


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


NAME
       perlguts - Introduction to the Perl API

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

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

           SV  Scalar Value
           AV  Array Value
           HV  Hash Value

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

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

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

   Working with SVs
       An SV can be created and loaded with one command.  There
       are five types of values that can be loaded: an integer
       value (IV), an unsigned integer value (UV), a double (NV),
       a string (PV), and another scalar (SV).

       The seven routines are:

           SV*  newSViv(IV);
           SV*  newSVuv(UV);
           SV*  newSVnv(double);
           SV*  newSVpv(const char*, STRLEN);
           SV*  newSVpvn(const char*, STRLEN);
           SV*  newSVpvf(const char*, ...);
           SV*  newSVsv(SV*);

       "STRLEN" is an integer type (Size_t, usually defined as
       size_t in config.h) guaranteed to be large enough to
       represent the size of any string that perl can handle.




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

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

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

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

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

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

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

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

       All SVs that contain strings should be terminated with a
       NUL character.  If it is not NUL-terminated there is a
       risk of core dumps and corruptions from code which passes
       the string to C functions or system calls which expect a
       NUL-terminated string.  Perl's own functions typically add
       a trailing NUL for this reason.  Nevertheless, you should



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       be very careful when you pass a string stored in an SV to
       a C function or system call.

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

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

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

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

       Also remember that C doesn't allow you to safely say
       "foo(SvPV(s, len), len);". It might work with your
       compiler, but it won't work for everyone.  Break this sort
       of statement up into separate assignments:

           SV *s;
           STRLEN len;
           char * ptr;
           ptr = SvPV(s, len);
           foo(ptr, len);

       If you want to know if the scalar value is TRUE, you can
       use:

           SvTRUE(SV*)

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

           SvGROW(SV*, STRLEN newlen)

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



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

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

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

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

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

           SvEND(SV*)

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

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

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

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

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

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

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

       This returns NULL if the variable does not exist.

       If you want to know if this variable (or any other SV) is



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       actually "defined", you can call:

           SvOK(SV*)

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

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

         foo(undef);

       But won't work when called as:

         $x = undef;
         foo($x);

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

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

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

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

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

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

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

   Offsets
       Perl provides the function "sv_chop" to efficiently remove
       characters from the beginning of a string; you give it an



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       SV and a pointer to somewhere inside the PV, and it
       discards everything before the pointer. The efficiency
       comes by means of a little hack: instead of actually
       removing the characters, "sv_chop" sets the flag "OOK"
       (offset OK) to signal to other functions that the offset
       hack is in effect, and it puts the number of bytes chopped
       off into the IV field of the SV. It then moves the PV
       pointer (called "SvPVX") forward that many bytes, and
       adjusts "SvCUR" and "SvLEN".

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

       This is best demonstrated by example:

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

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

       Something similar to the offset hack is performed on AVs
       to enable efficient shifting and splicing off the
       beginning of the array; while "AvARRAY" points to the
       first element in the array that is visible from Perl,
       "AvALLOC" points to the real start of the C array. These
       are usually the same, but a "shift" operation can be
       carried out by increasing "AvARRAY" by one and decreasing
       "AvFILL" and "AvLEN".  Again, the location of the real
       start of the C array only comes into play when freeing the
       array. See "av_shift" in av.c.

   What's Really Stored in an SV?
       Recall that the usual method of determining the type of
       scalar you have is to use "Sv*OK" macros.  Because a
       scalar can be both a number and a string, usually these
       macros will always return TRUE and calling the "Sv*V"
       macros will do the appropriate conversion of string to
       integer/double or integer/double to string.

       If you really need to know if you have an integer, double,
       or string pointer in an SV, you can use the following



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       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
       underlying value in the IV slot (so SvIOKp is true), but
       the data should be accessed via the FETCH routine rather
       than directly, so SvIOK is false. Another is when numeric
       conversion has occurred and precision has been lost: only
       the private flag is set on 'lossy' values. So when an NV
       is converted to an IV with loss, SvIOKp, SvNOKp and SvNOK
       will be set, while SvIOK wont be.

       In general, though, it's best to use the "Sv*V" macros.

   Working with AVs
       There are two ways to create and load an AV.  The first
       method creates an empty AV:

           AV*  newAV();

       The second method both creates the AV and initially
       populates it with SVs:

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

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

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

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

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

       Here are some other functions:





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

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

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

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

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

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

       This returns NULL if the variable does not exist.

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

   Working with HVs
       To create an HV, you use the following routine:

           HV*  newHV();

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



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       argument contains the SV pointer to the scalar being
       stored, and "hash" is the precomputed hash value (zero if
       you want "hv_store" to calculate it for you).  The "lval"
       parameter indicates whether this fetch is actually a part
       of a store operation, in which case a new undefined value
       will be added to the HV with the supplied key and
       "hv_fetch" will return as if the value had already
       existed.

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

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

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

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

       And more miscellaneous functions:

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

       Like their AV counterparts, "hv_clear" deletes all the
       entries in the hash table but does not actually delete the
       hash table.  The "hv_undef" deletes both the entries and
       the hash table itself.

