5 Consistent formatting of this file is achieved with:
6 perl ./Porting/podtidy pod/perlhacktips.pod
10 perlhacktips - Tips for Perl core C code hacking
14 This document will help you learn the best way to go about hacking on
15 the Perl core C code. It covers common problems, debugging, profiling,
18 If you haven't read L<perlhack> and L<perlhacktut> yet, you might want
21 =head1 COMMON PROBLEMS
23 Perl source plays by ANSI C89 rules: no C99 (or C++) extensions. In
24 some cases we have to take pre-ANSI requirements into consideration.
25 You don't care about some particular platform having broken Perl? I
26 hear there is still a strong demand for J2EE programmers.
28 =head2 Perl environment problems
34 Not compiling with threading
36 Compiling with threading (-Duseithreads) completely rewrites the
37 function prototypes of Perl. You better try your changes with that.
38 Related to this is the difference between "Perl_-less" and "Perl_-ly"
41 Perl_sv_setiv(aTHX_ ...);
44 The first one explicitly passes in the context, which is needed for
45 e.g. threaded builds. The second one does that implicitly; do not get
46 them mixed. If you are not passing in a aTHX_, you will need to do a
47 dTHX (or a dVAR) as the first thing in the function.
49 See L<perlguts/"How multiple interpreters and concurrency are
50 supported"> for further discussion about context.
54 Not compiling with -DDEBUGGING
56 The DEBUGGING define exposes more code to the compiler, therefore more
57 ways for things to go wrong. You should try it.
61 Introducing (non-read-only) globals
63 Do not introduce any modifiable globals, truly global or file static.
64 They are bad form and complicate multithreading and other forms of
65 concurrency. The right way is to introduce them as new interpreter
66 variables, see F<intrpvar.h> (at the very end for binary
69 Introducing read-only (const) globals is okay, as long as you verify
70 with e.g. C<nm libperl.a|egrep -v ' [TURtr] '> (if your C<nm> has
71 BSD-style output) that the data you added really is read-only. (If it
72 is, it shouldn't show up in the output of that command.)
74 If you want to have static strings, make them constant:
76 static const char etc[] = "...";
78 If you want to have arrays of constant strings, note carefully the
79 right combination of C<const>s:
81 static const char * const yippee[] =
82 {"hi", "ho", "silver"};
84 There is a way to completely hide any modifiable globals (they are all
85 moved to heap), the compilation setting
86 C<-DPERL_GLOBAL_STRUCT_PRIVATE>. It is not normally used, but can be
87 used for testing, read more about it in L<perlguts/"Background and
88 PERL_IMPLICIT_CONTEXT">.
92 Not exporting your new function
94 Some platforms (Win32, AIX, VMS, OS/2, to name a few) require any
95 function that is part of the public API (the shared Perl library) to be
96 explicitly marked as exported. See the discussion about F<embed.pl> in
101 Exporting your new function
103 The new shiny result of either genuine new functionality or your
104 arduous refactoring is now ready and correctly exported. So what could
107 Maybe simply that your function did not need to be exported in the
108 first place. Perl has a long and not so glorious history of exporting
109 functions that it should not have.
111 If the function is used only inside one source code file, make it
112 static. See the discussion about F<embed.pl> in L<perlguts>.
114 If the function is used across several files, but intended only for
115 Perl's internal use (and this should be the common case), do not export
116 it to the public API. See the discussion about F<embed.pl> in
121 =head2 Portability problems
123 The following are common causes of compilation and/or execution
124 failures, not common to Perl as such. The C FAQ is good bedtime
125 reading. Please test your changes with as many C compilers and
126 platforms as possible; we will, anyway, and it's nice to save oneself
127 from public embarrassment.
129 If using gcc, you can add the C<-std=c89> option which will hopefully
130 catch most of these unportabilities. (However it might also catch
131 incompatibilities in your system's header files.)
133 Use the Configure C<-Dgccansipedantic> flag to enable the gcc C<-ansi
134 -pedantic> flags which enforce stricter ANSI rules.
136 If using the C<gcc -Wall> note that not all the possible warnings (like
137 C<-Wuninitialized>) are given unless you also compile with C<-O>.
139 Note that if using gcc, starting from Perl 5.9.5 the Perl core source
140 code files (the ones at the top level of the source code distribution,
141 but not e.g. the extensions under ext/) are automatically compiled with
142 as many as possible of the C<-std=c89>, C<-ansi>, C<-pedantic>, and a
143 selection of C<-W> flags (see cflags.SH).
145 Also study L<perlport> carefully to avoid any bad assumptions about the
146 operating system, filesystems, character set, and so forth.
148 You may once in a while try a "make microperl" to see whether we can
149 still compile Perl with just the bare minimum of interfaces. (See
152 Do not assume an operating system indicates a certain compiler.
158 Casting pointers to integers or casting integers to pointers
170 Both are bad, and broken, and unportable. Use the PTR2IV() macro that
171 does it right. (Likewise, there are PTR2UV(), PTR2NV(), INT2PTR(), and
176 Casting between function pointers and data pointers
178 Technically speaking casting between function pointers and data
179 pointers is unportable and undefined, but practically speaking it seems
180 to work, but you should use the FPTR2DPTR() and DPTR2FPTR() macros.
181 Sometimes you can also play games with unions.
185 Assuming sizeof(int) == sizeof(long)
187 There are platforms where longs are 64 bits, and platforms where ints
188 are 64 bits, and while we are out to shock you, even platforms where
189 shorts are 64 bits. This is all legal according to the C standard. (In
190 other words, "long long" is not a portable way to specify 64 bits, and
191 "long long" is not even guaranteed to be any wider than "long".)
193 Instead, use the definitions IV, UV, IVSIZE, I32SIZE, and so forth.
194 Avoid things like I32 because they are B<not> guaranteed to be
195 I<exactly> 32 bits, they are I<at least> 32 bits, nor are they
196 guaranteed to be B<int> or B<long>. If you really explicitly need
197 64-bit variables, use I64 and U64, but only if guarded by HAS_QUAD.
201 Assuming one can dereference any type of pointer for any type of data
204 long pony = *(long *)p; /* BAD */
206 Many platforms, quite rightly so, will give you a core dump instead of
207 a pony if the p happens not to be correctly aligned.
213 (int)*p = ...; /* BAD */
215 Simply not portable. Get your lvalue to be of the right type, or maybe
216 use temporary variables, or dirty tricks with unions.
