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<-Wunitialized>) 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, 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 data 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 = *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 'A', but not 0x41. You can
279 safely say '\n', but not \012. If a character doesn't have a trivial
280 input form, you should add it to the list in
281 F<regen/unicode_constants.pl>, and have Perl create #defines for you,
282 based on the current platform.
284 Also, the range 'A' - 'Z' in ASCII is an unbroken sequence of 26 upper
285 case alphabetic characters. That is not true in EBCDIC. Nor for 'a' to
286 'z'. But '0' - '9' is an unbroken range in both systems. Don't assume
287 anything about other ranges.
289 Many of the comments in the existing code ignore the possibility of
290 EBCDIC, and may be wrong therefore, even if the code works. This is
291 actually a tribute to the successful transparent insertion of being
292 able to handle EBCDIC without having to change pre-existing code.
294 UTF-8 and UTF-EBCDIC are two different encodings used to represent
295 Unicode code points as sequences of bytes. Macros with the same names
296 (but different definitions) in C<utf8.h> and C<utfebcdic.h> are used to
297 allow the calling code to think that there is only one such encoding.
298 This is almost always referred to as C<utf8>, but it means the EBCDIC
299 version as well. Again, comments in the code may well be wrong even if
300 the code itself is right. For example, the concept of C<invariant
301 characters> differs between ASCII and EBCDIC. On ASCII platforms, only
302 characters that do not have the high-order bit set (i.e. whose ordinals
303 are strict ASCII, 0 - 127) are invariant, and the documentation and
304 comments in the code may assume that, often referring to something
305 like, say, C<hibit>. The situation differs and is not so simple on
306 EBCDIC machines, but as long as the code itself uses the
307 C<NATIVE_IS_INVARIANT()> macro appropriately, it works, even if the
312 Assuming the character set is just ASCII
314 ASCII is a 7 bit encoding, but bytes have 8 bits in them. The 128 extra
315 characters have different meanings depending on the locale. Absent a
316 locale, currently these extra characters are generally considered to be
317 unassigned, and this has presented some problems. This is being changed
318 starting in 5.12 so that these characters will be considered to be
319 Latin-1 (ISO-8859-1).
323 Mixing #define and #ifdef
325 #define BURGLE(x) ... \
326 #ifdef BURGLE_OLD_STYLE /* BAD */
327 ... do it the old way ... \
329 ... do it the new way ... \
332 You cannot portably "stack" cpp directives. For example in the above
333 you need two separate BURGLE() #defines, one for each #ifdef branch.
337 Adding non-comment stuff after #endif or #else
341 #else !SNOSH /* BAD */
343 #endif SNOSH /* BAD */
345 The #endif and #else cannot portably have anything non-comment after
346 them. If you want to document what is going (which is a good idea
347 especially if the branches are long), use (C) comments:
355 The gcc option C<-Wendif-labels> warns about the bad variant (by
356 default on starting from Perl 5.9.4).
360 Having a comma after the last element of an enum list
368 is not portable. Leave out the last comma.
370 Also note that whether enums are implicitly morphable to ints varies
371 between compilers, you might need to (int).
377 // This function bamfoodles the zorklator. /* BAD */
379 That is C99 or C++. Perl is C89. Using the //-comments is silently
380 allowed by many C compilers but cranking up the ANSI C89 strictness
381 (which we like to do) causes the compilation to fail.
385 Mixing declarations and code
390 set_zorkmids(n); /* BAD */
393 That is C99 or C++. Some C compilers allow that, but you shouldn't.
395 The gcc option C<-Wdeclaration-after-statements> scans for such
396 problems (by default on starting from Perl 5.9.4).
400 Introducing variables inside for()
402 for(int i = ...; ...; ...) { /* BAD */
404 That is C99 or C++. While it would indeed be awfully nice to have that
405 also in C89, to limit the scope of the loop variable, alas, we cannot.
409 Mixing signed char pointers with unsigned char pointers
411 int foo(char *s) { ... }
413 unsigned char *t = ...; /* Or U8* t = ... */
416 While this is legal practice, it is certainly dubious, and downright
417 fatal in at least one platform: for example VMS cc considers this a
418 fatal error. One cause for people often making this mistake is that a
419 "naked char" and therefore dereferencing a "naked char pointer" have an
420 undefined signedness: it depends on the compiler and the flags of the
421 compiler and the underlying platform whether the result is signed or
422 unsigned. For this very same reason using a 'char' as an array index is
427 Macros that have string constants and their arguments as substrings of
430 #define FOO(n) printf("number = %d\n", n) /* BAD */
433 Pre-ANSI semantics for that was equivalent to
435 printf("10umber = %d\10");
437 which is probably not what you were expecting. Unfortunately at least
438 one reasonably common and modern C compiler does "real backward
439 compatibility" here, in AIX that is what still happens even though the
440 rest of the AIX compiler is very happily C89.
