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

NAME
       perlhacktips - Tips for Perl core C code hacking

DESCRIPTION
       This document will help you learn the best way to go about hacking on
       the Perl core C code. It covers common problems, debugging, profiling,
       and more.

       If you haven't read perlhack and perlhacktut yet, you might want to do
       that first.

COMMON PROBLEMS
       Perl source plays by ANSI C89 rules: no C99 (or C++) extensions. In
       some cases we have to take pre-ANSI requirements into consideration.
       You don't care about some particular platform having broken Perl? I
       hear there is still a strong demand for J2EE programmers.

   Perl environment problems
       ·   Not compiling with threading

	   Compiling with threading (-Duseithreads) completely rewrites the
	   function prototypes of Perl. You better try your changes with that.
	   Related to this is the difference between "Perl_-less" and
	   "Perl_-ly" APIs, for example:

	     Perl_sv_setiv(aTHX_ ...);
	     sv_setiv(...);

	   The first one explicitly passes in the context, which is needed for
	   e.g. threaded builds. The second one does that implicitly; do not
	   get them mixed. If you are not passing in a aTHX_, you will need to
	   do a dTHX (or a dVAR) as the first thing in the function.

	   See "How multiple interpreters and concurrency are supported" in
	   perlguts for further discussion about context.

       ·   Not compiling with -DDEBUGGING

	   The DEBUGGING define exposes more code to the compiler, therefore
	   more ways for things to go wrong. You should try it.

       ·   Introducing (non-read-only) globals

	   Do not introduce any modifiable globals, truly global or file
	   static.  They are bad form and complicate multithreading and other
	   forms of concurrency. The right way is to introduce them as new
	   interpreter variables, see intrpvar.h (at the very end for binary
	   compatibility).

	   Introducing read-only (const) globals is okay, as long as you
	   verify with e.g. "nm libperl.a|egrep -v ' [TURtr] '" (if your "nm"
	   has BSD-style output) that the data you added really is read-only.
	   (If it is, it shouldn't show up in the output of that command.)

	   If you want to have static strings, make them constant:

	     static const char etc[] = "...";

	   If you want to have arrays of constant strings, note carefully the
	   right combination of "const"s:

	       static const char * const yippee[] =
		   {"hi", "ho", "silver"};

	   There is a way to completely hide any modifiable globals (they are
	   all moved to heap), the compilation setting
	   "-DPERL_GLOBAL_STRUCT_PRIVATE". It is not normally used, but can be
	   used for testing, read more about it in "Background and
	   PERL_IMPLICIT_CONTEXT" in perlguts.

       ·   Not exporting your new function

	   Some platforms (Win32, AIX, VMS, OS/2, to name a few) require any
	   function that is part of the public API (the shared Perl library)
	   to be explicitly marked as exported. See the discussion about
	   embed.pl in perlguts.

       ·   Exporting your new function

	   The new shiny result of either genuine new functionality or your
	   arduous refactoring is now ready and correctly exported. So what
	   could possibly go wrong?

	   Maybe simply that your function did not need to be exported in the
	   first place. Perl has a long and not so glorious history of
	   exporting functions that it should not have.

	   If the function is used only inside one source code file, make it
	   static. See the discussion about embed.pl in perlguts.

	   If the function is used across several files, but intended only for
	   Perl's internal use (and this should be the common case), do not
	   export it to the public API. See the discussion about embed.pl in
	   perlguts.

   Portability problems
       The following are common causes of compilation and/or execution
       failures, not common to Perl as such. The C FAQ is good bedtime
       reading. Please test your changes with as many C compilers and
       platforms as possible; we will, anyway, and it's nice to save oneself
       from public embarrassment.

       If using gcc, you can add the "-std=c89" option which will hopefully
       catch most of these unportabilities. (However it might also catch
       incompatibilities in your system's header files.)

       Use the Configure "-Dgccansipedantic" flag to enable the gcc "-ansi
       -pedantic" flags which enforce stricter ANSI rules.

       If using the "gcc -Wall" note that not all the possible warnings (like
       "-Wunitialized") are given unless you also compile with "-O".

       Note that if using gcc, starting from Perl 5.9.5 the Perl core source
       code files (the ones at the top level of the source code distribution,
       but not e.g. the extensions under ext/) are automatically compiled with
       as many as possible of the "-std=c89", "-ansi", "-pedantic", and a
       selection of "-W" flags (see cflags.SH).

       Also study perlport carefully to avoid any bad assumptions about the
       operating system, filesystems, and so forth.

       You may once in a while try a "make microperl" to see whether we can
       still compile Perl with just the bare minimum of interfaces. (See
       README.micro.)

       Do not assume an operating system indicates a certain compiler.

       ·   Casting pointers to integers or casting integers to pointers

	       void castaway(U8* p)
	       {
		 IV i = p;

	   or

	       void castaway(U8* p)
	       {
		 IV i = (IV)p;

	   Both are bad, and broken, and unportable. Use the PTR2IV() macro
	   that does it right. (Likewise, there are PTR2UV(), PTR2NV(),
	   INT2PTR(), and NUM2PTR().)

       ·   Casting between data function pointers and data pointers

	   Technically speaking casting between function pointers and data
	   pointers is unportable and undefined, but practically speaking it
	   seems to work, but you should use the FPTR2DPTR() and DPTR2FPTR()
	   macros.  Sometimes you can also play games with unions.

       ·   Assuming sizeof(int) == sizeof(long)

	   There are platforms where longs are 64 bits, and platforms where
	   ints are 64 bits, and while we are out to shock you, even platforms
	   where shorts are 64 bits. This is all legal according to the C
	   standard. (In other words, "long long" is not a portable way to
	   specify 64 bits, and "long long" is not even guaranteed to be any
	   wider than "long".)