220 Assume B<anything> about structs (especially the ones you don't
221 control, like the ones coming from the system headers)
227 That a certain field exists in a struct
231 That no other fields exist besides the ones you know of
235 That a field is of certain signedness, sizeof, or type
239 That the fields are in a certain order
245 While C guarantees the ordering specified in the struct definition,
246 between different platforms the definitions might differ
252 That the sizeof(struct) or the alignments are the same everywhere
258 There might be padding bytes between the fields to align the fields -
259 the bytes can be anything
263 Structs are required to be aligned to the maximum alignment required by
264 the fields - which for native types is for usually equivalent to
265 sizeof() of the field
273 Assuming the character set is ASCIIish
275 Perl can compile and run under EBCDIC platforms. See L<perlebcdic>.
276 This is transparent for the most part, but because the character sets
277 differ, you shouldn't use numeric (decimal, octal, nor hex) constants
278 to refer to characters. You can safely say C<'A'>, but not C<0x41>.
279 You can safely say C<'\n'>, but not C<\012>. However, you can use
280 macros defined in F<utf8.h> to specify any code point portably.
281 C<LATIN1_TO_NATIVE(0xDF)> is going to be the code point that means
282 LATIN SMALL LETTER SHARP S on whatever platform you are running on (on
283 ASCII platforms it compiles without adding any extra code, so there is
284 zero performance hit on those). The acceptable inputs to
285 C<LATIN1_TO_NATIVE> are from C<0x00> through C<0xFF>. If your input
286 isn't guaranteed to be in that range, use C<UNICODE_TO_NATIVE> instead.
287 C<NATIVE_TO_LATIN1> and C<NATIVE_TO_UNICODE> translate the opposite
290 If you need the string representation of a character that doesn't have a
291 mnemonic name in C, you should add it to the list in
292 F<regen/unicode_constants.pl>, and have Perl create C<#define>'s for you,
293 based on the current platform.
295 Note that the C<isI<FOO>> and C<toI<FOO>> macros in F<handy.h> work
296 properly on native code points and strings.
298 Also, the range 'A' - 'Z' in ASCII is an unbroken sequence of 26 upper
299 case 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
301 anything about other ranges. (Note that special handling of ranges in
302 regular expression patterns and transliterations makes it appear to Perl
303 code that the aforementioned ranges are all unbroken.)
305 Many of the comments in the existing code ignore the possibility of
306 EBCDIC, and may be wrong therefore, even if the code works. This is
307 actually a tribute to the successful transparent insertion of being
308 able to handle EBCDIC without having to change pre-existing code.
310 UTF-8 and UTF-EBCDIC are two different encodings used to represent
311 Unicode code points as sequences of bytes. Macros with the same names
312 (but different definitions) in F<utf8.h> and F<utfebcdic.h> are used to
313 allow the calling code to think that there is only one such encoding.
314 This is almost always referred to as C<utf8>, but it means the EBCDIC
315 version as well. Again, comments in the code may well be wrong even if
316 the code itself is right. For example, the concept of UTF-8 C<invariant
317 characters> differs between ASCII and EBCDIC. On ASCII platforms, only
318 characters that do not have the high-order bit set (i.e. whose ordinals
319 are strict ASCII, 0 - 127) are invariant, and the documentation and
320 comments in the code may assume that, often referring to something
321 like, say, C<hibit>. The situation differs and is not so simple on
322 EBCDIC machines, but as long as the code itself uses the
323 C<NATIVE_IS_INVARIANT()> macro appropriately, it works, even if the
326 As noted in L<perlhack/TESTING>, when writing test scripts, the file
327 F<t/charset_tools.pl> contains some helpful functions for writing tests
328 valid on both ASCII and EBCDIC platforms. Sometimes, though, a test
329 can't use a function and it's inconvenient to have different test
330 versions depending on the platform. There are 20 code points that are
331 the same in all 4 character sets currently recognized by Perl (the 3
332 EBCDIC code pages plus ISO 8859-1 (ASCII/Latin1)). These can be used in
333 such tests, though there is a small possibility that Perl will become
334 available in yet another character set, breaking your test. All but one
335 of these code points are C0 control characters. The most significant
336 controls that are the same are C<\0>, C<\r>, and C<\N{VT}> (also
337 specifiable as C<\cK>, C<\x0B>, C<\N{U+0B}>, or C<\013>). The single
338 non-control is U+00B6 PILCROW SIGN. The controls that are the same have
339 the same bit pattern in all 4 character sets, regardless of the UTF8ness
340 of the string containing them. The bit pattern for U+B6 is the same in
341 all 4 for non-UTF8 strings, but differs in each when its containing
342 string is UTF-8 encoded. The only other code points that have some sort
343 of sameness across all 4 character sets are the pair 0xDC and 0xFC.
344 Together these represent upper- and lowercase LATIN LETTER U WITH
345 DIAERESIS, but which is upper and which is lower may be reversed: 0xDC
346 is the capital in Latin1 and 0xFC is the small letter, while 0xFC is the
347 capital in EBCDIC and 0xDC is the small one. This factoid may be
348 exploited in writing case insensitive tests that are the same across all
353 Assuming the character set is just ASCII
355 ASCII is a 7 bit encoding, but bytes have 8 bits in them. The 128 extra
356 characters have different meanings depending on the locale. Absent a
357 locale, currently these extra characters are generally considered to be
358 unassigned, and this has presented some problems. This has being
359 changed starting in 5.12 so that these characters can be considered to
360 be Latin-1 (ISO-8859-1).
364 Mixing #define and #ifdef
366 #define BURGLE(x) ... \
367 #ifdef BURGLE_OLD_STYLE /* BAD */
368 ... do it the old way ... \
370 ... do it the new way ... \
373 You cannot portably "stack" cpp directives. For example in the above
374 you need two separate BURGLE() #defines, one for each #ifdef branch.
378 Adding non-comment stuff after #endif or #else
382 #else !SNOSH /* BAD */
384 #endif SNOSH /* BAD */
386 The #endif and #else cannot portably have anything non-comment after
387 them. If you want to document what is going (which is a good idea
388 especially if the branches are long), use (C) comments:
396 The gcc option C<-Wendif-labels> warns about the bad variant (by
397 default on starting from Perl 5.9.4).
401 Having a comma after the last element of an enum list
409 is not portable. Leave out the last comma.
411 Also note that whether enums are implicitly morphable to ints varies
412 between compilers, you might need to (int).
418 // This function bamfoodles the zorklator. /* BAD */
420 That is C99 or C++. Perl is C89. Using the //-comments is silently
421 allowed by many C compilers but cranking up the ANSI C89 strictness
422 (which we like to do) causes the compilation to fail.
