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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 | ||
adf7d503 | 23 | Perl source plays by ANSI C89 rules: no C99 (or C++) extensions. |
04c692a8 DR |
24 | You don't care about some particular platform having broken Perl? I |
25 | hear there is still a strong demand for J2EE programmers. | |
26 | ||
27 | =head2 Perl environment problems | |
28 | ||
29 | =over 4 | |
30 | ||
31 | =item * | |
32 | ||
33 | Not compiling with threading | |
34 | ||
35 | Compiling with threading (-Duseithreads) completely rewrites the | |
9b22382a | 36 | function prototypes of Perl. You better try your changes with that. |
04c692a8 DR |
37 | Related to this is the difference between "Perl_-less" and "Perl_-ly" |
38 | APIs, for example: | |
39 | ||
40 | Perl_sv_setiv(aTHX_ ...); | |
41 | sv_setiv(...); | |
42 | ||
43 | The first one explicitly passes in the context, which is needed for | |
9b22382a FC |
44 | e.g. threaded builds. The second one does that implicitly; do not get |
45 | them mixed. If you are not passing in a aTHX_, you will need to do a | |
04c692a8 DR |
46 | dTHX (or a dVAR) as the first thing in the function. |
47 | ||
48 | See L<perlguts/"How multiple interpreters and concurrency are | |
49 | supported"> for further discussion about context. | |
50 | ||
51 | =item * | |
52 | ||
53 | Not compiling with -DDEBUGGING | |
54 | ||
55 | The DEBUGGING define exposes more code to the compiler, therefore more | |
9b22382a | 56 | ways for things to go wrong. You should try it. |
04c692a8 DR |
57 | |
58 | =item * | |
59 | ||
60 | Introducing (non-read-only) globals | |
61 | ||
62 | Do not introduce any modifiable globals, truly global or file static. | |
63 | They are bad form and complicate multithreading and other forms of | |
9b22382a | 64 | concurrency. The right way is to introduce them as new interpreter |
04c692a8 DR |
65 | variables, see F<intrpvar.h> (at the very end for binary |
66 | compatibility). | |
67 | ||
68 | Introducing read-only (const) globals is okay, as long as you verify | |
69 | with e.g. C<nm libperl.a|egrep -v ' [TURtr] '> (if your C<nm> has | |
9b22382a | 70 | BSD-style output) that the data you added really is read-only. (If it |
04c692a8 DR |
71 | is, it shouldn't show up in the output of that command.) |
72 | ||
73 | If you want to have static strings, make them constant: | |
74 | ||
75 | static const char etc[] = "..."; | |
76 | ||
77 | If you want to have arrays of constant strings, note carefully the | |
78 | right combination of C<const>s: | |
79 | ||
80 | static const char * const yippee[] = | |
a63ef199 | 81 | {"hi", "ho", "silver"}; |
04c692a8 DR |
82 | |
83 | There is a way to completely hide any modifiable globals (they are all | |
84 | moved to heap), the compilation setting | |
9b22382a | 85 | C<-DPERL_GLOBAL_STRUCT_PRIVATE>. It is not normally used, but can be |
04c692a8 DR |
86 | used for testing, read more about it in L<perlguts/"Background and |
87 | PERL_IMPLICIT_CONTEXT">. | |
88 | ||
89 | =item * | |
90 | ||
91 | Not exporting your new function | |
92 | ||
93 | Some platforms (Win32, AIX, VMS, OS/2, to name a few) require any | |
94 | function that is part of the public API (the shared Perl library) to be | |
9b22382a | 95 | explicitly marked as exported. See the discussion about F<embed.pl> in |
04c692a8 DR |
96 | L<perlguts>. |
97 | ||
98 | =item * | |
99 | ||
100 | Exporting your new function | |
101 | ||
102 | The new shiny result of either genuine new functionality or your | |
9b22382a | 103 | arduous refactoring is now ready and correctly exported. So what could |
04c692a8 DR |
104 | possibly go wrong? |
105 | ||
106 | Maybe simply that your function did not need to be exported in the | |
9b22382a | 107 | first place. Perl has a long and not so glorious history of exporting |
04c692a8 DR |
108 | functions that it should not have. |
109 | ||
110 | If the function is used only inside one source code file, make it | |
9b22382a | 111 | static. See the discussion about F<embed.pl> in L<perlguts>. |
04c692a8 DR |
112 | |
113 | If the function is used across several files, but intended only for | |
114 | Perl's internal use (and this should be the common case), do not export | |
9b22382a | 115 | it to the public API. See the discussion about F<embed.pl> in |
04c692a8 DR |
116 | L<perlguts>. |
117 | ||
118 | =back | |
119 | ||
120 | =head2 Portability problems | |
121 | ||
122 | The following are common causes of compilation and/or execution | |
9b22382a FC |
123 | failures, not common to Perl as such. The C FAQ is good bedtime |
124 | reading. Please test your changes with as many C compilers and | |
04c692a8 DR |
125 | platforms as possible; we will, anyway, and it's nice to save oneself |
126 | from public embarrassment. | |
127 | ||
128 | If using gcc, you can add the C<-std=c89> option which will hopefully | |
9b22382a | 129 | catch most of these unportabilities. (However it might also catch |
04c692a8 DR |
130 | incompatibilities in your system's header files.) |
131 | ||
132 | Use the Configure C<-Dgccansipedantic> flag to enable the gcc C<-ansi | |
133 | -pedantic> flags which enforce stricter ANSI rules. | |
134 | ||
135 | If using the C<gcc -Wall> note that not all the possible warnings (like | |
404b28f3 | 136 | C<-Wuninitialized>) are given unless you also compile with C<-O>. |
04c692a8 DR |
137 | |
138 | Note that if using gcc, starting from Perl 5.9.5 the Perl core source | |
139 | code files (the ones at the top level of the source code distribution, | |
140 | but not e.g. the extensions under ext/) are automatically compiled with | |
141 | as many as possible of the C<-std=c89>, C<-ansi>, C<-pedantic>, and a | |
142 | selection of C<-W> flags (see cflags.SH). | |
143 | ||
144 | Also study L<perlport> carefully to avoid any bad assumptions about the | |
eb9df707 | 145 | operating system, filesystems, character set, and so forth. |
04c692a8 DR |
146 | |
147 | You may once in a while try a "make microperl" to see whether we can | |
9b22382a | 148 | still compile Perl with just the bare minimum of interfaces. (See |
04c692a8 DR |
149 | README.micro.) |
150 | ||
151 | Do not assume an operating system indicates a certain compiler. | |
152 | ||
153 | =over 4 | |
154 | ||
155 | =item * | |
156 | ||
157 | Casting pointers to integers or casting integers to pointers | |
158 | ||
159 | void castaway(U8* p) | |
160 | { | |
161 | IV i = p; | |
162 | ||
163 | or | |
164 | ||
165 | void castaway(U8* p) | |
166 | { | |
167 | IV i = (IV)p; | |
168 | ||
9b22382a FC |
169 | Both are bad, and broken, and unportable. Use the PTR2IV() macro that |
170 | does it right. (Likewise, there are PTR2UV(), PTR2NV(), INT2PTR(), and | |
04c692a8 DR |
171 | NUM2PTR().) |
172 | ||
173 | =item * | |
174 | ||
28ffebaf | 175 | Casting between function pointers and data pointers |
04c692a8 DR |
176 | |
177 | Technically speaking casting between function pointers and data | |
178 | pointers is unportable and undefined, but practically speaking it seems | |
179 | to work, but you should use the FPTR2DPTR() and DPTR2FPTR() macros. | |
180 | Sometimes you can also play games with unions. | |
181 | ||
182 | =item * | |
183 | ||
184 | Assuming sizeof(int) == sizeof(long) | |
185 | ||
186 | There are platforms where longs are 64 bits, and platforms where ints | |
187 | are 64 bits, and while we are out to shock you, even platforms where | |
9b22382a | 188 | shorts are 64 bits. This is all legal according to the C standard. (In |
04c692a8 DR |
189 | other words, "long long" is not a portable way to specify 64 bits, and |
190 | "long long" is not even guaranteed to be any wider than "long".) | |
191 | ||
192 | Instead, use the definitions IV, UV, IVSIZE, I32SIZE, and so forth. | |
193 | Avoid things like I32 because they are B<not> guaranteed to be | |
194 | I<exactly> 32 bits, they are I<at least> 32 bits, nor are they | |
9b22382a | 195 | guaranteed to be B<int> or B<long>. If you really explicitly need |
04c692a8 DR |
196 | 64-bit variables, use I64 and U64, but only if guarded by HAS_QUAD. |
197 | ||
198 | =item * | |
199 | ||
200 | Assuming one can dereference any type of pointer for any type of data | |
201 | ||
202 | char *p = ...; | |
56bb4b7b | 203 | long pony = *(long *)p; /* BAD */ |
04c692a8 DR |
204 | |
205 | Many platforms, quite rightly so, will give you a core dump instead of | |
768312ab | 206 | a pony if the p happens not to be correctly aligned. |
04c692a8 DR |
207 | |
208 | =item * | |
209 | ||
210 | Lvalue casts | |
211 | ||
212 | (int)*p = ...; /* BAD */ | |
213 | ||
9b22382a | 214 | Simply not portable. Get your lvalue to be of the right type, or maybe |
04c692a8 DR |
215 | use temporary variables, or dirty tricks with unions. |
216 | ||
217 | =item * | |
218 | ||
219 | Assume B<anything> about structs (especially the ones you don't | |
220 | control, like the ones coming from the system headers) | |
221 | ||
222 | =over 8 | |
223 | ||
224 | =item * | |
225 | ||
226 | That a certain field exists in a struct | |
227 | ||
228 | =item * | |
229 | ||
230 | That no other fields exist besides the ones you know of | |
231 | ||
232 | =item * | |
233 | ||
234 | That a field is of certain signedness, sizeof, or type | |
235 | ||
236 | =item * | |
237 | ||
