MirOS Manual: perlguts(1)


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

NAME

     perlguts - Introduction to the Perl API

DESCRIPTION

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

Variables

     Datatypes

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

         SV  Scalar Value
         AV  Array Value
         HV  Hash Value

     Each typedef has specific routines that manipulate the vari-
     ous 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).

     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*);

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

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

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

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

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

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

     "sv_vsetpvfn" is an analogue of "vsprintf", but it allows
     you to specify either a pointer to a variable argument list
     or the address and length of an array of SVs.  The last
     argument points to a boolean; on return, if that boolean is
     true, then locale-specific information has been used to for-
     mat 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.

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     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

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         SvGROW(SV*, STRLEN newlen)

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

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

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

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

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

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

         SvEND(SV*)

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

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

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

     The first function calculates the length of the string to be
     appended by using "strlen".  In the second, you specify the
     length of the string yourself.  The third function processes
     its arguments like "sprintf" and appends the formatted out-
     put.  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.

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

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

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

     This returns NULL if the variable does not exist.

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

         SvOK(SV*)

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

     Its address can be used whenever an "SV*" is needed. 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);

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     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 charac-
     ters, "sv_chop" sets the flag "OOK" (offset OK) to signal to
     other functions that the offset hack is in effect, and it
     puts the number of bytes chopped off into the IV field of
     the SV. It then moves the PV pointer (called "SvPVX") for-
     ward that many bytes, and adjusts "SvCUR" and "SvLEN".

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

     This is best demonstrated by example:

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

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

     Something similar to the offset hack is performed on AVs to
     enable efficient shifting and splicing off the beginning of
     the array; while "AvARRAY" points to the first element in
     the array that is visible from Perl, "AvALLOC" points to the
     real start of the C array. These are usually the same, but a

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     "shift" operation can be carried out by increasing "AvARRAY"
     by one and decreasing "AvFILL" and "AvLEN". Again, the loca-
     tion of the real start of the C array only comes into play
     when freeing the array. See "av_shift" in av.c.

     What's Really Stored in an SV?

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

     The are various ways in which the private and public flags
     may differ. For example, a tied SV may have a valid underly-
     ing value in the IV slot (so SvIOKp is true), but the data
     should be accessed via the FETCH routine rather than
     directly, so SvIOK is false. Another is when numeric conver-
     sion has occurred and precision has been lost: only the
     private flag is set on 'lossy' values. So when an NV is con-
     verted 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 popu-
     lates it with SVs:

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

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

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     destroyed, if so desired.

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

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

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

     Here are some other functions:

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

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

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

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

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

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

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     This returns NULL if the variable does not exist.

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

     Working with HVs

     To create an HV, you use the following routine:

         HV*  newHV();

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

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

     The "klen" parameter is the length of the key being passed
     in (Note that you cannot pass 0 in as a value of "klen" to
     tell Perl to measure the length of the key).  The "val"
     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 dere-
     ferencing it.

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

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

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

     And more miscellaneous functions:

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

     Like their AV counterparts, "hv_clear" deletes all the
     entries in the hash table but does not actually delete the

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     hash table.  The "hv_undef" deletes both the entries and the
     hash table itself.

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

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

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

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

     This returns NULL if the variable does not exist.

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

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

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

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

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     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*"), mak-
     ing 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 con-
     tents 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 descrip-
     tions 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 incre-
     ment 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 refer-
     ence count of "val" to avoid a memory leak.

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     AVs, HVs and undefined values

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

     For example, intuition tells you that this XS code:

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

     is equivalent to this Perl code:

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

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

     Other problems can occur when storing &PL_sv_undef in HVs:

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

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

         Modification of non-creatable hash value attempted

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

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

         Modification of a read-only value attempted

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

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

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         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 references).

     To create a reference, use either of the following func-
     tions:

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

         See the sv.h header file for more details.

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     Blessed References and Class Objects

     References are also used to support object-oriented program-
     ming.  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 */

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

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

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

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

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

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

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

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

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

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

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

             int  sv_isobject(SV* sv);

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

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

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

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

     Creating New Variables

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

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

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

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

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

           Name <varname> used only once: possible typo

         warning.

