


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 containing some info on the basic workings
       of the Perl core. It is far from complete and probably
       contains many errors. Please refer any questions or com-
       ments 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, respec-
       tively. (Again, there are U32 and U16, as well.)

       Working with SVs

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

       The six routines are:

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

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





2001-05-24                 perl v5.6.1                          1





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


           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*, int)
           void  sv_setpvf(SV*, const char*, ...);
           void  sv_setpvfn(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 argu-
       ment to "newSVpv".  Be warned, though, that Perl will
       determine the string's length by using "strlen", which
       depends on the string terminating with a NUL character.

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

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

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

       The "sv_set*()" functions are not generic enough to oper-
       ate on values that have "magic".  See the Magic Virtual
       Tables entry elsewhere in this document later in this doc-
       ument.

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

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






2001-05-24                 perl v5.6.1                          2





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


           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 com-
       piler, but it won't work for everyone.  Break this sort of
       statement up into separate assignments:

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

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

           SvTRUE(SV*)

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

           SvGROW(SV*, STRLEN newlen)

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

       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.





2001-05-24                 perl v5.6.1                          3





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


           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_catpvfn(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 oper-
       ate on values that have "magic".  See the Magic Virtual
       Tables entry elsewhere in this document later in this doc-
       ument.

       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*)



2001-05-24                 perl v5.6.1                          4





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


       The scalar "undef" value is stored in an SV instance
       called "PL_sv_undef".  Its address can be used whenever an
       "SV*" is needed.

       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 oth-
       erwise.  Instead it has returned a NULL pointer which,
       somewhere down the line, will cause a segmentation viola-
       tion, 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 "SvRE-
       FCNT_dec(SV*)".  Normally this call is not necessary (see
       the Reference Counts and Mortality entry elsewhere in this
       document).

       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 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 allo-
       cated storage.

       This is best demonstrated by example:







2001-05-24                 perl v5.6.1                          5





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


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

       What's Really Stored in an SV?

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

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

           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.

       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:



2001-05-24                 perl v5.6.1                          6





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


           AV*  newAV();

       The second method both creates the AV and initially popu-
       lates it with SVs:

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

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



2001-05-24                 perl v5.6.1                          7





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


       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 the Understanding the Magic of Tied Hashes and Arrays
       entry elsewhere in this document for more information on
       how to use the array access functions on tied arrays.

       Working with HVs

       To create an HV, you use the following routine:

           HV*  newHV();

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

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

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

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



2001-05-24                 perl v5.6.1                          8





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


           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 a 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 distri-
       bution of lower bits in the resulting hash value.

       See the Understanding the Magic of Tied Hashes and Arrays
       entry elsewhere in this document for more information on
       how to use the hash access functions on tied hashes.




2001-05-24                 perl v5.6.1                          9





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


       Hash API Extensions

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

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

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

           SV*     hv_iterkeysv  (HE* entry);

       Note that these functions take "SV*" keys, which simpli-
       fies writing of extension code that deals with hash struc-
       tures.  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 the perlapi manpage for detailed descriptions.

       The following macros must always be used to access the
       contents of hash entries.  Note that the arguments to
       these macros must be simple variables, since they may get
       evaluated more than once.  See the perlapi manpage 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 incre-
       ment the reference count of the stored "val", which is the
       caller's responsibility.  If these functions return a NULL
       value, the caller will usually have to decrement the ref-
       erence count of "val" to avoid a memory leak.

       References

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




2001-05-24                 perl v5.6.1                         10





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


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

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

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

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

           SvRV(SV*)

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

       To determine if an SV is a reference, you can use the fol-
       lowing macro:

           SvROK(SV*)

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

           SvTYPE(SvRV(SV*))

       The most useful types that will be returned are:

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

           See the sv.h header file for more details.


       Blessed References and Class Objects

       References are also used to support object-oriented pro-
       gramming.  In the OO lexicon, an object is simply a refer-
       ence 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



2001-05-24                 perl v5.6.1                         11





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


       following function:

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

       The "sv" argument must be a reference.  The "stash" argu-
       ment specifies which class the reference will belong to.
       See the Stashes and Globs entry elsewhere in this document
       for information on converting class names into stashes.