	   Instead, use the definitions IV, UV, IVSIZE, I32SIZE, and so forth.
	   Avoid things like I32 because they are not guaranteed to be exactly
	   32 bits, they are at least 32 bits, nor are they guaranteed to be
	   int or long. If you really explicitly need 64-bit variables, use
	   I64 and U64, but only if guarded by HAS_QUAD.

       ·   Assuming one can dereference any type of pointer for any type of
	   data

	     char *p = ...;
	     long pony = *p;	/* BAD */

	   Many platforms, quite rightly so, will give you a core dump instead
	   of a pony if the p happens not to be correctly aligned.

       ·   Lvalue casts

	     (int)*p = ...;    /* BAD */

	   Simply not portable. Get your lvalue to be of the right type, or
	   maybe use temporary variables, or dirty tricks with unions.

       ·   Assume anything about structs (especially the ones you don't
	   control, like the ones coming from the system headers)

	   ·	   That a certain field exists in a struct

	   ·	   That no other fields exist besides the ones you know of

	   ·	   That a field is of certain signedness, sizeof, or type

	   ·	   That the fields are in a certain order

		   ·	   While C guarantees the ordering specified in the
			   struct definition, between different platforms the
			   definitions might differ

	   ·	   That the sizeof(struct) or the alignments are the same
		   everywhere

		   ·	   There might be padding bytes between the fields to
			   align the fields - the bytes can be anything

		   ·	   Structs are required to be aligned to the maximum
			   alignment required by the fields - which for native
			   types is for usually equivalent to sizeof() of the
			   field

       ·   Assuming the character set is ASCIIish

	   Perl can compile and run under EBCDIC platforms. See perlebcdic.
	   This is transparent for the most part, but because the character
	   sets differ, you shouldn't use numeric (decimal, octal, nor hex)
	   constants to refer to characters. You can safely say 'A', but not
	   0x41. You can safely say '\n', but not \012. If a character doesn't
	   have a trivial input form, you should add it to the list in
	   regen/unicode_constants.pl, and have Perl create #defines for you,
	   based on the current platform.

	   Also, the range 'A' - 'Z' in ASCII is an unbroken sequence of 26
	   upper case alphabetic characters. That is not true in EBCDIC. Nor
	   for 'a' to 'z'. But '0' - '9' is an unbroken range in both systems.
	   Don't assume anything about other ranges.

	   Many of the comments in the existing code ignore the possibility of
	   EBCDIC, and may be wrong therefore, even if the code works. This is
	   actually a tribute to the successful transparent insertion of being
	   able to handle EBCDIC without having to change pre-existing code.

	   UTF-8 and UTF-EBCDIC are two different encodings used to represent
	   Unicode code points as sequences of bytes. Macros  with the same
	   names (but different definitions) in "utf8.h" and "utfebcdic.h" are
	   used to allow the calling code to think that there is only one such
	   encoding.  This is almost always referred to as "utf8", but it
	   means the EBCDIC version as well. Again, comments in the code may
	   well be wrong even if the code itself is right. For example, the
	   concept of "invariant characters" differs between ASCII and EBCDIC.
	   On ASCII platforms, only characters that do not have the high-order
	   bit set (i.e. whose ordinals are strict ASCII, 0 - 127) are
	   invariant, and the documentation and comments in the code may
	   assume that, often referring to something like, say, "hibit". The
	   situation differs and is not so simple on EBCDIC machines, but as
	   long as the code itself uses the "NATIVE_IS_INVARIANT()" macro
	   appropriately, it works, even if the comments are wrong.

       ·   Assuming the character set is just ASCII

	   ASCII is a 7 bit encoding, but bytes have 8 bits in them. The 128
	   extra characters have different meanings depending on the locale.
	   Absent a locale, currently these extra characters are generally
	   considered to be unassigned, and this has presented some problems.
	   This is being changed starting in 5.12 so that these characters
	   will be considered to be Latin-1 (ISO-8859-1).

       ·   Mixing #define and #ifdef

	     #define BURGLE(x) ... \
	     #ifdef BURGLE_OLD_STYLE	    /* BAD */
	     ... do it the old way ... \
	     #else
	     ... do it the new way ... \
	     #endif

	   You cannot portably "stack" cpp directives. For example in the
	   above you need two separate BURGLE() #defines, one for each #ifdef
	   branch.

       ·   Adding non-comment stuff after #endif or #else

	     #ifdef SNOSH
	     ...
	     #else !SNOSH    /* BAD */
	     ...
	     #endif SNOSH    /* BAD */

	   The #endif and #else cannot portably have anything non-comment
	   after them. If you want to document what is going (which is a good
	   idea especially if the branches are long), use (C) comments:

	     #ifdef SNOSH
	     ...
	     #else /* !SNOSH */
	     ...
	     #endif /* SNOSH */

	   The gcc option "-Wendif-labels" warns about the bad variant (by
	   default on starting from Perl 5.9.4).

       ·   Having a comma after the last element of an enum list

	     enum color {
	       CERULEAN,
	       CHARTREUSE,
	       CINNABAR,     /* BAD */
	     };

	   is not portable. Leave out the last comma.

	   Also note that whether enums are implicitly morphable to ints
	   varies between compilers, you might need to (int).

       ·   Using //-comments

	     // This function bamfoodles the zorklator.	  /* BAD */

	   That is C99 or C++. Perl is C89. Using the //-comments is silently
	   allowed by many C compilers but cranking up the ANSI C89 strictness
	   (which we like to do) causes the compilation to fail.

       ·   Mixing declarations and code

	     void zorklator()
	     {
	       int n = 3;
	       set_zorkmids(n);	   /* BAD */
	       int q = 4;

	   That is C99 or C++. Some C compilers allow that, but you shouldn't.

	   The gcc option "-Wdeclaration-after-statements" scans for such
	   problems (by default on starting from Perl 5.9.4).

