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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 want to write to an existing SV's buffer and set its value to a string, use
       SvPV_force() or one of its variants to force the SV to be a PV.  This will remove any of
       various types of non-stringness from the SV while preserving the content of the SV in the
       PV.  This can be used, for example, to append data from an API function to a buffer
       without extra copying:

           (void)SvPVbyte_force(sv, len);
           s = SvGROW(sv, len + needlen + 1);
           /* something that modifies up to needlen bytes at s+len, but
              modifies newlen bytes
                eg. newlen = read(fd, s + len, needlen);
              ignoring errors for these examples
            */
           s[len + newlen] = '\0';
           SvCUR_set(sv, len + newlen);
           SvUTF8_off(sv);
           SvSETMAGIC(sv);

       If you already have the data in memory or if you want to keep your code simple, you can
       use one of the sv_cat*() variants, such as sv_catpvn().  If you want to insert anywhere in
       the string you can use sv_insert() or sv_insert_flags().

       If you don't need the existing content of the SV, you can avoid some copying with:

           SvPVCLEAR(sv);
           s = SvGROW(sv, needlen + 1);
           /* something that modifies up to needlen bytes at s, but modifies
              newlen bytes
                eg. newlen = read(fd, s. needlen);
            */
           s[newlen] = '\0';
           SvCUR_set(sv, newlen);
           SvPOK_only(sv); /* also clears SVf_UTF8 */
           SvSETMAGIC(sv);

       Again, if you already have the data in memory or want to avoid the complexity of the
       above, you can use sv_setpvn().

       If you have a buffer allocated with Newx() and want to set that as the SV's value, you can
       use sv_usepvn_flags().  That has some requirements if you want to avoid perl re-allocating
       the buffer to fit the trailing NUL:

          Newx(buf, somesize+1, char);
          /* ... fill in buf ... */
          buf[somesize] = '\0';
          sv_usepvn_flags(sv, buf, somesize, SV_SMAGIC | SV_HAS_TRAILING_NUL);
          /* buf now belongs to perl, don't release it */

       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.  Normally copy-on-write will prevent the
       substitution from operator from using this hack, but if you can craft a string for which
       copy-on-write is not possible, you can see it in play.  In the current implementation, the
       final byte of a string buffer is used as a copy-on-write reference count.  If the buffer
       is not big enough, then copy-on-write is skipped.  First have a look at an empty string:

         % ./perl -Ilib -MDevel::Peek -le '$a=""; $a .= ""; Dump $a'
         SV = PV(0x7ffb7c008a70) at 0x7ffb7c030390
           REFCNT = 1
           FLAGS = (POK,pPOK)
           PV = 0x7ffb7bc05b50 ""\0
           CUR = 0
           LEN = 10

       Notice here the LEN is 10.  (It may differ on your platform.)  Extend the length of the
       string to one less than 10, and do a substitution:

        % ./perl -Ilib -MDevel::Peek -le '$a=""; $a.="123456789"; $a=~s/.//; \
                                                                   Dump($a)'
        SV = PV(0x7ffa04008a70) at 0x7ffa04030390
          REFCNT = 1
          FLAGS = (POK,OOK,pPOK)
          OFFSET = 1
          PV = 0x7ffa03c05b61 ( "\1" . ) "23456789"\0
          CUR = 8
          LEN = 9

       Here the number of bytes chopped off (1) is shown next as the 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.  (The first
       character of the string buffer happens to have changed to "\1" here, not "1", because the
       current implementation stores the offset count in the string buffer.  This is subject to
       change.)