       /* Still under construction */

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

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

       Copies integer 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_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" functionality.

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

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




2001-05-24                 perl v5.6.1                         12





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


               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 cre-
       ated in the current package.

       Reference Counts and Mortality

       Perl uses an reference count-driven garbage collection
       mechanism. SVs, AVs, or HVs (xV for short in the follow-
       ing) 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 refer-
       ence to it is changed or overwritten.  At the internal
       level, however, reference counts can be manipulated with
       the following macros:

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

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




2001-05-24                 perl v5.6.1                         13





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


       For example, imagine you want to return a reference from
       an XSUB function.  Inside the XSUB routine, you create an
       SV which initially has a reference count of one.  Then you
       call "newRV_inc", passing it the just-created SV.  This
       returns the reference as a new SV, but the reference count
       of the SV you passed to "newRV_inc" has been incremented
       to two.  Now you return the reference from the XSUB rou-
       tine 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 hap-
       pens.  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 refer-
       ence 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 intro-
       duce the concept of "mortality".  An xV that is mortal has
       had its reference count marked to be decremented, but not
       actually decremented, until "a short time later".  Gener-
       ally 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 the perlcall manpage and the perlxs manpage for more
       details on these macros.

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

       You should be careful about creating mortal variables.
       Strange things can happen if you make the same value mor-
       tal within multiple contexts, or if you make a variable
       mortal multiple times.

       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, the second converts an
       existing SV to a mortal SV (and thus defers a call to
       "SvREFCNT_dec"), and the third creates a mortal copy of an
       existing SV.

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



2001-05-24                 perl v5.6.1                         14





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


       Stashes and Globs

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

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

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

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

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

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

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

       Alternately, if you have an SV that is a blessed refer-
       ence, 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)



2001-05-24                 perl v5.6.1                         15





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


       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
       the perlref manpage.

       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 "dber-
       ror" that contains both the numeric and descriptive string
       error values, you could use the following code:

           extern int  dberror;
           extern char *dberror_list;

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

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






2001-05-24                 perl v5.6.1                         16





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


       Magic Variables

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

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

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

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

       Assigning Magic

       Perl adds magic to an SV using the sv_magic function:

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

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

       If "sv" is not already magical, Perl uses the "SvUPGRADE"
       macro to set the "SVt_PVMG" flag for the "sv".  Perl then
       continues by adding it 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 asso-
       ciated with an SV.

       The "name" and "namlen" arguments are used to associate a
       string with the magic, typically the name of a variable.
       "namlen" is stored in the "mg_len" field and if "name" is
       non-null and "namlen" >= 0 a malloc'd copy of the name is
       stored in "mg_ptr" field.

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




2001-05-24                 perl v5.6.1                         17





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


       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 "#", or if it is a NULL pointer, then "obj" is merely
       stored, without the reference count being incremented.

       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 a "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 vari-
       able.

       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 after 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



2001-05-24                 perl v5.6.1                         18





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


       corresponds to an "mg_type" of '\0') contains:

           { magic_get, magic_set, magic_len, 0, 0 }

       Thus, when an SV is determined to be magical and of type
       '\0', if a get operation is being performed, the routine
       "magic_get" is called.  All the various routines for the
       various magical types begin with "magic_".  NOTE: the
       magic routines are not considered part of the Perl API,
       and may not be exported by the Perl library.