       ·   Introducing variables inside for()

	     for(int i = ...; ...; ...) {    /* BAD */

	   That is C99 or C++. While it would indeed be awfully nice to have
	   that also in C89, to limit the scope of the loop variable, alas, we
	   cannot.

       ·   Mixing signed char pointers with unsigned char pointers

	     int foo(char *s) { ... }
	     ...
	     unsigned char *t = ...; /* Or U8* t = ... */
	     foo(t);   /* BAD */

	   While this is legal practice, it is certainly dubious, and
	   downright fatal in at least one platform: for example VMS cc
	   considers this a fatal error. One cause for people often making
	   this mistake is that a "naked char" and therefore dereferencing a
	   "naked char pointer" have an undefined signedness: it depends on
	   the compiler and the flags of the compiler and the underlying
	   platform whether the result is signed or unsigned. For this very
	   same reason using a 'char' as an array index is bad.

       ·   Macros that have string constants and their arguments as substrings
	   of the string constants

	     #define FOO(n) printf("number = %d\n", n)	  /* BAD */
	     FOO(10);

	   Pre-ANSI semantics for that was equivalent to

	     printf("10umber = %d\10");

	   which is probably not what you were expecting. Unfortunately at
	   least one reasonably common and modern C compiler does "real
	   backward compatibility" here, in AIX that is what still happens
	   even though the rest of the AIX compiler is very happily C89.

       ·   Using printf formats for non-basic C types

	      IV i = ...;
	      printf("i = %d\n", i);	/* BAD */

	   While this might by accident work in some platform (where IV
	   happens to be an "int"), in general it cannot. IV might be
	   something larger. Even worse the situation is with more specific
	   types (defined by Perl's configuration step in config.h):

	      Uid_t who = ...;
	      printf("who = %d\n", who);    /* BAD */

	   The problem here is that Uid_t might be not only not "int"-wide but
	   it might also be unsigned, in which case large uids would be
	   printed as negative values.

	   There is no simple solution to this because of printf()'s limited
	   intelligence, but for many types the right format is available as
	   with either 'f' or '_f' suffix, for example:

	      IVdf /* IV in decimal */
	      UVxf /* UV is hexadecimal */

	      printf("i = %"IVdf"\n", i); /* The IVdf is a string constant. */

	      Uid_t_f /* Uid_t in decimal */

	      printf("who = %"Uid_t_f"\n", who);

	   Or you can try casting to a "wide enough" type:

	      printf("i = %"IVdf"\n", (IV)something_very_small_and_signed);

	   Also remember that the %p format really does require a void
	   pointer:

	      U8* p = ...;
	      printf("p = %p\n", (void*)p);

	   The gcc option "-Wformat" scans for such problems.

       ·   Blindly using variadic macros

	   gcc has had them for a while with its own syntax, and C99 brought
	   them with a standardized syntax. Don't use the former, and use the
	   latter only if the HAS_C99_VARIADIC_MACROS is defined.

       ·   Blindly passing va_list

	   Not all platforms support passing va_list to further varargs
	   (stdarg) functions. The right thing to do is to copy the va_list
	   using the Perl_va_copy() if the NEED_VA_COPY is defined.

       ·   Using gcc statement expressions

	      val = ({...;...;...});	/* BAD */

	   While a nice extension, it's not portable. The Perl code does
	   admittedly use them if available to gain some extra speed
	   (essentially as a funky form of inlining), but you shouldn't.

       ·   Binding together several statements in a macro

	   Use the macros STMT_START and STMT_END.

	      STMT_START {
		 ...
	      } STMT_END

       ·   Testing for operating systems or versions when should be testing
	   for features

	     #ifdef __FOONIX__	  /* BAD */
	     foo = quux();
	     #endif

	   Unless you know with 100% certainty that quux() is only ever
	   available for the "Foonix" operating system and that is available
	   and correctly working for all past, present, and future versions of
	   "Foonix", the above is very wrong. This is more correct (though
	   still not perfect, because the below is a compile-time check):

	     #ifdef HAS_QUUX
	     foo = quux();
	     #endif

	   How does the HAS_QUUX become defined where it needs to be?  Well,
	   if Foonix happens to be Unixy enough to be able to run the
	   Configure script, and Configure has been taught about detecting and
	   testing quux(), the HAS_QUUX will be correctly defined. In other
	   platforms, the corresponding configuration step will hopefully do
	   the same.

	   In a pinch, if you cannot wait for Configure to be educated, or if
	   you have a good hunch of where quux() might be available, you can
	   temporarily try the following:

	     #if (defined(__FOONIX__) || defined(__BARNIX__))
	     # define HAS_QUUX
	     #endif

	     ...

	     #ifdef HAS_QUUX
	     foo = quux();
	     #endif

	   But in any case, try to keep the features and operating systems
	   separate.

   Problematic System Interfaces
       ·   malloc(0), realloc(0), calloc(0, 0) are non-portable. To be
	   portable allocate at least one byte. (In general you should rarely
	   need to work at this low level, but instead use the various malloc
	   wrappers.)

       ·   snprintf() - the return type is unportable. Use my_snprintf()
	   instead.

   Security problems
       Last but not least, here are various tips for safer coding.

       ·   Do not use gets()

	   Or we will publicly ridicule you. Seriously.

       ·   Do not use strcpy() or strcat() or strncpy() or strncat()

	   Use my_strlcpy() and my_strlcat() instead: they either use the
	   native implementation, or Perl's own implementation (borrowed from
	   the public domain implementation of INN).

       ·   Do not use sprintf() or vsprintf()

	   If you really want just plain byte strings, use my_snprintf() and
	   my_vsnprintf() instead, which will try to use snprintf() and
	   vsnprintf() if those safer APIs are available. If you want
	   something fancier than a plain byte string, use SVs and
	   Perl_sv_catpvf().