444 Using printf formats for non-basic C types
447 printf("i = %d\n", i); /* BAD */
449 While this might by accident work in some platform (where IV happens to
450 be an C<int>), in general it cannot. IV might be something larger. Even
451 worse the situation is with more specific types (defined by Perl's
452 configuration step in F<config.h>):
455 printf("who = %d\n", who); /* BAD */
457 The problem here is that Uid_t might be not only not C<int>-wide but it
458 might also be unsigned, in which case large uids would be printed as
461 There is no simple solution to this because of printf()'s limited
462 intelligence, but for many types the right format is available as with
463 either 'f' or '_f' suffix, for example:
465 IVdf /* IV in decimal */
466 UVxf /* UV is hexadecimal */
468 printf("i = %"IVdf"\n", i); /* The IVdf is a string constant. */
470 Uid_t_f /* Uid_t in decimal */
472 printf("who = %"Uid_t_f"\n", who);
474 Or you can try casting to a "wide enough" type:
476 printf("i = %"IVdf"\n", (IV)something_very_small_and_signed);
478 Also remember that the C<%p> format really does require a void pointer:
481 printf("p = %p\n", (void*)p);
483 The gcc option C<-Wformat> scans for such problems.
487 Blindly using variadic macros
489 gcc has had them for a while with its own syntax, and C99 brought them
490 with a standardized syntax. Don't use the former, and use the latter
491 only if the HAS_C99_VARIADIC_MACROS is defined.
495 Blindly passing va_list
497 Not all platforms support passing va_list to further varargs (stdarg)
498 functions. The right thing to do is to copy the va_list using the
499 Perl_va_copy() if the NEED_VA_COPY is defined.
503 Using gcc statement expressions
505 val = ({...;...;...}); /* BAD */
507 While a nice extension, it's not portable. The Perl code does
508 admittedly use them if available to gain some extra speed (essentially
509 as a funky form of inlining), but you shouldn't.
513 Binding together several statements in a macro
515 Use the macros STMT_START and STMT_END.
523 Testing for operating systems or versions when should be testing for
526 #ifdef __FOONIX__ /* BAD */
530 Unless you know with 100% certainty that quux() is only ever available
531 for the "Foonix" operating system B<and> that is available B<and>
532 correctly working for B<all> past, present, B<and> future versions of
533 "Foonix", the above is very wrong. This is more correct (though still
534 not perfect, because the below is a compile-time check):
540 How does the HAS_QUUX become defined where it needs to be? Well, if
541 Foonix happens to be Unixy enough to be able to run the Configure
542 script, and Configure has been taught about detecting and testing
543 quux(), the HAS_QUUX will be correctly defined. In other platforms, the
544 corresponding configuration step will hopefully do the same.
546 In a pinch, if you cannot wait for Configure to be educated, or if you
547 have a good hunch of where quux() might be available, you can
548 temporarily try the following:
550 #if (defined(__FOONIX__) || defined(__BARNIX__))
560 But in any case, try to keep the features and operating systems
565 =head2 Problematic System Interfaces
571 malloc(0), realloc(0), calloc(0, 0) are non-portable. To be portable
572 allocate at least one byte. (In general you should rarely need to work
573 at this low level, but instead use the various malloc wrappers.)
577 snprintf() - the return type is unportable. Use my_snprintf() instead.
581 =head2 Security problems
583 Last but not least, here are various tips for safer coding.
591 Or we will publicly ridicule you. Seriously.
595 Do not use strcpy() or strcat() or strncpy() or strncat()
597 Use my_strlcpy() and my_strlcat() instead: they either use the native
598 implementation, or Perl's own implementation (borrowed from the public
599 domain implementation of INN).
603 Do not use sprintf() or vsprintf()
605 If you really want just plain byte strings, use my_snprintf() and
606 my_vsnprintf() instead, which will try to use snprintf() and
607 vsnprintf() if those safer APIs are available. If you want something
608 fancier than a plain byte string, use
609 L<C<Perl_form>()|perlapi/form> or SVs and
610 L<C<Perl_sv_catpvf()>|perlapi/sv_catpvf>.
612 Note that glibc C<printf()>, C<sprintf()>, etc. are buggy before glibc
613 version 2.17. They won't allow a C<%.s> format with a precision to
614 create a string that isn't valid UTF-8 if the current underlying locale
615 of the program is UTF-8. What happens is that the C<%s> and its operand are
616 simply skipped without any notice.
