perlguts



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 ques-
       tions 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 inte-
       ger 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 con-
       fig.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 rou-
       tines:

           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 for-
       matted output becomes the value.

       "sv_vsetpvfn" is an analogue of "vsprintf", but it allows you to spec-
       ify 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 informa-
       tion 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 corrup-
       tions 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 sys-
       tem 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 every-
       one.  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)").

       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", FALSE);

       This returns NULL if the variable does not exist.

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

           SvOK(SV*)

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

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

         foo(undef);

       But won’t work when called as:

         $x = undef;
         foo($x);

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

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

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

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

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

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

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

       Offsets

       Perl provides the function "sv_chop" to efficiently remove characters
       from the beginning of a string; you give it an SV and a pointer to
       somewhere inside the PV, and it discards everything before the pointer.
       The efficiency comes by means of a little hack: instead of actually
       removing the characters, "sv_chop" sets the flag "OOK" (offset OK) to
       signal to other functions that the offset hack is in effect, and it
       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 usu-
       ally the same, but a "shift" operation can be carried out by increasing
       "AvARRAY" by one and decreasing "AvFILL" and "AvLEN".  Again, the loca-
       tion of the real start of the C array only comes into play when freeing
       the array. See "av_shift" in av.c.

       Whats 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 inte-
       ger/double or integer/double to string.

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

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

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

       The are various ways in which the private and public flags may differ.
       For example, a tied SV may have a valid underlying value in the IV slot
       (so SvIOKp is true), but the data should be accessed via the FETCH rou-
       tine 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

       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 infor-
       mation on how to use the array access functions on tied arrays.

       Working with HVs

       To create an HV, you use the following routine:

           HV*  newHV();

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

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

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

       Remember that "hv_store" and "hv_fetch" return "SV**"’s and not just
       "SV*".  To access the scalar value, you must first dereference the
       return value.  However, you should check to make sure that the return
       value is not NULL before 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);
                   /* This convenience routine combines hv_iternext,
                      hv_iterkey, and hv_iterval.  The key and retlen
                      arguments are return values for the key and its
                      length.  The value is returned in the SV* argument */

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

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

       This returns NULL if the variable does not exist.

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

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

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

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

       Hash API Extensions

       Beginning with version 5.004, the following functions are also sup-
       ported:

           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 vari-
       ables, 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 ref-
       erence 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 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 refer-
       ence count of the "thing", while "newRV_noinc" does not.  For histori-
       cal reasons, "newRV" is a synonym for "newRV_inc".

       Once you have a reference, you can use the following macro to derefer-
       ence 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 follow-
       ing macro and then check the return value.

           SvTYPE(SvRV(SV*))

       The most useful types that will be returned are:

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

           See the sv.h header file for more details.

       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 spec-
       ified class.  SV is returned.

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

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

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

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

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

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

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

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

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

       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" function-
       ality.

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

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

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

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

             Name <varname> used only once: possible typo

           warning.

       GV_ADDWARN
           Issues the warning:

             Had to create <varname> unexpectedly

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

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

       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, cre-
       ates 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 func-
       tion.  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 ref-
       erence count of the SV you passed to "newRV_inc" has been incremented
       to two.  Now you return the reference from the XSUB routine and forget
       about the SV.  But Perl hasn’t!  Whenever the returned reference is
       destroyed, the reference count of the original SV is decreased to one
       and nothing happens.  The SV will hang around without any way to access
       it until Perl itself terminates.  This is a memory leak.

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

       There are some convenience functions available that can help with the
       destruction of xVs.  These functions introduce the concept of "mortal-
       ity".  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 "SvRE-
       FCNT_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 "Mortal-
       ization" as deferred "SvREFCNT_dec" should help to minimize such prob-
       lems.  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 mor-
       talization.  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_2mor-
       tal" 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 refer-
       ences to the various objects of that name, including (but not limited
       to) the following:

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

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

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

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

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

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

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

           HV*  SvSTASH(SvRV(SV*));

       then use the following to get the package name itself:

           char*  HvNAME(HV* stash);

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

           SV*  sv_bless(SV*, HV* stash)

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

       For more information on references and blessings, consult perlref.

       Double-Typed SVs

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

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

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

               SvIOK_on
               SvNOK_on
               SvPOK_on
               SvROK_on

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

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

           extern int  dberror;
           extern char *dberror_list;

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

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

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

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

       Assigning Magic

       Perl adds magic to an SV using the sv_magic function:

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

       The "sv" argument is a pointer to the SV that is to acquire a new magi-
       cal feature.

       If "sv" is not already magical, Perl uses the "SvUPGRADE" macro to con-
       vert "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 incre-
       mented.

