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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 | |
9b22382a | 15 | the Perl core C code. It covers common problems, debugging, profiling, |
04c692a8 DR |
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 | ||
9b22382a | 23 | Perl source plays by ANSI C89 rules: no C99 (or C++) extensions. In |
04c692a8 DR |
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 | |
9b22382a | 37 | function prototypes of Perl. You better try your changes with that. |
04c692a8 DR |
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 | |
9b22382a FC |
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 | |
04c692a8 DR |
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 | |
9b22382a | 57 | ways for things to go wrong. You should try it. |
04c692a8 DR |
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 | |
9b22382a | 65 | concurrency. The right way is to introduce them as new interpreter |
04c692a8 DR |
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 | |
9b22382a | 71 | BSD-style output) that the data you added really is read-only. (If it |
04c692a8 DR |
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 | |
9b22382a | 86 | C<-DPERL_GLOBAL_STRUCT_PRIVATE>. It is not normally used, but can be |
04c692a8 DR |
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 | |
9b22382a | 96 | explicitly marked as exported. See the discussion about F<embed.pl> in |
04c692a8 DR |
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 | |
9b22382a | 104 | arduous refactoring is now ready and correctly exported. So what could |
04c692a8 DR |
105 | possibly go wrong? |
106 | ||
107 | Maybe simply that your function did not need to be exported in the | |
9b22382a | 108 | first place. Perl has a long and not so glorious history of exporting |
04c692a8 DR |
109 | functions that it should not have. |
110 | ||
111 | If the function is used only inside one source code file, make it | |
9b22382a | 112 | static. See the discussion about F<embed.pl> in L<perlguts>. |
04c692a8 DR |
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 | |
9b22382a | 116 | it to the public API. See the discussion about F<embed.pl> in |
04c692a8 DR |
117 | L<perlguts>. |
118 | ||
119 | =back | |
120 | ||
121 | =head2 Portability problems | |
122 | ||
123 | The following are common causes of compilation and/or execution | |
9b22382a FC |
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 | |
04c692a8 DR |
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 | |
9b22382a | 130 | catch most of these unportabilities. (However it might also catch |
04c692a8 DR |
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 | |
9b22382a | 149 | still compile Perl with just the bare minimum of interfaces. (See |
04c692a8 DR |
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 | ||
9b22382a FC |
170 | Both are bad, and broken, and unportable. Use the PTR2IV() macro that |
171 | does it right. (Likewise, there are PTR2UV(), PTR2NV(), INT2PTR(), and | |
04c692a8 DR |
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 | |
9b22382a | 189 | shorts are 64 bits. This is all legal according to the C standard. (In |
04c692a8 DR |
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 | |
9b22382a | 196 | guaranteed to be B<int> or B<long>. If you really explicitly need |
04c692a8 DR |
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 | ||
9b22382a | 215 | Simply not portable. Get your lvalue to be of the right type, or maybe |
04c692a8 DR |
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 | ||
9b22382a | 275 | Perl can compile and run under EBCDIC platforms. See L<perlebcdic>. |
04c692a8 DR |
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 | |
9b22382a FC |
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 | |
9b22382a FC |
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 | |
04c692a8 DR |
287 | anything about other ranges. |
288 | ||
289 | Many of the comments in the existing code ignore the possibility of | |
9b22382a | 290 | EBCDIC, and may be wrong therefore, even if the code works. This is |
04c692a8 DR |
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 | |
9b22382a | 295 | Unicode code points as sequences of bytes. Macros with the same names |
04c692a8 DR |
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 | |
9b22382a FC |
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 | |
04c692a8 DR |
303 | are strict ASCII, 0 - 127) are invariant, and the documentation and |
304 | comments in the code may assume that, often referring to something | |
9b22382a | 305 | like, say, C<hibit>. The situation differs and is not so simple on |
04c692a8 DR |
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 | ||
9b22382a | 314 | ASCII is a 7 bit encoding, but bytes have 8 bits in them. The 128 extra |
04c692a8 DR |
315 | characters have different meanings depending on the locale. Absent a |
316 | locale, currently these extra characters are generally considered to be | |
9b22382a | 317 | unassigned, and this has presented some problems. This is being changed |
04c692a8 DR |
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 | ||
9b22382a | 332 | You cannot portably "stack" cpp directives. For example in the above |
04c692a8 DR |
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 | |
9b22382a | 346 | them. If you want to document what is going (which is a good idea |
04c692a8 DR |
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 | ||
9b22382a | 368 | is not portable. Leave out the last comma. |
04c692a8 DR |
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 | ||
9b22382a | 379 | That is C99 or C++. Perl is C89. Using the //-comments is silently |
04c692a8 DR |
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 | ||
9b22382a | 393 | That is C99 or C++. Some C compilers allow that, but you shouldn't. |
04c692a8 DR |
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 | ||
9b22382a | 404 | That is C99 or C++. While it would indeed be awfully nice to have that |
04c692a8 DR |
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 | |
9b22382a | 418 | fatal error. One cause for people often making this mistake is that a |
04c692a8 DR |
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 | |
9b22382a | 422 | unsigned. For this very same reason using a 'char' as an array index is |
04c692a8 DR |
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 | ||
9b22382a | 437 | which is probably not what you were expecting. Unfortunately at least |
04c692a8 DR |