617 L<https://sourceware.org/bugzilla/show_bug.cgi?id=6530>.
623 You can compile a special debugging version of Perl, which allows you
624 to use the C<-D> option of Perl to tell more about what Perl is doing.
625 But sometimes there is no alternative than to dive in with a debugger,
626 either to see the stack trace of a core dump (very useful in a bug
627 report), or trying to figure out what went wrong before the core dump
628 happened, or how did we end up having wrong or unexpected results.
630 =head2 Poking at Perl
632 To really poke around with Perl, you'll probably want to build Perl for
633 debugging, like this:
635 ./Configure -d -D optimize=-g
638 C<-g> is a flag to the C compiler to have it produce debugging
639 information which will allow us to step through a running program, and
640 to see in which C function we are at (without the debugging information
641 we might see only the numerical addresses of the functions, which is
644 F<Configure> will also turn on the C<DEBUGGING> compilation symbol
645 which enables all the internal debugging code in Perl. There are a
646 whole bunch of things you can debug with this: L<perlrun> lists them
647 all, and the best way to find out about them is to play about with
648 them. The most useful options are probably
650 l Context (loop) stack processing
652 o Method and overloading resolution
653 c String/numeric conversions
655 Some of the functionality of the debugging code can be achieved using
658 -Dr => use re 'debug'
661 =head2 Using a source-level debugger
663 If the debugging output of C<-D> doesn't help you, it's time to step
664 through perl's execution with a source-level debugger.
670 We'll use C<gdb> for our examples here; the principles will apply to
671 any debugger (many vendors call their debugger C<dbx>), but check the
672 manual of the one you're using.
676 To fire up the debugger, type
680 Or if you have a core dump:
684 You'll want to do that in your Perl source tree so the debugger can
685 read the source code. You should see the copyright message, followed by
690 C<help> will get you into the documentation, but here are the most
697 Run the program with the given arguments.
699 =item * break function_name
701 =item * break source.c:xxx
703 Tells the debugger that we'll want to pause execution when we reach
704 either the named function (but see L<perlguts/Internal Functions>!) or
705 the given line in the named source file.
709 Steps through the program a line at a time.
713 Steps through the program a line at a time, without descending into
718 Run until the next breakpoint.
722 Run until the end of the current function, then stop again.
726 Just pressing Enter will do the most recent operation again - it's a
727 blessing when stepping through miles of source code.
731 Prints the C definition of the argument given.
737 OP *(*op_ppaddr)(void);
739 unsigned int op_type : 9;
740 unsigned int op_opt : 1;
741 unsigned int op_slabbed : 1;
742 unsigned int op_savefree : 1;
743 unsigned int op_static : 1;
744 unsigned int op_folded : 1;
745 unsigned int op_spare : 2;
752 Execute the given C code and print its results. B<WARNING>: Perl makes
753 heavy use of macros, and F<gdb> does not necessarily support macros
754 (see later L</"gdb macro support">). You'll have to substitute them
755 yourself, or to invoke cpp on the source code files (see L</"The .i
756 Targets">) So, for instance, you can't say
762 print Perl_sv_2pv_nolen(sv)
766 You may find it helpful to have a "macro dictionary", which you can
767 produce by saying C<cpp -dM perl.c | sort>. Even then, F<cpp> won't
768 recursively apply those macros for you.
770 =head2 gdb macro support
772 Recent versions of F<gdb> have fairly good macro support, but in order
773 to use it you'll need to compile perl with macro definitions included
774 in the debugging information. Using F<gcc> version 3.1, this means
775 configuring with C<-Doptimize=-g3>. Other compilers might use a
776 different switch (if they support debugging macros at all).
778 =head2 Dumping Perl Data Structures
780 One way to get around this macro hell is to use the dumping functions
781 in F<dump.c>; these work a little like an internal
782 L<Devel::Peek|Devel::Peek>, but they also cover OPs and other
783 structures that you can't get at from Perl. Let's take an example.
784 We'll use the C<$a = $b + $c> we used before, but give it a bit of
785 context: C<$b = "6XXXX"; $c = 2.3;>. Where's a good place to stop and
788 What about C<pp_add>, the function we examined earlier to implement the
791 (gdb) break Perl_pp_add
792 Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.