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

           I32    hv_iterinit(HV*);
                   /* Prepares starting point to traverse hash table */
           HE*    hv_iternext(HV*);
                   /* Get the next entry, and return a pointer to a
                      structure that has both the key and value */
           char*  hv_iterkey(HE* entry, I32* retlen);
                   /* Get the key from an HE structure and also return
                      the length of the key string */
           SV*    hv_iterval(HV*, HE* entry);
                   /* Return an SV pointer to the value of the HE
                      structure */
           SV*    hv_iternextsv(HV*, char** key, I32* retlen);



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                   /* This convenience routine combines hv_iternext,
                      hv_iterkey, and hv_iterval.  The key and retlen
                      arguments are return values for the key and its
                      length.  The value is returned in the SV* argument */

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

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

       This returns NULL if the variable does not exist.

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

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

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

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

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

           HE*     hv_fetch_ent  (HV* tb, SV* key, I32 lval, U32 hash);
           HE*     hv_store_ent  (HV* tb, SV* key, SV* val, U32 hash);

           bool    hv_exists_ent (HV* tb, SV* key, U32 hash);
           SV*     hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash);

           SV*     hv_iterkeysv  (HE* entry);

       Note that these functions take "SV*" keys, which
       simplifies writing of extension code that deals with hash
       structures.  These functions also allow passing of "SV*"
       keys to "tie" functions without forcing you to stringify
       the keys (unlike the previous set of functions).

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

       The following macros must always be used to access the
       contents of hash entries.  Note that the arguments to
       these macros must be simple variables, since they may get
       evaluated more than once.  See perlapi for detailed



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       descriptions of these macros.

           HePV(HE* he, STRLEN len)
           HeVAL(HE* he)
           HeHASH(HE* he)
           HeSVKEY(HE* he)
           HeSVKEY_force(HE* he)
           HeSVKEY_set(HE* he, SV* sv)

       These two lower level macros are defined, but must only be
       used when dealing with keys that are not "SV*"s:

           HeKEY(HE* he)
           HeKLEN(HE* he)

       Note that both "hv_store" and "hv_store_ent" do not
       increment the reference count of the stored "val", which
       is the caller's responsibility.  If these functions return
       a NULL value, the caller will usually have to decrement
       the reference count of "val" to avoid a memory leak.

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

       For example, intuition tells you that this XS code:

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

       is equivalent to this Perl code:

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

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

       Other problems can occur when storing &PL_sv_undef in HVs:

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

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

           Modification of non-creatable hash value attempted

       In perl 5.8.0, &PL_sv_undef was also used to mark
       placeholders in restricted hashes. This caused such hash



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       entries not to appear when iterating over the hash or when
       checking for the keys with the "hv_exists" function.

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

           Modification of a read-only value attempted

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

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

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

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

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

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

       The "thing" argument can be any of an "SV*", "AV*", or
       "HV*".  The functions are identical except that
       "newRV_inc" increments the reference count of the "thing",
       while "newRV_noinc" does not.  For historical reasons,
       "newRV" is a synonym for "newRV_inc".

       Once you have a reference, you can use the following macro
       to dereference the reference:

           SvRV(SV*)

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

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

           SvROK(SV*)

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




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           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.

   Blessed References and Class Objects
       References are also used to support object-oriented
       programming.  In perl's OO lexicon, an object is simply a
       reference that has been blessed into a package (or class).
       Once blessed, the programmer may now use the reference to
       access the various methods in the class.

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

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

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

       /* Still under construction */

       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);



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       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);

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

               int  sv_isobject(SV* sv);

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

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

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

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

   Creating New Variables
       To create a new Perl variable with an undef value which
       can be accessed from your Perl script, use the following
       routines, depending on the variable type.

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

       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:



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             Had to create <varname> unexpectedly

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

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

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

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

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

       However, there is one other function which manipulates the
       reference count of its argument.  The "newRV_inc"
       function, you will recall, creates a reference to the
       specified argument.  As a side effect, it increments the
       argument's reference count.  If this is not what you want,
       use "newRV_noinc" instead.

       For example, imagine you want to return a reference from
       an XSUB function.  Inside the XSUB routine, you create an
       SV which initially has a reference count of one.  Then you
       call "newRV_inc", passing it the just-created SV.  This
       returns the reference as a new SV, but the reference count
       of the SV you passed to "newRV_inc" has been incremented
       to two.  Now you return the reference from the XSUB
       routine and forget about the SV.  But Perl hasn't!
       Whenever the returned reference is destroyed, the
       reference count of the original SV is decreased to one and
       nothing happens.  The SV will hang around without any way
       to access it until Perl itself terminates.  This is a
       memory leak.

       The correct procedure, then, is to use "newRV_noinc"
       instead of "newRV_inc".  Then, if and when the last
       reference is destroyed, the reference count of the SV will
       go to zero and it will be destroyed, stopping any memory
       leak.

       There are some convenience functions available that can
       help with the destruction of xVs.  These functions



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       introduce the concept of "mortality".  An xV that is
       mortal has had its reference count marked to be
       decremented, but not actually decremented, until "a short
       time later".  Generally the term "short time later" means
       a single Perl statement, such as a call to an XSUB
       function.  The actual determinant for when mortal xVs have
       their reference count decremented depends on two macros,
       SAVETMPS and FREETMPS.  See perlcall and perlxs for more
       details on these macros.

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

       "Mortal" SVs are mainly used for SVs that are placed on
       perl's stack.  For example an SV which is created just to
       pass a number to a called sub is made mortal to have it
       cleaned up automatically when it's popped off the stack.
       Similarly, results returned by XSUBs (which are pushed on
       the stack) are often made mortal.

