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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
eb9df707 146operating system, filesystems, character set, and so forth.
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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
28ffebaf 176Casting between function pointers and data pointers
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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 C<'A'>, but not C<0x41>.
279You can safely say C<'\n'>, but not C<\012>. However, you can use
280macros defined in F<utf8.h> to specify any code point portably.
281C<LATIN1_TO_NATIVE(0xDF)> is going to be the code point that means
282LATIN SMALL LETTER SHARP S on whatever platform you are running on (on
283ASCII platforms it compiles without adding any extra code, so there is
284zero performance hit on those). The acceptable inputs to
285C<LATIN1_TO_NATIVE> are from C<0x00> through C<0xFF>. If your input
286isn't guaranteed to be in that range, use C<UNICODE_TO_NATIVE> instead.
287C<NATIVE_TO_LATIN1> and C<NATIVE_TO_UNICODE> translate the opposite
288direction.
289
290If you need the string representation of a character that doesn't have a
291mnemonic name in C, you should add it to the list in
c22aa07d 292F<regen/unicode_constants.pl>, and have Perl create C<#define>'s for you,
eb6d698b 293based on the current platform.
04c692a8 294
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295Note that the C<isI<FOO>> and C<toI<FOO>> macros in F<handy.h> work
296properly on native code points and strings.
297
04c692a8 298Also, the range 'A' - 'Z' in ASCII is an unbroken sequence of 26 upper
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299case alphabetic characters. That is not true in EBCDIC. Nor for 'a' to
300'z'. But '0' - '9' is an unbroken range in both systems. Don't assume
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301anything about other ranges. (Note that special handling of ranges in
302regular expression patterns makes it appear to Perl
303code that the aforementioned ranges are all unbroken.)
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304
305Many of the comments in the existing code ignore the possibility of
9b22382a 306EBCDIC, and may be wrong therefore, even if the code works. This is
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307actually a tribute to the successful transparent insertion of being
308able to handle EBCDIC without having to change pre-existing code.
309
310UTF-8 and UTF-EBCDIC are two different encodings used to represent
9b22382a 311Unicode code points as sequences of bytes. Macros with the same names
eb9df707 312(but different definitions) in F<utf8.h> and F<utfebcdic.h> are used to
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313allow the calling code to think that there is only one such encoding.
314This is almost always referred to as C<utf8>, but it means the EBCDIC
9b22382a 315version as well. Again, comments in the code may well be wrong even if
eb9df707 316the code itself is right. For example, the concept of UTF-8 C<invariant
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317characters> differs between ASCII and EBCDIC. On ASCII platforms, only
318characters that do not have the high-order bit set (i.e. whose ordinals
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319are strict ASCII, 0 - 127) are invariant, and the documentation and
320comments in the code may assume that, often referring to something
9b22382a 321like, say, C<hibit>. The situation differs and is not so simple on
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322EBCDIC machines, but as long as the code itself uses the
323C<NATIVE_IS_INVARIANT()> macro appropriately, it works, even if the
324comments are wrong.
325
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326As noted in L<perlhack/TESTING>, when writing test scripts, the file
327F<t/charset_tools.pl> contains some helpful functions for writing tests
328valid on both ASCII and EBCDIC platforms. Sometimes, though, a test
329can't use a function and it's inconvenient to have different test
330versions depending on the platform. There are 20 code points that are
331the same in all 4 character sets currently recognized by Perl (the 3
332EBCDIC code pages plus ISO 8859-1 (ASCII/Latin1)). These can be used in
333such tests, though there is a small possibility that Perl will become
334available in yet another character set, breaking your test. All but one
335of these code points are C0 control characters. The most significant
336controls that are the same are C<\0>, C<\r>, and C<\N{VT}> (also
337specifiable as C<\cK>, C<\x0B>, C<\N{U+0B}>, or C<\013>). The single
338non-control is U+00B6 PILCROW SIGN. The controls that are the same have
339the same bit pattern in all 4 character sets, regardless of the UTF8ness
340of the string containing them. The bit pattern for U+B6 is the same in
341all 4 for non-UTF8 strings, but differs in each when its containing
342string is UTF-8 encoded. The only other code points that have some sort
343of sameness across all 4 character sets are the pair 0xDC and 0xFC.
344Together these represent upper- and lowercase LATIN LETTER U WITH
345DIAERESIS, but which is upper and which is lower may be reversed: 0xDC
346is the capital in Latin1 and 0xFC is the small letter, while 0xFC is the
347capital in EBCDIC and 0xDC is the small one. This factoid may be
348exploited in writing case insensitive tests that are the same across all
3494 character sets.
350
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351=item *
352
353Assuming the character set is just ASCII
354
9b22382a 355ASCII is a 7 bit encoding, but bytes have 8 bits in them. The 128 extra
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356characters have different meanings depending on the locale. Absent a
357locale, currently these extra characters are generally considered to be
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358unassigned, and this has presented some problems. This has being
359changed starting in 5.12 so that these characters can be considered to
360be Latin-1 (ISO-8859-1).
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361
362=item *
363
364Mixing #define and #ifdef
365
366 #define BURGLE(x) ... \
367 #ifdef BURGLE_OLD_STYLE /* BAD */
368 ... do it the old way ... \
369 #else
370 ... do it the new way ... \
371 #endif
372
9b22382a 373You cannot portably "stack" cpp directives. For example in the above
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374you need two separate BURGLE() #defines, one for each #ifdef branch.
375
376=item *
377
378Adding non-comment stuff after #endif or #else
379
380 #ifdef SNOSH
381 ...
382 #else !SNOSH /* BAD */
383 ...
384 #endif SNOSH /* BAD */
385
386The #endif and #else cannot portably have anything non-comment after
9b22382a 387them. If you want to document what is going (which is a good idea
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388especially if the branches are long), use (C) comments:
389
390 #ifdef SNOSH
391 ...
392 #else /* !SNOSH */
393 ...
394 #endif /* SNOSH */
395
396The gcc option C<-Wendif-labels> warns about the bad variant (by
397default on starting from Perl 5.9.4).
398
399=item *
400
401Having a comma after the last element of an enum list
402
403 enum color {
404 CERULEAN,
405 CHARTREUSE,
406 CINNABAR, /* BAD */
407 };
408
9b22382a 409is not portable. Leave out the last comma.
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410
411Also note that whether enums are implicitly morphable to ints varies
412between compilers, you might need to (int).
413
414=item *
415
416Using //-comments
417
418 // This function bamfoodles the zorklator. /* BAD */
419
9b22382a 420That is C99 or C++. Perl is C89. Using the //-comments is silently
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421allowed by many C compilers but cranking up the ANSI C89 strictness
422(which we like to do) causes the compilation to fail.