     GV_ADDWARN
         Issues the warning:

           Had to create <varname> unexpectedly

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

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

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     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 refer-
     ence count of an xV ever drops to 0, then it will be des-
     troyed 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 refer-
     ence 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 argu-
     ment.  As a side effect, it increments the argument's refer-
     ence 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 refer-
     ence 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 ter-
     minates.  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

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     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 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.

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

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

     The name that "gv_stash*v" wants is the name of the package
     whose 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:

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         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 automati-
     cally 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", TRUE);
         sv_setiv(sv, (IV) dberror);
         sv_setpv(sv, dberror_list[dberror]);
         SvIOK_on(sv);

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

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     Magic Variables

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

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

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

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

     Assigning Magic

     Perl adds magic to an SV using the sv_magic function:

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

     The "sv" argument is a pointer to the SV that is to acquire
     a new magical feature.

     If "sv" is not already magical, Perl uses the "SvUPGRADE"
     macro to convert "sv" to type "SVt_PVMG". Perl then contin-
     ues 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 associ-
     ated 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.

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     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" argu-
     ment, the reference count of the "obj" object is incre-
     mented.  If it is the same, or if the "how" argument is
     "PERL_MAGIC_arylen", or if it is a NULL pointer, then "obj"
     is merely stored, without the reference count being incre-
     mented.

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

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

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

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

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

         void sv_unmagic(SV *sv, int type);

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

     Magic Virtual Tables

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

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

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

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

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

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

         { magic_get, magic_set, magic_len, 0, 0 }

     Thus, when an SV is determined to be magical and of type
     "PERL_MAGIC_sv", if a get operation is being performed, the
     routine "magic_get" is called.  All the various routines for
     the various 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 current kinds of Magic Virtual Tables are:

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         mg_type
         (old-style char and macro)   MGVTBL         Type of magic
         --------------------------   ------         ----------------------------
         \0 PERL_MAGIC_sv             vtbl_sv        Special scalar variable
         A  PERL_MAGIC_overload       vtbl_amagic    %OVERLOAD hash
         a  PERL_MAGIC_overload_elem  vtbl_amagicelem %OVERLOAD hash element
         c  PERL_MAGIC_overload_table (none)         Holds overload table (AMT)
                                                     on stash
         B  PERL_MAGIC_bm             vtbl_bm        Boyer-Moore (fast string search)
         D  PERL_MAGIC_regdata        vtbl_regdata   Regex match position data
                                                     (@+ and @- vars)
         d  PERL_MAGIC_regdatum       vtbl_regdatum  Regex match position data
                                                     element
         E  PERL_MAGIC_env            vtbl_env       %ENV hash
         e  PERL_MAGIC_envelem        vtbl_envelem   %ENV hash element
         f  PERL_MAGIC_fm             vtbl_fm        Formline ('compiled' format)
         g  PERL_MAGIC_regex_global   vtbl_mglob     m//g target / study()ed string
         I  PERL_MAGIC_isa            vtbl_isa       @ISA array
         i  PERL_MAGIC_isaelem        vtbl_isaelem   @ISA array element
         k  PERL_MAGIC_nkeys          vtbl_nkeys     scalar(keys()) lvalue
         L  PERL_MAGIC_dbfile         (none)         Debugger %_<filename
         l  PERL_MAGIC_dbline         vtbl_dbline    Debugger %_<filename element
         m  PERL_MAGIC_mutex          vtbl_mutex     ???
         o  PERL_MAGIC_collxfrm       vtbl_collxfrm  Locale collate transformation
         P  PERL_MAGIC_tied           vtbl_pack      Tied array or hash
         p  PERL_MAGIC_tiedelem       vtbl_packelem  Tied array or hash element
         q  PERL_MAGIC_tiedscalar     vtbl_packelem  Tied scalar or handle
         r  PERL_MAGIC_qr             vtbl_qr        precompiled qr// regex
         S  PERL_MAGIC_sig            vtbl_sig       %SIG hash
         s  PERL_MAGIC_sigelem        vtbl_sigelem   %SIG hash element
         t  PERL_MAGIC_taint          vtbl_taint     Taintedness
         U  PERL_MAGIC_uvar           vtbl_uvar      Available for use by extensions
         v  PERL_MAGIC_vec            vtbl_vec       vec() lvalue
         V  PERL_MAGIC_vstring        (none)         v-string scalars
         w  PERL_MAGIC_utf8           vtbl_utf8      UTF-8 length+offset cache
         x  PERL_MAGIC_substr         vtbl_substr    substr() lvalue
         y  PERL_MAGIC_defelem        vtbl_defelem   Shadow "foreach" iterator
                                                     variable / smart parameter
                                                     vivification
         *  PERL_MAGIC_glob           vtbl_glob      GV (typeglob)
         #  PERL_MAGIC_arylen         vtbl_arylen    Array length ($#ary)
         .  PERL_MAGIC_pos            vtbl_pos       pos() lvalue
         <  PERL_MAGIC_backref        vtbl_backref   ???
         ~  PERL_MAGIC_ext            (none)         Available for use by extensions

