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5Consistent formatting of this file is achieved with:
6 perl ./Porting/podtidy pod/perlhacktips.pod
7
8=head1 NAME
9
10perlhacktips - Tips for Perl core C code hacking
11
12=head1 DESCRIPTION
13
14This document will help you learn the best way to go about hacking on
9b22382a 15the Perl core C code. It covers common problems, debugging, profiling,
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16and more.
17
18If you haven't read L<perlhack> and L<perlhacktut> yet, you might want
19to do that first.
20
21=head1 COMMON PROBLEMS
22
9b22382a 23Perl source plays by ANSI C89 rules: no C99 (or C++) extensions. In
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24some cases we have to take pre-ANSI requirements into consideration.
25You don't care about some particular platform having broken Perl? I
26hear there is still a strong demand for J2EE programmers.
27
28=head2 Perl environment problems
29
30=over 4
31
32=item *
33
34Not compiling with threading
35
36Compiling with threading (-Duseithreads) completely rewrites the
9b22382a 37function prototypes of Perl. You better try your changes with that.
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38Related to this is the difference between "Perl_-less" and "Perl_-ly"
39APIs, for example:
40
41 Perl_sv_setiv(aTHX_ ...);
42 sv_setiv(...);
43
44The first one explicitly passes in the context, which is needed for
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45e.g. threaded builds. The second one does that implicitly; do not get
46them mixed. If you are not passing in a aTHX_, you will need to do a
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47dTHX (or a dVAR) as the first thing in the function.
48
49See L<perlguts/"How multiple interpreters and concurrency are
50supported"> for further discussion about context.
51
52=item *
53
54Not compiling with -DDEBUGGING
55
56The DEBUGGING define exposes more code to the compiler, therefore more
9b22382a 57ways for things to go wrong. You should try it.
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58
59=item *
60
61Introducing (non-read-only) globals
62
63Do not introduce any modifiable globals, truly global or file static.
64They are bad form and complicate multithreading and other forms of
9b22382a 65concurrency. The right way is to introduce them as new interpreter
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66variables, see F<intrpvar.h> (at the very end for binary
67compatibility).
68
69Introducing read-only (const) globals is okay, as long as you verify
70with e.g. C<nm libperl.a|egrep -v ' [TURtr] '> (if your C<nm> has
9b22382a 71BSD-style output) that the data you added really is read-only. (If it
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72is, it shouldn't show up in the output of that command.)
73
74If you want to have static strings, make them constant:
75
76 static const char etc[] = "...";
77
78If you want to have arrays of constant strings, note carefully the
79right combination of C<const>s:
80
81 static const char * const yippee[] =
82 {"hi", "ho", "silver"};
83
84There is a way to completely hide any modifiable globals (they are all
85moved to heap), the compilation setting
9b22382a 86C<-DPERL_GLOBAL_STRUCT_PRIVATE>. It is not normally used, but can be
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87used for testing, read more about it in L<perlguts/"Background and
88PERL_IMPLICIT_CONTEXT">.
89
90=item *
91
92Not exporting your new function
93
94Some platforms (Win32, AIX, VMS, OS/2, to name a few) require any
95function that is part of the public API (the shared Perl library) to be
9b22382a 96explicitly marked as exported. See the discussion about F<embed.pl> in
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97L<perlguts>.
98
99=item *
100
101Exporting your new function
102
103The new shiny result of either genuine new functionality or your
9b22382a 104arduous refactoring is now ready and correctly exported. So what could
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105possibly go wrong?
106
107Maybe simply that your function did not need to be exported in the
9b22382a 108first place. Perl has a long and not so glorious history of exporting
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109functions that it should not have.
110
111If the function is used only inside one source code file, make it
9b22382a 112static. See the discussion about F<embed.pl> in L<perlguts>.
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113
114If the function is used across several files, but intended only for
115Perl's internal use (and this should be the common case), do not export
9b22382a 116it to the public API. See the discussion about F<embed.pl> in
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117L<perlguts>.
118
119=back
120
121=head2 Portability problems
122
123The following are common causes of compilation and/or execution
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124failures, not common to Perl as such. The C FAQ is good bedtime
125reading. Please test your changes with as many C compilers and
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126platforms as possible; we will, anyway, and it's nice to save oneself
127from public embarrassment.
128
129If using gcc, you can add the C<-std=c89> option which will hopefully
9b22382a 130catch most of these unportabilities. (However it might also catch
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131incompatibilities in your system's header files.)
132
133Use the Configure C<-Dgccansipedantic> flag to enable the gcc C<-ansi
134-pedantic> flags which enforce stricter ANSI rules.
135
136If using the C<gcc -Wall> note that not all the possible warnings (like
137C<-Wunitialized>) are given unless you also compile with C<-O>.
138
139Note that if using gcc, starting from Perl 5.9.5 the Perl core source
140code files (the ones at the top level of the source code distribution,
141but not e.g. the extensions under ext/) are automatically compiled with
142as many as possible of the C<-std=c89>, C<-ansi>, C<-pedantic>, and a
143selection of C<-W> flags (see cflags.SH).
144
145Also study L<perlport> carefully to avoid any bad assumptions about the
146operating system, filesystems, and so forth.
147
148You may once in a while try a "make microperl" to see whether we can
9b22382a 149still compile Perl with just the bare minimum of interfaces. (See
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150README.micro.)
151
152Do not assume an operating system indicates a certain compiler.
153
154=over 4
155
156=item *
157
158Casting pointers to integers or casting integers to pointers
159
160 void castaway(U8* p)
161 {
162 IV i = p;
163
164or
165
166 void castaway(U8* p)
167 {
168 IV i = (IV)p;
169
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170Both are bad, and broken, and unportable. Use the PTR2IV() macro that
171does it right. (Likewise, there are PTR2UV(), PTR2NV(), INT2PTR(), and
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172NUM2PTR().)
173
174=item *
175
176Casting between data function pointers and data pointers
177
178Technically speaking casting between function pointers and data
179pointers is unportable and undefined, but practically speaking it seems
180to work, but you should use the FPTR2DPTR() and DPTR2FPTR() macros.
181Sometimes you can also play games with unions.
182
183=item *
184
185Assuming sizeof(int) == sizeof(long)
186
187There are platforms where longs are 64 bits, and platforms where ints
188are 64 bits, and while we are out to shock you, even platforms where
9b22382a 189shorts are 64 bits. This is all legal according to the C standard. (In
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190other 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
193Instead, use the definitions IV, UV, IVSIZE, I32SIZE, and so forth.
