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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.	 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
       o   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.

       o   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.

       o   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.

       o   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.

       o   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
       "-Wuninitialized") 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, character	set, 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.

       o   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().)

       o   Casting between 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.

       o   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.

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

	     char *p = ...;
	     long pony = *(long	*)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.

       o   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.

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

	   o	   That	a certain field	exists in a struct

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

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

	   o	   That	the fields are in a certain order

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

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

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

		   o	   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

       o   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".  However, you can
	   use macros defined in utf8.h	to specify any code point portably.
	   "LATIN1_TO_NATIVE(0xDF)" is going to	be the code point that means
	   LATIN SMALL LETTER SHARP S on whatever platform you are running on
	   (on ASCII platforms it compiles without adding any extra code, so
	   there is zero performance hit on those).  The acceptable inputs to
	   "LATIN1_TO_NATIVE" are from 0x00 through 0xFF.  If your input isn't
	   guaranteed to be in that range, use "UNICODE_TO_NATIVE" instead.
	   "NATIVE_TO_LATIN1" and "NATIVE_TO_UNICODE" translate	the opposite
	   direction.

	   If you need the string representation of a character	that doesn't
	   have	a mnemonic name	in C, you should add it	to the list in
	   regen/unicode_constants.pl, and have	Perl create "#define"'s	for
	   you,	based on the current platform.

	   Note	that the "isFOO" and "toFOO" macros in handy.h work properly
	   on native code points and strings.

	   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.	(Note that
	   special handling of ranges in regular expression patterns and
	   transliterations makes it appear to Perl code that the
	   aforementioned ranges are all unbroken.)

	   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 UTF-8 "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.

	   As noted in "TESTING" in perlhack, when writing test	scripts, the
	   file	t/charset_tools.pl contains some helpful functions for writing
	   tests valid on both ASCII and EBCDIC	platforms.  Sometimes, though,
	   a test can't	use a function and it's	inconvenient to	have different
	   test	versions depending on the platform.  There are 20 code points
	   that	are the	same in	all 4 character	sets currently recognized by
	   Perl	(the 3 EBCDIC code pages plus ISO 8859-1 (ASCII/Latin1)).
	   These can be	used in	such tests, though there is a small
	   possibility that Perl will become available in yet another
	   character set, breaking your	test.  All but one of these code
	   points are C0 control characters.  The most significant controls
	   that	are the	same are "\0", "\r", and "\N{VT}" (also	specifiable as
	   "\cK", "\x0B", "\N{U+0B}", or "\013").  The single non-control is
	   U+00B6 PILCROW SIGN.	 The controls that are the same	have the same
	   bit pattern in all 4	character sets,	regardless of the UTF8ness of
	   the string containing them.	The bit	pattern	for U+B6 is the	same
	   in all 4 for	non-UTF8 strings, but differs in each when its
	   containing string is	UTF-8 encoded.	The only other code points
	   that	have some sort of sameness across all 4	character sets are the
	   pair	0xDC and 0xFC.	Together these represent upper-	and lowercase
	   LATIN LETTER	U WITH DIAERESIS, but which is upper and which is
	   lower may be	reversed: 0xDC is the capital in Latin1	and 0xFC is
	   the small letter, while 0xFC	is the capital in EBCDIC and 0xDC is
	   the small one.  This	factoid	may be exploited in writing case
	   insensitive tests that are the same across all 4 character sets.

       o   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	has being changed starting in 5.12 so that these characters
	   can be considered to	be Latin-1 (ISO-8859-1).

       o   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.

       o   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).

       o   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).

       o   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.

       o   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-statement" scans	for such
	   problems (by	default	on starting from Perl 5.9.4).

       o   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.

       o   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.

       o   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.

       o   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);

	   See "Formatted Printing of Size_t and SSize_t" in perlguts for how
	   to print those.

	   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.

       o   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.

       o   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.

       o   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.

       o   Binding together several statements in a macro

	   Use the macros STMT_START and STMT_END.

	      STMT_START {
		 ...
	      }	STMT_END

       o   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.