238 | That the fields are in a certain order | |
239 | ||
240 | =over 8 | |
241 | ||
242 | =item * | |
243 | ||
244 | While C guarantees the ordering specified in the struct definition, | |
245 | between different platforms the definitions might differ | |
246 | ||
247 | =back | |
248 | ||
249 | =item * | |
250 | ||
251 | That the sizeof(struct) or the alignments are the same everywhere | |
252 | ||
253 | =over 8 | |
254 | ||
255 | =item * | |
256 | ||
257 | There might be padding bytes between the fields to align the fields - | |
258 | the bytes can be anything | |
259 | ||
260 | =item * | |
261 | ||
262 | Structs are required to be aligned to the maximum alignment required by | |
263 | the fields - which for native types is for usually equivalent to | |
264 | sizeof() of the field | |
265 | ||
266 | =back | |
267 | ||
268 | =back | |
269 | ||
270 | =item * | |
271 | ||
272 | Assuming the character set is ASCIIish | |
273 | ||
9b22382a | 274 | Perl can compile and run under EBCDIC platforms. See L<perlebcdic>. |
04c692a8 DR |
275 | This is transparent for the most part, but because the character sets |
276 | differ, you shouldn't use numeric (decimal, octal, nor hex) constants | |
eb9df707 KW |
277 | to refer to characters. You can safely say C<'A'>, but not C<0x41>. |
278 | You can safely say C<'\n'>, but not C<\012>. However, you can use | |
279 | macros defined in F<utf8.h> to specify any code point portably. | |
280 | C<LATIN1_TO_NATIVE(0xDF)> is going to be the code point that means | |
281 | LATIN SMALL LETTER SHARP S on whatever platform you are running on (on | |
282 | ASCII platforms it compiles without adding any extra code, so there is | |
283 | zero performance hit on those). The acceptable inputs to | |
284 | C<LATIN1_TO_NATIVE> are from C<0x00> through C<0xFF>. If your input | |
285 | isn't guaranteed to be in that range, use C<UNICODE_TO_NATIVE> instead. | |
286 | C<NATIVE_TO_LATIN1> and C<NATIVE_TO_UNICODE> translate the opposite | |
287 | direction. | |
288 | ||
289 | If you need the string representation of a character that doesn't have a | |
290 | mnemonic name in C, you should add it to the list in | |
c22aa07d | 291 | F<regen/unicode_constants.pl>, and have Perl create C<#define>'s for you, |
eb6d698b | 292 | based on the current platform. |
04c692a8 | 293 | |
eb9df707 KW |
294 | Note that the C<isI<FOO>> and C<toI<FOO>> macros in F<handy.h> work |
295 | properly on native code points and strings. | |
296 | ||
04c692a8 | 297 | Also, the range 'A' - 'Z' in ASCII is an unbroken sequence of 26 upper |
9b22382a FC |
298 | case alphabetic characters. That is not true in EBCDIC. Nor for 'a' to |
299 | 'z'. But '0' - '9' is an unbroken range in both systems. Don't assume | |
c22aa07d | 300 | anything about other ranges. (Note that special handling of ranges in |
f4240379 | 301 | regular expression patterns and transliterations makes it appear to Perl |
c22aa07d | 302 | code that the aforementioned ranges are all unbroken.) |
04c692a8 DR |
303 | |
304 | Many of the comments in the existing code ignore the possibility of | |
9b22382a | 305 | EBCDIC, and may be wrong therefore, even if the code works. This is |
04c692a8 DR |
306 | actually a tribute to the successful transparent insertion of being |
307 | able to handle EBCDIC without having to change pre-existing code. | |
308 | ||
309 | UTF-8 and UTF-EBCDIC are two different encodings used to represent | |
9b22382a | 310 | Unicode code points as sequences of bytes. Macros with the same names |
eb9df707 | 311 | (but different definitions) in F<utf8.h> and F<utfebcdic.h> are used to |
04c692a8 DR |
312 | allow the calling code to think that there is only one such encoding. |
313 | This is almost always referred to as C<utf8>, but it means the EBCDIC | |
9b22382a | 314 | version as well. Again, comments in the code may well be wrong even if |
eb9df707 | 315 | the code itself is right. For example, the concept of UTF-8 C<invariant |
9b22382a FC |
316 | characters> differs between ASCII and EBCDIC. On ASCII platforms, only |
317 | characters that do not have the high-order bit set (i.e. whose ordinals | |
04c692a8 DR |
318 | are strict ASCII, 0 - 127) are invariant, and the documentation and |
319 | comments in the code may assume that, often referring to something | |
9b22382a | 320 | like, say, C<hibit>. The situation differs and is not so simple on |
04c692a8 DR |
321 | EBCDIC machines, but as long as the code itself uses the |
322 | C<NATIVE_IS_INVARIANT()> macro appropriately, it works, even if the | |
323 | comments are wrong. | |
324 | ||
257844b9 KW |
325 | As noted in L<perlhack/TESTING>, when writing test scripts, the file |
326 | F<t/charset_tools.pl> contains some helpful functions for writing tests | |
327 | valid on both ASCII and EBCDIC platforms. Sometimes, though, a test | |
328 | can't use a function and it's inconvenient to have different test | |
329 | versions depending on the platform. There are 20 code points that are | |
330 | the same in all 4 character sets currently recognized by Perl (the 3 | |
331 | EBCDIC code pages plus ISO 8859-1 (ASCII/Latin1)). These can be used in | |
332 | such tests, though there is a small possibility that Perl will become | |
333 | available in yet another character set, breaking your test. All but one | |
334 | of these code points are C0 control characters. The most significant | |
335 | controls that are the same are C<\0>, C<\r>, and C<\N{VT}> (also | |
336 | specifiable as C<\cK>, C<\x0B>, C<\N{U+0B}>, or C<\013>). The single | |
337 | non-control is U+00B6 PILCROW SIGN. The controls that are the same have | |
338 | the same bit pattern in all 4 character sets, regardless of the UTF8ness | |
339 | of the string containing them. The bit pattern for U+B6 is the same in | |
340 | all 4 for non-UTF8 strings, but differs in each when its containing | |
341 | string is UTF-8 encoded. The only other code points that have some sort | |
342 | of sameness across all 4 character sets are the pair 0xDC and 0xFC. | |
343 | Together these represent upper- and lowercase LATIN LETTER U WITH | |
344 | DIAERESIS, but which is upper and which is lower may be reversed: 0xDC | |
345 | is the capital in Latin1 and 0xFC is the small letter, while 0xFC is the | |
346 | capital in EBCDIC and 0xDC is the small one. This factoid may be | |
347 | exploited in writing case insensitive tests that are the same across all | |
348 | 4 character sets. | |
349 | ||
04c692a8 DR |
350 | =item * |
351 | ||
352 | Assuming the character set is just ASCII | |
353 | ||
9b22382a | 354 | ASCII is a 7 bit encoding, but bytes have 8 bits in them. The 128 extra |
04c692a8 DR |
355 | characters have different meanings depending on the locale. Absent a |
356 | locale, currently these extra characters are generally considered to be | |
eb9df707 KW |
357 | unassigned, and this has presented some problems. This has being |
358 | changed starting in 5.12 so that these characters can be considered to | |
359 | be Latin-1 (ISO-8859-1). | |
04c692a8 DR |
360 | |
361 | =item * | |
362 | ||
363 | Mixing #define and #ifdef | |
364 | ||
365 | #define BURGLE(x) ... \ | |
366 | #ifdef BURGLE_OLD_STYLE /* BAD */ | |
367 | ... do it the old way ... \ | |
368 | #else | |
369 | ... do it the new way ... \ | |
370 | #endif | |
371 | ||
9b22382a | 372 | You cannot portably "stack" cpp directives. For example in the above |
04c692a8 DR |
373 | you need two separate BURGLE() #defines, one for each #ifdef branch. |
374 | ||
375 | =item * | |
376 | ||
377 | Adding non-comment stuff after #endif or #else | |
378 | ||
379 | #ifdef SNOSH | |
380 | ... | |
381 | #else !SNOSH /* BAD */ | |
382 | ... | |
383 | #endif SNOSH /* BAD */ | |
384 | ||
385 | The #endif and #else cannot portably have anything non-comment after | |
9b22382a | 386 | them. If you want to document what is going (which is a good idea |
04c692a8 DR |
387 | especially if the branches are long), use (C) comments: |
388 | ||
389 | #ifdef SNOSH | |
390 | ... | |
391 | #else /* !SNOSH */ | |
392 | ... | |
393 | #endif /* SNOSH */ | |
394 | ||
395 | The gcc option C<-Wendif-labels> warns about the bad variant (by | |
396 | default on starting from Perl 5.9.4). | |
397 | ||
398 | =item * | |
399 | ||
400 | Having a comma after the last element of an enum list | |
401 | ||
402 | enum color { | |
403 | CERULEAN, | |
404 | CHARTREUSE, | |
405 | CINNABAR, /* BAD */ | |
406 | }; | |
407 | ||
9b22382a | 408 | is not portable. Leave out the last comma. |
04c692a8 DR |
409 | |
410 | Also note that whether enums are implicitly morphable to ints varies | |
411 | between compilers, you might need to (int). | |
412 | ||
413 | =item * | |
414 | ||
415 | Using //-comments | |
416 | ||
417 | // This function bamfoodles the zorklator. /* BAD */ | |
418 | ||
9b22382a | 419 | That is C99 or C++. Perl is C89. Using the //-comments is silently |
04c692a8 DR |
420 | allowed by many C compilers but cranking up the ANSI C89 strictness |
421 | (which we like to do) causes the compilation to fail. | |
422 | ||
423 | =item * | |
424 | ||
425 | Mixing declarations and code | |
426 | ||
427 | void zorklator() | |
428 | { | |
429 | int n = 3; | |
430 | set_zorkmids(n); /* BAD */ | |
431 | int q = 4; | |
432 | ||