DEBUGGING
       You can compile a special debugging version of Perl, which allows you
       to use the "-D" option of Perl to tell more about what Perl is doing.
       But sometimes there is no alternative than to dive in with a debugger,
       either to see the stack trace of a core dump (very useful in a bug
       report), or trying to figure out what went wrong before the core dump
       happened, or how did we end up having wrong or unexpected results.

   Poking at Perl
       To really poke around with Perl, you'll probably want to build Perl for
       debugging, like this:

	   ./Configure -d -D optimize=-g
	   make

       "-g" is a flag to the C compiler to have it produce debugging
       information which will allow us to step through a running program, and
       to see in which C function we are at (without the debugging information
       we might see only the numerical addresses of the functions, which is
       not very helpful).

       Configure will also turn on the "DEBUGGING" compilation symbol which
       enables all the internal debugging code in Perl. There are a whole
       bunch of things you can debug with this: perlrun lists them all, and
       the best way to find out about them is to play about with them. The
       most useful options are probably

	   l  Context (loop) stack processing
	   t  Trace execution
	   o  Method and overloading resolution
	   c  String/numeric conversions

       Some of the functionality of the debugging code can be achieved using
       XS modules.

	   -Dr => use re 'debug'
	   -Dx => use O 'Debug'

   Using a source-level debugger
       If the debugging output of "-D" doesn't help you, it's time to step
       through perl's execution with a source-level debugger.

       ·  We'll use "gdb" for our examples here; the principles will apply to
	  any debugger (many vendors call their debugger "dbx"), but check the
	  manual of the one you're using.

       To fire up the debugger, type

	   gdb ./perl

       Or if you have a core dump:

	   gdb ./perl core

       You'll want to do that in your Perl source tree so the debugger can
       read the source code. You should see the copyright message, followed by
       the prompt.

	   (gdb)

       "help" will get you into the documentation, but here are the most
       useful commands:

       ·  run [args]

	  Run the program with the given arguments.

       ·  break function_name

       ·  break source.c:xxx

	  Tells the debugger that we'll want to pause execution when we reach
	  either the named function (but see "Internal Functions" in
	  perlguts!) or the given line in the named source file.

       ·  step

	  Steps through the program a line at a time.

       ·  next

	  Steps through the program a line at a time, without descending into
	  functions.

       ·  continue

	  Run until the next breakpoint.

       ·  finish

	  Run until the end of the current function, then stop again.

       ·  'enter'

	  Just pressing Enter will do the most recent operation again - it's a
	  blessing when stepping through miles of source code.

       ·  print

	  Execute the given C code and print its results. WARNING: Perl makes
	  heavy use of macros, and gdb does not necessarily support macros
	  (see later "gdb macro support"). You'll have to substitute them
	  yourself, or to invoke cpp on the source code files (see "The .i
	  Targets") So, for instance, you can't say

	      print SvPV_nolen(sv)

	  but you have to say

	      print Perl_sv_2pv_nolen(sv)

       You may find it helpful to have a "macro dictionary", which you can
       produce by saying "cpp -dM perl.c | sort". Even then, cpp won't
       recursively apply those macros for you.

   gdb macro support
       Recent versions of gdb have fairly good macro support, but in order to
       use it you'll need to compile perl with macro definitions included in
       the debugging information. Using gcc version 3.1, this means
       configuring with "-Doptimize=-g3". Other compilers might use a
       different switch (if they support debugging macros at all).

   Dumping Perl Data Structures
       One way to get around this macro hell is to use the dumping functions
       in dump.c; these work a little like an internal Devel::Peek, but they
       also cover OPs and other structures that you can't get at from Perl.
       Let's take an example.  We'll use the "$a = $b + $c" we used before,
       but give it a bit of context: "$b = "6XXXX"; $c = 2.3;". Where's a good
       place to stop and poke around?

       What about "pp_add", the function we examined earlier to implement the
       "+" operator:

	   (gdb) break Perl_pp_add
	   Breakpoint 1 at 0x46249f: file pp_hot.c, line 309.

       Notice we use "Perl_pp_add" and not "pp_add" - see "Internal Functions"
       in perlguts. With the breakpoint in place, we can run our program:

	   (gdb) run -e '$b = "6XXXX"; $c = 2.3; $a = $b + $c'

       Lots of junk will go past as gdb reads in the relevant source files and
       libraries, and then:

	   Breakpoint 1, Perl_pp_add () at pp_hot.c:309
	   309	       dSP; dATARGET; tryAMAGICbin(add,opASSIGN);
	   (gdb) step
	   311		 dPOPTOPnnrl_ul;
	   (gdb)

       We looked at this bit of code before, and we said that "dPOPTOPnnrl_ul"
       arranges for two "NV"s to be placed into "left" and "right" - let's
       slightly expand it:

	#define dPOPTOPnnrl_ul	NV right = POPn; \
				SV *leftsv = TOPs; \
				NV left = USE_LEFT(leftsv) ? SvNV(leftsv) : 0.0

       "POPn" takes the SV from the top of the stack and obtains its NV either
       directly (if "SvNOK" is set) or by calling the "sv_2nv" function.
       "TOPs" takes the next SV from the top of the stack - yes, "POPn" uses
       "TOPs" - but doesn't remove it. We then use "SvNV" to get the NV from
       "leftsv" in the same way as before - yes, "POPn" uses "SvNV".