       To create a mortal variable, use the functions:

           SV*  sv_newmortal()
           SV*  sv_2mortal(SV*)
           SV*  sv_mortalcopy(SV*)

       The first call creates a mortal SV (with no value), the
       second converts an existing SV to a mortal SV (and thus
       defers a call to "SvREFCNT_dec"), and the third creates a
       mortal copy of an existing SV.  Because "sv_newmortal"
       gives the new SV no value,it must normally be given one
       via "sv_setpv", "sv_setiv", etc. :

           SV *tmp = sv_newmortal();
           sv_setiv(tmp, an_integer);

       As that is multiple C statements it is quite common so see
       this idiom instead:

           SV *tmp = sv_2mortal(newSViv(an_integer));

       You should be careful about creating mortal variables.
       Strange things can happen if you make the same value
       mortal within multiple contexts, or if you make a variable
       mortal multiple times. Thinking of "Mortalization" as
       deferred "SvREFCNT_dec" should help to minimize such
       problems.  For example if you are passing an SV which you
       know has 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.




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       The mortal routines are not just for SVs -- AVs and HVs
       can be made mortal by passing their address (type-casted
       to "SV*") to the "sv_2mortal" or "sv_mortalcopy" routines.

   Stashes and Globs
       A stash is a hash that contains all variables that are
       defined within a package.  Each key of the stash is a
       symbol name (shared by all the different types of objects
       that have the same name), and each value in the hash table
       is a GV (Glob Value).  This GV in turn contains references
       to the various objects of that name, including (but not
       limited to) the following:

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

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

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

           HV*  gv_stashpv(const char* name, I32 flags)
           HV*  gv_stashsv(SV*, I32 flags)

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

       The name that "gv_stash*v" wants is the name of the
       package whose symbol table you want.  The default package
       is called "main".  If you have multiply nested packages,
       pass their names to "gv_stash*v", separated by "::" as in
       the Perl language itself.

       Alternately, if you have an SV that is a blessed
       reference, you can find out the stash pointer by using:

           HV*  SvSTASH(SvRV(SV*));

       then use the following to get the package name itself:

           char*  HvNAME(HV* stash);

       If you need to bless or re-bless an object you can use the



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       following function:

           SV*  sv_bless(SV*, HV* stash)

       where the first argument, an "SV*", must be a reference,
       and the second argument is a stash.  The returned "SV*"
       can now be used in the same way as any other SV.

       For more information on references and blessings, consult
       perlref.

   Double-Typed SVs
       Scalar variables normally contain only one type of value,
       an integer, double, pointer, or reference.  Perl will
       automatically convert the actual scalar data from the
       stored type into the requested type.

       Some scalar variables contain more than one type of scalar
       data.  For example, the variable $! contains either the
       numeric value of "errno" or its string equivalent from
       either "strerror" or "sys_errlist[]".

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

               SvIOK_on
               SvNOK_on
               SvPOK_on
               SvROK_on

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

       For example, to create a new Perl variable called
       "dberror" that contains both the numeric and descriptive
       string error values, you could use the following code:

           extern int  dberror;
           extern char *dberror_list;

           SV* sv = get_sv("dberror", GV_ADD);
           sv_setiv(sv, (IV) dberror);
           sv_setpv(sv, dberror_list[dberror]);
           SvIOK_on(sv);

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



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

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

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

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

   Assigning Magic
       Perl adds magic to an SV using the sv_magic function:

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

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

       If "sv" is not already magical, Perl uses the "SvUPGRADE"
       macro to convert "sv" to type "SVt_PVMG". Perl then
       continues by adding new magic to the beginning of the
       linked list of magical features.  Any prior entry of the
       same type of magic is deleted.  Note that this can be
       overridden, and multiple instances of the same type of
       magic can be associated with an SV.

       The "name" and "namlen" arguments are used to associate a
       string with the magic, typically the name of a variable.
       "namlen" is stored in the "mg_len" field and if "name" is
       non-null then either a "savepvn" copy of "name" or "name"
       itself is stored in the "mg_ptr" field, depending on
       whether "namlen" is greater than zero or equal to zero
       respectively.  As a special case, if "(name && namlen ==
       HEf_SVKEY)" then "name" is assumed to contain an "SV*" and
       is stored as-is with its REFCNT incremented.

       The sv_magic function uses "how" to determine which, if
       any, predefined "Magic Virtual Table" should be assigned
       to the "mg_virtual" field.  See the "Magic Virtual Tables"
       section below.  The "how" argument is also stored in the



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       "mg_type" field. The value of "how" should be chosen from
       the set of macros "PERL_MAGIC_foo" found in perl.h. Note
       that before these macros were added, Perl internals used
       to directly use character literals, so you may
       occasionally come across old code or documentation
       referring to 'U' magic rather than "PERL_MAGIC_uvar" for
       example.

       The "obj" argument is stored in the "mg_obj" field of the
       "MAGIC" structure.  If it is not the same as the "sv"
       argument, the reference count of the "obj" object is
       incremented.  If it is the same, or if the "how" argument
       is "PERL_MAGIC_arylen", or if it is a NULL pointer, then
       "obj" is merely stored, without the reference count being
       incremented.