194Avoid things like I32 because they are B<not> guaranteed to be
195I<exactly> 32 bits, they are I<at least> 32 bits, nor are they
9b22382a 196guaranteed to be B<int> or B<long>. If you really explicitly need
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19764-bit variables, use I64 and U64, but only if guarded by HAS_QUAD.
198
199=item *
200
201Assuming one can dereference any type of pointer for any type of data
202
203 char *p = ...;
204 long pony = *p; /* BAD */
205
206Many platforms, quite rightly so, will give you a core dump instead of
768312ab 207a pony if the p happens not to be correctly aligned.
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208
209=item *
210
211Lvalue casts
212
213 (int)*p = ...; /* BAD */
214
9b22382a 215Simply not portable. Get your lvalue to be of the right type, or maybe
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216use temporary variables, or dirty tricks with unions.
217
218=item *
219
220Assume B<anything> about structs (especially the ones you don't
221control, like the ones coming from the system headers)
222
223=over 8
224
225=item *
226
227That a certain field exists in a struct
228
229=item *
230
231That no other fields exist besides the ones you know of
232
233=item *
234
235That a field is of certain signedness, sizeof, or type
236
237=item *
238
239That the fields are in a certain order
240
241=over 8
242
243=item *
244
245While C guarantees the ordering specified in the struct definition,
246between different platforms the definitions might differ
247
248=back
249
250=item *
251
252That the sizeof(struct) or the alignments are the same everywhere
253
254=over 8
255
256=item *
257
258There might be padding bytes between the fields to align the fields -
259the bytes can be anything
260
261=item *
262
263Structs are required to be aligned to the maximum alignment required by
264the fields - which for native types is for usually equivalent to
265sizeof() of the field
266
267=back
268
269=back
270
271=item *
272
273Assuming the character set is ASCIIish
274
9b22382a 275Perl can compile and run under EBCDIC platforms. See L<perlebcdic>.
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276This is transparent for the most part, but because the character sets
277differ, you shouldn't use numeric (decimal, octal, nor hex) constants
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278to refer to characters. You can safely say 'A', but not 0x41. You can
279safely say '\n', but not \012. If a character doesn't have a trivial
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280input form, you should add it to the list in
281F<regen/unicode_constants.pl>, and have Perl create #defines for you,
282based on the current platform.
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283
284Also, the range 'A' - 'Z' in ASCII is an unbroken sequence of 26 upper
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285case 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
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287anything about other ranges.
288
289Many of the comments in the existing code ignore the possibility of
9b22382a 290EBCDIC, and may be wrong therefore, even if the code works. This is
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291actually a tribute to the successful transparent insertion of being
292able to handle EBCDIC without having to change pre-existing code.
293
294UTF-8 and UTF-EBCDIC are two different encodings used to represent
9b22382a 295Unicode code points as sequences of bytes. Macros with the same names
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296(but different definitions) in C<utf8.h> and C<utfebcdic.h> are used to
297allow the calling code to think that there is only one such encoding.
298This is almost always referred to as C<utf8>, but it means the EBCDIC
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299version as well. Again, comments in the code may well be wrong even if
300the code itself is right. For example, the concept of C<invariant
301characters> differs between ASCII and EBCDIC. On ASCII platforms, only
302characters that do not have the high-order bit set (i.e. whose ordinals
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303are strict ASCII, 0 - 127) are invariant, and the documentation and
304comments in the code may assume that, often referring to something
9b22382a 305like, say, C<hibit>. The situation differs and is not so simple on
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306EBCDIC machines, but as long as the code itself uses the
307C<NATIVE_IS_INVARIANT()> macro appropriately, it works, even if the
308comments are wrong.
309
310=item *
311
312Assuming the character set is just ASCII
313
9b22382a 314ASCII is a 7 bit encoding, but bytes have 8 bits in them. The 128 extra
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315characters have different meanings depending on the locale. Absent a
316locale, currently these extra characters are generally considered to be
9b22382a 317unassigned, and this has presented some problems. This is being changed
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318starting in 5.12 so that these characters will be considered to be
319Latin-1 (ISO-8859-1).
320
321=item *
322
323Mixing #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 332You cannot portably "stack" cpp directives. For example in the above
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333you need two separate BURGLE() #defines, one for each #ifdef branch.
334
335=item *
336
337Adding non-comment stuff after #endif or #else
338
339 #ifdef SNOSH
340 ...
341 #else !SNOSH /* BAD */
342 ...
343 #endif SNOSH /* BAD */
344
345The #endif and #else cannot portably have anything non-comment after
9b22382a 346them. If you want to document what is going (which is a good idea
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347especially if the branches are long), use (C) comments:
348
349 #ifdef SNOSH
350 ...
351 #else /* !SNOSH */
352 ...
353 #endif /* SNOSH */
354
355The gcc option C<-Wendif-labels> warns about the bad variant (by
356default on starting from Perl 5.9.4).
357
358=item *
359
360Having a comma after the last element of an enum list
361
362 enum color {
363 CERULEAN,
364 CHARTREUSE,
365 CINNABAR, /* BAD */
366 };
367
9b22382a 368is not portable. Leave out the last comma.
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369
370Also note that whether enums are implicitly morphable to ints varies
371between compilers, you might need to (int).
372
373=item *
374
375Using //-comments
376
377 // This function bamfoodles the zorklator. /* BAD */
378
9b22382a 379That is C99 or C++. Perl is C89. Using the //-comments is silently
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380allowed 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
385Mixing declarations and code
386
387 void zorklator()
388 {
389 int n = 3;
390 set_zorkmids(n); /* BAD */
391 int q = 4;
392
9b22382a 393That is C99 or C++. Some C compilers allow that, but you shouldn't.
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394
395The gcc option C<-Wdeclaration-after-statements> scans for such
396problems (by default on starting from Perl 5.9.4).
397
398=item *
399
400Introducing variables inside for()
401
402 for(int i = ...; ...; ...) { /* BAD */
403
9b22382a 404That is C99 or C++. While it would indeed be awfully nice to have that
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405also in C89, to limit the scope of the loop variable, alas, we cannot.
406
407=item *
408
409Mixing 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
416While this is legal practice, it is certainly dubious, and downright
417fatal in at least one platform: for example VMS cc considers this a
9b22382a 418fatal error. One cause for people often making this mistake is that a
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419"naked char" and therefore dereferencing a "naked char pointer" have an
420undefined signedness: it depends on the compiler and the flags of the
421compiler and the underlying platform whether the result is signed or
9b22382a 422unsigned. For this very same reason using a 'char' as an array index is
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423bad.