9b22382a | 433 | That is C99 or C++. Some C compilers allow that, but you shouldn't. |
04c692a8 DR |
434 | |
435 | The gcc option C<-Wdeclaration-after-statements> scans for such | |
436 | problems (by default on starting from Perl 5.9.4). | |
437 | ||
438 | =item * | |
439 | ||
440 | Introducing variables inside for() | |
441 | ||
442 | for(int i = ...; ...; ...) { /* BAD */ | |
443 | ||
9b22382a | 444 | That is C99 or C++. While it would indeed be awfully nice to have that |
04c692a8 DR |
445 | also in C89, to limit the scope of the loop variable, alas, we cannot. |
446 | ||
447 | =item * | |
448 | ||
449 | Mixing signed char pointers with unsigned char pointers | |
450 | ||
451 | int foo(char *s) { ... } | |
452 | ... | |
453 | unsigned char *t = ...; /* Or U8* t = ... */ | |
454 | foo(t); /* BAD */ | |
455 | ||
456 | While this is legal practice, it is certainly dubious, and downright | |
457 | fatal in at least one platform: for example VMS cc considers this a | |
9b22382a | 458 | fatal error. One cause for people often making this mistake is that a |
04c692a8 DR |
459 | "naked char" and therefore dereferencing a "naked char pointer" have an |
460 | undefined signedness: it depends on the compiler and the flags of the | |
461 | compiler and the underlying platform whether the result is signed or | |
9b22382a | 462 | unsigned. For this very same reason using a 'char' as an array index is |
04c692a8 DR |
463 | bad. |
464 | ||
465 | =item * | |
466 | ||
467 | Macros that have string constants and their arguments as substrings of | |
468 | the string constants | |
469 | ||
470 | #define FOO(n) printf("number = %d\n", n) /* BAD */ | |
471 | FOO(10); | |
472 | ||
473 | Pre-ANSI semantics for that was equivalent to | |
474 | ||
475 | printf("10umber = %d\10"); | |
476 | ||
9b22382a | 477 | which is probably not what you were expecting. Unfortunately at least |
04c692a8 DR |
478 | one reasonably common and modern C compiler does "real backward |
479 | compatibility" here, in AIX that is what still happens even though the | |
480 | rest of the AIX compiler is very happily C89. | |
481 | ||
482 | =item * | |
483 | ||
484 | Using printf formats for non-basic C types | |
485 | ||
486 | IV i = ...; | |
487 | printf("i = %d\n", i); /* BAD */ | |
488 | ||
489 | While this might by accident work in some platform (where IV happens to | |
9b22382a | 490 | be an C<int>), in general it cannot. IV might be something larger. Even |
04c692a8 DR |
491 | worse the situation is with more specific types (defined by Perl's |
492 | configuration step in F<config.h>): | |
493 | ||
494 | Uid_t who = ...; | |
495 | printf("who = %d\n", who); /* BAD */ | |
496 | ||
497 | The problem here is that Uid_t might be not only not C<int>-wide but it | |
498 | might also be unsigned, in which case large uids would be printed as | |
499 | negative values. | |
500 | ||
501 | There is no simple solution to this because of printf()'s limited | |
502 | intelligence, but for many types the right format is available as with | |
503 | either 'f' or '_f' suffix, for example: | |
504 | ||
505 | IVdf /* IV in decimal */ | |
506 | UVxf /* UV is hexadecimal */ | |
507 | ||
508 | printf("i = %"IVdf"\n", i); /* The IVdf is a string constant. */ | |
509 | ||
510 | Uid_t_f /* Uid_t in decimal */ | |
511 | ||
512 | printf("who = %"Uid_t_f"\n", who); | |
513 | ||
514 | Or you can try casting to a "wide enough" type: | |
515 | ||
516 | printf("i = %"IVdf"\n", (IV)something_very_small_and_signed); | |
517 | ||
9dd1a77d KW |
518 | See L<perlguts/Formatted Printing of Size_t and SSize_t> for how to |
519 | print those. | |
520 | ||
04c692a8 DR |
521 | Also remember that the C<%p> format really does require a void pointer: |
522 | ||
523 | U8* p = ...; | |
524 | printf("p = %p\n", (void*)p); | |
525 | ||
526 | The gcc option C<-Wformat> scans for such problems. | |
527 | ||
528 | =item * | |
529 | ||
530 | Blindly using variadic macros | |
531 | ||
532 | gcc has had them for a while with its own syntax, and C99 brought them | |
9b22382a | 533 | with a standardized syntax. Don't use the former, and use the latter |
04c692a8 DR |
534 | only if the HAS_C99_VARIADIC_MACROS is defined. |
535 | ||
536 | =item * | |
537 | ||
538 | Blindly passing va_list | |
539 | ||
540 | Not all platforms support passing va_list to further varargs (stdarg) | |
9b22382a | 541 | functions. The right thing to do is to copy the va_list using the |
04c692a8 DR |
542 | Perl_va_copy() if the NEED_VA_COPY is defined. |
543 | ||
544 | =item * | |
545 | ||
546 | Using gcc statement expressions | |
547 | ||
548 | val = ({...;...;...}); /* BAD */ | |
549 | ||
9b22382a | 550 | While a nice extension, it's not portable. The Perl code does |
04c692a8 DR |
551 | admittedly use them if available to gain some extra speed (essentially |
552 | as a funky form of inlining), but you shouldn't. | |
553 | ||
554 | =item * | |
555 | ||
556 | Binding together several statements in a macro | |
557 | ||
558 | Use the macros STMT_START and STMT_END. | |
559 | ||
560 | STMT_START { | |
561 | ... | |
562 | } STMT_END | |
563 | ||
564 | =item * | |
565 | ||
566 | Testing for operating systems or versions when should be testing for | |
567 | features | |
568 | ||
569 | #ifdef __FOONIX__ /* BAD */ | |
570 | foo = quux(); | |
571 | #endif | |
572 | ||
573 | Unless you know with 100% certainty that quux() is only ever available | |
574 | for the "Foonix" operating system B<and> that is available B<and> | |
575 | correctly working for B<all> past, present, B<and> future versions of | |
9b22382a | 576 | "Foonix", the above is very wrong. This is more correct (though still |
04c692a8 DR |
577 | not perfect, because the below is a compile-time check): |
578 | ||
579 | #ifdef HAS_QUUX | |
580 | foo = quux(); | |
581 | #endif | |
582 | ||
583 | How does the HAS_QUUX become defined where it needs to be? Well, if | |
584 | Foonix happens to be Unixy enough to be able to run the Configure | |
585 | script, and Configure has been taught about detecting and testing | |
9b22382a | 586 | quux(), the HAS_QUUX will be correctly defined. In other platforms, the |
04c692a8 DR |
587 | corresponding configuration step will hopefully do the same. |
588 | ||
589 | In a pinch, if you cannot wait for Configure to be educated, or if you | |
590 | have a good hunch of where quux() might be available, you can | |
591 | temporarily try the following: | |
592 | ||
593 | #if (defined(__FOONIX__) || defined(__BARNIX__)) | |
594 | # define HAS_QUUX | |
595 | #endif | |
596 | ||
597 | ... | |
598 | ||
599 | #ifdef HAS_QUUX | |
600 | foo = quux(); | |
601 | #endif | |
602 | ||
603 | But in any case, try to keep the features and operating systems | |
604 | separate. | |
605 | ||
b39b5b0c JH |
606 | A good resource on the predefined macros for various operating |
607 | systems, compilers, and so forth is | |
608 | L<http://sourceforge.net/p/predef/wiki/Home/> | |
609 | ||
38f18a30 KW |
610 | =item * |
611 | ||
612 | Assuming the contents of static memory pointed to by the return values | |
613 | of Perl wrappers for C library functions doesn't change. Many C library | |
614 | functions return pointers to static storage that can be overwritten by | |
615 | subsequent calls to the same or related functions. Perl has | |
616 | light-weight wrappers for some of these functions, and which don't make | |
617 | copies of the static memory. A good example is the interface to the | |
618 | environment variables that are in effect for the program. Perl has | |
619 | C<PerlEnv_getenv> to get values from the environment. But the return is | |
620 | a pointer to static memory in the C library. If you are using the value | |
621 | to immediately test for something, that's fine, but if you save the | |
622 | value and expect it to be unchanged by later processing, you would be | |
623 | wrong, but perhaps you wouldn't know it because different C library | |
624 | implementations behave differently, and the one on the platform you're | |
625 | testing on might work for your situation. But on some platforms, a | |
626 | subsequent call to C<PerlEnv_getenv> or related function WILL overwrite | |
627 | the memory that your first call points to. This has led to some | |
628 | hard-to-debug problems. Do a L<perlapi/savepv> to make a copy, thus | |
629 | avoiding these problems. You will have to free the copy when you're | |
630 | done to avoid memory leaks. If you don't have control over when it gets | |
631 | freed, you'll need to make the copy in a mortal scalar, like so: | |
632 | ||
633 | if ((s = PerlEnv_getenv("foo") == NULL) { | |
634 | ... /* handle NULL case */ | |
635 | } | |
636 | else { | |
637 | s = SvPVX(sv_2mortal(newSVpv(s, 0))); | |
638 | } | |
639 | ||
640 | The above example works only if C<"s"> is C<NUL>-terminated; otherwise | |
641 | you have to pass its length to C<newSVpv>. | |
642 | ||
04c692a8 DR |
643 | =back |
644 | ||
645 | =head2 Problematic System Interfaces | |
646 | ||
647 | =over 4 | |
648 | ||
649 | =item * | |
650 | ||
9b22382a FC |
651 | malloc(0), realloc(0), calloc(0, 0) are non-portable. To be portable |
652 | allocate at least one byte. (In general you should rarely need to work | |
04c692a8 DR |
653 | at this low level, but instead use the various malloc wrappers.) |
654 | ||