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

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

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

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

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

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

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

           void sv_unmagic(SV *sv, int type);

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

       Magic Virtual Tables

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

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

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

       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 magi-
       cal types begin with "magic_".  NOTE: the magic routines are not con-
       sidered part of the Perl API, and may not be exported by the Perl
       library.

       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
           E  PERL_MAGIC_env            vtbl_env       %ENV hash
           e  PERL_MAGIC_envelem        vtbl_envelem   %ENV hash element
           f  PERL_MAGIC_fm             vtbl_fm        Formline (’compiled’ format)
           g  PERL_MAGIC_regex_global   vtbl_mglob     m//g target / study()ed string
           I  PERL_MAGIC_isa            vtbl_isa       @ISA array
           i  PERL_MAGIC_isaelem        vtbl_isaelem   @ISA array element
           k  PERL_MAGIC_nkeys          vtbl_nkeys     scalar(keys()) lvalue
           L  PERL_MAGIC_dbfile         (none)         Debugger %_<filename
           l  PERL_MAGIC_dbline         vtbl_dbline    Debugger %_<filename element
           m  PERL_MAGIC_mutex          vtbl_mutex     ???
           o  PERL_MAGIC_collxfrm       vtbl_collxfrm  Locale collate transformation
           P  PERL_MAGIC_tied           vtbl_pack      Tied array or hash
           p  PERL_MAGIC_tiedelem       vtbl_packelem  Tied array or hash element
           q  PERL_MAGIC_tiedscalar     vtbl_packelem  Tied scalar or handle
           r  PERL_MAGIC_qr             vtbl_qr        precompiled qr// regex
           S  PERL_MAGIC_sig            vtbl_sig       %SIG hash
           s  PERL_MAGIC_sigelem        vtbl_sigelem   %SIG hash element
           t  PERL_MAGIC_taint          vtbl_taint     Taintedness
           U  PERL_MAGIC_uvar           vtbl_uvar      Available for use by extensions
           v  PERL_MAGIC_vec            vtbl_vec       vec() lvalue
           V  PERL_MAGIC_vstring        (none)         v-string scalars
           w  PERL_MAGIC_utf8           vtbl_utf8      UTF-8 length+offset cache
           x  PERL_MAGIC_substr         vtbl_substr    substr() lvalue
           y  PERL_MAGIC_defelem        vtbl_defelem   Shadow "foreach" iterator
                                                       variable / smart parameter
                                                       vivification
           *  PERL_MAGIC_glob           vtbl_glob      GV (typeglob)
           #  PERL_MAGIC_arylen         vtbl_arylen    Array length ($#ary)
           .  PERL_MAGIC_pos            vtbl_pos       pos() lvalue
           <  PERL_MAGIC_backref        vtbl_backref   ???
           ~  PERL_MAGIC_ext            (none)         Available for use by extensions

       When an uppercase and lowercase letter both exist in the table, then
       the uppercase letter is typically used to represent some kind of com-
       posite 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 informa-
       tion to variables (typically objects).  This is especially useful
       because there is no way for normal perl code to corrupt this private
       information (unlike using extra elements of a hash object).

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

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

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

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

       Note that because multiple extensions may be using "PERL_MAGIC_ext" or
       "PERL_MAGIC_uvar" magic, it is important for extensions to take extra
       care to avoid conflict.  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 ’sig-
       nature’ at the top of the private data area and check that.

       Also note that the "sv_set*()" and "sv_cat*()" functions described ear-
       lier do not invoke ’set’ magic on their targets.  This must be done by
       the user either by calling the "SvSETMAGIC()" macro after calling these
       functions, or by using one of the "sv_set*_mg()" or "sv_cat*_mg()"
       functions.  Similarly, generic C code must call the "SvGETMAGIC()"
       macro to invoke any ’get’ magic if they use an SV obtained from
       external sources in functions that don’t handle magic.  See perlapi for
       a description of these functions.  For example, calls to the
       "sv_cat*()" functions typically need to be followed by "SvSETMAGIC()",
       but they don’t need a prior "SvGETMAGIC()" since their implementation
       handles ’get’ magic.

       Finding Magic

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

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

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

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

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

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

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

       You would do well to understand that the TIEARRAY and TIEHASH inter-
       faces 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 cre-
       ated specially for some important localized task, or an existing one
       (like boundaries of enclosing Perl subroutine/block, or an existing
       pair for freeing TMPs) may be used. (In the second case the overhead of
       additional localization must be almost negligible.) Note that any XSUB
       is automatically enclosed in an "ENTER"/"LEAVE" pair.

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

       "SAVEINT(int i)"
       "SAVEIV(IV i)"
       "SAVEI32(I32 i)"
       "SAVELONG(long i)"
           These macros arrange things to restore the value of integer vari-
           able "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 mecha-
           nism for doing a delayed "SvREFCNT_dec".  However, while "sv_2mor-
           tal" 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 cur-
           rent scope instead of decrementing its reference count.  This usu-
           ally has the effect of keeping "sv" alive until the statement that
           called the currently live scope has finished executing.