424
425=item *
426
427Macros that have string constants and their arguments as substrings of
428the string constants
429
430 #define FOO(n) printf("number = %d\n", n) /* BAD */
431 FOO(10);
432
433Pre-ANSI semantics for that was equivalent to
434
435 printf("10umber = %d\10");
436
9b22382a 437which is probably not what you were expecting. Unfortunately at least
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438one reasonably common and modern C compiler does "real backward
439compatibility" here, in AIX that is what still happens even though the
440rest of the AIX compiler is very happily C89.
441
442=item *
443
444Using printf formats for non-basic C types
445
446 IV i = ...;
447 printf("i = %d\n", i); /* BAD */
448
449While this might by accident work in some platform (where IV happens to
9b22382a 450be an C<int>), in general it cannot. IV might be something larger. Even
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451worse the situation is with more specific types (defined by Perl's
452configuration step in F<config.h>):
453
454 Uid_t who = ...;
455 printf("who = %d\n", who); /* BAD */
456
457The problem here is that Uid_t might be not only not C<int>-wide but it
458might also be unsigned, in which case large uids would be printed as
459negative values.
460
461There is no simple solution to this because of printf()'s limited
462intelligence, but for many types the right format is available as with
463either '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
474Or you can try casting to a "wide enough" type:
475
476 printf("i = %"IVdf"\n", (IV)something_very_small_and_signed);
477
478Also remember that the C<%p> format really does require a void pointer:
479
480 U8* p = ...;
481 printf("p = %p\n", (void*)p);
482
483The gcc option C<-Wformat> scans for such problems.
484
485=item *
486
487Blindly using variadic macros
488
489gcc has had them for a while with its own syntax, and C99 brought them
9b22382a 490with a standardized syntax. Don't use the former, and use the latter
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491only if the HAS_C99_VARIADIC_MACROS is defined.
492
493=item *
494
495Blindly passing va_list
496
497Not all platforms support passing va_list to further varargs (stdarg)
9b22382a 498functions. The right thing to do is to copy the va_list using the
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499Perl_va_copy() if the NEED_VA_COPY is defined.
500
501=item *
502
503Using gcc statement expressions
504
505 val = ({...;...;...}); /* BAD */
506
9b22382a 507While a nice extension, it's not portable. The Perl code does
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508admittedly use them if available to gain some extra speed (essentially
509as a funky form of inlining), but you shouldn't.
510
511=item *
512
513Binding together several statements in a macro
514
515Use the macros STMT_START and STMT_END.
516
517 STMT_START {
518 ...
519 } STMT_END
520
521=item *
522
523Testing for operating systems or versions when should be testing for
524features
525
526 #ifdef __FOONIX__ /* BAD */
527 foo = quux();
528 #endif
529
530Unless you know with 100% certainty that quux() is only ever available
531for the "Foonix" operating system B<and> that is available B<and>
532correctly 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
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534not perfect, because the below is a compile-time check):
535
536 #ifdef HAS_QUUX
537 foo = quux();
538 #endif
539
540How does the HAS_QUUX become defined where it needs to be? Well, if
541Foonix happens to be Unixy enough to be able to run the Configure
542script, and Configure has been taught about detecting and testing
9b22382a 543quux(), the HAS_QUUX will be correctly defined. In other platforms, the
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544corresponding configuration step will hopefully do the same.
545
546In a pinch, if you cannot wait for Configure to be educated, or if you
547have a good hunch of where quux() might be available, you can
548temporarily 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
560But in any case, try to keep the features and operating systems
561separate.
562
563=back
564
565=head2 Problematic System Interfaces
566
567=over 4
568
569=item *
570
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571malloc(0), realloc(0), calloc(0, 0) are non-portable. To be portable
572allocate at least one byte. (In general you should rarely need to work
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573at this low level, but instead use the various malloc wrappers.)
574
575=item *
576
9b22382a 577snprintf() - the return type is unportable. Use my_snprintf() instead.
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578
579=back
580
581=head2 Security problems
582
583Last but not least, here are various tips for safer coding.
584
585=over 4
586
587=item *
588
589Do not use gets()
590
9b22382a 591Or we will publicly ridicule you. Seriously.
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592
593=item *
594
595Do not use strcpy() or strcat() or strncpy() or strncat()
596
597Use my_strlcpy() and my_strlcat() instead: they either use the native
598implementation, or Perl's own implementation (borrowed from the public
599domain implementation of INN).
600
601=item *
602
603Do not use sprintf() or vsprintf()
604
605If you really want just plain byte strings, use my_snprintf() and
606my_vsnprintf() instead, which will try to use snprintf() and
9b22382a 607vsnprintf() if those safer APIs are available. If you want something
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608fancier than a plain byte string, use
609L<C<Perl_form>()|perlapi/form> or SVs and
610L<C<Perl_sv_catpvf()>|perlapi/sv_catpvf>.
611
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612Note that glibc C<printf()>, C<sprintf()>, etc. are buggy before glibc
613version 2.17. They won't allow a C<%.s> format with a precision to
614create a string that isn't valid UTF-8 if the current underlying locale
615of the program is UTF-8. What happens is that the C<%s> and its operand are
6bfe0388 616simply skipped without any notice.
2e642750 617L<https://sourceware.org/bugzilla/show_bug.cgi?id=6530>.
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618
619=back
620
621=head1 DEBUGGING
622
623You can compile a special debugging version of Perl, which allows you
624to use the C<-D> option of Perl to tell more about what Perl is doing.
625But sometimes there is no alternative than to dive in with a debugger,
626either to see the stack trace of a core dump (very useful in a bug
627report), or trying to figure out what went wrong before the core dump
628happened, or how did we end up having wrong or unexpected results.
629
630=head2 Poking at Perl
631
632To really poke around with Perl, you'll probably want to build Perl for
633debugging, like this:
634
635 ./Configure -d -D optimize=-g
636 make
637
638C<-g> is a flag to the C compiler to have it produce debugging
639information which will allow us to step through a running program, and
640to see in which C function we are at (without the debugging information
641we might see only the numerical addresses of the functions, which is
642not very helpful).