4059ba87 AC |
655 | =item * |
656 | ||
657 | snprintf() - the return type is unportable. Use my_snprintf() instead. | |
658 | ||
04c692a8 DR |
659 | =back |
660 | ||
661 | =head2 Security problems | |
662 | ||
663 | Last but not least, here are various tips for safer coding. | |
bbc89b61 | 664 | See also L<perlclib> for libc/stdio replacements one should use. |
04c692a8 DR |
665 | |
666 | =over 4 | |
667 | ||
668 | =item * | |
669 | ||
670 | Do not use gets() | |
671 | ||
9b22382a | 672 | Or we will publicly ridicule you. Seriously. |
04c692a8 DR |
673 | |
674 | =item * | |
675 | ||
bbc89b61 JH |
676 | Do not use tmpfile() |
677 | ||
678 | Use mkstemp() instead. | |
679 | ||
680 | =item * | |
681 | ||
04c692a8 DR |
682 | Do not use strcpy() or strcat() or strncpy() or strncat() |
683 | ||
684 | Use my_strlcpy() and my_strlcat() instead: they either use the native | |
685 | implementation, or Perl's own implementation (borrowed from the public | |
686 | domain implementation of INN). | |
687 | ||
688 | =item * | |
689 | ||
690 | Do not use sprintf() or vsprintf() | |
691 | ||
4059ba87 AC |
692 | If you really want just plain byte strings, use my_snprintf() and |
693 | my_vsnprintf() instead, which will try to use snprintf() and | |
694 | vsnprintf() if those safer APIs are available. If you want something | |
6bfe0388 KW |
695 | fancier than a plain byte string, use |
696 | L<C<Perl_form>()|perlapi/form> or SVs and | |
697 | L<C<Perl_sv_catpvf()>|perlapi/sv_catpvf>. | |
698 | ||
4059ba87 AC |
699 | Note that glibc C<printf()>, C<sprintf()>, etc. are buggy before glibc |
700 | version 2.17. They won't allow a C<%.s> format with a precision to | |
701 | create a string that isn't valid UTF-8 if the current underlying locale | |
702 | of the program is UTF-8. What happens is that the C<%s> and its operand are | |
703 | simply skipped without any notice. | |
704 | L<https://sourceware.org/bugzilla/show_bug.cgi?id=6530>. | |
705 | ||
c98823ff JH |
706 | =item * |
707 | ||
708 | Do not use atoi() | |
709 | ||
22ff3130 | 710 | Use grok_atoUV() instead. atoi() has ill-defined behavior on overflows, |
c98823ff | 711 | and cannot be used for incremental parsing. It is also affected by locale, |
338aa8b0 JH |
712 | which is bad. |
713 | ||
714 | =item * | |
715 | ||
716 | Do not use strtol() or strtoul() | |
717 | ||
22ff3130 | 718 | Use grok_atoUV() instead. strtol() or strtoul() (or their IV/UV-friendly |
338aa8b0 JH |
719 | macro disguises, Strtol() and Strtoul(), or Atol() and Atoul() are |
720 | affected by locale, which is bad. | |
c98823ff | 721 | |
04c692a8 DR |
722 | =back |
723 | ||
724 | =head1 DEBUGGING | |
725 | ||
726 | You can compile a special debugging version of Perl, which allows you | |
727 | to use the C<-D> option of Perl to tell more about what Perl is doing. | |
728 | But sometimes there is no alternative than to dive in with a debugger, | |
729 | either to see the stack trace of a core dump (very useful in a bug | |
730 | report), or trying to figure out what went wrong before the core dump | |
731 | happened, or how did we end up having wrong or unexpected results. | |
732 | ||
733 | =head2 Poking at Perl | |
734 | ||
735 | To really poke around with Perl, you'll probably want to build Perl for | |
736 | debugging, like this: | |
737 | ||
f075db89 | 738 | ./Configure -d -DDEBUGGING |
04c692a8 DR |
739 | make |
740 | ||
f075db89 DM |
741 | C<-DDEBUGGING> turns on the C compiler's C<-g> flag to have it produce |
742 | debugging information which will allow us to step through a running | |
743 | program, and to see in which C function we are at (without the debugging | |
744 | information we might see only the numerical addresses of the functions, | |
745 | which is not very helpful). It will also turn on the C<DEBUGGING> | |
746 | compilation symbol which enables all the internal debugging code in Perl. | |
747 | There are a whole bunch of things you can debug with this: L<perlrun> | |
748 | lists them all, and the best way to find out about them is to play about | |
749 | with them. The most useful options are probably | |
04c692a8 DR |
750 | |
751 | l Context (loop) stack processing | |
f075db89 | 752 | s Stack snapshots (with v, displays all stacks) |
04c692a8 DR |
753 | t Trace execution |
754 | o Method and overloading resolution | |
755 | c String/numeric conversions | |
756 | ||
f075db89 DM |
757 | For example |
758 | ||
759 | $ perl -Dst -e '$a + 1' | |
760 | .... | |
761 | (-e:1) gvsv(main::a) | |
762 | => UNDEF | |
763 | (-e:1) const(IV(1)) | |
764 | => UNDEF IV(1) | |
765 | (-e:1) add | |
766 | => NV(1) | |
767 | ||
768 | ||
769 | Some of the functionality of the debugging code can be achieved with a | |
770 | non-debugging perl by using XS modules: | |
04c692a8 DR |
771 | |
772 | -Dr => use re 'debug' | |
773 | -Dx => use O 'Debug' | |
774 | ||
775 | =head2 Using a source-level debugger | |
776 | ||
777 | If the debugging output of C<-D> doesn't help you, it's time to step | |
778 | through perl's execution with a source-level debugger. | |
779 | ||
780 | =over 3 | |
781 | ||
782 | =item * | |
783 | ||
784 | We'll use C<gdb> for our examples here; the principles will apply to | |
785 | any debugger (many vendors call their debugger C<dbx>), but check the | |
786 | manual of the one you're using. | |
787 | ||
788 | =back | |
789 | ||
790 | To fire up the debugger, type | |
791 | ||
792 | gdb ./perl | |
793 | ||
794 | Or if you have a core dump: | |
795 | ||
796 | gdb ./perl core | |
797 | ||
798 | You'll want to do that in your Perl source tree so the debugger can | |
9b22382a | 799 | read the source code. You should see the copyright message, followed by |
04c692a8 DR |
800 | the prompt. |
801 | ||
802 | (gdb) | |
803 | ||
804 | C<help> will get you into the documentation, but here are the most | |
805 | useful commands: | |
806 | ||
807 | =over 3 | |
808 | ||
809 | =item * run [args] | |
810 | ||
811 | Run the program with the given arguments. | |
812 | ||
813 | =item * break function_name | |
814 | ||
815 | =item * break source.c:xxx | |
816 | ||
817 | Tells the debugger that we'll want to pause execution when we reach | |
818 | either the named function (but see L<perlguts/Internal Functions>!) or | |
819 | the given line in the named source file. | |
820 | ||
821 | =item * step | |
822 | ||
823 | Steps through the program a line at a time. | |
824 | ||
825 | =item * next | |
826 | ||
827 | Steps through the program a line at a time, without descending into | |
828 | functions. | |
829 | ||
830 | =item * continue | |
831 | ||
832 | Run until the next breakpoint. | |
833 | ||
834 | =item * finish | |
835 | ||
836 | Run until the end of the current function, then stop again. | |
837 | ||
838 | =item * 'enter' | |
839 | ||
840 | Just pressing Enter will do the most recent operation again - it's a | |
841 | blessing when stepping through miles of source code. | |
842 | ||
8b029fdf MH |
843 | =item * ptype |
844 | ||
845 | Prints the C definition of the argument given. | |
846 | ||
847 | (gdb) ptype PL_op | |
848 | type = struct op { | |
849 | OP *op_next; | |
86cd3a13 | 850 | OP *op_sibparent; |
8b029fdf MH |
851 | OP *(*op_ppaddr)(void); |
852 | PADOFFSET op_targ; | |
853 | unsigned int op_type : 9; | |
854 | unsigned int op_opt : 1; | |
855 | unsigned int op_slabbed : 1; | |
856 | unsigned int op_savefree : 1; | |
857 | unsigned int op_static : 1; | |
858 | unsigned int op_folded : 1; | |
859 | unsigned int op_spare : 2; | |
860 | U8 op_flags; | |
861 | U8 op_private; | |
862 | } * | |
863 | ||
04c692a8 DR |
864 | =item * print |
865 | ||
9b22382a | 866 | Execute the given C code and print its results. B<WARNING>: Perl makes |
04c692a8 | 867 | heavy use of macros, and F<gdb> does not necessarily support macros |
9b22382a | 868 | (see later L</"gdb macro support">). You'll have to substitute them |
04c692a8 DR |
869 | yourself, or to invoke cpp on the source code files (see L</"The .i |
870 | Targets">) So, for instance, you can't say | |
871 | ||
872 | print SvPV_nolen(sv) | |
873 | ||
874 | but you have to say | |
875 | ||
876 | print Perl_sv_2pv_nolen(sv) | |
877 | ||
878 | =back | |
879 | ||
880 | You may find it helpful to have a "macro dictionary", which you can | |
9b22382a | 881 | produce by saying C<cpp -dM perl.c | sort>. Even then, F<cpp> won't |
04c692a8 DR |
882 | recursively apply those macros for you. |
883 | ||
884 | =head2 gdb macro support | |
885 | ||
886 | Recent versions of F<gdb> have fairly good macro support, but in order | |
887 | to use it you'll need to compile perl with macro definitions included | |
9b22382a FC |
888 | in the debugging information. Using F<gcc> version 3.1, this means |
889 | configuring with C<-Doptimize=-g3>. Other compilers might use a | |
04c692a8 DR |
890 | different switch (if they support debugging macros at all). |
891 | ||
892 | =head2 Dumping Perl Data Structures | |
893 | ||
894 | One way to get around this macro hell is to use the dumping functions | |
895 | in F<dump.c>; these work a little like an internal | |
896 | L<Devel::Peek|Devel::Peek>, but they also cover OPs and other | |
9b22382a | 897 | structures that you can't get at from Perl. Let's take an example. |
04c692a8 | 898 | We'll use the C<$a = $b + $c> we used before, but give it a bit of |
