 | execution and data flows. As a matter of fact, this is exactly how C | |
849 | compilers know to give warnings about dubious code. | |
850 | ||
851 | =head2 lint, splint | |
852 | ||
853 | The good old C code quality inspector, C<lint>, is available in several | |
854 | platforms, but please be aware that there are several different | |
855 | implementations of it by different vendors, which means that the flags | |
856 | are not identical across different platforms. | |
857 | ||
858 | There is a lint variant called C<splint> (Secure Programming Lint) | |
859 | available from http://www.splint.org/ that should compile on any | |
860 | Unix-like platform. | |
861 | ||
862 | There are C<lint> and <splint> targets in Makefile, but you may have to | |
863 | diddle with the flags (see above). | |
864 | ||
865 | =head2 Coverity | |
866 | ||
867 | Coverity (http://www.coverity.com/) is a product similar to lint and as | |
868 | a testbed for their product they periodically check several open source | |
869 | projects, and they give out accounts to open source developers to the | |
870 | defect databases. | |
871 | ||
872 | =head2 cpd (cut-and-paste detector) | |
873 | ||
874 | The cpd tool detects cut-and-paste coding. If one instance of the | |
875 | cut-and-pasted code changes, all the other spots should probably be | |
876 | changed, too. Therefore such code should probably be turned into a | |
877 | subroutine or a macro. | |
878 | ||
879 | cpd (http://pmd.sourceforge.net/cpd.html) is part of the pmd project | |
880 | (http://pmd.sourceforge.net/). pmd was originally written for static | |
881 | analysis of Java code, but later the cpd part of it was extended to | |
882 | parse also C and C++. | |
883 | ||
884 | Download the pmd-bin-X.Y.zip () from the SourceForge site, extract the | |
885 | pmd-X.Y.jar from it, and then run that on source code thusly: | |
886 | ||
0cbf2b31 FC |
887 | java -cp pmd-X.Y.jar net.sourceforge.pmd.cpd.CPD \ |
888 | --minimum-tokens 100 --files /some/where/src --language c > cpd.txt | |
04c692a8 DR |
889 | |
890 | You may run into memory limits, in which case you should use the -Xmx | |
891 | option: | |
892 | ||
893 | java -Xmx512M ... | |
894 | ||
895 | =head2 gcc warnings | |
896 | ||
897 | Though much can be written about the inconsistency and coverage | |
898 | problems of gcc warnings (like C<-Wall> not meaning "all the warnings", | |
899 | or some common portability problems not being covered by C<-Wall>, or | |
900 | C<-ansi> and C<-pedantic> both being a poorly defined collection of | |
901 | warnings, and so forth), gcc is still a useful tool in keeping our | |
902 | coding nose clean. | |
903 | ||
904 | The C<-Wall> is by default on. | |
905 | ||
906 | The C<-ansi> (and its sidekick, C<-pedantic>) would be nice to be on | |
907 | always, but unfortunately they are not safe on all platforms, they can | |
908 | for example cause fatal conflicts with the system headers (Solaris | |
909 | being a prime example). If Configure C<-Dgccansipedantic> is used, the | |
910 | C<cflags> frontend selects C<-ansi -pedantic> for the platforms where | |
911 | they are known to be safe. | |
912 | ||
913 | Starting from Perl 5.9.4 the following extra flags are added: | |
914 | ||
915 | =over 4 | |
916 | ||
917 | =item * | |
918 | ||
919 | C<-Wendif-labels> | |
920 | ||
921 | =item * | |
922 | ||
923 | C<-Wextra> | |
924 | ||
925 | =item * | |
926 | ||
927 | C<-Wdeclaration-after-statement> | |
928 | ||
929 | =back | |
930 | ||
931 | The following flags would be nice to have but they would first need | |
932 | their own Augean stablemaster: | |
933 | ||
934 | =over 4 | |
935 | ||
936 | =item * | |
937 | ||
938 | C<-Wpointer-arith> | |
939 | ||
940 | =item * | |
941 | ||
942 | C<-Wshadow> | |
943 | ||
944 | =item * | |
945 | ||
946 | C<-Wstrict-prototypes> | |
947 | ||
948 | =back | |
949 | ||
950 | The C<-Wtraditional> is another example of the annoying tendency of gcc | |