643
644F<Configure> will also turn on the C<DEBUGGING> compilation symbol
9b22382a 645which enables all the internal debugging code in Perl. There are a
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646whole bunch of things you can debug with this: L<perlrun> lists them
647all, and the best way to find out about them is to play about with
9b22382a 648them. The most useful options are probably
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649
650 l Context (loop) stack processing
651 t Trace execution
652 o Method and overloading resolution
653 c String/numeric conversions
654
655Some of the functionality of the debugging code can be achieved using
656XS modules.
657
658 -Dr => use re 'debug'
659 -Dx => use O 'Debug'
660
661=head2 Using a source-level debugger
662
663If the debugging output of C<-D> doesn't help you, it's time to step
664through perl's execution with a source-level debugger.
665
666=over 3
667
668=item *
669
670We'll use C<gdb> for our examples here; the principles will apply to
671any debugger (many vendors call their debugger C<dbx>), but check the
672manual of the one you're using.
673
674=back
675
676To fire up the debugger, type
677
678 gdb ./perl
679
680Or if you have a core dump:
681
682 gdb ./perl core
683
684You'll want to do that in your Perl source tree so the debugger can
9b22382a 685read the source code. You should see the copyright message, followed by
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686the prompt.
687
688 (gdb)
689
690C<help> will get you into the documentation, but here are the most
691useful commands:
692
693=over 3
694
695=item * run [args]
696
697Run the program with the given arguments.
698
699=item * break function_name
700
701=item * break source.c:xxx
702
703Tells the debugger that we'll want to pause execution when we reach
704either the named function (but see L<perlguts/Internal Functions>!) or
705the given line in the named source file.
706
707=item * step
708
709Steps through the program a line at a time.
710
711=item * next
712
713Steps through the program a line at a time, without descending into
714functions.
715
716=item * continue
717
718Run until the next breakpoint.
719
720=item * finish
721
722Run until the end of the current function, then stop again.
723
724=item * 'enter'
725
726Just pressing Enter will do the most recent operation again - it's a
727blessing when stepping through miles of source code.
728
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729=item * ptype
730
731Prints the C definition of the argument given.
732
733 (gdb) ptype PL_op
734 type = struct op {
735 OP *op_next;
736 OP *op_sibling;
737 OP *(*op_ppaddr)(void);
738 PADOFFSET op_targ;
739 unsigned int op_type : 9;
740 unsigned int op_opt : 1;
741 unsigned int op_slabbed : 1;
742 unsigned int op_savefree : 1;
743 unsigned int op_static : 1;
744 unsigned int op_folded : 1;
745 unsigned int op_spare : 2;
746 U8 op_flags;
747 U8 op_private;
748 } *
749
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750=item * print
751
9b22382a 752Execute the given C code and print its results. B<WARNING>: Perl makes
04c692a8 753heavy use of macros, and F<gdb> does not necessarily support macros
9b22382a 754(see later L</"gdb macro support">). You'll have to substitute them
04c692a8
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755yourself, or to invoke cpp on the source code files (see L</"The .i
756Targets">) So, for instance, you can't say
757
758 print SvPV_nolen(sv)
759
760but you have to say
761
762 print Perl_sv_2pv_nolen(sv)
763
764=back
765
766You may find it helpful to have a "macro dictionary", which you can
9b22382a 767produce by saying C<cpp -dM perl.c | sort>. Even then, F<cpp> won't
04c692a8
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768recursively apply those macros for you.
769
770=head2 gdb macro support
771
772Recent versions of F<gdb> have fairly good macro support, but in order
773to use it you'll need to compile perl with macro definitions included
9b22382a
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774in the debugging information. Using F<gcc> version 3.1, this means
775configuring with C<-Doptimize=-g3>. Other compilers might use a
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776different switch (if they support debugging macros at all).
777
778=head2 Dumping Perl Data Structures
779
780One way to get around this macro hell is to use the dumping functions
781in F<dump.c>; these work a little like an internal
782L<Devel::Peek|Devel::Peek>, but they also cover OPs and other
9b22382a 783structures that you can't get at from Perl. Let's take an example.
04c692a8 784We'll use the C<$a = $b + $c> we used before, but give it a bit of
9b22382a 785context: C<$b = "6XXXX"; $c = 2.3;>. Where's a good place to stop and
04c692a8
DR
786poke around?
787
788What about C<pp_add>, the function we examined earlier to implement the
789C<+> operator:
790
791 (gdb) break Perl_pp_add
792 Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.
793
794Notice we use C<Perl_pp_add> and not C<pp_add> - see
9b22382a 795L<perlguts/Internal Functions>. With the breakpoint in place, we can
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796run our program:
797
798 (gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c'
799
800Lots of junk will go past as gdb reads in the relevant source files and
801libraries, and then:
802
803 Breakpoint 1, Perl_pp_add () at pp_hot.c:309
804 309 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
805 (gdb) step
806 311 dPOPTOPnnrl_ul;
807 (gdb)
808
809We looked at this bit of code before, and we said that
810C<dPOPTOPnnrl_ul> arranges for two C<NV>s to be placed into C<left> and
811C<right> - let's slightly expand it:
812
813 #define dPOPTOPnnrl_ul NV right = POPn; \
814 SV *leftsv = TOPs; \
815 NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0
816
817C<POPn> takes the SV from the top of the stack and obtains its NV
818either directly (if C<SvNOK> is set) or by calling the C<sv_2nv>
9b22382a
FC
819function. C<TOPs> takes the next SV from the top of the stack - yes,
820C<POPn> uses C<TOPs> - but doesn't remove it. We then use C<SvNV> to
04c692a8
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821get the NV from C<leftsv> in the same way as before - yes, C<POPn> uses
822C<SvNV>.
823
824Since we don't have an NV for C<$b>, we'll have to use C<sv_2nv> to
9b22382a 825convert it. If we step again, we'll find ourselves there:
04c692a8 826
8b029fdf 827 (gdb) step
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828 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669
829 1669 if (!sv)
830 (gdb)
831
832We can now use C<Perl_sv_dump> to investigate the SV:
833
8b029fdf 834 (gdb) print Perl_sv_dump(sv)
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835 SV = PV(0xa057cc0) at 0xa0675d0
836 REFCNT = 1
837 FLAGS = (POK,pPOK)
838 PV = 0xa06a510 "6XXXX"\0
839 CUR = 5
840 LEN = 6
841 $1 = void
842
843We know we're going to get C<6> from this, so let's finish the
844subroutine:
845
846 (gdb) finish
847 Run till exit from #0 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671
848 0x462669 in Perl_pp_add () at pp_hot.c:311
849 311 dPOPTOPnnrl_ul;
850
851We can also dump out this op: the current op is always stored in
9b22382a 852C<PL_op>, and we can dump it with C<Perl_op_dump>. This'll give us
04c692a8
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853similar output to L<B::Debug|B::Debug>.