9b22382a | 899 | context: C<$b = "6XXXX"; $c = 2.3;>. Where's a good place to stop and |
04c692a8 DR |
900 | poke around? |
901 | ||
902 | What about C<pp_add>, the function we examined earlier to implement the | |
903 | C<+> operator: | |
904 | ||
905 | (gdb) break Perl_pp_add | |
906 | Breakpoint 1 at 0x46249f: file pp_hot.c, line 309. | |
907 | ||
908 | Notice we use C<Perl_pp_add> and not C<pp_add> - see | |
9b22382a | 909 | L<perlguts/Internal Functions>. With the breakpoint in place, we can |
04c692a8 DR |
910 | run our program: |
911 | ||
912 | (gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c' | |
913 | ||
914 | Lots of junk will go past as gdb reads in the relevant source files and | |
915 | libraries, and then: | |
916 | ||
917 | Breakpoint 1, Perl_pp_add () at pp_hot.c:309 | |
72876cce | 918 | 1396 dSP; dATARGET; bool useleft; SV *svl, *svr; |
04c692a8 DR |
919 | (gdb) step |
920 | 311 dPOPTOPnnrl_ul; | |
921 | (gdb) | |
922 | ||
923 | We looked at this bit of code before, and we said that | |
924 | C<dPOPTOPnnrl_ul> arranges for two C<NV>s to be placed into C<left> and | |
925 | C<right> - let's slightly expand it: | |
926 | ||
927 | #define dPOPTOPnnrl_ul NV right = POPn; \ | |
928 | SV *leftsv = TOPs; \ | |
929 | NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0 | |
930 | ||
931 | C<POPn> takes the SV from the top of the stack and obtains its NV | |
932 | either directly (if C<SvNOK> is set) or by calling the C<sv_2nv> | |
9b22382a FC |
933 | function. C<TOPs> takes the next SV from the top of the stack - yes, |
934 | C<POPn> uses C<TOPs> - but doesn't remove it. We then use C<SvNV> to | |
04c692a8 DR |
935 | get the NV from C<leftsv> in the same way as before - yes, C<POPn> uses |
936 | C<SvNV>. | |
937 | ||
938 | Since we don't have an NV for C<$b>, we'll have to use C<sv_2nv> to | |
9b22382a | 939 | convert it. If we step again, we'll find ourselves there: |
04c692a8 | 940 | |
8b029fdf | 941 | (gdb) step |
04c692a8 DR |
942 | Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669 |
943 | 1669 if (!sv) | |
944 | (gdb) | |
945 | ||
946 | We can now use C<Perl_sv_dump> to investigate the SV: | |
947 | ||
8b029fdf | 948 | (gdb) print Perl_sv_dump(sv) |
04c692a8 DR |
949 | SV = PV(0xa057cc0) at 0xa0675d0 |
950 | REFCNT = 1 | |
951 | FLAGS = (POK,pPOK) | |
952 | PV = 0xa06a510 "6XXXX"\0 | |
953 | CUR = 5 | |
954 | LEN = 6 | |
955 | $1 = void | |
956 | ||
957 | We know we're going to get C<6> from this, so let's finish the | |
958 | subroutine: | |
959 | ||
960 | (gdb) finish | |
961 | Run till exit from #0 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671 | |
962 | 0x462669 in Perl_pp_add () at pp_hot.c:311 | |
963 | 311 dPOPTOPnnrl_ul; | |
964 | ||
965 | We can also dump out this op: the current op is always stored in | |
9b22382a | 966 | C<PL_op>, and we can dump it with C<Perl_op_dump>. This'll give us |
903b1101 | 967 | similar output to CPAN module B::Debug. |
04c692a8 | 968 | |
8b029fdf | 969 | (gdb) print Perl_op_dump(PL_op) |
04c692a8 DR |
970 | { |
971 | 13 TYPE = add ===> 14 | |
972 | TARG = 1 | |
973 | FLAGS = (SCALAR,KIDS) | |
974 | { | |
975 | TYPE = null ===> (12) | |
976 | (was rv2sv) | |
977 | FLAGS = (SCALAR,KIDS) | |
978 | { | |
979 | 11 TYPE = gvsv ===> 12 | |
980 | FLAGS = (SCALAR) | |
981 | GV = main::b | |
982 | } | |
983 | } | |
984 | ||
985 | # finish this later # | |
986 | ||
8b029fdf MH |
987 | =head2 Using gdb to look at specific parts of a program |
988 | ||
73013070 SF |
989 | With the example above, you knew to look for C<Perl_pp_add>, but what if |
990 | there were multiple calls to it all over the place, or you didn't know what | |
8b029fdf MH |
991 | the op was you were looking for? |
992 | ||
73013070 | 993 | One way to do this is to inject a rare call somewhere near what you're looking |
9b22382a | 994 | for. For example, you could add C<study> before your method: |
8b029fdf MH |
995 | |
996 | study; | |
997 | ||
998 | And in gdb do: | |
999 | ||
1000 | (gdb) break Perl_pp_study | |
1001 | ||
9b22382a | 1002 | And then step until you hit what you're |
73013070 | 1003 | looking for. This works well in a loop |
8b029fdf MH |
1004 | if you want to only break at certain iterations: |
1005 | ||
1006 | for my $c (1..100) { | |
1007 | study if $c == 50; | |
1008 | } | |
1009 | ||
1010 | =head2 Using gdb to look at what the parser/lexer are doing | |
1011 | ||
73013070 | 1012 | If you want to see what perl is doing when parsing/lexing your code, you can |
72b22e55 | 1013 | use C<BEGIN {}>: |
8b029fdf MH |
1014 | |
1015 | print "Before\n"; | |
1016 | BEGIN { study; } | |
1017 | print "After\n"; | |
1018 | ||
1019 | And in gdb: | |
1020 | ||
1021 | (gdb) break Perl_pp_study | |
1022 | ||
1023 | If you want to see what the parser/lexer is doing inside of C<if> blocks and | |
1024 | the like you need to be a little trickier: | |
1025 | ||
73013070 | 1026 | if ($a && $b && do { BEGIN { study } 1 } && $c) { ... } |
8b029fdf | 1027 | |
04c692a8 DR |
1028 | =head1 SOURCE CODE STATIC ANALYSIS |
1029 | ||
1030 | Various tools exist for analysing C source code B<statically>, as | |
9b22382a | 1031 | opposed to B<dynamically>, that is, without executing the code. It is |
04c692a8 DR |
1032 | possible to detect resource leaks, undefined behaviour, type |
1033 | mismatches, portability problems, code paths that would cause illegal | |
1034 | memory accesses, and other similar problems by just parsing the C code | |
1035 | and looking at the resulting graph, what does it tell about the | |
9b22382a | 1036 | execution and data flows. As a matter of fact, this is exactly how C |
04c692a8 DR |
1037 | compilers know to give warnings about dubious code. |
1038 | ||
c707756e | 1039 | =head2 lint |
04c692a8 DR |
1040 | |
1041 | The good old C code quality inspector, C<lint>, is available in several | |
1042 | platforms, but please be aware that there are several different | |
1043 | implementations of it by different vendors, which means that the flags | |
1044 | are not identical across different platforms. | |
1045 | ||
c707756e | 1046 | There is a C<lint> target in Makefile, but you may have to |
04c692a8 DR |
1047 | diddle with the flags (see above). |
1048 | ||
1049 | =head2 Coverity | |
1050 | ||
4b05bc8e | 1051 | Coverity (L<http://www.coverity.com/>) is a product similar to lint and as |
04c692a8 DR |
1052 | a testbed for their product they periodically check several open source |
1053 | projects, and they give out accounts to open source developers to the | |
1054 | defect databases. | |
1055 | ||
d3c1eddb JH |
1056 | There is Coverity setup for the perl5 project: |
1057 | L<https://scan.coverity.com/projects/perl5> | |
1058 | ||
a72f2680 | 1059 | =head2 HP-UX cadvise (Code Advisor) |
65c4791f JH |
1060 | |
1061 | HP has a C/C++ static analyzer product for HP-UX caller Code Advisor. | |
1062 | (Link not given here because the URL is horribly long and seems horribly | |
1063 | unstable; use the search engine of your choice to find it.) The use of | |
1064 | the C<cadvise_cc> recipe with C<Configure ... -Dcc=./cadvise_cc> | |
1065 | (see cadvise "User Guide") is recommended; as is the use of C<+wall>. | |
1066 | ||
04c692a8 DR |
1067 | =head2 cpd (cut-and-paste detector) |
1068 | ||
9b22382a | 1069 | The cpd tool detects cut-and-paste coding. If one instance of the |
04c692a8 | 1070 | cut-and-pasted code changes, all the other spots should probably be |
9b22382a | 1071 | changed, too. Therefore such code should probably be turned into a |
04c692a8 DR |
1072 | subroutine or a macro. |
1073 | ||
4b05bc8e KW |
1074 | cpd (L<http://pmd.sourceforge.net/cpd.html>) is part of the pmd project |
1075 | (L<http://pmd.sourceforge.net/>). pmd was originally written for static | |
04c692a8 DR |
1076 | analysis of Java code, but later the cpd part of it was extended to |
1077 | parse also C and C++. | |
1078 | ||
1079 | Download the pmd-bin-X.Y.zip () from the SourceForge site, extract the | |
1080 | pmd-X.Y.jar from it, and then run that on source code thusly: | |
1081 | ||
0cbf2b31 FC |
1082 | java -cp pmd-X.Y.jar net.sourceforge.pmd.cpd.CPD \ |
1083 | --minimum-tokens 100 --files /some/where/src --language c > cpd.txt | |
04c692a8 DR |
1084 | |
1085 | You may run into memory limits, in which case you should use the -Xmx | |
1086 | option: | |
1087 | ||
1088 | java -Xmx512M ... | |
1089 | ||
1090 | =head2 gcc warnings | |
1091 | ||
1092 | Though much can be written about the inconsistency and coverage | |
1093 | problems of gcc warnings (like C<-Wall> not meaning "all the warnings", | |
1094 | or some common portability problems not being covered by C<-Wall>, or | |
1095 | C<-ansi> and C<-pedantic> both being a poorly defined collection of | |
1096 | warnings, and so forth), gcc is still a useful tool in keeping our | |
1097 | coding nose clean. | |
1098 | ||
1099 | The C<-Wall> is by default on. | |
1100 | ||
1101 | The C<-ansi> (and its sidekick, C<-pedantic>) would be nice to be on | |
1102 | always, but unfortunately they are not safe on all platforms, they can | |
1103 | for example cause fatal conflicts with the system headers (Solaris | |
9b22382a | 1104 | being a prime example). If Configure C<-Dgccansipedantic> is used, the |
04c692a8 DR |