       Since we don't have an NV for $b, we'll have to use "sv_2nv" to convert
       it. If we step again, we'll find ourselves there:

	   Perl_sv_2nv (sv=0xa0675d0) at sv.c:1669
	   1669	       if (!sv)
	   (gdb)

       We can now use "Perl_sv_dump" to investigate the SV:

	   SV = PV(0xa057cc0) at 0xa0675d0
	   REFCNT = 1
	   FLAGS = (POK,pPOK)
	   PV = 0xa06a510 "6XXXX"\0
	   CUR = 5
	   LEN = 6
	   $1 = void

       We know we're going to get 6 from this, so let's finish the subroutine:

	   (gdb) finish
	   Run till exit from #0  Perl_sv_2nv (sv=0xa0675d0) at sv.c:1671
	   0x462669 in Perl_pp_add () at pp_hot.c:311
	   311		 dPOPTOPnnrl_ul;

       We can also dump out this op: the current op is always stored in
       "PL_op", and we can dump it with "Perl_op_dump". This'll give us
       similar output to B::Debug.

	   {
	   13  TYPE = add  ===> 14
	       TARG = 1
	       FLAGS = (SCALAR,KIDS)
	       {
		   TYPE = null	===> (12)
		     (was rv2sv)
		   FLAGS = (SCALAR,KIDS)
		   {
	   11	       TYPE = gvsv  ===> 12
		       FLAGS = (SCALAR)
		       GV = main::b
		   }
	       }

       # finish this later #

SOURCE CODE STATIC ANALYSIS
       Various tools exist for analysing C source code statically, as opposed
       to dynamically, that is, without executing the code. It is possible to
       detect resource leaks, undefined behaviour, type mismatches,
       portability problems, code paths that would cause illegal memory
       accesses, and other similar problems by just parsing the C code and
       looking at the resulting graph, what does it tell about the execution
       and data flows. As a matter of fact, this is exactly how C compilers
       know to give warnings about dubious code.

   lint, splint
       The good old C code quality inspector, "lint", is available in several
       platforms, but please be aware that there are several different
       implementations of it by different vendors, which means that the flags
       are not identical across different platforms.

       There is a lint variant called "splint" (Secure Programming Lint)
       available from http://www.splint.org/ that should compile on any Unix-
       like platform.

       There are "lint" and <splint> targets in Makefile, but you may have to
       diddle with the flags (see above).

   Coverity
       Coverity (http://www.coverity.com/) is a product similar to lint and as
       a testbed for their product they periodically check several open source
       projects, and they give out accounts to open source developers to the
       defect databases.

   cpd (cut-and-paste detector)
       The cpd tool detects cut-and-paste coding. If one instance of the cut-
       and-pasted code changes, all the other spots should probably be
       changed, too. Therefore such code should probably be turned into a
       subroutine or a macro.

       cpd (http://pmd.sourceforge.net/cpd.html) is part of the pmd project
       (http://pmd.sourceforge.net/). pmd was originally written for static
       analysis of Java code, but later the cpd part of it was extended to
       parse also C and C++.

       Download the pmd-bin-X.Y.zip () from the SourceForge site, extract the
       pmd-X.Y.jar from it, and then run that on source code thusly:

	 java -cp pmd-X.Y.jar net.sourceforge.pmd.cpd.CPD \
	  --minimum-tokens 100 --files /some/where/src --language c > cpd.txt

       You may run into memory limits, in which case you should use the -Xmx
       option:

	 java -Xmx512M ...

   gcc warnings
       Though much can be written about the inconsistency and coverage
       problems of gcc warnings (like "-Wall" not meaning "all the warnings",
       or some common portability problems not being covered by "-Wall", or
       "-ansi" and "-pedantic" both being a poorly defined collection of
       warnings, and so forth), gcc is still a useful tool in keeping our
       coding nose clean.

       The "-Wall" is by default on.

       The "-ansi" (and its sidekick, "-pedantic") would be nice to be on
       always, but unfortunately they are not safe on all platforms, they can
       for example cause fatal conflicts with the system headers (Solaris
       being a prime example). If Configure "-Dgccansipedantic" is used, the
       "cflags" frontend selects "-ansi -pedantic" for the platforms where
       they are known to be safe.

       Starting from Perl 5.9.4 the following extra flags are added:

       ·   "-Wendif-labels"

       ·   "-Wextra"

       ·   "-Wdeclaration-after-statement"

       The following flags would be nice to have but they would first need
       their own Augean stablemaster:

       ·   "-Wpointer-arith"

       ·   "-Wshadow"

       ·   "-Wstrict-prototypes"

       The "-Wtraditional" is another example of the annoying tendency of gcc
       to bundle a lot of warnings under one switch (it would be impossible to
       deploy in practice because it would complain a lot) but it does contain
       some warnings that would be beneficial to have available on their own,
       such as the warning about string constants inside macros containing the
       macro arguments: this behaved differently pre-ANSI than it does in
       ANSI, and some C compilers are still in transition, AIX being an
       example.

   Warnings of other C compilers
       Other C compilers (yes, there are other C compilers than gcc) often
       have their "strict ANSI" or "strict ANSI with some portability
       extensions" modes on, like for example the Sun Workshop has its "-Xa"
       mode on (though implicitly), or the DEC (these days, HP...) has its
       "-std1" mode on.

MEMORY DEBUGGERS
       NOTE 1: Running under older memory debuggers such as Purify, valgrind
       or Third Degree greatly slows down the execution: seconds become
       minutes, minutes become hours. For example as of Perl 5.8.1, the
       ext/Encode/t/Unicode.t takes extraordinarily long to complete under
       e.g. Purify, Third Degree, and valgrind. Under valgrind it takes more
       than six hours, even on a snappy computer. The said test must be doing
       something that is quite unfriendly for memory debuggers. If you don't
       feel like waiting, that you can simply kill away the perl process.
       Roughly valgrind slows down execution by factor 10, AddressSanitizer by
       factor 2.

       NOTE 2: To minimize the number of memory leak false alarms (see
       "PERL_DESTRUCT_LEVEL" for more information), you have to set the
       environment variable PERL_DESTRUCT_LEVEL to 2.