       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", "PERL_MAGIC_regdatum",
       "PERL_MAGIC_regdata", 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)  (pTHX_ SV* sv, MAGIC* mg);
           int  (*svt_set)  (pTHX_ SV* sv, MAGIC* mg);
           U32  (*svt_len)  (pTHX_ SV* sv, MAGIC* mg);
           int  (*svt_clear)(pTHX_ SV* sv, MAGIC* mg);
           int  (*svt_free) (pTHX_ SV* sv, MAGIC* mg);

           int  (*svt_copy) (pTHX_ SV *sv, MAGIC* mg, SV *nsv,
                                                 const char *name, I32 namlen);
           int  (*svt_dup)  (pTHX_ MAGIC *mg, CLONE_PARAMS *param);
           int  (*svt_local)(pTHX_ 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_debugvar       vtbl_debugvar  $DB::single, signal, trace
                                                    vars
        .  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_lvref          vtbl_lvref     Lvalue reference
                                                    constructor
        ]  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" will 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 has 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.  As of perl 5.21.5, the "SVs_PADMY" flag is no longer used and is defined as 0.
       "SvPADMY()" now returns true for anything without "SVs_PADTMP".

       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 "OpSIBLING" pointer from the first child
       to the last (but see below).

       There are also some 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.

       Finally, there is a "LOGOP", or logic op. Like a "LISTOP", this has one or more children,
       but it doesn't have an "op_last" field: so you have to follow "op_first" and then the
       "OpSIBLING" chain itself to find the last child. Instead it has an "op_other" field, which
       is comparable to the "op_next" field described below, and represents an alternate
       execution path. Operators like "and", "or" and "?" are "LOGOP"s. Note that in general,
       "op_other" may not point to any of the direct children of the "LOGOP".

       Starting in version 5.21.2, perls built with the experimental define "-DPERL_OP_PARENT"
       add an extra boolean flag for each op, "op_moresib".  When not set, this indicates that
       this is the last op in an "OpSIBLING" chain. This frees up the "op_sibling" field on the
       last sibling to point back to the parent op. Under this build, that field is also renamed
       "op_sibparent" to reflect its joint role. The macro OpSIBLING(o) wraps this special
       behaviour, and always returns NULL on the last sibling.  With this build the op_parent(o)
       function can be used to find the parent of any op. Thus for forward compatibility, you
       should always use the OpSIBLING(o) macro rather than accessing "op_sibling" directly.

       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 (or GNU) compatible "nm":

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

       If this displays any "D" or "d" symbols (or possibly "C" or "c"), you have non-const data.
       The symbols the "grep" removed are as follows: "Tt" are text, or code, the "Rr" are read-
       only (const) data, and the "U" is <undefined>, external symbols referred to.

       The test t/porting/libperl.t does this kind of symbol sanity checking on "libperl.a".

       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.

       Note that there are different "long doubles": Perl will use whatever the compiler has.

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

   Formatted Printing of Size_t and SSize_t
       The most general way to do this is to cast them to a UV or IV, and print as in the
       previous section.

       But if you're using "PerlIO_printf()", it's less typing and visual clutter to use the "%z"
       length modifier (for siZe):

               PerlIO_printf("STRLEN is %zu\n", len);

       This modifier is not portable, so its use should be restricted to "PerlIO_printf()".

   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
        L<perlapi/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.

       (On EBCDIC platforms, Perl uses instead UTF-EBCDIC, which is a form of UTF-8 adapted for
       EBCDIC platforms.  Below, we just talk about UTF-8.  UTF-EBCDIC is like UTF-8, but the
       details are different.  The macros hide the differences from you, just remember that the
       particular numbers and bit patterns presented below will differ in UTF-EBCDIC.)

   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, and the chances of a non-UTF-8 string looking like valid UTF-8 become
       very small very quickly with increasing string length.  On a character-by-character basis,
       "isUTF8_CHAR" will tell you whether the current character in a string is valid UTF-8.

   How does UTF-8 represent Unicode characters?
       As mentioned above, UTF-8 uses a variable number of bytes to store a character.
       Characters with values 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; character
       192 is "v195.128".  And so it goes on, moving to three bytes at character 2048.  "Unicode
       Encodings" in perlunicode has pictures of how this works.

       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 (!UVCHR_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!

       (Note that we don't have to test for invariant characters in the examples above.  The
       functions work on any well-formed UTF-8 input.  It's just that its faster to avoid the
       function overhead when it's not needed.)