1105 | C<cflags> frontend selects C<-ansi -pedantic> for the platforms where |
1106 | they are known to be safe. | |
1107 | ||
2884c977 | 1108 | The following extra flags are added: |
04c692a8 DR |
1109 | |
1110 | =over 4 | |
1111 | ||
1112 | =item * | |
1113 | ||
1114 | C<-Wendif-labels> | |
1115 | ||
1116 | =item * | |
1117 | ||
1118 | C<-Wextra> | |
1119 | ||
1120 | =item * | |
1121 | ||
2884c977 DIM |
1122 | C<-Wc++-compat> |
1123 | ||
1124 | =item * | |
1125 | ||
1126 | C<-Wwrite-strings> | |
1127 | ||
1128 | =item * | |
1129 | ||
1130 | C<-Werror=declaration-after-statement> | |
04c692a8 | 1131 | |
5997475b DIM |
1132 | =item * |
1133 | ||
1134 | C<-Werror=pointer-arith> | |
1135 | ||
04c692a8 DR |
1136 | =back |
1137 | ||
1138 | The following flags would be nice to have but they would first need | |
1139 | their own Augean stablemaster: | |
1140 | ||
1141 | =over 4 | |
1142 | ||
1143 | =item * | |
1144 | ||
04c692a8 DR |
1145 | C<-Wshadow> |
1146 | ||
1147 | =item * | |
1148 | ||
1149 | C<-Wstrict-prototypes> | |
1150 | ||
1151 | =back | |
1152 | ||
1153 | The C<-Wtraditional> is another example of the annoying tendency of gcc | |
1154 | to bundle a lot of warnings under one switch (it would be impossible to | |
1155 | deploy in practice because it would complain a lot) but it does contain | |
1156 | some warnings that would be beneficial to have available on their own, | |
1157 | such as the warning about string constants inside macros containing the | |
1158 | macro arguments: this behaved differently pre-ANSI than it does in | |
1159 | ANSI, and some C compilers are still in transition, AIX being an | |
1160 | example. | |
1161 | ||
1162 | =head2 Warnings of other C compilers | |
1163 | ||
1164 | Other C compilers (yes, there B<are> other C compilers than gcc) often | |
1165 | have their "strict ANSI" or "strict ANSI with some portability | |
1166 | extensions" modes on, like for example the Sun Workshop has its C<-Xa> | |
1167 | mode on (though implicitly), or the DEC (these days, HP...) has its | |
1168 | C<-std1> mode on. | |
1169 | ||
1170 | =head1 MEMORY DEBUGGERS | |
1171 | ||
d1fd4856 VP |
1172 | B<NOTE 1>: Running under older memory debuggers such as Purify, |
1173 | valgrind or Third Degree greatly slows down the execution: seconds | |
9b22382a | 1174 | become minutes, minutes become hours. For example as of Perl 5.8.1, the |
04c692a8 | 1175 | ext/Encode/t/Unicode.t takes extraordinarily long to complete under |
9b22382a FC |
1176 | e.g. Purify, Third Degree, and valgrind. Under valgrind it takes more |
1177 | than six hours, even on a snappy computer. The said test must be doing | |
1178 | something that is quite unfriendly for memory debuggers. If you don't | |
04c692a8 | 1179 | feel like waiting, that you can simply kill away the perl process. |
d1fd4856 VP |
1180 | Roughly valgrind slows down execution by factor 10, AddressSanitizer by |
1181 | factor 2. | |
04c692a8 DR |
1182 | |
1183 | B<NOTE 2>: To minimize the number of memory leak false alarms (see | |
1184 | L</PERL_DESTRUCT_LEVEL> for more information), you have to set the | |
9b22382a | 1185 | environment variable PERL_DESTRUCT_LEVEL to 2. For example, like this: |
04c692a8 DR |
1186 | |
1187 | env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib ... | |
1188 | ||
1189 | B<NOTE 3>: There are known memory leaks when there are compile-time | |
1190 | errors within eval or require, seeing C<S_doeval> in the call stack is | |
9b22382a | 1191 | a good sign of these. Fixing these leaks is non-trivial, unfortunately, |
04c692a8 DR |
1192 | but they must be fixed eventually. |
1193 | ||
1194 | B<NOTE 4>: L<DynaLoader> will not clean up after itself completely | |
1195 | unless Perl is built with the Configure option | |
1196 | C<-Accflags=-DDL_UNLOAD_ALL_AT_EXIT>. | |
1197 | ||
04c692a8 DR |
1198 | =head2 valgrind |
1199 | ||
d1fd4856 | 1200 | The valgrind tool can be used to find out both memory leaks and illegal |
9b22382a | 1201 | heap memory accesses. As of version 3.3.0, Valgrind only supports Linux |
0263e49a | 1202 | on x86, x86-64 and PowerPC and Darwin (OS X) on x86 and x86-64. The |
d1fd4856 | 1203 | special "test.valgrind" target can be used to run the tests under |
9b22382a | 1204 | valgrind. Found errors and memory leaks are logged in files named |
037ab3f1 MH |
1205 | F<testfile.valgrind> and by default output is displayed inline. |
1206 | ||
1207 | Example usage: | |
1208 | ||
1209 | make test.valgrind | |
1210 | ||
1211 | Since valgrind adds significant overhead, tests will take much longer to | |
1212 | run. The valgrind tests support being run in parallel to help with this: | |
1213 | ||
1214 | TEST_JOBS=9 make test.valgrind | |
1215 | ||
1216 | Note that the above two invocations will be very verbose as reachable | |
1217 | memory and leak-checking is enabled by default. If you want to just see | |
1218 | pure errors, try: | |
73013070 | 1219 | |
037ab3f1 MH |
1220 | VG_OPTS='-q --leak-check=no --show-reachable=no' TEST_JOBS=9 \ |
1221 | make test.valgrind | |
04c692a8 DR |
1222 | |
1223 | Valgrind also provides a cachegrind tool, invoked on perl as: | |
1224 | ||
1225 | VG_OPTS=--tool=cachegrind make test.valgrind | |
1226 | ||
1227 | As system libraries (most notably glibc) are also triggering errors, | |
9b22382a | 1228 | valgrind allows to suppress such errors using suppression files. The |
04c692a8 | 1229 | default suppression file that comes with valgrind already catches a lot |
9b22382a | 1230 | of them. Some additional suppressions are defined in F<t/perl.supp>. |
04c692a8 DR |
1231 | |
1232 | To get valgrind and for more information see | |
1233 | ||
0061d4fa | 1234 | http://valgrind.org/ |
04c692a8 | 1235 | |
81c3bbe7 RU |
1236 | =head2 AddressSanitizer |
1237 | ||
4dd56148 | 1238 | AddressSanitizer is a clang and gcc extension, included in clang since |
9b22382a | 1239 | v3.1 and gcc since v4.8. It checks illegal heap pointers, global |
4dd56148 NC |
1240 | pointers, stack pointers and use after free errors, and is fast enough |
1241 | that you can easily compile your debugging or optimized perl with it. | |
9b22382a | 1242 | It does not check memory leaks though. AddressSanitizer is available |
4dd56148 | 1243 | for Linux, Mac OS X and soon on Windows. |
81c3bbe7 | 1244 | |
8a64fbaa VP |
1245 | To build perl with AddressSanitizer, your Configure invocation should |
1246 | look like: | |
81c3bbe7 | 1247 | |
e8596d90 VP |
1248 | sh Configure -des -Dcc=clang \ |
1249 | -Accflags=-faddress-sanitizer -Aldflags=-faddress-sanitizer \ | |
1250 | -Alddlflags=-shared\ -faddress-sanitizer | |
81c3bbe7 RU |
1251 | |
1252 | where these arguments mean: | |
1253 | ||
1254 | =over 4 | |
1255 | ||
1256 | =item * -Dcc=clang | |
1257 | ||
8a64fbaa VP |
1258 | This should be replaced by the full path to your clang executable if it |
1259 | is not in your path. | |
81c3bbe7 RU |
1260 | |
1261 | =item * -Accflags=-faddress-sanitizer | |
1262 | ||
8a64fbaa | 1263 | Compile perl and extensions sources with AddressSanitizer. |
81c3bbe7 RU |
1264 | |
1265 | =item * -Aldflags=-faddress-sanitizer | |
1266 | ||
8a64fbaa | 1267 | Link the perl executable with AddressSanitizer. |
81c3bbe7 | 1268 | |
e8596d90 | 1269 | =item * -Alddlflags=-shared\ -faddress-sanitizer |
81c3bbe7 | 1270 | |
9b22382a | 1271 | Link dynamic extensions with AddressSanitizer. You must manually |
e8596d90 VP |
1272 | specify C<-shared> because using C<-Alddlflags=-shared> will prevent |
1273 | Configure from setting a default value for C<lddlflags>, which usually | |
5dfc6e97 | 1274 | contains C<-shared> (at least on Linux). |
81c3bbe7 RU |
1275 | |
1276 | =back | |
1277 | ||
8a64fbaa VP |
1278 | See also |
1279 | L<http://code.google.com/p/address-sanitizer/wiki/AddressSanitizer>. | |
81c3bbe7 RU |
1280 | |
1281 | ||
04c692a8 DR |
1282 | =head1 PROFILING |
1283 | ||
1284 | Depending on your platform there are various ways of profiling Perl. | |
1285 | ||
1286 | There are two commonly used techniques of profiling executables: | |
1287 | I<statistical time-sampling> and I<basic-block counting>. | |
1288 | ||
1289 | The first method takes periodically samples of the CPU program counter, | |
1290 | and since the program counter can be correlated with the code generated | |
1291 | for functions, we get a statistical view of in which functions the | |
9b22382a | 1292 | program is spending its time. The caveats are that very small/fast |
04c692a8 DR |
1293 | functions have lower probability of showing up in the profile, and that |
1294 | periodically interrupting the program (this is usually done rather | |
1295 | frequently, in the scale of milliseconds) imposes an additional | |
9b22382a | 1296 | overhead that may skew the results. The first problem can be alleviated |
04c692a8 DR |
1297 | by running the code for longer (in general this is a good idea for |
1298 | profiling), the second problem is usually kept in guard by the | |
1299 | profiling tools themselves. | |
1300 | ||
1301 | The second method divides up the generated code into I<basic blocks>. | |
1302 | Basic blocks are sections of code that are entered only in the | |
9b22382a FC |
1303 | beginning and exited only at the end. For example, a conditional jump |
1304 | starts a basic block. Basic block profiling usually works by | |
