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PERLGUTS(1)	       Perl Programmers	Reference Guide		   PERLGUTS(1)

NAME
       perlguts	- Introduction to the Perl API

DESCRIPTION
       This document attempts to describe how to use the Perl API, as well as
       containing some info on the basic workings of the Perl core. It is far
       from complete and probably contains many	errors.	Please refer any ques-
       tions or	comments to the	author below.

Variables
       Datatypes

       Perl has	three typedefs that handle Perl's three	main data types:

	   SV  Scalar Value
	   AV  Array Value
	   HV  Hash Value

       Each typedef has	specific routines that manipulate the various data
       types.

       What is an "IV"?

       Perl uses a special typedef IV which is a simple	signed integer type
       that is guaranteed to be	large enough to	hold a pointer (as well	as an
       integer).  Additionally,	there is the UV, which is simply an unsigned
       IV.

       Perl also uses two special typedefs, I32	and I16, which will always be
       at least	32-bits	and 16-bits long, respectively.	(Again,	there are U32
       and U16,	as well.)  They	will usually be	exactly	32 and 16 bits long,
       but on Crays they will both be 64 bits.

       Working with SVs

       An SV can be created and	loaded with one	command.  There	are five types
       of values that can be loaded: an	integer	value (IV), an unsigned	inte-
       ger value (UV), a double	(NV), a	string (PV), and another scalar	(SV).

       The seven routines are:

	   SV*	newSViv(IV);
	   SV*	newSVuv(UV);
	   SV*	newSVnv(double);
	   SV*	newSVpv(const char*, int);
	   SV*	newSVpvn(const char*, int);
	   SV*	newSVpvf(const char*, ...);
	   SV*	newSVsv(SV*);

       If you require more complex initialisation you can create an empty SV
       with newSV(len).	 If "len" is 0 an empty	SV of type NULL	is returned,
       else an SV of type PV is	returned with len + 1 (for the NUL) bytes of
       storage allocated, accessible via SvPVX.	 In both cases the SV has
       value undef.

	   SV*	newSV(0);   /* no storage allocated  */
	   SV*	newSV(10);  /* 10 (+1) bytes of	uninitialised storage allocated	 */

       To change the value of an *already-existing* SV,	there are eight	rou-
       tines:

	   void	 sv_setiv(SV*, IV);
	   void	 sv_setuv(SV*, UV);
	   void	 sv_setnv(SV*, double);
	   void	 sv_setpv(SV*, const char*);
	   void	 sv_setpvn(SV*,	const char*, int)
	   void	 sv_setpvf(SV*,	const char*, ...);
	   void	 sv_vsetpvfn(SV*, const	char*, STRLEN, va_list *, SV **, I32, bool *);
	   void	 sv_setsv(SV*, SV*);

       Notice that you can choose to specify the length	of the string to be
       assigned	by using "sv_setpvn", "newSVpvn", or "newSVpv",	or you may
       allow Perl to calculate the length by using "sv_setpv" or by specifying
       0 as the	second argument	to "newSVpv".  Be warned, though, that Perl
       will determine the string's length by using "strlen", which depends on
       the string terminating with a NUL character.

       The arguments of	"sv_setpvf" are	processed like "sprintf", and the for-
       matted output becomes the value.

       "sv_vsetpvfn" is	an analogue of "vsprintf", but it allows you to	spec-
       ify either a pointer to a variable argument list	or the address and
       length of an array of SVs.  The last argument points to a boolean; on
       return, if that boolean is true,	then locale-specific information has
       been used to format the string, and the string's	contents are therefore
       untrustworthy (see perlsec).  This pointer may be NULL if that informa-
       tion is not important.  Note that this function requires	you to specify
       the length of the format.

       STRLEN is an integer type (Size_t, usually defined as size_t in con-
       fig.h) guaranteed to be large enough to represent the size of any
       string that perl	can handle.

       The "sv_set*()" functions are not generic enough	to operate on values
       that have "magic".  See "Magic Virtual Tables" later in this document.

       All SVs that contain strings should be terminated with a	NUL character.
       If it is	not NUL-terminated there is a risk of core dumps and corrup-
       tions from code which passes the	string to C functions or system	calls
       which expect a NUL-terminated string.  Perl's own functions typically
       add a trailing NUL for this reason.  Nevertheless, you should be	very
       careful when you	pass a string stored in	an SV to a C function or sys-
       tem call.

       To access the actual value that an SV points to,	you can	use the
       macros:

	   SvIV(SV*)
	   SvUV(SV*)
	   SvNV(SV*)
	   SvPV(SV*, STRLEN len)
	   SvPV_nolen(SV*)

       which will automatically	coerce the actual scalar type into an IV, UV,
       double, or string.

       In the "SvPV" macro, the	length of the string returned is placed	into
       the variable "len" (this	is a macro, so you do not use &len).  If you
       do not care what	the length of the data is, use the "SvPV_nolen"	macro.
       Historically the	"SvPV" macro with the global variable "PL_na" has been
       used in this case.  But that can	be quite inefficient because "PL_na"
       must be accessed	in thread-local	storage	in threaded Perl.  In any
       case, remember that Perl	allows arbitrary strings of data that may both
       contain NULs and	might not be terminated	by a NUL.

       Also remember that C doesn't allow you to safely	say "foo(SvPV(s, len),
       len);". It might	work with your compiler, but it	won't work for every-
       one.  Break this	sort of	statement up into separate assignments:

	       SV *s;
	       STRLEN len;
	       char * ptr;
	       ptr = SvPV(s, len);
	       foo(ptr,	len);

       If you want to know if the scalar value is TRUE,	you can	use:

	   SvTRUE(SV*)

       Although	Perl will automatically	grow strings for you, if you need to
       force Perl to allocate more memory for your SV, you can use the macro

	   SvGROW(SV*, STRLEN newlen)

       which will determine if more memory needs to be allocated.  If so, it
       will call the function "sv_grow".  Note that "SvGROW" can only
       increase, not decrease, the allocated memory of an SV and that it does
       not automatically add a byte for	the a trailing NUL (perl's own string
       functions typically do "SvGROW(sv, len +	1)").

       If you have an SV and want to know what kind of data Perl thinks	is
       stored in it, you can use the following macros to check the type	of SV
       you have.

	   SvIOK(SV*)
	   SvNOK(SV*)
	   SvPOK(SV*)

       You can get and set the current length of the string stored in an SV
       with the	following macros:

	   SvCUR(SV*)
	   SvCUR_set(SV*, I32 val)

       You can also get	a pointer to the end of	the string stored in the SV
       with the	macro:

	   SvEND(SV*)

       But note	that these last	three macros are valid only if "SvPOK()" is
       true.

       If you want to append something to the end of string stored in an
       "SV*", you can use the following	functions:

	   void	 sv_catpv(SV*, const char*);
	   void	 sv_catpvn(SV*,	const char*, STRLEN);
	   void	 sv_catpvf(SV*,	const char*, ...);
	   void	 sv_vcatpvfn(SV*, const	char*, STRLEN, va_list *, SV **, I32, bool);
	   void	 sv_catsv(SV*, SV*);

       The first function calculates the length	of the string to be appended
       by using	"strlen".  In the second, you specify the length of the	string
       yourself.  The third function processes its arguments like "sprintf"
       and appends the formatted output.  The fourth function works like
       "vsprintf".  You	can specify the	address	and length of an array of SVs
       instead of the va_list argument.	The fifth function extends the string
       stored in the first SV with the string stored in	the second SV.	It
       also forces the second SV to be interpreted as a	string.

       The "sv_cat*()" functions are not generic enough	to operate on values
       that have "magic".  See "Magic Virtual Tables" later in this document.

       If you know the name of a scalar	variable, you can get a	pointer	to its
       SV by using the following:

	   SV*	get_sv("package::varname", FALSE);

       This returns NULL if the	variable does not exist.

       If you want to know if this variable (or	any other SV) is actually
       "defined", you can call:

	   SvOK(SV*)

       The scalar "undef" value	is stored in an	SV instance called
       "PL_sv_undef".  Its address can be used whenever	an "SV*" is needed.

       There are also the two values "PL_sv_yes" and "PL_sv_no", which contain
       Boolean TRUE and	FALSE values, respectively.  Like "PL_sv_undef", their
       addresses can be	used whenever an "SV*" is needed.

       Do not be fooled	into thinking that "(SV	*) 0" is the same as
       &PL_sv_undef.  Take this	code:

	   SV* sv = (SV*) 0;
	   if (I-am-to-return-a-real-value) {
		   sv =	sv_2mortal(newSViv(42));
	   }
	   sv_setsv(ST(0), sv);

       This code tries to return a new SV (which contains the value 42)	if it
       should return a real value, or undef otherwise.	Instead	it has
       returned	a NULL pointer which, somewhere	down the line, will cause a
       segmentation violation, bus error, or just weird	results.  Change the
       zero to &PL_sv_undef in the first line and all will be well.

       To free an SV that you've created, call "SvREFCNT_dec(SV*)".  Normally
       this call is not	necessary (see "Reference Counts and Mortality").

       Offsets

       Perl provides the function "sv_chop" to efficiently remove characters
       from the	beginning of a string; you give	it an SV and a pointer to
       somewhere inside	the PV,	and it discards	everything before the pointer.
       The efficiency comes by means of	a little hack: instead of actually
       removing	the characters,	"sv_chop" sets the flag	"OOK" (offset OK) to
       signal to other functions that the offset hack is in effect, and	it
       puts the	number of bytes	chopped	off into the IV	field of the SV. It
       then moves the PV pointer (called "SvPVX") forward that many bytes, and
       adjusts "SvCUR" and "SvLEN".

       Hence, at this point, the start of the buffer that we allocated lives
       at "SvPVX(sv) - SvIV(sv)" in memory and the PV pointer is pointing into
       the middle of this allocated storage.

       This is best demonstrated by example:

	 % ./perl -Ilib	-MDevel::Peek -le '$a="12345"; $a=~s/.//; Dump($a)'
	 SV = PVIV(0x8128450) at 0x81340f0
	   REFCNT = 1
	   FLAGS = (POK,OOK,pPOK)
	   IV =	1  (OFFSET)
	   PV =	0x8135781 ( "1"	. ) "2345"\0
	   CUR = 4
	   LEN = 5

       Here the	number of bytes	chopped	off (1)	is put into IV,	and
       "Devel::Peek::Dump" helpfully reminds us	that this is an	offset.	The
       portion of the string between the "real"	and the	"fake" beginnings is
       shown in	parentheses, and the values of "SvCUR" and "SvLEN" reflect the
       fake beginning, not the real one.

       Something similar to the	offset hack is performed on AVs	to enable
       efficient shifting and splicing off the beginning of the	array; while
       "AvARRAY" points	to the first element in	the array that is visible from
       Perl, "AvALLOC" points to the real start	of the C array.	These are usu-
       ally the	same, but a "shift" operation can be carried out by increasing
       "AvARRAY" by one	and decreasing "AvFILL"	and "AvLEN".  Again, the loca-
       tion of the real	start of the C array only comes	into play when freeing
       the array. See "av_shift" in av.c.

       What's Really Stored in an SV?

       Recall that the usual method of determining the type of scalar you have
       is to use "Sv*OK" macros.  Because a scalar can be both a number	and a
       string, usually these macros will always	return TRUE and	calling	the
       "Sv*V" macros will do the appropriate conversion	of string to inte-
       ger/double or integer/double to string.

       If you really need to know if you have an integer, double, or string
       pointer in an SV, you can use the following three macros	instead:

	   SvIOKp(SV*)
	   SvNOKp(SV*)
	   SvPOKp(SV*)

       These will tell you if you truly	have an	integer, double, or string
       pointer stored in your SV.  The "p" stands for private.