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 | |
9b22382a | 450 | be an C<int>), in general it cannot. IV might be something larger. Even |
04c692a8 DR |
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 | |
9b22382a | 490 | with a standardized syntax. Don't use the former, and use the latter |
04c692a8 DR |
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) | |
9b22382a | 498 | functions. The right thing to do is to copy the va_list using the |
04c692a8 DR |
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 | ||
9b22382a | 507 | While a nice extension, it's not portable. The Perl code does |
04c692a8 DR |
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 | |
9b22382a | 533 | "Foonix", the above is very wrong. This is more correct (though still |
04c692a8 DR |
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 | |
9b22382a | 543 | quux(), the HAS_QUUX will be correctly defined. In other platforms, the |
04c692a8 DR |
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 | ||
9b22382a FC |
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 | |
04c692a8 DR |
573 | at this low level, but instead use the various malloc wrappers.) |
574 | ||
575 | =item * | |
576 | ||
9b22382a | 577 | snprintf() - the return type is unportable. Use my_snprintf() instead. |
04c692a8 DR |
578 | |
579 | =back | |
580 | ||
581 | =head2 Security problems | |
582 | ||
583 | Last but not least, here are various tips for safer coding. | |
bbc89b61 | 584 | See also L<perlclib> for libc/stdio replacements one should use. |
04c692a8 DR |
585 | |
586 | =over 4 | |
587 | ||
588 | =item * | |
589 | ||
590 | Do not use gets() | |
591 | ||
9b22382a | 592 | Or we will publicly ridicule you. Seriously. |
04c692a8 DR |
593 | |
594 | =item * | |
595 | ||
bbc89b61 JH |
596 | Do not use tmpfile() |
597 | ||
598 | Use mkstemp() instead. | |
599 | ||
600 | =item * | |
601 | ||
04c692a8 DR |
602 | Do not use strcpy() or strcat() or strncpy() or strncat() |
603 | ||
604 | Use my_strlcpy() and my_strlcat() instead: they either use the native | |
605 | implementation, or Perl's own implementation (borrowed from the public | |
606 | domain implementation of INN). | |
607 | ||
608 | =item * | |
609 | ||
610 | Do not use sprintf() or vsprintf() | |
611 | ||
612 | If you really want just plain byte strings, use my_snprintf() and | |
613 | my_vsnprintf() instead, which will try to use snprintf() and | |
9b22382a | 614 | vsnprintf() if those safer APIs are available. If you want something |
6bfe0388 KW |
615 | fancier than a plain byte string, use |
616 | L<C<Perl_form>()|perlapi/form> or SVs and | |
617 | L<C<Perl_sv_catpvf()>|perlapi/sv_catpvf>. | |
618 | ||
2e642750 KW |
619 | Note that glibc C<printf()>, C<sprintf()>, etc. are buggy before glibc |
620 | version 2.17. They won't allow a C<%.s> format with a precision to | |
621 | create a string that isn't valid UTF-8 if the current underlying locale | |
622 | of the program is UTF-8. What happens is that the C<%s> and its operand are | |
6bfe0388 | 623 | simply skipped without any notice. |
2e642750 | 624 | L<https://sourceware.org/bugzilla/show_bug.cgi?id=6530>. |
04c692a8 | 625 | |
c98823ff JH |
626 | =item * |
627 | ||
628 | Do not use atoi() | |
629 | ||
630 | Use grok_atou() instead. atoi() has ill-defined behavior on overflows, | |
631 | and cannot be used for incremental parsing. It is also affected by locale, | |
338aa8b0 JH |
632 | which is bad. |
633 | ||
634 | =item * | |
635 | ||
636 | Do not use strtol() or strtoul() | |
637 | ||
638 | Use grok_atou() instead. strtol() or strtoul() (or their IV/UV-friendly | |
639 | macro disguises, Strtol() and Strtoul(), or Atol() and Atoul() are | |
640 | affected by locale, which is bad. | |
c98823ff | 641 | |
04c692a8 DR |
642 | =back |
643 | ||
644 | =head1 DEBUGGING | |
645 | ||
646 | You can compile a special debugging version of Perl, which allows you | |
647 | to use the C<-D> option of Perl to tell more about what Perl is doing. | |
648 | But sometimes there is no alternative than to dive in with a debugger, | |
649 | either to see the stack trace of a core dump (very useful in a bug | |
650 | report), or trying to figure out what went wrong before the core dump | |
651 | happened, or how did we end up having wrong or unexpected results. | |
652 | ||
653 | =head2 Poking at Perl | |
654 | ||
655 | To really poke around with Perl, you'll probably want to build Perl for | |
656 | debugging, like this: | |
657 | ||
658 | ./Configure -d -D optimize=-g | |
659 | make | |
660 | ||
661 | C<-g> is a flag to the C compiler to have it produce debugging | |
662 | information which will allow us to step through a running program, and | |
663 | to see in which C function we are at (without the debugging information | |
664 | we might see only the numerical addresses of the functions, which is | |
665 | not very helpful). | |
666 | ||
667 | F<Configure> will also turn on the C<DEBUGGING> compilation symbol | |
9b22382a | 668 | which enables all the internal debugging code in Perl. There are a |
04c692a8 DR |
669 | whole bunch of things you can debug with this: L<perlrun> lists them |
670 | all, and the best way to find out about them is to play about with | |
9b22382a | 671 | them. The most useful options are probably |
04c692a8 DR |
672 | |
673 | l Context (loop) stack processing | |
674 | t Trace execution | |
675 | o Method and overloading resolution | |
676 | c String/numeric conversions | |
677 | ||
678 | Some of the functionality of the debugging code can be achieved using | |
679 | XS modules. | |
680 | ||
681 | -Dr => use re 'debug' | |
682 | -Dx => use O 'Debug' | |
683 | ||
684 | =head2 Using a source-level debugger | |
685 | ||
686 | If the debugging output of C<-D> doesn't help you, it's time to step | |
687 | through perl's execution with a source-level debugger. | |
688 | ||
689 | =over 3 | |
690 | ||
691 | =item * | |
692 | ||
693 | We'll use C<gdb> for our examples here; the principles will apply to | |
694 | any debugger (many vendors call their debugger C<dbx>), but check the | |
695 | manual of the one you're using. | |
696 | ||
697 | =back | |
698 | ||
699 | To fire up the debugger, type | |
700 | ||
701 | gdb ./perl | |
702 | ||
703 | Or if you have a core dump: | |
704 | ||
705 | gdb ./perl core | |
706 | ||
707 | You'll want to do that in your Perl source tree so the debugger can | |
9b22382a | 708 | read the source code. You should see the copyright message, followed by |
04c692a8 DR |
709 | the prompt. |
710 | ||
