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perlguts(1)            Perl Programmers Reference Guide            perlguts(1)




NAME

       perlguts - Introduction to the Perl API


DESCRIPTION

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


Variables

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

           SV  Scalar Value
           AV  Array Value
           HV  Hash Value

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

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

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

   Working with SVs
       An SV can be created and loaded with one command.  There are five types
       of values that can be loaded: an integer value (IV), an unsigned
       integer value (UV), a double (NV), a string (PV), and another scalar
       (SV).  ("PV" stands for "Pointer Value".  You might think that it is
       misnamed because it is described as pointing only to strings.  However,
       it is possible to have it point to other things  For example, it could
       point to an array of UVs.  But, using it for non-strings requires care,
       as the underlying assumption of much of the internals is that PVs are
       just for strings.  Often, for example, a trailing NUL is tacked on
       automatically.  The non-string use is documented only in this
       paragraph.)

       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 initialization, 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 the undef value.

           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, and not otherwise
       containing NULs.

       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 argument points to a boolean; on
       return, if that boolean is true, then locale-specific information has
       been used to format the string, and the string's contents are therefore
       untrustworthy (see perlsec).  This pointer may be NULL if that
       information is not important.  Note that this function requires you to
       specify the length of the format.

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

       All SVs that contain strings should be terminated with a NUL character.
       If it is not NUL-terminated there is a risk of core dumps and
       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:

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

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

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

       Also remember that C doesn't allow you to safely say "foo(SvPV(s, len),
       len);".  It might work with your compiler, but it won't work for
       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:

           SvTRUE(SV*)

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

           SvGROW(SV*, STRLEN newlen)

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

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

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

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

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

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

           SvEND(SV*)

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

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

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

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

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

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

           SV*  get_sv("package::varname", 0);

       This returns NULL if the variable does not exist.

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

           SvOK(SV*)

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

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

         foo(undef);

       But won't work when called as:

         $x = undef;
         foo($x);

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

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

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

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

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

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

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

   Offsets
       Perl provides the function "sv_chop" to efficiently remove characters
       from the beginning of a string; you give it an SV and a pointer to
       somewhere inside the PV, and it discards everything before the pointer.
       The efficiency comes by means of a little hack: instead of actually
       removing the characters, "sv_chop" sets the flag "OOK" (offset OK) to
       signal to other functions that the offset hack is in effect, and it
       moves the PV pointer (called "SvPVX") forward by the number of bytes
       chopped off, and adjusts "SvCUR" and "SvLEN" accordingly.  (A portion
       of the space between the old and new PV pointers is used to store the
       count of chopped bytes.)

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

       This is best demonstrated by example:

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

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

       Something similar to the offset hack is performed on AVs to enable
       efficient shifting and splicing off the beginning of the array; while
       "AvARRAY" points to the first element in the array that is visible from
       Perl, "AvALLOC" points to the real start of the C array.  These are
       usually the same, but a "shift" operation can be carried out by
       increasing "AvARRAY" by one and decreasing "AvFILL" and "AvMAX".
       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:

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

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

       There are various ways in which the private and public flags may
       differ.  For example, in perl 5.16 and earlier 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.  (In perl 5.18 onwards, tied scalars use the flags the same
       way as untied scalars.)  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(SSize_t 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
       it:

           void  av_push(AV*, SV*);
           SV*   av_pop(AV*);
           SV*   av_shift(AV*);
           void  av_unshift(AV*, SSize_t 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:

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

       The "av_top_index" function returns the highest index value in an 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.

       A few more:

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

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

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

           AV*  get_av("package::varname", 0);

       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
       it:

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

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

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

       The first of these two functions checks if a hash table entry exists,
       and the second 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 a linked list of structures with a
       typedef of HE.  These contain the actual key and value pointers (plus
       extra administrative overhead).  The key is a string pointer; the value
       is an "SV*".  However, once you have an "HE*", to get the actual key
       and value, use the routines specified below.

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

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

           HV*  get_hv("package::varname", 0);

       This returns NULL if the variable does not exist.

       The hash algorithm is defined in the "PERL_HASH" macro:

           PERL_HASH(hash, key, klen)

       The exact implementation of this macro varies by architecture and
       version of perl, and the return value may change per invocation, so the
       value is only valid for the duration of a single perl process.

       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 functions).