	   A good resource on the predefined macros for	various	operating
	   systems, compilers, and so forth is
	   <http://sourceforge.net/p/predef/wiki/Home/>

       o   Assuming the	contents of static memory pointed to by	the return
	   values of Perl wrappers for C library functions doesn't change.
	   Many	C library functions return pointers to static storage that can
	   be overwritten by subsequent	calls to the same or related
	   functions.  Perl has	light-weight wrappers for some of these
	   functions, and which	don't make copies of the static	memory.	 A
	   good	example	is the interface to the	environment variables that are
	   in effect for the program.  Perl has	"PerlEnv_getenv" to get	values
	   from	the environment.  But the return is a pointer to static	memory
	   in the C library.  If you are using the value to immediately	test
	   for something, that's fine, but if you save the value and expect it
	   to be unchanged by later processing,	you would be wrong, but
	   perhaps you wouldn't	know it	because	different C library
	   implementations behave differently, and the one on the platform
	   you're testing on might work	for your situation.  But on some
	   platforms, a	subsequent call	to "PerlEnv_getenv" or related
	   function WILL overwrite the memory that your	first call points to.
	   This	has led	to some	hard-to-debug problems.	 Do a "savepv" in
	   perlapi to make a copy, thus	avoiding these problems.  You will
	   have	to free	the copy when you're done to avoid memory leaks.  If
	   you don't have control over when it gets freed, you'll need to make
	   the copy in a mortal	scalar,	like so:

	    if ((s = PerlEnv_getenv("foo") == NULL) {
	       ... /* handle NULL case */
	    }
	    else {
		s = SvPVX(sv_2mortal(newSVpv(s,	0)));
	    }

	   The above example works only	if "s" is "NUL"-terminated; otherwise
	   you have to pass its	length to "newSVpv".

   Problematic System Interfaces
       o   Perl	strings	are NOT	the same as C strings:	They may contain "NUL"
	   characters, whereas a C string is terminated	by the first "NUL".
	   That	is why Perl API	functions that deal with strings generally
	   take	a pointer to the first byte and	either a length	or a pointer
	   to the byte just beyond the final one.

	   And this is the reason that many of the C library string handling
	   functions should not	be used.  They don't cope with the full
	   generality of Perl strings.	It may be that your test cases don't
	   have	embedded "NUL"s, and so	the tests pass,	whereas	there may well
	   eventually arise real-world cases where they	fail.  A lesson	here
	   is to include "NUL"s	in your	tests.	Now it's fairly	rare in	most
	   real	world cases to get "NUL"s, so your code	may seem to work,
	   until one day a "NUL" comes along.

	   Here's an example.  It used to be a common paradigm,	for decades,
	   in the perl core to use "strchr("list", c)" to see if the character
	   "c" is any of the ones given	in "list", a double-quote-enclosed
	   string of the set of	characters that	we are seeing if "c" is	one
	   of.	As long	as "c" isn't a "NUL", it works.	 But when "c" is a
	   "NUL", "strchr" returns a pointer to	the terminating	"NUL" in
	   "list".   This likely will result in	a segfault or a	security issue
	   when	the caller uses	that end pointer as the	starting point to read
	   from.

	   A solution to this and many similar issues is to use	the "mem"-foo
	   C library functions instead.	 In this case "memchr" can be used to
	   see if "c" is in "list" and works even if "c" is "NUL".  These
	   functions need an additional	parameter to give the string length.
	   In the case of literal string parameters, perl has defined macros
	   that	calculate the length for you.  See "Miscellaneous Functions"
	   in perlapi.

       o   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.)

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

   Security problems
       Last but	not least, here	are various tips for safer coding.  See	also
       perlclib	for libc/stdio replacements one	should use.

       o   Do not use gets()

	   Or we will publicly ridicule	you.  Seriously.

       o   Do not use tmpfile()

	   Use mkstemp() instead.

       o   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).

       o   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 "Perl_form"() or
	   SVs and "Perl_sv_catpvf()".