711 | (gdb) | |
712 | ||
713 | C<help> will get you into the documentation, but here are the most | |
714 | useful commands: | |
715 | ||
716 | =over 3 | |
717 | ||
718 | =item * run [args] | |
719 | ||
720 | Run the program with the given arguments. | |
721 | ||
722 | =item * break function_name | |
723 | ||
724 | =item * break source.c:xxx | |
725 | ||
726 | Tells the debugger that we'll want to pause execution when we reach | |
727 | either the named function (but see L<perlguts/Internal Functions>!) or | |
728 | the given line in the named source file. | |
729 | ||
730 | =item * step | |
731 | ||
732 | Steps through the program a line at a time. | |
733 | ||
734 | =item * next | |
735 | ||
736 | Steps through the program a line at a time, without descending into | |
737 | functions. | |
738 | ||
739 | =item * continue | |
740 | ||
741 | Run until the next breakpoint. | |
742 | ||
743 | =item * finish | |
744 | ||
745 | Run until the end of the current function, then stop again. | |
746 | ||
747 | =item * 'enter' | |
748 | ||
749 | Just pressing Enter will do the most recent operation again - it's a | |
750 | blessing when stepping through miles of source code. | |
751 | ||
8b029fdf MH |
752 | =item * ptype |
753 | ||
754 | Prints the C definition of the argument given. | |
755 | ||
756 | (gdb) ptype PL_op | |
757 | type = struct op { | |
758 | OP *op_next; | |
759 | OP *op_sibling; | |
760 | OP *(*op_ppaddr)(void); | |
761 | PADOFFSET op_targ; | |
762 | unsigned int op_type : 9; | |
763 | unsigned int op_opt : 1; | |
764 | unsigned int op_slabbed : 1; | |
765 | unsigned int op_savefree : 1; | |
766 | unsigned int op_static : 1; | |
767 | unsigned int op_folded : 1; | |
768 | unsigned int op_spare : 2; | |
769 | U8 op_flags; | |
770 | U8 op_private; | |
771 | } * | |
772 | ||
04c692a8 DR |
773 | =item * print |
774 | ||
9b22382a | 775 | Execute the given C code and print its results. B<WARNING>: Perl makes |
04c692a8 | 776 | heavy use of macros, and F<gdb> does not necessarily support macros |
9b22382a | 777 | (see later L</"gdb macro support">). You'll have to substitute them |
04c692a8 DR |
778 | yourself, or to invoke cpp on the source code files (see L</"The .i |
779 | Targets">) So, for instance, you can't say | |
780 | ||
781 | print SvPV_nolen(sv) | |
782 | ||
783 | but you have to say | |
784 | ||
785 | print Perl_sv_2pv_nolen(sv) | |
786 | ||
787 | =back | |
788 | ||
789 | You may find it helpful to have a "macro dictionary", which you can | |
9b22382a | 790 | produce by saying C<cpp -dM perl.c | sort>. Even then, F<cpp> won't |
04c692a8 DR |
791 | recursively apply those macros for you. |
792 | ||
793 | =head2 gdb macro support | |
794 | ||
795 | Recent versions of F<gdb> have fairly good macro support, but in order | |
796 | to use it you'll need to compile perl with macro definitions included | |
9b22382a FC |
797 | in the debugging information. Using F<gcc> version 3.1, this means |
798 | configuring with C<-Doptimize=-g3>. Other compilers might use a | |
04c692a8 DR |
799 | different switch (if they support debugging macros at all). |
800 | ||
801 | =head2 Dumping Perl Data Structures | |
802 | ||
803 | One way to get around this macro hell is to use the dumping functions | |
804 | in F<dump.c>; these work a little like an internal | |
805 | L<Devel::Peek|Devel::Peek>, but they also cover OPs and other | |
9b22382a | 806 | structures that you can't get at from Perl. Let's take an example. |
04c692a8 | 807 | We'll use the C<$a = $b + $c> we used before, but give it a bit of |
9b22382a | 808 | context: C<$b = "6XXXX"; $c = 2.3;>. Where's a good place to stop and |
04c692a8 DR |
809 | poke around? |
810 | ||
811 | What about C<pp_add>, the function we examined earlier to implement the | |
812 | C<+> operator: | |
813 | ||
814 | (gdb) break Perl_pp_add | |
815 | Breakpoint 1 at 0x46249f: file pp_hot.c, line 309. | |
816 | ||
817 | Notice we use C<Perl_pp_add> and not C<pp_add> - see | |
9b22382a | 818 | L<perlguts/Internal Functions>. With the breakpoint in place, we can |
04c692a8 DR |
819 | run our program: |
820 | ||
821 | (gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c' | |
822 | ||
823 | Lots of junk will go past as gdb reads in the relevant source files and | |
824 | libraries, and then: | |
825 | ||
826 | Breakpoint 1, Perl_pp_add () at pp_hot.c:309 | |
827 | 309 dSP; dATARGET; tryAMAGICbin(add,opASSIGN); | |
828 | (gdb) step | |
829 | 311 dPOPTOPnnrl_ul; | |
830 | (gdb) | |
831 | ||
832 | We looked at this bit of code before, and we said that | |
833 | C<dPOPTOPnnrl_ul> arranges for two C<NV>s to be placed into C<left> and | |
834 | C<right> - let's slightly expand it: | |
835 | ||
836 | #define dPOPTOPnnrl_ul NV right = POPn; \ | |
837 | SV *leftsv = TOPs; \ | |
838 | NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0 | |
839 | ||
840 | C<POPn> takes the SV from the top of the stack and obtains its NV | |
841 | either directly (if C<SvNOK> is set) or by calling the C<sv_2nv> | |
9b22382a FC |
842 | function. C<TOPs> takes the next SV from the top of the stack - yes, |
843 | C<POPn> uses C<TOPs> - but doesn't remove it. We then use C<SvNV> to | |
04c692a8 DR |
844 | get the NV from C<leftsv> in the same way as before - yes, C<POPn> uses |
845 | C<SvNV>. | |
846 | ||
847 | Since we don't have an NV for C<$b>, we'll have to use C<sv_2nv> to | |
9b22382a | 848 | convert it. If we step again, we'll find ourselves there: |
04c692a8 | 849 | |
8b029fdf | 850 | (gdb) step |
04c692a8 DR |
851 | Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669 |
852 | 1669 if (!sv) | |
853 | (gdb) | |
854 | ||
855 | We can now use C<Perl_sv_dump> to investigate the SV: | |
856 | ||
8b029fdf | 857 | (gdb) print Perl_sv_dump(sv) |
04c692a8 DR |
858 | SV = PV(0xa057cc0) at 0xa0675d0 |
859 | REFCNT = 1 | |
860 | FLAGS = (POK,pPOK) | |
861 | PV = 0xa06a510 "6XXXX"\0 | |
862 | CUR = 5 | |
863 | LEN = 6 | |
864 | $1 = void | |
865 | ||
866 | We know we're going to get C<6> from this, so let's finish the | |
867 | subroutine: | |
868 | ||
869 | (gdb) finish | |
870 | Run till exit from #0 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671 | |
871 | 0x462669 in Perl_pp_add () at pp_hot.c:311 | |
872 | 311 dPOPTOPnnrl_ul; | |
873 | ||
874 | We can also dump out this op: the current op is always stored in | |
9b22382a | 875 | C<PL_op>, and we can dump it with C<Perl_op_dump>. This'll give us |
04c692a8 DR |
876 | similar output to L<B::Debug|B::Debug>. |
877 | ||
8b029fdf | 878 | (gdb) print Perl_op_dump(PL_op) |