   How does Perl store UTF-8 strings?
       Currently, Perl deals with UTF-8 strings and non-UTF-8 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.  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 UTF-8 data is not
       properly distinguished, regular expressions, "length", "substr" and other string handling
       operations will have undesirable (wrong) 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 from the PV value; you need to 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);
           is_utf8 = SvUTF8(sv);
           frobnicate(p, is_utf8);
           nsv = newSVpvn(p, len);
           if (is_utf8)
               SvUTF8_on(nsv);

       In the above, your "frobnicate" function has been changed to 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.

       For full generality, use the "DO_UTF8" macro to see if the string in an SV is to be
       treated as UTF-8.  This takes into account if the call to the XS function is being made
       from within the scope of "use bytes".  If so, the underlying bytes that comprise the UTF-8
       string are to be exposed, rather than the character they represent.  But this pragma
       should only really be used for debugging and perhaps low-level testing at the byte level.
       Hence most XS code need not concern itself with this, but various areas of the perl core
       do need to support it.

       And this isn't the whole story.  Starting in Perl v5.12, strings that aren't encoded in
       UTF-8 may also be treated as Unicode under various conditions (see "ASCII Rules versus
       Unicode Rules" in perlunicode).  This is only really a problem for characters whose
       ordinals are between 128 and 255, and their behavior varies under ASCII versus Unicode
       rules in ways that your code cares about (see "The "Unicode Bug"" in perlunicode).  There
       is no published API for dealing with this, as it is subject to change, but you can look at
       the code for "pp_lc" in pp.c for an example as to how it's currently done.

   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 the non-UTF-8
       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.

   How do I compare strings?
       "sv_cmp" in perlapi and "sv_cmp_flags" in perlapi do a lexigraphic comparison of two SV's,
       and handle UTF-8ness properly.  Note, however, that Unicode specifies a much fancier
       mechanism for collation, available via the Unicode::Collate module.

       To just compare two strings for equality/non-equality, you can just use "memEQ()" and
       "memNE()" as usual, except the strings must be both UTF-8 or not UTF-8 encoded.

       To compare two strings case-insensitively, use "foldEQ_utf8()" (the strings don't have to
       have the same UTF-8ness).

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

       ·  There's no way to tell if a "char *" or "U8 *" string is UTF-8 or not.  But you can
          tell if an SV is to be treated as UTF-8 by calling "DO_UTF8" on it, after stringifying
          it with "SvPV" or a similar macro.  And, you can tell if SV is actually UTF-8 (even if
          it is not to be treated as such) by looking at its "SvUTF8" flag (again after
          stringifying it).  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.

       ·  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.

       ·  When writing a character UV to a UTF-8 string, always use "uvchr_to_utf8", unless
          "UVCHR_IS_INVARIANT(uv))" in which case you can use "*s = uv".

       ·  Mixing UTF-8 and non-UTF-8 strings is tricky.  Use "bytes_to_utf8" to get a new string
          which is UTF-8 encoded, 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.

Dynamic Scope and the Context Stack

       Note: this section describes a non-public internal API that is subject to change without
       notice.

   Introduction to the context stack
       In Perl, dynamic scoping refers to the runtime nesting of things like subroutine calls,
       evals etc, as well as the entering and exiting of block scopes. For example, the restoring
       of a "local"ised variable is determined by the dynamic scope.

       Perl tracks the dynamic scope by a data structure called the context stack, which is an
       array of "PERL_CONTEXT" structures, and which is itself a big union for all the types of
       context. Whenever a new scope is entered (such as a block, a "for" loop, or a subroutine
       call), a new context entry is pushed onto the stack. Similarly when leaving a block or
       returning from a subroutine call etc. a context is popped. Since the context stack
       represents the current dynamic scope, it can be searched.  For example, "next LABEL"
       searches back through the stack looking for a loop context that matches the label;
       "return" pops contexts until it finds a sub or eval context or similar; "caller" examines
       sub contexts on the stack.

       Each context entry is labelled with a context type, "cx_type". Typical context types are
       "CXt_SUB", "CXt_EVAL" etc., as well as "CXt_BLOCK" and "CXt_NULL" which represent a basic
       scope (as pushed by "pp_enter") and a sort block. The type determines which part of the
       context union are valid.