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

       The following macros must always be used to access the contents of hash
       entries.  Note that the arguments to these macros must be simple
       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.  In perl 5.18 and earlier, 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.  In
       perl 5.20, storing &PL_sv_undef will create a read-only element,
       because the scalar &PL_sv_undef itself is stored, not a copy.

       Similar 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_yes or
       &PL_sv_no 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_yes and &PL_sv_no 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 example:

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

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

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

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

       The "thing" argument can be any of an "SV*", "AV*", or "HV*".  The
       functions are identical except that "newRV_inc" increments the
       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:

           SvRV(SV*)

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

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

           SvROK(SV*)

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

           SvTYPE(SvRV(SV*))

       The most useful types that will be returned are:

           < SVt_PVAV  Scalar
           SVt_PVAV    Array
           SVt_PVHV    Hash
           SVt_PVCV    Code
           SVt_PVGV    Glob (possibly a file handle)

       See "svtype" in perlapi 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 class.

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

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

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

       /* Still under construction */

       The following function upgrades rv to reference if not already one.
       Creates a 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);

       The following three functions copy 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);

       The following function 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, void* pv);

       The following function copies a 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, char* pv,
                                                                STRLEN length);

       The following function tests whether the SV is blessed into the
       specified class.  It does not check inheritance relationships.

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

       The following function tests whether the SV is a reference to a blessed
       object.

               int  sv_isobject(SV* sv);

       The following function 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", GV_ADD);
           AV*  get_av("package::varname", GV_ADD);
           HV*  get_hv("package::varname", GV_ADD);

       Notice the use of GV_ADD 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
       "GV_ADD" argument to enable certain extra features.  Those bits are:

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

             Name <varname> used only once: possible typo

           warning.

       GV_ADDWARN
           Issues the warning:

             Had to create <varname> unexpectedly

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

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

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

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

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

       However, there is one other function which manipulates the reference
       count of its argument.  The "newRV_inc" function, you will recall,
       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 reference count decremented depends on
       two macros, SAVETMPS and FREETMPS.  See perlcall and perlxs for more
       details on these macros.

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

       "Mortal" SVs are mainly used for SVs that are placed on perl's stack.
       For example an SV which is created just to pass a number to a called
       sub is made mortal to have it cleaned up automatically when 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 multiple times.  Thinking of
       "Mortalization" as deferred "SvREFCNT_dec" should help to minimize such
       problems.  For example if you are passing an SV which you know has a
       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 contains
       references to the various objects of that name, including (but not
       limited to) the following:

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

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

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

           HV*  gv_stashpv(const char* name, I32 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 variable $! contains either the numeric value of "errno"
       or its string equivalent from either "strerror" or "sys_errlist[]".

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

               SvIOK_on
               SvNOK_on
               SvPOK_on
               SvROK_on

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

       For example, to create a new Perl variable called "dberror" that
       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", GV_ADD);
           sv_setiv(sv, (IV) dberror);
           sv_setpv(sv, dberror_list[dberror]);
           SvIOK_on(sv);

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

   Read-Only Values
       In Perl 5.16 and earlier, copy-on-write (see the next section) shared a
       flag bit with read-only scalars.  So the only way to test whether
       "sv_setsv", etc., will raise a "Modification of a read-only value"
       error in those versions is:

           SvREADONLY(sv) && !SvIsCOW(sv)

       Under Perl 5.18 and later, SvREADONLY only applies to read-only
       variables, and, under 5.20, copy-on-write scalars can also be read-
       only, so the above check is incorrect.  You just want:

           SvREADONLY(sv)

       If you need to do this check often, define your own macro like this:

           #if PERL_VERSION >= 18
           # define SvTRULYREADONLY(sv) SvREADONLY(sv)
           #else
           # define SvTRULYREADONLY(sv) (SvREADONLY(sv) && !SvIsCOW(sv))
           #endif

   Copy on Write
       Perl implements a copy-on-write (COW) mechanism for scalars, in which
       string copies are not immediately made when requested, but are deferred
       until made necessary by one or the other scalar changing.  This is
       mostly transparent, but one must take care not to modify string buffers
       that are shared by multiple SVs.

       You can test whether an SV is using copy-on-write with "SvIsCOW(sv)".

       You can force an SV to make its own copy of its string buffer by
       calling "sv_force_normal(sv)" or SvPV_force_nolen(sv).

       If you want to make the SV drop its string buffer, use
       "sv_force_normal_flags(sv, SV_COW_DROP_PV)" or simply "sv_setsv(sv,
       NULL)".