794 Notice we use C<Perl_pp_add> and not C<pp_add> - see
795 L<perlguts/Internal Functions>. With the breakpoint in place, we can
798 (gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c'
800 Lots of junk will go past as gdb reads in the relevant source files and
803 Breakpoint 1, Perl_pp_add () at pp_hot.c:309
804 309 dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
809 We looked at this bit of code before, and we said that
810 C<dPOPTOPnnrl_ul> arranges for two C<NV>s to be placed into C<left> and
811 C<right> - let's slightly expand it:
813 #define dPOPTOPnnrl_ul NV right = POPn; \
815 NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0
817 C<POPn> takes the SV from the top of the stack and obtains its NV
818 either directly (if C<SvNOK> is set) or by calling the C<sv_2nv>
819 function. C<TOPs> takes the next SV from the top of the stack - yes,
820 C<POPn> uses C<TOPs> - but doesn't remove it. We then use C<SvNV> to
821 get the NV from C<leftsv> in the same way as before - yes, C<POPn> uses
824 Since we don't have an NV for C<$b>, we'll have to use C<sv_2nv> to
825 convert it. If we step again, we'll find ourselves there:
828 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669
832 We can now use C<Perl_sv_dump> to investigate the SV:
834 (gdb) print Perl_sv_dump(sv)
835 SV = PV(0xa057cc0) at 0xa0675d0
838 PV = 0xa06a510 "6XXXX"\0
843 We know we're going to get C<6> from this, so let's finish the
847 Run till exit from #0 Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671
848 0x462669 in Perl_pp_add () at pp_hot.c:311
851 We can also dump out this op: the current op is always stored in
852 C<PL_op>, and we can dump it with C<Perl_op_dump>. This'll give us
853 similar output to L<B::Debug|B::Debug>.
855 (gdb) print Perl_op_dump(PL_op)
857 13 TYPE = add ===> 14
859 FLAGS = (SCALAR,KIDS)
861 TYPE = null ===> (12)
863 FLAGS = (SCALAR,KIDS)
865 11 TYPE = gvsv ===> 12
871 # finish this later #
873 =head2 Using gdb to look at specific parts of a program
875 With the example above, you knew to look for C<Perl_pp_add>, but what if
876 there were multiple calls to it all over the place, or you didn't know what
877 the op was you were looking for?
879 One way to do this is to inject a rare call somewhere near what you're looking
880 for. For example, you could add C<study> before your method:
886 (gdb) break Perl_pp_study
888 And then step until you hit what you're
889 looking for. This works well in a loop
890 if you want to only break at certain iterations:
896 =head2 Using gdb to look at what the parser/lexer are doing
898 If you want to see what perl is doing when parsing/lexing your code, you can
907 (gdb) break Perl_pp_study
909 If you want to see what the parser/lexer is doing inside of C<if> blocks and
910 the like you need to be a little trickier:
912 if ($a && $b && do { BEGIN { study } 1 } && $c) { ... }
914 =head1 SOURCE CODE STATIC ANALYSIS
916 Various tools exist for analysing C source code B<statically>, as
917 opposed to B<dynamically>, that is, without executing the code. It is
918 possible to detect resource leaks, undefined behaviour, type
919 mismatches, portability problems, code paths that would cause illegal
920 memory accesses, and other similar problems by just parsing the C code
921 and looking at the resulting graph, what does it tell about the
922 execution and data flows. As a matter of fact, this is exactly how C
923 compilers know to give warnings about dubious code.
927 The good old C code quality inspector, C<lint>, is available in several
928 platforms, but please be aware that there are several different
929 implementations of it by different vendors, which means that the flags
930 are not identical across different platforms.
932 There is a lint variant called C<splint> (Secure Programming Lint)
933 available from http://www.splint.org/ that should compile on any
936 There are C<lint> and <splint> targets in Makefile, but you may have to
937 diddle with the flags (see above).
941 Coverity (http://www.coverity.com/) is a product similar to lint and as
942 a testbed for their product they periodically check several open source
943 projects, and they give out accounts to open source developers to the
946 =head2 cpd (cut-and-paste detector)
948 The cpd tool detects cut-and-paste coding. If one instance of the
949 cut-and-pasted code changes, all the other spots should probably be
950 changed, too. Therefore such code should probably be turned into a
951 subroutine or a macro.
953 cpd (http://pmd.sourceforge.net/cpd.html) is part of the pmd project
954 (http://pmd.sourceforge.net/). pmd was originally written for static
955 analysis of Java code, but later the cpd part of it was extended to
956 parse also C and C++.