       For csh-like shells:

	   setenv PERL_DESTRUCT_LEVEL 2

       For Bourne-type shells:

	   PERL_DESTRUCT_LEVEL=2
	   export PERL_DESTRUCT_LEVEL

       In Unixy environments you can also use the "env" command:

	   env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -Ilib ...

       NOTE 3: There are known memory leaks when there are compile-time errors
       within eval or require, seeing "S_doeval" in the call stack is a good
       sign of these. Fixing these leaks is non-trivial, unfortunately, but
       they must be fixed eventually.

       NOTE 4: DynaLoader will not clean up after itself completely unless
       Perl is built with the Configure option
       "-Accflags=-DDL_UNLOAD_ALL_AT_EXIT".

   Rational Software's Purify
       Purify is a commercial tool that is helpful in identifying memory
       overruns, wild pointers, memory leaks and other such badness. Perl must
       be compiled in a specific way for optimal testing with Purify.  Purify
       is available under Windows NT, Solaris, HP-UX, SGI, and Siemens Unix.

       Purify on Unix

       On Unix, Purify creates a new Perl binary. To get the most benefit out
       of Purify, you should create the perl to Purify using:

	   sh Configure -Accflags=-DPURIFY -Doptimize='-g' \
	    -Uusemymalloc -Dusemultiplicity

       where these arguments mean:

       ·   -Accflags=-DPURIFY

	   Disables Perl's arena memory allocation functions, as well as
	   forcing use of memory allocation functions derived from the system
	   malloc.

       ·   -Doptimize='-g'

	   Adds debugging information so that you see the exact source
	   statements where the problem occurs. Without this flag, all you
	   will see is the source filename of where the error occurred.

       ·   -Uusemymalloc

	   Disable Perl's malloc so that Purify can more closely monitor
	   allocations and leaks. Using Perl's malloc will make Purify report
	   most leaks in the "potential" leaks category.

       ·   -Dusemultiplicity

	   Enabling the multiplicity option allows perl to clean up thoroughly
	   when the interpreter shuts down, which reduces the number of bogus
	   leak reports from Purify.

       Once you've compiled a perl suitable for Purify'ing, then you can just:

	   make pureperl

       which creates a binary named 'pureperl' that has been Purify'ed. This
       binary is used in place of the standard 'perl' binary when you want to
       debug Perl memory problems.

       As an example, to show any memory leaks produced during the standard
       Perl testset you would create and run the Purify'ed perl as:

	   make pureperl
	   cd t
	   ../pureperl -I../lib harness

       which would run Perl on test.pl and report any memory problems.

       Purify outputs messages in "Viewer" windows by default. If you don't
       have a windowing environment or if you simply want the Purify output to
       unobtrusively go to a log file instead of to the interactive window,
       use these following options to output to the log file "perl.log":

	   setenv PURIFYOPTIONS "-chain-length=25 -windows=no \
	    -log-file=perl.log -append-logfile=yes"

       If you plan to use the "Viewer" windows, then you only need this
       option:

	   setenv PURIFYOPTIONS "-chain-length=25"

       In Bourne-type shells:

	   PURIFYOPTIONS="..."
	   export PURIFYOPTIONS

       or if you have the "env" utility:

	   env PURIFYOPTIONS="..." ../pureperl ...

       Purify on NT

       Purify on Windows NT instruments the Perl binary 'perl.exe' on the fly.
	There are several options in the makefile you should change to get the
       most use out of Purify:

       ·   DEFINES

	   You should add -DPURIFY to the DEFINES line so the DEFINES line
	   looks something like:

	      DEFINES = -DWIN32 -D_CONSOLE -DNO_STRICT $(CRYPT_FLAG) -DPURIFY=1

	   to disable Perl's arena memory allocation functions, as well as to
	   force use of memory allocation functions derived from the system
	   malloc.

       ·   USE_MULTI = define

	   Enabling the multiplicity option allows perl to clean up thoroughly
	   when the interpreter shuts down, which reduces the number of bogus
	   leak reports from Purify.

       ·   #PERL_MALLOC = define

	   Disable Perl's malloc so that Purify can more closely monitor
	   allocations and leaks. Using Perl's malloc will make Purify report
	   most leaks in the "potential" leaks category.

       ·   CFG = Debug

	   Adds debugging information so that you see the exact source
	   statements where the problem occurs. Without this flag, all you
	   will see is the source filename of where the error occurred.

       As an example, to show any memory leaks produced during the standard
       Perl testset you would create and run Purify as:

	   cd win32
	   make
	   cd ../t
	   purify ../perl -I../lib harness

       which would instrument Perl in memory, run Perl on test.pl, then
       finally report any memory problems.

   valgrind
       The valgrind tool can be used to find out both memory leaks and illegal
       heap memory accesses. As of version 3.3.0, Valgrind only supports Linux
       on x86, x86-64 and PowerPC and Darwin (OS X) on x86 and x86-64). The
       special "test.valgrind" target can be used to run the tests under
       valgrind. Found errors and memory leaks are logged in files named
       testfile.valgrind.

       Valgrind also provides a cachegrind tool, invoked on perl as:

	   VG_OPTS=--tool=cachegrind make test.valgrind

       As system libraries (most notably glibc) are also triggering errors,
       valgrind allows to suppress such errors using suppression files. The
       default suppression file that comes with valgrind already catches a lot
       of them. Some additional suppressions are defined in t/perl.supp.

       To get valgrind and for more information see

	   http://valgrind.org/

   AddressSanitizer
       AddressSanitizer is a clang extension, included in clang since v3.1. It
       checks illegal heap pointers, global pointers, stack pointers and use
       after free errors, and is fast enough that you can easily compile your
       debugging or optimized perl with it. It does not check memory leaks
       though. AddressSanitizer is available for linux, Mac OS X and soon on
       Windows.