       All of these functions will croak on read-only scalars (see the
       previous section for more on those).

       To test that your code is behaving correctly and not modifying COW
       buffers, on systems that support mmap(2) (i.e., Unix) you can configure
       perl with "-Accflags=-DPERL_DEBUG_READONLY_COW" and it will turn buffer
       violations into crashes.  You will find it to be marvellously slow, so
       you may want to skip perl's own tests.

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

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

           struct magic {
               MAGIC*      mg_moremagic;
               MGVTBL*     mg_virtual;
               U16         mg_private;
               char        mg_type;
               U8          mg_flags;
               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 Table" should be assigned to the "mg_virtual" field.
       See the "Magic Virtual Tables" section below.  The "how" argument is
       also stored in the "mg_type" field.  The value of "how" should be
       chosen from the set of macros "PERL_MAGIC_foo" found in perl.h.  Note
       that before these macros were added, Perl internals used to directly
       use character literals, so you may occasionally come across old code or
       documentation referring to 'U' magic rather than "PERL_MAGIC_uvar" for
       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:

           int sv_unmagic(SV *sv, int type);

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

       However, note that "sv_unmagic" removes all magic of a certain "type"
       from the "SV".  If you want to remove only certain magic of a "type"
       based on the magic virtual table, use "sv_unmagicext" instead:

           int sv_unmagicext(SV *sv, int type, MGVTBL *vtbl);

   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, I32 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 32 types.  These different structures contain pointers to
       various routines that perform additional actions depending on which
       function is being called.

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

          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 operation 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
       considered 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:

        mg_type
        (old-style char and macro)   MGVTBL         Type of magic
        --------------------------   ------         -------------
        \0 PERL_MAGIC_sv             vtbl_sv        Special scalar variable
        #  PERL_MAGIC_arylen         vtbl_arylen    Array length ($#ary)
        %  PERL_MAGIC_rhash          (none)         extra data for restricted
                                                    hashes
        &  PERL_MAGIC_proto          (none)         my sub prototype CV
        .  PERL_MAGIC_pos            vtbl_pos       pos() lvalue
        :  PERL_MAGIC_symtab         (none)         extra data for symbol
                                                    tables
        <  PERL_MAGIC_backref        vtbl_backref   for weak ref data
        @  PERL_MAGIC_arylen_p       (none)         to move arylen out of XPVAV
        B  PERL_MAGIC_bm             vtbl_regexp    Boyer-Moore
                                                    (fast string search)
        c  PERL_MAGIC_overload_table vtbl_ovrld     Holds overload table
                                                    (AMT) on stash
        D  PERL_MAGIC_regdata        vtbl_regdata   Regex match position data
                                                    (@+ and @- vars)
        d  PERL_MAGIC_regdatum       vtbl_regdatum  Regex match position data
                                                    element
        E  PERL_MAGIC_env            vtbl_env       %ENV hash
        e  PERL_MAGIC_envelem        vtbl_envelem   %ENV hash element
        f  PERL_MAGIC_fm             vtbl_regexp    Formline
                                                    ('compiled' format)
        g  PERL_MAGIC_regex_global   vtbl_mglob     m//g target
        H  PERL_MAGIC_hints          vtbl_hints     %^H hash
        h  PERL_MAGIC_hintselem      vtbl_hintselem %^H hash element
        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
        N  PERL_MAGIC_shared         (none)         Shared between threads
        n  PERL_MAGIC_shared_scalar  (none)         Shared between threads
        o  PERL_MAGIC_collxfrm       vtbl_collxfrm  Locale 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_regexp    precompiled qr// regex
        S  PERL_MAGIC_sig            (none)         %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
        u  PERL_MAGIC_uvar_elem      (none)         Reserved for use by
                                                    extensions
        V  PERL_MAGIC_vstring        (none)         SV was vstring literal
        v  PERL_MAGIC_vec            vtbl_vec       vec() lvalue
        w  PERL_MAGIC_utf8           vtbl_utf8      Cached UTF-8 information
        x  PERL_MAGIC_substr         vtbl_substr    substr() lvalue
        y  PERL_MAGIC_defelem        vtbl_defelem   Shadow "foreach" iterator
                                                    variable / smart parameter
                                                    vivification
        ]  PERL_MAGIC_checkcall      vtbl_checkcall inlining/mutation of call
                                                    to this CV
        ~  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
       useful because there is no way for normal perl code to corrupt this
       private information (unlike using extra elements of a hash object).