     When an uppercase and lowercase letter both exist in the
     table, then the uppercase letter is 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.

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     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 (typi-
     cally objects).  This is especially useful because there is
     no way for normal perl code to corrupt this private informa-
     tion (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));

     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.  Typi-
     cally only using the magic on objects blessed into the same
     class as the extension is sufficient. For "PERL_MAGIC_ext"
     magic, it may also be appropriate to add an I32 'signature'
     at the top of the private data area and check that.

     Also note that the "sv_set*()" and "sv_cat*()" functions
     described earlier do not invoke 'set' magic on their tar-
     gets.  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

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     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 fol-
     lowed by "SvSETMAGIC()", but they don't need a prior "SvGET-
     MAGIC()" since their implementation handles 'get' magic.

     Finding Magic

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

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

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

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

     Understanding the Magic of Tied Hashes and Arrays

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

     WARNING: As of the 5.004 release, proper usage of the array
     and hash access functions requires understanding a few
     caveats.  Some of these caveats are actually considered bugs
     in the API, to be fixed in later releases, and are bracketed
     with [MAYCHANGE] below. If you find yourself actually apply-
     ing 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 per-
     form 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.

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

     The "av_store" function, when given a tied array argument,
     merely copies the magic of the array onto the value to be
     "stored", using "mg_copy".  It may also return NULL, indi-
     cating 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 TIEAR-
     RAY 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.

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     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 "Return-
     ing 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

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     enclosed in an "ENTER"/"LEAVE" pair.

     Inside such a pseudo-block the following service is avail-
     able:

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

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

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

         Also compare "SAVEMORTALIZESV".

     "SAVEMORTALIZESV(SV *sv)"
         Just like "SAVEFREESV", but mortalizes "sv" at the end
         of the current scope instead of decrementing its refer-
         ence 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-

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         block. The string pointed to by "key" is Safefree()ed.
         If one has a key in short-lived storage, the correspond-
         ing 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.

     "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.

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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)))

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

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

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

         XPUSHs(SV*)

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

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     the stack.

     Despite their suggestions in earlier versions of this docu-
     ment the macros "(X)PUSH[iunp]" are not suited to XSUBs
     which return multiple results. For that, either stick to the
     "(X)PUSHs" macros shown above, or use the new
     "m(X)PUSH[iunp]" macros instead; see "Putting a C value on
     Perl stack".

     For more information, consult perlxs and perlxstut.

     Calling Perl Routines from within C Programs

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

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

     The routine most often used is "call_sv".  The "SV*" argu-
     ment contains either the name of the Perl subroutine to be
     called, or a reference to the subroutine.  The second argu-
     ment 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; mac-
     ros 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*()

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

     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 vari-
     able 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 argu-
     ments to "Renew" and "Renewc" match those of "New" and
     "Newc" with the exception of not needing the "magic cookie"
     argument.

     Moving

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         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 pre-
     viously 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 has been exper-
     imenting 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, con-
     sult perlapio.

     Putting a C value on Perl stack

     A lot of opcodes (this is an elementary operation in the
     internal perl stack machine) put an SV* on the stack. How-
     ever, 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 con-
     stantly 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 tar-
     get 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 push-
     ing 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

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     "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 mor-
     tal 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 vari-
     able "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. Dur-
     ing this time a special anonymous Perl array is created,
     which is called a scratchpad for the current unit.

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

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

     Scratchpads and recursion

     In fact it is not 100% true that a compiled unit contains a
     pointer to the scratchpad AV. In fact it contains a pointer

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     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 sub-
     routine. For the subroutine-child not write over the tem-
     poraries for the subroutine-parent (lifespan of which covers
     the call to the child), the parent and the child should have
     different scratchpads. (And the lexicals should be separate
     anyway!)