       The are various ways in which the private and public flags may differ.
       For example, a tied SV may have a valid underlying value	in the IV slot
       (so SvIOKp is true), but	the data should	be accessed via	the FETCH rou-
       tine rather than	directly, so SvIOK is false. Another is	when numeric
       conversion has occured and precision has	been lost: only	the private
       flag is set on 'lossy' values. So when an NV is converted to an IV with
       loss, SvIOKp, SvNOKp and	SvNOK will be set, while SvIOK wont be.

       In general, though, it's	best to	use the	"Sv*V" macros.

       Working with AVs

       There are two ways to create and	load an	AV.  The first method creates
       an empty	AV:

	   AV*	newAV();

       The second method both creates the AV and initially populates it	with
       SVs:

	   AV*	av_make(I32 num, SV **ptr);

       The second argument points to an	array containing "num" "SV*"'s.	 Once
       the AV has been created,	the SVs	can be destroyed, if so	desired.

       Once the	AV has been created, the following operations are possible on
       AVs:

	   void	 av_push(AV*, SV*);
	   SV*	 av_pop(AV*);
	   SV*	 av_shift(AV*);
	   void	 av_unshift(AV*, I32 num);

       These should be familiar	operations, with the exception of
       "av_unshift".  This routine adds	"num" elements at the front of the
       array with the "undef" value.  You must then use	"av_store" (described
       below) to assign	values to these	new elements.

       Here are	some other functions:

	   I32	 av_len(AV*);
	   SV**	 av_fetch(AV*, I32 key,	I32 lval);
	   SV**	 av_store(AV*, I32 key,	SV* val);

       The "av_len" function returns the highest index value in	array (just
       like $#array in Perl).  If the array is empty, -1 is returned.  The
       "av_fetch" function returns the value at	index "key", but if "lval" is
       non-zero, then "av_fetch" will store an undef value at that index.  The
       "av_store" function stores the value "val" at index "key", and does not
       increment the reference count of	"val".	Thus the caller	is responsible
       for taking care of that,	and if "av_store" returns NULL,	the caller
       will have to decrement the reference count to avoid a memory leak.
       Note that "av_fetch" and	"av_store" both	return "SV**"'s, not "SV*"'s
       as their	return value.

	   void	 av_clear(AV*);
	   void	 av_undef(AV*);
	   void	 av_extend(AV*,	I32 key);

       The "av_clear" function deletes all the elements	in the AV* array, but
       does not	actually delete	the array itself.  The "av_undef" function
       will delete all the elements in the array plus the array	itself.	 The
       "av_extend" function extends the	array so that it contains at least
       "key+1" elements.  If "key+1" is	less than the currently	allocated
       length of the array, then nothing is done.

       If you know the name of an array	variable, you can get a	pointer	to its
       AV by using the following:

	   AV*	get_av("package::varname", FALSE);

       This returns NULL if the	variable does not exist.

       See "Understanding the Magic of Tied Hashes and Arrays" for more	infor-
       mation on how to	use the	array access functions on tied arrays.

       Working with HVs

       To create an HV,	you use	the following routine:

	   HV*	newHV();

       Once the	HV has been created, the following operations are possible on
       HVs:

	   SV**	 hv_store(HV*, const char* key,	U32 klen, SV* val, U32 hash);
	   SV**	 hv_fetch(HV*, const char* key,	U32 klen, I32 lval);

       The "klen" parameter is the length of the key being passed in (Note
       that you	cannot pass 0 in as a value of "klen" to tell Perl to measure
       the length of the key).	The "val" argument contains the	SV pointer to
       the scalar being	stored,	and "hash" is the precomputed hash value (zero
       if you want "hv_store" to calculate it for you).	 The "lval" parameter
       indicates whether this fetch is actually	a part of a store operation,
       in which	case a new undefined value will	be added to the	HV with	the
       supplied	key and	"hv_fetch" will	return as if the value had already
       existed.

       Remember	that "hv_store"	and "hv_fetch" return "SV**"'s and not just
       "SV*".  To access the scalar value, you must first dereference the
       return value.  However, you should check	to make	sure that the return
       value is	not NULL before	dereferencing it.

       These two functions check if a hash table entry exists, and deletes it.

	   bool	 hv_exists(HV*,	const char* key, U32 klen);
	   SV*	 hv_delete(HV*,	const char* key, U32 klen, I32 flags);

       If "flags" does not include the "G_DISCARD" flag	then "hv_delete" will
       create and return a mortal copy of the deleted value.

       And more	miscellaneous functions:

	   void	  hv_clear(HV*);
	   void	  hv_undef(HV*);

       Like their AV counterparts, "hv_clear" deletes all the entries in the
       hash table but does not actually	delete the hash	table.	The "hv_undef"
       deletes both the	entries	and the	hash table itself.

       Perl keeps the actual data in linked list of structures with a typedef
       of HE.  These contain the actual	key and	value pointers (plus extra
       administrative overhead).  The key is a string pointer; the value is an
       "SV*".  However,	once you have an "HE*",	to get the actual key and
       value, use the routines specified below.

	   I32	  hv_iterinit(HV*);
		   /* Prepares starting	point to traverse hash table */
	   HE*	  hv_iternext(HV*);
		   /* Get the next entry, and return a pointer to a
		      structure	that has both the key and value	*/
	   char*  hv_iterkey(HE* entry,	I32* retlen);
		   /* Get the key from an HE structure and also	return
		      the length of the	key string */
	   SV*	  hv_iterval(HV*, HE* entry);
		   /* Return an	SV pointer to the value	of the HE
		      structure	*/
	   SV*	  hv_iternextsv(HV*, char** key, I32* retlen);
		   /* This convenience routine combines	hv_iternext,
		      hv_iterkey, and hv_iterval.  The key and retlen
		      arguments	are return values for the key and its
		      length.  The value is returned in	the SV*	argument */

       If you know the name of a hash variable,	you can	get a pointer to its
       HV by using the following:

	   HV*	get_hv("package::varname", FALSE);

       This returns NULL if the	variable does not exist.

       The hash	algorithm is defined in	the "PERL_HASH(hash, key, klen)"
       macro:

	   hash	= 0;
	   while (klen--)
	       hash = (hash * 33) + *key++;
	   hash	= hash + (hash >> 5);		       /* after	5.6 */

       The last	step was added in version 5.6 to improve distribution of lower
       bits in the resulting hash value.

       See "Understanding the Magic of Tied Hashes and Arrays" for more	infor-
       mation on how to	use the	hash access functions on tied hashes.

       Hash API	Extensions

       Beginning with version 5.004, the following functions are also sup-
       ported:

	   HE*	   hv_fetch_ent	 (HV* tb, SV* key, I32 lval, U32 hash);
	   HE*	   hv_store_ent	 (HV* tb, SV* key, SV* val, U32	hash);

	   bool	   hv_exists_ent (HV* tb, SV* key, U32 hash);
	   SV*	   hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash);

	   SV*	   hv_iterkeysv	 (HE* entry);

       Note that these functions take "SV*" keys, which	simplifies writing of
       extension code that deals with hash structures.	These functions	also
       allow passing of	"SV*" keys to "tie" functions without forcing you to
       stringify the keys (unlike the previous set of functions).

       They also return	and accept whole hash entries ("HE*"), making their
       use more	efficient (since the hash number for a particular string
       doesn't have to be recomputed every time).  See perlapi for detailed
       descriptions.

       The following macros must always	be used	to access the contents of hash
       entries.	 Note that the arguments to these macros must be simple	vari-
       ables, since they may get evaluated more	than once.  See	perlapi	for
       detailed	descriptions of	these macros.

	   HePV(HE* he,	STRLEN len)
	   HeVAL(HE* he)
	   HeHASH(HE* he)
	   HeSVKEY(HE* he)
	   HeSVKEY_force(HE* he)
	   HeSVKEY_set(HE* he, SV* sv)

       These two lower level macros are	defined, but must only be used when
       dealing with keys that are not "SV*"s:

	   HeKEY(HE* he)
	   HeKLEN(HE* he)

       Note that both "hv_store" and "hv_store_ent" do not increment the ref-
       erence count of the stored "val", which is the caller's responsibility.
       If these	functions return a NULL	value, the caller will usually have to
       decrement the reference count of	"val" to avoid a memory	leak.

       References

       References are a	special	type of	scalar that point to other data	types
       (including references).

       To create a reference, use either of the	following functions:

	   SV* newRV_inc((SV*) thing);
	   SV* newRV_noinc((SV*) thing);

       The "thing" argument can	be any of an "SV*", "AV*", or "HV*".  The
       functions are identical except that "newRV_inc" increments the refer-
       ence count of the "thing", while	"newRV_noinc" does not.	 For histori-
       cal reasons, "newRV" is a synonym for "newRV_inc".

       Once you	have a reference, you can use the following macro to derefer-
       ence the	reference:

	   SvRV(SV*)

       then call the appropriate routines, casting the returned	"SV*" to
       either an "AV*" or "HV*", if required.

       To determine if an SV is	a reference, you can use the following macro:

	   SvROK(SV*)

       To discover what	type of	value the reference refers to, use the follow-
       ing macro and then check	the return value.

	   SvTYPE(SvRV(SV*))

       The most	useful types that will be returned are:

	   SVt_IV    Scalar
	   SVt_NV    Scalar
	   SVt_PV    Scalar
	   SVt_RV    Scalar
	   SVt_PVAV  Array
	   SVt_PVHV  Hash
	   SVt_PVCV  Code
	   SVt_PVGV  Glob (possible a file handle)
	   SVt_PVMG  Blessed or	Magical	Scalar

	   See the sv.h	header file for	more details.

       Blessed References and Class Objects

       References are also used	to support object-oriented programming.	 In
       the OO lexicon, an object is simply a reference that has	been blessed
       into a package (or class).  Once	blessed, the programmer	may now	use
       the reference to	access the various methods in the class.

       A reference can be blessed into a package with the following function:

	   SV* sv_bless(SV* sv,	HV* stash);

       The "sv"	argument must be a reference.  The "stash" argument specifies
       which class the reference will belong to.  See "Stashes and Globs" for
       information on converting class names into stashes.

       /* Still	under construction */

       Upgrades	rv to reference	if not already one.  Creates new SV for	rv to
       point to.  If "classname" is non-null, the SV is	blessed	into the spec-
       ified class.  SV	is returned.

	       SV* newSVrv(SV* rv, const char* classname);

       Copies integer, unsigned	integer	or double into an SV whose reference
       is "rv".	 SV is blessed if "classname" is non-null.

	       SV* sv_setref_iv(SV* rv,	const char* classname, IV iv);
	       SV* sv_setref_uv(SV* rv,	const char* classname, UV uv);
	       SV* sv_setref_nv(SV* rv,	const char* classname, NV iv);

       Copies the pointer value	(the address, not the string!) into an SV
       whose reference is rv.  SV is blessed if	"classname" is non-null.

	       SV* sv_setref_pv(SV* rv,	const char* classname, PV iv);

       Copies string into an SV	whose reference	is "rv".  Set length to	0 to
       let Perl	calculate the string length.  SV is blessed if "classname" is
       non-null.

	       SV* sv_setref_pvn(SV* rv, const char* classname,	PV iv, STRLEN length);

       Tests whether the SV is blessed into the	specified class.  It does not
       check inheritance relationships.

	       int  sv_isa(SV* sv, const char* name);

       Tests whether the SV is a reference to a	blessed	object.

	       int  sv_isobject(SV* sv);

       Tests whether the SV is derived from the	specified class. SV can	be
       either a	reference to a blessed object or a string containing a class
       name. This is the function implementing the "UNIVERSAL::isa" function-
       ality.

	       bool sv_derived_from(SV*	sv, const char*	name);

       To check	if you've got an object	derived	from a specific	class you have
       to write:

	       if (sv_isobject(sv) && sv_derived_from(sv, class)) { ...	}

       Creating	New Variables

       To create a new Perl variable with an undef value which can be accessed
       from your Perl script, use the following	routines, depending on the
       variable	type.