854
8b029fdf 855 (gdb) print Perl_op_dump(PL_op)
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DR
856 {
857 13 TYPE = add ===> 14
858 TARG = 1
859 FLAGS = (SCALAR,KIDS)
860 {
861 TYPE = null ===> (12)
862 (was rv2sv)
863 FLAGS = (SCALAR,KIDS)
864 {
865 11 TYPE = gvsv ===> 12
866 FLAGS = (SCALAR)
867 GV = main::b
868 }
869 }
870
871# finish this later #
872
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MH
873=head2 Using gdb to look at specific parts of a program
874
875With the example above, you knew to look for C<Perl_pp_add>, but what if
876there were multiple calls to it all over the place, or you didn't know what
877the op was you were looking for?
878
879One way to do this is to inject a rare call somewhere near what you're looking
9b22382a 880for. For example, you could add C<study> before your method:
8b029fdf
MH
881
882 study;
883
884And in gdb do:
885
886 (gdb) break Perl_pp_study
887
9b22382a
FC
888And then step until you hit what you're
889looking for. This works well in a loop
8b029fdf
MH
890if you want to only break at certain iterations:
891
892 for my $c (1..100) {
893 study if $c == 50;
894 }
895
896=head2 Using gdb to look at what the parser/lexer are doing
897
898If you want to see what perl is doing when parsing/lexing your code, you can
72b22e55 899use C<BEGIN {}>:
8b029fdf
MH
900
901 print "Before\n";
902 BEGIN { study; }
903 print "After\n";
904
905And in gdb:
906
907 (gdb) break Perl_pp_study
908
909If you want to see what the parser/lexer is doing inside of C<if> blocks and
910the like you need to be a little trickier:
911
912 if ($a && $b && do { BEGIN { study } 1 } && $c) { ... }
913
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914=head1 SOURCE CODE STATIC ANALYSIS
915
916Various tools exist for analysing C source code B<statically>, as
9b22382a 917opposed to B<dynamically>, that is, without executing the code. It is
04c692a8
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918possible to detect resource leaks, undefined behaviour, type
919mismatches, portability problems, code paths that would cause illegal
920memory accesses, and other similar problems by just parsing the C code
921and looking at the resulting graph, what does it tell about the
9b22382a 922execution and data flows. As a matter of fact, this is exactly how C
04c692a8
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923compilers know to give warnings about dubious code.
924
925=head2 lint, splint
926
927The good old C code quality inspector, C<lint>, is available in several
928platforms, but please be aware that there are several different
929implementations of it by different vendors, which means that the flags
930are not identical across different platforms.
931
932There is a lint variant called C<splint> (Secure Programming Lint)
933available from http://www.splint.org/ that should compile on any
934Unix-like platform.
935
936There are C<lint> and <splint> targets in Makefile, but you may have to
937diddle with the flags (see above).
938
939=head2 Coverity
940
941Coverity (http://www.coverity.com/) is a product similar to lint and as
942a testbed for their product they periodically check several open source
943projects, and they give out accounts to open source developers to the
944defect databases.
945
946=head2 cpd (cut-and-paste detector)
947
9b22382a 948The cpd tool detects cut-and-paste coding. If one instance of the
04c692a8 949cut-and-pasted code changes, all the other spots should probably be
9b22382a 950changed, too. Therefore such code should probably be turned into a
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951subroutine or a macro.
952
953cpd (http://pmd.sourceforge.net/cpd.html) is part of the pmd project
9b22382a 954(http://pmd.sourceforge.net/). pmd was originally written for static
04c692a8
DR
955analysis of Java code, but later the cpd part of it was extended to
956parse also C and C++.
957
958Download the pmd-bin-X.Y.zip () from the SourceForge site, extract the
959pmd-X.Y.jar from it, and then run that on source code thusly:
960
0cbf2b31
FC
961 java -cp pmd-X.Y.jar net.sourceforge.pmd.cpd.CPD \
962 --minimum-tokens 100 --files /some/where/src --language c > cpd.txt
04c692a8
DR
963
964You may run into memory limits, in which case you should use the -Xmx
965option:
966
967 java -Xmx512M ...
968
969=head2 gcc warnings
970
971Though much can be written about the inconsistency and coverage
972problems of gcc warnings (like C<-Wall> not meaning "all the warnings",
973or some common portability problems not being covered by C<-Wall>, or
974C<-ansi> and C<-pedantic> both being a poorly defined collection of
975warnings, and so forth), gcc is still a useful tool in keeping our
976coding nose clean.
977
978The C<-Wall> is by default on.
979
980The C<-ansi> (and its sidekick, C<-pedantic>) would be nice to be on
981always, but unfortunately they are not safe on all platforms, they can
982for example cause fatal conflicts with the system headers (Solaris
9b22382a 983being a prime example). If Configure C<-Dgccansipedantic> is used, the
04c692a8
DR
984C<cflags> frontend selects C<-ansi -pedantic> for the platforms where
985they are known to be safe.
986
987Starting from Perl 5.9.4 the following extra flags are added:
988
989=over 4
990
991=item *
992
993C<-Wendif-labels>
994
995=item *
996
997C<-Wextra>
998
999=item *
1000
1001C<-Wdeclaration-after-statement>
1002
1003=back
1004
1005The following flags would be nice to have but they would first need
1006their own Augean stablemaster:
1007
1008=over 4
1009
1010=item *
1011
1012C<-Wpointer-arith>
1013
1014=item *
1015
1016C<-Wshadow>
1017
1018=item *
1019
1020C<-Wstrict-prototypes>
1021
1022=back
1023
1024The C<-Wtraditional> is another example of the annoying tendency of gcc
1025to bundle a lot of warnings under one switch (it would be impossible to
1026deploy in practice because it would complain a lot) but it does contain
1027some warnings that would be beneficial to have available on their own,
1028such as the warning about string constants inside macros containing the
1029macro arguments: this behaved differently pre-ANSI than it does in
1030ANSI, and some C compilers are still in transition, AIX being an
1031example.
1032
1033=head2 Warnings of other C compilers
1034
1035Other C compilers (yes, there B<are> other C compilers than gcc) often
1036have their "strict ANSI" or "strict ANSI with some portability
1037extensions" modes on, like for example the Sun Workshop has its C<-Xa>
1038mode on (though implicitly), or the DEC (these days, HP...) has its
1039C<-std1> mode on.