426 Mixing declarations and code
431 set_zorkmids(n); /* BAD */
434 That is C99 or C++. Some C compilers allow that, but you shouldn't.
436 The gcc option C<-Wdeclaration-after-statements> scans for such
437 problems (by default on starting from Perl 5.9.4).
441 Introducing variables inside for()
443 for(int i = ...; ...; ...) { /* BAD */
445 That is C99 or C++. While it would indeed be awfully nice to have that
446 also in C89, to limit the scope of the loop variable, alas, we cannot.
450 Mixing signed char pointers with unsigned char pointers
452 int foo(char *s) { ... }
454 unsigned char *t = ...; /* Or U8* t = ... */
457 While this is legal practice, it is certainly dubious, and downright
458 fatal in at least one platform: for example VMS cc considers this a
459 fatal error. One cause for people often making this mistake is that a
460 "naked char" and therefore dereferencing a "naked char pointer" have an
461 undefined signedness: it depends on the compiler and the flags of the
462 compiler and the underlying platform whether the result is signed or
463 unsigned. For this very same reason using a 'char' as an array index is
468 Macros that have string constants and their arguments as substrings of
471 #define FOO(n) printf("number = %d\n", n) /* BAD */
474 Pre-ANSI semantics for that was equivalent to
476 printf("10umber = %d\10");
478 which is probably not what you were expecting. Unfortunately at least
479 one reasonably common and modern C compiler does "real backward
480 compatibility" here, in AIX that is what still happens even though the
481 rest of the AIX compiler is very happily C89.
485 Using printf formats for non-basic C types
488 printf("i = %d\n", i); /* BAD */
490 While this might by accident work in some platform (where IV happens to
491 be an C<int>), in general it cannot. IV might be something larger. Even
492 worse the situation is with more specific types (defined by Perl's
493 configuration step in F<config.h>):
496 printf("who = %d\n", who); /* BAD */
498 The problem here is that Uid_t might be not only not C<int>-wide but it
499 might also be unsigned, in which case large uids would be printed as
502 There is no simple solution to this because of printf()'s limited
503 intelligence, but for many types the right format is available as with
504 either 'f' or '_f' suffix, for example:
506 IVdf /* IV in decimal */
507 UVxf /* UV is hexadecimal */
509 printf("i = %"IVdf"\n", i); /* The IVdf is a string constant. */
511 Uid_t_f /* Uid_t in decimal */
513 printf("who = %"Uid_t_f"\n", who);
515 Or you can try casting to a "wide enough" type:
517 printf("i = %"IVdf"\n", (IV)something_very_small_and_signed);
519 See L<perlguts/Formatted Printing of Size_t and SSize_t> for how to
522 Also remember that the C<%p> format really does require a void pointer:
525 printf("p = %p\n", (void*)p);
527 The gcc option C<-Wformat> scans for such problems.
531 Blindly using variadic macros
533 gcc has had them for a while with its own syntax, and C99 brought them
534 with a standardized syntax. Don't use the former, and use the latter
535 only if the HAS_C99_VARIADIC_MACROS is defined.
539 Blindly passing va_list
541 Not all platforms support passing va_list to further varargs (stdarg)
542 functions. The right thing to do is to copy the va_list using the
543 Perl_va_copy() if the NEED_VA_COPY is defined.
547 Using gcc statement expressions
549 val = ({...;...;...}); /* BAD */
551 While a nice extension, it's not portable. The Perl code does
552 admittedly use them if available to gain some extra speed (essentially
553 as a funky form of inlining), but you shouldn't.
557 Binding together several statements in a macro
559 Use the macros STMT_START and STMT_END.
567 Testing for operating systems or versions when should be testing for
570 #ifdef __FOONIX__ /* BAD */
574 Unless you know with 100% certainty that quux() is only ever available
575 for the "Foonix" operating system B<and> that is available B<and>
576 correctly working for B<all> past, present, B<and> future versions of
577 "Foonix", the above is very wrong. This is more correct (though still
578 not perfect, because the below is a compile-time check):
584 How does the HAS_QUUX become defined where it needs to be? Well, if
585 Foonix happens to be Unixy enough to be able to run the Configure
586 script, and Configure has been taught about detecting and testing
587 quux(), the HAS_QUUX will be correctly defined. In other platforms, the
588 corresponding configuration step will hopefully do the same.
590 In a pinch, if you cannot wait for Configure to be educated, or if you
591 have a good hunch of where quux() might be available, you can
592 temporarily try the following:
594 #if (defined(__FOONIX__) || defined(__BARNIX__))
604 But in any case, try to keep the features and operating systems
607 A good resource on the predefined macros for various operating
608 systems, compilers, and so forth is
609 L<http://sourceforge.net/p/predef/wiki/Home/>
613 Assuming the contents of static memory pointed to by the return values
614 of Perl wrappers for C library functions doesn't change. Many C library
615 functions return pointers to static storage that can be overwritten by
616 subsequent calls to the same or related functions. Perl has
617 light-weight wrappers for some of these functions, and which don't make
618 copies of the static memory. A good example is the interface to the
619 environment variables that are in effect for the program. Perl has
620 C<PerlEnv_getenv> to get values from the environment. But the return is
621 a pointer to static memory in the C library. If you are using the value
622 to immediately test for something, that's fine, but if you save the
623 value and expect it to be unchanged by later processing, you would be
624 wrong, but perhaps you wouldn't know it because different C library
625 implementations behave differently, and the one on the platform you're
626 testing on might work for your situation. But on some platforms, a
627 subsequent call to C<PerlEnv_getenv> or related function WILL overwrite
628 the memory that your first call points to. This has led to some
629 hard-to-debug problems. Do a L<perlapi/savepv> to make a copy, thus
630 avoiding these problems. You will have to free the copy when you're
631 done to avoid memory leaks. If you don't have control over when it gets
632 freed, you'll need to make the copy in a mortal scalar, like so:
634 if ((s = PerlEnv_getenv("foo") == NULL) {
635 ... /* handle NULL case */
638 s = SvPVX(sv_2mortal(newSVpv(s, 0)));
641 The above example works only if C<"s"> is C<NUL>-terminated; otherwise
642 you have to pass its length to C<newSVpv>.
646 =head2 Problematic System Interfaces
652 malloc(0), realloc(0), calloc(0, 0) are non-portable. To be portable
653 allocate at least one byte. (In general you should rarely need to work
654 at this low level, but instead use the various malloc wrappers.)
658 snprintf() - the return type is unportable. Use my_snprintf() instead.