       The main division in the context struct is between a substitution scope ("CXt_SUBST") and
       block scopes, which are everything else. The former is just used while executing "s///e",
       and won't be discussed further here.

       All the block scope types share a common base, which corresponds to "CXt_BLOCK". This
       stores the old values of various scope-related variables like "PL_curpm", as well as
       information about the current scope, such as "gimme". On scope exit, the old variables are
       restored.

       Particular block scope types store extra per-type information. For example, "CXt_SUB"
       stores the currently executing CV, while the various for loop types might hold the
       original loop variable SV. On scope exit, the per-type data is processed; for example the
       CV has its reference count decremented, and the original loop variable is restored.

       The macro "cxstack" returns the base of the current context stack, while "cxstack_ix" is
       the index of the current frame within that stack.

       In fact, the context stack is actually part of a stack-of-stacks system; whenever
       something unusual is done such as calling a "DESTROY" or tie handler, a new stack is
       pushed, then popped at the end.

       Note that the API described here changed considerably in perl 5.24; prior to that, big
       macros like "PUSHBLOCK" and "POPSUB" were used; in 5.24 they were replaced by the inline
       static functions described below. In addition, the ordering and detail of how these
       macros/function work changed in many ways, often subtly. In particular they didn't handle
       saving the savestack and temps stack positions, and required additional "ENTER",
       "SAVETMPS" and "LEAVE" compared to the new functions. The old-style macros will not be
       described further.

   Pushing contexts
       For pushing a new context, the two basic functions are "cx = cx_pushblock()", which pushes
       a new basic context block and returns its address, and a family of similar functions with
       names like "cx_pushsub(cx)" which populate the additional type-dependent fields in the
       "cx" struct. Note that "CXt_NULL" and "CXt_BLOCK" don't have their own push functions, as
       they don't store any data beyond that pushed by "cx_pushblock".

       The fields of the context struct and the arguments to the "cx_*" functions are subject to
       change between perl releases, representing whatever is convenient or efficient for that
       release.

       A typical context stack pushing can be found in "pp_entersub"; the following shows a
       simplified and stripped-down example of a non-XS call, along with comments showing roughly
       what each function does.

        dMARK;
        U8 gimme      = GIMME_V;
        bool hasargs  = cBOOL(PL_op->op_flags & OPf_STACKED);
        OP *retop     = PL_op->op_next;
        I32 old_ss_ix = PL_savestack_ix;
        CV *cv        = ....;

        /* ... make mortal copies of stack args which are PADTMPs here ... */

        /* ... do any additional savestack pushes here ... */

        /* Now push a new context entry of type 'CXt_SUB'; initially just
         * doing the actions common to all block types: */

        cx = cx_pushblock(CXt_SUB, gimme, MARK, old_ss_ix);

            /* this does (approximately):
                CXINC;              /* cxstack_ix++ (grow if necessary) */
                cx = CX_CUR();      /* and get the address of new frame */
                cx->cx_type        = CXt_SUB;
                cx->blk_gimme      = gimme;
                cx->blk_oldsp      = MARK - PL_stack_base;
                cx->blk_oldsaveix  = old_ss_ix;
                cx->blk_oldcop     = PL_curcop;
                cx->blk_oldmarksp  = PL_markstack_ptr - PL_markstack;
                cx->blk_oldscopesp = PL_scopestack_ix;
                cx->blk_oldpm      = PL_curpm;
                cx->blk_old_tmpsfloor = PL_tmps_floor;

                PL_tmps_floor        = PL_tmps_ix;
            */

        /* then update the new context frame with subroutine-specific info,
         * such as the CV about to be executed: */

        cx_pushsub(cx, cv, retop, hasargs);

            /* this does (approximately):
                cx->blk_sub.cv          = cv;
                cx->blk_sub.olddepth    = CvDEPTH(cv);
                cx->blk_sub.prevcomppad = PL_comppad;
                cx->cx_type            |= (hasargs) ? CXp_HASARGS : 0;
                cx->blk_sub.retop       = retop;
                SvREFCNT_inc_simple_void_NN(cv);
            */

       Note that "cx_pushblock()" sets two new floors: for the args stack (to "MARK") and the
       temps stack (to "PL_tmps_ix"). While executing at this scope level, every "nextstate"
       (amongst others) will reset the args and tmps stack levels to these floors. Note that
       since "cx_pushblock" uses the current value of "PL_tmps_ix" rather than it being passed as
       an arg, this dictates at what point "cx_pushblock" should be called. In particular, any
       new mortals which should be freed only on scope exit (rather than at the next "nextstate")
       should be created first.