       The current kinds of Magic Virtual Tables are:

           mg_type  MGVTBL              Type of magic
           -------  ------              ----------------------------
           \0       vtbl_sv             Special scalar variable
           A        vtbl_amagic         %OVERLOAD hash
           a        vtbl_amagicelem     %OVERLOAD hash element
           c        (none)              Holds overload table (AMT) on stash
           B        vtbl_bm             Boyer-Moore (fast string search)
           D        vtbl_regdata        Regex match position data (@+ and @- vars)
           d        vtbl_regdatum       Regex match position data element
           E        vtbl_env            %ENV hash
           e        vtbl_envelem        %ENV hash element
           f        vtbl_fm             Formline ('compiled' format)
           g        vtbl_mglob          m//g target / study()ed string
           I        vtbl_isa            @ISA array
           i        vtbl_isaelem        @ISA array element
           k        vtbl_nkeys          scalar(keys()) lvalue
           L        (none)              Debugger %_<filename
           l        vtbl_dbline         Debugger %_<filename element
           o        vtbl_collxfrm       Locale transformation
           P        vtbl_pack           Tied array or hash
           p        vtbl_packelem       Tied array or hash element
           q        vtbl_packelem       Tied scalar or handle
           S        vtbl_sig            %SIG hash
           s        vtbl_sigelem        %SIG hash element
           t        vtbl_taint          Taintedness
           U        vtbl_uvar           Available for use by extensions
           v        vtbl_vec            vec() lvalue
           x        vtbl_substr         substr() lvalue
           y        vtbl_defelem        Shadow "foreach" iterator variable /
                                         smart parameter vivification
           *        vtbl_glob           GV (typeglob)
           #        vtbl_arylen         Array length ($#ary)
           .        vtbl_pos            pos() lvalue
           ~        (none)              Available for use by extensions

       When an uppercase and lowercase letter both exist in the
       table, then the uppercase letter is used to represent some
       kind of composite type (a list or a hash), and the lower-
       case letter is used to represent an element of that com-
       posite type.




2001-05-24                 perl v5.6.1                         19





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


       The '~' and 'U' magic types are defined specifically for
       use by extensions and will not be used by perl itself.
       Extensions can use '~' 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 ele-
       ments of a hash object).

       Similarly, 'U' 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" struc-
       ture:

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

       When the SV is read from or written to, the "uf_val" or
       "uf_set" function will be called with "uf_index" as the
       first arg and a pointer to the SV as the second.  A simple
       example of how to add 'U' 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, 'U', (char*)&uf, sizeof(uf));

       Note that because multiple extensions may be using '~' or
       'U' 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 '~' magic, it may also be appropriate to
       add an I32 'signature' at the top of the private data area
       and check that.

       Also note that the "sv_set*()" and "sv_cat*()" functions
       described earlier do not invoke 'set' magic on their tar-
       gets.  This must be done by the user either by calling the
       "SvSETMAGIC()" macro after calling these functions, or by
       using one of the "sv_set*_mg()" or "sv_cat*_mg()" func-
       tions.  Similarly, generic C code must call the "SvGET-
       MAGIC()" macro to invoke any 'get' magic if they use an SV
       obtained from external sources in functions that don't
       handle magic.  See the perlapi manpage for a description



2001-05-24                 perl v5.6.1                         20





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


       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 'P' 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 consid-
       ered bugs in the API, to be fixed in later releases, and
       are bracketed with [MAYCHANGE] below. If you find yourself
       actually applying such information in this section, be
       aware that the behavior may change in the future, umm,
       without warning.

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










2001-05-24                 perl v5.6.1                         21





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


           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, tie, 'P');
               RETVAL = newRV_noinc(hash);
           OUTPUT:
               RETVAL

       The "av_store" function, when given a tied array argument,
       merely copies the magic of the array onto the value to be
       "stored", using "mg_copy".  It may also return NULL, indi-
       cating that the value did not actually need to be stored
       in the array.  [MAYCHANGE] After a call to "av_store" on a
       tied array, the caller will usually need to call
       "mg_set(val)" to actually invoke the perl level "STORE"
       method on the 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 deallo-
       cated, 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 cur-
       rently implements a kind of lazy access to arrays and
       hashes.

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



2001-05-24                 perl v5.6.1                         22





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


       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 pro-
       vide more transparent access to both tied and normal data
       types in future versions.  [/MAYCHANGE]

       You would do well to understand that the TIEARRAY and
       TIEHASH interfaces are mere sugar to invoke some perl
       method calls while using the uniform hash and array syn-
       tax.  The use of this sugar imposes some overhead (typi-
       cally 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 them-
       selves 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 the Returning a Scalar entry
       in the perlcall manpage).  Such a construct may be created
       specially for some important localized task, or an exist-
       ing one (like boundaries of enclosing Perl subrou-
       tine/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.