04c692a8 DR |
879 | { |
880 | 13 TYPE = add ===> 14 | |
881 | TARG = 1 | |
882 | FLAGS = (SCALAR,KIDS) | |
883 | { | |
884 | TYPE = null ===> (12) | |
885 | (was rv2sv) | |
886 | FLAGS = (SCALAR,KIDS) | |
887 | { | |
888 | 11 TYPE = gvsv ===> 12 | |
889 | FLAGS = (SCALAR) | |
890 | GV = main::b | |
891 | } | |
892 | } | |
893 | ||
894 | # finish this later # | |
895 | ||
8b029fdf MH |
896 | =head2 Using gdb to look at specific parts of a program |
897 | ||
898 | With the example above, you knew to look for C<Perl_pp_add>, but what if | |
899 | there were multiple calls to it all over the place, or you didn't know what | |
900 | the op was you were looking for? | |
901 | ||
902 | One way to do this is to inject a rare call somewhere near what you're looking | |
9b22382a | 903 | for. For example, you could add C<study> before your method: |
8b029fdf MH |
904 | |
905 | study; | |
906 | ||
907 | And in gdb do: | |
908 | ||
909 | (gdb) break Perl_pp_study | |
910 | ||
9b22382a FC |
911 | And then step until you hit what you're |
912 | looking for. This works well in a loop | |
8b029fdf MH |
913 | if you want to only break at certain iterations: |
914 | ||
915 | for my $c (1..100) { | |
916 | study if $c == 50; | |
917 | } | |
918 | ||
919 | =head2 Using gdb to look at what the parser/lexer are doing | |
920 | ||
921 | If you want to see what perl is doing when parsing/lexing your code, you can | |
72b22e55 | 922 | use C<BEGIN {}>: |
8b029fdf MH |
923 | |
924 | print "Before\n"; | |
925 | BEGIN { study; } | |
926 | print "After\n"; | |
927 | ||
928 | And in gdb: | |
929 | ||
930 | (gdb) break Perl_pp_study | |
931 | ||
932 | If you want to see what the parser/lexer is doing inside of C<if> blocks and | |
933 | the like you need to be a little trickier: | |
934 | ||
935 | if ($a && $b && do { BEGIN { study } 1 } && $c) { ... } | |
936 | ||
04c692a8 DR |
937 | =head1 SOURCE CODE STATIC ANALYSIS |
938 | ||
939 | Various tools exist for analysing C source code B<statically>, as | |
9b22382a | 940 | opposed to B<dynamically>, that is, without executing the code. It is |
04c692a8 DR |
941 | possible to detect resource leaks, undefined behaviour, type |
942 | mismatches, portability problems, code paths that would cause illegal | |
943 | memory accesses, and other similar problems by just parsing the C code | |
944 | and looking at the resulting graph, what does it tell about the | |
9b22382a | 945 | execution and data flows. As a matter of fact, this is exactly how C |
04c692a8 DR |
946 | compilers know to give warnings about dubious code. |
947 | ||
948 | =head2 lint, splint | |
949 | ||
950 | The good old C code quality inspector, C<lint>, is available in several | |
951 | platforms, but please be aware that there are several different | |
952 | implementations of it by different vendors, which means that the flags | |
953 | are not identical across different platforms. | |
954 | ||
955 | There is a lint variant called C<splint> (Secure Programming Lint) | |
956 | available from http://www.splint.org/ that should compile on any | |
957 | Unix-like platform. | |
958 | ||
959 | There are C<lint> and <splint> targets in Makefile, but you may have to | |
960 | diddle with the flags (see above). | |
961 | ||
962 | =head2 Coverity | |
963 | ||
964 | Coverity (http://www.coverity.com/) is a product similar to lint and as | |
965 | a testbed for their product they periodically check several open source | |
966 | projects, and they give out accounts to open source developers to the | |
967 | defect databases. | |
968 | ||
969 | =head2 cpd (cut-and-paste detector) | |
970 | ||
9b22382a | 971 | The cpd tool detects cut-and-paste coding. If one instance of the |
04c692a8 | 972 | cut-and-pasted code changes, all the other spots should probably be |
9b22382a | 973 | changed, too. Therefore such code should probably be turned into a |
04c692a8 DR |
974 | subroutine or a macro. |
975 | ||
976 | cpd (http://pmd.sourceforge.net/cpd.html) is part of the pmd project | |
9b22382a | 977 | (http://pmd.sourceforge.net/). pmd was originally written for static |
04c692a8 DR |
978 | analysis of Java code, but later the cpd part of it was extended to |
979 | parse also C and C++. | |
980 | ||
981 | Download the pmd-bin-X.Y.zip () from the SourceForge site, extract the | |
982 | pmd-X.Y.jar from it, and then run that on source code thusly: | |
983 | ||
0cbf2b31 FC |
984 | java -cp pmd-X.Y.jar net.sourceforge.pmd.cpd.CPD \ |
985 | --minimum-tokens 100 --files /some/where/src --language c > cpd.txt | |
04c692a8 DR |
986 | |
987 | You may run into memory limits, in which case you should use the -Xmx | |
988 | option: | |
989 | ||
990 | java -Xmx512M ... | |
991 | ||
992 | =head2 gcc warnings | |
993 | ||
994 | Though much can be written about the inconsistency and coverage | |
995 | problems of gcc warnings (like C<-Wall> not meaning "all the warnings", | |
996 | or some common portability problems not being covered by C<-Wall>, or | |
997 | C<-ansi> and C<-pedantic> both being a poorly defined collection of | |
998 | warnings, and so forth), gcc is still a useful tool in keeping our | |
999 | coding nose clean. | |
1000 | ||
1001 | The C<-Wall> is by default on. | |
1002 | ||
1003 | The C<-ansi> (and its sidekick, C<-pedantic>) would be nice to be on | |
1004 | always, but unfortunately they are not safe on all platforms, they can | |
1005 | for example cause fatal conflicts with the system headers (Solaris | |
9b22382a | 1006 | being a prime example). If Configure C<-Dgccansipedantic> is used, the |
04c692a8 DR |
1007 | C<cflags> frontend selects C<-ansi -pedantic> for the platforms where |
1008 | they are known to be safe. | |
1009 | ||
1010 | Starting from Perl 5.9.4 the following extra flags are added: | |
1011 | ||
1012 | =over 4 | |
1013 | ||
1014 | =item * | |
1015 | ||
1016 | C<-Wendif-labels> | |
1017 | ||
1018 | =item * | |
1019 | ||
1020 | C<-Wextra> | |
1021 | ||
1022 | =item * | |
1023 | ||
1024 | C<-Wdeclaration-after-statement> | |
1025 | ||
1026 | =back | |
1027 | ||
1028 | The following flags would be nice to have but they would first need | |
1029 | their own Augean stablemaster: | |
1030 | ||
1031 | =over 4 | |
1032 | ||
1033 | =item * | |
1034 | ||
1035 | C<-Wpointer-arith> | |
1036 | ||
1037 | =item * | |
1038 | ||
1039 | C<-Wshadow> | |
1040 | ||
1041 | =item * | |
1042 | ||
1043 | C<-Wstrict-prototypes> | |
1044 | ||
1045 | =back | |
1046 | ||
1047 | The C<-Wtraditional> is another example of the annoying tendency of gcc | |