	   Note	that glibc "printf()", "sprintf()", etc. are buggy before
	   glibc version 2.17.	They won't allow a "%.s" format	with a
	   precision to	create a string	that isn't valid UTF-8 if the current
	   underlying locale of	the program is UTF-8.  What happens is that
	   the %s and its operand are simply skipped without any notice.
	   <https://sourceware.org/bugzilla/show_bug.cgi?id=6530>.

       o   Do not use atoi()

	   Use grok_atoUV() instead.  atoi() has ill-defined behavior on
	   overflows, and cannot be used for incremental parsing.  It is also
	   affected by locale, which is	bad.

       o   Do not use strtol() or strtoul()

	   Use grok_atoUV() instead.  strtol() or strtoul() (or	their
	   IV/UV-friendly macro	disguises, Strtol() and	Strtoul(), or Atol()
	   and Atoul() are affected by locale, which is	bad.

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 -DDEBUGGING
	   make

       "-DDEBUGGING" turns on the C compiler's "-g" flag 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). It 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
	   s  Stack snapshots (with v, displays	all stacks)
	   t  Trace execution
	   o  Method and overloading resolution
	   c  String/numeric conversions

       For example

	   $ perl -Dst -e '$a +	1'
	   ....
	   (-e:1)      gvsv(main::a)
	       =>  UNDEF
	   (-e:1)      const(IV(1))
	       =>  UNDEF  IV(1)
	   (-e:1)      add
	       =>  NV(1)

       Some of the functionality of the	debugging code can be achieved with a
       non-debugging perl by 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.

       o  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:

       o  run [args]

	  Run the program with the given arguments.

       o  break	function_name

       o  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.

       o  step

	  Steps	through	the program a line at a	time.

       o  next

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

       o  continue

	  Run until the	next breakpoint.

       o  finish

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

       o  'enter'

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

       o  ptype

	  Prints the C definition of the argument given.

	    (gdb) ptype	PL_op
	    type = struct op {
		OP *op_next;
		OP *op_sibparent;
		OP *(*op_ppaddr)(void);
		PADOFFSET op_targ;
		unsigned int op_type : 9;
		unsigned int op_opt : 1;
		unsigned int op_slabbed	: 1;
		unsigned int op_savefree : 1;
		unsigned int op_static : 1;
		unsigned int op_folded : 1;
		unsigned int op_spare :	2;
		U8 op_flags;
		U8 op_private;
	    } *

       o  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
	   1396	   dSP;	dATARGET; bool useleft;	SV *svl, *svr;
	   (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:

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

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

	   (gdb) print Perl_sv_dump(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 CPAN module B::Debug.

	   (gdb) print Perl_op_dump(PL_op)
	   {
	   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 #

   Using gdb to	look at	specific parts of a program
       With the	example	above, you knew	to look	for "Perl_pp_add", but what if
       there were multiple calls to it all over	the place, or you didn't know
       what the	op was you were	looking	for?

       One way to do this is to	inject a rare call somewhere near what you're
       looking for.  For example, you could add	"study"	before your method:

	   study;

       And in gdb do:

	   (gdb) break Perl_pp_study

       And then	step until you hit what	you're looking for.  This works	well
       in a loop if you	want to	only break at certain iterations:

	   for my $c (1..100) {
	       study if	$c == 50;
	   }

   Using gdb to	look at	what the parser/lexer are doing
       If you want to see what perl is doing when parsing/lexing your code,
       you can use "BEGIN {}":

	   print "Before\n";
	   BEGIN { study; }
	   print "After\n";

       And in gdb:

	   (gdb) break Perl_pp_study

       If you want to see what the parser/lexer	is doing inside	of "if"	blocks
       and the like you	need to	be a little trickier:

	   if ($a && $b	&& do {	BEGIN {	study }	1 } && $c) { ... }

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
       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" target	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.

       There is	Coverity setup for the perl5 project:
       <https://scan.coverity.com/projects/perl5>

   HP-UX cadvise (Code Advisor)
       HP has a	C/C++ static analyzer product for HP-UX	caller Code Advisor.
       (Link not given here because the	URL is horribly	long and seems
       horribly	unstable; use the search engine	of your	choice to find it.)
       The use of the "cadvise_cc" recipe with "Configure ...
       -Dcc=./cadvise_cc" (see cadvise "User Guide") is	recommended; as	is the
       use of "+wall".

   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.