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

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

Compiled code

     Code tree

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

       $a = $b + $c;

     This is converted to a tree similar to this one:

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

     (but slightly more complicated).  This tree reflects the way
     Perl parsed your code, but has nothing to do with the execu-
     tion 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 simpli-
     fied example, the execution order is the same as in our
     example.

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     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 opera-
     tion 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

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     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 dif-
     ferent ways.

     The simplest type of op structure is "OP": this has no chil-
     dren. 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 itera-
     tively following the "op_sibling" pointer from the first
     child to the last.

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

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

     Compile pass 1: check routines

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

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

     A check routine is called when the node is fully constructed
     except for the execution-order thread.  Since at this time
     there are no back-links to the currently constructed node,
     one can do most any operation to the top-level node, includ-
     ing 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).

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     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 exe-
     cuted, 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 per-
     formed. This pass is neither top-down or bottom-up, but in
     the execution order (with additional complications for con-
     ditionals).  These optimizations are done in the subroutine
     peep().  Optimizations performed at this stage are subject
     to the same restrictions as in the pass 2.

     Pluggable runops

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

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

       PL_runops = my_runops;

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

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

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

     "Perl_op_dump" can be used to dump an "OP" structure or any
     of its 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

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     so that you can have multiple interpreters, with one inter-
     preter represented either as a C structure, or inside a
     thread-specific structure.  These structures contain all the
     context, the state of that interpreter.

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

     All this obviously requires a way for the Perl internal
     functions to be either subroutines taking some kind of
     structure as the first argument, or subroutines taking noth-
     ing as the first argument.  To enable these two very dif-
     ferent ways of building the interpreter, the Perl source (as
     it does in so many other situations) makes heavy use of mac-
     ros 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 explain-
     ing why you think it should be.

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

       STATIC void
       S_incline(pTHX_ char *s)

     STATIC becomes "static" in C, and may be #define'd to noth-
     ing in some configurations in future.

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

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       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 con-
     text.  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

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     extensions get source compatibility at the expense of per-
     formance.  (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 inef-
     ficient way, which is also the default, in order to maintain
     source compatibility with extensions: whenever XSUB.h is
     #included, it redefines the aTHX and aTHX_ macros to call a
     function that will return the context. Thus, something like:

             sv_setiv(sv, num);

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

             Perl_sv_setiv(Perl_get_context(), sv, num);

     or to this otherwise:

             Perl_sv_setiv(sv, num);

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

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

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

             static my_private_function(int arg1, int arg2);

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

             [... etc ...]

             MODULE = Foo            PACKAGE = Foo

             /* typical XSUB */

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

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

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

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

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

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

             [... etc ...]

             MODULE = Foo            PACKAGE = Foo

             /* typical XSUB */

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

     This implementation never has to fetch the context using a
     function call, since it is always passed as an extra argu-
     ment.  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.

     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 func-
     tions 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);

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

     Future Plans and PERL_IMPLICIT_SYS

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

     This allows the ability to provide an extra pointer (called
     the "host" environment) for all the system calls.  This
     makes it possible for all the system stuff to maintain their
     own state, broken down into seven C structures.  These are
     thin wrappers around the usual system calls (see
     win32/perllib.c) for the default perl executable, but for a
     more ambitious host (like the one that would do fork() emu-
     lation) 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 pro-
     cess, 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 pro-
     gram in which Perl is embedded. Similarly, all global vari-
     ables begin with "PL_". (By convention, static functions
     start with "S_".)

     Inside the Perl core, you can get at the functions either
     with or without the "Perl_" prefix, thanks to a bunch of
     defines that live in embed.h. This header file is generated
     automatically from embed.pl and embed.fnc. embed.pl also
     creates the prototyping header files for the internal func-
     tions, 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:

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

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

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

     Other available flags are:

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

     n  This does not need a interpreter context, so the defini-
        tion has no "pTHX", and it follows that callers don't use
        "aTHX".  (See "Background and PERL_IMPLICIT_CONTEXT" in
        perlguts.)

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

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

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

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

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

     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.

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

     Formatted Printing of IVs, UVs, and NVs

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

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

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

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

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

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

     Pointer-To-Integer and Integer-To-Pointer

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

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

     For example:

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

     and

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

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     Source Documentation

     There's an effort going on to document the internal func-
     tions 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 auto-
     generated manual for the functions which are not part of the
     API and are supposedly for internal use only.

