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NAME    [Toc]    [Back]

       perlguts - Introduction to the Perl API

DESCRIPTION    [Toc]    [Back]

       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    [Toc]    [Back]


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

           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    [Toc]    [Back]

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

       The seven routines are:

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

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

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

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

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

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

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

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

       "sv_vsetpvfn" is an analogue of "vsprintf", but it allows
       you to specify either a pointer to a variable argument
       list or the address and length of an array of SVs.  The
       last 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

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

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

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

           SvPV(SV*, STRLEN len)

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

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


       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)").

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


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

           SvCUR_set(SV*, I32 val)

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


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

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

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

       The first function calculates the length of the string to
       be appended by using "strlen".  In the second, you specify
       the length of the string yourself.  The third function
       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", FALSE);
       This returns NULL if the variable does not exist.

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


       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:


       But won't work when called as:

         $x = undef;

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

       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").

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

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

       This is best demonstrated by example:

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

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

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

       What's Really Stored in an SV?

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

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


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

       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 occured 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    [Toc]    [Back]

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

           AV*  newAV();

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

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

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

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

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

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

       Here are some other functions:

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

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

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

       The "av_clear" function deletes all the elements in the
       AV*  array, but does not actually delete the array itself.
       The "av_undef" function will delete all the elements in
       the array plus the array itself.  The "av_extend" function
       extends the array so that it 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", FALSE);

       This returns NULL if the variable does not exist.

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

       Working with HVs    [Toc]    [Back]

       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

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

       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

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

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

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

       This returns NULL if the variable does not exist.

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

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

       The last step was added in version 5.6 to improve 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    [Toc]    [Back]

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

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

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

           SV*     hv_iterkeysv  (HE* entry);

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

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

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

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

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

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

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

       AVs, HVs and undefined values

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

       For example, intuition tells you that this XS code:

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

       is equivalent to this Perl code:

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

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

       Other problems can occur when storing &PL_sv_undef in HVs:

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

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

           Modification of non-creatable hash value attempted

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

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

           Modification of a read-only value attempted

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

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

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

       References    [Toc]    [Back]

       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:


       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


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


       The most useful types that will be returned are:

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

           See the sv.h header file for more details.

       Blessed References and Class Objects    [Toc]    [Back]

       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

           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

               SV* sv_setref_iv(SV* rv, const char* classname, IV
               SV* sv_setref_uv(SV* rv, const char* classname, UV
               SV* sv_setref_nv(SV* rv, const char* classname, NV

       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

       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    [Toc]    [Back]

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

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

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

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

           Marks the variable as multiply defined, thus preventing

             Name <varname> used only once: possible typo


           Issues the warning:

             Had to create <varname> unexpectedly

           if the variable did not exist before the function was

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

       Reference Counts and Mortality    [Toc]    [Back]

       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

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

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

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

       To create a mortal variable, use the functions:

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

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

           SV *tmp = sv_newmortal();
           sv_setiv(tmp, an_integer);
       As that is multiple C statements it is quite common so see
       this idiom instead:

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

       You should be careful about creating mortal variables.
       Strange things can happen if you make the same value mortal
 within multiple contexts, or if you make a variable
       mortal multiple times. Thinking of "Mortalization" as
       deferred "SvREFCNT_dec" should help to minimize such problems.
  For example if you are passing an SV which you know
       has high enough REFCNT to survive its use on the stack you
       need not do any mortalization.  If you are not sure then
       doing an "SvREFCNT_inc" and "sv_2mortal", or making a
       "sv_mortalcopy" is safer.

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

       Stashes and Globs    [Toc]    [Back]

       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

       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

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

       The first function takes a literal string, the second uses
       the string stored in the SV.  Remember that a stash is
       just a hash table, so you get back an "HV*".  The "create"
       flag will create a new package if it is set.
       The name that "gv_stash*v" wants is the name of the package
 whose symbol table you want.  The default package is
       called "main".  If you have multiply nested packages, pass
       their names to "gv_stash*v", separated by "::" as in the
       Perl language itself.