1040
1041=head1 MEMORY DEBUGGERS
1042
d1fd4856
VP
1043B<NOTE 1>: Running under older memory debuggers such as Purify,
1044valgrind or Third Degree greatly slows down the execution: seconds
9b22382a 1045become minutes, minutes become hours. For example as of Perl 5.8.1, the
04c692a8 1046ext/Encode/t/Unicode.t takes extraordinarily long to complete under
9b22382a
FC
1047e.g. Purify, Third Degree, and valgrind. Under valgrind it takes more
1048than six hours, even on a snappy computer. The said test must be doing
1049something that is quite unfriendly for memory debuggers. If you don't
04c692a8 1050feel like waiting, that you can simply kill away the perl process.
d1fd4856
VP
1051Roughly valgrind slows down execution by factor 10, AddressSanitizer by
1052factor 2.
04c692a8
DR
1053
1054B<NOTE 2>: To minimize the number of memory leak false alarms (see
1055L</PERL_DESTRUCT_LEVEL> for more information), you have to set the
9b22382a 1056environment variable PERL_DESTRUCT_LEVEL to 2. For example, like this:
04c692a8
DR
1057
1058 env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib ...
1059
1060B<NOTE 3>: There are known memory leaks when there are compile-time
1061errors within eval or require, seeing C<S_doeval> in the call stack is
9b22382a 1062a good sign of these. Fixing these leaks is non-trivial, unfortunately,
04c692a8
DR
1063but they must be fixed eventually.
1064
1065B<NOTE 4>: L<DynaLoader> will not clean up after itself completely
1066unless Perl is built with the Configure option
1067C<-Accflags=-DDL_UNLOAD_ALL_AT_EXIT>.
1068
04c692a8
DR
1069=head2 valgrind
1070
d1fd4856 1071The valgrind tool can be used to find out both memory leaks and illegal
9b22382a
FC
1072heap memory accesses. As of version 3.3.0, Valgrind only supports Linux
1073on x86, x86-64 and PowerPC and Darwin (OS X) on x86 and x86-64). The
d1fd4856 1074special "test.valgrind" target can be used to run the tests under
9b22382a 1075valgrind. Found errors and memory leaks are logged in files named
d1fd4856 1076F<testfile.valgrind>.
04c692a8
DR
1077
1078Valgrind also provides a cachegrind tool, invoked on perl as:
1079
1080 VG_OPTS=--tool=cachegrind make test.valgrind
1081
1082As system libraries (most notably glibc) are also triggering errors,
9b22382a 1083valgrind allows to suppress such errors using suppression files. The
04c692a8 1084default suppression file that comes with valgrind already catches a lot
9b22382a 1085of them. Some additional suppressions are defined in F<t/perl.supp>.
04c692a8
DR
1086
1087To get valgrind and for more information see
1088
0061d4fa 1089 http://valgrind.org/
04c692a8 1090
81c3bbe7
RU
1091=head2 AddressSanitizer
1092
4dd56148 1093AddressSanitizer is a clang and gcc extension, included in clang since
9b22382a 1094v3.1 and gcc since v4.8. It checks illegal heap pointers, global
4dd56148
NC
1095pointers, stack pointers and use after free errors, and is fast enough
1096that you can easily compile your debugging or optimized perl with it.
9b22382a 1097It does not check memory leaks though. AddressSanitizer is available
4dd56148 1098for Linux, Mac OS X and soon on Windows.
81c3bbe7 1099
8a64fbaa
VP
1100To build perl with AddressSanitizer, your Configure invocation should
1101look like:
81c3bbe7 1102
e8596d90
VP
1103 sh Configure -des -Dcc=clang \
1104 -Accflags=-faddress-sanitizer -Aldflags=-faddress-sanitizer \
1105 -Alddlflags=-shared\ -faddress-sanitizer
81c3bbe7
RU
1106
1107where these arguments mean:
1108
1109=over 4
1110
1111=item * -Dcc=clang
1112
8a64fbaa
VP
1113This should be replaced by the full path to your clang executable if it
1114is not in your path.
81c3bbe7
RU
1115
1116=item * -Accflags=-faddress-sanitizer
1117
8a64fbaa 1118Compile perl and extensions sources with AddressSanitizer.
81c3bbe7
RU
1119
1120=item * -Aldflags=-faddress-sanitizer
1121
8a64fbaa 1122Link the perl executable with AddressSanitizer.
81c3bbe7 1123
e8596d90 1124=item * -Alddlflags=-shared\ -faddress-sanitizer
81c3bbe7 1125
9b22382a 1126Link dynamic extensions with AddressSanitizer. You must manually
e8596d90
VP
1127specify C<-shared> because using C<-Alddlflags=-shared> will prevent
1128Configure from setting a default value for C<lddlflags>, which usually
5dfc6e97 1129contains C<-shared> (at least on Linux).
81c3bbe7
RU
1130
1131=back
1132
8a64fbaa
VP
1133See also
1134L<http://code.google.com/p/address-sanitizer/wiki/AddressSanitizer>.
81c3bbe7
RU
1135
1136
04c692a8
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1137=head1 PROFILING
1138
1139Depending on your platform there are various ways of profiling Perl.
1140
1141There are two commonly used techniques of profiling executables:
1142I<statistical time-sampling> and I<basic-block counting>.
1143
1144The first method takes periodically samples of the CPU program counter,
1145and since the program counter can be correlated with the code generated
1146for functions, we get a statistical view of in which functions the
9b22382a 1147program is spending its time. The caveats are that very small/fast
04c692a8
DR
1148functions have lower probability of showing up in the profile, and that
1149periodically interrupting the program (this is usually done rather
1150frequently, in the scale of milliseconds) imposes an additional
9b22382a 1151overhead that may skew the results. The first problem can be alleviated
04c692a8
DR
1152by running the code for longer (in general this is a good idea for
1153profiling), the second problem is usually kept in guard by the
1154profiling tools themselves.
1155
1156The second method divides up the generated code into I<basic blocks>.
1157Basic blocks are sections of code that are entered only in the
9b22382a
FC
1158beginning and exited only at the end. For example, a conditional jump
1159starts a basic block. Basic block profiling usually works by
04c692a8 1160I<instrumenting> the code by adding I<enter basic block #nnnn>
9b22382a
FC
1161book-keeping code to the generated code. During the execution of the
1162code the basic block counters are then updated appropriately. The
04c692a8
DR
1163caveat is that the added extra code can skew the results: again, the
1164profiling tools usually try to factor their own effects out of the
1165results.
