1PERLHACKTIPS(1)        Perl Programmers Reference Guide        PERLHACKTIPS(1)
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3
4

NAME

6       perlhacktips - Tips for Perl core C code hacking
7

DESCRIPTION

9       This document will help you learn the best way to go about hacking on
10       the Perl core C code.  It covers common problems, debugging, profiling,
11       and more.
12
13       If you haven't read perlhack and perlhacktut yet, you might want to do
14       that first.
15

COMMON PROBLEMS

17       Perl source plays by ANSI C89 rules: no C99 (or C++) extensions.  You
18       don't care about some particular platform having broken Perl? I hear
19       there is still a strong demand for J2EE programmers.
20
21   Perl environment problems
22       ·   Not compiling with threading
23
24           Compiling with threading (-Duseithreads) completely rewrites the
25           function prototypes of Perl.  You better try your changes with
26           that.  Related to this is the difference between "Perl_-less" and
27           "Perl_-ly" APIs, for example:
28
29             Perl_sv_setiv(aTHX_ ...);
30             sv_setiv(...);
31
32           The first one explicitly passes in the context, which is needed for
33           e.g. threaded builds.  The second one does that implicitly; do not
34           get them mixed.  If you are not passing in a aTHX_, you will need
35           to do a dTHX (or a dVAR) as the first thing in the function.
36
37           See "How multiple interpreters and concurrency are supported" in
38           perlguts for further discussion about context.
39
40       ·   Not compiling with -DDEBUGGING
41
42           The DEBUGGING define exposes more code to the compiler, therefore
43           more ways for things to go wrong.  You should try it.
44
45       ·   Introducing (non-read-only) globals
46
47           Do not introduce any modifiable globals, truly global or file
48           static.  They are bad form and complicate multithreading and other
49           forms of concurrency.  The right way is to introduce them as new
50           interpreter variables, see intrpvar.h (at the very end for binary
51           compatibility).
52
53           Introducing read-only (const) globals is okay, as long as you
54           verify with e.g. "nm libperl.a|egrep -v ' [TURtr] '" (if your "nm"
55           has BSD-style output) that the data you added really is read-only.
56           (If it is, it shouldn't show up in the output of that command.)
57
58           If you want to have static strings, make them constant:
59
60             static const char etc[] = "...";
61
62           If you want to have arrays of constant strings, note carefully the
63           right combination of "const"s:
64
65               static const char * const yippee[] =
66                   {"hi", "ho", "silver"};
67
68           There is a way to completely hide any modifiable globals (they are
69           all moved to heap), the compilation setting
70           "-DPERL_GLOBAL_STRUCT_PRIVATE".  It is not normally used, but can
71           be used for testing, read more about it in "Background and
72           PERL_IMPLICIT_CONTEXT" in perlguts.
73
74       ·   Not exporting your new function
75
76           Some platforms (Win32, AIX, VMS, OS/2, to name a few) require any
77           function that is part of the public API (the shared Perl library)
78           to be explicitly marked as exported.  See the discussion about
79           embed.pl in perlguts.
80
81       ·   Exporting your new function
82
83           The new shiny result of either genuine new functionality or your
84           arduous refactoring is now ready and correctly exported.  So what
85           could possibly go wrong?
86
87           Maybe simply that your function did not need to be exported in the
88           first place.  Perl has a long and not so glorious history of
89           exporting functions that it should not have.
90
91           If the function is used only inside one source code file, make it
92           static.  See the discussion about embed.pl in perlguts.
93
94           If the function is used across several files, but intended only for
95           Perl's internal use (and this should be the common case), do not
96           export it to the public API.  See the discussion about embed.pl in
97           perlguts.
98
99   Portability problems
100       The following are common causes of compilation and/or execution
101       failures, not common to Perl as such.  The C FAQ is good bedtime
102       reading.  Please test your changes with as many C compilers and
103       platforms as possible; we will, anyway, and it's nice to save oneself
104       from public embarrassment.
105
106       If using gcc, you can add the "-std=c89" option which will hopefully
107       catch most of these unportabilities.  (However it might also catch
108       incompatibilities in your system's header files.)
109
110       Use the Configure "-Dgccansipedantic" flag to enable the gcc "-ansi
111       -pedantic" flags which enforce stricter ANSI rules.
112
113       If using the "gcc -Wall" note that not all the possible warnings (like
114       "-Wuninitialized") are given unless you also compile with "-O".
115
116       Note that if using gcc, starting from Perl 5.9.5 the Perl core source
117       code files (the ones at the top level of the source code distribution,
118       but not e.g. the extensions under ext/) are automatically compiled with
119       as many as possible of the "-std=c89", "-ansi", "-pedantic", and a
120       selection of "-W" flags (see cflags.SH).
121
122       Also study perlport carefully to avoid any bad assumptions about the
123       operating system, filesystems, character set, and so forth.
124
125       You may once in a while try a "make microperl" to see whether we can
126       still compile Perl with just the bare minimum of interfaces.  (See
127       README.micro.)
128
129       Do not assume an operating system indicates a certain compiler.
130
131       ·   Casting pointers to integers or casting integers to pointers
132
133               void castaway(U8* p)
134               {
135                 IV i = p;
136
137           or
138
139               void castaway(U8* p)
140               {
141                 IV i = (IV)p;
142
143           Both are bad, and broken, and unportable.  Use the PTR2IV() macro
144           that does it right.  (Likewise, there are PTR2UV(), PTR2NV(),
145           INT2PTR(), and NUM2PTR().)
146
147       ·   Casting between function pointers and data pointers
148
149           Technically speaking casting between function pointers and data
150           pointers is unportable and undefined, but practically speaking it
151           seems to work, but you should use the FPTR2DPTR() and DPTR2FPTR()
152           macros.  Sometimes you can also play games with unions.
153
154       ·   Assuming sizeof(int) == sizeof(long)
155
156           There are platforms where longs are 64 bits, and platforms where
157           ints are 64 bits, and while we are out to shock you, even platforms
158           where shorts are 64 bits.  This is all legal according to the C
159           standard.  (In other words, "long long" is not a portable way to
160           specify 64 bits, and "long long" is not even guaranteed to be any
161           wider than "long".)
162
163           Instead, use the definitions IV, UV, IVSIZE, I32SIZE, and so forth.
164           Avoid things like I32 because they are not guaranteed to be exactly
165           32 bits, they are at least 32 bits, nor are they guaranteed to be
166           int or long.  If you really explicitly need 64-bit variables, use
167           I64 and U64, but only if guarded by HAS_QUAD.
168
169       ·   Assuming one can dereference any type of pointer for any type of
170           data
171
172             char *p = ...;
173             long pony = *(long *)p;    /* BAD */
174
175           Many platforms, quite rightly so, will give you a core dump instead
176           of a pony if the p happens not to be correctly aligned.
177
178       ·   Lvalue casts
179
180             (int)*p = ...;    /* BAD */
181
182           Simply not portable.  Get your lvalue to be of the right type, or
183           maybe use temporary variables, or dirty tricks with unions.
184
185       ·   Assume anything about structs (especially the ones you don't
186           control, like the ones coming from the system headers)
187
188           ·       That a certain field exists in a struct
189
190           ·       That no other fields exist besides the ones you know of
191
192           ·       That a field is of certain signedness, sizeof, or type
193
194           ·       That the fields are in a certain order
195
196                   ·       While C guarantees the ordering specified in the
197                           struct definition, between different platforms the
198                           definitions might differ
199
200           ·       That the sizeof(struct) or the alignments are the same
201                   everywhere
202
203                   ·       There might be padding bytes between the fields to
204                           align the fields - the bytes can be anything
205
206                   ·       Structs are required to be aligned to the maximum
207                           alignment required by the fields - which for native
208                           types is for usually equivalent to sizeof() of the
209                           field
210
211       ·   Assuming the character set is ASCIIish
212