951 | to bundle a lot of warnings under one switch (it would be impossible to | |
952 | deploy in practice because it would complain a lot) but it does contain | |
953 | some warnings that would be beneficial to have available on their own, | |
954 | such as the warning about string constants inside macros containing the | |
955 | macro arguments: this behaved differently pre-ANSI than it does in | |
956 | ANSI, and some C compilers are still in transition, AIX being an | |
957 | example. | |
958 | ||
959 | =head2 Warnings of other C compilers | |
960 | ||
961 | Other C compilers (yes, there B<are> other C compilers than gcc) often | |
962 | have their "strict ANSI" or "strict ANSI with some portability | |
963 | extensions" modes on, like for example the Sun Workshop has its C<-Xa> | |
964 | mode on (though implicitly), or the DEC (these days, HP...) has its | |
965 | C<-std1> mode on. | |
966 | ||
967 | =head1 MEMORY DEBUGGERS | |
968 | ||
d1fd4856 VP |
969 | B<NOTE 1>: Running under older memory debuggers such as Purify, |
970 | valgrind or Third Degree greatly slows down the execution: seconds | |
971 | become minutes, minutes become hours. For example as of Perl 5.8.1, the | |
04c692a8 DR |
972 | ext/Encode/t/Unicode.t takes extraordinarily long to complete under |
973 | e.g. Purify, Third Degree, and valgrind. Under valgrind it takes more | |
974 | than six hours, even on a snappy computer. The said test must be doing | |
975 | something that is quite unfriendly for memory debuggers. If you don't | |
976 | feel like waiting, that you can simply kill away the perl process. | |
d1fd4856 VP |
977 | Roughly valgrind slows down execution by factor 10, AddressSanitizer by |
978 | factor 2. | |
04c692a8 DR |
979 | |
980 | B<NOTE 2>: To minimize the number of memory leak false alarms (see | |
981 | L</PERL_DESTRUCT_LEVEL> for more information), you have to set the | |
d8651d0d | 982 | environment variable PERL_DESTRUCT_LEVEL to 2. For example, like this: |
04c692a8 DR |
983 | |
984 | env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib ... | |
985 | ||
986 | B<NOTE 3>: There are known memory leaks when there are compile-time | |
987 | errors within eval or require, seeing C<S_doeval> in the call stack is | |
988 | a good sign of these. Fixing these leaks is non-trivial, unfortunately, | |
989 | but they must be fixed eventually. | |
990 | ||
991 | B<NOTE 4>: L<DynaLoader> will not clean up after itself completely | |
992 | unless Perl is built with the Configure option | |
993 | C<-Accflags=-DDL_UNLOAD_ALL_AT_EXIT>. | |
994 | ||
04c692a8 DR |
995 | =head2 valgrind |
996 | ||
d1fd4856 VP |
997 | The valgrind tool can be used to find out both memory leaks and illegal |
998 | heap memory accesses. As of version 3.3.0, Valgrind only supports Linux | |
999 | on x86, x86-64 and PowerPC and Darwin (OS X) on x86 and x86-64). The | |
1000 | special "test.valgrind" target can be used to run the tests under | |
1001 | valgrind. Found errors and memory leaks are logged in files named | |
1002 | F<testfile.valgrind>. | |
04c692a8 DR |
1003 | |
1004 | Valgrind also provides a cachegrind tool, invoked on perl as: | |
1005 | ||
1006 | VG_OPTS=--tool=cachegrind make test.valgrind | |
1007 | ||
1008 | As system libraries (most notably glibc) are also triggering errors, | |
1009 | valgrind allows to suppress such errors using suppression files. The | |
1010 | default suppression file that comes with valgrind already catches a lot | |
1011 | of them. Some additional suppressions are defined in F<t/perl.supp>. | |
1012 | ||
1013 | To get valgrind and for more information see | |
1014 | ||
0061d4fa | 1015 | http://valgrind.org/ |
04c692a8 | 1016 | |
81c3bbe7 RU |
1017 | =head2 AddressSanitizer |
1018 | ||
4dd56148 NC |
1019 | AddressSanitizer is a clang and gcc extension, included in clang since |
1020 | v3.1 and gcc since v4.8. It checks illegal heap pointers, global | |
1021 | pointers, stack pointers and use after free errors, and is fast enough | |