1048 | to bundle a lot of warnings under one switch (it would be impossible to | |
1049 | deploy in practice because it would complain a lot) but it does contain | |
1050 | some warnings that would be beneficial to have available on their own, | |
1051 | such as the warning about string constants inside macros containing the | |
1052 | macro arguments: this behaved differently pre-ANSI than it does in | |
1053 | ANSI, and some C compilers are still in transition, AIX being an | |
1054 | example. | |
1055 | ||
1056 | =head2 Warnings of other C compilers | |
1057 | ||
1058 | Other C compilers (yes, there B<are> other C compilers than gcc) often | |
1059 | have their "strict ANSI" or "strict ANSI with some portability | |
1060 | extensions" modes on, like for example the Sun Workshop has its C<-Xa> | |
1061 | mode on (though implicitly), or the DEC (these days, HP...) has its | |
1062 | C<-std1> mode on. | |
1063 | ||
1064 | =head1 MEMORY DEBUGGERS | |
1065 | ||
d1fd4856 VP |
1066 | B<NOTE 1>: Running under older memory debuggers such as Purify, |
1067 | valgrind or Third Degree greatly slows down the execution: seconds | |
9b22382a | 1068 | become minutes, minutes become hours. For example as of Perl 5.8.1, the |
04c692a8 | 1069 | ext/Encode/t/Unicode.t takes extraordinarily long to complete under |
9b22382a FC |
1070 | e.g. Purify, Third Degree, and valgrind. Under valgrind it takes more |
1071 | than six hours, even on a snappy computer. The said test must be doing | |
1072 | something that is quite unfriendly for memory debuggers. If you don't | |
04c692a8 | 1073 | feel like waiting, that you can simply kill away the perl process. |
d1fd4856 VP |
1074 | Roughly valgrind slows down execution by factor 10, AddressSanitizer by |
1075 | factor 2. | |
04c692a8 DR |
1076 | |
1077 | B<NOTE 2>: To minimize the number of memory leak false alarms (see | |
1078 | L</PERL_DESTRUCT_LEVEL> for more information), you have to set the | |
9b22382a | 1079 | environment variable PERL_DESTRUCT_LEVEL to 2. For example, like this: |
04c692a8 DR |
1080 | |
1081 | env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib ... | |
1082 | ||
1083 | B<NOTE 3>: There are known memory leaks when there are compile-time | |
1084 | errors within eval or require, seeing C<S_doeval> in the call stack is | |
9b22382a | 1085 | a good sign of these. Fixing these leaks is non-trivial, unfortunately, |
04c692a8 DR |
1086 | but they must be fixed eventually. |
1087 | ||
1088 | B<NOTE 4>: L<DynaLoader> will not clean up after itself completely | |
1089 | unless Perl is built with the Configure option | |
1090 | C<-Accflags=-DDL_UNLOAD_ALL_AT_EXIT>. | |
1091 | ||
04c692a8 DR |
1092 | =head2 valgrind |
1093 | ||
d1fd4856 | 1094 | The valgrind tool can be used to find out both memory leaks and illegal |
9b22382a FC |
1095 | heap memory accesses. As of version 3.3.0, Valgrind only supports Linux |
1096 | on x86, x86-64 and PowerPC and Darwin (OS X) on x86 and x86-64). The | |
d1fd4856 | 1097 | special "test.valgrind" target can be used to run the tests under |
9b22382a | 1098 | valgrind. Found errors and memory leaks are logged in files named |
037ab3f1 MH |
1099 | F<testfile.valgrind> and by default output is displayed inline. |
1100 | ||
1101 | Example usage: | |
1102 | ||
1103 | make test.valgrind | |
1104 | ||
1105 | Since valgrind adds significant overhead, tests will take much longer to | |
1106 | run. The valgrind tests support being run in parallel to help with this: | |
1107 | ||
1108 | TEST_JOBS=9 make test.valgrind | |
1109 | ||
1110 | Note that the above two invocations will be very verbose as reachable | |
1111 | memory and leak-checking is enabled by default. If you want to just see | |
1112 | pure errors, try: | |
1113 | ||
1114 | VG_OPTS='-q --leak-check=no --show-reachable=no' TEST_JOBS=9 \ | |
1115 | make test.valgrind | |
04c692a8 DR |
1116 | |
1117 | Valgrind also provides a cachegrind tool, invoked on perl as: | |
1118 | ||
1119 | VG_OPTS=--tool=cachegrind make test.valgrind | |
1120 | ||
1121 | As system libraries (most notably glibc) are also triggering errors, | |
9b22382a | 1122 | valgrind allows to suppress such errors using suppression files. The |
04c692a8 | 1123 | default suppression file that comes with valgrind already catches a lot |
9b22382a | 1124 | of them. Some additional suppressions are defined in F<t/perl.supp>. |
04c692a8 DR |
1125 | |
1126 | To get valgrind and for more information see | |
1127 | ||
0061d4fa | 1128 | http://valgrind.org/ |
04c692a8 | 1129 | |
81c3bbe7 RU |
1130 | =head2 AddressSanitizer |
1131 | ||
4dd56148 | 1132 | AddressSanitizer is a clang and gcc extension, included in clang since |
9b22382a | 1133 | v3.1 and gcc since v4.8. It checks illegal heap pointers, global |
4dd56148 NC |
1134 | pointers, stack pointers and use after free errors, and is fast enough |
1135 | that you can easily compile your debugging or optimized perl with it. | |
9b22382a | 1136 | It does not check memory leaks though. AddressSanitizer is available |
4dd56148 | 1137 | for Linux, Mac OS X and soon on Windows. |
81c3bbe7 | 1138 | |
8a64fbaa VP |
1139 | To build perl with AddressSanitizer, your Configure invocation should |
1140 | look like: | |
81c3bbe7 | 1141 | |
e8596d90 VP |
1142 | sh Configure -des -Dcc=clang \ |
1143 | -Accflags=-faddress-sanitizer -Aldflags=-faddress-sanitizer \ | |
1144 | -Alddlflags=-shared\ -faddress-sanitizer | |
81c3bbe7 RU |
1145 | |
1146 | where these arguments mean: | |
1147 | ||
1148 | =over 4 | |
1149 | ||
1150 | =item * -Dcc=clang | |
1151 | ||
8a64fbaa VP |
1152 | This should be replaced by the full path to your clang executable if it |
1153 | is not in your path. | |
81c3bbe7 RU |
1154 | |
1155 | =item * -Accflags=-faddress-sanitizer | |
1156 | ||
8a64fbaa | 1157 | Compile perl and extensions sources with AddressSanitizer. |
81c3bbe7 RU |
1158 | |
1159 | =item * -Aldflags=-faddress-sanitizer | |
1160 | ||
8a64fbaa | 1161 | Link the perl executable with AddressSanitizer. |
81c3bbe7 | 1162 | |
e8596d90 | 1163 | =item * -Alddlflags=-shared\ -faddress-sanitizer |
81c3bbe7 | 1164 | |
9b22382a | 1165 | Link dynamic extensions with AddressSanitizer. You must manually |
e8596d90 VP |
1166 | specify C<-shared> because using C<-Alddlflags=-shared> will prevent |
1167 | Configure from setting a default value for C<lddlflags>, which usually | |
5dfc6e97 | 1168 | contains C<-shared> (at least on Linux). |
81c3bbe7 RU |
1169 | |
1170 | =back | |
1171 | ||