	   SV*	get_sv("package::varname", TRUE);
	   AV*	get_av("package::varname", TRUE);
	   HV*	get_hv("package::varname", TRUE);

       Notice the use of TRUE as the second parameter.	The new	variable can
       now be set, using the routines appropriate to the data type.

       There are additional macros whose values	may be bitwise OR'ed with the
       "TRUE" argument to enable certain extra features.  Those	bits are:

       GV_ADDMULTI
	   Marks the variable as multiply defined, thus	preventing the:

	     Name <varname> used only once: possible typo

	   warning.

       GV_ADDWARN
	   Issues the warning:

	     Had to create <varname> unexpectedly

	   if the variable did not exist before	the function was called.

       If you do not specify a package name, the variable is created in	the
       current package.

       Reference Counts	and Mortality

       Perl uses a reference count-driven garbage collection mechanism.	SVs,
       AVs, or HVs (xV for short in the	following) start their life with a
       reference count of 1.  If the reference count of	an xV ever drops to 0,
       then it will be destroyed and its memory	made available for reuse.

       This normally doesn't happen at the Perl	level unless a variable	is
       undef'ed	or the last variable holding a reference to it is changed or
       overwritten.  At	the internal level, however, reference counts can be
       manipulated with	the following macros:

	   int SvREFCNT(SV* sv);
	   SV* SvREFCNT_inc(SV*	sv);
	   void	SvREFCNT_dec(SV* sv);

       However,	there is one other function which manipulates the reference
       count of	its argument.  The "newRV_inc" function, you will recall, cre-
       ates a reference	to the specified argument.  As a side effect, it
       increments the argument's reference count.  If this is not what you
       want, use "newRV_noinc" instead.

       For example, imagine you	want to	return a reference from	an XSUB	func-
       tion.  Inside the XSUB routine, you create an SV	which initially	has a
       reference count of one.	Then you call "newRV_inc", passing it the
       just-created SV.	 This returns the reference as a new SV, but the ref-
       erence count of the SV you passed to "newRV_inc"	has been incremented
       to two.	Now you	return the reference from the XSUB routine and forget
       about the SV.  But Perl hasn't!	Whenever the returned reference	is
       destroyed, the reference	count of the original SV is decreased to one
       and nothing happens.  The SV will hang around without any way to	access
       it until	Perl itself terminates.	 This is a memory leak.

       The correct procedure, then, is to use "newRV_noinc" instead of
       "newRV_inc".  Then, if and when the last	reference is destroyed,	the
       reference count of the SV will go to zero and it	will be	destroyed,
       stopping	any memory leak.

       There are some convenience functions available that can help with the
       destruction of xVs.  These functions introduce the concept of "mortal-
       ity".  An xV that is mortal has had its reference count marked to be
       decremented, but	not actually decremented, until	"a short time later".
       Generally the term "short time later" means a single Perl statement,
       such as a call to an XSUB function.  The	actual determinant for when
       mortal xVs have their reference count decremented depends on two
       macros, SAVETMPS	and FREETMPS.  See perlcall and	perlxs for more
       details on these	macros.

       "Mortalization" then is at its simplest a deferred "SvREFCNT_dec".
       However,	if you mortalize a variable twice, the reference count will
       later be	decremented twice.

       "Mortal"	SVs are	mainly used for	SVs that are placed on perl's stack.
       For example an SV which is created just to pass a number	to a called
       sub is made mortal to have it cleaned up	automatically when stack is
       popped.	Similarly results returned by XSUBs (which go in the stack)
       are often made mortal.

       To create a mortal variable, use	the functions:

	   SV*	sv_newmortal()
	   SV*	sv_2mortal(SV*)
	   SV*	sv_mortalcopy(SV*)

       The first call creates a	mortal SV (with	no value), the second converts
       an existing SV to a mortal SV (and thus defers a	call to	"SvRE-
       FCNT_dec"), and the third creates a mortal copy of an existing SV.
       Because "sv_newmortal" gives the	new SV no value,it must	normally be
       given one via "sv_setpv", "sv_setiv", etc. :

	   SV *tmp = sv_newmortal();
	   sv_setiv(tmp, an_integer);

       As that is multiple C statements	it is quite common so see this idiom
       instead:

	   SV *tmp = sv_2mortal(newSViv(an_integer));

       You should be careful about creating mortal variables.  Strange things
       can happen if you make the same value mortal within multiple contexts,
       or if you make a	variable mortal	multiple times.	Thinking of "Mortal-
       ization"	as deferred "SvREFCNT_dec" should help to minimize such	prob-
       lems.  For example if you are passing an	SV which you know has high
       enough REFCNT to	survive	its use	on the stack you need not do any mor-
       talization.  If you are not sure	then doing an "SvREFCNT_inc" and
       "sv_2mortal", or	making a "sv_mortalcopy" is safer.

       The mortal routines are not just	for SVs	-- AVs and HVs can be made
       mortal by passing their address (type-casted to "SV*") to the "sv_2mor-
       tal" or "sv_mortalcopy" routines.

       Stashes and Globs

       A "stash" is a hash that	contains all of	the different objects that are
       contained within	a package.  Each key of	the stash is a symbol name
       (shared by all the different types of objects that have the same	name),
       and each	value in the hash table	is a GV	(Glob Value).  This GV in turn
       contains	references to the various objects of that name,	including (but
       not limited to) the following:

	   Scalar Value
	   Array Value
	   Hash	Value
	   I/O Handle
	   Format
	   Subroutine

       There is	a single stash called "PL_defstash" that holds the items that
       exist in	the "main" package.  To	get at the items in other packages,
       append the string "::" to the package name.  The	items in the "Foo"
       package are in the stash	"Foo::"	in PL_defstash.	 The items in the
       "Bar::Baz" package are in the stash "Baz::" in "Bar::"'s	stash.

       To get the stash	pointer	for a particular package, use the function:

	   HV*	gv_stashpv(const char* name, I32 create)
	   HV*	gv_stashsv(SV*,	I32 create)

       The first function takes	a literal string, the second uses the string
       stored in the SV.  Remember that	a stash	is just	a hash table, so you
       get back	an "HV*".  The "create"	flag will create a new package if it
       is set.

       The name	that "gv_stash*v" wants	is the name of the package whose sym-
       bol table you want.  The	default	package	is called "main".  If you have
       multiply	nested packages, pass their names to "gv_stash*v", separated
       by "::" as in the Perl language itself.

       Alternately, if you have	an SV that is a	blessed	reference, you can
       find out	the stash pointer by using:

	   HV*	SvSTASH(SvRV(SV*));

       then use	the following to get the package name itself:

	   char*  HvNAME(HV* stash);

       If you need to bless or re-bless	an object you can use the following
       function:

	   SV*	sv_bless(SV*, HV* stash)

       where the first argument, an "SV*", must	be a reference,	and the	second
       argument	is a stash.  The returned "SV*"	can now	be used	in the same
       way as any other	SV.

       For more	information on references and blessings, consult perlref.

       Double-Typed SVs

       Scalar variables	normally contain only one type of value, an integer,
       double, pointer,	or reference.  Perl will automatically convert the
       actual scalar data from the stored type into the	requested type.

       Some scalar variables contain more than one type	of scalar data.	 For
       example,	the variable $!	contains either	the numeric value of "errno"
       or its string equivalent	from either "strerror" or "sys_errlist[]".

       To force	multiple data values into an SV, you must do two things: use
       the "sv_set*v" routines to add the additional scalar type, then set a
       flag so that Perl will believe it contains more than one	type of	data.
       The four	macros to set the flags	are:

	       SvIOK_on
	       SvNOK_on
	       SvPOK_on
	       SvROK_on

       The particular macro you	must use depends on which "sv_set*v" routine
       you called first.  This is because every	"sv_set*v" routine turns on
       only the	bit for	the particular type of data being set, and turns off
       all the rest.

       For example, to create a	new Perl variable called "dberror" that	con-
       tains both the numeric and descriptive string error values, you could
       use the following code:

	   extern int  dberror;
	   extern char *dberror_list;

	   SV* sv = get_sv("dberror", TRUE);
	   sv_setiv(sv,	(IV) dberror);
	   sv_setpv(sv,	dberror_list[dberror]);
	   SvIOK_on(sv);

       If the order of "sv_setiv" and "sv_setpv" had been reversed, then the
       macro "SvPOK_on"	would need to be called	instead	of "SvIOK_on".

       Magic Variables

       [This section still under construction.	Ignore everything here.	 Post
       no bills.  Everything not permitted is forbidden.]

       Any SV may be magical, that is, it has special features that a normal
       SV does not have.  These	features are stored in the SV structure	in a
       linked list of "struct magic"'s,	typedef'ed to "MAGIC".

	   struct magic	{
	       MAGIC*	   mg_moremagic;
	       MGVTBL*	   mg_virtual;
	       U16	   mg_private;
	       char	   mg_type;
	       U8	   mg_flags;
	       SV*	   mg_obj;
	       char*	   mg_ptr;
	       I32	   mg_len;
	   };

       Note this is current as of patchlevel 0,	and could change at any	time.

       Assigning Magic

       Perl adds magic to an SV	using the sv_magic function:

	   void	sv_magic(SV* sv, SV* obj, int how, const char* name, I32 namlen);

       The "sv"	argument is a pointer to the SV	that is	to acquire a new magi-
       cal feature.

       If "sv" is not already magical, Perl uses the "SvUPGRADE" macro to con-
       vert "sv" to type "SVt_PVMG". Perl then continues by adding new magic
       to the beginning	of the linked list of magical features.	 Any prior
       entry of	the same type of magic is deleted.  Note that this can be
       overridden, and multiple	instances of the same type of magic can	be
       associated with an SV.

       The "name" and "namlen" arguments are used to associate a string	with
       the magic, typically the	name of	a variable. "namlen" is	stored in the
       "mg_len"	field and if "name" is non-null	and "namlen" >=	0 a malloc'd
       copy of the name	is stored in "mg_ptr" field.

       The sv_magic function uses "how"	to determine which, if any, predefined
       "Magic Virtual Table" should be assigned	to the "mg_virtual" field.
       See the "Magic Virtual Table" section below.  The "how" argument	is
       also stored in the "mg_type" field. The value of	"how" should be	chosen
       from the	set of macros "PERL_MAGIC_foo" found perl.h. Note that before
       these macros were added,	Perl internals used to directly	use character
       literals, so you	may occasionally come across old code or documentation
       referring to 'U'	magic rather than "PERL_MAGIC_uvar" for	example.

       The "obj" argument is stored in the "mg_obj" field of the "MAGIC"
       structure.  If it is not	the same as the	"sv" argument, the reference
       count of	the "obj" object is incremented.  If it	is the same, or	if the
       "how" argument is "PERL_MAGIC_arylen", or if it is a NULL pointer, then
       "obj" is	merely stored, without the reference count being incremented.

       There is	also a function	to add magic to	an "HV":

	   void	hv_magic(HV *hv, GV *gv, int how);

       This simply calls "sv_magic" and	coerces	the "gv" argument into an
       "SV".

       To remove the magic from	an SV, call the	function sv_unmagic:

	   void	sv_unmagic(SV *sv, int type);

       The "type" argument should be equal to the "how"	value when the "SV"
       was initially made magical.

       Magic Virtual Tables

       The "mg_virtual"	field in the "MAGIC" structure is a pointer to an
       "MGVTBL", which is a structure of function pointers and stands for
       "Magic Virtual Table" to	handle the various operations that might be
       applied to that variable.

       The "MGVTBL" has	five pointers to the following routine types:

	   int	(*svt_get)(SV* sv, MAGIC* mg);
	   int	(*svt_set)(SV* sv, MAGIC* mg);
	   U32	(*svt_len)(SV* sv, MAGIC* mg);
	   int	(*svt_clear)(SV* sv, MAGIC* mg);
	   int	(*svt_free)(SV*	sv, MAGIC* mg);

       This MGVTBL structure is	set at compile-time in "perl.h"	and there are
       currently 19 types (or 21 with overloading turned on).  These different
       structures contain pointers to various routines that perform additional
       actions depending on which function is being called.