213           Perl can compile and run under EBCDIC platforms.  See perlebcdic.
214           This is transparent for the most part, but because the character
215           sets differ, you shouldn't use numeric (decimal, octal, nor hex)
216           constants to refer to characters.  You can safely say 'A', but not
217           0x41.  You can safely say '\n', but not "\012".  However, you can
218           use macros defined in utf8.h to specify any code point portably.
219           "LATIN1_TO_NATIVE(0xDF)" is going to be the code point that means
220           LATIN SMALL LETTER SHARP S on whatever platform you are running on
221           (on ASCII platforms it compiles without adding any extra code, so
222           there is zero performance hit on those).  The acceptable inputs to
223           "LATIN1_TO_NATIVE" are from 0x00 through 0xFF.  If your input isn't
224           guaranteed to be in that range, use "UNICODE_TO_NATIVE" instead.
225           "NATIVE_TO_LATIN1" and "NATIVE_TO_UNICODE" translate the opposite
226           direction.
227
228           If you need the string representation of a character that doesn't
229           have a mnemonic name in C, you should add it to the list in
230           regen/unicode_constants.pl, and have Perl create "#define"'s for
231           you, based on the current platform.
232
233           Note that the "isFOO" and "toFOO" macros in handy.h work properly
234           on native code points and strings.
235
236           Also, the range 'A' - 'Z' in ASCII is an unbroken sequence of 26
237           upper case alphabetic characters.  That is not true in EBCDIC.  Nor
238           for 'a' to 'z'.  But '0' - '9' is an unbroken range in both
239           systems.  Don't assume anything about other ranges.  (Note that
240           special handling of ranges in regular expression patterns and
241           transliterations makes it appear to Perl code that the
242           aforementioned ranges are all unbroken.)
243
244           Many of the comments in the existing code ignore the possibility of
245           EBCDIC, and may be wrong therefore, even if the code works.  This
246           is actually a tribute to the successful transparent insertion of
247           being able to handle EBCDIC without having to change pre-existing
248           code.
249
250           UTF-8 and UTF-EBCDIC are two different encodings used to represent
251           Unicode code points as sequences of bytes.  Macros  with the same
252           names (but different definitions) in utf8.h and utfebcdic.h are
253           used to allow the calling code to think that there is only one such
254           encoding.  This is almost always referred to as "utf8", but it
255           means the EBCDIC version as well.  Again, comments in the code may
256           well be wrong even if the code itself is right.  For example, the
257           concept of UTF-8 "invariant characters" differs between ASCII and
258           EBCDIC.  On ASCII platforms, only characters that do not have the
259           high-order bit set (i.e.  whose ordinals are strict ASCII, 0 - 127)
260           are invariant, and the documentation and comments in the code may
261           assume that, often referring to something like, say, "hibit".  The
262           situation differs and is not so simple on EBCDIC machines, but as
263           long as the code itself uses the "NATIVE_IS_INVARIANT()" macro
264           appropriately, it works, even if the comments are wrong.
265
266           As noted in "TESTING" in perlhack, when writing test scripts, the
267           file t/charset_tools.pl contains some helpful functions for writing
268           tests valid on both ASCII and EBCDIC platforms.  Sometimes, though,
269           a test can't use a function and it's inconvenient to have different
270           test versions depending on the platform.  There are 20 code points
271           that are the same in all 4 character sets currently recognized by
272           Perl (the 3 EBCDIC code pages plus ISO 8859-1 (ASCII/Latin1)).
273           These can be used in such tests, though there is a small
274           possibility that Perl will become available in yet another
275           character set, breaking your test.  All but one of these code
276           points are C0 control characters.  The most significant controls
277           that are the same are "\0", "\r", and "\N{VT}" (also specifiable as
278           "\cK", "\x0B", "\N{U+0B}", or "\013").  The single non-control is
279           U+00B6 PILCROW SIGN.  The controls that are the same have the same
280           bit pattern in all 4 character sets, regardless of the UTF8ness of
281           the string containing them.  The bit pattern for U+B6 is the same
282           in all 4 for non-UTF8 strings, but differs in each when its
283           containing string is UTF-8 encoded.  The only other code points
284           that have some sort of sameness across all 4 character sets are the
285           pair 0xDC and 0xFC.  Together these represent upper- and lowercase
286           LATIN LETTER U WITH DIAERESIS, but which is upper and which is
287           lower may be reversed: 0xDC is the capital in Latin1 and 0xFC is
288           the small letter, while 0xFC is the capital in EBCDIC and 0xDC is
289           the small one.  This factoid may be exploited in writing case
290           insensitive tests that are the same across all 4 character sets.
291
292       ·   Assuming the character set is just ASCII
293
294           ASCII is a 7 bit encoding, but bytes have 8 bits in them.  The 128
295           extra characters have different meanings depending on the locale.
296           Absent a locale, currently these extra characters are generally
297           considered to be unassigned, and this has presented some problems.
298           This has being changed starting in 5.12 so that these characters
299           can be considered to be Latin-1 (ISO-8859-1).
300
301       ·   Mixing #define and #ifdef
302
303             #define BURGLE(x) ... \
304             #ifdef BURGLE_OLD_STYLE        /* BAD */
305             ... do it the old way ... \
306             #else
307             ... do it the new way ... \
308             #endif
309
310           You cannot portably "stack" cpp directives.  For example in the
311           above you need two separate BURGLE() #defines, one for each #ifdef
312           branch.
313
314       ·   Adding non-comment stuff after #endif or #else
315
316             #ifdef SNOSH
317             ...
318             #else !SNOSH    /* BAD */
319             ...
320             #endif SNOSH    /* BAD */
321
322           The #endif and #else cannot portably have anything non-comment
323           after them.  If you want to document what is going (which is a good
324           idea especially if the branches are long), use (C) comments:
325
326             #ifdef SNOSH
327             ...
328             #else /* !SNOSH */
329             ...
330             #endif /* SNOSH */
331
332           The gcc option "-Wendif-labels" warns about the bad variant (by
333           default on starting from Perl 5.9.4).
334
335       ·   Having a comma after the last element of an enum list
336
337             enum color {
338               CERULEAN,
339               CHARTREUSE,
340               CINNABAR,     /* BAD */
341             };
342
343           is not portable.  Leave out the last comma.
344
345           Also note that whether enums are implicitly morphable to ints
346           varies between compilers, you might need to (int).
347
348       ·   Using //-comments
349
350             // This function bamfoodles the zorklator.   /* BAD */
351
352           That is C99 or C++.  Perl is C89.  Using the //-comments is
353           silently allowed by many C compilers but cranking up the ANSI C89
354           strictness (which we like to do) causes the compilation to fail.
355
356       ·   Mixing declarations and code
357
358             void zorklator()
359             {
360               int n = 3;
361               set_zorkmids(n);    /* BAD */
362               int q = 4;
363
364           That is C99 or C++.  Some C compilers allow that, but you
365           shouldn't.
366
367           The gcc option "-Wdeclaration-after-statements" scans for such
368           problems (by default on starting from Perl 5.9.4).
369
370       ·   Introducing variables inside for()
371
372             for(int i = ...; ...; ...) {    /* BAD */
373
374           That is C99 or C++.  While it would indeed be awfully nice to have
375           that also in C89, to limit the scope of the loop variable, alas, we
376           cannot.
377
378       ·   Mixing signed char pointers with unsigned char pointers
379
380             int foo(char *s) { ... }
381             ...
382             unsigned char *t = ...; /* Or U8* t = ... */
383             foo(t);   /* BAD */
384
385           While this is legal practice, it is certainly dubious, and
386           downright fatal in at least one platform: for example VMS cc
387           considers this a fatal error.  One cause for people often making
388           this mistake is that a "naked char" and therefore dereferencing a
389           "naked char pointer" have an undefined signedness: it depends on
390           the compiler and the flags of the compiler and the underlying
391           platform whether the result is signed or unsigned.  For this very
392           same reason using a 'char' as an array index is bad.
393
394       ·   Macros that have string constants and their arguments as substrings
395           of the string constants
396