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

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

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

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

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

           void sv_unmagic(SV *sv, int type);

       The "type" argument should be equal to the "how" value
       when the "SV" was initially made 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 (or sometimes eight) pointers to the
       following routine types:

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

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




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       This MGVTBL structure is set at compile-time in perl.h and
       there are currently 32 types.  These different structures
       contain pointers to various routines that perform
       additional actions depending on which function is being
       called.

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

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

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

           { magic_get, magic_set, magic_len, 0, 0 }

       Thus, when an SV is determined to be magical and of type
       "PERL_MAGIC_sv", if a get operation is being performed,
       the routine "magic_get" is called.  All the various
       routines for the various magical types begin with
       "magic_".  NOTE: the magic routines are not considered
       part of the Perl API, and may not be exported by the Perl
       library.

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

       The current kinds of Magic Virtual Tables are:

           mg_type
           (old-style char and macro)   MGVTBL          Type of magic
           --------------------------   ------          -------------
           \0 PERL_MAGIC_sv             vtbl_sv         Special scalar variable
           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



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           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
           H  PERL_MAGIC_hints          vtbl_sig        %^H hash
           h  PERL_MAGIC_hintselem      vtbl_hintselem  %^H hash element
           I  PERL_MAGIC_isa            vtbl_isa        @ISA array
           i  PERL_MAGIC_isaelem        vtbl_isaelem    @ISA array element
           k  PERL_MAGIC_nkeys          vtbl_nkeys      scalar(keys()) lvalue
           L  PERL_MAGIC_dbfile         (none)          Debugger %_<filename
           l  PERL_MAGIC_dbline         vtbl_dbline     Debugger %_<filename element
           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_arylen         vtbl_arylen     Array length ($#ary)
           .  PERL_MAGIC_pos            vtbl_pos        pos() lvalue
           <  PERL_MAGIC_backref        vtbl_backref    back pointer to a weak ref
           ~  PERL_MAGIC_ext            (none)          Available for use by extensions
           :  PERL_MAGIC_symtab         (none)          hash used as symbol table
           %  PERL_MAGIC_rhash          (none)          hash used as restricted hash
           @  PERL_MAGIC_arylen_p       vtbl_arylen_p   pointer to $#a from @a

       When an uppercase and lowercase letter both exist in the
       table, then the uppercase letter is typically used to
       represent some kind of composite type (a list or a hash),
       and the lowercase letter is used to represent an element
       of that composite type. Some internals code makes use of
       this case relationship.  However, 'v' and 'V' (vec and
       v-string) are in no way related.

       The "PERL_MAGIC_ext" and "PERL_MAGIC_uvar" magic types are
       defined specifically for use by extensions and will not be
       used by perl itself.  Extensions can use "PERL_MAGIC_ext"
       magic to 'attach' private information to variables
       (typically objects).  This is especially useful because
       there is no way for normal perl code to corrupt this
       private information (unlike using extra elements of a hash
       object).

       Similarly, "PERL_MAGIC_uvar" magic can be used much like
       tie() to call a C function any time a scalar's value is



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       used or changed.  The "MAGIC"'s "mg_ptr" field points to a
       "ufuncs" structure:

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

       When the SV is read from or written to, the "uf_val" or
       "uf_set" function will be called with "uf_index" as the
       first arg and a pointer to the SV as the second.  A simple
       example of how to add "PERL_MAGIC_uvar" magic is shown
       below.  Note that the ufuncs structure is copied by
       sv_magic, so you can safely allocate it on the stack.

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

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

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

       Note that because multiple extensions may be using
       "PERL_MAGIC_ext" or "PERL_MAGIC_uvar" magic, it is
       important for extensions to take extra care to avoid
       conflict.  Typically only using the magic on objects
       blessed into the same class as the extension is
       sufficient.  For "PERL_MAGIC_ext" magic, it may also be
       appropriate to add an I32 'signature' at the top of the
       private data area and check that.

       Also note that the "sv_set*()" and "sv_cat*()" functions
       described earlier do not invoke 'set' magic on their
       targets.  This must be done by the user either by calling
       the "SvSETMAGIC()" macro after calling these functions, or



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       by using one of the "sv_set*_mg()" or "sv_cat*_mg()"
       functions.  Similarly, generic C code must call the
       "SvGETMAGIC()" macro to invoke any 'get' magic if they use
       an SV obtained from external sources in functions that
       don't handle magic.  See perlapi for a description of
       these functions.  For example, calls to the "sv_cat*()"
       functions typically need to be followed by "SvSETMAGIC()",
       but they don't need a prior "SvGETMAGIC()" since their
       implementation handles 'get' magic.

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

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

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

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

   Understanding the Magic of Tied Hashes and Arrays
       Tied hashes and arrays are magical beasts of the
       "PERL_MAGIC_tied" magic type.

       WARNING: As of the 5.004 release, proper usage of the
       array and hash access functions requires understanding a
       few caveats.  Some of these caveats are actually
       considered bugs in the API, to be fixed in later releases,
       and are bracketed with [MAYCHANGE] below. If you find
       yourself actually applying such information in this
       section, be aware that the behavior may change in the
       future, umm, without warning.

       The perl tie function associates a variable with an object
       that implements the various GET, SET, etc methods.  To
       perform the equivalent of the perl tie function from an
       XSUB, you must mimic this behaviour.  The code below
       carries out the necessary steps - firstly it creates a new
       hash, and then creates a second hash which it blesses into
       the class which will implement the tie methods. Lastly it
       ties the two hashes together, and returns a reference to
       the new tied hash.  Note that the code below does NOT call
       the TIEHASH method in the MyTie class - see "Calling Perl
       Routines from within C Programs" for details on how to do
       this.