	   Function pointer    Action taken
	   ----------------    ------------
	   svt_get	       Do something before the value of	the SV is retrieved.
	   svt_set	       Do something after the SV is assigned a value.
	   svt_len	       Report on the SV's length.
	   svt_clear	       Clear something the SV represents.
	   svt_free	       Free any	extra storage associated with the SV.

       For instance, the MGVTBL	structure called "vtbl_sv" (which corresponds
       to an "mg_type" of "PERL_MAGIC_sv") contains:

	   { magic_get,	magic_set, magic_len, 0, 0 }

       Thus, when an SV	is determined to be magical and	of type
       "PERL_MAGIC_sv",	if a get operation is being performed, the routine
       "magic_get" is called.  All the various routines	for the	various	magi-
       cal types begin with "magic_".  NOTE: the magic routines	are not	con-
       sidered part of the Perl	API, and may not be exported by	the Perl
       library.

       The current kinds of Magic Virtual Tables are:

	   mg_type
	   (old-style char and macro)	MGVTBL	       Type of magic
	   --------------------------	------	       ----------------------------
	   \0 PERL_MAGIC_sv		vtbl_sv	       Special scalar variable
	   A  PERL_MAGIC_overload	vtbl_amagic    %OVERLOAD hash
	   a  PERL_MAGIC_overload_elem	vtbl_amagicelem	%OVERLOAD hash element
	   c  PERL_MAGIC_overload_table	(none)	       Holds overload table (AMT)
						       on stash
	   B  PERL_MAGIC_bm		vtbl_bm	       Boyer-Moore (fast string	search)
	   D  PERL_MAGIC_regdata	vtbl_regdata   Regex match position data
						       (@+ and @- vars)
	   d  PERL_MAGIC_regdatum	vtbl_regdatum  Regex match position data
						       element
	   E  PERL_MAGIC_env		vtbl_env       %ENV hash
	   e  PERL_MAGIC_envelem	vtbl_envelem   %ENV hash element
	   f  PERL_MAGIC_fm		vtbl_fm	       Formline	('compiled' format)
	   g  PERL_MAGIC_regex_global	vtbl_mglob     m//g target / study()ed string
	   I  PERL_MAGIC_isa		vtbl_isa       @ISA array
	   i  PERL_MAGIC_isaelem	vtbl_isaelem   @ISA array element
	   k  PERL_MAGIC_nkeys		vtbl_nkeys     scalar(keys()) lvalue
	   L  PERL_MAGIC_dbfile		(none)	       Debugger	%_<filename
	   l  PERL_MAGIC_dbline		vtbl_dbline    Debugger	%_<filename element
	   m  PERL_MAGIC_mutex		vtbl_mutex     ???
	   o  PERL_MAGIC_collxfrm	vtbl_collxfrm  Locale collate transformation
	   P  PERL_MAGIC_tied		vtbl_pack      Tied array or hash
	   p  PERL_MAGIC_tiedelem	vtbl_packelem  Tied array or hash element
	   q  PERL_MAGIC_tiedscalar	vtbl_packelem  Tied scalar or handle
	   r  PERL_MAGIC_qr		vtbl_qr	       precompiled qr//	regex
	   S  PERL_MAGIC_sig		vtbl_sig       %SIG hash
	   s  PERL_MAGIC_sigelem	vtbl_sigelem   %SIG hash element
	   t  PERL_MAGIC_taint		vtbl_taint     Taintedness
	   U  PERL_MAGIC_uvar		vtbl_uvar      Available for use by extensions
	   v  PERL_MAGIC_vec		vtbl_vec       vec() lvalue
	   x  PERL_MAGIC_substr		vtbl_substr    substr()	lvalue
	   y  PERL_MAGIC_defelem	vtbl_defelem   Shadow "foreach"	iterator
						       variable	/ smart	parameter
						       vivification
	   *  PERL_MAGIC_glob		vtbl_glob      GV (typeglob)
	   #  PERL_MAGIC_arylen		vtbl_arylen    Array length ($#ary)
	   .  PERL_MAGIC_pos		vtbl_pos       pos() lvalue
	   <  PERL_MAGIC_backref	vtbl_backref   ???
	   ~  PERL_MAGIC_ext		(none)	       Available for use by extensions

       When an uppercase and lowercase letter both exist in the	table, then
       the uppercase letter is used to represent some kind of composite	type
       (a list or a hash), and the lowercase letter is used to represent an
       element of that composite type. Some internals code makes use of	this
       case relationship.

       The "PERL_MAGIC_ext" and	"PERL_MAGIC_uvar" magic	types are defined
       specifically for	use by extensions and will not be used by perl itself.
       Extensions can use "PERL_MAGIC_ext" magic to 'attach' private informa-
       tion to variables (typically objects).  This is especially useful
       because there is	no way for normal perl code to corrupt this private
       information (unlike using extra elements	of a hash object).

       Similarly, "PERL_MAGIC_uvar" magic can be used much like	tie() to call
       a C function any	time a scalar's	value is used or changed.  The
       "MAGIC"'s "mg_ptr" field	points to a "ufuncs" structure:

	   struct ufuncs {
	       I32 (*uf_val)(pTHX_ IV, SV*);
	       I32 (*uf_set)(pTHX_ IV, SV*);
	       IV uf_index;
	   };

       When the	SV is read from	or written to, the "uf_val" or "uf_set"	func-
       tion will be called with	"uf_index" as the first	arg and	a pointer to
       the SV as the second.  A	simple example of how to add "PERL_MAGIC_uvar"
       magic is	shown below.  Note that	the ufuncs structure is	copied by
       sv_magic, so you	can safely allocate it on the stack.

	   void
	   Umagic(sv)
	       SV *sv;
	   PREINIT:
	       struct ufuncs uf;
	   CODE:
	       uf.uf_val   = &my_get_fn;
	       uf.uf_set   = &my_set_fn;
	       uf.uf_index = 0;
	       sv_magic(sv, 0, PERL_MAGIC_uvar,	(char*)&uf, sizeof(uf));

       Note that because multiple extensions may be using "PERL_MAGIC_ext" or
       "PERL_MAGIC_uvar" magic,	it is important	for extensions to take extra
       care to avoid conflict.	Typically only using the magic on objects
       blessed into the	same class as the extension is sufficient.  For
       "PERL_MAGIC_ext"	magic, it may also be appropriate to add an I32	'sig-
       nature' at the top of the private data area and check that.

       Also note that the "sv_set*()" and "sv_cat*()" functions	described ear-
       lier do not invoke 'set'	magic on their targets.	 This must be done by
       the user	either by calling the "SvSETMAGIC()" macro after calling these
       functions, or by	using one of the "sv_set*_mg()"	or "sv_cat*_mg()"
       functions.  Similarly, generic C	code must call the "SvGETMAGIC()"
       macro to	invoke any 'get' magic if they use an SV obtained from exter-
       nal sources in functions	that don't handle magic.  See perlapi for a
       description of these functions.	For example, calls to the "sv_cat*()"
       functions typically need	to be followed by "SvSETMAGIC()", but they
       don't need a prior "SvGETMAGIC()" since their implementation handles
       'get' magic.

       Finding Magic

	   MAGIC* mg_find(SV*, int type); /* Finds the magic pointer of	that type */

       This routine returns a pointer to the "MAGIC" structure stored in the
       SV.  If the SV does not have that magical feature, "NULL" is returned.
       Also, if	the SV is not of type SVt_PVMG,	Perl may core dump.

	   int mg_copy(SV* sv, SV* nsv,	const char* key, STRLEN	klen);

       This routine checks to see what types of	magic "sv" has.	 If the
       mg_type field is	an uppercase letter, then the mg_obj is	copied to
       "nsv", but the mg_type field is changed to be the lowercase letter.

       Understanding the Magic of Tied Hashes and Arrays

       Tied hashes and arrays are magical beasts of the	"PERL_MAGIC_tied"
       magic type.

       WARNING:	As of the 5.004	release, proper	usage of the array and hash
       access functions	requires understanding a few caveats.  Some of these
       caveats are actually considered bugs in the API,	to be fixed in later
       releases, and are bracketed with	[MAYCHANGE] below. If you find your-
       self actually applying such information in this section,	be aware that
       the behavior may	change in the future, umm, without warning.

       The perl	tie function associates	a variable with	an object that imple-
       ments the various GET, SET, etc methods.	 To perform the	equivalent of
       the perl	tie function from an XSUB, you must mimic this behaviour.  The
       code below carries out the necessary steps - firstly it creates a new
       hash, and then creates a	second hash which it blesses into the class
       which will implement the	tie methods. Lastly it ties the	two hashes
       together, and returns a reference to the	new tied hash.	Note that the
       code below does NOT call	the TIEHASH method in the MyTie	class -	see
       "Calling	Perl Routines from within C Programs" for details on how to do
       this.

	   SV*
	   mytie()
	   PREINIT:
	       HV *hash;
	       HV *stash;
	       SV *tie;
	   CODE:
	       hash = newHV();
	       tie = newRV_noinc((SV*)newHV());
	       stash = gv_stashpv("MyTie", TRUE);
	       sv_bless(tie, stash);
	       hv_magic(hash, (GV*)tie,	PERL_MAGIC_tied);
	       RETVAL =	newRV_noinc(hash);
	   OUTPUT:
	       RETVAL

       The "av_store" function,	when given a tied array	argument, merely
       copies the magic	of the array onto the value to be "stored", using
       "mg_copy".  It may also return NULL, indicating that the	value did not
       actually	need to	be stored in the array.	 [MAYCHANGE] After a call to
       "av_store" on a tied array, the caller will usually need	to call
       "mg_set(val)" to	actually invoke	the perl level "STORE" method on the
       TIEARRAY	object.	 If "av_store" did return NULL,	a call to "SvRE-
       FCNT_dec(val)" will also	be usually necessary to	avoid a	memory leak.
       [/MAYCHANGE]

       The previous paragraph is applicable verbatim to	tied hash access using
       the "hv_store" and "hv_store_ent" functions as well.

       "av_fetch" and the corresponding	hash functions "hv_fetch" and
       "hv_fetch_ent" actually return an undefined mortal value	whose magic
       has been	initialized using "mg_copy".  Note the value so	returned does
       not need	to be deallocated, as it is already mortal.  [MAYCHANGE] But
       you will	need to	call "mg_get()"	on the returned	value in order to
       actually	invoke the perl	level "FETCH" method on	the underlying TIE
       object.	Similarly, you may also	call "mg_set()"	on the return value
       after possibly assigning	a suitable value to it using "sv_setsv",
       which will invoke the "STORE" method on the TIE object. [/MAYCHANGE]

       [MAYCHANGE] In other words, the array or	hash fetch/store functions
       don't really fetch and store actual values in the case of tied arrays
       and hashes.  They merely	call "mg_copy" to attach magic to the values
       that were meant to be "stored" or "fetched".  Later calls to "mg_get"
       and "mg_set" actually do	the job	of invoking the	TIE methods on the
       underlying objects.  Thus the magic mechanism currently implements a
       kind of lazy access to arrays and hashes.

       Currently (as of	perl version 5.004), use of the	hash and array access
       functions requires the user to be aware of whether they are operating
       on "normal" hashes and arrays, or on their tied variants.  The API may
       be changed to provide more transparent access to	both tied and normal
       data types in future versions.  [/MAYCHANGE]

       You would do well to understand that the	TIEARRAY and TIEHASH inter-
       faces are mere sugar to invoke some perl	method calls while using the
       uniform hash and	array syntax.  The use of this sugar imposes some
       overhead	(typically about two to	four extra opcodes per FETCH/STORE
       operation, in addition to the creation of all the mortal	variables
       required	to invoke the methods).	 This overhead will be comparatively
       small if	the TIE	methods	are themselves substantial, but	if they	are
       only a few statements long, the overhead	will not be insignificant.