423
424=item *
425
426Mixing declarations and code
427
428 void zorklator()
429 {
430 int n = 3;
431 set_zorkmids(n); /* BAD */
432 int q = 4;
433
9b22382a 434That is C99 or C++. Some C compilers allow that, but you shouldn't.
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435
436The gcc option C<-Wdeclaration-after-statements> scans for such
437problems (by default on starting from Perl 5.9.4).
438
439=item *
440
441Introducing variables inside for()
442
443 for(int i = ...; ...; ...) { /* BAD */
444
9b22382a 445That is C99 or C++. While it would indeed be awfully nice to have that
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446also in C89, to limit the scope of the loop variable, alas, we cannot.
447
448=item *
449
450Mixing signed char pointers with unsigned char pointers
451
452 int foo(char *s) { ... }
453 ...
454 unsigned char *t = ...; /* Or U8* t = ... */
455 foo(t); /* BAD */
456
457While this is legal practice, it is certainly dubious, and downright
458fatal in at least one platform: for example VMS cc considers this a
9b22382a 459fatal error. One cause for people often making this mistake is that a
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460"naked char" and therefore dereferencing a "naked char pointer" have an
461undefined signedness: it depends on the compiler and the flags of the
462compiler and the underlying platform whether the result is signed or
9b22382a 463unsigned. For this very same reason using a 'char' as an array index is
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464bad.
465
466=item *
467
468Macros that have string constants and their arguments as substrings of
469the string constants
470
471 #define FOO(n) printf("number = %d\n", n) /* BAD */
472 FOO(10);
473
474Pre-ANSI semantics for that was equivalent to
475
476 printf("10umber = %d\10");
477
9b22382a 478which is probably not what you were expecting. Unfortunately at least
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479one reasonably common and modern C compiler does "real backward
480compatibility" here, in AIX that is what still happens even though the
481rest of the AIX compiler is very happily C89.
482
483=item *
484
485Using printf formats for non-basic C types
486
487 IV i = ...;
488 printf("i = %d\n", i); /* BAD */
489
490While this might by accident work in some platform (where IV happens to
9b22382a 491be an C<int>), in general it cannot. IV might be something larger. Even
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492worse the situation is with more specific types (defined by Perl's
493configuration step in F<config.h>):
494
495 Uid_t who = ...;
496 printf("who = %d\n", who); /* BAD */
497
498The problem here is that Uid_t might be not only not C<int>-wide but it
499might also be unsigned, in which case large uids would be printed as
500negative values.
501
502There is no simple solution to this because of printf()'s limited
503intelligence, but for many types the right format is available as with
504either 'f' or '_f' suffix, for example:
505
506 IVdf /* IV in decimal */
507 UVxf /* UV is hexadecimal */
508
509 printf("i = %"IVdf"\n", i); /* The IVdf is a string constant. */
510
511 Uid_t_f /* Uid_t in decimal */
512
513 printf("who = %"Uid_t_f"\n", who);
514
515Or you can try casting to a "wide enough" type:
516
517 printf("i = %"IVdf"\n", (IV)something_very_small_and_signed);
518
519Also remember that the C<%p> format really does require a void pointer:
520
521 U8* p = ...;
522 printf("p = %p\n", (void*)p);
523
524The gcc option C<-Wformat> scans for such problems.
525
526=item *
527
528Blindly using variadic macros
529
530gcc has had them for a while with its own syntax, and C99 brought them
9b22382a 531with a standardized syntax. Don't use the former, and use the latter
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532only if the HAS_C99_VARIADIC_MACROS is defined.
533
534=item *
535
536Blindly passing va_list
537
538Not all platforms support passing va_list to further varargs (stdarg)
9b22382a 539functions. The right thing to do is to copy the va_list using the
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540Perl_va_copy() if the NEED_VA_COPY is defined.
541
542=item *
543
544Using gcc statement expressions
545
546 val = ({...;...;...}); /* BAD */
547
9b22382a 548While a nice extension, it's not portable. The Perl code does
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549admittedly use them if available to gain some extra speed (essentially
550as a funky form of inlining), but you shouldn't.
551
552=item *
553
554Binding together several statements in a macro
555
556Use the macros STMT_START and STMT_END.
557
558 STMT_START {
559 ...
560 } STMT_END
561
562=item *
563
564Testing for operating systems or versions when should be testing for
565features
566
567 #ifdef __FOONIX__ /* BAD */
568 foo = quux();
569 #endif
570
571Unless you know with 100% certainty that quux() is only ever available
572for the "Foonix" operating system B<and> that is available B<and>
573correctly working for B<all> past, present, B<and> future versions of
9b22382a 574"Foonix", the above is very wrong. This is more correct (though still
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575not perfect, because the below is a compile-time check):
576
577 #ifdef HAS_QUUX
578 foo = quux();
579 #endif
580
581How does the HAS_QUUX become defined where it needs to be? Well, if
582Foonix happens to be Unixy enough to be able to run the Configure
583script, and Configure has been taught about detecting and testing
9b22382a 584quux(), the HAS_QUUX will be correctly defined. In other platforms, the
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585corresponding configuration step will hopefully do the same.
586
587In a pinch, if you cannot wait for Configure to be educated, or if you
588have a good hunch of where quux() might be available, you can
589temporarily try the following:
590
591 #if (defined(__FOONIX__) || defined(__BARNIX__))
592 # define HAS_QUUX
593 #endif
594
595 ...
596
597 #ifdef HAS_QUUX
598 foo = quux();
599 #endif
600
601But in any case, try to keep the features and operating systems
602separate.
603
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604=item *
605
606Assuming the contents of static memory pointed to by the return values
607of Perl wrappers for C library functions doesn't change. Many C library
608functions return pointers to static storage that can be overwritten by
609subsequent calls to the same or related functions. Perl has
610light-weight wrappers for some of these functions, and which don't make
611copies of the static memory. A good example is the interface to the
612environment variables that are in effect for the program. Perl has
613C<PerlEnv_getenv> to get values from the environment. But the return is
614a pointer to static memory in the C library. If you are using the value
615to immediately test for something, that's fine, but if you save the
616value and expect it to be unchanged by later processing, you would be
617wrong, but perhaps you wouldn't know it because different C library
618implementations behave differently, and the one on the platform you're
619testing on might work for your situation. But on some platforms, a
620subsequent call to C<PerlEnv_getenv> or related function WILL overwrite
621the memory that your first call points to. This has led to some
622hard-to-debug problems. Do a L<perlapi/savepv> to make a copy, thus
623avoiding these problems. You will have to free the copy when you're
624done to avoid memory leaks. If you don't have control over when it gets
625freed, you'll need to make the copy in a mortal scalar, like so:
626
627 if ((s = PerlEnv_getenv("foo") == NULL) {
628 ... /* handle NULL case */
629 }
630 else {
631 s = SvPVX(sv_2mortal(newSVpv(s, 0)));
632 }
633
634The above example works only if C<"s"> is C<NUL>-terminated; otherwise
635you have to pass its length to C<newSVpv>.