8a64fbaa VP |
1172 | See also |
1173 | L<http://code.google.com/p/address-sanitizer/wiki/AddressSanitizer>. | |
81c3bbe7 RU |
1174 | |
1175 | ||
04c692a8 DR |
1176 | =head1 PROFILING |
1177 | ||
1178 | Depending on your platform there are various ways of profiling Perl. | |
1179 | ||
1180 | There are two commonly used techniques of profiling executables: | |
1181 | I<statistical time-sampling> and I<basic-block counting>. | |
1182 | ||
1183 | The first method takes periodically samples of the CPU program counter, | |
1184 | and since the program counter can be correlated with the code generated | |
1185 | for functions, we get a statistical view of in which functions the | |
9b22382a | 1186 | program is spending its time. The caveats are that very small/fast |
04c692a8 DR |
1187 | functions have lower probability of showing up in the profile, and that |
1188 | periodically interrupting the program (this is usually done rather | |
1189 | frequently, in the scale of milliseconds) imposes an additional | |
9b22382a | 1190 | overhead that may skew the results. The first problem can be alleviated |
04c692a8 DR |
1191 | by running the code for longer (in general this is a good idea for |
1192 | profiling), the second problem is usually kept in guard by the | |
1193 | profiling tools themselves. | |
1194 | ||
1195 | The second method divides up the generated code into I<basic blocks>. | |
1196 | Basic blocks are sections of code that are entered only in the | |
9b22382a FC |
1197 | beginning and exited only at the end. For example, a conditional jump |
1198 | starts a basic block. Basic block profiling usually works by | |
04c692a8 | 1199 | I<instrumenting> the code by adding I<enter basic block #nnnn> |
9b22382a FC |
1200 | book-keeping code to the generated code. During the execution of the |
1201 | code the basic block counters are then updated appropriately. The | |
04c692a8 DR |
1202 | caveat is that the added extra code can skew the results: again, the |
1203 | profiling tools usually try to factor their own effects out of the | |
1204 | results. | |
1205 | ||
1206 | =head2 Gprof Profiling | |
1207 | ||
e2aed43d | 1208 | I<gprof> is a profiling tool available in many Unix platforms which |
9b22382a FC |
1209 | uses I<statistical time-sampling>. You can build a profiled version of |
1210 | F<perl> by compiling using gcc with the flag C<-pg>. Either edit | |
1211 | F<config.sh> or re-run F<Configure>. Running the profiled version of | |
e2aed43d NC |
1212 | Perl will create an output file called F<gmon.out> which contains the |
1213 | profiling data collected during the execution. | |
04c692a8 | 1214 | |
e2aed43d NC |
1215 | quick hint: |
1216 | ||
1217 | $ sh Configure -des -Dusedevel -Accflags='-pg' \ | |
1218 | -Aldflags='-pg' -Alddlflags='-pg -shared' \ | |
1219 | && make perl | |
1220 | $ ./perl ... # creates gmon.out in current directory | |
1221 | $ gprof ./perl > out | |
1222 | $ less out | |
1223 | ||
1224 | (you probably need to add C<-shared> to the <-Alddlflags> line until RT | |
1225 | #118199 is resolved) | |
04c692a8 | 1226 | |
e2aed43d NC |
1227 | The F<gprof> tool can then display the collected data in various ways. |
1228 | Usually F<gprof> understands the following options: | |
04c692a8 DR |
1229 | |
1230 | =over 4 | |
1231 | ||
1232 | =item * -a | |
1233 | ||
1234 | Suppress statically defined functions from the profile. | |
1235 | ||
1236 | =item * -b | |
1237 | ||
1238 | Suppress the verbose descriptions in the profile. | |
1239 | ||
1240 | =item * -e routine | |
1241 | ||
1242 | Exclude the given routine and its descendants from the profile. | |
1243 | ||
1244 | =item * -f routine | |
1245 | ||
1246 | Display only the given routine and its descendants in the profile. | |
1247 | ||
1248 | =item * -s | |
1249 | ||
1250 | Generate a summary file called F<gmon.sum> which then may be given to | |
1251 | subsequent gprof runs to accumulate data over several runs. | |
1252 | ||
1253 | =item * -z | |
1254 | ||
1255 | Display routines that have zero usage. | |
1256 | ||
1257 | =back | |
1258 | ||
1259 | For more detailed explanation of the available commands and output | |
e2aed43d | 1260 | formats, see your own local documentation of F<gprof>. |
04c692a8 | 1261 | |
e2aed43d | 1262 | =head2 GCC gcov Profiling |
04c692a8 | 1263 | |
e2aed43d NC |
1264 | I<basic block profiling> is officially available in gcc 3.0 and later. |
1265 | You can build a profiled version of F<perl> by compiling using gcc with | |
9b22382a | 1266 | the flags C<-fprofile-arcs -ftest-coverage>. Either edit F<config.sh> |
e2aed43d | 1267 | or re-run F<Configure>. |
04c692a8 | 1268 | |
e2aed43d | 1269 | quick hint: |
04c692a8 | 1270 | |
e2aed43d NC |
1271 | $ sh Configure -des -Dusedevel -Doptimize='-g' \ |
1272 | -Accflags='-fprofile-arcs -ftest-coverage' \ | |
1273 | -Aldflags='-fprofile-arcs -ftest-coverage' \ | |
1274 | -Alddlflags='-fprofile-arcs -ftest-coverage -shared' \ | |
1275 | && make perl | |
1276 | $ rm -f regexec.c.gcov regexec.gcda | |
1277 | $ ./perl ... | |
1278 | $ gcov regexec.c | |
1279 | $ less regexec.c.gcov | |
04c692a8 | 1280 | |
e2aed43d NC |
1281 | (you probably need to add C<-shared> to the <-Alddlflags> line until RT |
1282 | #118199 is resolved) | |
04c692a8 DR |
1283 | |
1284 | Running the profiled version of Perl will cause profile output to be | |
9b22382a | 1285 | generated. For each source file an accompanying F<.gcda> file will be |
04c692a8 DR |
1286 | created. |
1287 | ||
e2aed43d | 1288 | To display the results you use the I<gcov> utility (which should be |
9b22382a | 1289 | installed if you have gcc 3.0 or newer installed). F<gcov> is run on |
04c692a8 DR |
1290 | source code files, like this |
1291 | ||
1292 | gcov sv.c | |
1293 | ||
9b22382a | 1294 | which will cause F<sv.c.gcov> to be created. The F<.gcov> files contain |
04c692a8 | 1295 | the source code annotated with relative frequencies of execution |
9b22382a | 1296 | indicated by "#" markers. If you want to generate F<.gcov> files for |
6f134219 NC |
1297 | all profiled object files, you can run something like this: |
1298 | ||
1299 | for file in `find . -name \*.gcno` | |
1300 | do sh -c "cd `dirname $file` && gcov `basename $file .gcno`" | |
1301 | done | |
04c692a8 DR |
1302 | |
1303 | Useful options of F<gcov> include C<-b> which will summarise the basic | |
1304 | block, branch, and function call coverage, and C<-c> which instead of | |
9b22382a | 1305 | relative frequencies will use the actual counts. For more information |