397             #define FOO(n) printf("number = %d\n", n)    /* BAD */
398             FOO(10);
399
400           Pre-ANSI semantics for that was equivalent to
401
402             printf("10umber = %d\10");
403
404           which is probably not what you were expecting.  Unfortunately at
405           least one reasonably common and modern C compiler does "real
406           backward compatibility" here, in AIX that is what still happens
407           even though the rest of the AIX compiler is very happily C89.
408
409       ·   Using printf formats for non-basic C types
410
411              IV i = ...;
412              printf("i = %d\n", i);    /* BAD */
413
414           While this might by accident work in some platform (where IV
415           happens to be an "int"), in general it cannot.  IV might be
416           something larger.  Even worse the situation is with more specific
417           types (defined by Perl's configuration step in config.h):
418
419              Uid_t who = ...;
420              printf("who = %d\n", who);    /* BAD */
421
422           The problem here is that Uid_t might be not only not "int"-wide but
423           it might also be unsigned, in which case large uids would be
424           printed as negative values.
425
426           There is no simple solution to this because of printf()'s limited
427           intelligence, but for many types the right format is available as
428           with either 'f' or '_f' suffix, for example:
429
430              IVdf /* IV in decimal */
431              UVxf /* UV is hexadecimal */
432
433              printf("i = %"IVdf"\n", i); /* The IVdf is a string constant. */
434
435              Uid_t_f /* Uid_t in decimal */
436
437              printf("who = %"Uid_t_f"\n", who);
438
439           Or you can try casting to a "wide enough" type:
440
441              printf("i = %"IVdf"\n", (IV)something_very_small_and_signed);
442
443           See "Formatted Printing of Size_t and SSize_t" in perlguts for how
444           to print those.
445
446           Also remember that the %p format really does require a void
447           pointer:
448
449              U8* p = ...;
450              printf("p = %p\n", (void*)p);
451
452           The gcc option "-Wformat" scans for such problems.
453
454       ·   Blindly using variadic macros
455
456           gcc has had them for a while with its own syntax, and C99 brought
457           them with a standardized syntax.  Don't use the former, and use the
458           latter only if the HAS_C99_VARIADIC_MACROS is defined.
459
460       ·   Blindly passing va_list
461
462           Not all platforms support passing va_list to further varargs
463           (stdarg) functions.  The right thing to do is to copy the va_list
464           using the Perl_va_copy() if the NEED_VA_COPY is defined.
465
466       ·   Using gcc statement expressions
467
468              val = ({...;...;...});    /* BAD */
469
470           While a nice extension, it's not portable.  The Perl code does
471           admittedly use them if available to gain some extra speed
472           (essentially as a funky form of inlining), but you shouldn't.
473
474       ·   Binding together several statements in a macro
475
476           Use the macros STMT_START and STMT_END.
477
478              STMT_START {
479                 ...
480              } STMT_END
481
482       ·   Testing for operating systems or versions when should be testing
483           for features
484
485             #ifdef __FOONIX__    /* BAD */
486             foo = quux();
487             #endif
488
489           Unless you know with 100% certainty that quux() is only ever
490           available for the "Foonix" operating system and that is available
491           and correctly working for all past, present, and future versions of
492           "Foonix", the above is very wrong.  This is more correct (though
493           still not perfect, because the below is a compile-time check):
494
495             #ifdef HAS_QUUX
496             foo = quux();
497             #endif
498
499           How does the HAS_QUUX become defined where it needs to be?  Well,
500           if Foonix happens to be Unixy enough to be able to run the
501           Configure script, and Configure has been taught about detecting and
502           testing quux(), the HAS_QUUX will be correctly defined.  In other
503           platforms, the corresponding configuration step will hopefully do
504           the same.
505
506           In a pinch, if you cannot wait for Configure to be educated, or if
507           you have a good hunch of where quux() might be available, you can
508           temporarily try the following:
509
510             #if (defined(__FOONIX__) || defined(__BARNIX__))
511             # define HAS_QUUX
512             #endif
513
514             ...
515
516             #ifdef HAS_QUUX
517             foo = quux();
518             #endif
519
520           But in any case, try to keep the features and operating systems
521           separate.
522
523           A good resource on the predefined macros for various operating
524           systems, compilers, and so forth is
525           <http://sourceforge.net/p/predef/wiki/Home/>
526
527       ·   Assuming the contents of static memory pointed to by the return
528           values of Perl wrappers for C library functions doesn't change.
529           Many C library functions return pointers to static storage that can
530           be overwritten by subsequent calls to the same or related
531           functions.  Perl has light-weight wrappers for some of these
532           functions, and which don't make copies of the static memory.  A
533           good example is the interface to the environment variables that are
534           in effect for the program.  Perl has "PerlEnv_getenv" to get values
535           from the environment.  But the return is a pointer to static memory
536           in the C library.  If you are using the value to immediately test
537           for something, that's fine, but if you save the value and expect it
538           to be unchanged by later processing, you would be wrong, but
539           perhaps you wouldn't know it because different C library
540           implementations behave differently, and the one on the platform
541           you're testing on might work for your situation.  But on some
542           platforms, a subsequent call to "PerlEnv_getenv" or related
543           function WILL overwrite the memory that your first call points to.
544           This has led to some hard-to-debug problems.  Do a "savepv" in
545           perlapi to make a copy, thus avoiding these problems.  You will
546           have to free the copy when you're done to avoid memory leaks.  If
547           you don't have control over when it gets freed, you'll need to make
548           the copy in a mortal scalar, like so:
549
550            if ((s = PerlEnv_getenv("foo") == NULL) {
551               ... /* handle NULL case */
552            }
553            else {
554                s = SvPVX(sv_2mortal(newSVpv(s, 0)));
555            }
556
557           The above example works only if "s" is "NUL"-terminated; otherwise
558           you have to pass its length to "newSVpv".
559
560   Problematic System Interfaces
561       ·   malloc(0), realloc(0), calloc(0, 0) are non-portable.  To be
562           portable allocate at least one byte.  (In general you should rarely
563           need to work at this low level, but instead use the various malloc
564           wrappers.)
565
566       ·   snprintf() - the return type is unportable.  Use my_snprintf()
567           instead.
568
569   Security problems
570       Last but not least, here are various tips for safer coding.  See also
571       perlclib for libc/stdio replacements one should use.
572
573       ·   Do not use gets()
574
575           Or we will publicly ridicule you.  Seriously.
576
577       ·   Do not use tmpfile()
578
579           Use mkstemp() instead.
580
581       ·   Do not use strcpy() or strcat() or strncpy() or strncat()
582
583           Use my_strlcpy() and my_strlcat() instead: they either use the
584           native implementation, or Perl's own implementation (borrowed from
585           the public domain implementation of INN).
586
587       ·   Do not use sprintf() or vsprintf()
588
589           If you really want just plain byte strings, use my_snprintf() and
590           my_vsnprintf() instead, which will try to use snprintf() and
591           vsnprintf() if those safer APIs are available.  If you want
592           something fancier than a plain byte string, use "Perl_form"() or
593           SVs and "Perl_sv_catpvf()".
594
595           Note that glibc "printf()", "sprintf()", etc. are buggy before
596           glibc version 2.17.  They won't allow a "%.s" format with a
597           precision to create a string that isn't valid UTF-8 if the current
598           underlying locale of the program is UTF-8.  What happens is that
599           the %s and its operand are simply skipped without any notice.
600           <https://sourceware.org/bugzilla/show_bug.cgi?id=6530>.
601
602       ·   Do not use atoi()
603
604           Use grok_atoUV() instead.  atoi() has ill-defined behavior on
605           overflows, and cannot be used for incremental parsing.  It is also
606           affected by locale, which is bad.
607
608       ·   Do not use strtol() or strtoul()
609
610           Use grok_atoUV() instead.  strtol() or strtoul() (or their
611           IV/UV-friendly macro disguises, Strtol() and Strtoul(), or Atol()
612           and Atoul() are affected by locale, which is bad.
613