2001-05-24                 perl v5.6.1                         23





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


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

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

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

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

           Also compare "SAVEMORTALIZESV".

       ""SAVEMORTALIZESV(SV *sv)""
           Just like "SAVEFREESV", but mortalizes "sv" at the end
           of the current scope instead of decrementing its ref-
           erence count.  This usually has the effect of keeping
           "sv" alive until the statement that called the cur-
           rently 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 corre-
           sponds 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 corre-
           sponding string may be reallocated like this:



2001-05-24                 perl v5.6.1                         24





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


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


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

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

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

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

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

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

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

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

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

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

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

Subroutines





2001-05-24                 perl v5.6.1                         25





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


       XSUBs and the Argument Stack

       The XSUB mechanism is a simple way for Perl programs to
       access C subroutines.  An XSUB routine will have a stack
       that contains the arguments from the Perl program, and a
       way to map from the Perl data structures to a C equiva-
       lent.

       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 argu-
       ments, 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 the macros to push IVs, doubles, strings, and
       SV pointers respectively:

           PUSHi(IV)
           PUSHn(double)
           PUSHp(char*, I32)
           PUSHs(SV*)

       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 val-
       ues on the stack is to use the macros:

           XPUSHi(IV)
           XPUSHn(double)
           XPUSHp(char*, I32)
           XPUSHs(SV*)

       These macros automatically adjust the stack for you, if



2001-05-24                 perl v5.6.1                         26





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


       needed.  Thus, you do not need to call "EXTEND" to extend
       the stack.  However, see the Putting a C value on Perl
       stack entry elsewhere in this document

       For more information, consult the perlxs manpage and the
       perlxstut manpage.

       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*" argu-
       ment 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, consult the perlcall manpage.







2001-05-24                 perl v5.6.1                         27





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


       Memory Allocation

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

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

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

       These three macros are used to initially allocate memory.

       The first argument "x" was a "magic cookie" that was used
       to keep track of who called the macro, to help when debug-
       ging memory problems.  However, the current code makes no
       use of this feature (most Perl developers now use run-time
       memory checkers), so this argument can be any number.

       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" argu-
       ment.

       Unlike the "New" and "Newc" macros, the "Newz" macro calls
       "memzero" to zero out all the newly allocated memory.

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

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

           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



2001-05-24                 perl v5.6.1                         28





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


       previously allocated memory.  The "source" and "dest"
       arguments point to the source and destination starting
       points.  Perl will move, copy, or zero out "number"
       instances of the size of the "type" data structure (using
       the "sizeof" function).

       PerlIO

       The most recent development releases of Perl has been
       experimenting with removing Perl's dependency on the "nor-
       mal" standard I/O suite and allowing other stdio implemen-
       tations to be used.  This involves creating a new abstrac-
       tion layer that then calls whichever implementation of
       stdio Perl was compiled with.  All XSUBs should now use
       the functions in the PerlIO abstraction layer and not make
       any assumptions about what kind of stdio is being used.

       For a complete description of the PerlIO abstraction, con-
       sult the perlapio manpage.

       Putting a C value on Perl stack

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

       Each of the targets is created only once (but see the
       Scratchpads and recursion entry elsewhere in this document
       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[pni]".

       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, use
       "XPUSHs", which bypasses "TARG".




2001-05-24                 perl v5.6.1                         29





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


       On a related note, if you do use "(X)PUSH[npi]", then
       you're going to need a "dTARG" in your variable declara-
       tions so that the "*PUSH*" macros can make use of the
       local variable "TARG".