       Most callers of "cx_pushblock" simply set the new args stack floor to the top of the
       previous stack frame, but for "CXt_LOOP_LIST" it stores the items being iterated over on
       the stack, and so sets "blk_oldsp" to the top of these items instead. Note that, contrary
       to its name, "blk_oldsp" doesn't always represent the value to restore "PL_stack_sp" to on
       scope exit.

       Note the early capture of "PL_savestack_ix" to "old_ss_ix", which is later passed as an
       arg to "cx_pushblock". In the case of "pp_entersub", this is because, although most values
       needing saving are stored in fields of the context struct, an extra value needs saving
       only when the debugger is running, and it doesn't make sense to bloat the struct for this
       rare case. So instead it is saved on the savestack. Since this value gets calculated and
       saved before the context is pushed, it is necessary to pass the old value of
       "PL_savestack_ix" to "cx_pushblock", to ensure that the saved value gets freed during
       scope exit.  For most users of "cx_pushblock", where nothing needs pushing on the save
       stack, "PL_savestack_ix" is just passed directly as an arg to "cx_pushblock".

       Note that where possible, values should be saved in the context struct rather than on the
       save stack; it's much faster that way.

       Normally "cx_pushblock" should be immediately followed by the appropriate "cx_pushfoo",
       with nothing between them; this is because if code in-between could die (e.g. a warning
       upgraded to fatal), then the context stack unwinding code in "dounwind" would see (in the
       example above) a "CXt_SUB" context frame, but without all the subroutine-specific fields
       set, and crashes would soon ensue.

       Where the two must be separate, initially set the type to "CXt_NULL" or "CXt_BLOCK", and
       later change it to "CXt_foo" when doing the "cx_pushfoo". This is exactly what
       "pp_enteriter" does, once it's determined which type of loop it's pushing.

   Popping contexts
       Contexts are popped using "cx_popsub()" etc. and "cx_popblock()". Note however, that
       unlike "cx_pushblock", neither of these functions actually decrement the current context
       stack index; this is done separately using "CX_POP()".

       There are two main ways that contexts are popped. During normal execution as scopes are
       exited, functions like "pp_leave", "pp_leaveloop" and "pp_leavesub" process and pop just
       one context using "cx_popfoo" and "cx_popblock". On the other hand, things like
       "pp_return" and "next" may have to pop back several scopes until a sub or loop context is
       found, and exceptions (such as "die") need to pop back contexts until an eval context is
       found. Both of these are accomplished by "dounwind()", which is capable of processing and
       popping all contexts above the target one.

       Here is a typical example of context popping, as found in "pp_leavesub" (simplified
       slightly):

        U8 gimme;
        PERL_CONTEXT *cx;
        SV **oldsp;
        OP *retop;

        cx = CX_CUR();

        gimme = cx->blk_gimme;
        oldsp = PL_stack_base + cx->blk_oldsp; /* last arg of previous frame */

        if (gimme == G_VOID)
            PL_stack_sp = oldsp;
        else
            leave_adjust_stacks(oldsp, oldsp, gimme, 0);

        CX_LEAVE_SCOPE(cx);
        cx_popsub(cx);
        cx_popblock(cx);
        retop = cx->blk_sub.retop;
        CX_POP(cx);

        return retop;

