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X11R7.4 - man page for perlguts (x11r4 section 1)

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 questions or comments to the author below.


       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 usu-
       ally 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 integer 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 config.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 routines:

	   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 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 formatted output
       becomes the value.

       "sv_vsetpvfn" is an analogue of "vsprintf", but it allows you to specify either a pointer
       to a variable argument list or the address and length of an array of SVs.  The last argu-
       ment points to a boolean; on return, if that boolean is true, then locale-specific infor-
       mation has been used to format the string, and the string's contents are therefore
       untrustworthy (see perlsec).  This pointer may be NULL if that information is not impor-
       tant.  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 corruptions 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 system call.

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

	   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 termi-
       nated 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 everyone.  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 allo-
       cate 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 func-
       tion "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.


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

	   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 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 pro-
       cesses 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

       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

	   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


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

       Its address can be used whenever an "SV*" is needed. Make sure that you don't try to com-
       pare a random sv with &PL_sv_undef. For example when interfacing Perl code, it'll work
       correctly for:


       But won't 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

	   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").


       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 stor-

       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 usually the same, but a "shift" operation can be carried out by increasing
       "AvARRAY" by one and decreasing "AvFILL" and "AvLEN".  Again, the location 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 integer/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 routine 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, 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 con-
       tains 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

	   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 information 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" argu-
       ment 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 con-
       tain 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 fol-

	   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 information on how to use
       the hash access functions on tied hashes.

       Hash API Extensions

       Beginning with version 5.004, the following functions are also supported:

	   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 func-

       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 variables, 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 reference 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.

       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. That's because you're 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 isn't 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",
       you'll 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 you're 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

	   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 refer-

       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 reference count of the "thing", while "newRV_noinc"
       does not.  For historical reasons, "newRV" is a synonym for "newRV_inc".

       Once you have a reference, you can use the following macro to dereference 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 following 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 perl's 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

       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 specified 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

	       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" functionality.

	       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:

	   Marks the variable as multiply defined, thus preventing the:

	     Name <varname> used only once: possible typo


	   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, how-
       ever, 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 argu-
       ment.  The "newRV_inc" function, you will recall, creates 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 function.  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 reference 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 "mortality".  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 refer-
       ence 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 mor-
       talize 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 it's 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 "SvREFCNT_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 mul-
       tiple times. Thinking of "Mortalization" as deferred "SvREFCNT_dec" should help to mini-
       mize such problems.  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 mortalization.  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_2mortal" 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 con-
       tains references to the various objects of that name, including (but not limited to) the

	   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 flags)
	   HV*	gv_stashsv(SV*, I32 flags)

       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 "flags" flag
       will create a new package if it is set to GV_ADD.

       The name that "gv_stash*v" wants is the name of the package whose symbol 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 vari-
       able $! 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" rou-
       tines to add the additional scalar type, then set a flag so that Perl will believe it con-
       tains 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 contains 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.  Every-
       thing 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, type-
       def'ed to "MAGIC".

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

       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 magical feature.

       If "sv" is not already magical, Perl uses the "SvUPGRADE" macro to convert "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 incremented.

       The sv_magic function uses "how" to determine which, if any, predefined "Magic Virtual Ta-
       ble" 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 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.

       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 "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

       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 (or sometimes eight) 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);

	   int	(*svt_copy)(SV *sv, MAGIC* mg, SV *nsv, const char *name, int namlen);
	   int	(*svt_dup)(MAGIC *mg, CLONE_PARAMS *param);
	   int	(*svt_local)(SV *nsv, 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 vari-
       ous 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.

	   svt_copy	       copy tied variable magic to a tied element
	   svt_dup	       duplicate a magic structure during thread cloning
	   svt_local	       copy magic to local value during 'local'

       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 opera-
       tion is being performed, the routine "magic_get" is called.  All the various routines for
       the various magical types begin with "magic_".  NOTE: the magic routines are not consid-
       ered part of the Perl API, and may not be exported by the Perl library.

       The last three slots are a recent addition, and for source code compatibility they are
       only checked for if one of the three flags MGf_COPY, MGf_DUP or MGf_LOCAL is set in
       mg_flags. This means that most code can continue declaring a vtable as a 5-element value.
       These three are currently used exclusively by the threading code, and are highly subject
       to change.