04c692a8 | 1305 | I<instrumenting> the code by adding I<enter basic block #nnnn> |
9b22382a FC |
1306 | book-keeping code to the generated code. During the execution of the |
1307 | code the basic block counters are then updated appropriately. The | |
04c692a8 DR |
1308 | caveat is that the added extra code can skew the results: again, the |
1309 | profiling tools usually try to factor their own effects out of the | |
1310 | results. | |
1311 | ||
1312 | =head2 Gprof Profiling | |
1313 | ||
e2aed43d | 1314 | I<gprof> is a profiling tool available in many Unix platforms which |
9b22382a FC |
1315 | uses I<statistical time-sampling>. You can build a profiled version of |
1316 | F<perl> by compiling using gcc with the flag C<-pg>. Either edit | |
1317 | F<config.sh> or re-run F<Configure>. Running the profiled version of | |
e2aed43d NC |
1318 | Perl will create an output file called F<gmon.out> which contains the |
1319 | profiling data collected during the execution. | |
04c692a8 | 1320 | |
e2aed43d NC |
1321 | quick hint: |
1322 | ||
1323 | $ sh Configure -des -Dusedevel -Accflags='-pg' \ | |
1324 | -Aldflags='-pg' -Alddlflags='-pg -shared' \ | |
1325 | && make perl | |
1326 | $ ./perl ... # creates gmon.out in current directory | |
1327 | $ gprof ./perl > out | |
1328 | $ less out | |
1329 | ||
1330 | (you probably need to add C<-shared> to the <-Alddlflags> line until RT | |
1331 | #118199 is resolved) | |
04c692a8 | 1332 | |
e2aed43d NC |
1333 | The F<gprof> tool can then display the collected data in various ways. |
1334 | Usually F<gprof> understands the following options: | |
04c692a8 DR |
1335 | |
1336 | =over 4 | |
1337 | ||
1338 | =item * -a | |
1339 | ||
1340 | Suppress statically defined functions from the profile. | |
1341 | ||
1342 | =item * -b | |
1343 | ||
1344 | Suppress the verbose descriptions in the profile. | |
1345 | ||
1346 | =item * -e routine | |
1347 | ||
1348 | Exclude the given routine and its descendants from the profile. | |
1349 | ||
1350 | =item * -f routine | |
1351 | ||
1352 | Display only the given routine and its descendants in the profile. | |
1353 | ||
1354 | =item * -s | |
1355 | ||
1356 | Generate a summary file called F<gmon.sum> which then may be given to | |
1357 | subsequent gprof runs to accumulate data over several runs. | |
1358 | ||
1359 | =item * -z | |
1360 | ||
1361 | Display routines that have zero usage. | |
1362 | ||
1363 | =back | |
1364 | ||
1365 | For more detailed explanation of the available commands and output | |
e2aed43d | 1366 | formats, see your own local documentation of F<gprof>. |
04c692a8 | 1367 | |
e2aed43d | 1368 | =head2 GCC gcov Profiling |
04c692a8 | 1369 | |
e2aed43d NC |
1370 | I<basic block profiling> is officially available in gcc 3.0 and later. |
1371 | You can build a profiled version of F<perl> by compiling using gcc with | |
9b22382a | 1372 | the flags C<-fprofile-arcs -ftest-coverage>. Either edit F<config.sh> |
e2aed43d | 1373 | or re-run F<Configure>. |
04c692a8 | 1374 | |
e2aed43d | 1375 | quick hint: |
04c692a8 | 1376 | |
e2aed43d NC |
1377 | $ sh Configure -des -Dusedevel -Doptimize='-g' \ |
1378 | -Accflags='-fprofile-arcs -ftest-coverage' \ | |
1379 | -Aldflags='-fprofile-arcs -ftest-coverage' \ | |
1380 | -Alddlflags='-fprofile-arcs -ftest-coverage -shared' \ | |
1381 | && make perl | |
1382 | $ rm -f regexec.c.gcov regexec.gcda | |
1383 | $ ./perl ... | |
1384 | $ gcov regexec.c | |
1385 | $ less regexec.c.gcov | |
04c692a8 | 1386 | |
e2aed43d NC |
1387 | (you probably need to add C<-shared> to the <-Alddlflags> line until RT |
1388 | #118199 is resolved) | |
04c692a8 DR |
1389 | |
1390 | Running the profiled version of Perl will cause profile output to be | |
9b22382a | 1391 | generated. For each source file an accompanying F<.gcda> file will be |
04c692a8 DR |
1392 | created. |
1393 | ||
e2aed43d | 1394 | To display the results you use the I<gcov> utility (which should be |
9b22382a | 1395 | installed if you have gcc 3.0 or newer installed). F<gcov> is run on |
04c692a8 DR |
1396 | source code files, like this |
1397 | ||
1398 | gcov sv.c | |
1399 | ||
9b22382a | 1400 | which will cause F<sv.c.gcov> to be created. The F<.gcov> files contain |
04c692a8 | 1401 | the source code annotated with relative frequencies of execution |
9b22382a | 1402 | indicated by "#" markers. If you want to generate F<.gcov> files for |
6f134219 NC |
1403 | all profiled object files, you can run something like this: |
1404 | ||
1405 | for file in `find . -name \*.gcno` | |
1406 | do sh -c "cd `dirname $file` && gcov `basename $file .gcno`" | |
1407 | done | |
04c692a8 DR |
1408 | |
1409 | Useful options of F<gcov> include C<-b> which will summarise the basic | |
1410 | block, branch, and function call coverage, and C<-c> which instead of | |
9b22382a | 1411 | relative frequencies will use the actual counts. For more information |
04c692a8 | 1412 | on the use of F<gcov> and basic block profiling with gcc, see the |
9b22382a | 1413 | latest GNU CC manual. As of gcc 4.8, this is at |
e2aed43d | 1414 | L<http://gcc.gnu.org/onlinedocs/gcc/Gcov-Intro.html#Gcov-Intro> |
04c692a8 DR |
1415 | |
1416 | =head1 MISCELLANEOUS TRICKS | |
1417 | ||
1418 | =head2 PERL_DESTRUCT_LEVEL | |
1419 | ||
1420 | If you want to run any of the tests yourself manually using e.g. | |
4dd56148 NC |
1421 | valgrind, please note that by default perl B<does not> explicitly |
1422 | cleanup all the memory it has allocated (such as global memory arenas) | |
1423 | but instead lets the exit() of the whole program "take care" of such | |
1424 | allocations, also known as "global destruction of objects". | |
04c692a8 DR |
1425 | |
1426 | There is a way to tell perl to do complete cleanup: set the environment | |
9b22382a | 1427 | variable PERL_DESTRUCT_LEVEL to a non-zero value. The t/TEST wrapper |
04c692a8 | 1428 | does set this to 2, and this is what you need to do too, if you don't |
f01ecde8 | 1429 | want to see the "global leaks": For example, for running under valgrind |
04c692a8 | 1430 | |
a63ef199 | 1431 | env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib t/foo/bar.t |
04c692a8 DR |
1432 | |
1433 | (Note: the mod_perl apache module uses also this environment variable | |
9b22382a FC |
1434 | for its own purposes and extended its semantics. Refer to the mod_perl |
1435 | documentation for more information. Also, spawned threads do the | |
04c692a8 DR |
1436 | equivalent of setting this variable to the value 1.) |
1437 | ||
1438 | If, at the end of a run you get the message I<N scalars leaked>, you | |
b4986286 DM |
1439 | can recompile with C<-DDEBUG_LEAKING_SCALARS>, |
1440 | (C<Configure -Accflags=-DDEBUG_LEAKING_SCALARS>), which will cause the | |
04c692a8 | 1441 | addresses of all those leaked SVs to be dumped along with details as to |
9b22382a FC |
1442 | where each SV was originally allocated. This information is also |
1443 | displayed by Devel::Peek. Note that the extra details recorded with | |
04c692a8 | 1444 | each SV increases memory usage, so it shouldn't be used in production |
9b22382a | 1445 | environments. It also converts C<new_SV()> from a macro into a real |
04c692a8 DR |
1446 | function, so you can use your favourite debugger to discover where |
1447 | those pesky SVs were allocated. | |
1448 | ||
1449 | If you see that you're leaking memory at runtime, but neither valgrind | |
1450 | nor C<-DDEBUG_LEAKING_SCALARS> will find anything, you're probably | |
1451 | leaking SVs that are still reachable and will be properly cleaned up | |
9b22382a FC |
1452 | during destruction of the interpreter. In such cases, using the C<-Dm> |
1453 | switch can point you to the source of the leak. If the executable was | |
04c692a8 | 1454 | built with C<-DDEBUG_LEAKING_SCALARS>, C<-Dm> will output SV |
9b22382a | 1455 | allocations in addition to memory allocations. Each SV allocation has a |
04c692a8 | 1456 | distinct serial number that will be written on creation and destruction |
9b22382a | 1457 | of the SV. So if you're executing the leaking code in a loop, you need |
04c692a8 | 1458 | to look for SVs that are created, but never destroyed between each |
9b22382a | 1459 | cycle. If such an SV is found, set a conditional breakpoint within |
04c692a8 | 1460 | C<new_SV()> and make it break only when C<PL_sv_serial> is equal to the |
9b22382a | 1461 | serial number of the leaking SV. Then you will catch the interpreter in |
04c692a8 DR |
1462 | exactly the state where the leaking SV is allocated, which is |
1463 | sufficient in many cases to find the source of the leak. | |
1464 | ||
1465 | As C<-Dm> is using the PerlIO layer for output, it will by itself | |
9b22382a | 1466 | allocate quite a bunch of SVs, which are hidden to avoid recursion. You |
04c692a8 DR |
1467 | can bypass the PerlIO layer if you use the SV logging provided by |
1468 | C<-DPERL_MEM_LOG> instead. | |
1469 | ||
1470 | =head2 PERL_MEM_LOG | |
1471 | ||
6fb87544 MH |
1472 | If compiled with C<-DPERL_MEM_LOG> (C<-Accflags=-DPERL_MEM_LOG>), both |
1473 | memory and SV allocations go through logging functions, which is | |
1474 | handy for breakpoint setting. | |
04c692a8 | 1475 | |
6fb87544 MH |
1476 | Unless C<-DPERL_MEM_LOG_NOIMPL> (C<-Accflags=-DPERL_MEM_LOG_NOIMPL>) is |