       Localizing changes

       Perl has	a very handy construction

	 {
	   local $var =	2;
	   ...
	 }

       This construction is approximately equivalent to

	 {
	   my $oldvar =	$var;
	   $var	= 2;
	   ...
	   $var	= $oldvar;
	 }

       The biggest difference is that the first	construction would reinstate
       the initial value of $var, irrespective of how control exits the	block:
       "goto", "return", "die"/"eval", etc. It is a little bit more efficient
       as well.

       There is	a way to achieve a similar task	from C via Perl	API: create a
       pseudo-block, and arrange for some changes to be	automatically undone
       at the end of it, either	explicit, or via a non-local exit (via die()).
       A block-like construct is created by a pair of "ENTER"/"LEAVE" macros
       (see "Returning a Scalar" in perlcall).	Such a construct may be	cre-
       ated specially for some important localized task, or an existing	one
       (like boundaries	of enclosing Perl subroutine/block, or an existing
       pair for	freeing	TMPs) may be used. (In the second case the overhead of
       additional localization must be almost negligible.) Note	that any XSUB
       is automatically	enclosed in an "ENTER"/"LEAVE" pair.

       Inside such a pseudo-block the following	service	is available:

       "SAVEINT(int i)"
       "SAVEIV(IV i)"
       "SAVEI32(I32 i)"
       "SAVELONG(long i)"
	   These macros	arrange	things to restore the value of integer vari-
	   able	"i" at the end of enclosing pseudo-block.

       SAVESPTR(s)
       SAVEPPTR(p)
	   These macros	arrange	things to restore the value of pointers	"s"
	   and "p". "s"	must be	a pointer of a type which survives conversion
	   to "SV*" and	back, "p" should be able to survive conversion to
	   "char*" and back.

       "SAVEFREESV(SV *sv)"
	   The refcount	of "sv"	would be decremented at	the end	of pseudo-
	   block.  This	is similar to "sv_2mortal" in that it is also a	mecha-
	   nism	for doing a delayed "SvREFCNT_dec".  However, while "sv_2mor-
	   tal"	extends	the lifetime of	"sv" until the beginning of the	next
	   statement, "SAVEFREESV" extends it until the	end of the enclosing
	   scope.  These lifetimes can be wildly different.

	   Also	compare	"SAVEMORTALIZESV".

       "SAVEMORTALIZESV(SV *sv)"
	   Just	like "SAVEFREESV", but mortalizes "sv" at the end of the cur-
	   rent	scope instead of decrementing its reference count.  This usu-
	   ally	has the	effect of keeping "sv" alive until the statement that
	   called the currently	live scope has finished	executing.

       "SAVEFREEOP(OP *op)"
	   The "OP *" is op_free()ed at	the end	of pseudo-block.

       SAVEFREEPV(p)
	   The chunk of	memory which is	pointed	to by "p" is Safefree()ed at
	   the end of pseudo-block.

       "SAVECLEARSV(SV *sv)"
	   Clears a slot in the	current	scratchpad which corresponds to	"sv"
	   at the end of pseudo-block.

       "SAVEDELETE(HV *hv, char	*key, I32 length)"
	   The key "key" of "hv" is deleted at the end of pseudo-block.	The
	   string pointed to by	"key" is Safefree()ed.	If one has a key in
	   short-lived storage,	the corresponding string may be	reallocated
	   like	this:

	     SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));

       "SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)"
	   At the end of pseudo-block the function "f" is called with the only
	   argument "p".

       "SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)"
	   At the end of pseudo-block the function "f" is called with the
	   implicit context argument (if any), and "p".

       "SAVESTACK_POS()"
	   The current offset on the Perl internal stack (cf. "SP") is
	   restored at the end of pseudo-block.

       The following API list contains functions, thus one needs to provide
       pointers	to the modifiable data explicitly (either C pointers, or Perl-
       ish "GV *"s).  Where the	above macros take "int", a similar function
       takes "int *".

       "SV* save_scalar(GV *gv)"
	   Equivalent to Perl code "local $gv".

       "AV* save_ary(GV	*gv)"
       "HV* save_hash(GV *gv)"
	   Similar to "save_scalar", but localize @gv and %gv.

       "void save_item(SV *item)"
	   Duplicates the current value	of "SV", on the	exit from the current
	   "ENTER"/"LEAVE" pseudo-block	will restore the value of "SV" using
	   the stored value.

       "void save_list(SV **sarg, I32 maxsarg)"
	   A variant of	"save_item" which takes	multiple arguments via an
	   array "sarg"	of "SV*" of length "maxsarg".

       "SV* save_svref(SV **sptr)"
	   Similar to "save_scalar", but will reinstate	an "SV *".

       "void save_aptr(AV **aptr)"
       "void save_hptr(HV **hptr)"
	   Similar to "save_svref", but	localize "AV *"	and "HV	*".

       The "Alias" module implements localization of the basic types within
       the caller's scope.  People who are interested in how to	localize
       things in the containing	scope should take a look there too.

Subroutines
       XSUBs and the Argument Stack

       The XSUB	mechanism is a simple way for Perl programs to access C	sub-
       routines.  An XSUB routine will have a stack that contains the argu-
       ments from the Perl program, and	a way to map from the Perl data	struc-
       tures to	a C equivalent.

       The stack arguments are accessible through the ST(n) macro, which
       returns the "n"'th stack	argument.  Argument 0 is the first argument
       passed in the Perl subroutine call.  These arguments are	"SV*", and can
       be used anywhere	an "SV*" is used.

       Most of the time, output	from the C routine can be handled through use
       of the RETVAL and OUTPUT	directives.  However, there are	some cases
       where the argument stack	is not already long enough to handle all the
       return values.  An example is the POSIX tzname()	call, which takes no
       arguments, but returns two, the local time zone's standard and summer
       time abbreviations.

       To handle this situation, the PPCODE directive is used and the stack is
       extended	using the macro:

	   EXTEND(SP, num);

       where "SP" is the macro that represents the local copy of the stack
       pointer,	and "num" is the number	of elements the	stack should be
       extended	by.

       Now that	there is room on the stack, values can be pushed on it using
       "PUSHs" macro. The values pushed	will often need	to be "mortal" (See
       "Reference Counts and Mortality").

	   PUSHs(sv_2mortal(newSViv(an_integer)))
	   PUSHs(sv_2mortal(newSVpv("Some String",0)))
	   PUSHs(sv_2mortal(newSVnv(3.141592)))

       And now the Perl	program	calling	"tzname", the two values will be
       assigned	as in:

	   ($standard_abbrev, $summer_abbrev) =	POSIX::tzname;

       An alternate (and possibly simpler) method to pushing values on the
       stack is	to use the macro:

	   XPUSHs(SV*)

       This macro automatically	adjust the stack for you, if needed.  Thus,
       you do not need to call "EXTEND"	to extend the stack.

       Despite their suggestions in earlier versions of	this document the
       macros "PUSHi", "PUSHn" and "PUSHp" are not suited to XSUBs which
       return multiple results,	see "Putting a C value on Perl stack".

       For more	information, consult perlxs and	perlxstut.

       Calling Perl Routines from within C Programs

       There are four routines that can	be used	to call	a Perl subroutine from
       within a	C program.  These four are:

	   I32	call_sv(SV*, I32);
	   I32	call_pv(const char*, I32);
	   I32	call_method(const char*, I32);
	   I32	call_argv(const	char*, I32, register char**);

       The routine most	often used is "call_sv".  The "SV*" argument contains
       either the name of the Perl subroutine to be called, or a reference to
       the subroutine.	The second argument consists of	flags that control the
       context in which	the subroutine is called, whether or not the subrou-
       tine is being passed arguments, how errors should be trapped, and how
       to treat	return values.

       All four	routines return	the number of arguments	that the subroutine
       returned	on the Perl stack.

       These routines used to be called	"perl_call_sv",	etc., before Perl
       v5.6.0, but those names are now deprecated; macros of the same name are
       provided	for compatibility.

       When using any of these routines	(except	"call_argv"), the programmer
       must manipulate the Perl	stack.	These include the following macros and
       functions:

	   dSP
	   SP
	   PUSHMARK()
	   PUTBACK
	   SPAGAIN
	   ENTER
	   SAVETMPS
	   FREETMPS
	   LEAVE
	   XPUSH*()
	   POP*()

       For a detailed description of calling conventions from C	to Perl, con-
       sult perlcall.

       Memory Allocation

       All memory meant	to be used with	the Perl API functions should be
       manipulated using the macros described in this section.	The macros
       provide the necessary transparency between differences in the actual
       malloc implementation that is used within perl.

       It is suggested that you	enable the version of malloc that is distrib-
       uted with Perl.	It keeps pools of various sizes	of unallocated memory
       in order	to satisfy allocation requests more quickly.  However, on some
       platforms, it may cause spurious	malloc or free errors.

	   New(x, pointer, number, type);
	   Newc(x, pointer, number, type, cast);
	   Newz(x, pointer, number, type);

       These three macros are used to initially	allocate memory.

       The first argument "x" was a "magic cookie" that	was used to keep track
       of who called the macro,	to help	when debugging memory problems.	 How-
       ever, the current code makes no use of this feature (most Perl develop-
       ers now use run-time memory checkers), so this argument can be any num-
       ber.

       The second argument "pointer" should be the name	of a variable that
       will point to the newly allocated memory.

       The third and fourth arguments "number" and "type" specify how many of
       the specified type of data structure should be allocated.  The argument
       "type" is passed	to "sizeof".  The final	argument to "Newc", "cast",
       should be used if the "pointer" argument	is different from the "type"
       argument.

       Unlike the "New"	and "Newc" macros, the "Newz" macro calls "memzero" to
       zero out	all the	newly allocated	memory.

	   Renew(pointer, number, type);
	   Renewc(pointer, number, type, cast);
	   Safefree(pointer)

       These three macros are used to change a memory buffer size or to	free a
       piece of	memory no longer needed.  The arguments	to "Renew" and
       "Renewc"	match those of "New" and "Newc"	with the exception of not
       needing the "magic cookie" argument.

	   Move(source,	dest, number, type);
	   Copy(source,	dest, number, type);
	   Zero(dest, number, type);

       These three macros are used to move, copy, or zero out previously allo-
       cated memory.  The "source" and "dest" arguments	point to the source
       and destination starting	points.	 Perl will move, copy, or zero out
       "number"	instances of the size of the "type" data structure (using the
       "sizeof"	function).

       PerlIO

       The most	recent development releases of Perl has	been experimenting
       with removing Perl's dependency on the "normal" standard	I/O suite and
       allowing	other stdio implementations to be used.	 This involves creat-
       ing a new abstraction layer that	then calls whichever implementation of
       stdio Perl was compiled with.  All XSUBs	should now use the functions
       in the PerlIO abstraction layer and not make any	assumptions about what
       kind of stdio is	being used.

       For a complete description of the PerlIO	abstraction, consult perlapio.

       Putting a C value on Perl stack

       A lot of	opcodes	(this is an elementary operation in the	internal perl
       stack machine) put an SV* on the	stack. However,	as an optimization the
       corresponding SV	is (usually) not recreated each	time. The opcodes re-
       use specially assigned SVs (targets) which are (as a corollary) not
       constantly freed/created.

       Each of the targets is created only once	(but see "Scratchpads and
       recursion" below), and when an opcode needs to put an integer, a	dou-
       ble, or a string	on stack, it just sets the corresponding parts of its
       target and puts the target on stack.

       The macro to put	this target on stack is	"PUSHTARG", and	it is directly
       used in some opcodes, as	well as	indirectly in zillions of others,
       which use it via	"(X)PUSH[pni]".