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

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

       When the SV is read from or written to, the "uf_val" or "uf_set"
       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.

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

       Attaching "PERL_MAGIC_uvar" to arrays is permissible but has no effect.

       For hashes there is a specialized hook that gives control over hash
       keys (but not values).  This hook calls "PERL_MAGIC_uvar" 'get' magic
       if the "set" function in the "ufuncs" structure is NULL.  The hook is
       activated whenever the hash is accessed with a key specified as an "SV"
       through the functions "hv_store_ent", "hv_fetch_ent", "hv_delete_ent",
       and "hv_exists_ent".  Accessing the key as a string through the
       functions without the "..._ent" suffix circumvents the hook.  See
       "GUTS" in Hash::Util::FieldHash for a detailed description.

       Note that because multiple extensions may be using "PERL_MAGIC_ext" or
       "PERL_MAGIC_uvar" magic, it is important for extensions to take extra
       care to avoid conflict.  Typically only using the magic on objects
       blessed into the same class as the extension is sufficient.  For
       "PERL_MAGIC_ext" magic, it is usually a good idea to define an
       "MGVTBL", even if all its fields will be 0, so that individual "MAGIC"
       pointers can be identified as a particular kind of magic using their
       magic virtual table.  "mg_findext" provides an easy way to do that:

           STATIC MGVTBL my_vtbl = { 0, 0, 0, 0, 0, 0, 0, 0 };

           MAGIC *mg;
           if ((mg = mg_findext(sv, PERL_MAGIC_ext, &my_vtbl))) {
               /* this is really ours, not another module's PERL_MAGIC_ext */
               my_priv_data_t *priv = (my_priv_data_t *)mg->mg_ptr;
               ...
           }

       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 "SvSETMAGIC()" macro after calling
       these functions, or by using one of the "sv_set*_mg()" or
       "sv_cat*_mg()" functions.  Similarly, generic C code must call the
       "SvGETMAGIC()" macro to invoke any 'get' magic if they use an SV
       obtained from 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 *sv, int type); /* Finds the magic pointer of that
                                              * type */

       This routine returns a pointer to a "MAGIC" structure stored in the SV.
       If the SV does not have that magical feature, "NULL" is returned.  If
       the SV has multiple instances of that magical feature, the first one
       will be returned.  "mg_findext" can be used to find a "MAGIC" structure
       of an SV based on both its magic type and its magic virtual table:

           MAGIC *mg_findext(SV *sv, int type, MGVTBL *vtbl);

       Also, if the SV passed to "mg_find" or "mg_findext" 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.

           SV*
           mytie()
           PREINIT:
               HV *hash;
               HV *stash;
               SV *tie;
           CODE:
               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);
           OUTPUT:
               RETVAL

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

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

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

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

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

       You would do well to understand that the TIEARRAY and TIEHASH
       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.

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

       "SAVEFREESV(SV *sv)"
           The refcount of "sv" would be decremented at the end of pseudo-
           block.  This is similar to "sv_2mortal" in that it is also a
           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".

       "SAVEMORTALIZESV(SV *sv)"
           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.

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

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

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

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

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

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

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

       The following API list contains functions, thus one needs to provide
       pointers to the modifiable data explicitly (either C pointers, or
       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.  It doesn't handle magic.  Use "save_scalar" if
           magic is affected.

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

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

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

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


Subroutines

   XSUBs and the Argument Stack
       The XSUB mechanism is a simple way for Perl programs to access C
       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 abbreviations.

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

           EXTEND(SP, num);

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

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

           PUSHs(sv_2mortal(newSViv(an_integer)))
           PUSHs(sv_2mortal(newSVuv(an_unsigned_integer)))
           PUSHs(sv_2mortal(newSVnv(a_double)))
           PUSHs(sv_2mortal(newSVpv("Some String",0)))
           /* Although the last example is better written as the more
            * efficient: */
           PUSHs(newSVpvs_flags("Some String", SVs_TEMP))

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

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

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

           XPUSHs(SV*)

       This macro automatically adjusts 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.

   Autoloading with XSUBs
       If an AUTOLOAD routine is an XSUB, as with Perl subroutines, Perl puts
       the fully-qualified name of the autoloaded subroutine in the $AUTOLOAD
       variable of the XSUB's package.