662 =head2 Security problems
664 Last but not least, here are various tips for safer coding.
665 See also L<perlclib> for libc/stdio replacements one should use.
673 Or we will publicly ridicule you. Seriously.
679 Use mkstemp() instead.
683 Do not use strcpy() or strcat() or strncpy() or strncat()
685 Use my_strlcpy() and my_strlcat() instead: they either use the native
686 implementation, or Perl's own implementation (borrowed from the public
687 domain implementation of INN).
691 Do not use sprintf() or vsprintf()
693 If you really want just plain byte strings, use my_snprintf() and
694 my_vsnprintf() instead, which will try to use snprintf() and
695 vsnprintf() if those safer APIs are available. If you want something
696 fancier than a plain byte string, use
697 L<C<Perl_form>()|perlapi/form> or SVs and
698 L<C<Perl_sv_catpvf()>|perlapi/sv_catpvf>.
700 Note that glibc C<printf()>, C<sprintf()>, etc. are buggy before glibc
701 version 2.17. They won't allow a C<%.s> format with a precision to
702 create a string that isn't valid UTF-8 if the current underlying locale
703 of the program is UTF-8. What happens is that the C<%s> and its operand are
704 simply skipped without any notice.
705 L<https://sourceware.org/bugzilla/show_bug.cgi?id=6530>.
711 Use grok_atoUV() instead. atoi() has ill-defined behavior on overflows,
712 and cannot be used for incremental parsing. It is also affected by locale,
717 Do not use strtol() or strtoul()
719 Use grok_atoUV() instead. strtol() or strtoul() (or their IV/UV-friendly
720 macro disguises, Strtol() and Strtoul(), or Atol() and Atoul() are
721 affected by locale, which is bad.
727 You can compile a special debugging version of Perl, which allows you
728 to use the C<-D> option of Perl to tell more about what Perl is doing.
729 But sometimes there is no alternative than to dive in with a debugger,
730 either to see the stack trace of a core dump (very useful in a bug
731 report), or trying to figure out what went wrong before the core dump
732 happened, or how did we end up having wrong or unexpected results.
734 =head2 Poking at Perl
736 To really poke around with Perl, you'll probably want to build Perl for
737 debugging, like this:
739 ./Configure -d -DDEBUGGING
742 C<-DDEBUGGING> turns on the C compiler's C<-g> flag to have it produce
743 debugging information which will allow us to step through a running
744 program, and to see in which C function we are at (without the debugging
745 information we might see only the numerical addresses of the functions,
746 which is not very helpful). It will also turn on the C<DEBUGGING>
747 compilation symbol which enables all the internal debugging code in Perl.
748 There are a whole bunch of things you can debug with this: L<perlrun>
749 lists them all, and the best way to find out about them is to play about
750 with them. The most useful options are probably
752 l Context (loop) stack processing
753 s Stack snapshots (with v, displays all stacks)
755 o Method and overloading resolution
756 c String/numeric conversions
760 $ perl -Dst -e '$a + 1'
770 Some of the functionality of the debugging code can be achieved with a
771 non-debugging perl by using XS modules:
773 -Dr => use re 'debug'
776 =head2 Using a source-level debugger
778 If the debugging output of C<-D> doesn't help you, it's time to step
779 through perl's execution with a source-level debugger.
785 We'll use C<gdb> for our examples here; the principles will apply to
786 any debugger (many vendors call their debugger C<dbx>), but check the
787 manual of the one you're using.
791 To fire up the debugger, type
795 Or if you have a core dump:
799 You'll want to do that in your Perl source tree so the debugger can
800 read the source code. You should see the copyright message, followed by
805 C<help> will get you into the documentation, but here are the most
812 Run the program with the given arguments.
814 =item * break function_name
816 =item * break source.c:xxx
818 Tells the debugger that we'll want to pause execution when we reach
819 either the named function (but see L<perlguts/Internal Functions>!) or
820 the given line in the named source file.
824 Steps through the program a line at a time.
828 Steps through the program a line at a time, without descending into
833 Run until the next breakpoint.
837 Run until the end of the current function, then stop again.
841 Just pressing Enter will do the most recent operation again - it's a
842 blessing when stepping through miles of source code.
846 Prints the C definition of the argument given.
852 OP *(*op_ppaddr)(void);
854 unsigned int op_type : 9;
855 unsigned int op_opt : 1;
856 unsigned int op_slabbed : 1;
857 unsigned int op_savefree : 1;
858 unsigned int op_static : 1;
859 unsigned int op_folded : 1;
860 unsigned int op_spare : 2;
867 Execute the given C code and print its results. B<WARNING>: Perl makes
868 heavy use of macros, and F<gdb> does not necessarily support macros
869 (see later L</"gdb macro support">). You'll have to substitute them
870 yourself, or to invoke cpp on the source code files (see L</"The .i
871 Targets">) So, for instance, you can't say
877 print Perl_sv_2pv_nolen(sv)
881 You may find it helpful to have a "macro dictionary", which you can
882 produce by saying C<cpp -dM perl.c | sort>. Even then, F<cpp> won't
883 recursively apply those macros for you.
885 =head2 gdb macro support
887 Recent versions of F<gdb> have fairly good macro support, but in order
888 to use it you'll need to compile perl with macro definitions included
889 in the debugging information. Using F<gcc> version 3.1, this means
890 configuring with C<-Doptimize=-g3>. Other compilers might use a
891 different switch (if they support debugging macros at all).
893 =head2 Dumping Perl Data Structures
895 One way to get around this macro hell is to use the dumping functions
896 in F<dump.c>; these work a little like an internal
897 L<Devel::Peek|Devel::Peek>, but they also cover OPs and other
898 structures that you can't get at from Perl. Let's take an example.
899 We'll use the C<$a = $b + $c> we used before, but give it a bit of
900 context: C<$b = "6XXXX"; $c = 2.3;>. Where's a good place to stop and
903 What about C<pp_add>, the function we examined earlier to implement the
906 (gdb) break Perl_pp_add
907 Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.