958 Download the pmd-bin-X.Y.zip () from the SourceForge site, extract the
959 pmd-X.Y.jar from it, and then run that on source code thusly:
961 java -cp pmd-X.Y.jar net.sourceforge.pmd.cpd.CPD \
962 --minimum-tokens 100 --files /some/where/src --language c > cpd.txt
964 You may run into memory limits, in which case you should use the -Xmx
971 Though much can be written about the inconsistency and coverage
972 problems of gcc warnings (like C<-Wall> not meaning "all the warnings",
973 or some common portability problems not being covered by C<-Wall>, or
974 C<-ansi> and C<-pedantic> both being a poorly defined collection of
975 warnings, and so forth), gcc is still a useful tool in keeping our
978 The C<-Wall> is by default on.
980 The C<-ansi> (and its sidekick, C<-pedantic>) would be nice to be on
981 always, but unfortunately they are not safe on all platforms, they can
982 for example cause fatal conflicts with the system headers (Solaris
983 being a prime example). If Configure C<-Dgccansipedantic> is used, the
984 C<cflags> frontend selects C<-ansi -pedantic> for the platforms where
985 they are known to be safe.
987 Starting from Perl 5.9.4 the following extra flags are added:
1001 C<-Wdeclaration-after-statement>
1005 The following flags would be nice to have but they would first need
1006 their own Augean stablemaster:
1020 C<-Wstrict-prototypes>
1024 The C<-Wtraditional> is another example of the annoying tendency of gcc
1025 to bundle a lot of warnings under one switch (it would be impossible to
1026 deploy in practice because it would complain a lot) but it does contain
1027 some warnings that would be beneficial to have available on their own,
1028 such as the warning about string constants inside macros containing the
1029 macro arguments: this behaved differently pre-ANSI than it does in
1030 ANSI, and some C compilers are still in transition, AIX being an
1033 =head2 Warnings of other C compilers
1035 Other C compilers (yes, there B<are> other C compilers than gcc) often
1036 have their "strict ANSI" or "strict ANSI with some portability
1037 extensions" modes on, like for example the Sun Workshop has its C<-Xa>
1038 mode on (though implicitly), or the DEC (these days, HP...) has its
1041 =head1 MEMORY DEBUGGERS
1043 B<NOTE 1>: Running under older memory debuggers such as Purify,
1044 valgrind or Third Degree greatly slows down the execution: seconds
1045 become minutes, minutes become hours. For example as of Perl 5.8.1, the
1046 ext/Encode/t/Unicode.t takes extraordinarily long to complete under
1047 e.g. Purify, Third Degree, and valgrind. Under valgrind it takes more
1048 than six hours, even on a snappy computer. The said test must be doing
1049 something that is quite unfriendly for memory debuggers. If you don't
1050 feel like waiting, that you can simply kill away the perl process.
1051 Roughly valgrind slows down execution by factor 10, AddressSanitizer by
1054 B<NOTE 2>: To minimize the number of memory leak false alarms (see
1055 L</PERL_DESTRUCT_LEVEL> for more information), you have to set the
1056 environment variable PERL_DESTRUCT_LEVEL to 2. For example, like this:
1058 env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib ...
1060 B<NOTE 3>: There are known memory leaks when there are compile-time
1061 errors within eval or require, seeing C<S_doeval> in the call stack is
1062 a good sign of these. Fixing these leaks is non-trivial, unfortunately,
1063 but they must be fixed eventually.
1065 B<NOTE 4>: L<DynaLoader> will not clean up after itself completely
1066 unless Perl is built with the Configure option
1067 C<-Accflags=-DDL_UNLOAD_ALL_AT_EXIT>.
1071 The valgrind tool can be used to find out both memory leaks and illegal
1072 heap memory accesses. As of version 3.3.0, Valgrind only supports Linux
1073 on x86, x86-64 and PowerPC and Darwin (OS X) on x86 and x86-64). The
1074 special "test.valgrind" target can be used to run the tests under
1075 valgrind. Found errors and memory leaks are logged in files named
1076 F<testfile.valgrind>.
1078 Valgrind also provides a cachegrind tool, invoked on perl as:
1080 VG_OPTS=--tool=cachegrind make test.valgrind
1082 As system libraries (most notably glibc) are also triggering errors,
1083 valgrind allows to suppress such errors using suppression files. The
1084 default suppression file that comes with valgrind already catches a lot
1085 of them. Some additional suppressions are defined in F<t/perl.supp>.