1022 | that you can easily compile your debugging or optimized perl with it. | |
1023 | It does not check memory leaks though. AddressSanitizer is available | |
1024 | for Linux, Mac OS X and soon on Windows. | |
81c3bbe7 | 1025 | |
8a64fbaa VP |
1026 | To build perl with AddressSanitizer, your Configure invocation should |
1027 | look like: | |
81c3bbe7 | 1028 | |
e8596d90 VP |
1029 | sh Configure -des -Dcc=clang \ |
1030 | -Accflags=-faddress-sanitizer -Aldflags=-faddress-sanitizer \ | |
1031 | -Alddlflags=-shared\ -faddress-sanitizer | |
81c3bbe7 RU |
1032 | |
1033 | where these arguments mean: | |
1034 | ||
1035 | =over 4 | |
1036 | ||
1037 | =item * -Dcc=clang | |
1038 | ||
8a64fbaa VP |
1039 | This should be replaced by the full path to your clang executable if it |
1040 | is not in your path. | |
81c3bbe7 RU |
1041 | |
1042 | =item * -Accflags=-faddress-sanitizer | |
1043 | ||
8a64fbaa | 1044 | Compile perl and extensions sources with AddressSanitizer. |
81c3bbe7 RU |
1045 | |
1046 | =item * -Aldflags=-faddress-sanitizer | |
1047 | ||
8a64fbaa | 1048 | Link the perl executable with AddressSanitizer. |
81c3bbe7 | 1049 | |
e8596d90 | 1050 | =item * -Alddlflags=-shared\ -faddress-sanitizer |
81c3bbe7 | 1051 | |
e8596d90 VP |
1052 | Link dynamic extensions with AddressSanitizer. You must manually |
1053 | specify C<-shared> because using C<-Alddlflags=-shared> will prevent | |
1054 | Configure from setting a default value for C<lddlflags>, which usually | |
5dfc6e97 | 1055 | contains C<-shared> (at least on Linux). |
81c3bbe7 RU |
1056 | |
1057 | =back | |
1058 | ||
8a64fbaa VP |
1059 | See also |
1060 | L<http://code.google.com/p/address-sanitizer/wiki/AddressSanitizer>. | |
81c3bbe7 RU |
1061 | |
1062 | ||
04c692a8 DR |
1063 | =head1 PROFILING |
1064 | ||
1065 | Depending on your platform there are various ways of profiling Perl. | |
1066 | ||
1067 | There are two commonly used techniques of profiling executables: | |
1068 | I<statistical time-sampling> and I<basic-block counting>. | |
1069 | ||
1070 | The first method takes periodically samples of the CPU program counter, | |
1071 | and since the program counter can be correlated with the code generated | |
1072 | for functions, we get a statistical view of in which functions the | |
1073 | program is spending its time. The caveats are that very small/fast | |
1074 | functions have lower probability of showing up in the profile, and that | |
1075 | periodically interrupting the program (this is usually done rather | |
1076 | frequently, in the scale of milliseconds) imposes an additional | |
1077 | overhead that may skew the results. The first problem can be alleviated | |
1078 | by running the code for longer (in general this is a good idea for | |
1079 | profiling), the second problem is usually kept in guard by the | |
1080 | profiling tools themselves. | |
1081 | ||
1082 | The second method divides up the generated code into I<basic blocks>. | |
1083 | Basic blocks are sections of code that are entered only in the | |
1084 | beginning and exited only at the end. For example, a conditional jump | |
1085 | starts a basic block. Basic block profiling usually works by | |
1086 | I<instrumenting> the code by adding I<enter basic block #nnnn> | |
1087 | book-keeping code to the generated code. During the execution of the | |
1088 | code the basic block counters are then updated appropriately. The | |
1089 | caveat is that the added extra code can skew the results: again, the | |
1090 | profiling tools usually try to factor their own effects out of the | |
1091 | results. | |
1092 | ||
1093 | =head2 Gprof Profiling | |
1094 | ||
e2aed43d NC |
1095 | I<gprof> is a profiling tool available in many Unix platforms which |
1096 | uses I<statistical time-sampling>. You can build a profiled version of | |
1097 | F<perl> by compiling using gcc with the flag C<-pg>. Either edit | |
1098 | F<config.sh> or re-run F<Configure>. Running the profiled version of | |