       Because the target is reused, you must be careful when pushing multiple
       values on the stack. The	following code will not	do what	you think:

	   XPUSHi(10);
	   XPUSHi(20);

       This translates as "set "TARG" to 10, push a pointer to "TARG" onto the
       stack; set "TARG" to 20,	push a pointer to "TARG" onto the stack".  At
       the end of the operation, the stack does	not contain the	values 10 and
       20, but actually	contains two pointers to "TARG", which we have set to
       20. If you need to push multiple	different values, use "XPUSHs",	which
       bypasses	"TARG".

       On a related note, if you do use	"(X)PUSH[npi]",	then you're going to
       need a "dTARG" in your variable declarations so that the	"*PUSH*"
       macros can make use of the local	variable "TARG".

       Scratchpads

       The question remains on when the	SVs which are targets for opcodes are
       created.	The answer is that they	are created when the current unit -- a
       subroutine or a file (for opcodes for statements	outside	of subrou-
       tines) -- is compiled. During this time a special anonymous Perl	array
       is created, which is called a scratchpad	for the	current	unit.

       A scratchpad keeps SVs which are	lexicals for the current unit and are
       targets for opcodes. One	can deduce that	an SV lives on a scratchpad by
       looking on its flags: lexicals have "SVs_PADMY" set, and	targets	have
       "SVs_PADTMP" set.

       The correspondence between OPs and targets is not 1-to-1. Different OPs
       in the compile tree of the unit can use the same	target,	if this	would
       not conflict with the expected life of the temporary.

       Scratchpads and recursion

       In fact it is not 100% true that	a compiled unit	contains a pointer to
       the scratchpad AV. In fact it contains a	pointer	to an AV of (ini-
       tially) one element, and	this element is	the scratchpad AV. Why do we
       need an extra level of indirection?

       The answer is recursion,	and maybe threads. Both	these can create sev-
       eral execution pointers going into the same subroutine. For the subrou-
       tine-child not write over the temporaries for the subroutine-parent
       (lifespan of which covers the call to the child), the parent and	the
       child should have different scratchpads.	(And the lexicals should be
       separate	anyway!)

       So each subroutine is born with an array	of scratchpads (of length 1).
       On each entry to	the subroutine it is checked that the current depth of
       the recursion is	not more than the length of this array,	and if it is,
       new scratchpad is created and pushed into the array.

       The targets on this scratchpad are "undef"s, but	they are already
       marked with correct flags.

Compiled code
       Code tree

       Here we describe	the internal form your code is converted to by Perl.
       Start with a simple example:

	 $a = $b + $c;

       This is converted to a tree similar to this one:

		    assign-to
		  /	      \
		 +	       $a
	       /   \
	     $b	    $c

       (but slightly more complicated).	 This tree reflects the	way Perl
       parsed your code, but has nothing to do with the	execution order.
       There is	an additional "thread" going through the nodes of the tree
       which shows the order of	execution of the nodes.	 In our	simplified
       example above it	looks like:

	    $b ---> $c ---> + ---> $a ---> assign-to

       But with	the actual compile tree	for "$a	= $b + $c" it is different:
       some nodes optimized away.  As a	corollary, though the actual tree con-
       tains more nodes	than our simplified example, the execution order is
       the same	as in our example.

       Examining the tree

       If you have your	perl compiled for debugging (usually done with "-D
       optimize=-g" on "Configure" command line), you may examine the compiled
       tree by specifying "-Dx"	on the Perl command line.  The output takes
       several lines per node, and for "$b+$c" it looks	like this:

	   5	       TYPE = add  ===>	6
		       TARG = 1
		       FLAGS = (SCALAR,KIDS)
		       {
			   TYPE	= null	===> (4)
			     (was rv2sv)
			   FLAGS = (SCALAR,KIDS)
			   {
	   3		       TYPE = gvsv  ===> 4
			       FLAGS = (SCALAR)
			       GV = main::b
			   }
		       }
		       {
			   TYPE	= null	===> (5)
			     (was rv2sv)
			   FLAGS = (SCALAR,KIDS)
			   {
	   4		       TYPE = gvsv  ===> 5
			       FLAGS = (SCALAR)
			       GV = main::c
			   }
		       }

       This tree has 5 nodes (one per "TYPE" specifier), only 3	of them	are
       not optimized away (one per number in the left column).	The immediate
       children	of the given node correspond to	"{}" pairs on the same level
       of indentation, thus this listing corresponds to	the tree:

			  add
			/     \
		      null    null
		       |       |
		      gvsv    gvsv

       The execution order is indicated	by "===>" marks, thus it is "3 4 5 6"
       (node 6 is not included into above listing), i.e., "gvsv	gvsv add what-
       ever".

       Each of these nodes represents an op, a fundamental operation inside
       the Perl	core. The code which implements	each operation can be found in
       the pp*.c files;	the function which implements the op with type "gvsv"
       is "pp_gvsv", and so on.	As the tree above shows, different ops have
       different numbers of children: "add" is a binary	operator, as one would
       expect, and so has two children.	To accommodate the various different
       numbers of children, there are various types of op data structure, and
       they link together in different ways.

       The simplest type of op structure is "OP": this has no children.	Unary
       operators, "UNOP"s, have	one child, and this is pointed to by the
       "op_first" field. Binary	operators ("BINOP"s) have not only an
       "op_first" field	but also an "op_last" field. The most complex type of
       op is a "LISTOP", which has any number of children. In this case, the
       first child is pointed to by "op_first" and the last child by
       "op_last". The children in between can be found by iteratively follow-
       ing the "op_sibling" pointer from the first child to the	last.

       There are also two other	op types: a "PMOP" holds a regular expression,
       and has no children, and	a "LOOP" may or	may not	have children. If the
       "op_children" field is non-zero,	it behaves like	a "LISTOP". To compli-
       cate matters, if	a "UNOP" is actually a "null" op after optimization
       (see "Compile pass 2: context propagation") it will still have children
       in accordance with its former type.

       Compile pass 1: check routines

       The tree	is created by the compiler while yacc code feeds it the	con-
       structions it recognizes. Since yacc works bottom-up, so	does the first
       pass of perl compilation.

       What makes this pass interesting	for perl developers is that some opti-
       mization	may be performed on this pass.	This is	optimization by	so-
       called "check routines".	 The correspondence between node names and
       corresponding check routines is described in opcode.pl (do not forget
       to run "make regen_headers" if you modify this file).

       A check routine is called when the node is fully	constructed except for
       the execution-order thread.  Since at this time there are no back-links
       to the currently	constructed node, one can do most any operation	to the
       top-level node, including freeing it and/or creating new	nodes
       above/below it.

       The check routine returns the node which	should be inserted into	the
       tree (if	the top-level node was not modified, check routine returns its
       argument).

       By convention, check routines have names	"ck_*".	They are usually
       called from "new*OP" subroutines	(or "convert") (which in turn are
       called from perly.y).

       Compile pass 1a:	constant folding

       Immediately after the check routine is called the returned node is
       checked for being compile-time executable.  If it is (the value is
       judged to be constant) it is immediately	executed, and a	constant node
       with the	"return	value" of the corresponding subtree is substituted
       instead.	 The subtree is	deleted.

       If constant folding was not performed, the execution-order thread is
       created.

       Compile pass 2: context propagation

       When a context for a part of compile tree is known, it is propagated
       down through the	tree.  At this time the	context	can have 5 values
       (instead	of 2 for runtime context): void, boolean, scalar, list,	and
       lvalue.	In contrast with the pass 1 this pass is processed from	top to
       bottom: a node's	context	determines the context for its children.

       Additional context-dependent optimizations are performed	at this	time.
       Since at	this moment the	compile	tree contains back-references (via
       "thread"	pointers), nodes cannot	be free()d now.	 To allow optimized-
       away nodes at this stage, such nodes are	null()ified instead of
       free()ing (i.e. their type is changed to	OP_NULL).

       Compile pass 3: peephole	optimization

       After the compile tree for a subroutine (or for an "eval" or a file) is
       created,	an additional pass over	the code is performed. This pass is
       neither top-down	or bottom-up, but in the execution order (with addi-
       tional complications for	conditionals).	These optimizations are	done
       in the subroutine peep().  Optimizations	performed at this stage	are
       subject to the same restrictions	as in the pass 2.

       Pluggable runops

       The compile tree	is executed in a runops	function.  There are two
       runops functions	in run.c.  "Perl_runops_debug" is used with DEBUGGING
       and "Perl_runops_standard" is used otherwise.  For fine control over
       the execution of	the compile tree it is possible	to provide your	own
       runops function.

       It's probably best to copy one of the existing runops functions and
       change it to suit your needs.  Then, in the BOOT	section	of your	XS
       file, add the line:

	 PL_runops = my_runops;

       This function should be as efficient as possible	to keep	your programs
       running as fast as possible.

Examining internal data	structures with	the "dump" functions
       To aid debugging, the source file dump.c	contains a number of functions
       which produce formatted output of internal data structures.

       The most	commonly used of these functions is "Perl_sv_dump"; it's used
       for dumping SVs,	AVs, HVs, and CVs. The "Devel::Peek" module calls
       "sv_dump" to produce debugging output from Perl-space, so users of that
       module should already be	familiar with its format.

       "Perl_op_dump" can be used to dump an "OP" structure or any of its de-
       rivatives, and produces output similar to "perl -Dx"; in	fact,
       "Perl_dump_eval"	will dump the main root	of the code being evaluated,
       exactly like "-Dx".

       Other useful functions are "Perl_dump_sub", which turns a "GV" into an
       op tree,	"Perl_dump_packsubs" which calls "Perl_dump_sub" on all	the
       subroutines in a	package	like so: (Thankfully, these are	all xsubs, so
       there is	no op tree)

	   (gdb) print Perl_dump_packsubs(PL_defstash)

	   SUB attributes::bootstrap = (xsub 0x811fedc 0)

	   SUB UNIVERSAL::can =	(xsub 0x811f50c	0)

	   SUB UNIVERSAL::isa =	(xsub 0x811f304	0)

	   SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0)

	   SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0)

       and "Perl_dump_all", which dumps	all the	subroutines in the stash and
       the op tree of the main root.

How multiple interpreters and concurrency are supported
       Background and PERL_IMPLICIT_CONTEXT

       The Perl	interpreter can	be regarded as a closed	box: it	has an API for
       feeding it code or otherwise making it do things, but it	also has func-
       tions for its own use.  This smells a lot like an object, and there are
       ways for	you to build Perl so that you can have multiple	interpreters,
       with one	interpreter represented	either as a C structure, or inside a
       thread-specific structure.  These structures contain all	the context,
       the state of that interpreter.

       Two macros control the major Perl build flavors:	MULTIPLICITY and
       USE_5005THREADS.	 The MULTIPLICITY build	has a C	structure that pack-
       ages all	the interpreter	state, and there is a similar thread-specific
       data structure under USE_5005THREADS.  In both cases,
       PERL_IMPLICIT_CONTEXT is	also normally defined, and enables the support
       for passing in a	"hidden" first argument	that represents	all three data
       structures.

       All this	obviously requires a way for the Perl internal functions to be
       either subroutines taking some kind of structure	as the first argument,
       or subroutines taking nothing as	the first argument.  To	enable these
       two very	different ways of building the interpreter, the	Perl source
       (as it does in so many other situations)	makes heavy use	of macros and
       subroutine naming conventions.

       First problem: deciding which functions will be public API functions
       and which will be private.  All functions whose names begin "S_"	are
       private (think "S" for "secret" or "static").  All other	functions
       begin with "Perl_", but just because a function begins with "Perl_"
       does not	mean it	is part	of the API. (See "Internal Functions".)	The
       easiest way to be sure a	function is part of the	API is to find its
       entry in	perlapi.  If it	exists in perlapi, it's	part of	the API.  If
       it doesn't, and you think it should be (i.e., you need it for your
       extension), send	mail via perlbug explaining why	you think it should
       be.