909 Notice we use C<Perl_pp_add> and not C<pp_add> - see
910 L<perlguts/Internal Functions>. With the breakpoint in place, we can
913 (gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c'
915 Lots of junk will go past as gdb reads in the relevant source files and
918 Breakpoint 1, Perl_pp_add () at pp_hot.c:309
919 309 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
924 We looked at this bit of code before, and we said that
925 C<dPOPTOPnnrl_ul> arranges for two C<NV>s to be placed into C<left> and
926 C<right> - let's slightly expand it:
928 #define dPOPTOPnnrl_ul NV right = POPn; \
930 NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0
932 C<POPn> takes the SV from the top of the stack and obtains its NV
933 either directly (if C<SvNOK> is set) or by calling the C<sv_2nv>
934 function. C<TOPs> takes the next SV from the top of the stack - yes,
935 C<POPn> uses C<TOPs> - but doesn't remove it. We then use C<SvNV> to
936 get the NV from C<leftsv> in the same way as before - yes, C<POPn> uses
939 Since we don't have an NV for C<$b>, we'll have to use C<sv_2nv> to
940 convert it. If we step again, we'll find ourselves there:
943 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669
947 We can now use C<Perl_sv_dump> to investigate the SV:
949 (gdb) print Perl_sv_dump(sv)
950 SV = PV(0xa057cc0) at 0xa0675d0
953 PV = 0xa06a510 "6XXXX"\0
958 We know we're going to get C<6> from this, so let's finish the
962 Run till exit from #0 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671
963 0x462669 in Perl_pp_add () at pp_hot.c:311
966 We can also dump out this op: the current op is always stored in
967 C<PL_op>, and we can dump it with C<Perl_op_dump>. This'll give us
968 similar output to L<B::Debug|B::Debug>.
970 (gdb) print Perl_op_dump(PL_op)
972 13 TYPE = add ===> 14
974 FLAGS = (SCALAR,KIDS)
976 TYPE = null ===> (12)
978 FLAGS = (SCALAR,KIDS)
980 11 TYPE = gvsv ===> 12
986 # finish this later #
988 =head2 Using gdb to look at specific parts of a program
990 With the example above, you knew to look for C<Perl_pp_add>, but what if
991 there were multiple calls to it all over the place, or you didn't know what
992 the op was you were looking for?
994 One way to do this is to inject a rare call somewhere near what you're looking
995 for. For example, you could add C<study> before your method:
1001 (gdb) break Perl_pp_study
1003 And then step until you hit what you're
1004 looking for. This works well in a loop
1005 if you want to only break at certain iterations:
1007 for my $c (1..100) {
1011 =head2 Using gdb to look at what the parser/lexer are doing
1013 If you want to see what perl is doing when parsing/lexing your code, you can
1022 (gdb) break Perl_pp_study
1024 If you want to see what the parser/lexer is doing inside of C<if> blocks and
1025 the like you need to be a little trickier:
1027 if ($a && $b && do { BEGIN { study } 1 } && $c) { ... }
1029 =head1 SOURCE CODE STATIC ANALYSIS
1031 Various tools exist for analysing C source code B<statically>, as
1032 opposed to B<dynamically>, that is, without executing the code. It is
1033 possible to detect resource leaks, undefined behaviour, type
1034 mismatches, portability problems, code paths that would cause illegal
1035 memory accesses, and other similar problems by just parsing the C code
1036 and looking at the resulting graph, what does it tell about the
1037 execution and data flows. As a matter of fact, this is exactly how C
1038 compilers know to give warnings about dubious code.
1042 The good old C code quality inspector, C<lint>, is available in several
1043 platforms, but please be aware that there are several different
1044 implementations of it by different vendors, which means that the flags
1045 are not identical across different platforms.
1047 There is a C<lint> target in Makefile, but you may have to
1048 diddle with the flags (see above).
1052 Coverity (L<http://www.coverity.com/>) is a product similar to lint and as
1053 a testbed for their product they periodically check several open source
1054 projects, and they give out accounts to open source developers to the
1057 There is Coverity setup for the perl5 project:
1058 L<https://scan.coverity.com/projects/perl5>
1060 =head2 HP-UX cadvise (Code Advisor)
1062 HP has a C/C++ static analyzer product for HP-UX caller Code Advisor.
1063 (Link not given here because the URL is horribly long and seems horribly
1064 unstable; use the search engine of your choice to find it.) The use of
1065 the C<cadvise_cc> recipe with C<Configure ... -Dcc=./cadvise_cc>
1066 (see cadvise "User Guide") is recommended; as is the use of C<+wall>.
1068 =head2 cpd (cut-and-paste detector)
1070 The cpd tool detects cut-and-paste coding. If one instance of the
1071 cut-and-pasted code changes, all the other spots should probably be
1072 changed, too. Therefore such code should probably be turned into a
1073 subroutine or a macro.
1075 cpd (L<http://pmd.sourceforge.net/cpd.html>) is part of the pmd project
1076 (L<http://pmd.sourceforge.net/>). pmd was originally written for static
1077 analysis of Java code, but later the cpd part of it was extended to
1078 parse also C and C++.
1080 Download the pmd-bin-X.Y.zip () from the SourceForge site, extract the
1081 pmd-X.Y.jar from it, and then run that on source code thusly:
1083 java -cp pmd-X.Y.jar net.sourceforge.pmd.cpd.CPD \
1084 --minimum-tokens 100 --files /some/where/src --language c > cpd.txt
1086 You may run into memory limits, in which case you should use the -Xmx
1093 Though much can be written about the inconsistency and coverage
1094 problems of gcc warnings (like C<-Wall> not meaning "all the warnings",
1095 or some common portability problems not being covered by C<-Wall>, or
1096 C<-ansi> and C<-pedantic> both being a poorly defined collection of
1097 warnings, and so forth), gcc is still a useful tool in keeping our
1100 The C<-Wall> is by default on.
1102 The C<-ansi> (and its sidekick, C<-pedantic>) would be nice to be on
1103 always, but unfortunately they are not safe on all platforms, they can
1104 for example cause fatal conflicts with the system headers (Solaris
1105 being a prime example). If Configure C<-Dgccansipedantic> is used, the
1106 C<cflags> frontend selects C<-ansi -pedantic> for the platforms where
1107 they are known to be safe.