04c692a8 | 1306 | on the use of F<gcov> and basic block profiling with gcc, see the |
9b22382a | 1307 | latest GNU CC manual. As of gcc 4.8, this is at |
e2aed43d | 1308 | L<http://gcc.gnu.org/onlinedocs/gcc/Gcov-Intro.html#Gcov-Intro> |
04c692a8 DR |
1309 | |
1310 | =head1 MISCELLANEOUS TRICKS | |
1311 | ||
1312 | =head2 PERL_DESTRUCT_LEVEL | |
1313 | ||
1314 | If you want to run any of the tests yourself manually using e.g. | |
4dd56148 NC |
1315 | valgrind, please note that by default perl B<does not> explicitly |
1316 | cleanup all the memory it has allocated (such as global memory arenas) | |
1317 | but instead lets the exit() of the whole program "take care" of such | |
1318 | allocations, also known as "global destruction of objects". | |
04c692a8 DR |
1319 | |
1320 | There is a way to tell perl to do complete cleanup: set the environment | |
9b22382a | 1321 | variable PERL_DESTRUCT_LEVEL to a non-zero value. The t/TEST wrapper |
04c692a8 | 1322 | does set this to 2, and this is what you need to do too, if you don't |
f01ecde8 | 1323 | want to see the "global leaks": For example, for running under valgrind |
04c692a8 | 1324 | |
f01ecde8 | 1325 | env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib t/foo/bar.t |
04c692a8 DR |
1326 | |
1327 | (Note: the mod_perl apache module uses also this environment variable | |
9b22382a FC |
1328 | for its own purposes and extended its semantics. Refer to the mod_perl |
1329 | documentation for more information. Also, spawned threads do the | |
04c692a8 DR |
1330 | equivalent of setting this variable to the value 1.) |
1331 | ||
1332 | If, at the end of a run you get the message I<N scalars leaked>, you | |
1333 | can recompile with C<-DDEBUG_LEAKING_SCALARS>, which will cause the | |
1334 | addresses of all those leaked SVs to be dumped along with details as to | |
9b22382a FC |
1335 | where each SV was originally allocated. This information is also |
1336 | displayed by Devel::Peek. Note that the extra details recorded with | |
04c692a8 | 1337 | each SV increases memory usage, so it shouldn't be used in production |
9b22382a | 1338 | environments. It also converts C<new_SV()> from a macro into a real |
04c692a8 DR |
1339 | function, so you can use your favourite debugger to discover where |
1340 | those pesky SVs were allocated. | |
1341 | ||
1342 | If you see that you're leaking memory at runtime, but neither valgrind | |
1343 | nor C<-DDEBUG_LEAKING_SCALARS> will find anything, you're probably | |
1344 | leaking SVs that are still reachable and will be properly cleaned up | |
9b22382a FC |
1345 | during destruction of the interpreter. In such cases, using the C<-Dm> |
1346 | switch can point you to the source of the leak. If the executable was | |
04c692a8 | 1347 | built with C<-DDEBUG_LEAKING_SCALARS>, C<-Dm> will output SV |
9b22382a | 1348 | allocations in addition to memory allocations. Each SV allocation has a |
04c692a8 | 1349 | distinct serial number that will be written on creation and destruction |
9b22382a | 1350 | of the SV. So if you're executing the leaking code in a loop, you need |
04c692a8 | 1351 | to look for SVs that are created, but never destroyed between each |
9b22382a | 1352 | cycle. If such an SV is found, set a conditional breakpoint within |
04c692a8 | 1353 | C<new_SV()> and make it break only when C<PL_sv_serial> is equal to the |
9b22382a | 1354 | serial number of the leaking SV. Then you will catch the interpreter in |
04c692a8 DR |
1355 | exactly the state where the leaking SV is allocated, which is |
1356 | sufficient in many cases to find the source of the leak. | |
1357 | ||
1358 | As C<-Dm> is using the PerlIO layer for output, it will by itself | |
9b22382a | 1359 | allocate quite a bunch of SVs, which are hidden to avoid recursion. You |
04c692a8 DR |
1360 | can bypass the PerlIO layer if you use the SV logging provided by |
1361 | C<-DPERL_MEM_LOG> instead. | |
1362 | ||
1363 | =head2 PERL_MEM_LOG | |
1364 | ||
1365 | If compiled with C<-DPERL_MEM_LOG>, both memory and SV allocations go | |
1366 | through logging functions, which is handy for breakpoint setting. | |
1367 | ||
1368 | Unless C<-DPERL_MEM_LOG_NOIMPL> is also compiled, the logging functions | |
1369 | read $ENV{PERL_MEM_LOG} to determine whether to log the event, and if | |
1370 | so how: | |
1371 | ||
1372 | $ENV{PERL_MEM_LOG} =~ /m/ Log all memory ops | |
1373 | $ENV{PERL_MEM_LOG} =~ /s/ Log all SV ops | |
1374 | $ENV{PERL_MEM_LOG} =~ /t/ include timestamp in Log | |
1375 | $ENV{PERL_MEM_LOG} =~ /^(\d+)/ write to FD given (default is 2) | |
1376 | ||
1377 | Memory logging is somewhat similar to C<-Dm> but is independent of | |
1378 | C<-DDEBUGGING>, and at a higher level; all uses of Newx(), Renew(), and | |
1379 | Safefree() are logged with the caller's source code file and line | |
9b22382a FC |
1380 | number (and C function name, if supported by the C compiler). In |
1381 | contrast, C<-Dm> is directly at the point of C<malloc()>. SV logging is | |
04c692a8 DR |
1382 | similar. |
1383 | ||
1384 | Since the logging doesn't use PerlIO, all SV allocations are logged and | |
9b22382a | 1385 | no extra SV allocations are introduced by enabling the logging. If |
04c692a8 DR |
1386 | compiled with C<-DDEBUG_LEAKING_SCALARS>, the serial number for each SV |
1387 | allocation is also logged. | |
1388 | ||
1389 | =head2 DDD over gdb | |
1390 | ||
1391 | Those debugging perl with the DDD frontend over gdb may find the | |
1392 | following useful: | |
1393 | ||
1394 | You can extend the data conversion shortcuts menu, so for example you | |
1395 | can display an SV's IV value with one click, without doing any typing. | |
1396 | To do that simply edit ~/.ddd/init file and add after: | |
1397 | ||
1398 | ! Display shortcuts. | |
1399 | Ddd*gdbDisplayShortcuts: \ | |
1400 | /t () // Convert to Bin\n\ | |
1401 | /d () // Convert to Dec\n\ | |
1402 | /x () // Convert to Hex\n\ | |
1403 | /o () // Convert to Oct(\n\ | |
1404 | ||
1405 | the following two lines: | |
1406 | ||
1407 | ((XPV*) (())->sv_any )->xpv_pv // 2pvx\n\ | |
1408 | ((XPVIV*) (())->sv_any )->xiv_iv // 2ivx | |
1409 | ||
1410 | so now you can do ivx and pvx lookups or you can plug there the sv_peek | |
1411 | "conversion": | |
1412 | ||
1413 | Perl_sv_peek(my_perl, (SV*)()) // sv_peek | |
1414 | ||
9b22382a | 1415 | (The my_perl is for threaded builds.) Just remember that every line, |
04c692a8 DR |
1416 | but the last one, should end with \n\ |
1417 | ||