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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", GV_ADD);
               sv_bless(tie, stash);
               hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
               RETVAL = newRV_noinc(hash);
           OUTPUT:
               RETVAL

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

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

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

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

       Currently (as of perl version 5.004), use of the hash and



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

       You would do well to understand that the TIEARRAY and
       TIEHASH interfaces are mere sugar to invoke some perl
       method calls while using the uniform hash and array
       syntax.  The use of this sugar imposes some overhead
       (typically about two to four extra opcodes per FETCH/STORE
       operation, in addition to the creation of all the mortal
       variables required to invoke the methods).  This overhead
       will be comparatively small if the TIE methods are
       themselves substantial, but if they are only a few
       statements long, the overhead will not be insignificant.

   Localizing changes
       Perl has a very handy construction

         {
           local $var = 2;
           ...
         }

       This construction is approximately equivalent to

         {
           my $oldvar = $var;
           $var = 2;
           ...
           $var = $oldvar;
         }

       The biggest difference is that the first construction
       would reinstate the initial value of $var, irrespective of
       how control exits the block: "goto", "return",
       "die"/"eval", etc. It is a little bit more efficient as
       well.

       There is a way to achieve a similar task from C via Perl
       API: create a pseudo-block, and arrange for some changes
       to be automatically undone at the end of it, either
       explicit, or via a non-local exit (via die()). A
       block-like construct is created by a pair of
       "ENTER"/"LEAVE" macros (see "Returning a Scalar" in
       perlcall).  Such a construct may be created specially for
       some important localized task, or an existing one (like
       boundaries of enclosing Perl subroutine/block, or an
       existing pair for freeing TMPs) may be used. (In the
       second case the overhead of additional localization must
       be almost negligible.) Note that any XSUB is automatically
       enclosed in an "ENTER"/"LEAVE" pair.




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       Inside such a pseudo-block the following service is
       available:

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

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

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

           Also compare "SAVEMORTALIZESV".

       "SAVEMORTALIZESV(SV *sv)"
           Just like "SAVEFREESV", but mortalizes "sv" at the end
           of the current scope instead of decrementing its
           reference count.  This usually has the effect of
           keeping "sv" alive until the statement that called the
           currently live scope has finished executing.

       "SAVEFREEOP(OP *op)"
           The "OP *" is op_free()ed at the end of pseudo-block.

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

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

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



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

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

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

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

       The following API list contains functions, thus one needs
       to provide pointers to the modifiable data explicitly
       (either C pointers, or Perlish "GV *"s).  Where the above
       macros take "int", a similar function takes "int *".

       "SV* save_scalar(GV *gv)"
           Equivalent to Perl code "local $gv".

       "AV* save_ary(GV *gv)"
       "HV* save_hash(GV *gv)"
           Similar to "save_scalar", but localize @gv and %gv.

       "void save_item(SV *item)"
           Duplicates the current value of "SV", on the exit from
           the current "ENTER"/"LEAVE" pseudo-block will restore
           the value of "SV" using the stored value. It doesn't
           handle magic. Use "save_scalar" if magic is affected.

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

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

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

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

Subroutines
   XSUBs and the Argument Stack
       The XSUB mechanism is a simple way for Perl programs to



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       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
       the stack.

       Despite their suggestions in earlier versions of this
       document the macros "(X)PUSH[iunp]" are not suited to



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       XSUBs which return multiple results.  For that, either
       stick to the "(X)PUSHs" macros shown above, or use the new
       "m(X)PUSH[iunp]" macros instead; see "Putting a C value on
       Perl stack".

       For more information, consult perlxs and perlxstut.

   Calling Perl Routines from within C Programs
       There are four routines that can be used to call a Perl
       subroutine from within a C 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*"
       argument contains either the name of the Perl subroutine
       to be called, or a reference to the subroutine.  The
       second argument consists of flags that control the context
       in which the subroutine is called, whether or not the
       subroutine is being passed arguments, how errors should be
       trapped, and how to treat return values.

       All four routines return the number of arguments that the
       subroutine returned on the Perl stack.

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

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

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

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

   Memory Allocation
       Allocation

       All memory meant to be used with the Perl API functions



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       should be manipulated using the macros described in this
       section.  The macros provide the necessary transparency
       between differences in the actual malloc implementation
       that is used within perl.

       It is suggested that you enable the version of malloc that
       is distributed with Perl.  It keeps pools of various sizes
       of unallocated memory in order to satisfy allocation
       requests more quickly.  However, on some platforms, it may
       cause spurious malloc or free errors.

       The following three macros are used to initially allocate
       memory :

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

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

       The second and third arguments "number" and "type" specify
       how many of the specified type of data structure should be
       allocated.  The argument "type" is passed to "sizeof".
       The final argument to "Newxc", "cast", should be used if
       the "pointer" argument is different from the "type"
       argument.

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

       Reallocation

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

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

       Moving

           Move(source, dest, number, type);
           Copy(source, dest, number, type);
           Zero(dest, number, type);

       These three macros are used to move, copy, or zero out
       previously allocated memory.  The "source" and "dest"
       arguments point to the source and destination starting
       points.  Perl will move, copy, or zero out "number"



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       instances of the size of the "type" data structure (using
       the "sizeof" function).