       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 %_<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
	   V  PERL_MAGIC_vstring	(none)		v-string scalars
	   w  PERL_MAGIC_utf8		vtbl_utf8	UTF-8 length+offset cache
	   x  PERL_MAGIC_substr 	vtbl_substr	substr() lvalue
	   y  PERL_MAGIC_defelem	vtbl_defelem	Shadow "foreach" iterator
							variable / smart parameter
	   *  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	back pointer to a weak ref
	   ~  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 typically 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.  However, 'v' and 'V' (vec and v-string) are in
       no way related.

       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 information to variables (typically objects).  This is especially use-
       ful 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" function 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.

	       SV *sv;
	       struct ufuncs uf;
	       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 suffi-
       cient.  For "PERL_MAGIC_ext" magic, it may also be appropriate to add an I32 'signature'
       at the top of the private data area and check that.

       Also note that the "sv_set*()" and "sv_cat*()" functions described earlier do not invoke
       'set' magic on their targets.  This must be done by the user either by calling the "SvSET-
       MAGIC()" 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 external 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 yourself 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 implements 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.

	       HV *hash;
	       HV *stash;
	       SV *tie;
	       hash = newHV();
	       tie = newRV_noinc((SV*)newHV());
	       stash = gv_stashpv("MyTie", GV_ADD);
	       sv_bless(tie, stash);
	       hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
	       RETVAL = newRV_noinc(hash);

       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 actu-
       ally invoke the perl level "STORE" method on the TIEARRAY object.  If "av_store" did
       return NULL, a call to "SvREFCNT_dec(val)" will also be usually necessary to avoid a mem-
       ory 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.	[MAY-
       CHANGE] 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 interfaces 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
       created 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 variable "i" at the end of
	   enclosing pseudo-block.

	   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 mechanism for doing a delayed "SvREFCNT_dec".
	   However, while "sv_2mortal" 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".

	   Just like "SAVEFREESV", but mortalizes "sv" at the end of the current scope instead of
	   decrementing its reference count.  This usually 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.

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

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

       "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));

	   At the end of pseudo-block the function "f" is called with the only argument "p".

	   At the end of pseudo-block the function "f" is called with the implicit context argu-
	   ment (if any), and "p".

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

       The following API list contains functions, thus one needs to provide pointers to the modi-
       fiable data explicitly (either C pointers, or Perlish "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.

       XSUBs and the Argument Stack

       The XSUB mechanism is a simple way for Perl programs to access C subroutines.  An XSUB
       routine will have a stack that contains the arguments from the Perl program, and a way to
       map from the Perl data structures 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 abbre-

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

	   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
       pushed values will often need to be "mortal" (See "Reference Counts and Mortality"):

	   PUSHs(sv_2mortal(newSVpv("Some String",0)))

       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


       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 "(X)PUSH[iunp]"
       are not suited to XSUBs which return multiple results.  For that, either stick to the
       "(X)PUSHs" macros shown above, or use the new "m(X)PUSH[iunp]" macros instead; 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 pro-
       gram.  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 subroutine 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

       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:


       For a detailed description of calling conventions from C to Perl, consult 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 distributed 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.

       The following three macros are used to initially allocate memory :

	   Newx(pointer, number, type);
	   Newxc(pointer, number, type, cast);
	   Newxz(pointer, number, type);

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

       The second and third 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 "Newxc", "cast", should be used if the "pointer" argument is different
       from the "type" argument.

       Unlike the "Newx" and "Newxc" macros, the "Newxz" macro calls "memzero" to zero out all
       the newly allocated memory.


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

       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 allocated 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).


       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 creating a new abstraction layer that then calls whichever imple-
       mentation 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

       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 reuse 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 double, 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[iunp]".

       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:


       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 then you should either use the "(X)PUSHs"
       macros, or else use the new "m(X)PUSH[iunp]" macros, none of which make use of "TARG".
       The "(X)PUSHs" macros simply push an SV* on the stack, which, as noted under "XSUBs and
       the Argument Stack", will often need to be "mortal".  The new "m(X)PUSH[iunp]" macros make
       this a little easier to achieve by creating a new mortal for you (via "(X)PUSHmortal"),
       pushing that onto the stack (extending it if necessary in the case of the "mXPUSH[iunp]"
       macros), and then setting its value.  Thus, instead of writing this to "fix" the example


       you can simply write:


       On a related note, if you do use "(X)PUSH[iunp]", 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".	See also "dTARGET" and "dXSTARG".