1166
1167=head2 Gprof Profiling
1168
e2aed43d 1169I<gprof> is a profiling tool available in many Unix platforms which
9b22382a
FC
1170uses I<statistical time-sampling>. You can build a profiled version of
1171F<perl> by compiling using gcc with the flag C<-pg>. Either edit
1172F<config.sh> or re-run F<Configure>. Running the profiled version of
e2aed43d
NC
1173Perl will create an output file called F<gmon.out> which contains the
1174profiling data collected during the execution.
04c692a8 1175
e2aed43d
NC
1176quick hint:
1177
1178 $ sh Configure -des -Dusedevel -Accflags='-pg' \
1179 -Aldflags='-pg' -Alddlflags='-pg -shared' \
1180 && make perl
1181 $ ./perl ... # creates gmon.out in current directory
1182 $ gprof ./perl > out
1183 $ less out
1184
1185(you probably need to add C<-shared> to the <-Alddlflags> line until RT
1186#118199 is resolved)
04c692a8 1187
e2aed43d
NC
1188The F<gprof> tool can then display the collected data in various ways.
1189Usually F<gprof> understands the following options:
04c692a8
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1190
1191=over 4
1192
1193=item * -a
1194
1195Suppress statically defined functions from the profile.
1196
1197=item * -b
1198
1199Suppress the verbose descriptions in the profile.
1200
1201=item * -e routine
1202
1203Exclude the given routine and its descendants from the profile.
1204
1205=item * -f routine
1206
1207Display only the given routine and its descendants in the profile.
1208
1209=item * -s
1210
1211Generate a summary file called F<gmon.sum> which then may be given to
1212subsequent gprof runs to accumulate data over several runs.
1213
1214=item * -z
1215
1216Display routines that have zero usage.
1217
1218=back
1219
1220For more detailed explanation of the available commands and output
e2aed43d 1221formats, see your own local documentation of F<gprof>.
04c692a8 1222
e2aed43d 1223=head2 GCC gcov Profiling
04c692a8 1224
e2aed43d
NC
1225I<basic block profiling> is officially available in gcc 3.0 and later.
1226You can build a profiled version of F<perl> by compiling using gcc with
9b22382a 1227the flags C<-fprofile-arcs -ftest-coverage>. Either edit F<config.sh>
e2aed43d 1228or re-run F<Configure>.
04c692a8 1229
e2aed43d 1230quick hint:
04c692a8 1231
e2aed43d
NC
1232 $ sh Configure -des -Dusedevel -Doptimize='-g' \
1233 -Accflags='-fprofile-arcs -ftest-coverage' \
1234 -Aldflags='-fprofile-arcs -ftest-coverage' \
1235 -Alddlflags='-fprofile-arcs -ftest-coverage -shared' \
1236 && make perl
1237 $ rm -f regexec.c.gcov regexec.gcda
1238 $ ./perl ...
1239 $ gcov regexec.c
1240 $ less regexec.c.gcov
04c692a8 1241
e2aed43d
NC
1242(you probably need to add C<-shared> to the <-Alddlflags> line until RT
1243#118199 is resolved)
04c692a8
DR
1244
1245Running the profiled version of Perl will cause profile output to be
9b22382a 1246generated. For each source file an accompanying F<.gcda> file will be
04c692a8
DR
1247created.
1248
e2aed43d 1249To display the results you use the I<gcov> utility (which should be
9b22382a 1250installed if you have gcc 3.0 or newer installed). F<gcov> is run on
04c692a8
DR
1251source code files, like this
1252
1253 gcov sv.c
1254
9b22382a 1255which will cause F<sv.c.gcov> to be created. The F<.gcov> files contain
04c692a8 1256the source code annotated with relative frequencies of execution
9b22382a 1257indicated by "#" markers. If you want to generate F<.gcov> files for
6f134219
NC
1258all profiled object files, you can run something like this:
1259
1260 for file in `find . -name \*.gcno`
1261 do sh -c "cd `dirname $file` && gcov `basename $file .gcno`"
1262 done
04c692a8
DR
1263
1264Useful options of F<gcov> include C<-b> which will summarise the basic
1265block, branch, and function call coverage, and C<-c> which instead of
9b22382a 1266relative frequencies will use the actual counts. For more information
04c692a8 1267on the use of F<gcov> and basic block profiling with gcc, see the
9b22382a 1268latest GNU CC manual. As of gcc 4.8, this is at
e2aed43d 1269L<http://gcc.gnu.org/onlinedocs/gcc/Gcov-Intro.html#Gcov-Intro>
04c692a8
DR
1270
1271=head1 MISCELLANEOUS TRICKS
1272
1273=head2 PERL_DESTRUCT_LEVEL
1274
1275If you want to run any of the tests yourself manually using e.g.
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1276valgrind, please note that by default perl B<does not> explicitly
1277cleanup all the memory it has allocated (such as global memory arenas)
1278but instead lets the exit() of the whole program "take care" of such
1279allocations, also known as "global destruction of objects".
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1280
1281There is a way to tell perl to do complete cleanup: set the environment
9b22382a 1282variable PERL_DESTRUCT_LEVEL to a non-zero value. The t/TEST wrapper
04c692a8 1283does set this to 2, and this is what you need to do too, if you don't
f01ecde8 1284want to see the "global leaks": For example, for running under valgrind
04c692a8 1285
f01ecde8 1286 env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib t/foo/bar.t
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1287
1288(Note: the mod_perl apache module uses also this environment variable
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1289for its own purposes and extended its semantics. Refer to the mod_perl
1290documentation for more information. Also, spawned threads do the
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1291equivalent of setting this variable to the value 1.)
1292
1293If, at the end of a run you get the message I<N scalars leaked>, you
1294can recompile with C<-DDEBUG_LEAKING_SCALARS>, which will cause the
1295addresses of all those leaked SVs to be dumped along with details as to
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FC
1296where each SV was originally allocated. This information is also
1297displayed by Devel::Peek. Note that the extra details recorded with
04c692a8 1298each SV increases memory usage, so it shouldn't be used in production
9b22382a 1299environments. It also converts C<new_SV()> from a macro into a real
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1300function, so you can use your favourite debugger to discover where
1301those pesky SVs were allocated.