       To build perl with AddressSanitizer, your Configure invocation should
       look like:

	   sh Configure -des -Dcc=clang \
	      -Accflags=-faddress-sanitizer -Aldflags=-faddress-sanitizer \
	      -Alddlflags=-shared\ -faddress-sanitizer

       where these arguments mean:

       ·   -Dcc=clang

	   This should be replaced by the full path to your clang executable
	   if it is not in your path.

       ·   -Accflags=-faddress-sanitizer

	   Compile perl and extensions sources with AddressSanitizer.

       ·   -Aldflags=-faddress-sanitizer

	   Link the perl executable with AddressSanitizer.

       ·   -Alddlflags=-shared\ -faddress-sanitizer

	   Link dynamic extensions with AddressSanitizer. You must manually
	   specify "-shared" because using "-Alddlflags=-shared" will prevent
	   Configure from setting a default value for "lddlflags", which
	   usually contains "-shared" (at least on linux).

       See also
       <http://code.google.com/p/address-sanitizer/wiki/AddressSanitizer>.

PROFILING
       Depending on your platform there are various ways of profiling Perl.

       There are two commonly used techniques of profiling executables:
       statistical time-sampling and basic-block counting.

       The first method takes periodically samples of the CPU program counter,
       and since the program counter can be correlated with the code generated
       for functions, we get a statistical view of in which functions the
       program is spending its time. The caveats are that very small/fast
       functions have lower probability of showing up in the profile, and that
       periodically interrupting the program (this is usually done rather
       frequently, in the scale of milliseconds) imposes an additional
       overhead that may skew the results. The first problem can be alleviated
       by running the code for longer (in general this is a good idea for
       profiling), the second problem is usually kept in guard by the
       profiling tools themselves.

       The second method divides up the generated code into basic blocks.
       Basic blocks are sections of code that are entered only in the
       beginning and exited only at the end. For example, a conditional jump
       starts a basic block. Basic block profiling usually works by
       instrumenting the code by adding enter basic block #nnnn book-keeping
       code to the generated code. During the execution of the code the basic
       block counters are then updated appropriately. The caveat is that the
       added extra code can skew the results: again, the profiling tools
       usually try to factor their own effects out of the results.

   Gprof Profiling
       gprof is a profiling tool available in many Unix platforms, it uses
       statistical time-sampling.

       You can build a profiled version of perl called "perl.gprof" by
       invoking the make target "perl.gprof"  (What is required is that Perl
       must be compiled using the "-pg" flag, you may need to re-Configure).
       Running the profiled version of Perl will create an output file called
       gmon.out is created which contains the profiling data collected during
       the execution.

       The gprof tool can then display the collected data in various ways.
       Usually gprof understands the following options:

       ·   -a

	   Suppress statically defined functions from the profile.

       ·   -b

	   Suppress the verbose descriptions in the profile.

       ·   -e routine

	   Exclude the given routine and its descendants from the profile.

       ·   -f routine

	   Display only the given routine and its descendants in the profile.

       ·   -s

	   Generate a summary file called gmon.sum which then may be given to
	   subsequent gprof runs to accumulate data over several runs.

       ·   -z

	   Display routines that have zero usage.

       For more detailed explanation of the available commands and output
       formats, see your own local documentation of gprof.

       quick hint:

	   $ sh Configure -des -Dusedevel -Doptimize='-pg' && make perl.gprof
	   $ ./perl.gprof someprog # creates gmon.out in current directory
	   $ gprof ./perl.gprof > out
	   $ view out

   GCC gcov Profiling
       Starting from GCC 3.0 basic block profiling is officially available for
       the GNU CC.

       You can build a profiled version of perl called perl.gcov by invoking
       the make target "perl.gcov" (what is required that Perl must be
       compiled using gcc with the flags "-fprofile-arcs -ftest-coverage", you
       may need to re-Configure).

       Running the profiled version of Perl will cause profile output to be
       generated. For each source file an accompanying ".da" file will be
       created.

       To display the results you use the "gcov" utility (which should be
       installed if you have gcc 3.0 or newer installed). gcov is run on
       source code files, like this

	   gcov sv.c

       which will cause sv.c.gcov to be created. The .gcov files contain the
       source code annotated with relative frequencies of execution indicated
       by "#" markers.

       Useful options of gcov include "-b" which will summarise the basic
       block, branch, and function call coverage, and "-c" which instead of
       relative frequencies will use the actual counts. For more information
       on the use of gcov and basic block profiling with gcc, see the latest
       GNU CC manual, as of GCC 3.0 see

	   http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc.html

       and its section titled "8. gcov: a Test Coverage Program"

	   http://gcc.gnu.org/onlinedocs/gcc-3.0/gcc_8.html#SEC132

       quick hint:

	   $ sh Configure -des -Dusedevel -Doptimize='-g' \
	       -Accflags='-fprofile-arcs -ftest-coverage' \
	       -Aldflags='-fprofile-arcs -ftest-coverage' && make perl.gcov
	   $ rm -f regexec.c.gcov regexec.gcda
	   $ ./perl.gcov
	   $ gcov regexec.c
	   $ view regexec.c.gcov

MISCELLANEOUS TRICKS
   PERL_DESTRUCT_LEVEL
       If you want to run any of the tests yourself manually using e.g.
       valgrind, or the pureperl or perl.third executables, please note that
       by default perl does not explicitly cleanup all the memory it has
       allocated (such as global memory arenas) but instead lets the exit() of
       the whole program "take care" of such allocations, also known as
       "global destruction of objects".