       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 subroutines) -- 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 (initially) 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 several execution point-
       ers going into the same subroutine. For the subroutine-child not write over the tempo-
       raries 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 sepa-
       rate 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

Compiled code
       Code tree

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

	 $a = $b + $c;

       This is converted to a tree similar to this one:

		  /	      \
		 +	       $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 contains 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 "-DDEBUGGING" on the "Con-
       figure" 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

	   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 whatever".

       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, dif-
       ferent 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 following the "op_sibling" pointer from the first child to the

       There are also two other op types: a "PMOP" holds a regular expression, and has no chil-
       dren, and a "LOOP" may or may not have children. If the "op_children" field is non-zero,
       it behaves like a "LISTOP". To complicate 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 constructions it recog-
       nizes. Since yacc works bottom-up, so does the first pass of perl compilation.

       What makes this pass interesting for perl developers is that some optimization may be per-
       formed on this pass.  This is optimization by so-called "check routines".  The correspon-
       dence 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 com-
       pile-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 sub-
       stituted 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 pro-
       cessed 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 addi-
       tional pass over the code is performed. This pass is neither top-down or bottom-up, but in
       the execution order (with additional 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_stan-
       dard" 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

Examining internal data structures with the ";dump" functions
       To aid debugging, the source file dump.c contains a number of functions which produce for-
       matted 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 derivatives, and pro-
       duces 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 functions 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

       Two macros control the major Perl build flavors: MULTIPLICITY and USE_5005THREADS.  The
       MULTIPLICITY build has a C structure that packages 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 subrou-
       tine 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

       Second problem: there must be a syntax so that the same subroutine declarations and calls
       can pass a structure as their first argument, or pass nothing.  To solve this, the subrou-
       tines 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

       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 inter-
       preter'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

       When Perl is built without options that set PERL_IMPLICIT_CONTEXT, there is no first argu-
       ment containing the interpreter's context.  The trailing underscore in the pTHX_ macro
       indicates that the macro expansion 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 argument.  The form of the macro with-
       out 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)
	     /* can't 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 compiled with.

       This doesn't work so cleanly for varargs functions, though, as macros imply that the num-
       ber of arguments is known in advance.  Instead we either need to spell them out fully,
       passing "aTHX_" as the first argument (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_nocontext, 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

       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 use-
       ful 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 argument 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_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 void my_private_function(int arg1, int arg2);

	       STATIC void
	       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 extension is the addition
       of a "#define PERL_NO_GET_CONTEXT" before including the Perl headers, followed by a
       "dTHX;" declaration at the start of every function that will call the Perl API.	(You'll
       know which functions need this, because the C compiler will complain that there's an unde-
       clared 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 void my_private_function(pTHX_ int arg1, int arg2);

	       STATIC void
	       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 under-
       score for functions that take explicit arguments, or the form without the argument for
       functions with no explicit arguments.

       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 inter-
       preter is always accessed in the same thread that created it, and that thread did not cre-
       ate 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 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 pre-
       tend that different interpreters are actually different "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

Internal Functions
       All of Perl's 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 conven-
       tion, static functions start with "S_".)

       Inside the Perl core, you can get at the functions either with or without the "Perl_" pre-
       fix, 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. 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 in embed.fnc 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. Columns after that are
       the arguments. The first column is a set of flags:

       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 "Perl_av_fetch".

       d  This function has documentation using the "apidoc" feature which we'll 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 fol-
	  lows that callers don't use "aTHX".  (See "Background and PERL_IMPLICIT_CONTEXT" in

       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 isn't 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_" imple-
	  mentation (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 files.

       Formatted Printing of IVs, UVs, and NVs

       If you are printing IVs, UVs, or NVS instead of the stdio(3) style formatting 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.