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

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

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

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

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

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

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

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

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

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

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

       "void save_item(SV *item)"
           Duplicates the current value of "SV", on the exit from the current
           "ENTER"/"LEAVE" pseudo-block will restore the value of "SV" using
           the stored value.

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

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

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

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


Subroutines

       XSUBs and the Argument Stack

       The XSUB mechanism is a simple way for Perl programs to access C sub-
       routines.  An XSUB routine will have a stack that contains the argu-
       ments from the Perl program, and a way to map from the Perl data struc-
       tures 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 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 subrou-
       tine 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, con-
       sult perlcall.

       Memory Allocation

       Allocation

       All memory meant to be used with the Perl API functions should be
       manipulated using the macros described in this section.  The macros
       provide the necessary transparency between differences in the actual
       malloc implementation that is used within perl.

       It is suggested that you enable the version of malloc that is dis-
       tributed 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 :

           New(x, pointer, number, type);
           Newc(x, pointer, number, type, cast);
           Newz(x, pointer, number, type);

       The first argument "x" was a "magic cookie" that was used to keep track
       of who called the macro, to help when debugging memory problems.  How-
       ever, the current code makes no use of this feature (most Perl develop-
       ers now use run-time memory checkers), so this argument can be any num-
       ber.

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

       The third and fourth 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 "Newc", "cast",
       should be used if the "pointer" argument is different from the "type"
       argument.

       Unlike the "New" and "Newc" macros, the "Newz" 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 allo-
       cated memory.  The "source" and "dest" arguments point to the source
       and destination starting points.  Perl will move, copy, or zero out
       "number" instances of the size of the "type" data structure (using the
       "sizeof" function).

       PerlIO

       The most recent development releases of Perl has been experimenting
       with removing Perl’s dependency on the "normal" standard I/O suite and
       allowing other stdio implementations to be used.  This involves creat-
       ing 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 dou-
       ble, or a string on stack, it just sets the corresponding parts of its
       target and puts the target on stack.

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

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

           XPUSHi(10);
           XPUSHi(20);

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

       If you need to push multiple different values then you should either
       use the "(X)PUSHs" macros, or else use the new "m(X)PUSH[iunp]" macros,
       none of which make use of "TARG".  The "(X)PUSHs" macros simply push an
       SV* on the stack, which, as noted under "XSUBs and the Argument Stack",
       will often need to be "mortal".  The new "m(X)PUSH[iunp]" macros make
       this a little easier to achieve by creating a new mortal for you (via
       "(X)PUSHmortal"), pushing that onto the stack (extending it if neces-
       sary 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 subrou-
       tines) -- 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 (ini-
       tially) 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 sev-
       eral execution pointers going into the same subroutine. For the subrou-
       tine-child not write over the temporaries for the subroutine-parent
       (lifespan of which covers the call to the child), the parent and the
       child should have different scratchpads. (And the lexicals should be
       separate anyway!)

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

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


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 con-
       tains 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 "-DDE-
       BUGGING" on the "Configure" command line), you may examine the compiled
       tree by specifying "-Dx" on the Perl command line.  The output takes
       several lines per node, and for "$b+$c" it looks like this:

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

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

                          add
                        /     \
                      null    null
                       │       │
                      gvsv    gvsv

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

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

       The simplest type of op structure is "OP": this has no children. Unary
       operators, "UNOP"s, have one child, and this is pointed to by the
       "op_first" field. Binary operators ("BINOP"s) have not only an
       "op_first" field but also an "op_last" field. The most complex type of
       op is a "LISTOP", which has any number of children. In this case, the
       first child is pointed to by "op_first" and the last child by
       "op_last". The children in between can be found by iteratively follow-
       ing 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 compli-
       cate 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 con-
       structions 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 opti-
       mization may be performed on this pass.  This is optimization by so-
       called "check routines".  The correspondence between node names and
       corresponding check routines is described in opcode.pl (do not forget
       to run "make regen_headers" if you modify this file).

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

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

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

       Compile pass 1a: constant folding

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

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

       Compile pass 2: context propagation

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

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

       Compile pass 3: peephole optimization

       After the compile tree for a subroutine (or for an "eval" or a file) is
       created, an additional pass over the code is performed. This pass is
       neither top-down or bottom-up, but in the execution order (with addi-
       tional 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 pro-
       vide 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 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 func-
       tions 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.