1418 | Alternatively edit the init file interactively via: 3rd mouse button -> | |
1419 | New Display -> Edit Menu | |
1420 | ||
1421 | Note: you can define up to 20 conversion shortcuts in the gdb section. | |
1422 | ||
470dd224 JH |
1423 | =head2 C backtrace |
1424 | ||
0762e42f JH |
1425 | On some platforms Perl supports retrieving the C level backtrace |
1426 | (similar to what symbolic debuggers like gdb do). | |
470dd224 JH |
1427 | |
1428 | The backtrace returns the stack trace of the C call frames, | |
1429 | with the symbol names (function names), the object names (like "perl"), | |
1430 | and if it can, also the source code locations (file:line). | |
1431 | ||
0762e42f JH |
1432 | The supported platforms are Linux, and OS X (some *BSD might |
1433 | work at least partly, but they have not yet been tested). | |
1434 | ||
1435 | This feature hasn't been tested with multiple threads, but it will | |
1436 | only show the backtrace of the thread doing the backtracing. | |
470dd224 JH |
1437 | |
1438 | The feature needs to be enabled with C<Configure -Dusecbacktrace>. | |
1439 | ||
0762e42f JH |
1440 | The C<-Dusecbacktrace> also enables keeping the debug information when |
1441 | compiling/linking (often: C<-g>). Many compilers/linkers do support | |
1442 | having both optimization and keeping the debug information. The debug | |
1443 | information is needed for the symbol names and the source locations. | |
1444 | ||
1445 | Static functions might not be visible for the backtrace. | |
470dd224 JH |
1446 | |
1447 | Source code locations, even if available, can often be missing or | |
0762e42f JH |
1448 | misleading if the compiler has e.g. inlined code. Optimizer can |
1449 | make matching the source code and the object code quite challenging. | |
470dd224 JH |
1450 | |
1451 | =over 4 | |
1452 | ||
1453 | =item Linux | |
1454 | ||
59b3baca | 1455 | You B<must> have the BFD (-lbfd) library installed, otherwise C<perl> will |
0762e42f | 1456 | fail to link. The BFD is usually distributed as part of the GNU binutils. |
470dd224 JH |
1457 | |
1458 | Summary: C<Configure ... -Dusecbacktrace> | |
1459 | and you need C<-lbfd>. | |
1460 | ||
1461 | =item OS X | |
1462 | ||
0762e42f JH |
1463 | The source code locations are supported B<only> if you have |
1464 | the Developer Tools installed. (BFD is B<not> needed.) | |
470dd224 JH |
1465 | |
1466 | Summary: C<Configure ... -Dusecbacktrace> | |
1467 | and installing the Developer Tools would be good. | |
1468 | ||
1469 | =back | |
1470 | ||
1471 | Optionally, for trying out the feature, you may want to enable | |
0762e42f JH |
1472 | automatic dumping of the backtrace just before a warning or croak (die) |
1473 | message is emitted, by adding C<-Accflags=-DUSE_C_BACKTRACE_ON_ERROR> | |
1474 | for Configure. | |
470dd224 JH |
1475 | |
1476 | Unless the above additional feature is enabled, nothing about the | |
1477 | backtrace functionality is visible, except for the Perl/XS level. | |
1478 | ||
1479 | Furthermore, even if you have enabled this feature to be compiled, | |
1480 | you need to enable it in runtime with an environment variable: | |
0762e42f JH |
1481 | C<PERL_C_BACKTRACE_ON_ERROR=10>. It must be an integer higher |
1482 | than zero, telling the desired frame count. | |
470dd224 JH |
1483 | |
1484 | Retrieving the backtrace from Perl level (using for example an XS | |
1485 | extension) would be much less exciting than one would hope: normally | |
1486 | you would see C<runops>, C<entersub>, and not much else. This API is | |
1487 | intended to be called B<from within> the Perl implementation, not from | |
1488 | Perl level execution. | |
1489 | ||
0762e42f | 1490 | The C API for the backtrace is as follows: |
470dd224 JH |
1491 | |
1492 | =over 4 | |
1493 | ||
1494 | =item get_c_backtrace | |
1495 | ||
1496 | =item free_c_backtrace | |
1497 | ||
1498 | =item get_c_backtrace_dump | |
1499 | ||
1500 | =item dump_c_backtrace | |
1501 | ||
1502 | =back | |
1503 | ||
04c692a8 DR |
1504 | =head2 Poison |
1505 | ||
1506 | If you see in a debugger a memory area mysteriously full of 0xABABABAB | |
1507 | or 0xEFEFEFEF, you may be seeing the effect of the Poison() macros, see | |
1508 | L<perlclib>. | |
1509 | ||
1510 | =head2 Read-only optrees | |
1511 | ||
9b22382a | 1512 | Under ithreads the optree is read only. If you want to enforce this, to |
04c692a8 | 1513 | check for write accesses from buggy code, compile with |
91fc0422 FC |
1514 | C<-Accflags=-DPERL_DEBUG_READONLY_OPS> |
1515 | to enable code that allocates op memory | |
4dd56148 NC |
1516 | via C<mmap>, and sets it read-only when it is attached to a subroutine. |
1517 | Any write access to an op results in a C<SIGBUS> and abort. | |
04c692a8 DR |
1518 | |
1519 | This code is intended for development only, and may not be portable | |
9b22382a FC |
1520 | even to all Unix variants. Also, it is an 80% solution, in that it |
1521 | isn't able to make all ops read only. Specifically it does not apply to | |
4dd56148 | 1522 | op slabs belonging to C<BEGIN> blocks. |
04c692a8 | 1523 | |
4dd56148 NC |
1524 | However, as an 80% solution it is still effective, as it has caught |
1525 | bugs in the past. | |
04c692a8 | 1526 | |
f789f6a4 FC |
1527 | =head2 When is a bool not a bool? |
1528 | ||
1529 | On pre-C99 compilers, C<bool> is defined as equivalent to C<char>. | |
1530 | Consequently assignment of any larger type to a C<bool> is unsafe and may | |
1531 | be truncated. The C<cBOOL> macro exists to cast it correctly. | |
1532 | ||
1533 | On those platforms and compilers where C<bool> really is a boolean (C++, | |
1534 | C99), it is easy to forget the cast. You can force C<bool> to be a C<char> | |
1535 | by compiling with C<-Accflags=-DPERL_BOOL_AS_CHAR>. You may also wish to | |
50e4f4d4 CB |
1536 | run C<Configure> with something like |
1537 | ||
cbc13c3d | 1538 | -Accflags='-Wconversion -Wno-sign-conversion -Wno-shorten-64-to-32' |
50e4f4d4 CB |
1539 | |
1540 | or your compiler's equivalent to make it easier to spot any unsafe truncations | |
1541 | that show up. | |
f789f6a4 | 1542 | |
04c692a8 DR |
1543 | =head2 The .i Targets |
1544 | ||
1545 | You can expand the macros in a F<foo.c> file by saying | |
1546 | ||
1547 | make foo.i | |
1548 | ||
d1fd4856 VP |
1549 | which will expand the macros using cpp. Don't be scared by the |
1550 | results. | |
04c692a8 DR |
1551 | |
1552 | =head1 AUTHOR | |
1553 | ||
1554 | This document was originally written by Nathan Torkington, and is | |
1555 | maintained by the perl5-porters mailing list. |