636
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637=back
638
639=head2 Problematic System Interfaces
640
641=over 4
642
643=item *
644
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645malloc(0), realloc(0), calloc(0, 0) are non-portable. To be portable
646allocate at least one byte. (In general you should rarely need to work
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647at this low level, but instead use the various malloc wrappers.)
648
649=item *
650
9b22382a 651snprintf() - the return type is unportable. Use my_snprintf() instead.
04c692a8
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652
653=back
654
655=head2 Security problems
656
657Last but not least, here are various tips for safer coding.
bbc89b61 658See also L<perlclib> for libc/stdio replacements one should use.
04c692a8
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659
660=over 4
661
662=item *
663
664Do not use gets()
665
9b22382a 666Or we will publicly ridicule you. Seriously.
04c692a8
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667
668=item *
669
bbc89b61
JH
670Do not use tmpfile()
671
672Use mkstemp() instead.
673
674=item *
675
04c692a8
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676Do not use strcpy() or strcat() or strncpy() or strncat()
677
678Use my_strlcpy() and my_strlcat() instead: they either use the native
679implementation, or Perl's own implementation (borrowed from the public
680domain implementation of INN).
681
682=item *
683
684Do not use sprintf() or vsprintf()
685
686If you really want just plain byte strings, use my_snprintf() and
687my_vsnprintf() instead, which will try to use snprintf() and
9b22382a 688vsnprintf() if those safer APIs are available. If you want something
6bfe0388
KW
689fancier than a plain byte string, use
690L<C<Perl_form>()|perlapi/form> or SVs and
691L<C<Perl_sv_catpvf()>|perlapi/sv_catpvf>.
692
2e642750
KW
693Note that glibc C<printf()>, C<sprintf()>, etc. are buggy before glibc
694version 2.17. They won't allow a C<%.s> format with a precision to
695create a string that isn't valid UTF-8 if the current underlying locale
696of the program is UTF-8. What happens is that the C<%s> and its operand are
6bfe0388 697simply skipped without any notice.
2e642750 698L<https://sourceware.org/bugzilla/show_bug.cgi?id=6530>.
04c692a8 699
c98823ff
JH
700=item *
701
702Do not use atoi()
703
22ff3130 704Use grok_atoUV() instead. atoi() has ill-defined behavior on overflows,
c98823ff 705and cannot be used for incremental parsing. It is also affected by locale,
338aa8b0
JH
706which is bad.
707
708=item *
709
710Do not use strtol() or strtoul()
711
22ff3130 712Use grok_atoUV() instead. strtol() or strtoul() (or their IV/UV-friendly
338aa8b0
JH
713macro disguises, Strtol() and Strtoul(), or Atol() and Atoul() are
714affected by locale, which is bad.
c98823ff 715
04c692a8
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716=back
717
718=head1 DEBUGGING
719
720You can compile a special debugging version of Perl, which allows you
721to use the C<-D> option of Perl to tell more about what Perl is doing.
722But sometimes there is no alternative than to dive in with a debugger,
723either to see the stack trace of a core dump (very useful in a bug
724report), or trying to figure out what went wrong before the core dump
725happened, or how did we end up having wrong or unexpected results.
726
727=head2 Poking at Perl
728
729To really poke around with Perl, you'll probably want to build Perl for
730debugging, like this:
731
732 ./Configure -d -D optimize=-g
733 make
734
735C<-g> is a flag to the C compiler to have it produce debugging
736information which will allow us to step through a running program, and
737to see in which C function we are at (without the debugging information
738we might see only the numerical addresses of the functions, which is
739not very helpful).
740
741F<Configure> will also turn on the C<DEBUGGING> compilation symbol
9b22382a 742which enables all the internal debugging code in Perl. There are a
04c692a8
DR
743whole bunch of things you can debug with this: L<perlrun> lists them
744all, and the best way to find out about them is to play about with
9b22382a 745them. The most useful options are probably
04c692a8
DR
746
747 l Context (loop) stack processing
748 t Trace execution
749 o Method and overloading resolution
750 c String/numeric conversions
751
752Some of the functionality of the debugging code can be achieved using
753XS modules.
754
755 -Dr => use re 'debug'
756 -Dx => use O 'Debug'
757
758=head2 Using a source-level debugger
759
760If the debugging output of C<-D> doesn't help you, it's time to step
761through perl's execution with a source-level debugger.
762
763=over 3
764
765=item *
766
767We'll use C<gdb> for our examples here; the principles will apply to
768any debugger (many vendors call their debugger C<dbx>), but check the
769manual of the one you're using.
770
771=back
772
773To fire up the debugger, type
774
775 gdb ./perl
776
777Or if you have a core dump:
778
779 gdb ./perl core
780
781You'll want to do that in your Perl source tree so the debugger can
9b22382a 782read the source code. You should see the copyright message, followed by
04c692a8
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783the prompt.
784
785 (gdb)
786
787C<help> will get you into the documentation, but here are the most
788useful commands:
789
790=over 3
791
792=item * run [args]
793
794Run the program with the given arguments.
795
796=item * break function_name
797
798=item * break source.c:xxx
799
800Tells the debugger that we'll want to pause execution when we reach
801either the named function (but see L<perlguts/Internal Functions>!) or
802the given line in the named source file.
803
804=item * step
805
806Steps through the program a line at a time.
807
808=item * next
809
810Steps through the program a line at a time, without descending into
811functions.
812
813=item * continue
814
815Run until the next breakpoint.
816
817=item * finish
818
819Run until the end of the current function, then stop again.
820
821=item * 'enter'
822
823Just pressing Enter will do the most recent operation again - it's a
824blessing when stepping through miles of source code.
825
8b029fdf
MH
826=item * ptype
827
828Prints the C definition of the argument given.
829
830 (gdb) ptype PL_op
831 type = struct op {
832 OP *op_next;
86cd3a13 833 OP *op_sibparent;
8b029fdf
MH
834 OP *(*op_ppaddr)(void);
835 PADOFFSET op_targ;
836 unsigned int op_type : 9;
837 unsigned int op_opt : 1;
838 unsigned int op_slabbed : 1;
839 unsigned int op_savefree : 1;
840 unsigned int op_static : 1;
841 unsigned int op_folded : 1;
842 unsigned int op_spare : 2;
843 U8 op_flags;
844 U8 op_private;
845 } *
846
04c692a8
DR
847=item * print
848
9b22382a 849Execute the given C code and print its results. B<WARNING>: Perl makes
04c692a8 850heavy use of macros, and F<gdb> does not necessarily support macros
9b22382a 851(see later L</"gdb macro support">). You'll have to substitute them
04c692a8
DR
852yourself, or to invoke cpp on the source code files (see L</"The .i
853Targets">) So, for instance, you can't say
854
855 print SvPV_nolen(sv)
856
857but you have to say
858
859 print Perl_sv_2pv_nolen(sv)
860
861=back
862
863You may find it helpful to have a "macro dictionary", which you can
9b22382a 864produce by saying C<cpp -dM perl.c | sort>. Even then, F<cpp> won't
04c692a8
DR
865recursively apply those macros for you.