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

       All this obviously requires a way for the Perl internal functions to be
       either subroutines taking some kind of structure as the first argument,
       or subroutines taking 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 dec-
       larations and calls can pass a structure as their first argument, or
       pass nothing.  To solve this, the subroutines are named and declared in
       a particular way.  Here’s a typical start of a static function used
       within the Perl guts:

         STATIC void
         S_incline(pTHX_ char *s)

       STATIC becomes "static" in C, and may be #define’d to nothing in some
       configurations in 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 expan-
       sion 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 argu-
       ment.  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 com-
       piled with.

       This doesn’t work so cleanly for varargs functions, though, as macros
       imply that the number of arguments is known in advance.  Instead we
       either need to spell them out fully, passing "aTHX_" as the first argu-
       ment (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_nocon-
       text, 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 argu-
       ment 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 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 my_private_function(int arg1, int arg2);

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

               [... etc ...]

               MODULE = Foo            PACKAGE = Foo

               /* typical XSUB */

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

       Note that the only two changes from the normal way of writing an exten-
       sion is the addition of a "#define PERL_NO_GET_CONTEXT" before includ-
       ing the Perl headers, followed by a "dTHX;" declaration at the start of
       every function that will call the Perl API.  (You’ll know which func-
       tions need this, because the C compiler will complain that there’s an
       undeclared identifier in those functions.)  No changes are needed for
       the XSUBs themselves, because the XS() macro is correctly defined to
       pass in the implicit 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 my_private_function(pTHX_ int arg1, int arg2);

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

               [... etc ...]

               MODULE = Foo            PACKAGE = Foo

               /* typical XSUB */

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

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

       Should I do anything special if I call perl from multiple threads?

       If you create interpreters in one thread and then proceed to call them
       in another, you need to make sure perl’s own Thread Local Storage (TLS)
       slot is initialized correctly in each of those threads.

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

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

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

       Future Plans and PERL_IMPLICIT_SYS

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

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

       p  This function has a "Perl_" prefix; ie, it is defined as
          "Perl_av_fetch"

       d  This function has documentation using the "apidoc" feature which
          we’ll look at in a second.

       Other available flags are:

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

       n  This does not use "aTHX_" and "pTHX" to pass interpreter context.
          (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).

       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 for-
       matting 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);

       Source Documentation

       There’s an effort going on to document the internal functions and auto-
       matically produce reference manuals from them - perlapi is one such
       manual which details all the functions which are available to XS writ-
       ers. 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.


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 par-
       ticularly 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 charac-
       ter.

       To fix this, some people formed Unicode, Inc. and produced a new char-
       acter 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, instead of just one. You can learn more about
       Unicode at http://www.unicode.org/

       How can I recognise a UTF-8 string?

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

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

       How does UTF-8 represent Unicode characters?

       As mentioned above, UTF-8 uses a variable number of bytes to store a
       character. Characters with values 1...128 are stored in one byte, just
       like good ol’ ASCII. Character 129 is stored as "v194.129"; this con-
       tinues 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 */
               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 charac-
       ters. 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. If a string has been identified as being UTF-8
       encoded, Perl will set a flag in the SV, "SVf_UTF8". You can check and
       manipulate this flag with the following macros:

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

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

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

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

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

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

       The "char*" string does not tell you the whole story, and you can’t
       copy or reconstruct an SV just by copying the string value. Check if
       the old SV has the UTF-8 flag set, and act accordingly:

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

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

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

       How do I convert a string to UTF-8?

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

           sv_utf8_upgrade(sv);

       However, you must not do this, for example:

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

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

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

       Is there anything else I need to know?

       Not really. Just remember these things:

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

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

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

       ·  Mixing UTF-8 and non-UTF-8 strings is tricky. Use "bytes_to_utf8" to
          get a new string which is UTF-8 encoded. There are tricks you can
          use to delay deciding whether you need to use a UTF-8 string until
          you get to a high character - "HALF_UPGRADE" is one of those.


Custom Operators

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

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

       It’s important to know what custom operators won’t do for you. They
       won’t let you add new syntax to Perl, directly. They won’t even let you
       add new keywords, directly. In fact, they won’t change the way Perl
       compiles a program at all. You have to do those changes yourself, after
       Perl has 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 cre-
       ating ops with the type "OP_CUSTOM" and the "pp_addr" of your own PP
       function. This should be defined in XS code, and should look like the
       PP ops in "pp_*.c". You are responsible for ensuring that your op takes
       the appropriate number of values from the stack, and you are responsi-
       ble 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_CUS-
       TOM", Perl uses the value of "o->op_ppaddr" as a key into the "PL_cus-
       tom_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; "Opcodes::Custom"
       will provide functions which make it trivial to "register" custom ops
       to the Perl interpreter.


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 Bow-
       ers, Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer, Stephen
       McCamant, and Gurusamy Sarathy.


SEE ALSO

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



perl v5.8.6                       2004-11-05                       PERLGUTS(1)

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