1109 Starting from Perl 5.9.4 the following extra flags are added:
1123 C<-Wdeclaration-after-statement>
1127 The following flags would be nice to have but they would first need
1128 their own Augean stablemaster:
1142 C<-Wstrict-prototypes>
1146 The C<-Wtraditional> is another example of the annoying tendency of gcc
1147 to bundle a lot of warnings under one switch (it would be impossible to
1148 deploy in practice because it would complain a lot) but it does contain
1149 some warnings that would be beneficial to have available on their own,
1150 such as the warning about string constants inside macros containing the
1151 macro arguments: this behaved differently pre-ANSI than it does in
1152 ANSI, and some C compilers are still in transition, AIX being an
1155 =head2 Warnings of other C compilers
1157 Other C compilers (yes, there B<are> other C compilers than gcc) often
1158 have their "strict ANSI" or "strict ANSI with some portability
1159 extensions" modes on, like for example the Sun Workshop has its C<-Xa>
1160 mode on (though implicitly), or the DEC (these days, HP...) has its
1163 =head1 MEMORY DEBUGGERS
1165 B<NOTE 1>: Running under older memory debuggers such as Purify,
1166 valgrind or Third Degree greatly slows down the execution: seconds
1167 become minutes, minutes become hours. For example as of Perl 5.8.1, the
1168 ext/Encode/t/Unicode.t takes extraordinarily long to complete under
1169 e.g. Purify, Third Degree, and valgrind. Under valgrind it takes more
1170 than six hours, even on a snappy computer. The said test must be doing
1171 something that is quite unfriendly for memory debuggers. If you don't
1172 feel like waiting, that you can simply kill away the perl process.
1173 Roughly valgrind slows down execution by factor 10, AddressSanitizer by
1176 B<NOTE 2>: To minimize the number of memory leak false alarms (see
1177 L</PERL_DESTRUCT_LEVEL> for more information), you have to set the
1178 environment variable PERL_DESTRUCT_LEVEL to 2. For example, like this:
1180 env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib ...
1182 B<NOTE 3>: There are known memory leaks when there are compile-time
1183 errors within eval or require, seeing C<S_doeval> in the call stack is
1184 a good sign of these. Fixing these leaks is non-trivial, unfortunately,
1185 but they must be fixed eventually.
1187 B<NOTE 4>: L<DynaLoader> will not clean up after itself completely
1188 unless Perl is built with the Configure option
1189 C<-Accflags=-DDL_UNLOAD_ALL_AT_EXIT>.
1193 The valgrind tool can be used to find out both memory leaks and illegal
1194 heap memory accesses. As of version 3.3.0, Valgrind only supports Linux
1195 on x86, x86-64 and PowerPC and Darwin (OS X) on x86 and x86-64. The
1196 special "test.valgrind" target can be used to run the tests under
1197 valgrind. Found errors and memory leaks are logged in files named
1198 F<testfile.valgrind> and by default output is displayed inline.
1204 Since valgrind adds significant overhead, tests will take much longer to
1205 run. The valgrind tests support being run in parallel to help with this:
1207 TEST_JOBS=9 make test.valgrind
1209 Note that the above two invocations will be very verbose as reachable
1210 memory and leak-checking is enabled by default. If you want to just see
1213 VG_OPTS='-q --leak-check=no --show-reachable=no' TEST_JOBS=9 \
1216 Valgrind also provides a cachegrind tool, invoked on perl as:
1218 VG_OPTS=--tool=cachegrind make test.valgrind
1220 As system libraries (most notably glibc) are also triggering errors,
1221 valgrind allows to suppress such errors using suppression files. The
1222 default suppression file that comes with valgrind already catches a lot
1223 of them. Some additional suppressions are defined in F<t/perl.supp>.
1225 To get valgrind and for more information see
1227 http://valgrind.org/
1229 =head2 AddressSanitizer
1231 AddressSanitizer is a clang and gcc extension, included in clang since
1232 v3.1 and gcc since v4.8. It checks illegal heap pointers, global
1233 pointers, stack pointers and use after free errors, and is fast enough
1234 that you can easily compile your debugging or optimized perl with it.
1235 It does not check memory leaks though. AddressSanitizer is available
1236 for Linux, Mac OS X and soon on Windows.
1238 To build perl with AddressSanitizer, your Configure invocation should
1241 sh Configure -des -Dcc=clang \
1242 -Accflags=-faddress-sanitizer -Aldflags=-faddress-sanitizer \
1243 -Alddlflags=-shared\ -faddress-sanitizer
1245 where these arguments mean:
1251 This should be replaced by the full path to your clang executable if it
1252 is not in your path.
1254 =item * -Accflags=-faddress-sanitizer
1256 Compile perl and extensions sources with AddressSanitizer.
1258 =item * -Aldflags=-faddress-sanitizer
1260 Link the perl executable with AddressSanitizer.
1262 =item * -Alddlflags=-shared\ -faddress-sanitizer
1264 Link dynamic extensions with AddressSanitizer. You must manually
1265 specify C<-shared> because using C<-Alddlflags=-shared> will prevent
1266 Configure from setting a default value for C<lddlflags>, which usually
1267 contains C<-shared> (at least on Linux).
1272 L<http://code.google.com/p/address-sanitizer/wiki/AddressSanitizer>.
1277 Depending on your platform there are various ways of profiling Perl.
1279 There are two commonly used techniques of profiling executables:
1280 I<statistical time-sampling> and I<basic-block counting>.
1282 The first method takes periodically samples of the CPU program counter,
1283 and since the program counter can be correlated with the code generated
1284 for functions, we get a statistical view of in which functions the
1285 program is spending its time. The caveats are that very small/fast
1286 functions have lower probability of showing up in the profile, and that
1287 periodically interrupting the program (this is usually done rather
1288 frequently, in the scale of milliseconds) imposes an additional
1289 overhead that may skew the results. The first problem can be alleviated
1290 by running the code for longer (in general this is a good idea for
1291 profiling), the second problem is usually kept in guard by the
1292 profiling tools themselves.
1294 The second method divides up the generated code into I<basic blocks>.
1295 Basic blocks are sections of code that are entered only in the
1296 beginning and exited only at the end. For example, a conditional jump
1297 starts a basic block. Basic block profiling usually works by
1298 I<instrumenting> the code by adding I<enter basic block #nnnn>
1299 book-keeping code to the generated code. During the execution of the
1300 code the basic block counters are then updated appropriately. The
1301 caveat is that the added extra code can skew the results: again, the
1302 profiling tools usually try to factor their own effects out of the
1305 =head2 Gprof Profiling
1307 I<gprof> is a profiling tool available in many Unix platforms which
1308 uses I<statistical time-sampling>. You can build a profiled version of
1309 F<perl> by compiling using gcc with the flag C<-pg>. Either edit
1310 F<config.sh> or re-run F<Configure>. Running the profiled version of
1311 Perl will create an output file called F<gmon.out> which contains the
1312 profiling data collected during the execution.