DEBUGGING

615       You can compile a special debugging version of Perl, which allows you
616       to use the "-D" option of Perl to tell more about what Perl is doing.
617       But sometimes there is no alternative than to dive in with a debugger,
618       either to see the stack trace of a core dump (very useful in a bug
619       report), or trying to figure out what went wrong before the core dump
620       happened, or how did we end up having wrong or unexpected results.
621
622   Poking at Perl
623       To really poke around with Perl, you'll probably want to build Perl for
624       debugging, like this:
625
626           ./Configure -d -DDEBUGGING
627           make
628
629       "-DDEBUGGING" turns on the C compiler's "-g" flag to have it produce
630       debugging information which will allow us to step through a running
631       program, and to see in which C function we are at (without the
632       debugging information we might see only the numerical addresses of the
633       functions, which is not very helpful). It will also turn on the
634       "DEBUGGING" compilation symbol which enables all the internal debugging
635       code in Perl.  There are a whole bunch of things you can debug with
636       this: perlrun lists them all, and the best way to find out about them
637       is to play about with them.  The most useful options are probably
638
639           l  Context (loop) stack processing
640           s  Stack snapshots (with v, displays all stacks)
641           t  Trace execution
642           o  Method and overloading resolution
643           c  String/numeric conversions
644
645       For example
646
647           $ perl -Dst -e '$a + 1'
648           ....
649           (-e:1)      gvsv(main::a)
650               =>  UNDEF
651           (-e:1)      const(IV(1))
652               =>  UNDEF  IV(1)
653           (-e:1)      add
654               =>  NV(1)
655
656       Some of the functionality of the debugging code can be achieved with a
657       non-debugging perl by using XS modules:
658
659           -Dr => use re 'debug'
660           -Dx => use O 'Debug'
661
662   Using a source-level debugger
663       If the debugging output of "-D" doesn't help you, it's time to step
664       through perl's execution with a source-level debugger.
665
666       ·  We'll use "gdb" for our examples here; the principles will apply to
667          any debugger (many vendors call their debugger "dbx"), but check the
668          manual of the one you're using.
669
670       To fire up the debugger, type
671
672           gdb ./perl
673
674       Or if you have a core dump:
675
676           gdb ./perl core
677
678       You'll want to do that in your Perl source tree so the debugger can
679       read the source code.  You should see the copyright message, followed
680       by the prompt.
681
682           (gdb)
683
684       "help" will get you into the documentation, but here are the most
685       useful commands:
686
687       ·  run [args]
688
689          Run the program with the given arguments.
690
691       ·  break function_name
692
693       ·  break source.c:xxx
694
695          Tells the debugger that we'll want to pause execution when we reach
696          either the named function (but see "Internal Functions" in
697          perlguts!) or the given line in the named source file.
698
699       ·  step
700
701          Steps through the program a line at a time.
702
703       ·  next
704
705          Steps through the program a line at a time, without descending into
706          functions.
707
708       ·  continue
709
710          Run until the next breakpoint.
711
712       ·  finish
713
714          Run until the end of the current function, then stop again.
715
716       ·  'enter'
717
718          Just pressing Enter will do the most recent operation again - it's a
719          blessing when stepping through miles of source code.
720
721       ·  ptype
722
723          Prints the C definition of the argument given.
724
725            (gdb) ptype PL_op
726            type = struct op {
727                OP *op_next;
728                OP *op_sibparent;
729                OP *(*op_ppaddr)(void);
730                PADOFFSET op_targ;
731                unsigned int op_type : 9;
732                unsigned int op_opt : 1;
733                unsigned int op_slabbed : 1;
734                unsigned int op_savefree : 1;
735                unsigned int op_static : 1;
736                unsigned int op_folded : 1;
737                unsigned int op_spare : 2;
738                U8 op_flags;
739                U8 op_private;
740            } *
741
742       ·  print
743
744          Execute the given C code and print its results.  WARNING: Perl makes
745          heavy use of macros, and gdb does not necessarily support macros
746          (see later "gdb macro support").  You'll have to substitute them
747          yourself, or to invoke cpp on the source code files (see "The .i
748          Targets") So, for instance, you can't say
749
750              print SvPV_nolen(sv)
751
752          but you have to say
753
754              print Perl_sv_2pv_nolen(sv)
755
756       You may find it helpful to have a "macro dictionary", which you can
757       produce by saying "cpp -dM perl.c | sort".  Even then, cpp won't
758       recursively apply those macros for you.
759
760   gdb macro support
761       Recent versions of gdb have fairly good macro support, but in order to
762       use it you'll need to compile perl with macro definitions included in
763       the debugging information.  Using gcc version 3.1, this means
764       configuring with "-Doptimize=-g3".  Other compilers might use a
765       different switch (if they support debugging macros at all).
766
767   Dumping Perl Data Structures
768       One way to get around this macro hell is to use the dumping functions
769       in dump.c; these work a little like an internal Devel::Peek, but they
770       also cover OPs and other structures that you can't get at from Perl.
771       Let's take an example.  We'll use the "$a = $b + $c" we used before,
772       but give it a bit of context: "$b = "6XXXX"; $c = 2.3;".  Where's a
773       good place to stop and poke around?
774
775       What about "pp_add", the function we examined earlier to implement the
776       "+" operator:
777
778           (gdb) break Perl_pp_add
779           Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.
780
781       Notice we use "Perl_pp_add" and not "pp_add" - see "Internal Functions"
782       in perlguts.  With the breakpoint in place, we can run our program:
783
784           (gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c'
785
786       Lots of junk will go past as gdb reads in the relevant source files and
787       libraries, and then:
788
789           Breakpoint 1, Perl_pp_add () at pp_hot.c:309
790           1396    dSP; dATARGET; bool useleft; SV *svl, *svr;
791           (gdb) step
792           311           dPOPTOPnnrl_ul;
793           (gdb)
794
795       We looked at this bit of code before, and we said that "dPOPTOPnnrl_ul"
796       arranges for two "NV"s to be placed into "left" and "right" - let's
797       slightly expand it:
798
799        #define dPOPTOPnnrl_ul  NV right = POPn; \
800                                SV *leftsv = TOPs; \
801                                NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0
802
803       "POPn" takes the SV from the top of the stack and obtains its NV either
804       directly (if "SvNOK" is set) or by calling the "sv_2nv" function.
805       "TOPs" takes the next SV from the top of the stack - yes, "POPn" uses
806       "TOPs" - but doesn't remove it.  We then use "SvNV" to get the NV from
807       "leftsv" in the same way as before - yes, "POPn" uses "SvNV".
808
809       Since we don't have an NV for $b, we'll have to use "sv_2nv" to convert
810       it.  If we step again, we'll find ourselves there:
811
812           (gdb) step
813           Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669
814           1669        if (!sv)
815           (gdb)
816
817       We can now use "Perl_sv_dump" to investigate the SV:
818
819           (gdb) print Perl_sv_dump(sv)
820           SV = PV(0xa057cc0) at 0xa0675d0
821           REFCNT = 1
822           FLAGS = (POK,pPOK)
823           PV = 0xa06a510 "6XXXX"\0
824           CUR = 5
825           LEN = 6
826           $1 = void
827
828       We know we're going to get 6 from this, so let's finish the subroutine:
829
830           (gdb) finish
831           Run till exit from #0  Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671
832           0x462669 in Perl_pp_add () at pp_hot.c:311
833           311           dPOPTOPnnrl_ul;
834
835       We can also dump out this op: the current op is always stored in
836       "PL_op", and we can dump it with "Perl_op_dump".  This'll give us
837       similar output to CPAN module B::Debug.
838
839           (gdb) print Perl_op_dump(PL_op)
840           {
841           13  TYPE = add  ===> 14
842               TARG = 1
843               FLAGS = (SCALAR,KIDS)
844               {
845                   TYPE = null  ===> (12)
846                     (was rv2sv)
847                   FLAGS = (SCALAR,KIDS)
848                   {
849           11          TYPE = gvsv  ===> 12
850                       FLAGS = (SCALAR)
851                       GV = main::b
852                   }
853               }
854
855       # finish this later #
856
857   Using gdb to look at specific parts of a program
858       With the example above, you knew to look for "Perl_pp_add", but what if
859       there were multiple calls to it all over the place, or you didn't know
860       what the op was you were looking for?
861
862       One way to do this is to inject a rare call somewhere near what you're
863       looking for.  For example, you could add "study" before your method:
864
865           study;
866
867       And in gdb do:
868
869           (gdb) break Perl_pp_study
870
871       And then step until you hit what you're looking for.  This works well
872       in a loop if you want to only break at certain iterations:
873
874           for my $c (1..100) {
875               study if $c == 50;
876           }
877
878   Using gdb to look at what the parser/lexer are doing
879       If you want to see what perl is doing when parsing/lexing your code,
880       you can use "BEGIN {}":
881
882           print "Before\n";
883           BEGIN { study; }
884           print "After\n";
885
886       And in gdb:
887
888           (gdb) break Perl_pp_study
889
890       If you want to see what the parser/lexer is doing inside of "if" blocks
891       and the like you need to be a little trickier:
892
893           if ($a && $b && do { BEGIN { study } 1 } && $c) { ... }
894