866
867=head2 gdb macro support
868
869Recent versions of F<gdb> have fairly good macro support, but in order
870to use it you'll need to compile perl with macro definitions included
9b22382a
FC
871in the debugging information. Using F<gcc> version 3.1, this means
872configuring with C<-Doptimize=-g3>. Other compilers might use a
04c692a8
DR
873different switch (if they support debugging macros at all).
874
875=head2 Dumping Perl Data Structures
876
877One way to get around this macro hell is to use the dumping functions
878in F<dump.c>; these work a little like an internal
879L<Devel::Peek|Devel::Peek>, but they also cover OPs and other
9b22382a 880structures that you can't get at from Perl. Let's take an example.
04c692a8 881We'll use the C<$a = $b + $c> we used before, but give it a bit of
9b22382a 882context: C<$b = "6XXXX"; $c = 2.3;>. Where's a good place to stop and
04c692a8
DR
883poke around?
884
885What about C<pp_add>, the function we examined earlier to implement the
886C<+> operator:
887
888 (gdb) break Perl_pp_add
889 Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.
890
891Notice we use C<Perl_pp_add> and not C<pp_add> - see
9b22382a 892L<perlguts/Internal Functions>. With the breakpoint in place, we can
04c692a8
DR
893run our program:
894
895 (gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c'
896
897Lots of junk will go past as gdb reads in the relevant source files and
898libraries, and then:
899
900 Breakpoint 1, Perl_pp_add () at pp_hot.c:309
901 309 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
902 (gdb) step
903 311 dPOPTOPnnrl_ul;
904 (gdb)
905
906We looked at this bit of code before, and we said that
907C<dPOPTOPnnrl_ul> arranges for two C<NV>s to be placed into C<left> and
908C<right> - let's slightly expand it:
909
910 #define dPOPTOPnnrl_ul NV right = POPn; \
911 SV *leftsv = TOPs; \
912 NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0
913
914C<POPn> takes the SV from the top of the stack and obtains its NV
915either directly (if C<SvNOK> is set) or by calling the C<sv_2nv>
9b22382a
FC
916function. C<TOPs> takes the next SV from the top of the stack - yes,
917C<POPn> uses C<TOPs> - but doesn't remove it. We then use C<SvNV> to
04c692a8
DR
918get the NV from C<leftsv> in the same way as before - yes, C<POPn> uses
919C<SvNV>.
920
921Since we don't have an NV for C<$b>, we'll have to use C<sv_2nv> to
9b22382a 922convert it. If we step again, we'll find ourselves there:
04c692a8 923
8b029fdf 924 (gdb) step
04c692a8
DR
925 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669
926 1669 if (!sv)
927 (gdb)
928
929We can now use C<Perl_sv_dump> to investigate the SV:
930
8b029fdf 931 (gdb) print Perl_sv_dump(sv)
04c692a8
DR
932 SV = PV(0xa057cc0) at 0xa0675d0
933 REFCNT = 1
934 FLAGS = (POK,pPOK)
935 PV = 0xa06a510 "6XXXX"\0
936 CUR = 5
937 LEN = 6
938 $1 = void
939
940We know we're going to get C<6> from this, so let's finish the
941subroutine:
942
943 (gdb) finish
944 Run till exit from #0 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671
945 0x462669 in Perl_pp_add () at pp_hot.c:311
946 311 dPOPTOPnnrl_ul;
947
948We can also dump out this op: the current op is always stored in
9b22382a 949C<PL_op>, and we can dump it with C<Perl_op_dump>. This'll give us
04c692a8
DR
950similar output to L<B::Debug|B::Debug>.
951
8b029fdf 952 (gdb) print Perl_op_dump(PL_op)
04c692a8
DR
953 {
954 13 TYPE = add ===> 14
955 TARG = 1
956 FLAGS = (SCALAR,KIDS)
957 {
958 TYPE = null ===> (12)
959 (was rv2sv)
960 FLAGS = (SCALAR,KIDS)
961 {
962 11 TYPE = gvsv ===> 12
963 FLAGS = (SCALAR)
964 GV = main::b
965 }
966 }
967
968# finish this later #
969
8b029fdf
MH
970=head2 Using gdb to look at specific parts of a program
971
972With the example above, you knew to look for C<Perl_pp_add>, but what if
973there were multiple calls to it all over the place, or you didn't know what
974the op was you were looking for?
975
976One way to do this is to inject a rare call somewhere near what you're looking
9b22382a 977for. For example, you could add C<study> before your method:
8b029fdf
MH
978
979 study;
980
981And in gdb do:
982
983 (gdb) break Perl_pp_study
984
9b22382a
FC
985And then step until you hit what you're
986looking for. This works well in a loop
8b029fdf
MH
987if you want to only break at certain iterations:
988
989 for my $c (1..100) {
990 study if $c == 50;
991 }
992
993=head2 Using gdb to look at what the parser/lexer are doing
994
995If you want to see what perl is doing when parsing/lexing your code, you can
72b22e55 996use C<BEGIN {}>:
8b029fdf
MH
997
998 print "Before\n";
999 BEGIN { study; }
1000 print "After\n";
1001
1002And in gdb:
1003
1004 (gdb) break Perl_pp_study
1005
1006If you want to see what the parser/lexer is doing inside of C<if> blocks and
1007the like you need to be a little trickier:
1008
1009 if ($a && $b && do { BEGIN { study } 1 } && $c) { ... }
1010
04c692a8
DR
1011=head1 SOURCE CODE STATIC ANALYSIS
1012
1013Various tools exist for analysing C source code B<statically>, as
9b22382a 1014opposed to B<dynamically>, that is, without executing the code. It is
04c692a8
DR
1015possible to detect resource leaks, undefined behaviour, type
1016mismatches, portability problems, code paths that would cause illegal
1017memory accesses, and other similar problems by just parsing the C code
1018and looking at the resulting graph, what does it tell about the
9b22382a 1019execution and data flows. As a matter of fact, this is exactly how C
04c692a8
DR
1020compilers know to give warnings about dubious code.
1021
1022=head2 lint, splint
1023
1024The good old C code quality inspector, C<lint>, is available in several
1025platforms, but please be aware that there are several different
1026implementations of it by different vendors, which means that the flags
1027are not identical across different platforms.
1028
1029There is a lint variant called C<splint> (Secure Programming Lint)
1030available from http://www.splint.org/ that should compile on any
1031Unix-like platform.
1032
1033There are C<lint> and <splint> targets in Makefile, but you may have to
1034diddle with the flags (see above).