       But it also puts the same information in certain fields of the XSUB
       itself:

           HV *stash           = CvSTASH(cv);
           const char *subname = SvPVX(cv);
           STRLEN name_length  = SvCUR(cv); /* in bytes */
           U32 is_utf8         = SvUTF8(cv);

       "SvPVX(cv)" contains just the sub name itself, not including the
       package.  For an AUTOLOAD routine in UNIVERSAL or one of its
       superclasses, "CvSTASH(cv)" returns NULL during a method call on a
       nonexistent package.

       Note: Setting $AUTOLOAD stopped working in 5.6.1, which did not support
       XS AUTOLOAD subs at all.  Perl 5.8.0 introduced the use of fields in
       the XSUB itself.  Perl 5.16.0 restored the setting of $AUTOLOAD.  If
       you need to support 5.8-5.14, use the XSUB's fields.

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

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

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

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

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

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

   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:

           XPUSHi(10);
           XPUSHi(20);

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

       If you need to push multiple different values 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 above:

           XPUSHs(sv_2mortal(newSViv(10)))
           XPUSHs(sv_2mortal(newSViv(20)))

       you can simply write:

           mXPUSHi(10)
           mXPUSHi(20)

       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".

   Scratchpads
       The question remains on when the SVs which are targets for opcodes are
       created.  The answer is that they are created when the current unit--a
       subroutine or a file (for opcodes for statements outside of
       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.  A previous version of this document stated that
       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.  But this have never been fully true.  "SVs_PADMY" could be set on
       a variable that no longer resides in any pad.  While targets do have
       "SVs_PADTMP" set, it can also be set on variables that have never
       resided in a pad, but nonetheless act like targets.

       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 pointers going into the same subroutine.  For the
       subroutine-child not write over the temporaries for the subroutine-
       parent (lifespan of which covers the call to the child), the parent and
       the child should have different scratchpads.  (And the lexicals should
       be separate anyway!)

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

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


Memory Allocation

   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.

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

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

   Moving
           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).


PerlIO

       The most recent development releases of Perl have 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
       implementation of stdio Perl was compiled with.  All XSUBs should now
       use the functions in the PerlIO abstraction layer and not make any
       assumptions about what kind of stdio is being used.

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


Compiled code

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

         $a = $b + $c;

       This is converted to a tree similar to this one:

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

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

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

       But with the actual compile tree for "$a = $b + $c" it is different:
       some nodes optimized away.  As a corollary, though the actual tree
       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 "Configure" command line), you may examine the
       compiled tree by specifying "-Dx" on the Perl command line.  The output
       takes several lines per node, and for "$b+$c" it looks like this:

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

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

                          add
                        /     \
                      null    null
                       |       |
                      gvsv    gvsv

       The execution order is indicated by "===>" marks, thus it is "3 4 5 6"
       (node 6 is not included into above listing), i.e., "gvsv gvsv add
       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, different ops
       have different numbers of children: "add" is a binary operator, as one
       would expect, and so has two children.  To accommodate the various
       different numbers of children, there are various types of op data
       structure, and they link together in different ways.

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

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

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

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

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

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

   Compile pass 1a: constant folding
       Immediately after the check routine is called the returned node is
       checked for being compile-time executable.  If it is (the value is
       judged to be constant) it is immediately executed, and a constant node
       with the "return value" of the corresponding subtree is substituted
       instead.  The subtree is deleted.

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

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

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

   Compile pass 3: peephole optimization
       After the compile tree for a subroutine (or for an "eval" or a file) is
       created, an additional pass over the code is performed.  This pass is
       neither top-down or bottom-up, but in the execution order (with
       additional complications for conditionals).  Optimizations performed at
       this stage are subject to the same restrictions as in the pass 2.

       Peephole optimizations are done by calling the function pointed to by
       the global variable "PL_peepp".  By default, "PL_peepp" just calls the
       function pointed to by the global variable "PL_rpeepp".  By default,
       that performs some basic op fixups and optimisations along the
       execution-order op chain, and recursively calls "PL_rpeepp" for each
       side chain of ops (resulting from conditionals).  Extensions may
       provide additional optimisations or fixups, hooking into either the
       per-subroutine or recursive stage, like this:

           static peep_t prev_peepp;
           static void my_peep(pTHX_ OP *o)
           {
               /* custom per-subroutine optimisation goes here */
               prev_peepp(aTHX_ o);
               /* custom per-subroutine optimisation may also go here */
           }
           BOOT:
               prev_peepp = PL_peepp;
               PL_peepp = my_peep;

           static peep_t prev_rpeepp;
           static void my_rpeep(pTHX_ OP *o)
           {
               OP *orig_o = o;
               for(; o; o = o->op_next) {
                   /* custom per-op optimisation goes here */
               }
               prev_rpeepp(aTHX_ orig_o);
           }
           BOOT:
               prev_rpeepp = PL_rpeepp;
               PL_rpeepp = my_rpeep;