       Scratchpads

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

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

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

       Scratchpads and recursion

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

       The answer is recursion, and maybe (sometime soon)
       threads. Both these can create several execution pointers
       going into the same subroutine. For the subroutine-child
       not write over the temporaries for the subroutine-parent
       (lifespan of which covers the call to the child), the par-
       ent 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






2001-05-24                 perl v5.6.1                         30





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


       Code tree

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

         $a = $b + $c;

       This is converted to a tree similar to this one:

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

       (but slightly more complicated).  This tree reflects the
       way Perl parsed your code, but has nothing to do with the
       execution order.  There is an additional "thread" going
       through the nodes of the tree which shows the order of
       execution of the nodes.  In our simplified example above
       it looks like:

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

       But with the actual compile tree for "$a = $b + $c" it is
       different: some nodes optimized away.  As a corollary,
       though the actual tree contains more nodes than our sim-
       plified 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 "-D optimize=-g" on "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:




















2001-05-24                 perl v5.6.1                         31





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


           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 corre-
       spond 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 list-
       ing), i.e., "gvsv gvsv add whatever".

       Each of these nodes represents an op, a fundamental opera-
       tion inside the Perl core. The code which implements each
       operation can be found in the pp*.c files; the function
       which implements the op with type "gvsv" is "pp_gvsv", and
       so on. As the tree above shows, different ops have differ-
       ent 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 vari-
       ous 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



2001-05-24                 perl v5.6.1                         32





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


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

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

       Compile pass 1: check routines

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

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

       A check routine is called when the node is fully con-
       structed except for the execution-order thread.  Since at
       this time there are no back-links to the currently con-
       structed 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 mod-
       ified, 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.




2001-05-24                 perl v5.6.1                         33





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


       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 con-
       text 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 con-
       tains back-references (via "thread" pointers), nodes can-
       not be free()d now.  To allow optimized-away nodes at this
       stage, such nodes are null()ified instead of free()ing
       (i.e. their type is changed to OP_NULL).

       Compile pass 3: peephole optimization

       After the compile tree for a subroutine (or for an "eval"
       or a file) is created, an additional pass over the code is
       performed. This pass is neither top-down or bottom-up, but
       in the execution order (with additional complications for
       conditionals).  These optimizations are done in the sub-
       routine peep().  Optimizations performed at this stage are
       subject to the same restrictions as in the pass 2.

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 similiar 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)



2001-05-24                 perl v5.6.1                         34





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


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

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

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

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

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

How multiple interpreters and concurrency are supported
       Background and PERL_IMPLICIT_CONTEXT

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

       Three macros control the major Perl build flavors: MULTI-
       PLICITY, USE_THREADS and PERL_OBJECT.  The MULTIPLICITY
       build has a C structure that packages all the interpreter
       state, there is a similar thread-specific data structure
       under USE_THREADS, and the (now deprecated) PERL_OBJECT
       build has a C++ class to maintain interpreter state.  In
       all three 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 C++ methods, subroutines taking some kind
       of structure as the first argument, or subroutines taking
       nothing as the first argument.  To enable these three 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 the Internal Functions entry
       elsewhere in this document.) The easiest way to be sure a
       function is part of the API is to find its entry in the
       perlapi manpage.  If it exists in the perlapi manpage,
       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 the perlbug manpage explaining why you think it



2001-05-24                 perl v5.6.1                         35





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


       should be.

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

         STATIC void
         S_incline(pTHX_ char *s)

       STATIC becomes "static" in C, and is #define'd to nothing
       in C++.

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

         void
         Perl_sv_setsv(pTHX_ SV* dsv, SV* ssv)

       "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 contain-
       ing the interpreter's context.  The trailing underscore in
       the pTHX_ macro indicates that the macro expansion needs a
       comma after the context argument because other arguments
       follow it.  If PERL_IMPLICIT_CONTEXT is not defined, pTHX_
       will be ignored, and the subroutine is not prototyped to
       take the extra argument.  The form of the macro without
       the trailing underscore is used when there are no addi-
       tional explicit arguments.