       The following extra flags are added:

       o   "-Wendif-labels"

       o   "-Wextra"

       o   "-Wc++-compat"

       o   "-Wwrite-strings"

       o   "-Werror=declaration-after-statement"

       o   "-Werror=pointer-arith"

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

       o   "-Wshadow"

       o   "-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 example, like this:

	   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".

   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 and by	default	output is displayed inline.

       Example usage:

	   make	test.valgrind

       Since valgrind adds significant overhead, tests will take much longer
       to run.	The valgrind tests support being run in	parallel to help with
       this:

	   TEST_JOBS=9 make test.valgrind

       Note that the above two invocations will	be very	verbose	as reachable
       memory and leak-checking	is enabled by default.	If you want to just
       see pure	errors,	try:

	   VG_OPTS='-q --leak-check=no --show-reachable=no' TEST_JOBS=9	\
	       make test.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	("ASan") consists of a compiler	instrumentation	module
       and a run-time "malloc" library.	ASan is	available for a	variety	of
       architectures, operating	systems, and compilers (see project link
       below).	It checks for unsafe memory usage, such	as use after free and
       buffer overflow conditions, and is fast enough that you can easily
       compile your debugging or optimized perl	with it. Modern	versions of
       ASan check for memory leaks by default on most platforms, otherwise
       (e.g. x86_64 OS X) this feature can be enabled via
       "ASAN_OPTIONS=detect_leaks=1".

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

	   sh Configure	-des -Dcc=clang	\
	      -Accflags=-fsanitize=address -Aldflags=-fsanitize=address	\
	      -Alddlflags=-shared\ -fsanitize=address \
	      -fsanitize-blacklist=`pwd`/asan_ignore

       where these arguments mean:

       o   -Dcc=clang

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

       o   -Accflags=-fsanitize=address

	   Compile perl	and extensions sources with AddressSanitizer.

       o   -Aldflags=-fsanitize=address

	   Link	the perl executable with AddressSanitizer.

       o   -Alddlflags=-shared\	-fsanitize=address

	   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).

       o   -fsanitize-blacklist=`pwd`/asan_ignore

	   AddressSanitizer will ignore	functions listed in the	"asan_ignore"
	   file. (This file should contain a short explanation of why each of
	   the functions is listed.)

       See also	<https://github.com/google/sanitizers/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 which	uses
       statistical time-sampling.  You can build a profiled version of perl by
       compiling using gcc with	the flag "-pg".	 Either	edit config.sh or re-
       run Configure.  Running the profiled version of Perl will create	an
       output file called gmon.out which contains the profiling	data collected
       during the execution.

       quick hint:

	   $ sh	Configure -des -Dusedevel -Accflags='-pg' \
	       -Aldflags='-pg' -Alddlflags='-pg	-shared' \
	       && make perl
	   $ ./perl ...	# creates gmon.out in current directory
	   $ gprof ./perl > out
	   $ less out

       (you probably need to add "-shared" to the <-Alddlflags>	line until RT
       #118199 is resolved)

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

       o   -a

	   Suppress statically defined functions from the profile.

       o   -b

	   Suppress the	verbose	descriptions in	the profile.

       o   -e routine

	   Exclude the given routine and its descendants from the profile.

       o   -f routine

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

       o   -s

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

       o   -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.

   GCC gcov Profiling
       basic block profiling is	officially available in	gcc 3.0	and later.
       You can build a profiled	version	of perl	by compiling using gcc with
       the flags "-fprofile-arcs -ftest-coverage".  Either edit	config.sh or
       re-run Configure.

       quick hint:

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

       (you probably need to add "-shared" to the <-Alddlflags>	line until RT
       #118199 is resolved)

       Running the profiled version of Perl will cause profile output to be
       generated.  For each source file	an accompanying	.gcda 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.	If you want to generate	.gcov files for	all profiled
       object files, you can run something like	this:

	   for file in `find . -name \*.gcno`
	   do sh -c "cd	`dirname $file`	&& gcov	`basename $file	.gcno`"
	   done

       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 4.8, this is at
       <http://gcc.gnu.org/onlinedocs/gcc/Gcov-Intro.html#Gcov-Intro>

MISCELLANEOUS TRICKS
   PERL_DESTRUCT_LEVEL
       If you want to run any of the tests yourself manually using e.g.
       valgrind, 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	running	under valgrind

	   env PERL_DESTRUCT_LEVEL=2 valgrind ./perl -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", ("Configure
       -Accflags=-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" ("-Accflags=-DPERL_MEM_LOG"), both
       memory and SV allocations go through logging functions, which is	handy
       for breakpoint setting.