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

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

         PL_runops = my_runops;

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

   Compile-time scope hooks
       As of perl 5.14 it is possible to hook into the compile-time lexical
       scope mechanism using "Perl_blockhook_register".  This is used like
       this:

           STATIC void my_start_hook(pTHX_ int full);
           STATIC BHK my_hooks;

           BOOT:
               BhkENTRY_set(&my_hooks, bhk_start, my_start_hook);
               Perl_blockhook_register(aTHX_ &my_hooks);

       This will arrange to have "my_start_hook" called at the start of
       compiling every lexical scope.  The available hooks are:

       "void bhk_start(pTHX_ int full)"
           This is called just after starting a new lexical scope.  Note that
           Perl code like

               if ($x) { ... }

           creates two scopes: the first starts at the "(" and has "full ==
           1", the second starts at the "{" and has "full == 0".  Both end at
           the "}", so calls to "start" and "pre/post_end" will match.
           Anything pushed onto the save stack by this hook will be popped
           just before the scope ends (between the "pre_" and "post_end"
           hooks, in fact).

       "void bhk_pre_end(pTHX_ OP **o)"
           This is called at the end of a lexical scope, just before unwinding
           the stack.  o is the root of the optree representing the scope; it
           is a double pointer so you can replace the OP if you need to.

       "void bhk_post_end(pTHX_ OP **o)"
           This is called at the end of a lexical scope, just after unwinding
           the stack.  o is as above.  Note that it is possible for calls to
           "pre_" and "post_end" to nest, if there is something on the save
           stack that calls string eval.

       "void bhk_eval(pTHX_ OP *const o)"
           This is called just before starting to compile an "eval STRING",
           "do FILE", "require" or "use", after the eval has been set up.  o
           is the OP that requested the eval, and will normally be an
           "OP_ENTEREVAL", "OP_DOFILE" or "OP_REQUIRE".

       Once you have your hook functions, you need a "BHK" structure to put
       them in.  It's best to allocate it statically, since there is no way to
       free it once it's registered.  The function pointers should be inserted
       into this structure using the "BhkENTRY_set" macro, which will also set
       flags indicating which entries are valid.  If you do need to allocate
       your "BHK" dynamically for some reason, be sure to zero it before you
       start.

       Once registered, there is no mechanism to switch these hooks off, so if
       that is necessary you will need to do this yourself.  An entry in "%^H"
       is probably the best way, so the effect is lexically scoped; however it
       is also possible to use the "BhkDISABLE" and "BhkENABLE" macros to
       temporarily switch entries on and off.  You should also be aware that
       generally speaking at least one scope will have opened before your
       extension is loaded, so you will see some "pre/post_end" pairs that
       didn't have a matching "start".


Examining internal data structures with the "dump" functions

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

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

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

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

           (gdb) print Perl_dump_packsubs(PL_defstash)

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

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

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

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

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

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


How multiple interpreters and concurrency are supported

   Background and PERL_IMPLICIT_CONTEXT
       The Perl interpreter can be regarded as a closed box: it has an API for
       feeding it code or otherwise making it do things, but it also has
       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 interpreter.

       One macro controls the major Perl build flavor: MULTIPLICITY.  The
       MULTIPLICITY build has a C structure that packages all the interpreter
       state.  With multiplicity-enabled perls, PERL_IMPLICIT_CONTEXT is also
       normally defined, and enables the support for passing in a "hidden"
       first argument that represents all three data structures.  MULTIPLICITY
       makes multi-threaded perls possible (with the ithreads threading model,
       related to the macro USE_ITHREADS.)