1099 | Perl will create an output file called F<gmon.out> which contains the | |
1100 | profiling data collected during the execution. | |
04c692a8 | 1101 | |
e2aed43d NC |
1102 | quick hint: |
1103 | ||
1104 | $ sh Configure -des -Dusedevel -Accflags='-pg' \ | |
1105 | -Aldflags='-pg' -Alddlflags='-pg -shared' \ | |
1106 | && make perl | |
1107 | $ ./perl ... # creates gmon.out in current directory | |
1108 | $ gprof ./perl > out | |
1109 | $ less out | |
1110 | ||
1111 | (you probably need to add C<-shared> to the <-Alddlflags> line until RT | |
1112 | #118199 is resolved) | |
04c692a8 | 1113 | |
e2aed43d NC |
1114 | The F<gprof> tool can then display the collected data in various ways. |
1115 | Usually F<gprof> understands the following options: | |
04c692a8 DR |
1116 | |
1117 | =over 4 | |
1118 | ||
1119 | =item * -a | |
1120 | ||
1121 | Suppress statically defined functions from the profile. | |
1122 | ||
1123 | =item * -b | |
1124 | ||
1125 | Suppress the verbose descriptions in the profile. | |
1126 | ||
1127 | =item * -e routine | |
1128 | ||
1129 | Exclude the given routine and its descendants from the profile. | |
1130 | ||
1131 | =item * -f routine | |
1132 | ||
1133 | Display only the given routine and its descendants in the profile. | |
1134 | ||
1135 | =item * -s | |
1136 | ||
1137 | Generate a summary file called F<gmon.sum> which then may be given to | |
1138 | subsequent gprof runs to accumulate data over several runs. | |
1139 | ||
1140 | =item * -z | |
1141 | ||
1142 | Display routines that have zero usage. | |
1143 | ||
1144 | =back | |
1145 | ||
1146 | For more detailed explanation of the available commands and output | |
e2aed43d | 1147 | formats, see your own local documentation of F<gprof>. |
04c692a8 | 1148 | |
e2aed43d | 1149 | =head2 GCC gcov Profiling |
04c692a8 | 1150 | |
e2aed43d NC |
1151 | I<basic block profiling> is officially available in gcc 3.0 and later. |
1152 | You can build a profiled version of F<perl> by compiling using gcc with | |
1153 | the flags C<-fprofile-arcs -ftest-coverage>. Either edit F<config.sh> | |
1154 | or re-run F<Configure>. | |
04c692a8 | 1155 | |
e2aed43d | 1156 | quick hint: |
04c692a8 | 1157 | |
e2aed43d NC |
1158 | $ sh Configure -des -Dusedevel -Doptimize='-g' \ |
1159 | -Accflags='-fprofile-arcs -ftest-coverage' \ | |
1160 | -Aldflags='-fprofile-arcs -ftest-coverage' \ | |
1161 | -Alddlflags='-fprofile-arcs -ftest-coverage -shared' \ | |
1162 | && make perl | |
1163 | $ rm -f regexec.c.gcov regexec.gcda | |
1164 | $ ./perl ... | |
1165 | $ gcov regexec.c | |
1166 | $ less regexec.c.gcov | |
04c692a8 | 1167 | |
e2aed43d NC |
1168 | (you probably need to add C<-shared> to the <-Alddlflags> line until RT |
1169 | #118199 is resolved) | |
04c692a8 DR |
1170 | |
1171 | Running the profiled version of Perl will cause profile output to be | |
e2aed43d | 1172 | generated. For each source file an accompanying F<.gcda> file will be |
04c692a8 DR |
1173 | created. |
1174 | ||
e2aed43d | 1175 | To display the results you use the I<gcov> utility (which should be |
04c692a8 DR |
1176 | installed if you have gcc 3.0 or newer installed). F<gcov> is run on |
1177 | source code files, like this | |
1178 | ||
1179 | gcov sv.c | |
1180 | ||
1181 | which will cause F<sv.c.gcov> to be created. The F<.gcov> files contain | |
1182 | the source code annotated with relative frequencies of execution | |
6f134219 NC |
1183 | indicated by "#" markers. If you want to generate F<.gcov> files for |
1184 | all profiled object files, you can run something like this: | |
1185 | ||
1186 | for file in `find . -name \*.gcno` | |
1187 | do sh -c "cd `dirname $file` && gcov `basename $file .gcno`" | |
1188 | done | |
04c692a8 DR |
1189 | |
1190 | Useful options of F<gcov> include C<-b> which will summarise the basic | |
1191 | block, branch, and function call coverage, and C<-c> which instead of | |
1192 | relative frequencies will use the actual counts. For more information | |
1193 | on the use of F<gcov> and basic block profiling with gcc, see the | |