       There is a way to tell perl to do complete cleanup: set the environment
       variable PERL_DESTRUCT_LEVEL to a non-zero value. The t/TEST wrapper
       does set this to 2, and this is what you need to do too, if you don't
       want to see the "global leaks": For example, for "third-degreed" Perl:

	       env PERL_DESTRUCT_LEVEL=2 ./perl.third -Ilib t/foo/bar.t

       (Note: the mod_perl apache module uses also this environment variable
       for its own purposes and extended its semantics. Refer to the mod_perl
       documentation for more information. Also, spawned threads do the
       equivalent of setting this variable to the value 1.)

       If, at the end of a run you get the message N scalars leaked, you can
       recompile with "-DDEBUG_LEAKING_SCALARS", which will cause the
       addresses of all those leaked SVs to be dumped along with details as to
       where each SV was originally allocated. This information is also
       displayed by Devel::Peek. Note that the extra details recorded with
       each SV increases memory usage, so it shouldn't be used in production
       environments. It also converts "new_SV()" from a macro into a real
       function, so you can use your favourite debugger to discover where
       those pesky SVs were allocated.

       If you see that you're leaking memory at runtime, but neither valgrind
       nor "-DDEBUG_LEAKING_SCALARS" will find anything, you're probably
       leaking SVs that are still reachable and will be properly cleaned up
       during destruction of the interpreter. In such cases, using the "-Dm"
       switch can point you to the source of the leak. If the executable was
       built with "-DDEBUG_LEAKING_SCALARS", "-Dm" will output SV allocations
       in addition to memory allocations. Each SV allocation has a distinct
       serial number that will be written on creation and destruction of the
       SV. So if you're executing the leaking code in a loop, you need to look
       for SVs that are created, but never destroyed between each cycle. If
       such an SV is found, set a conditional breakpoint within "new_SV()" and
       make it break only when "PL_sv_serial" is equal to the serial number of
       the leaking SV. Then you will catch the interpreter in exactly the
       state where the leaking SV is allocated, which is sufficient in many
       cases to find the source of the leak.

       As "-Dm" is using the PerlIO layer for output, it will by itself
       allocate quite a bunch of SVs, which are hidden to avoid recursion. You
       can bypass the PerlIO layer if you use the SV logging provided by
       "-DPERL_MEM_LOG" instead.

   PERL_MEM_LOG
       If compiled with "-DPERL_MEM_LOG", both memory and SV allocations go
       through logging functions, which is handy for breakpoint setting.

       Unless "-DPERL_MEM_LOG_NOIMPL" is also compiled, the logging functions
       read $ENV{PERL_MEM_LOG} to determine whether to log the event, and if
       so how:

	   $ENV{PERL_MEM_LOG} =~ /m/	       Log all memory ops
	   $ENV{PERL_MEM_LOG} =~ /s/	       Log all SV ops
	   $ENV{PERL_MEM_LOG} =~ /t/	       include timestamp in Log
	   $ENV{PERL_MEM_LOG} =~ /^(\d+)/      write to FD given (default is 2)

       Memory logging is somewhat similar to "-Dm" but is independent of
       "-DDEBUGGING", and at a higher level; all uses of Newx(), Renew(), and
       Safefree() are logged with the caller's source code file and line
       number (and C function name, if supported by the C compiler). In
       contrast, "-Dm" is directly at the point of "malloc()". SV logging is
       similar.

       Since the logging doesn't use PerlIO, all SV allocations are logged and
       no extra SV allocations are introduced by enabling the logging. If
       compiled with "-DDEBUG_LEAKING_SCALARS", the serial number for each SV
       allocation is also logged.

   DDD over gdb
       Those debugging perl with the DDD frontend over gdb may find the
       following useful:

       You can extend the data conversion shortcuts menu, so for example you
       can display an SV's IV value with one click, without doing any typing.
       To do that simply edit ~/.ddd/init file and add after:

	 ! Display shortcuts.
	 Ddd*gdbDisplayShortcuts: \
	 /t ()	 // Convert to Bin\n\
	 /d ()	 // Convert to Dec\n\
	 /x ()	 // Convert to Hex\n\
	 /o ()	 // Convert to Oct(\n\

       the following two lines:

	 ((XPV*) (())->sv_any )->xpv_pv	 // 2pvx\n\
	 ((XPVIV*) (())->sv_any )->xiv_iv // 2ivx

       so now you can do ivx and pvx lookups or you can plug there the sv_peek
       "conversion":

	 Perl_sv_peek(my_perl, (SV*)()) // sv_peek

       (The my_perl is for threaded builds.) Just remember that every line,
       but the last one, should end with \n\

       Alternatively edit the init file interactively via: 3rd mouse button ->
       New Display -> Edit Menu

       Note: you can define up to 20 conversion shortcuts in the gdb section.

   Poison
       If you see in a debugger a memory area mysteriously full of 0xABABABAB
       or 0xEFEFEFEF, you may be seeing the effect of the Poison() macros, see
       perlclib.

   Read-only optrees
       Under ithreads the optree is read only. If you want to enforce this, to
       check for write accesses from buggy code, compile with
       "-DPERL_DEBUG_READONLY_OPS" to enable code that allocates op memory via
       "mmap", and sets it read-only when it is attached to a subroutine. Any
       write access to an op results in a "SIGBUS" and abort.

       This code is intended for development only, and may not be portable
       even to all Unix variants. Also, it is an 80% solution, in that it
       isn't able to make all ops read only. Specifically it does not apply to
       op slabs belonging to "BEGIN" blocks.

       However, as an 80% solution it is still effective, as it has caught
       bugs in the past.

   The .i Targets
       You can expand the macros in a foo.c file by saying

	   make foo.i

       which will expand the macros using cpp.	Don't be scared by the
       results.

AUTHOR
       This document was originally written by Nathan Torkington, and is
       maintained by the perl5-porters mailing list.

perl v5.18.1			  2013-08-11		       PERLHACKTIPS(1)
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