       Two other "encapsulation" macros are the PERL_GLOBAL_STRUCT and
       PERL_GLOBAL_STRUCT_PRIVATE (the latter turns on the former, and the
       former turns on MULTIPLICITY.)  The PERL_GLOBAL_STRUCT causes all the
       internal variables of Perl to be wrapped inside a single global struct,
       struct perl_vars, accessible as (globals) &PL_Vars or PL_VarsPtr or the
       function  Perl_GetVars().  The PERL_GLOBAL_STRUCT_PRIVATE goes one step
       further, there is still a single struct (allocated in main() either
       from heap or from stack) but there are no global data symbols pointing
       to it.  In either case the global struct should be initialized as the
       very first thing in main() using Perl_init_global_struct() and
       correspondingly tear it down after perl_free() using
       Perl_free_global_struct(), please see miniperlmain.c for usage details.
       You may also need to use "dVAR" in your coding to "declare the global
       variables" when you are using them.  dTHX does this for you
       automatically.

       To see whether you have non-const data you can use a BSD-compatible
       "nm":

         nm libperl.a | grep -v ' [TURtr] '

       If this displays any "D" or "d" symbols, you have non-const data.

       For backward compatibility reasons defining just PERL_GLOBAL_STRUCT
       doesn't actually hide all symbols inside a big global struct: some
       PerlIO_xxx vtables are left visible.  The PERL_GLOBAL_STRUCT_PRIVATE
       then hides everything (see how the PERLIO_FUNCS_DECL is used).

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

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

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

         STATIC void
         S_incline(pTHX_ char *s)

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

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

         void
         Perl_sv_setiv(pTHX_ SV* dsv, IV num)

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

       When Perl is built without options that set PERL_IMPLICIT_CONTEXT,
       there is no first argument containing the interpreter's context.  The
       trailing underscore in the pTHX_ macro indicates that the macro
       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 without the trailing underscore is
       used when there are no additional explicit arguments.

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

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

       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 number 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 storage.)

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

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

   How do I use all this in extensions?
       When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call any
       functions in the Perl API will need to pass the initial context
       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 don't have to do anything 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 */

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

       Note that the only two changes from the normal way of writing an
       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 undeclared identifier in those functions.)  No changes are
       needed for the XSUBs themselves, because the XS() macro is correctly
       defined to pass in the implicit context if needed.

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

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

               /* pTHX_ only needed for functions that call Perl API */
               STATIC 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 */

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

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

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

       If one is compiling Perl with the "-DPERL_GLOBAL_STRUCT" the "dVAR"
       definition is needed if the Perl global variables (see perlvars.h or
       globvar.sym) are accessed in the function and "dTHX" is not used (the
       "dTHX" includes the "dVAR" if necessary).  One notices the need for
       "dVAR" only with the said compile-time define, because otherwise the
       Perl global variables are visible as-is.

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

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

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

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

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

       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
       ambitious host (like the one that would do fork() emulation) all the
       extra work needed to pretend 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 them.


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 convention,
       static functions start with "S_".)

       Inside the Perl core ("PERL_CORE" defined), you can get at the
       functions either with or without the "Perl_" prefix, thanks to a bunch
       of defines that live in embed.h.  Note that extension code should not
       set "PERL_CORE"; this exposes the full perl internals, and is likely to
       cause breakage of the XS in each new perl release.

       The file embed.h 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 an interpreter context, so the definition has no
          "pTHX", and it follows that callers don't use "aTHX".  (See
          "Background and PERL_IMPLICIT_CONTEXT".)

       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_" implementation (which is exported).

       others
          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.

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

       For example:

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

       and

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

   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 necessary variables */

               XCPT_TRY_START {
                 code_that_may_croak();
               } XCPT_TRY_END

               XCPT_CATCH
               {
                 /* do cleanup here */
                 XCPT_RETHROW;
               }

       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 functions
       which are not part of the API and are supposedly for internal use only.

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

        /*
        =for apidoc sv_setiv

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

        =cut
        */

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

   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 multiple
       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 understand this support and make sure that the code they
       write does not corrupt Unicode data.

   What is Unicode, anyway?
       In the olden, less enlightened times, we all used to use ASCII.  Most
       of us did, anyway.  The big problem with ASCII is that it's American.
       Well, no, that's not actually the problem; the problem is that it's not
       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 thousands of characters, then you really can't fit them
       into a mere 256, so they had to forget about ASCII altogether, and
       build their own systems using pairs of numbers to refer to one
       character.

       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 representing 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 Unicode 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 represented by the two bytes
       "v196.172".  Unfortunately, the non-Unicode string "chr(196).chr(172)"
       has that byte sequence as well.  So you can't tell just by looking -
       this is what makes Unicode input an interesting problem.

       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_buf" will tell you whether the current character in a
       string is valid UTF-8.