1477 | also compiled, the logging functions read $ENV{PERL_MEM_LOG} to | |
1478 | determine whether to log the event, and if so how: | |
04c692a8 | 1479 | |
a63ef199 SF |
1480 | $ENV{PERL_MEM_LOG} =~ /m/ Log all memory ops |
1481 | $ENV{PERL_MEM_LOG} =~ /s/ Log all SV ops | |
1482 | $ENV{PERL_MEM_LOG} =~ /t/ include timestamp in Log | |
1483 | $ENV{PERL_MEM_LOG} =~ /^(\d+)/ write to FD given (default is 2) | |
04c692a8 DR |
1484 | |
1485 | Memory logging is somewhat similar to C<-Dm> but is independent of | |
1486 | C<-DDEBUGGING>, and at a higher level; all uses of Newx(), Renew(), and | |
1487 | Safefree() are logged with the caller's source code file and line | |
9b22382a FC |
1488 | number (and C function name, if supported by the C compiler). In |
1489 | contrast, C<-Dm> is directly at the point of C<malloc()>. SV logging is | |
04c692a8 DR |
1490 | similar. |
1491 | ||
1492 | Since the logging doesn't use PerlIO, all SV allocations are logged and | |
9b22382a | 1493 | no extra SV allocations are introduced by enabling the logging. If |
04c692a8 DR |
1494 | compiled with C<-DDEBUG_LEAKING_SCALARS>, the serial number for each SV |
1495 | allocation is also logged. | |
1496 | ||
1497 | =head2 DDD over gdb | |
1498 | ||
1499 | Those debugging perl with the DDD frontend over gdb may find the | |
1500 | following useful: | |
1501 | ||
1502 | You can extend the data conversion shortcuts menu, so for example you | |
1503 | can display an SV's IV value with one click, without doing any typing. | |
1504 | To do that simply edit ~/.ddd/init file and add after: | |
1505 | ||
1506 | ! Display shortcuts. | |
1507 | Ddd*gdbDisplayShortcuts: \ | |
1508 | /t () // Convert to Bin\n\ | |
1509 | /d () // Convert to Dec\n\ | |
1510 | /x () // Convert to Hex\n\ | |
1511 | /o () // Convert to Oct(\n\ | |
1512 | ||
1513 | the following two lines: | |
1514 | ||
1515 | ((XPV*) (())->sv_any )->xpv_pv // 2pvx\n\ | |
1516 | ((XPVIV*) (())->sv_any )->xiv_iv // 2ivx | |
1517 | ||
1518 | so now you can do ivx and pvx lookups or you can plug there the sv_peek | |
1519 | "conversion": | |
1520 | ||
1521 | Perl_sv_peek(my_perl, (SV*)()) // sv_peek | |
1522 | ||
9b22382a | 1523 | (The my_perl is for threaded builds.) Just remember that every line, |
04c692a8 DR |
1524 | but the last one, should end with \n\ |
1525 | ||
1526 | Alternatively edit the init file interactively via: 3rd mouse button -> | |
1527 | New Display -> Edit Menu | |
1528 | ||
1529 | Note: you can define up to 20 conversion shortcuts in the gdb section. | |
1530 | ||
470dd224 JH |
1531 | =head2 C backtrace |
1532 | ||
0762e42f JH |
1533 | On some platforms Perl supports retrieving the C level backtrace |
1534 | (similar to what symbolic debuggers like gdb do). | |
470dd224 JH |
1535 | |
1536 | The backtrace returns the stack trace of the C call frames, | |
1537 | with the symbol names (function names), the object names (like "perl"), | |
1538 | and if it can, also the source code locations (file:line). | |
1539 | ||
0762e42f JH |
1540 | The supported platforms are Linux, and OS X (some *BSD might |
1541 | work at least partly, but they have not yet been tested). | |
1542 | ||
1543 | This feature hasn't been tested with multiple threads, but it will | |
1544 | only show the backtrace of the thread doing the backtracing. | |
470dd224 JH |
1545 | |
1546 | The feature needs to be enabled with C<Configure -Dusecbacktrace>. | |
1547 | ||
0762e42f JH |
1548 | The C<-Dusecbacktrace> also enables keeping the debug information when |
1549 | compiling/linking (often: C<-g>). Many compilers/linkers do support | |
1550 | having both optimization and keeping the debug information. The debug | |
1551 | information is needed for the symbol names and the source locations. | |
1552 | ||
1553 | Static functions might not be visible for the backtrace. | |
470dd224 JH |
1554 | |
1555 | Source code locations, even if available, can often be missing or | |
0762e42f JH |
1556 | misleading if the compiler has e.g. inlined code. Optimizer can |
1557 | make matching the source code and the object code quite challenging. | |
470dd224 JH |
1558 | |
1559 | =over 4 | |
1560 | ||
1561 | =item Linux | |
1562 | ||
59b3baca | 1563 | You B<must> have the BFD (-lbfd) library installed, otherwise C<perl> will |
0762e42f | 1564 | fail to link. The BFD is usually distributed as part of the GNU binutils. |
470dd224 JH |
1565 | |
1566 | Summary: C<Configure ... -Dusecbacktrace> | |
1567 | and you need C<-lbfd>. | |
1568 | ||
1569 | =item OS X | |
1570 | ||
0762e42f JH |
1571 | The source code locations are supported B<only> if you have |
1572 | the Developer Tools installed. (BFD is B<not> needed.) | |
470dd224 JH |
1573 | |
1574 | Summary: C<Configure ... -Dusecbacktrace> | |
1575 | and installing the Developer Tools would be good. | |
1576 | ||
1577 | =back | |
1578 | ||
1579 | Optionally, for trying out the feature, you may want to enable | |
0762e42f JH |
1580 | automatic dumping of the backtrace just before a warning or croak (die) |
1581 | message is emitted, by adding C<-Accflags=-DUSE_C_BACKTRACE_ON_ERROR> | |
1582 | for Configure. | |
470dd224 JH |
1583 | |
1584 | Unless the above additional feature is enabled, nothing about the | |
1585 | backtrace functionality is visible, except for the Perl/XS level. | |
1586 | ||
1587 | Furthermore, even if you have enabled this feature to be compiled, | |
1588 | you need to enable it in runtime with an environment variable: | |
0762e42f JH |
1589 | C<PERL_C_BACKTRACE_ON_ERROR=10>. It must be an integer higher |
1590 | than zero, telling the desired frame count. | |
470dd224 JH |
1591 | |
1592 | Retrieving the backtrace from Perl level (using for example an XS | |
1593 | extension) would be much less exciting than one would hope: normally | |
1594 | you would see C<runops>, C<entersub>, and not much else. This API is | |
1595 | intended to be called B<from within> the Perl implementation, not from | |
1596 | Perl level execution. | |
1597 | ||
0762e42f | 1598 | The C API for the backtrace is as follows: |
470dd224 JH |
1599 | |
1600 | =over 4 | |
1601 | ||
1602 | =item get_c_backtrace | |
1603 | ||
1604 | =item free_c_backtrace | |
1605 | ||
1606 | =item get_c_backtrace_dump | |
1607 | ||
1608 | =item dump_c_backtrace | |
1609 | ||
1610 | =back | |
1611 | ||
04c692a8 DR |
1612 | =head2 Poison |
1613 | ||
1614 | If you see in a debugger a memory area mysteriously full of 0xABABABAB | |
1615 | or 0xEFEFEFEF, you may be seeing the effect of the Poison() macros, see | |
1616 | L<perlclib>. | |
1617 | ||
1618 | =head2 Read-only optrees | |
1619 | ||
9b22382a | 1620 | Under ithreads the optree is read only. If you want to enforce this, to |
04c692a8 | 1621 | check for write accesses from buggy code, compile with |
91fc0422 FC |
1622 | C<-Accflags=-DPERL_DEBUG_READONLY_OPS> |
1623 | to enable code that allocates op memory | |
4dd56148 NC |
1624 | via C<mmap>, and sets it read-only when it is attached to a subroutine. |
1625 | Any write access to an op results in a C<SIGBUS> and abort. | |
04c692a8 DR |
1626 | |
1627 | This code is intended for development only, and may not be portable | |
9b22382a FC |
1628 | even to all Unix variants. Also, it is an 80% solution, in that it |
1629 | isn't able to make all ops read only. Specifically it does not apply to | |
4dd56148 | 1630 | op slabs belonging to C<BEGIN> blocks. |
04c692a8 | 1631 | |
4dd56148 NC |
1632 | However, as an 80% solution it is still effective, as it has caught |
1633 | bugs in the past. | |
04c692a8 | 1634 | |
f789f6a4 FC |
1635 | =head2 When is a bool not a bool? |
1636 | ||
1637 | On pre-C99 compilers, C<bool> is defined as equivalent to C<char>. | |
b59008ae AC |
1638 | Consequently assignment of any larger type to a C<bool> is unsafe and may be |
1639 | truncated. The C<cBOOL> macro exists to cast it correctly; you may also find | |
1640 | that using it is shorter and clearer than writing out the equivalent | |
1641 | conditional expression longhand. | |
f789f6a4 FC |
1642 | |
1643 | On those platforms and compilers where C<bool> really is a boolean (C++, | |
1644 | C99), it is easy to forget the cast. You can force C<bool> to be a C<char> | |
1645 | by compiling with C<-Accflags=-DPERL_BOOL_AS_CHAR>. You may also wish to | |
50e4f4d4 CB |
1646 | run C<Configure> with something like |
1647 | ||
cbc13c3d | 1648 | -Accflags='-Wconversion -Wno-sign-conversion -Wno-shorten-64-to-32' |
50e4f4d4 CB |
1649 | |
1650 | or your compiler's equivalent to make it easier to spot any unsafe truncations | |
1651 | that show up. | |
f789f6a4 | 1652 | |
b59008ae AC |
1653 | The C<TRUE> and C<FALSE> macros are available for situations where using them |
1654 | would clarify intent. (But they always just mean the same as the integers 1 and | |
1655 | 0 regardless, so using them isn't compulsory.) | |
1656 | ||
04c692a8 DR |
1657 | =head2 The .i Targets |
1658 | ||
1659 | You can expand the macros in a F<foo.c> file by saying | |
1660 | ||
1661 | make foo.i | |
1662 | ||
d1fd4856 VP |
1663 | which will expand the macros using cpp. Don't be scared by the |
1664 | results. | |
04c692a8 DR |
1665 | |
1666 | =head1 AUTHOR | |
1667 | ||
1668 | This document was originally written by Nathan Torkington, and is | |
1669 | maintained by the perl5-porters mailing list. |