1087 To get valgrind and for more information see
1089 http://valgrind.org/
1091 =head2 AddressSanitizer
1093 AddressSanitizer is a clang and gcc extension, included in clang since
1094 v3.1 and gcc since v4.8. It checks illegal heap pointers, global
1095 pointers, stack pointers and use after free errors, and is fast enough
1096 that you can easily compile your debugging or optimized perl with it.
1097 It does not check memory leaks though. AddressSanitizer is available
1098 for Linux, Mac OS X and soon on Windows.
1100 To build perl with AddressSanitizer, your Configure invocation should
1103 sh Configure -des -Dcc=clang \
1104 -Accflags=-faddress-sanitizer -Aldflags=-faddress-sanitizer \
1105 -Alddlflags=-shared\ -faddress-sanitizer
1107 where these arguments mean:
1113 This should be replaced by the full path to your clang executable if it
1114 is not in your path.
1116 =item * -Accflags=-faddress-sanitizer
1118 Compile perl and extensions sources with AddressSanitizer.
1120 =item * -Aldflags=-faddress-sanitizer
1122 Link the perl executable with AddressSanitizer.
1124 =item * -Alddlflags=-shared\ -faddress-sanitizer
1126 Link dynamic extensions with AddressSanitizer. You must manually
1127 specify C<-shared> because using C<-Alddlflags=-shared> will prevent
1128 Configure from setting a default value for C<lddlflags>, which usually
1129 contains C<-shared> (at least on Linux).
1134 L<http://code.google.com/p/address-sanitizer/wiki/AddressSanitizer>.
1139 Depending on your platform there are various ways of profiling Perl.
1141 There are two commonly used techniques of profiling executables:
1142 I<statistical time-sampling> and I<basic-block counting>.
1144 The first method takes periodically samples of the CPU program counter,
1145 and since the program counter can be correlated with the code generated
1146 for functions, we get a statistical view of in which functions the
1147 program is spending its time. The caveats are that very small/fast
1148 functions have lower probability of showing up in the profile, and that
1149 periodically interrupting the program (this is usually done rather
1150 frequently, in the scale of milliseconds) imposes an additional
1151 overhead that may skew the results. The first problem can be alleviated
1152 by running the code for longer (in general this is a good idea for
1153 profiling), the second problem is usually kept in guard by the
1154 profiling tools themselves.
1156 The second method divides up the generated code into I<basic blocks>.
1157 Basic blocks are sections of code that are entered only in the
1158 beginning and exited only at the end. For example, a conditional jump
1159 starts a basic block. Basic block profiling usually works by
1160 I<instrumenting> the code by adding I<enter basic block #nnnn>
1161 book-keeping code to the generated code. During the execution of the
1162 code the basic block counters are then updated appropriately. The
1163 caveat is that the added extra code can skew the results: again, the
1164 profiling tools usually try to factor their own effects out of the
1167 =head2 Gprof Profiling
1169 I<gprof> is a profiling tool available in many Unix platforms which
1170 uses I<statistical time-sampling>. You can build a profiled version of
1171 F<perl> by compiling using gcc with the flag C<-pg>. Either edit
1172 F<config.sh> or re-run F<Configure>. Running the profiled version of
1173 Perl will create an output file called F<gmon.out> which contains the
1174 profiling data collected during the execution.
1178 $ sh Configure -des -Dusedevel -Accflags='-pg' \
1179 -Aldflags='-pg' -Alddlflags='-pg -shared' \
1181 $ ./perl ... # creates gmon.out in current directory
1182 $ gprof ./perl > out
1185 (you probably need to add C<-shared> to the <-Alddlflags> line until RT
1186 #118199 is resolved)
1188 The F<gprof> tool can then display the collected data in various ways.
1189 Usually F<gprof> understands the following options:
1195 Suppress statically defined functions from the profile.
1199 Suppress the verbose descriptions in the profile.
1203 Exclude the given routine and its descendants from the profile.
1207 Display only the given routine and its descendants in the profile.
1211 Generate a summary file called F<gmon.sum> which then may be given to
1212 subsequent gprof runs to accumulate data over several runs.
1216 Display routines that have zero usage.
1220 For more detailed explanation of the available commands and output
1221 formats, see your own local documentation of F<gprof>.
1223 =head2 GCC gcov Profiling
1225 I<basic block profiling> is officially available in gcc 3.0 and later.
1226 You can build a profiled version of F<perl> by compiling using gcc with
1227 the flags C<-fprofile-arcs -ftest-coverage>. Either edit F<config.sh>
1228 or re-run F<Configure>.