	       INT2PTR(pointertotype, integer)

       For example:

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


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

       Exception Handling

       There are a couple of macros to do very basic exception handling in XS modules. You have
       to define "NO_XSLOCKS" before including XSUB.h to be able to use these macros:

	       #define NO_XSLOCKS
	       #include "XSUB.h"

       You can use these macros if you call code that may croak, but you need to do some cleanup
       before giving control back to Perl. For example:

	       dXCPT;	 /* set up neccessary variables */

	       XCPT_TRY_START {
	       } XCPT_TRY_END

		 /* do cleanup here */

       Note that you always have to rethrow an exception that has been caught. Using these
       macros, it is not possible to just catch the exception and ignore it. If you have to
       ignore the exception, you have to use the "call_*" function.

       The advantage of using the above macros is that you don't have to setup an extra function
       for "call_*", and that using these macros is faster than using "call_*".

       Source Documentation

       There's an effort going on to document the internal functions and automatically produce
       reference manuals from them - perlapi is one such manual which details all the functions
       which are available to XS writers. perlintern is the autogenerated manual for the func-
       tions 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 interfaces of existing
       functions are changed. The "Devel::PPPort" module tries to provide compatibility code for
       some of these changes, so XS writers don't have to code it themselves when supporting mul-
       tiple versions 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. It's important for porters and XS writers to under-
       stand 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 prob-
       lem; the problem is that it's not particularly 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 thou-
       sands 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

       To fix this, some people formed Unicode, Inc. and produced a new character set containing
       all the characters you can possibly think of and more. There are several ways of repre-
       senting these characters, and the one Perl uses is called UTF-8. UTF-8 uses a variable
       number of bytes to represent a character. You can learn more about Unicode and Perl's Uni-
       code model in perlunicode.

       How can I recognise a UTF-8 string?

       You can't. 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 repre-
       sented 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.

       In general, you either have to know what you're dealing with, or you have to guess.  The
       API function "is_utf8_string" can help; it'll tell you if a string contains only valid
       UTF-8 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

       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 0...127 are stored in one byte, just like good ol' ASCII. Character 128 is
       stored as "v194.128"; this continues 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 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. You're on your own about bounds checking,
       though, so don't use it lightly.

       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 charac-
       ter; 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 you're ever in a situation
       where you have to match UTF-8 and non-UTF-8 characters. You may not skip over UTF-8 char-
       acters in this case. If you do this, you'll 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 don't do that!

       How does Perl store UTF-8 strings?

       Currently, Perl deals with Unicode strings and non-Unicode strings slightly differently. A
       flag in the SV, "SVf_UTF8", indicates that the string is internally encoded as UTF-8.
       Without it, the byte value is the codepoint number and vice versa (in other words, the
       string is encoded as iso-8859-1). You can check and manipulate this flag with the follow-
       ing macros:


       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 han-
       dling operations will have undesirable results.

       The problem comes when you have, for instance, a string that isn't flagged as 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
       don't accidentally knock it off while you're manipulating SVs. More specifically, you can-
       not 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 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

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

       In fact, your "frobnicate" function should be made aware of whether or not it's dealing
       with UTF-8 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 passing a "char *" to an XS function.

       How do I convert a string to UTF-8?

       If you're mixing UTF-8 and non-UTF-8 strings, it is necessary to upgrade one of the
       strings to UTF-8. If you've 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 shouldn't be noticeable by the end user, it can cause
       problems in deficient code.

       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 comparisons 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

       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 UTF-8 or not. You can tell if an SV is UTF-8 by
	  looking at is "SvUTF8" flag. Don't forget to set the flag if something should be UTF-8.
	  Treat the flag as part of the PV, even though it's not - if you pass on the PV to some-
	  where, pass on the flag too.

       o  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.

       o  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".

       o  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

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 creating 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 responsible for
       adding stack marks if necessary.

       You should also "register" your op with the Perl interpreter so that it can produce sensi-
       ble error and warning messages. Since it is possible to have multiple custom ops within
       the one "logical" op type "OP_CUSTOM", Perl uses the value of "o->op_ppaddr" as a key into
       the "PL_custom_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 <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 Bowers, Matthew Green, Tim Bunce, Spi-
       der Boardman, Ulrich Pfeifer, Stephen McCamant, and Gurusamy Sarathy.

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

perl v5.8.9				    2007-11-17				      PERLGUTS(1)

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