SOURCE CODE STATIC ANALYSIS

896       Various tools exist for analysing C source code statically, as opposed
897       to dynamically, that is, without executing the code.  It is possible to
898       detect resource leaks, undefined behaviour, type mismatches,
899       portability problems, code paths that would cause illegal memory
900       accesses, and other similar problems by just parsing the C code and
901       looking at the resulting graph, what does it tell about the execution
902       and data flows.  As a matter of fact, this is exactly how C compilers
903       know to give warnings about dubious code.
904
905   lint
906       The good old C code quality inspector, "lint", is available in several
907       platforms, but please be aware that there are several different
908       implementations of it by different vendors, which means that the flags
909       are not identical across different platforms.
910
911       There is a "lint" target in Makefile, but you may have to diddle with
912       the flags (see above).
913
914   Coverity
915       Coverity (<http://www.coverity.com/>) is a product similar to lint and
916       as a testbed for their product they periodically check several open
917       source projects, and they give out accounts to open source developers
918       to the defect databases.
919
920       There is Coverity setup for the perl5 project:
921       <https://scan.coverity.com/projects/perl5>
922
923   HP-UX cadvise (Code Advisor)
924       HP has a C/C++ static analyzer product for HP-UX caller Code Advisor.
925       (Link not given here because the URL is horribly long and seems
926       horribly unstable; use the search engine of your choice to find it.)
927       The use of the "cadvise_cc" recipe with "Configure ...
928       -Dcc=./cadvise_cc" (see cadvise "User Guide") is recommended; as is the
929       use of "+wall".
930
931   cpd (cut-and-paste detector)
932       The cpd tool detects cut-and-paste coding.  If one instance of the cut-
933       and-pasted code changes, all the other spots should probably be
934       changed, too.  Therefore such code should probably be turned into a
935       subroutine or a macro.
936
937       cpd (<http://pmd.sourceforge.net/cpd.html>) is part of the pmd project
938       (<http://pmd.sourceforge.net/>).  pmd was originally written for static
939       analysis of Java code, but later the cpd part of it was extended to
940       parse also C and C++.
941
942       Download the pmd-bin-X.Y.zip () from the SourceForge site, extract the
943       pmd-X.Y.jar from it, and then run that on source code thusly:
944
945         java -cp pmd-X.Y.jar net.sourceforge.pmd.cpd.CPD \
946          --minimum-tokens 100 --files /some/where/src --language c > cpd.txt
947
948       You may run into memory limits, in which case you should use the -Xmx
949       option:
950
951         java -Xmx512M ...
952
953   gcc warnings
954       Though much can be written about the inconsistency and coverage
955       problems of gcc warnings (like "-Wall" not meaning "all the warnings",
956       or some common portability problems not being covered by "-Wall", or
957       "-ansi" and "-pedantic" both being a poorly defined collection of
958       warnings, and so forth), gcc is still a useful tool in keeping our
959       coding nose clean.
960
961       The "-Wall" is by default on.
962
963       The "-ansi" (and its sidekick, "-pedantic") would be nice to be on
964       always, but unfortunately they are not safe on all platforms, they can
965       for example cause fatal conflicts with the system headers (Solaris
966       being a prime example).  If Configure "-Dgccansipedantic" is used, the
967       "cflags" frontend selects "-ansi -pedantic" for the platforms where
968       they are known to be safe.
969
970       The following extra flags are added:
971
972       ·   "-Wendif-labels"
973
974       ·   "-Wextra"
975
976       ·   "-Wc++-compat"
977
978       ·   "-Wwrite-strings"
979
980       ·   "-Werror=declaration-after-statement"
981
982       ·   "-Werror=pointer-arith"
983
984       The following flags would be nice to have but they would first need
985       their own Augean stablemaster:
986
987       ·   "-Wshadow"
988
989       ·   "-Wstrict-prototypes"
990
991       The "-Wtraditional" is another example of the annoying tendency of gcc
992       to bundle a lot of warnings under one switch (it would be impossible to
993       deploy in practice because it would complain a lot) but it does contain
994       some warnings that would be beneficial to have available on their own,
995       such as the warning about string constants inside macros containing the
996       macro arguments: this behaved differently pre-ANSI than it does in
997       ANSI, and some C compilers are still in transition, AIX being an
998       example.
999
1000   Warnings of other C compilers
1001       Other C compilers (yes, there are other C compilers than gcc) often
1002       have their "strict ANSI" or "strict ANSI with some portability
1003       extensions" modes on, like for example the Sun Workshop has its "-Xa"
1004       mode on (though implicitly), or the DEC (these days, HP...) has its
1005       "-std1" mode on.
1006

