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

       perlguts - Introduction to the Perl API

       This document attempts to describe how to use the Perl API, as well as
       to provide 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.


       Perl has three typedefs that handle Perls three main data types:

	   SV  Scalar Value
	   AV  Array Value
	   HV  Hash Value

       Each typedef has specific routines that manipulate the various data

       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

       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*, STRLEN);
	   SV*	newSVpvn(const char*, STRLEN);
	   SV*	newSVpvf(const char*, ...);
	   SV*	newSVsv(SV*);

       "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.

       In the unlikely case of a SV requiring 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 *sv = newSV(0);	/* no storage allocated  */
	   SV *sv = newSV(10);	/* 10 (+1) bytes of uninitialised storage allocated  */

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

	   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*, STRLEN)
	   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 strings 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 strings 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.

       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.  Perls 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

	   SvPV(SV*, STRLEN len)

       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 doesnt allow you to safely say "foo(SvPV(s, len),
       len);". It might work with your compiler, but it wont 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:


       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 (perls 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.


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

	   SvCUR_set(SV*, I32 val)

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


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

       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:


       The scalar "undef" value is stored in an SV instance called

       Its address can be used whenever an "SV*" is needed. Make sure that you
       dont try to compare a random sv with &PL_sv_undef. For example when
       interfacing Perl code, itll work correctly for:


       But wont work when called as:

	 $x = undef;

       So to repeat always use SvOK() to check whether an sv is defined.

       Also you have to be careful when using &PL_sv_undef as a value in AVs
       or HVs (see "AVs, HVs and undefined values").

       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 youve created, call "SvREFCNT_dec(SV*)".  Normally
       this call is not necessary (see "Reference Counts and Mortality").


       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
	   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.

       Whats 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:


       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 occurred 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, its 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

	   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

	   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

	   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

       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)"

	   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

	   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
       doesnt have to be recomputed every time).  See perlapi for detailed

       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 callers 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.

       AVs, HVs and undefined values

       Sometimes you have to store undefined values in AVs or HVs. Although
       this may be a rare case, it can be tricky. Thats because youre used
       to using &PL_sv_undef if you need an undefined SV.

       For example, intuition tells you that this XS code:

	   AV *av = newAV();
	   av_store( av, 0, &PL_sv_undef );

       is equivalent to this Perl code:

	   my @av;
	   $av[0] = undef;

       Unfortunately, this isnt true. AVs use &PL_sv_undef as a marker for
       indicating that an array element has not yet been initialized.  Thus,
       "exists $av[0]" would be true for the above Perl code, but false for
       the array generated by the XS code.

       Other problems can occur when storing &PL_sv_undef in HVs:

	   hv_store( hv, "key", 3, &PL_sv_undef, 0 );

       This will indeed make the value "undef", but if you try to modify the
       value of "key", youll get the following error:

	   Modification of non-creatable hash value attempted

       In perl 5.8.0, &PL_sv_undef was also used to mark placeholders in
       restricted hashes. This caused such hash entries not to appear when
       iterating over the hash or when checking for the keys with the
       "hv_exists" function.

       You can run into similar problems when you store &PL_sv_true or
       &PL_sv_false into AVs or HVs. Trying to modify such elements will give
       you the following error:

	   Modification of a read-only value attempted

       To make a long story short, you can use the special variables
       &PL_sv_undef, &PL_sv_true and &PL_sv_false with AVs and HVs, but you
       have to make sure you know what youre doing.

       Generally, if you want to store an undefined value in an AV or HV, you
       should not use &PL_sv_undef, but rather create a new undefined value
       using the "newSV" function, for example:

	   av_store( av, 42, newSV(0) );
	   hv_store( hv, "foo", 3, newSV(0), 0 );


       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:


       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:


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


       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
       perls 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 value.  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

	       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

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

       To check if youve 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 ORed with the
       "TRUE" argument to enable certain extra features.  Those bits are:

	   Marks the variable as multiply defined, thus preventing the:

	     Name  used only once: possible typo


	   Issues the warning:

	     Had to create  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 doesnt happen at the Perl level unless a variable is
       undefed 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 arguments 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 hasnt!  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 perls 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 its popped
       off the stack. Similarly, results returned by XSUBs (which are pushed
       on 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

	   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 variables that are defined 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 refer
       ences to the various objects of that name, including (but not limited
       to) the following:

	   Scalar Value
	   Array Value
	   Hash Value
	   I/O Handle

       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

	   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:


       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]);

       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, typedefed 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 then either a "savepvn" copy
       of "name" or "name" itself is stored in the "mg_ptr" field, depending
       on whether "namlen" is greater than zero or equal to zero respectively.
       As a special case, if "(name && namlen == HEf_SVKEY)" then "name" is
       assumed to contain an "SV*" and is stored as-is with its REFCNT incre

       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 Tables" 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 in 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

       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.