1316 $ sh Configure -des -Dusedevel -Accflags='-pg' \
1317 -Aldflags='-pg' -Alddlflags='-pg -shared' \
1319 $ ./perl ... # creates gmon.out in current directory
1320 $ gprof ./perl > out
1323 (you probably need to add C<-shared> to the <-Alddlflags> line until RT
1324 #118199 is resolved)
1326 The F<gprof> tool can then display the collected data in various ways.
1327 Usually F<gprof> understands the following options:
1333 Suppress statically defined functions from the profile.
1337 Suppress the verbose descriptions in the profile.
1341 Exclude the given routine and its descendants from the profile.
1345 Display only the given routine and its descendants in the profile.
1349 Generate a summary file called F<gmon.sum> which then may be given to
1350 subsequent gprof runs to accumulate data over several runs.
1354 Display routines that have zero usage.
1358 For more detailed explanation of the available commands and output
1359 formats, see your own local documentation of F<gprof>.
1361 =head2 GCC gcov Profiling
1363 I<basic block profiling> is officially available in gcc 3.0 and later.
1364 You can build a profiled version of F<perl> by compiling using gcc with
1365 the flags C<-fprofile-arcs -ftest-coverage>. Either edit F<config.sh>
1366 or re-run F<Configure>.
1370 $ sh Configure -des -Dusedevel -Doptimize='-g' \
1371 -Accflags='-fprofile-arcs -ftest-coverage' \
1372 -Aldflags='-fprofile-arcs -ftest-coverage' \
1373 -Alddlflags='-fprofile-arcs -ftest-coverage -shared' \
1375 $ rm -f regexec.c.gcov regexec.gcda
1378 $ less regexec.c.gcov
1380 (you probably need to add C<-shared> to the <-Alddlflags> line until RT
1381 #118199 is resolved)
1383 Running the profiled version of Perl will cause profile output to be
1384 generated. For each source file an accompanying F<.gcda> file will be
1387 To display the results you use the I<gcov> utility (which should be
1388 installed if you have gcc 3.0 or newer installed). F<gcov> is run on
1389 source code files, like this
1393 which will cause F<sv.c.gcov> to be created. The F<.gcov> files contain
1394 the source code annotated with relative frequencies of execution
1395 indicated by "#" markers. If you want to generate F<.gcov> files for
1396 all profiled object files, you can run something like this:
1398 for file in `find . -name \*.gcno`
1399 do sh -c "cd `dirname $file` && gcov `basename $file .gcno`"
1402 Useful options of F<gcov> include C<-b> which will summarise the basic
1403 block, branch, and function call coverage, and C<-c> which instead of
1404 relative frequencies will use the actual counts. For more information
1405 on the use of F<gcov> and basic block profiling with gcc, see the
1406 latest GNU CC manual. As of gcc 4.8, this is at
1407 L<http://gcc.gnu.org/onlinedocs/gcc/Gcov-Intro.html#Gcov-Intro>
1409 =head1 MISCELLANEOUS TRICKS
1411 =head2 PERL_DESTRUCT_LEVEL
1413 If you want to run any of the tests yourself manually using e.g.
1414 valgrind, please note that by default perl B<does not> explicitly
1415 cleanup all the memory it has allocated (such as global memory arenas)
1416 but instead lets the exit() of the whole program "take care" of such
1417 allocations, also known as "global destruction of objects".
1419 There is a way to tell perl to do complete cleanup: set the environment
1420 variable PERL_DESTRUCT_LEVEL to a non-zero value. The t/TEST wrapper
1421 does set this to 2, and this is what you need to do too, if you don't
1422 want to see the "global leaks": For example, for running under valgrind
1424 env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib t/foo/bar.t
1426 (Note: the mod_perl apache module uses also this environment variable
1427 for its own purposes and extended its semantics. Refer to the mod_perl
1428 documentation for more information. Also, spawned threads do the
1429 equivalent of setting this variable to the value 1.)
1431 If, at the end of a run you get the message I<N scalars leaked>, you
1432 can recompile with C<-DDEBUG_LEAKING_SCALARS>,
1433 (C<Configure -Accflags=-DDEBUG_LEAKING_SCALARS>), which will cause the
1434 addresses of all those leaked SVs to be dumped along with details as to
1435 where each SV was originally allocated. This information is also
1436 displayed by Devel::Peek. Note that the extra details recorded with
1437 each SV increases memory usage, so it shouldn't be used in production
1438 environments. It also converts C<new_SV()> from a macro into a real
1439 function, so you can use your favourite debugger to discover where
1440 those pesky SVs were allocated.
1442 If you see that you're leaking memory at runtime, but neither valgrind
1443 nor C<-DDEBUG_LEAKING_SCALARS> will find anything, you're probably
1444 leaking SVs that are still reachable and will be properly cleaned up
1445 during destruction of the interpreter. In such cases, using the C<-Dm>
1446 switch can point you to the source of the leak. If the executable was
1447 built with C<-DDEBUG_LEAKING_SCALARS>, C<-Dm> will output SV
1448 allocations in addition to memory allocations. Each SV allocation has a
1449 distinct serial number that will be written on creation and destruction
1450 of the SV. So if you're executing the leaking code in a loop, you need
1451 to look for SVs that are created, but never destroyed between each
1452 cycle. If such an SV is found, set a conditional breakpoint within
1453 C<new_SV()> and make it break only when C<PL_sv_serial> is equal to the
1454 serial number of the leaking SV. Then you will catch the interpreter in
1455 exactly the state where the leaking SV is allocated, which is
1456 sufficient in many cases to find the source of the leak.
1458 As C<-Dm> is using the PerlIO layer for output, it will by itself
1459 allocate quite a bunch of SVs, which are hidden to avoid recursion. You
1460 can bypass the PerlIO layer if you use the SV logging provided by
1461 C<-DPERL_MEM_LOG> instead.
1465 If compiled with C<-DPERL_MEM_LOG> (C<-Accflags=-DPERL_MEM_LOG>), both
1466 memory and SV allocations go through logging functions, which is
1467 handy for breakpoint setting.
1469 Unless C<-DPERL_MEM_LOG_NOIMPL> (C<-Accflags=-DPERL_MEM_LOG_NOIMPL>) is
1470 also compiled, the logging functions read $ENV{PERL_MEM_LOG} to
1471 determine whether to log the event, and if so how:
1473 $ENV{PERL_MEM_LOG} =~ /m/ Log all memory ops
1474 $ENV{PERL_MEM_LOG} =~ /s/ Log all SV ops
1475 $ENV{PERL_MEM_LOG} =~ /t/ include timestamp in Log
1476 $ENV{PERL_MEM_LOG} =~ /^(\d+)/ write to FD given (default is 2)
1478 Memory logging is somewhat similar to C<-Dm> but is independent of
1479 C<-DDEBUGGING>, and at a higher level; all uses of Newx(), Renew(), and
1480 Safefree() are logged with the caller's source code file and line
1481 number (and C function name, if supported by the C compiler). In
1482 contrast, C<-Dm> is directly at the point of C<malloc()>. SV logging is
1485 Since the logging doesn't use PerlIO, all SV allocations are logged and
1486 no extra SV allocations are introduced by enabling the logging. If
1487 compiled with C<-DDEBUG_LEAKING_SCALARS>, the serial number for each SV
1488 allocation is also logged.