MEMORY DEBUGGERS

1008       NOTE 1: Running under older memory debuggers such as Purify, valgrind
1009       or Third Degree greatly slows down the execution: seconds become
1010       minutes, minutes become hours.  For example as of Perl 5.8.1, the
1011       ext/Encode/t/Unicode.t takes extraordinarily long to complete under
1012       e.g. Purify, Third Degree, and valgrind.  Under valgrind it takes more
1013       than six hours, even on a snappy computer.  The said test must be doing
1014       something that is quite unfriendly for memory debuggers.  If you don't
1015       feel like waiting, that you can simply kill away the perl process.
1016       Roughly valgrind slows down execution by factor 10, AddressSanitizer by
1017       factor 2.
1018
1019       NOTE 2: To minimize the number of memory leak false alarms (see
1020       "PERL_DESTRUCT_LEVEL" for more information), you have to set the
1021       environment variable PERL_DESTRUCT_LEVEL to 2.  For example, like this:
1022
1023           env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib ...
1024
1025       NOTE 3: There are known memory leaks when there are compile-time errors
1026       within eval or require, seeing "S_doeval" in the call stack is a good
1027       sign of these.  Fixing these leaks is non-trivial, unfortunately, but
1028       they must be fixed eventually.
1029
1030       NOTE 4: DynaLoader will not clean up after itself completely unless
1031       Perl is built with the Configure option
1032       "-Accflags=-DDL_UNLOAD_ALL_AT_EXIT".
1033
1034   valgrind
1035       The valgrind tool can be used to find out both memory leaks and illegal
1036       heap memory accesses.  As of version 3.3.0, Valgrind only supports
1037       Linux on x86, x86-64 and PowerPC and Darwin (OS X) on x86 and x86-64.
1038       The special "test.valgrind" target can be used to run the tests under
1039       valgrind.  Found errors and memory leaks are logged in files named
1040       testfile.valgrind and by default output is displayed inline.
1041
1042       Example usage:
1043
1044           make test.valgrind
1045
1046       Since valgrind adds significant overhead, tests will take much longer
1047       to run.  The valgrind tests support being run in parallel to help with
1048       this:
1049
1050           TEST_JOBS=9 make test.valgrind
1051
1052       Note that the above two invocations will be very verbose as reachable
1053       memory and leak-checking is enabled by default.  If you want to just
1054       see pure errors, try:
1055
1056           VG_OPTS='-q --leak-check=no --show-reachable=no' TEST_JOBS=9 \
1057               make test.valgrind
1058
1059       Valgrind also provides a cachegrind tool, invoked on perl as:
1060
1061           VG_OPTS=--tool=cachegrind make test.valgrind
1062
1063       As system libraries (most notably glibc) are also triggering errors,
1064       valgrind allows to suppress such errors using suppression files.  The
1065       default suppression file that comes with valgrind already catches a lot
1066       of them.  Some additional suppressions are defined in t/perl.supp.
1067
1068       To get valgrind and for more information see
1069
1070           http://valgrind.org/
1071
1072   AddressSanitizer
1073       AddressSanitizer is a clang and gcc extension, included in clang since
1074       v3.1 and gcc since v4.8.  It checks illegal heap pointers, global
1075       pointers, stack pointers and use after free errors, and is fast enough
1076       that you can easily compile your debugging or optimized perl with it.
1077       It does not check memory leaks though.  AddressSanitizer is available
1078       for Linux, Mac OS X and soon on Windows.
1079
1080       To build perl with AddressSanitizer, your Configure invocation should
1081       look like:
1082
1083           sh Configure -des -Dcc=clang \
1084              -Accflags=-faddress-sanitizer -Aldflags=-faddress-sanitizer \
1085              -Alddlflags=-shared\ -faddress-sanitizer
1086
1087       where these arguments mean:
1088
1089       ·   -Dcc=clang
1090
1091           This should be replaced by the full path to your clang executable
1092           if it is not in your path.
1093
1094       ·   -Accflags=-faddress-sanitizer
1095
1096           Compile perl and extensions sources with AddressSanitizer.
1097
1098       ·   -Aldflags=-faddress-sanitizer
1099
1100           Link the perl executable with AddressSanitizer.
1101
1102       ·   -Alddlflags=-shared\ -faddress-sanitizer
1103
1104           Link dynamic extensions with AddressSanitizer.  You must manually
1105           specify "-shared" because using "-Alddlflags=-shared" will prevent
1106           Configure from setting a default value for "lddlflags", which
1107           usually contains "-shared" (at least on Linux).
1108
1109       See also
1110       <http://code.google.com/p/address-sanitizer/wiki/AddressSanitizer>.
1111