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

      /*
      =for apidoc sv_setiv

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

      =cut
      */

     Please try and supply some documentation if you add func-
     tions 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 exam-
     ple:

       % 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.

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     What is Unicode, anyway?

     In the olden, less enlightened times, we all used to use
     ASCII. Most of us did, anyway. The big problem with ASCII is
     that it's American. Well, no, that's not actually the prob-
     lem; the problem is that it's not particularly useful for
     people who don't use the Roman alphabet. What used to happen
     was that particular languages would stick their own alphabet
     in the upper range of the sequence, between 128 and 255. Of
     course, we then ended up with plenty of variants that
     weren't quite ASCII, and the whole point of it being a stan-
     dard 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 pos-
     sibly think of and more. There are several ways of
     representing these characters, and the one Perl uses is
     called UTF-8. UTF-8 uses a variable number of bytes to
     represent a character, instead of just one. You can learn
     more about Unicode at http://www.unicode.org/

     How can I recognise a UTF-8 string?

     You 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.

     The API function "is_utf8_string" can help; it'll tell you
     if a string contains only valid UTF-8 characters. However,
     it can't do the work for you. On a character-by-character
     basis, "is_utf8_char" will tell you whether the current
     character in a string is valid UTF-8.

     How does UTF-8 represent Unicode characters?

     As mentioned above, UTF-8 uses a variable number of bytes to
     store a character. Characters with values 1...128 are stored
     in one byte, just like good ol' ASCII. Character 129 is
     stored as "v194.129"; this continues up to character 191,
     which is "v194.191". Now we've run out of bits (191 is
     binary 10111111) so we move on; 192 is "v195.128". And so it
     goes on, moving to three bytes at character 2048.

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     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 charac-
     ters to skip over. You're on your own about bounds checking,
     though, so don't use it lightly.

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

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

         if (!UTF8_IS_INVARIANT(*utf))
             /* Must treat this as UTF-8 */
             uv = utf8_to_uv(utf);
         else
             /* OK to treat this character as a byte */
             uv = *utf;

     You can also see in that example that we use "utf8_to_uv" to
     get the value of the character; the inverse function
     "uv_to_utf8" is available for putting a UV into UTF-8:

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

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

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     How does Perl store UTF-8 strings?

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

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

     This flag has an important effect on Perl's treatment of the
     string: if Unicode data is not properly distinguished, regu-
     lar expressions, "length", "substr" and other string han-
     dling operations will have undesirable results.

     The problem comes when you have, for instance, a string that
     isn't flagged is UTF-8, and contains a byte sequence that
     could be UTF-8 - especially when combining non-UTF-8 and
     UTF-8 strings.

     Never forget that the "SVf_UTF8" flag is separate to the PV
     value; you need be sure you 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 UTF-8 flag set,
     and act accordingly:

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

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

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

     How do I convert a string to UTF-8?

     If you're mixing UTF-8 and non-UTF-8 strings, you might find
     it necessary to upgrade one of the strings to UTF-8. If
     you've got an SV, the easiest way to do this is:

         sv_utf8_upgrade(sv);

     However, you must not do this, for example:

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

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

     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 sin-
     gle byte.

     Is there anything else I need to know?

     Not really. Just remember these things:

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

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

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

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

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        whether you need to use a UTF-8 string until you get to a
        high character - "HALF_UPGRADE" is one of those.

Custom Operators

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

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

     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 com-
     piled the program. You do this either by manipulating the op
     tree using a "CHECK" block and the "B::Generate" module, or
     by adding a custom peephole optimizer with the "optimize"
     module.

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

     You should also "register" your op with the Perl interpreter
     so that it can produce 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" as a key into the "PL_custom_op_descs" and
     "PL_custom_op_names" hashes. This means you need to enter a
     name and description for your op at the appropriate place in
     the "PL_custom_op_names" and "PL_custom_op_descs" hashes.

     Forthcoming versions of "B::Generate" (version 1.0 and
     above) should directly support the creation of custom ops by
     name.

AUTHORS

     Until May 1997, this document was maintained by Jeff Okamoto
     <okamoto@corp.hp.com>.  It is now maintained as part of Perl

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     itself by the Perl 5 Porters <perl5-porters@perl.org>.

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

SEE ALSO

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

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