   How does UTF-8 represent Unicode characters?
       As mentioned above, UTF-8 uses a variable number of bytes to store a
       character.  Characters with values 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 is encoded as a single byte even in UTF-8):

           U8 *utf;
           U8 *utf_end; /* 1 beyond buffer pointed to by utf */
           UV uv;      /* Note: a UV, not a U8, not a char */
           STRLEN len; /* length of character in bytes */

           if (!UTF8_IS_INVARIANT(*utf))
               /* Must treat this as UTF-8 */
               uv = utf8_to_uvchr_buf(utf, utf_end, &len);
           else
               /* OK to treat this character as a byte */
               uv = *utf;

       You can also see in that example that we use "utf8_to_uvchr_buf" to get
       the value of the character; the inverse function "uvchr_to_utf8" is
       available for putting a UV into UTF-8:

           if (!UTF8_IS_INVARIANT(uv))
               /* Must treat this as UTF8 */
               utf8 = uvchr_to_utf8(utf8, uv);
           else
               /* OK to treat this character as a byte */
               *utf8++ = uv;

       You must convert characters to UVs using the above functions if you're
       ever in a situation where you have to match UTF-8 and non-UTF-8
       characters.  You may not skip over UTF-8 characters 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, but "use feature 'unicode_strings'" is needed to
       get iso-8859-1 semantics).  This flag is only meaningful if the SV is
       "SvPOK" or immediately after stringification via "SvPV" or a similar
       macro.  You can check and manipulate this flag with the following
       macros:

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

       This flag has an important effect on Perl's treatment of the string: if
       Unicode data is not properly distinguished, regular expressions,
       "length", "substr" and other string handling operations will have
       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 cannot expect to do this:

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

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

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

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

       In fact, your "frobnicate" function should be made aware of whether or
       not it's dealing with 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:

           sv_utf8_upgrade(sv);

       However, you must not do this, for example:

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

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

   Is there anything else I need to know?
       Not really.  Just remember these things:

       o  There's no way to tell if a string is UTF-8 or not.  You can tell if
          an SV is UTF-8 by looking at its "SvUTF8" flag after stringifying it
          with "SvPV" or a similar macro.  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 somewhere, pass on the
          flag too.

       o  If a string is UTF-8, always use "utf8_to_uvchr_buf" 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
          "uvchr_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, and then combine them.


Custom Operators

       Custom operator support is an 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 "op_ppaddr" 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 sensible 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" to determine which
       custom op it is dealing with.  You should create an "XOP" structure for
       each ppaddr you use, set the properties of the custom op with
       "XopENTRY_set", and register the structure against the ppaddr using
       "Perl_custom_op_register".  A trivial example might look like:

           static XOP my_xop;
           static OP *my_pp(pTHX);

           BOOT:
               XopENTRY_set(&my_xop, xop_name, "myxop");
               XopENTRY_set(&my_xop, xop_desc, "Useless custom op");
               Perl_custom_op_register(aTHX_ my_pp, &my_xop);

       The available fields in the structure are:

       xop_name
           A short name for your op.  This will be included in some error
           messages, and will also be returned as "$op->name" by the B module,
           so it will appear in the output of module like B::Concise.

       xop_desc
           A short description of the function of the op.

       xop_class
           Which of the various *OP structures this op uses.  This should be
           one of the "OA_*" constants from op.h, namely

           OA_BASEOP
           OA_UNOP
           OA_BINOP
           OA_LOGOP
           OA_LISTOP
           OA_PMOP
           OA_SVOP
           OA_PADOP
           OA_PVOP_OR_SVOP
               This should be interpreted as '"PVOP"' only.  The "_OR_SVOP" is
               because the only core "PVOP", "OP_TRANS", can sometimes be a
               "SVOP" instead.

           OA_LOOP
           OA_COP

           The other "OA_*" constants should not be used.

       xop_peep
           This member is of type "Perl_cpeep_t", which expands to "void
           (*Perl_cpeep_t)(aTHX_ OP *o, OP *oldop)".  If it is set, this
           function will be called from "Perl_rpeep" when ops of this type are
           encountered by the peephole optimizer.  o is the OP that needs
           optimizing; oldop is the previous OP optimized, whose "op_next"
           points to o.

       "B::Generate" directly supports the creation of custom ops by name.


AUTHORS

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

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


SEE ALSO

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



perl v5.20.0                      2014-05-26                       perlguts(1)

perl 5.20.0 - Generated Sat May 31 07:17:33 CDT 2014