1035
1036=head2 Coverity
1037
1038Coverity (http://www.coverity.com/) is a product similar to lint and as
1039a testbed for their product they periodically check several open source
1040projects, and they give out accounts to open source developers to the
1041defect databases.
1042
1043=head2 cpd (cut-and-paste detector)
1044
9b22382a 1045The cpd tool detects cut-and-paste coding. If one instance of the
04c692a8 1046cut-and-pasted code changes, all the other spots should probably be
9b22382a 1047changed, too. Therefore such code should probably be turned into a
04c692a8
DR
1048subroutine or a macro.
1049
1050cpd (http://pmd.sourceforge.net/cpd.html) is part of the pmd project
9b22382a 1051(http://pmd.sourceforge.net/). pmd was originally written for static
04c692a8
DR
1052analysis of Java code, but later the cpd part of it was extended to
1053parse also C and C++.
1054
1055Download the pmd-bin-X.Y.zip () from the SourceForge site, extract the
1056pmd-X.Y.jar from it, and then run that on source code thusly:
1057
0cbf2b31
FC
1058 java -cp pmd-X.Y.jar net.sourceforge.pmd.cpd.CPD \
1059 --minimum-tokens 100 --files /some/where/src --language c > cpd.txt
04c692a8
DR
1060
1061You may run into memory limits, in which case you should use the -Xmx
1062option:
1063
1064 java -Xmx512M ...
1065
1066=head2 gcc warnings
1067
1068Though much can be written about the inconsistency and coverage
1069problems of gcc warnings (like C<-Wall> not meaning "all the warnings",
1070or some common portability problems not being covered by C<-Wall>, or
1071C<-ansi> and C<-pedantic> both being a poorly defined collection of
1072warnings, and so forth), gcc is still a useful tool in keeping our
1073coding nose clean.
1074
1075The C<-Wall> is by default on.
1076
1077The C<-ansi> (and its sidekick, C<-pedantic>) would be nice to be on
1078always, but unfortunately they are not safe on all platforms, they can
1079for example cause fatal conflicts with the system headers (Solaris
9b22382a 1080being a prime example). If Configure C<-Dgccansipedantic> is used, the
04c692a8
DR
1081C<cflags> frontend selects C<-ansi -pedantic> for the platforms where
1082they are known to be safe.
1083
1084Starting from Perl 5.9.4 the following extra flags are added:
1085
1086=over 4
1087
1088=item *
1089
1090C<-Wendif-labels>
1091
1092=item *
1093
1094C<-Wextra>
1095
1096=item *
1097
1098C<-Wdeclaration-after-statement>
1099
1100=back
1101
1102The following flags would be nice to have but they would first need
1103their own Augean stablemaster:
1104
1105=over 4
1106
1107=item *
1108
1109C<-Wpointer-arith>
1110
1111=item *
1112
1113C<-Wshadow>
1114
1115=item *
1116
1117C<-Wstrict-prototypes>
1118
1119=back
1120
1121The C<-Wtraditional> is another example of the annoying tendency of gcc
1122to bundle a lot of warnings under one switch (it would be impossible to
1123deploy in practice because it would complain a lot) but it does contain
1124some warnings that would be beneficial to have available on their own,
1125such as the warning about string constants inside macros containing the
1126macro arguments: this behaved differently pre-ANSI than it does in
1127ANSI, and some C compilers are still in transition, AIX being an
1128example.
1129
1130=head2 Warnings of other C compilers
1131
1132Other C compilers (yes, there B<are> other C compilers than gcc) often
1133have their "strict ANSI" or "strict ANSI with some portability
1134extensions" modes on, like for example the Sun Workshop has its C<-Xa>
1135mode on (though implicitly), or the DEC (these days, HP...) has its
1136C<-std1> mode on.
1137
1138=head1 MEMORY DEBUGGERS
1139
d1fd4856
VP
1140B<NOTE 1>: Running under older memory debuggers such as Purify,
1141valgrind or Third Degree greatly slows down the execution: seconds
9b22382a 1142become minutes, minutes become hours. For example as of Perl 5.8.1, the
04c692a8 1143ext/Encode/t/Unicode.t takes extraordinarily long to complete under
9b22382a
FC
1144e.g. Purify, Third Degree, and valgrind. Under valgrind it takes more
1145than six hours, even on a snappy computer. The said test must be doing
1146something that is quite unfriendly for memory debuggers. If you don't
04c692a8 1147feel like waiting, that you can simply kill away the perl process.
d1fd4856
VP
1148Roughly valgrind slows down execution by factor 10, AddressSanitizer by
1149factor 2.
04c692a8
DR
1150
1151B<NOTE 2>: To minimize the number of memory leak false alarms (see
1152L</PERL_DESTRUCT_LEVEL> for more information), you have to set the
9b22382a 1153environment variable PERL_DESTRUCT_LEVEL to 2. For example, like this:
04c692a8
DR
1154
1155 env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib ...
1156
1157B<NOTE 3>: There are known memory leaks when there are compile-time
1158errors within eval or require, seeing C<S_doeval> in the call stack is
9b22382a 1159a good sign of these. Fixing these leaks is non-trivial, unfortunately,
04c692a8
DR
1160but they must be fixed eventually.
1161
1162B<NOTE 4>: L<DynaLoader> will not clean up after itself completely
1163unless Perl is built with the Configure option
1164C<-Accflags=-DDL_UNLOAD_ALL_AT_EXIT>.
1165
04c692a8
DR
1166=head2 valgrind
1167
d1fd4856 1168The valgrind tool can be used to find out both memory leaks and illegal
9b22382a 1169heap memory accesses. As of version 3.3.0, Valgrind only supports Linux
0263e49a 1170on x86, x86-64 and PowerPC and Darwin (OS X) on x86 and x86-64. The
d1fd4856 1171special "test.valgrind" target can be used to run the tests under
9b22382a 1172valgrind. Found errors and memory leaks are logged in files named
037ab3f1
MH
1173F<testfile.valgrind> and by default output is displayed inline.
1174
1175Example usage:
1176
1177 make test.valgrind
1178
1179Since valgrind adds significant overhead, tests will take much longer to
1180run. The valgrind tests support being run in parallel to help with this:
1181
1182 TEST_JOBS=9 make test.valgrind
1183
1184Note that the above two invocations will be very verbose as reachable
1185memory and leak-checking is enabled by default. If you want to just see
1186pure errors, try:
1187
1188 VG_OPTS='-q --leak-check=no --show-reachable=no' TEST_JOBS=9 \
1189 make test.valgrind
04c692a8
DR
1190
1191Valgrind also provides a cachegrind tool, invoked on perl as:
1192
1193 VG_OPTS=--tool=cachegrind make test.valgrind
1194
1195As system libraries (most notably glibc) are also triggering errors,
9b22382a 1196valgrind allows to suppress such errors using suppression files. The
04c692a8 1197default suppression file that comes with valgrind already catches a lot
9b22382a 1198of them. Some additional suppressions are defined in F<t/perl.supp>.