e2aed43d NC |
1194 | latest GNU CC manual. As of gcc 4.8, this is at |
1195 | L<http://gcc.gnu.org/onlinedocs/gcc/Gcov-Intro.html#Gcov-Intro> | |
04c692a8 DR |
1196 | |
1197 | =head1 MISCELLANEOUS TRICKS | |
1198 | ||
1199 | =head2 PERL_DESTRUCT_LEVEL | |
1200 | ||
1201 | If you want to run any of the tests yourself manually using e.g. | |
4dd56148 NC |
1202 | valgrind, please note that by default perl B<does not> explicitly |
1203 | cleanup all the memory it has allocated (such as global memory arenas) | |
1204 | but instead lets the exit() of the whole program "take care" of such | |
1205 | allocations, also known as "global destruction of objects". | |
04c692a8 DR |
1206 | |
1207 | There is a way to tell perl to do complete cleanup: set the environment | |
1208 | variable PERL_DESTRUCT_LEVEL to a non-zero value. The t/TEST wrapper | |
1209 | does set this to 2, and this is what you need to do too, if you don't | |
f01ecde8 | 1210 | want to see the "global leaks": For example, for running under valgrind |
04c692a8 | 1211 | |
f01ecde8 | 1212 | env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib t/foo/bar.t |
04c692a8 DR |
1213 | |
1214 | (Note: the mod_perl apache module uses also this environment variable | |
1215 | for its own purposes and extended its semantics. Refer to the mod_perl | |
1216 | documentation for more information. Also, spawned threads do the | |
1217 | equivalent of setting this variable to the value 1.) | |
1218 | ||
1219 | If, at the end of a run you get the message I<N scalars leaked>, you | |
1220 | can recompile with C<-DDEBUG_LEAKING_SCALARS>, which will cause the | |
1221 | addresses of all those leaked SVs to be dumped along with details as to | |
1222 | where each SV was originally allocated. This information is also | |
1223 | displayed by Devel::Peek. Note that the extra details recorded with | |
1224 | each SV increases memory usage, so it shouldn't be used in production | |
1225 | environments. It also converts C<new_SV()> from a macro into a real | |
1226 | function, so you can use your favourite debugger to discover where | |
1227 | those pesky SVs were allocated. | |
1228 | ||
1229 | If you see that you're leaking memory at runtime, but neither valgrind | |
1230 | nor C<-DDEBUG_LEAKING_SCALARS> will find anything, you're probably | |
1231 | leaking SVs that are still reachable and will be properly cleaned up | |
1232 | during destruction of the interpreter. In such cases, using the C<-Dm> | |
1233 | switch can point you to the source of the leak. If the executable was | |
1234 | built with C<-DDEBUG_LEAKING_SCALARS>, C<-Dm> will output SV | |
1235 | allocations in addition to memory allocations. Each SV allocation has a | |
1236 | distinct serial number that will be written on creation and destruction | |
1237 | of the SV. So if you're executing the leaking code in a loop, you need | |
1238 | to look for SVs that are created, but never destroyed between each | |
1239 | cycle. If such an SV is found, set a conditional breakpoint within | |
1240 | C<new_SV()> and make it break only when C<PL_sv_serial> is equal to the | |
1241 | serial number of the leaking SV. Then you will catch the interpreter in | |
1242 | exactly the state where the leaking SV is allocated, which is | |
1243 | sufficient in many cases to find the source of the leak. | |
1244 | ||
1245 | As C<-Dm> is using the PerlIO layer for output, it will by itself | |
1246 | allocate quite a bunch of SVs, which are hidden to avoid recursion. You | |
1247 | can bypass the PerlIO layer if you use the SV logging provided by | |
1248 | C<-DPERL_MEM_LOG> instead. | |
1249 | ||
1250 | =head2 PERL_MEM_LOG | |
1251 | ||
1252 | If compiled with C<-DPERL_MEM_LOG>, both memory and SV allocations go | |
1253 | through logging functions, which is handy for breakpoint setting. | |
1254 | ||
1255 | Unless C<-DPERL_MEM_LOG_NOIMPL> is also compiled, the logging functions | |