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

           HV*  SvSTASH(SvRV(SV*));

       then use the following to get the package name itself:

           char*  HvNAME(HV* stash);

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

           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

       Double-Typed SVs    [Toc]    [Back]

       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:


       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

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

           extern int  dberror;
           extern char *dberror_list;

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

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

       Magic Variables    [Toc]    [Back]

       [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;
               SV*         mg_obj;
               char*       mg_ptr;
               I32         mg_len;

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

       Assigning Magic    [Toc]    [Back]

       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 and "namlen" >= 0 a malloc'd copy of the name is
       stored in "mg_ptr" field.

       The sv_magic function uses "how" to determine which, if
       any, predefined "Magic Virtual Table" should be assigned
       to the "mg_virtual" field.  See the "Magic Virtual Tables"
       section below.  The "how" argument is also stored in the
       "mg_type" field. The value of "how" should be chosen from
       the set of macros "PERL_MAGIC_foo" found in perl.h. Note
       that before these macros were added, Perl internals used
       to directly use character literals, so you may occasionally
 come across old code or documentation referring to
       'U' magic rather than "PERL_MAGIC_uvar" for example.

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

       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

           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    [Toc]    [Back]

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

       The "MGVTBL" has five pointers to the following routine

           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);

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

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

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

           { magic_get, magic_set, magic_len, 0, 0 }

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

       The current kinds of Magic Virtual Tables are:
           (old-style  char  and  macro)   MGVTBL         Type of
           --------------------------                      ------
               PERL_MAGIC_sv              vtbl_sv         Special
scalar variable
           A  PERL_MAGIC_overload       vtbl_amagic     %OVERLOAD
           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         BoyerMoore (fast string search)
           D   PERL_MAGIC_regdata          vtbl_regdata     Regex
match position data
                                                       (@+ and @-
           d   PERL_MAGIC_regdatum         vtbl_regdatum    Regex
match position data
           E  PERL_MAGIC_env            vtbl_env       %ENV hash
           e   PERL_MAGIC_envelem        vtbl_envelem   %ENV hash
           f  PERL_MAGIC_fm              vtbl_fm         Formline
('compiled' format)
           g   PERL_MAGIC_regex_global   vtbl_mglob     m//g target / study()ed string
           I  PERL_MAGIC_isa            vtbl_isa       @ISA array
           i  PERL_MAGIC_isaelem        vtbl_isaelem   @ISA array
           k         PERL_MAGIC_nkeys                  vtbl_nkeys
scalar(keys()) lvalue
           L   PERL_MAGIC_dbfile          (none)         Debugger
           l  PERL_MAGIC_dbline          vtbl_dbline     Debugger
%_<filename element
           m  PERL_MAGIC_mutex          vtbl_mutex     ???
           o    PERL_MAGIC_collxfrm        vtbl_collxfrm   Locale
collate transformation
           P  PERL_MAGIC_tied           vtbl_pack      Tied array
or hash
           p  PERL_MAGIC_tiedelem       vtbl_packelem  Tied array
or hash element
           q    PERL_MAGIC_tiedscalar       vtbl_packelem    Tied
scalar or handle
           r   PERL_MAGIC_qr              vtbl_qr         precompiled qr// regex
           S  PERL_MAGIC_sig            vtbl_sig       %SIG hash
           s  PERL_MAGIC_sigelem        vtbl_sigelem   %SIG  hash
           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()
           V  PERL_MAGIC_vstring         (none)          v-string
           w    PERL_MAGIC_utf8             vtbl_utf8       UTF-8
length+offset cache
           x  PERL_MAGIC_substr          vtbl_substr     substr()
           y    PERL_MAGIC_defelem         vtbl_defelem    Shadow
"foreach" iterator
                                                       variable /
smart parameter
           *  PERL_MAGIC_glob           vtbl_glob      GV  (typeglob)
           #    PERL_MAGIC_arylen           vtbl_arylen     Array
length ($#ary)
           .   PERL_MAGIC_pos              vtbl_pos         pos()
           <  PERL_MAGIC_backref        vtbl_backref   ???
           ~   PERL_MAGIC_ext            (none)         Available
for use by extensions

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

       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

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

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

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

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

       Note that because multiple extensions may be using
       "PERL_MAGIC_ext" or "PERL_MAGIC_uvar" magic, it is important
 for extensions to take extra care to avoid  conflict.
       Typically only using the magic on objects blessed into the
       same class as the extension is 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 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    [Toc]    [Back]

           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

       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    [Toc]    [Back]

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

       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

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

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

       Localizing changes    [Toc]    
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