04c692a8
DR
1199
1200To get valgrind and for more information see
1201
0061d4fa 1202 http://valgrind.org/
04c692a8 1203
81c3bbe7
RU
1204=head2 AddressSanitizer
1205
4dd56148 1206AddressSanitizer is a clang and gcc extension, included in clang since
9b22382a 1207v3.1 and gcc since v4.8. It checks illegal heap pointers, global
4dd56148
NC
1208pointers, stack pointers and use after free errors, and is fast enough
1209that you can easily compile your debugging or optimized perl with it.
9b22382a 1210It does not check memory leaks though. AddressSanitizer is available
4dd56148 1211for Linux, Mac OS X and soon on Windows.
81c3bbe7 1212
8a64fbaa
VP
1213To build perl with AddressSanitizer, your Configure invocation should
1214look like:
81c3bbe7 1215
e8596d90
VP
1216 sh Configure -des -Dcc=clang \
1217 -Accflags=-faddress-sanitizer -Aldflags=-faddress-sanitizer \
1218 -Alddlflags=-shared\ -faddress-sanitizer
81c3bbe7
RU
1219
1220where these arguments mean:
1221
1222=over 4
1223
1224=item * -Dcc=clang
1225
8a64fbaa
VP
1226This should be replaced by the full path to your clang executable if it
1227is not in your path.
81c3bbe7
RU
1228
1229=item * -Accflags=-faddress-sanitizer
1230
8a64fbaa 1231Compile perl and extensions sources with AddressSanitizer.
81c3bbe7
RU
1232
1233=item * -Aldflags=-faddress-sanitizer
1234
8a64fbaa 1235Link the perl executable with AddressSanitizer.
81c3bbe7 1236
e8596d90 1237=item * -Alddlflags=-shared\ -faddress-sanitizer
81c3bbe7 1238
9b22382a 1239Link dynamic extensions with AddressSanitizer. You must manually
e8596d90
VP
1240specify C<-shared> because using C<-Alddlflags=-shared> will prevent
1241Configure from setting a default value for C<lddlflags>, which usually
5dfc6e97 1242contains C<-shared> (at least on Linux).
81c3bbe7
RU
1243
1244=back
1245
8a64fbaa
VP
1246See also
1247L<http://code.google.com/p/address-sanitizer/wiki/AddressSanitizer>.
81c3bbe7
RU
1248
1249
04c692a8
DR
1250=head1 PROFILING
1251
1252Depending on your platform there are various ways of profiling Perl.
1253
1254There are two commonly used techniques of profiling executables:
1255I<statistical time-sampling> and I<basic-block counting>.
1256
1257The first method takes periodically samples of the CPU program counter,
1258and since the program counter can be correlated with the code generated
1259for functions, we get a statistical view of in which functions the
9b22382a 1260program is spending its time. The caveats are that very small/fast
04c692a8
DR
1261functions have lower probability of showing up in the profile, and that
1262periodically interrupting the program (this is usually done rather
1263frequently, in the scale of milliseconds) imposes an additional
9b22382a 1264overhead that may skew the results. The first problem can be alleviated
04c692a8
DR
1265by running the code for longer (in general this is a good idea for
1266profiling), the second problem is usually kept in guard by the
1267profiling tools themselves.
1268
1269The second method divides up the generated code into I<basic blocks>.
1270Basic blocks are sections of code that are entered only in the
9b22382a
FC
1271beginning and exited only at the end. For example, a conditional jump
1272starts a basic block. Basic block profiling usually works by
04c692a8 1273I<instrumenting> the code by adding I<enter basic block #nnnn>
9b22382a
FC
1274book-keeping code to the generated code. During the execution of the
1275code the basic block counters are then updated appropriately. The
04c692a8
DR
1276caveat is that the added extra code can skew the results: again, the
1277profiling tools usually try to factor their own effects out of the
1278results.
1279
1280=head2 Gprof Profiling
1281
e2aed43d 1282I<gprof> is a profiling tool available in many Unix platforms which
9b22382a
FC
1283uses I<statistical time-sampling>. You can build a profiled version of
1284F<perl> by compiling using gcc with the flag C<-pg>. Either edit
1285F<config.sh> or re-run F<Configure>. Running the profiled version of
e2aed43d
NC
1286Perl will create an output file called F<gmon.out> which contains the
1287profiling data collected during the execution.
04c692a8 1288
e2aed43d
NC
1289quick hint:
1290
1291 $ sh Configure -des -Dusedevel -Accflags='-pg' \
1292 -Aldflags='-pg' -Alddlflags='-pg -shared' \
1293 && make perl
1294 $ ./perl ... # creates gmon.out in current directory
1295 $ gprof ./perl > out
1296 $ less out
1297
1298(you probably need to add C<-shared> to the <-Alddlflags> line until RT
1299#118199 is resolved)
04c692a8 1300
e2aed43d
NC
1301The F<gprof> tool can then display the collected data in various ways.
1302Usually F<gprof> understands the following options:
04c692a8
DR
1303
1304=over 4
1305
1306=item * -a
1307
1308Suppress statically defined functions from the profile.
1309
1310=item * -b
1311
1312Suppress the verbose descriptions in the profile.
1313
1314=item * -e routine
1315
1316Exclude the given routine and its descendants from the profile.
1317
1318=item * -f routine
1319
1320Display only the given routine and its descendants in the profile.
1321
1322=item * -s
1323
1324Generate a summary file called F<gmon.sum> which then may be given to
1325subsequent gprof runs to accumulate data over several runs.
1326
1327=item * -z
1328
1329Display routines that have zero usage.
1330
1331=back
1332
1333For more detailed explanation of the available commands and output
e2aed43d 1334formats, see your own local documentation of F<gprof>.
04c692a8 1335
e2aed43d 1336=head2 GCC gcov Profiling
04c692a8 1337
e2aed43d
NC
1338I<basic block profiling> is officially available in gcc 3.0 and later.
1339You can build a profiled version of F<perl> by compiling using gcc with
9b22382a 1340the flags C<-fprofile-arcs -ftest-coverage>. Either edit F<config.sh>
e2aed43d 1341or re-run F<Configure>.
04c692a8 1342
e2aed43d 1343quick hint:
04c692a8 1344
e2aed43d
NC
1345 $ sh Configure -des -Dusedevel -Doptimize='-g' \
1346 -Accflags='-fprofile-arcs -ftest-coverage' \
1347 -Aldflags='-fprofile-arcs -ftest-coverage' \
1348 -Alddlflags='-fprofile-arcs -ftest-coverage -shared' \
1349 && make perl
1350 $ rm -f regexec.c.gcov regexec.gcda
1351 $ ./perl ...