1232 $ sh Configure -des -Dusedevel -Doptimize='-g' \
1233 -Accflags='-fprofile-arcs -ftest-coverage' \
1234 -Aldflags='-fprofile-arcs -ftest-coverage' \
1235 -Alddlflags='-fprofile-arcs -ftest-coverage -shared' \
1237 $ rm -f regexec.c.gcov regexec.gcda
1240 $ less regexec.c.gcov
1242 (you probably need to add C<-shared> to the <-Alddlflags> line until RT
1243 #118199 is resolved)
1245 Running the profiled version of Perl will cause profile output to be
1246 generated. For each source file an accompanying F<.gcda> file will be
1249 To display the results you use the I<gcov> utility (which should be
1250 installed if you have gcc 3.0 or newer installed). F<gcov> is run on
1251 source code files, like this
1255 which will cause F<sv.c.gcov> to be created. The F<.gcov> files contain
1256 the source code annotated with relative frequencies of execution
1257 indicated by "#" markers. If you want to generate F<.gcov> files for
1258 all profiled object files, you can run something like this:
1260 for file in `find . -name \*.gcno`
1261 do sh -c "cd `dirname $file` && gcov `basename $file .gcno`"
1264 Useful options of F<gcov> include C<-b> which will summarise the basic
1265 block, branch, and function call coverage, and C<-c> which instead of
1266 relative frequencies will use the actual counts. For more information
1267 on the use of F<gcov> and basic block profiling with gcc, see the
1268 latest GNU CC manual. As of gcc 4.8, this is at
1269 L<http://gcc.gnu.org/onlinedocs/gcc/Gcov-Intro.html#Gcov-Intro>
1271 =head1 MISCELLANEOUS TRICKS
1273 =head2 PERL_DESTRUCT_LEVEL
1275 If you want to run any of the tests yourself manually using e.g.
1276 valgrind, please note that by default perl B<does not> explicitly
1277 cleanup all the memory it has allocated (such as global memory arenas)
1278 but instead lets the exit() of the whole program "take care" of such
1279 allocations, also known as "global destruction of objects".
1281 There is a way to tell perl to do complete cleanup: set the environment
1282 variable PERL_DESTRUCT_LEVEL to a non-zero value. The t/TEST wrapper
1283 does set this to 2, and this is what you need to do too, if you don't
1284 want to see the "global leaks": For example, for running under valgrind
1286 env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib t/foo/bar.t
1288 (Note: the mod_perl apache module uses also this environment variable
1289 for its own purposes and extended its semantics. Refer to the mod_perl
1290 documentation for more information. Also, spawned threads do the
1291 equivalent of setting this variable to the value 1.)
1293 If, at the end of a run you get the message I<N scalars leaked>, you
1294 can recompile with C<-DDEBUG_LEAKING_SCALARS>, which will cause the
1295 addresses of all those leaked SVs to be dumped along with details as to
1296 where each SV was originally allocated. This information is also
1297 displayed by Devel::Peek. Note that the extra details recorded with
1298 each SV increases memory usage, so it shouldn't be used in production
1299 environments. It also converts C<new_SV()> from a macro into a real
1300 function, so you can use your favourite debugger to discover where
1301 those pesky SVs were allocated.
1303 If you see that you're leaking memory at runtime, but neither valgrind
1304 nor C<-DDEBUG_LEAKING_SCALARS> will find anything, you're probably
1305 leaking SVs that are still reachable and will be properly cleaned up
1306 during destruction of the interpreter. In such cases, using the C<-Dm>
1307 switch can point you to the source of the leak. If the executable was
1308 built with C<-DDEBUG_LEAKING_SCALARS>, C<-Dm> will output SV
1309 allocations in addition to memory allocations. Each SV allocation has a
1310 distinct serial number that will be written on creation and destruction
1311 of the SV. So if you're executing the leaking code in a loop, you need
1312 to look for SVs that are created, but never destroyed between each
1313 cycle. If such an SV is found, set a conditional breakpoint within
1314 C<new_SV()> and make it break only when C<PL_sv_serial> is equal to the
1315 serial number of the leaking SV. Then you will catch the interpreter in
1316 exactly the state where the leaking SV is allocated, which is
1317 sufficient in many cases to find the source of the leak.