       See also "sv_magicext" in perlapi for a more flexible way to add magic
       to an SV.

       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

       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 SVs 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

       The current kinds of Magic Virtual Tables are:

	   (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
	   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 %_ $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 "-DDE
       BUGGING" on the "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)
	   3		       TYPE = gvsv  ===> 4
			       FLAGS = (SCALAR)
			       GV = main::b
			   TYPE = null	===> (5)
			     (was rv2sv)
	   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:

			/     \
		      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

       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.

       Another way to examine the tree is to use a compiler back-end module,
       such as B::Concise.

       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

       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

       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 nodes 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 and in dump.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 pro
       vide your own runops function.

       Its 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"; its 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
       derivatives, 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

       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, its part of the API.  If
       it doesnt, and you think it should be (i.e., you need it for your
       extension), send mail via perlbug explaining why you think it should

       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.  Heres 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 #defined 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:

	 Perl_sv_setiv(pTHX_ SV* dsv, IV num)

       "pTHX_" is one of a number of macros (in perl.h) that hide the details
       of the interpreters 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

       When Perl is built without options that set PERL_IMPLICIT_CONTEXT,
       there is no first argument containing the interpreters 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_setiv".  It expands into
       something like this:

	     #define sv_setiv(a,b)	Perl_sv_setiv(aTHX_ a, b)
	     /* cant do this for vararg functions, see below */
	     #define sv_setiv		Perl_sv_setiv

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

	   sv_setiv(foo, bar);

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

       This doesnt 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 hasnt been built with

       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_setiv(sv, num);

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

	       Perl_sv_setiv(Perl_get_context(), sv, num);

       or to this otherwise:

	       Perl_sv_setiv(sv, num);

       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 */

		       int arg
		       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.  (Youll know which func
       tions need this, because the C compiler will complain that theres 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 */

		       int arg
		       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

       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 perls 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 */

	       ... 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 its 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 Perls internal functions which will be exposed to the outside
       world are 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 and
       embed.fnc. embed.pl also creates the prototyping header files for the
       internal functions, generates the documentation and a lot of other bits
       and pieces. Its important that when you add a new function to the core
       or change an existing one, you change the data in the table in
       embed.fnc as well. Heres 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.
       Columns after that are the arguments. The first column is a set of

       A  This function is a part of the public API. All such functions should
	  also have d, very few do not.

       p  This function has a "Perl_" prefix; i.e. it is defined as

       d  This function has documentation using the "apidoc" feature which
	  well look at in a second.  Some functions have d but not A;
	  docs are good.

       Other available flags are:

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

       n  This does not need a interpreter context, so the definition has no
	  "pTHX", and it follows that callers dont use "aTHX".	(See "Back
	  ground 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".

       x  This function isnt exported out of the Perl core.

       m  This is implemented as a macro.

       X  This function is explicitly exported.

       E  This function is visible to extensions included in the Perl core.

       b  Binary backward compatibility; this function is a macro but also has
	  a "Perl_" implementation (which is exported).

	  See the comments at the top of "embed.fnc" for others.

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

       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

	       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.

	       INT2PTR(pointertotype, integer)

       For example:

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


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

       Source Documentation

       Theres 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


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

       Backwards compatibility

       The Perl API changes over time. New functions are added or the inter
       faces of existing functions are changed. The "Devel::PPPort" module
       tries to provide compatibility code for some of these changes, so XS
       writers dont have to code it themselves when supporting multiple ver
       sions of Perl.

       "Devel::PPPort" generates a C header file ppport.h that can also be run
       as a Perl script. To generate ppport.h, run:

	   perl -MDevel::PPPort -eDevel::PPPort::WriteFile

       Besides checking existing XS code, the script can also be used to
       retrieve compatibility information for various API calls using the
       "--api-info" command line switch. For example:

	 % perl ppport.h --api-info=sv_magicext

       For details, see "perldoc ppport.h".