       Second problem: there must be a syntax so that the same subroutine dec-
       larations and calls can pass a structure	as their first argument, or
       pass nothing.  To solve this, the subroutines are named and declared in
       a particular way.  Here's a typical start of a static function used
       within the Perl guts:

	 STATIC	void
	 S_incline(pTHX_ char *s)

       STATIC becomes "static" in C, and may be	#define'd to nothing in	some
       configurations in future.

       A public	function (i.e. part of the internal API, but not necessarily
       sanctioned for use in extensions) begins	like this:

	 void
	 Perl_sv_setsv(pTHX_ SV* dsv, SV* ssv)

       "pTHX_" is one of a number of macros (in	perl.h)	that hide the details
       of the interpreter's context.  THX stands for "thread", "this", or
       "thingy", as the	case may be.  (And no, George Lucas is not involved.
       :-) The first character could be	'p' for	a prototype, 'a' for argument,
       or 'd' for declaration, so we have "pTHX", "aTHX" and "dTHX", and their
       variants.

       When Perl is built without options that set PERL_IMPLICIT_CONTEXT,
       there is	no first argument containing the interpreter's context.	 The
       trailing	underscore in the pTHX_	macro indicates	that the macro expan-
       sion needs a comma after	the context argument because other arguments
       follow it.  If PERL_IMPLICIT_CONTEXT is not defined, pTHX_ will be
       ignored,	and the	subroutine is not prototyped to	take the extra argu-
       ment.  The form of the macro without the	trailing underscore is used
       when there are no additional explicit arguments.

       When a core function calls another, it must pass	the context.  This is
       normally	hidden via macros.  Consider "sv_setsv".  It expands into
       something like this:

	   ifdef PERL_IMPLICIT_CONTEXT
	     define sv_setsv(a,b)      Perl_sv_setsv(aTHX_ a, b)
	     /*	can't do this for vararg functions, see	below */
	   else
	     define sv_setsv	       Perl_sv_setsv
	   endif

       This works well,	and means that XS authors can gleefully	write:

	   sv_setsv(foo, bar);

       and still have it work under all	the modes Perl could have been com-
       piled with.

       This doesn't work so cleanly for	varargs	functions, though, as macros
       imply that the number of	arguments is known in advance.	Instead	we
       either need to spell them out fully, passing "aTHX_" as the first argu-
       ment (the Perl core tends to do this with functions like	Perl_warner),
       or use a	context-free version.

       The context-free	version	of Perl_warner is called Perl_warner_nocon-
       text, and does not take the extra argument.  Instead it does dTHX; to
       get the context from thread-local storage.  We "#define warner
       Perl_warner_nocontext" so that extensions get source compatibility at
       the expense of performance.  (Passing an	arg is cheaper than grabbing
       it from thread-local storage.)

       You can ignore [pad]THXx	when browsing the Perl headers/sources.	 Those
       are strictly for	use within the core.  Extensions and embedders need
       only be aware of	[pad]THX.

       So what happened	to dTHR?

       "dTHR" was introduced in	perl 5.005 to support the older	thread model.
       The older thread	model now uses the "THX" mechanism to pass context
       pointers	around,	so "dTHR" is not useful	any more.  Perl	5.6.0 and
       later still have	it for backward	source compatibility, but it is
       defined to be a no-op.

       How do I	use all	this in	extensions?

       When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call any
       functions in the	Perl API will need to pass the initial context argu-
       ment somehow.  The kicker is that you will need to write	it in such a
       way that	the extension still compiles when Perl hasn't been built with
       PERL_IMPLICIT_CONTEXT enabled.

       There are three ways to do this.	 First,	the easy but inefficient way,
       which is	also the default, in order to maintain source compatibility
       with extensions:	whenever XSUB.h	is #included, it redefines the aTHX
       and aTHX_ macros	to call	a function that	will return the	context.
       Thus, something like:

	       sv_setsv(asv, bsv);

       in your extension will translate	to this	when PERL_IMPLICIT_CONTEXT is
       in effect:

	       Perl_sv_setsv(Perl_get_context(), asv, bsv);

       or to this otherwise:

	       Perl_sv_setsv(asv, bsv);

       You have	to do nothing new in your extension to get this; since the
       Perl library provides Perl_get_context(), it will all just work.

       The second, more	efficient way is to use	the following template for
       your Foo.xs:

	       #define PERL_NO_GET_CONTEXT     /* we want efficiency */
	       #include	"EXTERN.h"
	       #include	"perl.h"
	       #include	"XSUB.h"

	       static my_private_function(int arg1, int	arg2);

	       static SV *
	       my_private_function(int arg1, int arg2)
	       {
		   dTHX;       /* fetch	context	*/
		   ... call many Perl API functions ...
	       }

	       [... etc	...]

	       MODULE =	Foo	       PACKAGE = Foo

	       /* typical XSUB */

	       void
	       my_xsub(arg)
		       int arg
		   CODE:
		       my_private_function(arg,	10);

       Note that the only two changes from the normal way of writing an	exten-
       sion is the addition of a "#define PERL_NO_GET_CONTEXT" before includ-
       ing the Perl headers, followed by a "dTHX;" declaration at the start of
       every function that will	call the Perl API.  (You'll know which func-
       tions need this,	because	the C compiler will complain that there's an
       undeclared identifier in	those functions.)  No changes are needed for
       the XSUBs themselves, because the XS() macro is correctly defined to
       pass in the implicit context if needed.

       The third, even more efficient way is to	ape how	it is done within the
       Perl guts:

	       #define PERL_NO_GET_CONTEXT     /* we want efficiency */
	       #include	"EXTERN.h"
	       #include	"perl.h"
	       #include	"XSUB.h"

	       /* pTHX_	only needed for	functions that call Perl API */
	       static my_private_function(pTHX_	int arg1, int arg2);

	       static SV *
	       my_private_function(pTHX_ int arg1, int arg2)
	       {
		   /* dTHX; not	needed here, because THX is an argument	*/
		   ... call Perl API functions ...
	       }

	       [... etc	...]

	       MODULE =	Foo	       PACKAGE = Foo

	       /* typical XSUB */

	       void
	       my_xsub(arg)
		       int arg
		   CODE:
		       my_private_function(aTHX_ arg, 10);

       This implementation never has to	fetch the context using	a function
       call, since it is always	passed as an extra argument.  Depending	on
       your needs for simplicity or efficiency,	you may	mix the	previous two
       approaches freely.

       Never add a comma after "pTHX" yourself--always use the form of the
       macro with the underscore for functions that take explicit arguments,
       or the form without the argument	for functions with no explicit argu-
       ments.

       Should I	do anything special if I call perl from	multiple threads?

       If you create interpreters in one thread	and then proceed to call them
       in another, you need to make sure perl's	own Thread Local Storage (TLS)
       slot is initialized correctly in	each of	those threads.

       The "perl_alloc"	and "perl_clone" API functions will automatically set
       the TLS slot to the interpreter they created, so	that there is no need
       to do anything special if the interpreter is always accessed in the
       same thread that	created	it, and	that thread did	not create or call any
       other interpreters afterwards.  If that is not the case,	you have to
       set the TLS slot	of the thread before calling any functions in the Perl
       API on that particular interpreter.  This is done by calling the
       "PERL_SET_CONTEXT" macro	in that	thread as the first thing you do:

	       /* do this before doing anything	else with some_perl */
	       PERL_SET_CONTEXT(some_perl);

	       ... other Perl API calls	on some_perl go	here ...

       Future Plans and	PERL_IMPLICIT_SYS

       Just as PERL_IMPLICIT_CONTEXT provides a	way to bundle up everything
       that the	interpreter knows about	itself and pass	it around, so too are
       there plans to allow the	interpreter to bundle up everything it knows
       about the environment it's running on.  This is enabled with the
       PERL_IMPLICIT_SYS macro.	 Currently it only works with USE_ITHREADS and
       USE_5005THREADS on Windows (see inside iperlsys.h).

       This allows the ability to provide an extra pointer (called the "host"
       environment) for	all the	system calls.  This makes it possible for all
       the system stuff	to maintain their own state, broken down into seven C
       structures.  These are thin wrappers around the usual system calls (see
       win32/perllib.c)	for the	default	perl executable, but for a more	ambi-
       tious host (like	the one	that would do fork() emulation)	all the	extra
       work needed to pretend that different interpreters are actually differ-
       ent "processes",	would be done here.

       The Perl	engine/interpreter and the host	are orthogonal entities.
       There could be one or more interpreters in a process, and one or	more
       "hosts",	with free association between them.

Internal Functions
       All of Perl's internal functions	which will be exposed to the outside
       world are be prefixed by	"Perl_"	so that	they will not conflict with XS
       functions or functions used in a	program	in which Perl is embedded.
       Similarly, all global variables begin with "PL_". (By convention,
       static functions	start with "S_")

       Inside the Perl core, you can get at the	functions either with or with-
       out the "Perl_" prefix, thanks to a bunch of defines that live in
       embed.h.	This header file is generated automatically from embed.pl.
       embed.pl	also creates the prototyping header files for the internal
       functions, generates the	documentation and a lot	of other bits and
       pieces. It's important that when	you add	a new function to the core or
       change an existing one, you change the data in the table	at the end of
       embed.pl	as well. Here's	a sample entry from that table:

	   Apd |SV**   |av_fetch   |AV*	ar|I32 key|I32 lval

       The second column is the	return type, the third column the name.	Col-
       umns after that are the arguments. The first column is a	set of flags:

       A  This function	is a part of the public	API.

       p  This function	has a "Perl_" prefix; ie, it is	defined	as
	  "Perl_av_fetch"

       d  This function	has documentation using	the "apidoc" feature which
	  we'll	look at	in a second.

       Other available flags are:

       s  This is a static function and	is defined as "S_whatever", and	usu-
	  ally called within the sources as "whatever(...)".

       n  This does not	use "aTHX_" and	"pTHX" to pass interpreter context.
	  (See "Background and PERL_IMPLICIT_CONTEXT" in perlguts.)

       r  This function	never returns; "croak",	"exit" and friends.

       f  This function	takes a	variable number	of arguments, "printf" style.
	  The argument list should end with "...", like	this:

	      Afprd   |void   |croak	      |const char* pat|...

       M  This function	is part	of the experimental development	API, and may
	  change or disappear without notice.

       o  This function	should not have	a compatibility	macro to define, say,
	  "Perl_parse" to "parse". It must be called as	"Perl_parse".

       j  This function	is not a member	of "CPerlObj". If you don't know what
	  this means, don't use	it.

       x  This function	isn't exported out of the Perl core.

       If you edit embed.pl, you will need to run "make	regen_headers" to
       force a rebuild of embed.h and other auto-generated files.

       Formatted Printing of IVs, UVs, and NVs

       If you are printing IVs,	UVs, or	NVS instead of the stdio(3) style for-
       matting codes like %d, %ld, %f, you should use the following macros for
       portability

	       IVdf	       IV in decimal
	       UVuf	       UV in decimal
	       UVof	       UV in octal
	       UVxf	       UV in hexadecimal
	       NVef	       NV %e-like
	       NVff	       NV %f-like
	       NVgf	       NV %g-like

       These will take care of 64-bit integers and long	doubles.  For example:

	       printf("IV is %"IVdf"\n", iv);

       The IVdf	will expand to whatever	is the correct format for the IVs.

       If you are printing addresses of	pointers, use UVxf combined with
       PTR2UV(), do not	use %lx	or %p.

       Pointer-To-Integer and Integer-To-Pointer

       Because pointer size does not necessarily equal integer size, use the
       follow macros to	do it right.

	       PTR2UV(pointer)
	       PTR2IV(pointer)
	       PTR2NV(pointer)
	       INT2PTR(pointertotype, integer)

       For example:

	       IV  iv =	...;
	       SV *sv =	INT2PTR(SV*, iv);

       and

	       AV *av =	...;
	       UV  uv =	PTR2UV(av);

       Source Documentation

       There's an effort going on to document the internal functions and auto-
       matically produce reference manuals from	them - perlapi is one such
       manual which details all	the functions which are	available to XS	writ-
       ers. perlintern is the autogenerated manual for the functions which are
       not part	of the API and are supposedly for internal use only.