   PerlIO
       The most recent development releases of Perl has been
       experimenting with removing Perl's dependency on the
       "normal" standard I/O suite and allowing other stdio
       implementations to be used.  This involves creating a new
       abstraction layer that then calls whichever implementation
       of stdio Perl was compiled with.  All XSUBs should now use
       the functions in the PerlIO abstraction layer and not make
       any assumptions about what kind of stdio is being used.

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

   Putting a C value on Perl stack
       A lot of opcodes (this is an elementary operation in the
       internal perl stack machine) put an SV* on the stack.
       However, as an optimization the corresponding SV is
       (usually) not recreated each time. The opcodes reuse
       specially assigned SVs (targets) which are (as a
       corollary) not constantly freed/created.

       Each of the targets is created only once (but see
       "Scratchpads and recursion" below), and when an opcode
       needs to put an integer, a double, or a string on stack,
       it just sets the corresponding parts of its target and
       puts the target on stack.

       The macro to put this target on stack is "PUSHTARG", and
       it is directly used in some opcodes, as well as indirectly
       in zillions of others, which use it via "(X)PUSH[iunp]".

       Because the target is reused, you must be careful when
       pushing multiple values on the stack. The following code
       will not do what you think:

           XPUSHi(10);
           XPUSHi(20);

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

       If you need to push multiple different values then you
       should either use the "(X)PUSHs" macros, or else use the
       new "m(X)PUSH[iunp]" macros, none of which make use of
       "TARG".  The "(X)PUSHs" macros simply push an SV* on the
       stack, which, as noted under "XSUBs and the Argument
       Stack", will often need to be "mortal".  The new
       "m(X)PUSH[iunp]" macros make this a little easier to



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       achieve by creating a new mortal for you (via
       "(X)PUSHmortal"), pushing that onto the stack (extending
       it if necessary in the case of the "mXPUSH[iunp]" macros),
       and then setting its value.  Thus, instead of writing this
       to "fix" the example above:

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

       you can simply write:

           mXPUSHi(10)
           mXPUSHi(20)

       On a related note, if you do use "(X)PUSH[iunp]", then
       you're going to need a "dTARG" in your variable
       declarations so that the "*PUSH*" macros can make use of
       the local variable "TARG".  See also "dTARGET" and
       "dXSTARG".

   Scratchpads
       The question remains on when the SVs which are targets for
       opcodes are created. The answer is that they are created
       when the current unit -- a subroutine or a file (for
       opcodes for statements outside of subroutines) -- is
       compiled. During this time a special anonymous Perl array
       is created, which is called a scratchpad for the current
       unit.

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

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

   Scratchpads and recursion
       In fact it is not 100% true that a compiled unit contains
       a pointer to the scratchpad AV. In fact it contains a
       pointer to an AV of (initially) one element, and this
       element is the scratchpad AV. Why do we need an extra
       level of indirection?

       The answer is recursion, and maybe threads. Both these can
       create several execution pointers going into the same
       subroutine. For the subroutine-child not write over the
       temporaries for the subroutine-parent (lifespan of which
       covers the call to the child), the parent and the child
       should have different scratchpads. (And the lexicals
       should be separate anyway!)




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       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
       execution order.  There is an additional "thread" going
       through the nodes of the tree which shows the order of
       execution of the nodes.  In our simplified example above
       it looks like:

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

       But with the actual compile tree for "$a = $b + $c" it is
       different: some nodes optimized away.  As a corollary,
       though the actual tree contains more nodes than our
       simplified example, the execution order is the same as in
       our example.

   Examining the tree
       If you have your perl compiled for debugging (usually done
       with "-DDEBUGGING" on the "Configure" command line), you
       may examine the compiled tree by specifying "-Dx" on the
       Perl command line.  The output takes several lines per
       node, and for "$b+$c" it looks like this:












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

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

                          add
                        /     \
                      null    null
                       |       |
                      gvsv    gvsv

       The execution order is indicated by "===>" marks, thus it
       is "3 4 5 6" (node 6 is not included into above listing),
       i.e., "gvsv gvsv add whatever".

       Each of these nodes represents an op, a fundamental
       operation inside the Perl core. The code which implements
       each operation can be found in the pp*.c files; the
       function which implements the op with type "gvsv" is
       "pp_gvsv", and so on. As the tree above shows, different
       ops have different numbers of children: "add" is a binary
       operator, as one would expect, and so has two children. To
       accommodate the various different numbers of children,
       there are various types of op data structure, and they
       link together in different ways.

       The simplest type of op structure is "OP": this has no
       children. Unary operators, "UNOP"s, have one child, and
       this is pointed to by the "op_first" field. Binary



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       operators ("BINOP"s) have not only an "op_first" field but
       also an "op_last" field. The most complex type of op is a
       "LISTOP", which has any number of children. In this case,
       the first child is pointed to by "op_first" and the last
       child by "op_last". The children in between can be found
       by iteratively following the "op_sibling" pointer from the
       first child to the last.

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

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

   Compile pass 1: check routines
       The tree is created by the compiler while yacc code feeds
       it the constructions it recognizes. Since yacc works
       bottom-up, so does the first pass of perl compilation.