       The steps above are in a very specific order, designed to be the reverse order of when the
       context was pushed. The first thing to do is to copy and/or protect any any return
       arguments and free any temps in the current scope. Scope exits like an rvalue sub normally
       return a mortal copy of their return args (as opposed to lvalue subs). It is important to
       make this copy before the save stack is popped or variables are restored, or bad things
       like the following can happen:

           sub f { my $x =...; $x }  # $x freed before we get to copy it
           sub f { /(...)/;    $1 }  # PL_curpm restored before $1 copied

       Although we wish to free any temps at the same time, we have to be careful not to free any
       temps which are keeping return args alive; nor to free the temps we have just created
       while mortal copying return args. Fortunately, "leave_adjust_stacks()" is capable of
       making mortal copies of return args, shifting args down the stack, and only processing
       those entries on the temps stack that are safe to do so.

       In void context no args are returned, so it's more efficient to skip calling
       "leave_adjust_stacks()". Also in void context, a "nextstate" op is likely to be imminently
       called which will do a "FREETMPS", so there's no need to do that either.

       The next step is to pop savestack entries: "CX_LEAVE_SCOPE(cx)" is just defined as
       "<LEAVE_SCOPE(cx-"blk_oldsaveix)>>. Note that during the popping, it's possible for perl
       to call destructors, call "STORE" to undo localisations of tied vars, and so on. Any of
       these can die or call "exit()". In this case, "dounwind()" will be called, and the current
       context stack frame will be re-processed. Thus it is vital that all steps in popping a
       context are done in such a way to support reentrancy.  The other alternative, of
       decrementing "cxstack_ix" before processing the frame, would lead to leaks and the like if
       something died halfway through, or overwriting of the current frame.

       "CX_LEAVE_SCOPE" itself is safely re-entrant: if only half the savestack items have been
       popped before dying and getting trapped by eval, then the "CX_LEAVE_SCOPE"s in "dounwind"
       or "pp_leaveeval" will continue where the first one left off.

       The next step is the type-specific context processing; in this case "cx_popsub". In part,
       this looks like:

           cv = cx->blk_sub.cv;
           CvDEPTH(cv) = cx->blk_sub.olddepth;
           cx->blk_sub.cv = NULL;
           SvREFCNT_dec(cv);

       where its processing the just-executed CV. Note that before it decrements the CV's
       reference count, it nulls the "blk_sub.cv". This means that if it re-enters, the CV won't
       be freed twice. It also means that you can't rely on such type-specific fields having
       useful values after the return from "cx_popfoo".

       Next, "cx_popblock" restores all the various interpreter vars to their previous values or
       previous high water marks; it expands to:

           PL_markstack_ptr = PL_markstack + cx->blk_oldmarksp;
           PL_scopestack_ix = cx->blk_oldscopesp;
           PL_curpm         = cx->blk_oldpm;
           PL_curcop        = cx->blk_oldcop;
           PL_tmps_floor    = cx->blk_old_tmpsfloor;

       Note that it doesn't restore "PL_stack_sp"; as mentioned earlier, which value to restore
       it to depends on the context type (specifically "for (list) {}"), and what args (if any)
       it returns; and that will already have been sorted out earlier by "leave_adjust_stacks()".

       Finally, the context stack pointer is actually decremented by "CX_POP(cx)".  After this
       point, it's possible that that the current context frame could be overwritten by other
       contexts being pushed. Although things like ties and "DESTROY" are supposed to work within
       a new context stack, it's best not to assume this. Indeed on debugging builds,
       "CX_POP(cx)" deliberately sets "cx" to null to detect code that is still relying on the
       field values in that context frame. Note in the "pp_leavesub()" example above, we grab
       "blk_sub.retop" before calling "CX_POP".

   Redoing contexts
       Finally, there is "cx_topblock(cx)", which acts like a super-"nextstate" as regards to
       resetting various vars to their base values. It is used in places like "pp_next",
       "pp_redo" and "pp_goto" where rather than exiting a scope, we want to re-initialise the
       scope. As well as resetting "PL_stack_sp" like "nextstate", it also resets
       "PL_markstack_ptr", "PL_scopestack_ix" and "PL_curpm". Note that it doesn't do a
       "FREETMPS".

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, perlintern, perlxs, perlembed