1352 $ gcov regexec.c
1353 $ less regexec.c.gcov
04c692a8 1354
e2aed43d
NC
1355(you probably need to add C<-shared> to the <-Alddlflags> line until RT
1356#118199 is resolved)
04c692a8
DR
1357
1358Running the profiled version of Perl will cause profile output to be
9b22382a 1359generated. For each source file an accompanying F<.gcda> file will be
04c692a8
DR
1360created.
1361
e2aed43d 1362To display the results you use the I<gcov> utility (which should be
9b22382a 1363installed if you have gcc 3.0 or newer installed). F<gcov> is run on
04c692a8
DR
1364source code files, like this
1365
1366 gcov sv.c
1367
9b22382a 1368which will cause F<sv.c.gcov> to be created. The F<.gcov> files contain
04c692a8 1369the source code annotated with relative frequencies of execution
9b22382a 1370indicated by "#" markers. If you want to generate F<.gcov> files for
6f134219
NC
1371all profiled object files, you can run something like this:
1372
1373 for file in `find . -name \*.gcno`
1374 do sh -c "cd `dirname $file` && gcov `basename $file .gcno`"
1375 done
04c692a8
DR
1376
1377Useful options of F<gcov> include C<-b> which will summarise the basic
1378block, branch, and function call coverage, and C<-c> which instead of
9b22382a 1379relative frequencies will use the actual counts. For more information
04c692a8 1380on the use of F<gcov> and basic block profiling with gcc, see the
9b22382a 1381latest GNU CC manual. As of gcc 4.8, this is at
e2aed43d 1382L<http://gcc.gnu.org/onlinedocs/gcc/Gcov-Intro.html#Gcov-Intro>
04c692a8
DR
1383
1384=head1 MISCELLANEOUS TRICKS
1385
1386=head2 PERL_DESTRUCT_LEVEL
1387
1388If you want to run any of the tests yourself manually using e.g.
4dd56148
NC
1389valgrind, please note that by default perl B<does not> explicitly
1390cleanup all the memory it has allocated (such as global memory arenas)
1391but instead lets the exit() of the whole program "take care" of such
1392allocations, also known as "global destruction of objects".
04c692a8
DR
1393
1394There is a way to tell perl to do complete cleanup: set the environment
9b22382a 1395variable PERL_DESTRUCT_LEVEL to a non-zero value. The t/TEST wrapper
04c692a8 1396does set this to 2, and this is what you need to do too, if you don't
f01ecde8 1397want to see the "global leaks": For example, for running under valgrind
04c692a8 1398
f01ecde8 1399 env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib t/foo/bar.t
04c692a8
DR
1400
1401(Note: the mod_perl apache module uses also this environment variable
9b22382a
FC
1402for its own purposes and extended its semantics. Refer to the mod_perl
1403documentation for more information. Also, spawned threads do the
04c692a8
DR
1404equivalent of setting this variable to the value 1.)
1405
1406If, at the end of a run you get the message I<N scalars leaked>, you
1407can recompile with C<-DDEBUG_LEAKING_SCALARS>, which will cause the
1408addresses of all those leaked SVs to be dumped along with details as to
9b22382a
FC
1409where each SV was originally allocated. This information is also
1410displayed by Devel::Peek. Note that the extra details recorded with
04c692a8 1411each SV increases memory usage, so it shouldn't be used in production
9b22382a 1412environments. It also converts C<new_SV()> from a macro into a real
04c692a8
DR
1413function, so you can use your favourite debugger to discover where
1414those pesky SVs were allocated.
1415
1416If you see that you're leaking memory at runtime, but neither valgrind
1417nor C<-DDEBUG_LEAKING_SCALARS> will find anything, you're probably
1418leaking SVs that are still reachable and will be properly cleaned up
9b22382a
FC
1419during destruction of the interpreter. In such cases, using the C<-Dm>
1420switch can point you to the source of the leak. If the executable was
04c692a8 1421built with C<-DDEBUG_LEAKING_SCALARS>, C<-Dm> will output SV
9b22382a 1422allocations in addition to memory allocations. Each SV allocation has a
04c692a8 1423distinct serial number that will be written on creation and destruction
9b22382a 1424of the SV. So if you're executing the leaking code in a loop, you need
04c692a8 1425to look for SVs that are created, but never destroyed between each
9b22382a 1426cycle. If such an SV is found, set a conditional breakpoint within
04c692a8 1427C<new_SV()> and make it break only when C<PL_sv_serial> is equal to the
9b22382a 1428serial number of the leaking SV. Then you will catch the interpreter in
04c692a8
DR
1429exactly the state where the leaking SV is allocated, which is
1430sufficient in many cases to find the source of the leak.
1431
1432As C<-Dm> is using the PerlIO layer for output, it will by itself
9b22382a 1433allocate quite a bunch of SVs, which are hidden to avoid recursion. You
04c692a8
DR
1434can bypass the PerlIO layer if you use the SV logging provided by
1435C<-DPERL_MEM_LOG> instead.
1436
1437=head2 PERL_MEM_LOG
1438
1439If compiled with C<-DPERL_MEM_LOG>, both memory and SV allocations go
1440through logging functions, which is handy for breakpoint setting.
1441
1442Unless C<-DPERL_MEM_LOG_NOIMPL> is also compiled, the logging functions
1443read $ENV{PERL_MEM_LOG} to determine whether to log the event, and if
1444so how:
1445
1446 $ENV{PERL_MEM_LOG} =~ /m/ Log all memory ops
1447 $ENV{PERL_MEM_LOG} =~ /s/ Log all SV ops
1448 $ENV{PERL_MEM_LOG} =~ /t/ include timestamp in Log
1449 $ENV{PERL_MEM_LOG} =~ /^(\d+)/ write to FD given (default is 2)
1450
1451Memory logging is somewhat similar to C<-Dm> but is independent of
1452C<-DDEBUGGING>, and at a higher level; all uses of Newx(), Renew(), and
1453Safefree() are logged with the caller's source code file and line
9b22382a
FC
1454number (and C function name, if supported by the C compiler). In
1455contrast, C<-Dm> is directly at the point of C<malloc()>. SV logging is
04c692a8
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1456similar.
1457
1458Since the logging doesn't use PerlIO, all SV allocations are logged and
9b22382a 1459no extra SV allocations are introduced by enabling the logging. If
04c692a8
DR
1460compiled with C<-DDEBUG_LEAKING_SCALARS>, the serial number for each SV
1461allocation is also logged.
1462
1463=head2 DDD over gdb
1464
1465Those debugging perl with the DDD frontend over gdb may find the
1466following useful:
1467
1468You can extend the data conversion shortcuts menu, so for example you
1469can display an SV's IV value with one click, without doing any typing.