Unicode Support
       Perl 5.6.0 introduced Unicode support. Its 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 its American. Well,
       no, thats not actually the problem; the problem is that its not par
       ticularly useful for people who dont 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 werent quite ASCII, and the
       whole point of it being a standard was lost.

       Worse still, if youve got a language like Chinese or Japanese that has
       hundreds or thousands of characters, then you really cant 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

       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 UTF-8. UTF-8 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 UTF-8 string?

       You cant. This is because UTF-8 data is stored in bytes just like
       non-UTF-8 data. The Unicode character 200, (0xC8 for you hex types)
       capital 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 cant tell just by looking -
       this is what makes Unicode input an interesting problem.

       The API function "is_utf8_string" can help; itll tell you if a string
       contains only valid UTF-8 characters. However, it cant 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 UTF-8.

       How does UTF-8 represent Unicode characters?

       As mentioned above, UTF-8 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 weve 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 youre dealing with a UTF-8 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 UTF-8 string is to use
       "utf8_hop", which takes a string and a number of characters to skip
       over. Youre on your own about bounds checking, though, so dont use it

       All bytes in a multi-byte UTF-8 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 UTF-8 */
	       uv = utf8_to_uv(utf);
	       /* 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 UTF-8:

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

       You must convert characters to UVs using the above functions if youre
       ever in a situation where you have to match UTF-8 and non-UTF-8 charac
       ters. You may not skip over UTF-8 characters in this case. If you do
       this, youll lose the ability to match hi-bit non-UTF-8 characters; for
       instance, if your UTF-8 string contains "v196.172", and you skip that
       character, you can never match a "chr(200)" in a non-UTF-8 string.  So
       dont do that!

       How does Perl store UTF-8 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:


       This flag has an important effect on Perls 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 isnt
       flagged is UTF-8, and contains a byte sequence that could be UTF-8 -
       especially when combining non-UTF-8 and UTF-8 strings.

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

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

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

       The "char*" string does not tell you the whole story, and you cant
       copy or reconstruct an SV just by copying the string value. Check if
       the old SV has the UTF-8 flag set, and act accordingly:

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

       In fact, your "frobnicate" function should be made aware of whether or
       not its dealing with UTF-8 data, so that it can handle the string

       Since just passing an SV to an XS function and copying the data of the
       SV is not enough to copy the UTF-8 flags, even less right is just pass
       ing a "char *" to an XS function.

       How do I convert a string to UTF-8?

       If youre mixing UTF-8 and non-UTF-8 strings, you might find it neces
       sary to upgrade one of the strings to UTF-8. If youve got an SV, the
       easiest way to do this is:


       However, you must not do this, for example:

	   if (!SvUTF8(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 shouldnt be
       noticeable by the end user, it can cause problems.

       Instead, "bytes_to_utf8" will give you a UTF-8-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. Theres also
       "utf8_to_bytes" to go the other way, but naturally, this will fail if
       the string contains any characters above 255 that cant be represented
       in a single byte.

       Is there anything else I need to know?

       Not really. Just remember these things:

	 Theres no way to tell if a string is UTF-8 or not. You can tell if
	  an SV is UTF-8 by looking at is "SvUTF8" flag. Dont forget to set
	  the flag if something should be UTF-8. Treat the flag as part of the
	  PV, even though its not - if you pass on the PV to somewhere, pass
	  on the flag too.

	 If a string is UTF-8, always use "utf8_to_uv" to get at the value,
	  unless "UTF8_IS_INVARIANT(*s)" in which case you can use *s.

	 When writing a character "uv" to a UTF-8 string, always use
	  "uv_to_utf8", unless "UTF8_IS_INVARIANT(uv))" in which case you can
	  use "*s = uv".

	 Mixing UTF-8 and non-UTF-8 strings is tricky. Use "bytes_to_utf8" to
	  get a new string which is UTF-8 encoded. There are tricks you can
	  use to delay deciding whether you need to use a UTF-8 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.

       Its important to know what custom operators wont do for you. They
       wont let you add new syntax to Perl, directly. They wont even let you
       add new keywords, directly. In fact, they wont 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.

       Until May 1997, this document was maintained by Jeff Okamoto
       .  It is now maintained as part of Perl itself by
       the Perl 5 Porters .

       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.

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

perl v5.8.8			  2008-04-25			   PERLGUTS(1)

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