PROFILING

1113       Depending on your platform there are various ways of profiling Perl.
1114
1115       There are two commonly used techniques of profiling executables:
1116       statistical time-sampling and basic-block counting.
1117
1118       The first method takes periodically samples of the CPU program counter,
1119       and since the program counter can be correlated with the code generated
1120       for functions, we get a statistical view of in which functions the
1121       program is spending its time.  The caveats are that very small/fast
1122       functions have lower probability of showing up in the profile, and that
1123       periodically interrupting the program (this is usually done rather
1124       frequently, in the scale of milliseconds) imposes an additional
1125       overhead that may skew the results.  The first problem can be
1126       alleviated by running the code for longer (in general this is a good
1127       idea for profiling), the second problem is usually kept in guard by the
1128       profiling tools themselves.
1129
1130       The second method divides up the generated code into basic blocks.
1131       Basic blocks are sections of code that are entered only in the
1132       beginning and exited only at the end.  For example, a conditional jump
1133       starts a basic block.  Basic block profiling usually works by
1134       instrumenting the code by adding enter basic block #nnnn book-keeping
1135       code to the generated code.  During the execution of the code the basic
1136       block counters are then updated appropriately.  The caveat is that the
1137       added extra code can skew the results: again, the profiling tools
1138       usually try to factor their own effects out of the results.
1139
1140   Gprof Profiling
1141       gprof is a profiling tool available in many Unix platforms which uses
1142       statistical time-sampling.  You can build a profiled version of perl by
1143       compiling using gcc with the flag "-pg".  Either edit config.sh or re-
1144       run Configure.  Running the profiled version of Perl will create an
1145       output file called gmon.out which contains the profiling data collected
1146       during the execution.
1147
1148       quick hint:
1149
1150           $ sh Configure -des -Dusedevel -Accflags='-pg' \
1151               -Aldflags='-pg' -Alddlflags='-pg -shared' \
1152               && make perl
1153           $ ./perl ... # creates gmon.out in current directory
1154           $ gprof ./perl > out
1155           $ less out
1156
1157       (you probably need to add "-shared" to the <-Alddlflags> line until RT
1158       #118199 is resolved)
1159
1160       The gprof tool can then display the collected data in various ways.
1161       Usually gprof understands the following options:
1162
1163       ·   -a
1164
1165           Suppress statically defined functions from the profile.
1166
1167       ·   -b
1168
1169           Suppress the verbose descriptions in the profile.
1170
1171       ·   -e routine
1172
1173           Exclude the given routine and its descendants from the profile.
1174
1175       ·   -f routine
1176
1177           Display only the given routine and its descendants in the profile.
1178
1179       ·   -s
1180
1181           Generate a summary file called gmon.sum which then may be given to
1182           subsequent gprof runs to accumulate data over several runs.
1183
1184       ·   -z
1185
1186           Display routines that have zero usage.
1187
1188       For more detailed explanation of the available commands and output
1189       formats, see your own local documentation of gprof.
1190
1191   GCC gcov Profiling
1192       basic block profiling is officially available in gcc 3.0 and later.
1193       You can build a profiled version of perl by compiling using gcc with
1194       the flags "-fprofile-arcs -ftest-coverage".  Either edit config.sh or
1195       re-run Configure.
1196
1197       quick hint:
1198
1199           $ sh Configure -des -Dusedevel -Doptimize='-g' \
1200               -Accflags='-fprofile-arcs -ftest-coverage' \
1201               -Aldflags='-fprofile-arcs -ftest-coverage' \
1202               -Alddlflags='-fprofile-arcs -ftest-coverage -shared' \
1203               && make perl
1204           $ rm -f regexec.c.gcov regexec.gcda
1205           $ ./perl ...
1206           $ gcov regexec.c
1207           $ less regexec.c.gcov
1208
1209       (you probably need to add "-shared" to the <-Alddlflags> line until RT
1210       #118199 is resolved)
1211
1212       Running the profiled version of Perl will cause profile output to be
1213       generated.  For each source file an accompanying .gcda file will be
1214       created.
1215
1216       To display the results you use the gcov utility (which should be
1217       installed if you have gcc 3.0 or newer installed).  gcov is run on
1218       source code files, like this
1219
1220           gcov sv.c
1221
1222       which will cause sv.c.gcov to be created.  The .gcov files contain the
1223       source code annotated with relative frequencies of execution indicated
1224       by "#" markers.  If you want to generate .gcov files for all profiled
1225       object files, you can run something like this:
1226
1227           for file in `find . -name \*.gcno`
1228           do sh -c "cd `dirname $file` && gcov `basename $file .gcno`"
1229           done
1230
1231       Useful options of gcov include "-b" which will summarise the basic
1232       block, branch, and function call coverage, and "-c" which instead of
1233       relative frequencies will use the actual counts.  For more information
1234       on the use of gcov and basic block profiling with gcc, see the latest
1235       GNU CC manual.  As of gcc 4.8, this is at
1236       <http://gcc.gnu.org/onlinedocs/gcc/Gcov-Intro.html#Gcov-Intro>
1237