1470To do that simply edit ~/.ddd/init file and add after:
1471
1472 ! Display shortcuts.
1473 Ddd*gdbDisplayShortcuts: \
1474 /t () // Convert to Bin\n\
1475 /d () // Convert to Dec\n\
1476 /x () // Convert to Hex\n\
1477 /o () // Convert to Oct(\n\
1478
1479the following two lines:
1480
1481 ((XPV*) (())->sv_any )->xpv_pv // 2pvx\n\
1482 ((XPVIV*) (())->sv_any )->xiv_iv // 2ivx
1483
1484so now you can do ivx and pvx lookups or you can plug there the sv_peek
1485"conversion":
1486
1487 Perl_sv_peek(my_perl, (SV*)()) // sv_peek
1488
9b22382a 1489(The my_perl is for threaded builds.) Just remember that every line,
04c692a8
DR
1490but the last one, should end with \n\
1491
1492Alternatively edit the init file interactively via: 3rd mouse button ->
1493New Display -> Edit Menu
1494
1495Note: you can define up to 20 conversion shortcuts in the gdb section.
1496
470dd224
JH
1497=head2 C backtrace
1498
0762e42f
JH
1499On some platforms Perl supports retrieving the C level backtrace
1500(similar to what symbolic debuggers like gdb do).
470dd224
JH
1501
1502The backtrace returns the stack trace of the C call frames,
1503with the symbol names (function names), the object names (like "perl"),
1504and if it can, also the source code locations (file:line).
1505
0762e42f
JH
1506The supported platforms are Linux, and OS X (some *BSD might
1507work at least partly, but they have not yet been tested).
1508
1509This feature hasn't been tested with multiple threads, but it will
1510only show the backtrace of the thread doing the backtracing.
470dd224
JH
1511
1512The feature needs to be enabled with C<Configure -Dusecbacktrace>.
1513
0762e42f
JH
1514The C<-Dusecbacktrace> also enables keeping the debug information when
1515compiling/linking (often: C<-g>). Many compilers/linkers do support
1516having both optimization and keeping the debug information. The debug
1517information is needed for the symbol names and the source locations.
1518
1519Static functions might not be visible for the backtrace.
470dd224
JH
1520
1521Source code locations, even if available, can often be missing or
0762e42f
JH
1522misleading if the compiler has e.g. inlined code. Optimizer can
1523make matching the source code and the object code quite challenging.
470dd224
JH
1524
1525=over 4
1526
1527=item Linux
1528
59b3baca 1529You B<must> have the BFD (-lbfd) library installed, otherwise C<perl> will
0762e42f 1530fail to link. The BFD is usually distributed as part of the GNU binutils.
470dd224
JH
1531
1532Summary: C<Configure ... -Dusecbacktrace>
1533and you need C<-lbfd>.
1534
1535=item OS X
1536
0762e42f
JH
1537The source code locations are supported B<only> if you have
1538the Developer Tools installed. (BFD is B<not> needed.)
470dd224
JH
1539
1540Summary: C<Configure ... -Dusecbacktrace>
1541and installing the Developer Tools would be good.
1542
1543=back
1544
1545Optionally, for trying out the feature, you may want to enable
0762e42f
JH
1546automatic dumping of the backtrace just before a warning or croak (die)
1547message is emitted, by adding C<-Accflags=-DUSE_C_BACKTRACE_ON_ERROR>
1548for Configure.
470dd224
JH
1549
1550Unless the above additional feature is enabled, nothing about the
1551backtrace functionality is visible, except for the Perl/XS level.
1552
1553Furthermore, even if you have enabled this feature to be compiled,
1554you need to enable it in runtime with an environment variable:
0762e42f
JH
1555C<PERL_C_BACKTRACE_ON_ERROR=10>. It must be an integer higher
1556than zero, telling the desired frame count.
470dd224
JH
1557
1558Retrieving the backtrace from Perl level (using for example an XS
1559extension) would be much less exciting than one would hope: normally
1560you would see C<runops>, C<entersub>, and not much else. This API is
1561intended to be called B<from within> the Perl implementation, not from
1562Perl level execution.
1563
0762e42f 1564The C API for the backtrace is as follows:
470dd224
JH
1565
1566=over 4
1567
1568=item get_c_backtrace
1569
1570=item free_c_backtrace
1571
1572=item get_c_backtrace_dump
1573
1574=item dump_c_backtrace
1575
1576=back
1577
04c692a8
DR
1578=head2 Poison
1579
1580If you see in a debugger a memory area mysteriously full of 0xABABABAB
1581or 0xEFEFEFEF, you may be seeing the effect of the Poison() macros, see
1582L<perlclib>.
1583
1584=head2 Read-only optrees
1585
9b22382a 1586Under ithreads the optree is read only. If you want to enforce this, to
04c692a8 1587check for write accesses from buggy code, compile with
91fc0422
FC
1588C<-Accflags=-DPERL_DEBUG_READONLY_OPS>
1589to enable code that allocates op memory
4dd56148
NC
1590via C<mmap>, and sets it read-only when it is attached to a subroutine.
1591Any write access to an op results in a C<SIGBUS> and abort.
04c692a8
DR
1592
1593This code is intended for development only, and may not be portable
9b22382a
FC
1594even to all Unix variants. Also, it is an 80% solution, in that it
1595isn't able to make all ops read only. Specifically it does not apply to
4dd56148 1596op slabs belonging to C<BEGIN> blocks.
04c692a8 1597
4dd56148
NC
1598However, as an 80% solution it is still effective, as it has caught
1599bugs in the past.
04c692a8 1600
f789f6a4
FC
1601=head2 When is a bool not a bool?
1602
1603On pre-C99 compilers, C<bool> is defined as equivalent to C<char>.
1604Consequently assignment of any larger type to a C<bool> is unsafe and may
1605be truncated. The C<cBOOL> macro exists to cast it correctly.
1606
1607On those platforms and compilers where C<bool> really is a boolean (C++,
1608C99), it is easy to forget the cast. You can force C<bool> to be a C<char>
1609by compiling with C<-Accflags=-DPERL_BOOL_AS_CHAR>. You may also wish to
50e4f4d4
CB
1610run C<Configure> with something like
1611
cbc13c3d 1612 -Accflags='-Wconversion -Wno-sign-conversion -Wno-shorten-64-to-32'
50e4f4d4
CB
1613
1614or your compiler's equivalent to make it easier to spot any unsafe truncations
1615that show up.
f789f6a4 1616
04c692a8
DR
1617=head2 The .i Targets
1618
1619You can expand the macros in a F<foo.c> file by saying
1620
1621 make foo.i
1622
d1fd4856
VP
1623which will expand the macros using cpp. Don't be scared by the
1624results.
04c692a8
DR
1625
1626=head1 AUTHOR
1627
1628This document was originally written by Nathan Torkington, and is
1629maintained by the perl5-porters mailing list.