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

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

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

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

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



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

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

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

   Compile pass 3: peephole optimization
       After the compile tree for a subroutine (or for an "eval"
       or a file) is created, an additional pass over the code is
       performed. This pass is neither top-down or bottom-up, but
       in the execution order (with additional complications for
       conditionals).  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.

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

         PL_runops = my_runops;

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

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



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       should already be familiar with its format.

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

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

           (gdb) print Perl_dump_packsubs(PL_defstash)

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

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

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

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

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

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

How multiple interpreters and concurrency are supported
   Background and PERL_IMPLICIT_CONTEXT
       The Perl interpreter can be regarded as a closed box: it
       has an API for feeding it code or otherwise making it do
       things, but it also has functions for its own use.  This
       smells a lot like an object, and there are ways for you to
       build Perl so that you can have multiple interpreters,
       with one interpreter represented either as a C structure,
       or inside a thread-specific structure.  These structures
       contain all the context, the state of that interpreter.

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

       Two other "encapsulation" macros are the
       PERL_GLOBAL_STRUCT and PERL_GLOBAL_STRUCT_PRIVATE (the
       latter turns on the former, and the former turns on
       MULTIPLICITY.)  The PERL_GLOBAL_STRUCT causes all the
       internal variables of Perl to be wrapped inside a single



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

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

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

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

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

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

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

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



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         STATIC void
         S_incline(pTHX_ char *s)

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

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

         void
         Perl_sv_setiv(pTHX_ SV* dsv, IV num)

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

       When Perl is built without options that set
       PERL_IMPLICIT_CONTEXT, there is no first argument
       containing the interpreter's context.  The trailing
       underscore in the pTHX_ macro indicates that the macro
       expansion needs a comma after the context argument because
       other arguments follow it.  If PERL_IMPLICIT_CONTEXT is
       not defined, pTHX_ will be ignored, and the subroutine is
       not prototyped to take the extra argument.  The form of
       the macro without the trailing underscore is used when
       there are no additional explicit arguments.

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

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

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

           sv_setiv(foo, bar);

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

       This doesn't work so cleanly for varargs functions,
       though, as macros imply that the number of arguments is
       known in advance.  Instead we either need to spell them



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       out fully, passing "aTHX_" as the first argument (the Perl
       core tends to do this with functions like Perl_warner), or
       use a context-free version.

       The context-free version of Perl_warner is called
       Perl_warner_nocontext, and does not take the extra
       argument.  Instead it does dTHX; to get the context from
       thread-local storage.  We "#define warner
       Perl_warner_nocontext" so that extensions get source
       compatibility at the expense of performance.  (Passing an
       arg is cheaper than grabbing it from thread-local
       storage.)

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

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

   How do I use all this in extensions?
       When Perl is built with PERL_IMPLICIT_CONTEXT, extensions
       that call any functions in the Perl API will need to pass
       the initial context argument somehow.  The kicker is that
       you will need to write it in such a way that the extension
       still compiles when Perl hasn't been built with
       PERL_IMPLICIT_CONTEXT enabled.

       There are three ways to do this.  First, the easy but
       inefficient way, which is also the default, in order to
       maintain source compatibility with extensions: whenever
       XSUB.h is #included, it redefines the aTHX and aTHX_
       macros to call a function that will return the context.
       Thus, something like:

               sv_setiv(sv, num);

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

               Perl_sv_setiv(Perl_get_context(), sv, num);

       or to this otherwise:

               Perl_sv_setiv(sv, num);

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



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       will all just work.

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

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

               STATIC void my_private_function(int arg1, int arg2);

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

               [... etc ...]

               MODULE = Foo            PACKAGE = Foo

               /* typical XSUB */

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

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

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

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

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

               STATIC void



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

               [... etc ...]

               MODULE = Foo            PACKAGE = Foo

               /* typical XSUB */

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

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

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

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

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

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



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       you do:

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

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

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

       This allows the ability to provide an extra pointer
       (called the "host" environment) for all the system calls.
       This makes it possible for all the system stuff to
       maintain their own state, broken down into seven C
       structures.  These are thin wrappers around the usual
       system calls (see win32/perllib.c) for the default perl
       executable, but for a more ambitious host (like the one
       that would do fork() emulation) all the extra work needed
       to pretend that different interpreters are actually
       different "processes", would be done here.

       The Perl engine/interpreter and the host are orthogonal
       entities.  There could be one or more interpreters in a
       process, and one or more "hosts", with free association
       between them.

Internal Functions
       All of Perl's internal functions which will be exposed to
       the outside world are prefixed by "Perl_" so that they
       will not conflict with XS functions or functions used in a
       program in which Perl is embedded.  Similarly, all global
       variables begin with "PL_". (By convention, static
       functions start with "S_".)

       Inside the Perl core, 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 functions, generates the documentation and a lot
       of other bits and pieces. It's important that when you add
       a new function to the core or change an existing one, you
       change the data in the table in embed.fnc as well. Here's
       a sample entry from that table:

           Apd |SV**   |av_fetch   |AV* ar|I32 key|I32 lval

       The second column is the return type, the third column the



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       name. Columns after that are the arguments. The first
       column is a set of flags:

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

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

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

       Other available flags are:

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

       n  This does not need a interpreter context, so the
          definition has no "pTHX", and it 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).





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

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

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

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

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

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

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

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

   Pointer-To-Integer and Integer-To-Pointer
       Because pointer size does not necessarily equal integer
       size, use the follow macros to do it right.