1256 | read $ENV{PERL_MEM_LOG} to determine whether to log the event, and if | |
1257 | so how: | |
1258 | ||
1259 | $ENV{PERL_MEM_LOG} =~ /m/ Log all memory ops | |
1260 | $ENV{PERL_MEM_LOG} =~ /s/ Log all SV ops | |
1261 | $ENV{PERL_MEM_LOG} =~ /t/ include timestamp in Log | |
1262 | $ENV{PERL_MEM_LOG} =~ /^(\d+)/ write to FD given (default is 2) | |
1263 | ||
1264 | Memory logging is somewhat similar to C<-Dm> but is independent of | |
1265 | C<-DDEBUGGING>, and at a higher level; all uses of Newx(), Renew(), and | |
1266 | Safefree() are logged with the caller's source code file and line | |
1267 | number (and C function name, if supported by the C compiler). In | |
1268 | contrast, C<-Dm> is directly at the point of C<malloc()>. SV logging is | |
1269 | similar. | |
1270 | ||
1271 | Since the logging doesn't use PerlIO, all SV allocations are logged and | |
1272 | no extra SV allocations are introduced by enabling the logging. If | |
1273 | compiled with C<-DDEBUG_LEAKING_SCALARS>, the serial number for each SV | |
1274 | allocation is also logged. | |
1275 | ||
1276 | =head2 DDD over gdb | |
1277 | ||
1278 | Those debugging perl with the DDD frontend over gdb may find the | |
1279 | following useful: | |
1280 | ||
1281 | You can extend the data conversion shortcuts menu, so for example you | |
1282 | can display an SV's IV value with one click, without doing any typing. | |
1283 | To do that simply edit ~/.ddd/init file and add after: | |
1284 | ||
1285 | ! Display shortcuts. | |
1286 | Ddd*gdbDisplayShortcuts: \ | |
1287 | /t () // Convert to Bin\n\ | |
1288 | /d () // Convert to Dec\n\ | |
1289 | /x () // Convert to Hex\n\ | |
1290 | /o () // Convert to Oct(\n\ | |
1291 | ||
1292 | the following two lines: | |
1293 | ||
1294 | ((XPV*) (())->sv_any )->xpv_pv // 2pvx\n\ | |
1295 | ((XPVIV*) (())->sv_any )->xiv_iv // 2ivx | |
1296 | ||
1297 | so now you can do ivx and pvx lookups or you can plug there the sv_peek | |
1298 | "conversion": | |
1299 | ||
1300 | Perl_sv_peek(my_perl, (SV*)()) // sv_peek | |
1301 | ||
1302 | (The my_perl is for threaded builds.) Just remember that every line, | |
1303 | but the last one, should end with \n\ | |
1304 | ||
1305 | Alternatively edit the init file interactively via: 3rd mouse button -> | |
1306 | New Display -> Edit Menu | |
1307 | ||
1308 | Note: you can define up to 20 conversion shortcuts in the gdb section. | |
1309 | ||
1310 | =head2 Poison | |
1311 | ||
1312 | If you see in a debugger a memory area mysteriously full of 0xABABABAB | |
1313 | or 0xEFEFEFEF, you may be seeing the effect of the Poison() macros, see | |
1314 | L<perlclib>. | |
1315 | ||
1316 | =head2 Read-only optrees | |
1317 | ||
1318 | Under ithreads the optree is read only. If you want to enforce this, to | |
1319 | check for write accesses from buggy code, compile with | |
04c692a8 | 1320 | C<-DPERL_DEBUG_READONLY_OPS> to enable code that allocates op memory |
4dd56148 NC |
1321 | via C<mmap>, and sets it read-only when it is attached to a subroutine. |
1322 | Any write access to an op results in a C<SIGBUS> and abort. | |
04c692a8 DR |
1323 | |
1324 | This code is intended for development only, and may not be portable | |
1325 | even to all Unix variants. Also, it is an 80% solution, in that it | |
4dd56148 NC |
1326 | isn't able to make all ops read only. Specifically it does not apply to |
1327 | op slabs belonging to C<BEGIN> blocks. | |
04c692a8 | 1328 | |
4dd56148 NC |
1329 | However, as an 80% solution it is still effective, as it has caught |
1330 | bugs in the past. | |
04c692a8 DR |
1331 | |
1332 | =head2 The .i Targets | |
1333 | ||
1334 | You can expand the macros in a F<foo.c> file by saying | |
1335 | ||
1336 | make foo.i | |
1337 | ||
d1fd4856 VP |
1338 | which will expand the macros using cpp. Don't be scared by the |
1339 | results. | |
04c692a8 DR |
1340 | |
1341 | =head1 AUTHOR | |
1342 | ||
1343 | This document was originally written by Nathan Torkington, and is | |
1344 | maintained by the perl5-porters mailing list. |