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

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

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



2001-05-24                 perl v5.6.1                         36





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


           sv_setsv(foo, bar);

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

       Under PERL_OBJECT in the core, that will translate to
       either:

           CPerlObj::Perl_sv_setsv(foo,bar);  # in CPerlObj functions,
                                              # C++ takes care of 'this'
         or

           pPerl->Perl_sv_setsv(foo,bar);     # in truly static functions,
                                              # see objXSUB.h

       Under PERL_OBJECT in extensions (aka PERL_CAPI), or under
       MULTIPLICITY/USE_THREADS with PERL_IMPLICIT_CONTEXT in
       both core and extensions, it will become:

           Perl_sv_setsv(aTHX_ foo, bar);     # the canonical Perl "API"
                                              # for all build flavors

       This doesn't work so cleanly for varargs functions,
       though, as macros imply that the number of arguments is
       known in advance.  Instead we either need to spell them
       out fully, passing "aTHX_" as the first argument (the Perl
       core tends to do this with functions like Perl_warner), or
       use a context-free version.

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

       You can ignore [pad]THX[xo] when browsing the Perl head-
       ers/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



2001-05-24                 perl v5.6.1                         37





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


       the initial context argument somehow.  The kicker is that
       you will need to write it in such a way that the extension
       still compiles when Perl hasn't been built with
       PERL_IMPLICIT_CONTEXT enabled.

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

               sv_setsv(asv, bsv);

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

               Perl_sv_setsv(Perl_get_context(), asv, bsv);

       or to this otherwise:

               Perl_sv_setsv(asv, bsv);

       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 */








2001-05-24                 perl v5.6.1                         38





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


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

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

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

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

               /* pTHX_ only needed for functions that call Perl API */
               static 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 argu-
       ment.  Depending on your needs for simplicity or effi-
       ciency, 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



2001-05-24                 perl v5.6.1                         39





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


       take explicit arguments, or the form without the argument
       for functions with no explicit arguments.

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

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

       The "perl_alloc" and "perl_clone" API functions will auto-
       matically set the TLS slot to the interpreter they cre-
       ated, 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 inter-
       preter.  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 inter-
       preter to bundle up everything it knows about the environ-
       ment it's running on.  This is enabled with the
       PERL_IMPLICIT_SYS macro.  Currently it only works with
       PERL_OBJECT and USE_THREADS on Windows (see inside iperl-
       sys.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 main-
       tain their own state, broken down into seven C structures.
       These are thin wrappers around the usual system calls (see
       win32/perllib.c) for the default perl executable, but for
       a more ambitious host (like the one that would do fork()
       emulation) all the extra work needed to pretend that dif-
       ferent interpreters are actually different "processes",
       would be done here.

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



2001-05-24                 perl v5.6.1                         40





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


Internal Functions
       All of Perl's internal functions which will be exposed to
       the outside world are be 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 func-
       tions start with "S_")

       Inside the Perl core, you can get at the functions either
       with or without the "Perl_" prefix, thanks to a bunch of
       defines that live in embed.h. This header file is gener-
       ated automatically from embed.pl. 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 at the end of embed.pl 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 col-
       umn 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" fea-
          ture which we'll look at in a second.

       Other available flags are:

       s  This is a static function and is defined as "S_what-
          ever", and usually called within the sources as "what-
          ever(...)".

       n  This does not use "aTHX_" and "pTHX" to pass inter-
          preter context. (See the Background and
          PERL_IMPLICIT_CONTEXT entry in the perlguts manpage.)

       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|...






2001-05-24                 perl v5.6.1                         41





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


       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".

       j  This function is not a member of "CPerlObj". If you
          don't know what this means, don't use it.

       x  This function isn't exported out of the Perl core.

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

       Formatted Printing of IVs, UVs, and NVs

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

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

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

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

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

       If you are printing addresses of pointers, use UVxf com-
       bined 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:





2001-05-24                 perl v5.6.1                         42





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


               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 func-
       tions and automatically produce reference manuals from
       them - the perlapi manpage is one such manual which
       details all the functions which are available to XS writ-
       ers. the perlintern manpage 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 func-
       tions 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 particularly useful
       for people who don't use the Roman alphabet. What used to
       happen was that particular languages would stick their own
       alphabet in the upper range of the sequence, between 128
       and 255. Of course, we then ended up with plenty of vari-
       ants 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



2001-05-24                 perl v5.6.1                         43





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


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

       To fix this, some people formed Unicode, Inc. and produced
       a new character set containing all the characters you can
       possibly think of and more. There are several ways of rep-
       resenting these characters, and the one Perl uses is
       called UTF8. UTF8 uses a variable number of bytes to rep-
       resent a character, instead of just one. You can learn
       more about Unicode at http://www.unicode.org/

       How can I recognise a UTF8 string?