       Unless "-DPERL_MEM_LOG_NOIMPL" ("-Accflags=-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.

   C backtrace
       On some platforms Perl supports retrieving the C	level backtrace
       (similar	to what	symbolic debuggers like	gdb do).

       The backtrace returns the stack trace of	the C call frames, with	the
       symbol names (function names), the object names (like "perl"), and if
       it can, also the	source code locations (file:line).

       The supported platforms are Linux, and OS X (some *BSD might work at
       least partly, but they have not yet been	tested).

       This feature hasn't been	tested with multiple threads, but it will only
       show the	backtrace of the thread	doing the backtracing.

       The feature needs to be enabled with "Configure -Dusecbacktrace".

       The "-Dusecbacktrace" also enables keeping the debug information	when
       compiling/linking (often: "-g").	 Many compilers/linkers	do support
       having both optimization	and keeping the	debug information.  The	debug
       information is needed for the symbol names and the source locations.

       Static functions	might not be visible for the backtrace.

       Source code locations, even if available, can often be missing or
       misleading if the compiler has e.g. inlined code.  Optimizer can	make
       matching	the source code	and the	object code quite challenging.

       Linux
	   You must have the BFD (-lbfd) library installed, otherwise "perl"
	   will	fail to	link.  The BFD is usually distributed as part of the
	   GNU binutils.

	   Summary: "Configure ... -Dusecbacktrace" and	you need "-lbfd".

       OS X
	   The source code locations are supported only	if you have the
	   Developer Tools installed.  (BFD is not needed.)

	   Summary: "Configure ... -Dusecbacktrace" and	installing the
	   Developer Tools would be good.

       Optionally, for trying out the feature, you may want to enable
       automatic dumping of the	backtrace just before a	warning	or croak (die)
       message is emitted, by adding "-Accflags=-DUSE_C_BACKTRACE_ON_ERROR"
       for Configure.

       Unless the above	additional feature is enabled, nothing about the
       backtrace functionality is visible, except for the Perl/XS level.

       Furthermore, even if you	have enabled this feature to be	compiled, you
       need to enable it in runtime with an environment	variable:
       "PERL_C_BACKTRACE_ON_ERROR=10".	It must	be an integer higher than
       zero, telling the desired frame count.

       Retrieving the backtrace	from Perl level	(using for example an XS
       extension) would	be much	less exciting than one would hope: normally
       you would see "runops", "entersub", and not much	else.  This API	is
       intended	to be called from within the Perl implementation, not from
       Perl level execution.

       The C API for the backtrace is as follows:

       get_c_backtrace
       free_c_backtrace
       get_c_backtrace_dump
       dump_c_backtrace

   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
       "-Accflags=-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.

   When	is a bool not a	bool?
       On pre-C99 compilers, "bool" is defined as equivalent to	"char".
       Consequently assignment of any larger type to a "bool" is unsafe	and
       may be truncated.  The "cBOOL" macro exists to cast it correctly; you
       may also	find that using	it is shorter and clearer than writing out the
       equivalent conditional expression longhand.

       On those	platforms and compilers	where "bool" really is a boolean (C++,
       C99), it	is easy	to forget the cast.  You can force "bool" to be	a
       "char" by compiling with	"-Accflags=-DPERL_BOOL_AS_CHAR".  You may also
       wish to run "Configure" with something like

	   -Accflags='-Wconversion -Wno-sign-conversion	-Wno-shorten-64-to-32'

       or your compiler's equivalent to	make it	easier to spot any unsafe
       truncations that	show up.

       The "TRUE" and "FALSE" macros are available for situations where	using
       them would clarify intent. (But they always just	mean the same as the
       integers	1 and 0	regardless, so using them isn't	compulsory.)

   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 v5.32.0			  2020-06-14		       PERLHACKTIPS(1)

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