1302
1303If you see that you're leaking memory at runtime, but neither valgrind
1304nor C<-DDEBUG_LEAKING_SCALARS> will find anything, you're probably
1305leaking SVs that are still reachable and will be properly cleaned up
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1306during destruction of the interpreter. In such cases, using the C<-Dm>
1307switch can point you to the source of the leak. If the executable was
04c692a8 1308built with C<-DDEBUG_LEAKING_SCALARS>, C<-Dm> will output SV
9b22382a 1309allocations in addition to memory allocations. Each SV allocation has a
04c692a8 1310distinct serial number that will be written on creation and destruction
9b22382a 1311of the SV. So if you're executing the leaking code in a loop, you need
04c692a8 1312to look for SVs that are created, but never destroyed between each
9b22382a 1313cycle. If such an SV is found, set a conditional breakpoint within
04c692a8 1314C<new_SV()> and make it break only when C<PL_sv_serial> is equal to the
9b22382a 1315serial number of the leaking SV. Then you will catch the interpreter in
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1316exactly the state where the leaking SV is allocated, which is
1317sufficient in many cases to find the source of the leak.
1318
1319As C<-Dm> is using the PerlIO layer for output, it will by itself
9b22382a 1320allocate quite a bunch of SVs, which are hidden to avoid recursion. You
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1321can bypass the PerlIO layer if you use the SV logging provided by
1322C<-DPERL_MEM_LOG> instead.
1323
1324=head2 PERL_MEM_LOG
1325
1326If compiled with C<-DPERL_MEM_LOG>, both memory and SV allocations go
1327through logging functions, which is handy for breakpoint setting.
1328
1329Unless C<-DPERL_MEM_LOG_NOIMPL> is also compiled, the logging functions
1330read $ENV{PERL_MEM_LOG} to determine whether to log the event, and if
1331so how:
1332
1333 $ENV{PERL_MEM_LOG} =~ /m/ Log all memory ops
1334 $ENV{PERL_MEM_LOG} =~ /s/ Log all SV ops
1335 $ENV{PERL_MEM_LOG} =~ /t/ include timestamp in Log
1336 $ENV{PERL_MEM_LOG} =~ /^(\d+)/ write to FD given (default is 2)
1337
1338Memory logging is somewhat similar to C<-Dm> but is independent of
1339C<-DDEBUGGING>, and at a higher level; all uses of Newx(), Renew(), and
1340Safefree() are logged with the caller's source code file and line
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FC
1341number (and C function name, if supported by the C compiler). In
1342contrast, C<-Dm> is directly at the point of C<malloc()>. SV logging is
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1343similar.
1344
1345Since the logging doesn't use PerlIO, all SV allocations are logged and
9b22382a 1346no extra SV allocations are introduced by enabling the logging. If
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1347compiled with C<-DDEBUG_LEAKING_SCALARS>, the serial number for each SV
1348allocation is also logged.
1349
1350=head2 DDD over gdb
1351
1352Those debugging perl with the DDD frontend over gdb may find the
1353following useful:
1354
1355You can extend the data conversion shortcuts menu, so for example you
1356can display an SV's IV value with one click, without doing any typing.
1357To do that simply edit ~/.ddd/init file and add after:
1358
1359 ! Display shortcuts.
1360 Ddd*gdbDisplayShortcuts: \
1361 /t () // Convert to Bin\n\
1362 /d () // Convert to Dec\n\
1363 /x () // Convert to Hex\n\
1364 /o () // Convert to Oct(\n\
1365
1366the following two lines:
1367
1368 ((XPV*) (())->sv_any )->xpv_pv // 2pvx\n\
1369 ((XPVIV*) (())->sv_any )->xiv_iv // 2ivx
1370
1371so now you can do ivx and pvx lookups or you can plug there the sv_peek
1372"conversion":
1373
1374 Perl_sv_peek(my_perl, (SV*)()) // sv_peek
1375
9b22382a 1376(The my_perl is for threaded builds.) Just remember that every line,
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1377but the last one, should end with \n\
1378
1379Alternatively edit the init file interactively via: 3rd mouse button ->
1380New Display -> Edit Menu
1381
1382Note: you can define up to 20 conversion shortcuts in the gdb section.
1383
1384=head2 Poison
1385
1386If you see in a debugger a memory area mysteriously full of 0xABABABAB
1387or 0xEFEFEFEF, you may be seeing the effect of the Poison() macros, see
1388L<perlclib>.
1389
1390=head2 Read-only optrees
1391
9b22382a 1392Under ithreads the optree is read only. If you want to enforce this, to
04c692a8 1393check for write accesses from buggy code, compile with
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1394C<-Accflags=-DPERL_DEBUG_READONLY_OPS>
1395to enable code that allocates op memory
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1396via C<mmap>, and sets it read-only when it is attached to a subroutine.
1397Any write access to an op results in a C<SIGBUS> and abort.
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1398
1399This code is intended for development only, and may not be portable
9b22382a
FC
1400even to all Unix variants. Also, it is an 80% solution, in that it
1401isn't able to make all ops read only. Specifically it does not apply to
4dd56148 1402op slabs belonging to C<BEGIN> blocks.
04c692a8 1403
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1404However, as an 80% solution it is still effective, as it has caught
1405bugs in the past.
04c692a8 1406
f789f6a4
FC
1407=head2 When is a bool not a bool?
1408
1409On pre-C99 compilers, C<bool> is defined as equivalent to C<char>.
1410Consequently assignment of any larger type to a C<bool> is unsafe and may
1411be truncated. The C<cBOOL> macro exists to cast it correctly.
1412
1413On those platforms and compilers where C<bool> really is a boolean (C++,
1414C99), it is easy to forget the cast. You can force C<bool> to be a C<char>
1415by compiling with C<-Accflags=-DPERL_BOOL_AS_CHAR>. You may also wish to
50e4f4d4
CB
1416run C<Configure> with something like
1417
cbc13c3d 1418 -Accflags='-Wconversion -Wno-sign-conversion -Wno-shorten-64-to-32'
50e4f4d4
CB
1419
1420or your compiler's equivalent to make it easier to spot any unsafe truncations
1421that show up.
f789f6a4 1422
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1423=head2 The .i Targets
1424
1425You can expand the macros in a F<foo.c> file by saying
1426
1427 make foo.i
1428
d1fd4856
VP
1429which will expand the macros using cpp. Don't be scared by the
1430results.
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1431
1432=head1 AUTHOR
1433
1434This document was originally written by Nathan Torkington, and is
1435maintained by the perl5-porters mailing list.