1319 As C<-Dm> is using the PerlIO layer for output, it will by itself
1320 allocate quite a bunch of SVs, which are hidden to avoid recursion. You
1321 can bypass the PerlIO layer if you use the SV logging provided by
1322 C<-DPERL_MEM_LOG> instead.
1326 If compiled with C<-DPERL_MEM_LOG>, both memory and SV allocations go
1327 through logging functions, which is handy for breakpoint setting.
1329 Unless C<-DPERL_MEM_LOG_NOIMPL> is also compiled, the logging functions
1330 read $ENV{PERL_MEM_LOG} to determine whether to log the event, and if
1333 $ENV{PERL_MEM_LOG} =~ /m/ Log all memory ops
1334 $ENV{PERL_MEM_LOG} =~ /s/ Log all SV ops
1335 $ENV{PERL_MEM_LOG} =~ /t/ include timestamp in Log
1336 $ENV{PERL_MEM_LOG} =~ /^(\d+)/ write to FD given (default is 2)
1338 Memory logging is somewhat similar to C<-Dm> but is independent of
1339 C<-DDEBUGGING>, and at a higher level; all uses of Newx(), Renew(), and
1340 Safefree() are logged with the caller's source code file and line
1341 number (and C function name, if supported by the C compiler). In
1342 contrast, C<-Dm> is directly at the point of C<malloc()>. SV logging is
1345 Since the logging doesn't use PerlIO, all SV allocations are logged and
1346 no extra SV allocations are introduced by enabling the logging. If
1347 compiled with C<-DDEBUG_LEAKING_SCALARS>, the serial number for each SV
1348 allocation is also logged.
1352 Those debugging perl with the DDD frontend over gdb may find the
1355 You can extend the data conversion shortcuts menu, so for example you
1356 can display an SV's IV value with one click, without doing any typing.
1357 To do that simply edit ~/.ddd/init file and add after:
1359 ! Display shortcuts.
1360 Ddd*gdbDisplayShortcuts: \
1361 /t () // Convert to Bin\n\
1362 /d () // Convert to Dec\n\
1363 /x () // Convert to Hex\n\
1364 /o () // Convert to Oct(\n\
1366 the following two lines:
1368 ((XPV*) (())->sv_any )->xpv_pv // 2pvx\n\
1369 ((XPVIV*) (())->sv_any )->xiv_iv // 2ivx
1371 so now you can do ivx and pvx lookups or you can plug there the sv_peek
1374 Perl_sv_peek(my_perl, (SV*)()) // sv_peek
1376 (The my_perl is for threaded builds.) Just remember that every line,
1377 but the last one, should end with \n\
1379 Alternatively edit the init file interactively via: 3rd mouse button ->
1380 New Display -> Edit Menu
1382 Note: you can define up to 20 conversion shortcuts in the gdb section.
1386 If you see in a debugger a memory area mysteriously full of 0xABABABAB
1387 or 0xEFEFEFEF, you may be seeing the effect of the Poison() macros, see
1390 =head2 Read-only optrees
1392 Under ithreads the optree is read only. If you want to enforce this, to
1393 check for write accesses from buggy code, compile with
1394 C<-Accflags=-DPERL_DEBUG_READONLY_OPS>
1395 to enable code that allocates op memory
1396 via C<mmap>, and sets it read-only when it is attached to a subroutine.
1397 Any write access to an op results in a C<SIGBUS> and abort.
1399 This code is intended for development only, and may not be portable
1400 even to all Unix variants. Also, it is an 80% solution, in that it
1401 isn't able to make all ops read only. Specifically it does not apply to
1402 op slabs belonging to C<BEGIN> blocks.
1404 However, as an 80% solution it is still effective, as it has caught
1407 =head2 When is a bool not a bool?
1409 On pre-C99 compilers, C<bool> is defined as equivalent to C<char>.
1410 Consequently assignment of any larger type to a C<bool> is unsafe and may
1411 be truncated. The C<cBOOL> macro exists to cast it correctly.
1413 On those platforms and compilers where C<bool> really is a boolean (C++,
1414 C99), it is easy to forget the cast. You can force C<bool> to be a C<char>
1415 by compiling with C<-Accflags=-DPERL_BOOL_AS_CHAR>. You may also wish to
1416 run C<Configure> with something like
1418 -Accflags='-Wconversion -Wno-sign-conversion -Wno-shorten-64-to-32'
1420 or your compiler's equivalent to make it easier to spot any unsafe truncations
1423 =head2 The .i Targets
1425 You can expand the macros in a F<foo.c> file by saying
1429 which will expand the macros using cpp. Don't be scared by the
1434 This document was originally written by Nathan Torkington, and is
1435 maintained by the perl5-porters mailing list.