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

       For example:

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

       and

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

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



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               #define NO_XSLOCKS
               #include "XSUB.h"

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

               dXCPT;    /* set up necessary variables */

               XCPT_TRY_START {
                 code_that_may_croak();
               } XCPT_TRY_END

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

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

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

   Source Documentation
       There's an effort going on to document the internal
       functions and automatically produce reference manuals from
       them - perlapi is one such manual which details all the
       functions which are available to XS writers. perlintern is
       the autogenerated manual for the functions which are not
       part of the API and are supposedly for internal use only.

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

        /*
        =for apidoc sv_setiv

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

        =cut
        */

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

   Backwards compatibility
       The Perl API changes over time. New functions are added or



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       the interfaces of existing functions are changed. The
       "Devel::PPPort" module tries to provide compatibility code
       for some of these changes, so XS writers don't have to
       code it themselves when supporting multiple versions of
       Perl.

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

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

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

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

       For details, see "perldoc ppport.h".

Unicode Support
       Perl 5.6.0 introduced Unicode support. It's important for
       porters and XS writers to understand this support and make
       sure that the code they write does not corrupt Unicode
       data.

   What is Unicode, anyway?
       In the olden, less enlightened times, we all used to use
       ASCII. Most of us did, anyway. The big problem with ASCII
       is that it's American. Well, no, that's not actually the
       problem; the problem is that it's not particularly useful
       for people who don't use the Roman alphabet. What used to
       happen was that particular languages would stick their own
       alphabet in the upper range of the sequence, between 128
       and 255. Of course, we then ended up with plenty of
       variants that weren't quite ASCII, and the whole point of
       it being a standard was lost.

       Worse still, if you've got a language like Chinese or
       Japanese that has hundreds or thousands of characters,
       then you really can't fit them into a mere 256, so they
       had to forget about ASCII altogether, and build their own
       systems using pairs of numbers to refer to one character.

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





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   How can I recognise a UTF-8 string?
       You can't. This is because UTF-8 data is stored in bytes
       just like non-UTF-8 data. The Unicode character 200, (0xC8
       for you hex types) capital E with a grave accent, is
       represented by the two bytes "v196.172". Unfortunately,
       the non-Unicode string "chr(196).chr(172)" has that byte
       sequence as well. So you can't tell just by looking - this
       is what makes Unicode input an interesting problem.

       In general, you either have to know what you're dealing
       with, or you have to guess.  The API function
       "is_utf8_string" can help; it'll tell you if a string
       contains only valid UTF-8 characters. However, it can't do
       the work for you. On a character-by-character basis,
       "is_utf8_char" will tell you whether the current character
       in a string is valid UTF-8.

   How does UTF-8 represent Unicode characters?
       As mentioned above, UTF-8 uses a variable number of bytes
       to store a character. Characters with values 0...127 are
       stored in one byte, just like good ol' ASCII. Character
       128 is stored as "v194.128"; this continues up to
       character 191, which is "v194.191". Now we've run out of
       bits (191 is binary 10111111) so we move on; 192 is
       "v195.128". And so it goes on, moving to three bytes at
       character 2048.

       Assuming you know you're dealing with a UTF-8 string, you
       can find out how long the first character in it is with
       the "UTF8SKIP" macro:

           char *utf = "\305\233\340\240\201";
           I32 len;

           len = UTF8SKIP(utf); /* len is 2 here */
           utf += len;
           len = UTF8SKIP(utf); /* len is 3 here */

       Another way to skip over characters in a UTF-8 string is
       to use "utf8_hop", which takes a string and a number of
       characters to skip over. You're on your own about bounds
       checking, though, so don't use it lightly.

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



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               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 characters in this case. If you do this, you'll
       lose the ability to match hi-bit non-UTF-8 characters; for
       instance, if your UTF-8 string contains "v196.172", and
       you skip that character, you can never match a "chr(200)"
       in a non-UTF-8 string.  So don't do that!

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

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

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

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

       Never forget that the "SVf_UTF8" flag is separate to the
       PV value; you need be sure you don't accidentally knock it
       off while you're manipulating SVs. More specifically, you
       cannot expect to do this:





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

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

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

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

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

       Since just passing an SV to an XS function and copying the
       data of the SV is not enough to copy the UTF8 flags, even
       less right is just passing a "char *" to an XS function.

   How do I convert a string to UTF-8?
       If you're mixing UTF-8 and non-UTF-8 strings, it is
       necessary to upgrade one of the strings to UTF-8. If
       you've got an SV, the easiest way to do this is:

           sv_utf8_upgrade(sv);

       However, you must not do this, for example:

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

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

       Instead, "bytes_to_utf8" will give you a UTF-8-encoded
       copy of its string argument. This is useful for having the
       data available for comparisons and so on, without harming
       the original SV. There's also "utf8_to_bytes" to go the
       other way, but naturally, this will fail if the string
       contains any characters above 255 that can't be
       represented in a single byte.





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

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

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

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

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

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

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

       It's important to know what custom operators won't do for
       you. They won't let you add new syntax to Perl, directly.
       They won't even let you add new keywords, directly. In
       fact, they won't change the way Perl compiles a program at
       all. You have to do those changes yourself, after Perl has
       compiled the program. You do this either by manipulating
       the op tree using a "CHECK" block and the "B::Generate"
       module, or by adding a custom peephole optimizer with the
       "optimize" module.

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



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

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

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



























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