       You can't. This is because UTF8 data is stored in bytes
       just like non-UTF8 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 UTF8 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 UTF8.

       How does UTF8 represent Unicode characters?

       As mentioned above, UTF8 uses a variable number of bytes
       to store a character. Characters with values 1...128 are
       stored in one byte, just like good ol' ASCII. Character
       129 is stored as "v194.129"; this continues up to charac-
       ter 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 UTF8 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 UTF8 string is to
       use "utf8_hop", which takes a string and a number of char-
       acters to skip over. You're on your own about bounds



2001-05-24                 perl v5.6.1                         44





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


       checking, though, so don't use it lightly.

       All bytes in a multi-byte UTF8 character will have the
       high bit set, so you can test if you need to do something
       special with this character like this:

           UV uv;

           if (utf & 0x80)
               /* Must treat this as UTF8 */
               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 UTF8:

           if (uv > 0x80)
               /* 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 func-
       tions if you're ever in a situation where you have to
       match UTF8 and non-UTF8 characters. You may not skip over
       UTF8 characters in this case. If you do this, you'll lose
       the ability to match hi-bit non-UTF8 characters; for
       instance, if your UTF8 string contains "v196.172", and you
       skip that character, you can never match a "chr(200)" in a
       non-UTF8 string.  So don't do that!

       How does Perl store UTF8 strings?

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

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

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

       The problem comes when you have, for instance, a string
       that isn't flagged is UTF8, and contains a byte sequence



2001-05-24                 perl v5.6.1                         45





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


       that could be UTF8 - especially when combining non-UTF8
       and UTF8 strings.

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

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

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

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

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

       In fact, your "frobnicate" function should be made aware
       of whether or not it's dealing with UTF8 data, so that it
       can handle the string appropriately.

       How do I convert a string to UTF8?

       If you're mixing UTF8 and non-UTF8 strings, you might find
       it necessary to upgrade one of the strings to UTF8. 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 UTF8-encoded copy
       of its string argument. This is useful for having the data
       available for comparisons and so on, without harming the
       original SV. There's also "utf8_to_bytes" to go the other



2001-05-24                 perl v5.6.1                         46





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


       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:

       o  There's no way to tell if a string is UTF8 or not. You
          can tell if an SV is UTF8 by looking at is "SvUTF8"
          flag. Don't forget to set the flag if something should
          be UTF8. Treat the flag as part of the PV, even though
          it's not - if you pass on the PV to somewhere, pass on
          the flag too.

       o  If a string is UTF8, always use "utf8_to_uv" to get at
          the value, unless "!(*s & 0x80)" in which case you can
          use "*s".

       o  When writing to a UTF8 string, always use "uv_to_utf8",
          unless "uv < 0x80" in which case you can use "*s = uv".

       o  Mixing UTF8 and non-UTF8 strings is tricky. Use
          "bytes_to_utf8" to get a new string which is UTF8
          encoded. There are tricks you can use to delay deciding
          whether you need to use a UTF8 string until you get to
          a high character - "HALF_UPGRADE" is one of those.

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, Mal-
       colm Beattie, Andreas Koenig, Paul Hudson, Ilya Zakhare-
       vich, Paul Marquess, Neil Bowers, Matthew Green, Tim
       Bunce, Spider Boardman, Ulrich Pfeifer, Stephen McCamant,
       and Gurusamy Sarathy.

       API Listing originally by Dean Roehrich
       <roehrich@cray.com>.

       Modifications to autogenerate the API listing (the perlapi
       manpage) by Benjamin Stuhl.

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









2001-05-24                 perl v5.6.1                         47