       Source documentation is created by putting POD comments into the	C
       source, like this:

	/*
	=for apidoc sv_setiv

	Copies an integer into the given SV.  Does not handle 'set' magic.  See
	C<sv_setiv_mg>.

	=cut
	*/

       Please try and supply some documentation	if you add functions to	the
       Perl core.

Unicode	Support
       Perl 5.6.0 introduced Unicode support. It's important for porters and
       XS writers to understand	this support and make sure that	the code they
       write does not corrupt Unicode data.

       What is Unicode,	anyway?

       In the olden, less enlightened times, we	all used to use	ASCII. Most of
       us did, anyway. The big problem with ASCII is that it's American. Well,
       no, that's not actually the problem; the	problem	is that	it's not par-
       ticularly useful	for people who don't use the Roman alphabet. What used
       to happen was that particular languages would stick their own alphabet
       in the upper range of the sequence, between 128 and 255.	Of course, we
       then ended up with plenty of variants that weren't quite	ASCII, and the
       whole point of it being a standard was lost.

       Worse still, if you've got a language like Chinese or Japanese that has
       hundreds	or thousands of	characters, then you really can't fit them
       into a mere 256,	so they	had to forget about ASCII altogether, and
       build their own systems using pairs of numbers to refer to one charac-
       ter.

       To fix this, some people	formed Unicode,	Inc. and produced a new	char-
       acter set containing all	the characters you can possibly	think of and
       more. There are several ways of representing these characters, and the
       one Perl	uses is	called UTF8. UTF8 uses a variable number of bytes to
       represent a character, instead of just one. You can learn more about
       Unicode at http://www.unicode.org/

       How can I recognise a UTF8 string?

       You can't. This is because UTF8 data is stored in bytes just like
       non-UTF8	data. The Unicode character 200, (0xC8 for you hex types) cap-
       ital E with a grave accent, is represented by the two bytes "v196.172".
       Unfortunately, the non-Unicode string "chr(196).chr(172)" has that byte
       sequence	as well. So you	can't tell just	by looking - this is what
       makes Unicode input an interesting problem.

       The API function	"is_utf8_string" can help; it'll tell you if a string
       contains	only valid UTF8	characters. However, it	can't do the work for
       you. On a character-by-character	basis, "is_utf8_char" will tell	you
       whether the current character in	a string is valid UTF8.

       How does	UTF8 represent Unicode characters?

       As mentioned above, UTF8	uses a variable	number of bytes	to store a
       character. Characters with values 1...128 are stored in one byte, just
       like good ol' ASCII. Character 129 is stored as "v194.129"; this	con-
       tinues up to character 191, which is "v194.191".	Now we've run out of
       bits (191 is binary 10111111) so	we move	on; 192	is "v195.128". And so
       it goes on, moving to three bytes at character 2048.

       Assuming	you know you're	dealing	with a UTF8 string, you	can find out
       how long	the first character in it is with the "UTF8SKIP" macro:

	   char	*utf = "\305\233\340\240\201";
	   I32 len;

	   len = UTF8SKIP(utf);	/* len is 2 here */
	   utf += len;
	   len = UTF8SKIP(utf);	/* len is 3 here */

       Another way to skip over	characters in a	UTF8 string is to use
       "utf8_hop", which takes a string	and a number of	characters to skip
       over. You're on your own	about bounds checking, though, so don't	use it
       lightly.

       All bytes in a multi-byte UTF8 character	will have the high bit set, so
       you can test if you need	to do something	special	with this character
       like this (the UTF8_IS_INVARIANT() is a macro that tests	whether	the
       byte can	be encoded as a	single byte even in UTF-8):

	   U8 *utf;
	   UV uv;      /* Note:	a UV, not a U8,	not a char */

	   if (!UTF8_IS_INVARIANT(*utf))
	       /* Must treat this as UTF8 */
	       uv = utf8_to_uv(utf);
	   else
	       /* OK to	treat this character as	a byte */
	       uv = *utf;

       You can also see	in that	example	that we	use "utf8_to_uv" to get	the
       value of	the character; the inverse function "uv_to_utf8" is available
       for putting a UV	into UTF8:

	   if (!UTF8_IS_INVARIANT(uv))
	       /* Must treat this as UTF8 */
	       utf8 = uv_to_utf8(utf8, uv);
	   else
	       /* OK to	treat this character as	a byte */
	       *utf8++ = uv;

       You must	convert	characters to UVs using	the above functions if you're
       ever in a situation where you have to match UTF8	and non-UTF8 charac-
       ters. You may not skip over UTF8	characters in this case. If you	do
       this, you'll lose the ability to	match hi-bit non-UTF8 characters; for
       instance, if your UTF8 string contains "v196.172", and you skip that
       character, you can never	match a	"chr(200)" in a	non-UTF8 string.  So
       don't do	that!

       How does	Perl store UTF8	strings?

       Currently, Perl deals with Unicode strings and non-Unicode strings
       slightly	differently. If	a string has been identified as	being UTF-8
       encoded,	Perl will set a	flag in	the SV,	"SVf_UTF8". You	can check and
       manipulate this flag with the following macros:

	   SvUTF8(sv)
	   SvUTF8_on(sv)
	   SvUTF8_off(sv)

       This flag has an	important effect on Perl's treatment of	the string: if
       Unicode data is not properly distinguished, regular expressions,
       "length", "substr" and other string handling operations will have unde-
       sirable results.

       The problem comes when you have,	for instance, a	string that isn't
       flagged is UTF8,	and contains a byte sequence that could	be UTF8	-
       especially when combining non-UTF8 and UTF8 strings.

       Never forget that the "SVf_UTF8"	flag is	separate to the	PV value; you
       need be sure you	don't accidentally knock it off	while you're manipu-
       lating SVs. More	specifically, you cannot expect	to do this:

	   SV *sv;
	   SV *nsv;
	   STRLEN len;
	   char	*p;

	   p = SvPV(sv,	len);
	   frobnicate(p);
	   nsv = newSVpvn(p, len);

       The "char*" string does not tell	you the	whole story, and you can't
       copy or reconstruct an SV just by copying the string value. Check if
       the old SV has the UTF8 flag set, and act accordingly:

	   p = SvPV(sv,	len);
	   frobnicate(p);
	   nsv = newSVpvn(p, len);
	   if (SvUTF8(sv))
	       SvUTF8_on(nsv);

       In fact,	your "frobnicate" function should be made aware	of whether or
       not it's	dealing	with UTF8 data,	so that	it can handle the string
       appropriately.

       Since just passing an SV	to an XS function and copying the data of the
       SV is not enough	to copy	the UTF8 flags,	even less right	is just	pass-
       ing a "char *" to an XS function.

       How do I	convert	a string to UTF8?

       If you're mixing	UTF8 and non-UTF8 strings, you might find it necessary
       to upgrade one of the strings to	UTF8. If you've	got an SV, the easiest
       way to do this is:

	   sv_utf8_upgrade(sv);

       However,	you must not do	this, for example:

	   if (!SvUTF8(left))
	       sv_utf8_upgrade(left);

       If you do this in a binary operator, you	will actually change one of
       the strings that	came into the operator,	and, while it shouldn't	be
       noticeable by the end user, it can cause	problems.

       Instead,	"bytes_to_utf8"	will give you a	UTF8-encoded copy of its
       string argument.	This is	useful for having the data available for com-
       parisons	and so on, without harming the original	SV. There's also
       "utf8_to_bytes" to go the other way, but	naturally, this	will fail if
       the string contains any characters above	255 that can't be represented
       in a single byte.

       Is there	anything else I	need to	know?

       Not really. Just	remember these things:

       o  There's no way to tell if a string is	UTF8 or	not. You can tell if
	  an SV	is UTF8	by looking at is "SvUTF8" flag.	Don't forget to	set
	  the flag if something	should be UTF8.	Treat the flag as part of the
	  PV, even though it's not - if	you pass on the	PV to somewhere, pass
	  on the flag too.

       o  If a string is UTF8, always use "utf8_to_uv" to get at the value,
	  unless "UTF8_IS_INVARIANT(*s)" in which case you can use *s.

       o  When writing a character "uv"	to a UTF8 string, always use
	  "uv_to_utf8",	unless "UTF8_IS_INVARIANT(uv))"	in which case you can
	  use "*s = uv".

       o  Mixing UTF8 and non-UTF8 strings is tricky. Use "bytes_to_utf8" to
	  get a	new string which is UTF8 encoded. There	are tricks you can use
	  to delay deciding whether you	need to	use a UTF8 string until	you
	  get to a high	character - "HALF_UPGRADE" is one of those.

Custom Operators
       Custom operator support is a new	experimental feature that allows you
       to define your own ops. This is primarily to allow the building of
       interpreters for	other languages	in the Perl core, but it also allows
       optimizations through the creation of "macro-ops" (ops which perform
       the functions of	multiple ops which are usually executed	together, such
       as "gvsv, gvsv, add".)

       This feature is implemented as a	new op type, "OP_CUSTOM". The Perl
       core does not "know" anything special about this	op type, and so	it
       will not	be involved in any optimizations. This also means that you can
       define your custom ops to be any	op structure - unary, binary, list and
       so on - you like.

       It's important to know what custom operators won't do for you. They
       won't let you add new syntax to Perl, directly. They won't even let you
       add new keywords, directly. In fact, they won't change the way Perl
       compiles	a program at all. You have to do those changes yourself, after
       Perl has	compiled the program. You do this either by manipulating the
       op tree using a "CHECK" block and the "B::Generate" module, or by
       adding a	custom peephole	optimizer with the "optimize" module.

       When you	do this, you replace ordinary Perl ops with custom ops by cre-
       ating ops with the type "OP_CUSTOM" and the "pp_addr" of	your own PP
       function. This should be	defined	in XS code, and	should look like the
       PP ops in "pp_*.c". You are responsible for ensuring that your op takes
       the appropriate number of values	from the stack,	and you	are responsi-
       ble for adding stack marks if necessary.

       You should also "register" your op with the Perl	interpreter so that it
       can produce sensible error and warning messages.	Since it is possible
       to have multiple	custom ops within the one "logical" op type "OP_CUS-
       TOM", Perl uses the value of "o->op_ppaddr" as a	key into the "PL_cus-
       tom_op_descs" and "PL_custom_op_names" hashes. This means you need to
       enter a name and	description for	your op	at the appropriate place in
       the "PL_custom_op_names"	and "PL_custom_op_descs" hashes.

       Forthcoming versions of "B::Generate" (version 1.0 and above) should
       directly	support	the creation of	custom ops by name; "Opcodes::Custom"
       will provide functions which make it trivial to "register" custom ops
       to the Perl interpreter.

AUTHORS
       Until May 1997, this document was maintained by Jeff Okamoto
       <okamoto@corp.hp.com>.  It is now maintained as part of Perl itself by
       the Perl	5 Porters <perl5-porters@perl.org>.

       With lots of help and suggestions from Dean Roehrich, Malcolm Beattie,
       Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil Bow-
       ers, Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer, Stephen
       McCamant, and Gurusamy Sarathy.

       API Listing originally by Dean Roehrich <roehrich@cray.com>.

       Modifications to	autogenerate the API listing (perlapi) by Benjamin
       Stuhl.

SEE ALSO
       perlapi(1), perlintern(1), perlxs(1), perlembed(1)

perl v5.8.0			  2003-02-18			   PERLGUTS(1)

NAME | DESCRIPTION | Variables | Subroutines | Compiled code | Examining internal data structures with the "dump" functions | How multiple interpreters and concurrency are supported | Internal Functions | Unicode Support | Custom Operators | AUTHORS | SEE ALSO

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