MISCELLANEOUS TRICKS

1239   PERL_DESTRUCT_LEVEL
1240       If you want to run any of the tests yourself manually using e.g.
1241       valgrind, please note that by default perl does not explicitly cleanup
1242       all the memory it has allocated (such as global memory arenas) but
1243       instead lets the exit() of the whole program "take care" of such
1244       allocations, also known as "global destruction of objects".
1245
1246       There is a way to tell perl to do complete cleanup: set the environment
1247       variable PERL_DESTRUCT_LEVEL to a non-zero value.  The t/TEST wrapper
1248       does set this to 2, and this is what you need to do too, if you don't
1249       want to see the "global leaks": For example, for running under valgrind
1250
1251           env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib t/foo/bar.t
1252
1253       (Note: the mod_perl apache module uses also this environment variable
1254       for its own purposes and extended its semantics.  Refer to the mod_perl
1255       documentation for more information.  Also, spawned threads do the
1256       equivalent of setting this variable to the value 1.)
1257
1258       If, at the end of a run you get the message N scalars leaked, you can
1259       recompile with "-DDEBUG_LEAKING_SCALARS", ("Configure
1260       -Accflags=-DDEBUG_LEAKING_SCALARS"), which will cause the addresses of
1261       all those leaked SVs to be dumped along with details as to where each
1262       SV was originally allocated.  This information is also displayed by
1263       Devel::Peek.  Note that the extra details recorded with each SV
1264       increases memory usage, so it shouldn't be used in production
1265       environments.  It also converts "new_SV()" from a macro into a real
1266       function, so you can use your favourite debugger to discover where
1267       those pesky SVs were allocated.
1268
1269       If you see that you're leaking memory at runtime, but neither valgrind
1270       nor "-DDEBUG_LEAKING_SCALARS" will find anything, you're probably
1271       leaking SVs that are still reachable and will be properly cleaned up
1272       during destruction of the interpreter.  In such cases, using the "-Dm"
1273       switch can point you to the source of the leak.  If the executable was
1274       built with "-DDEBUG_LEAKING_SCALARS", "-Dm" will output SV allocations
1275       in addition to memory allocations.  Each SV allocation has a distinct
1276       serial number that will be written on creation and destruction of the
1277       SV.  So if you're executing the leaking code in a loop, you need to
1278       look for SVs that are created, but never destroyed between each cycle.
1279       If such an SV is found, set a conditional breakpoint within "new_SV()"
1280       and make it break only when "PL_sv_serial" is equal to the serial
1281       number of the leaking SV.  Then you will catch the interpreter in
1282       exactly the state where the leaking SV is allocated, which is
1283       sufficient in many cases to find the source of the leak.
1284
1285       As "-Dm" is using the PerlIO layer for output, it will by itself
1286       allocate quite a bunch of SVs, which are hidden to avoid recursion.
1287       You can bypass the PerlIO layer if you use the SV logging provided by
1288       "-DPERL_MEM_LOG" instead.
1289
1290   PERL_MEM_LOG
1291       If compiled with "-DPERL_MEM_LOG" ("-Accflags=-DPERL_MEM_LOG"), both
1292       memory and SV allocations go through logging functions, which is handy
1293       for breakpoint setting.
1294
1295       Unless "-DPERL_MEM_LOG_NOIMPL" ("-Accflags=-DPERL_MEM_LOG_NOIMPL") is
1296       also compiled, the logging functions read $ENV{PERL_MEM_LOG} to
1297       determine whether to log the event, and if so how:
1298
1299           $ENV{PERL_MEM_LOG} =~ /m/           Log all memory ops
1300           $ENV{PERL_MEM_LOG} =~ /s/           Log all SV ops
1301           $ENV{PERL_MEM_LOG} =~ /t/           include timestamp in Log
1302           $ENV{PERL_MEM_LOG} =~ /^(\d+)/      write to FD given (default is 2)
1303
1304       Memory logging is somewhat similar to "-Dm" but is independent of
1305       "-DDEBUGGING", and at a higher level; all uses of Newx(), Renew(), and
1306       Safefree() are logged with the caller's source code file and line
1307       number (and C function name, if supported by the C compiler).  In
1308       contrast, "-Dm" is directly at the point of "malloc()".  SV logging is
1309       similar.
1310
1311       Since the logging doesn't use PerlIO, all SV allocations are logged and
1312       no extra SV allocations are introduced by enabling the logging.  If
1313       compiled with "-DDEBUG_LEAKING_SCALARS", the serial number for each SV
1314       allocation is also logged.
1315
1316   DDD over gdb
1317       Those debugging perl with the DDD frontend over gdb may find the
1318       following useful:
1319
1320       You can extend the data conversion shortcuts menu, so for example you
1321       can display an SV's IV value with one click, without doing any typing.
1322       To do that simply edit ~/.ddd/init file and add after:
1323
1324         ! Display shortcuts.
1325         Ddd*gdbDisplayShortcuts: \
1326         /t ()   // Convert to Bin\n\
1327         /d ()   // Convert to Dec\n\
1328         /x ()   // Convert to Hex\n\
1329         /o ()   // Convert to Oct(\n\
1330
1331       the following two lines:
1332
1333         ((XPV*) (())->sv_any )->xpv_pv  // 2pvx\n\
1334         ((XPVIV*) (())->sv_any )->xiv_iv // 2ivx
1335
1336       so now you can do ivx and pvx lookups or you can plug there the sv_peek
1337       "conversion":
1338
1339         Perl_sv_peek(my_perl, (SV*)()) // sv_peek
1340
1341       (The my_perl is for threaded builds.)  Just remember that every line,
1342       but the last one, should end with \n\
1343
1344       Alternatively edit the init file interactively via: 3rd mouse button ->
1345       New Display -> Edit Menu
1346
1347       Note: you can define up to 20 conversion shortcuts in the gdb section.
1348
1349   C backtrace
1350       On some platforms Perl supports retrieving the C level backtrace
1351       (similar to what symbolic debuggers like gdb do).
1352
1353       The backtrace returns the stack trace of the C call frames, with the
1354       symbol names (function names), the object names (like "perl"), and if
1355       it can, also the source code locations (file:line).
1356
1357       The supported platforms are Linux, and OS X (some *BSD might work at
1358       least partly, but they have not yet been tested).
1359
1360       This feature hasn't been tested with multiple threads, but it will only
1361       show the backtrace of the thread doing the backtracing.
1362
1363       The feature needs to be enabled with "Configure -Dusecbacktrace".
1364
1365       The "-Dusecbacktrace" also enables keeping the debug information when
1366       compiling/linking (often: "-g").  Many compilers/linkers do support
1367       having both optimization and keeping the debug information.  The debug
1368       information is needed for the symbol names and the source locations.
1369
1370       Static functions might not be visible for the backtrace.
1371
1372       Source code locations, even if available, can often be missing or
1373       misleading if the compiler has e.g. inlined code.  Optimizer can make
1374       matching the source code and the object code quite challenging.
1375
1376       Linux
1377           You must have the BFD (-lbfd) library installed, otherwise "perl"
1378           will fail to link.  The BFD is usually distributed as part of the
1379           GNU binutils.
1380
1381           Summary: "Configure ... -Dusecbacktrace" and you need "-lbfd".
1382
1383       OS X
1384           The source code locations are supported only if you have the
1385           Developer Tools installed.  (BFD is not needed.)
1386
1387           Summary: "Configure ... -Dusecbacktrace" and installing the
1388           Developer Tools would be good.
1389
1390       Optionally, for trying out the feature, you may want to enable
1391       automatic dumping of the backtrace just before a warning or croak (die)
1392       message is emitted, by adding "-Accflags=-DUSE_C_BACKTRACE_ON_ERROR"
1393       for Configure.
1394
1395       Unless the above additional feature is enabled, nothing about the
1396       backtrace functionality is visible, except for the Perl/XS level.
1397
1398       Furthermore, even if you have enabled this feature to be compiled, you
1399       need to enable it in runtime with an environment variable:
1400       "PERL_C_BACKTRACE_ON_ERROR=10".  It must be an integer higher than
1401       zero, telling the desired frame count.
1402
1403       Retrieving the backtrace from Perl level (using for example an XS
1404       extension) would be much less exciting than one would hope: normally
1405       you would see "runops", "entersub", and not much else.  This API is
1406       intended to be called from within the Perl implementation, not from
1407       Perl level execution.
1408
1409       The C API for the backtrace is as follows:
1410
1411       get_c_backtrace
1412       free_c_backtrace
1413       get_c_backtrace_dump
1414       dump_c_backtrace
1415
1416   Poison
1417       If you see in a debugger a memory area mysteriously full of 0xABABABAB
1418       or 0xEFEFEFEF, you may be seeing the effect of the Poison() macros, see
1419       perlclib.
1420
1421   Read-only optrees
1422       Under ithreads the optree is read only.  If you want to enforce this,
1423       to check for write accesses from buggy code, compile with
1424       "-Accflags=-DPERL_DEBUG_READONLY_OPS" to enable code that allocates op
1425       memory via "mmap", and sets it read-only when it is attached to a
1426       subroutine.  Any write access to an op results in a "SIGBUS" and abort.
1427
1428       This code is intended for development only, and may not be portable
1429       even to all Unix variants.  Also, it is an 80% solution, in that it
1430       isn't able to make all ops read only.  Specifically it does not apply
1431       to op slabs belonging to "BEGIN" blocks.
1432
1433       However, as an 80% solution it is still effective, as it has caught
1434       bugs in the past.
1435
1436   When is a bool not a bool?
1437       On pre-C99 compilers, "bool" is defined as equivalent to "char".
1438       Consequently assignment of any larger type to a "bool" is unsafe and
1439       may be truncated.  The "cBOOL" macro exists to cast it correctly; you
1440       may also find that using it is shorter and clearer than writing out the
1441       equivalent conditional expression longhand.
1442
1443       On those platforms and compilers where "bool" really is a boolean (C++,
1444       C99), it is easy to forget the cast.  You can force "bool" to be a
1445       "char" by compiling with "-Accflags=-DPERL_BOOL_AS_CHAR".  You may also
1446       wish to run "Configure" with something like
1447
1448           -Accflags='-Wconversion -Wno-sign-conversion -Wno-shorten-64-to-32'
1449
1450       or your compiler's equivalent to make it easier to spot any unsafe
1451       truncations that show up.
1452
1453       The "TRUE" and "FALSE" macros are available for situations where using
1454       them would clarify intent. (But they always just mean the same as the
1455       integers 1 and 0 regardless, so using them isn't compulsory.)
1456
1457   The .i Targets
1458       You can expand the macros in a foo.c file by saying
1459
1460           make foo.i
1461
1462       which will expand the macros using cpp.  Don't be scared by the
1463       results.
1464

AUTHOR

1466       This document was originally written by Nathan Torkington, and is
1467       maintained by the perl5-porters mailing list.
1468
1469
1470
1471perl v5.30.2                      2020-03-27                   PERLHACKTIPS(1)
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