1492 Those debugging perl with the DDD frontend over gdb may find the
1495 You can extend the data conversion shortcuts menu, so for example you
1496 can display an SV's IV value with one click, without doing any typing.
1497 To do that simply edit ~/.ddd/init file and add after:
1499 ! Display shortcuts.
1500 Ddd*gdbDisplayShortcuts: \
1501 /t () // Convert to Bin\n\
1502 /d () // Convert to Dec\n\
1503 /x () // Convert to Hex\n\
1504 /o () // Convert to Oct(\n\
1506 the following two lines:
1508 ((XPV*) (())->sv_any )->xpv_pv // 2pvx\n\
1509 ((XPVIV*) (())->sv_any )->xiv_iv // 2ivx
1511 so now you can do ivx and pvx lookups or you can plug there the sv_peek
1514 Perl_sv_peek(my_perl, (SV*)()) // sv_peek
1516 (The my_perl is for threaded builds.) Just remember that every line,
1517 but the last one, should end with \n\
1519 Alternatively edit the init file interactively via: 3rd mouse button ->
1520 New Display -> Edit Menu
1522 Note: you can define up to 20 conversion shortcuts in the gdb section.
1526 On some platforms Perl supports retrieving the C level backtrace
1527 (similar to what symbolic debuggers like gdb do).
1529 The backtrace returns the stack trace of the C call frames,
1530 with the symbol names (function names), the object names (like "perl"),
1531 and if it can, also the source code locations (file:line).
1533 The supported platforms are Linux, and OS X (some *BSD might
1534 work at least partly, but they have not yet been tested).
1536 This feature hasn't been tested with multiple threads, but it will
1537 only show the backtrace of the thread doing the backtracing.
1539 The feature needs to be enabled with C<Configure -Dusecbacktrace>.
1541 The C<-Dusecbacktrace> also enables keeping the debug information when
1542 compiling/linking (often: C<-g>). Many compilers/linkers do support
1543 having both optimization and keeping the debug information. The debug
1544 information is needed for the symbol names and the source locations.
1546 Static functions might not be visible for the backtrace.
1548 Source code locations, even if available, can often be missing or
1549 misleading if the compiler has e.g. inlined code. Optimizer can
1550 make matching the source code and the object code quite challenging.
1556 You B<must> have the BFD (-lbfd) library installed, otherwise C<perl> will
1557 fail to link. The BFD is usually distributed as part of the GNU binutils.
1559 Summary: C<Configure ... -Dusecbacktrace>
1560 and you need C<-lbfd>.
1564 The source code locations are supported B<only> if you have
1565 the Developer Tools installed. (BFD is B<not> needed.)
1567 Summary: C<Configure ... -Dusecbacktrace>
1568 and installing the Developer Tools would be good.
1572 Optionally, for trying out the feature, you may want to enable
1573 automatic dumping of the backtrace just before a warning or croak (die)
1574 message is emitted, by adding C<-Accflags=-DUSE_C_BACKTRACE_ON_ERROR>
1577 Unless the above additional feature is enabled, nothing about the
1578 backtrace functionality is visible, except for the Perl/XS level.
1580 Furthermore, even if you have enabled this feature to be compiled,
1581 you need to enable it in runtime with an environment variable:
1582 C<PERL_C_BACKTRACE_ON_ERROR=10>. It must be an integer higher
1583 than zero, telling the desired frame count.
1585 Retrieving the backtrace from Perl level (using for example an XS
1586 extension) would be much less exciting than one would hope: normally
1587 you would see C<runops>, C<entersub>, and not much else. This API is
1588 intended to be called B<from within> the Perl implementation, not from
1589 Perl level execution.
1591 The C API for the backtrace is as follows:
1595 =item get_c_backtrace
1597 =item free_c_backtrace
1599 =item get_c_backtrace_dump
1601 =item dump_c_backtrace
1607 If you see in a debugger a memory area mysteriously full of 0xABABABAB
1608 or 0xEFEFEFEF, you may be seeing the effect of the Poison() macros, see
1611 =head2 Read-only optrees
1613 Under ithreads the optree is read only. If you want to enforce this, to
1614 check for write accesses from buggy code, compile with
1615 C<-Accflags=-DPERL_DEBUG_READONLY_OPS>
1616 to enable code that allocates op memory
1617 via C<mmap>, and sets it read-only when it is attached to a subroutine.
1618 Any write access to an op results in a C<SIGBUS> and abort.
1620 This code is intended for development only, and may not be portable
1621 even to all Unix variants. Also, it is an 80% solution, in that it
1622 isn't able to make all ops read only. Specifically it does not apply to
1623 op slabs belonging to C<BEGIN> blocks.
1625 However, as an 80% solution it is still effective, as it has caught
1628 =head2 When is a bool not a bool?
1630 On pre-C99 compilers, C<bool> is defined as equivalent to C<char>.
1631 Consequently assignment of any larger type to a C<bool> is unsafe and may be
1632 truncated. The C<cBOOL> macro exists to cast it correctly; you may also find
1633 that using it is shorter and clearer than writing out the equivalent
1634 conditional expression longhand.
1636 On those platforms and compilers where C<bool> really is a boolean (C++,
1637 C99), it is easy to forget the cast. You can force C<bool> to be a C<char>
1638 by compiling with C<-Accflags=-DPERL_BOOL_AS_CHAR>. You may also wish to
1639 run C<Configure> with something like
1641 -Accflags='-Wconversion -Wno-sign-conversion -Wno-shorten-64-to-32'
1643 or your compiler's equivalent to make it easier to spot any unsafe truncations
1646 The C<TRUE> and C<FALSE> macros are available for situations where using them
1647 would clarify intent. (But they always just mean the same as the integers 1 and
1648 0 regardless, so using them isn't compulsory.)
1650 =head2 The .i Targets
1652 You can expand the macros in a F<foo.c> file by saying
1656 which will expand the macros using cpp. Don't be scared by the
1661 This document was originally written by Nathan Torkington, and is
1662 maintained by the perl5-porters mailing list.