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perlguts



DESCRIPTION

       This document attempts to describe how to use the Perl
       API, as well as to provide some info on the basic workings
       of the Perl core. It is far from complete and probably
       contains many errors. Please refer any questions or 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.)  They
       will usually be exactly 32 and 16 bits long, but on Crays
       they will both be 64 bits.

       Working with SVs

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

       The seven routines are:

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

       "STRLEN" is an integer type (Size_t, usually defined as
       size_t in config.h) guaranteed to be large enough to rep­

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

       Notice that you can choose to specify the length of the
       string to be assigned by using "sv_setpvn", "newSVpvn", or
       "newSVpv", or you may allow Perl to calculate the length
       by using "sv_setpv" or by specifying 0 as the second 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_vsetpvfn" is an analogue of "vsprintf", but it allows
       you to specify either a pointer to a variable argument
       list or the address and length of an array of SVs.  The
       last argument points to a boolean; on return, if that
       boolean is true, then locale-specific information has been
       used to format the string, and the string's contents are
       therefore untrustworthy (see perlsec).  This pointer may
       be NULL if that information is not important.  Note that
       this function requires you to specify the length of the
       format.

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

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

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

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

       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.

           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.
       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 "Magic Virtual
       Tables" later in this document.

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

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

       This returns NULL if the variable does not exist.

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

           SvOK(SV*)

       The scalar "undef" value is stored in an SV instance
       called "PL_sv_undef".  Its address can be used whenever an
       "SV*" is needed.  However, you have to be careful when
       using &PL_sv_undef as a value in AVs or HVs (see "AVs, HVs
       and undefined values").

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

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

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

       This code tries to return a new SV (which contains the
       value 42) if it should return a real value, or undef 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.
       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:

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

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

       Something similar to the offset hack is performed on AVs
       to enable efficient shifting and splicing off the begin­
       ning 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,
       the data should be accessed via the FETCH routine rather
       than directly, so SvIOK is false. Another is when numeric
       conversion has occured and precision has been lost: only
       the private flag is set on 'lossy' values. So when an NV
       is converted to an IV with loss, SvIOKp, SvNOKp and SvNOK
       will be set, while SvIOK wont be.

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

       Working with AVs

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

           AV*  newAV();

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

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

       The second argument points to an array containing "num"
       "SV*"'s.  Once the AV has been created, the SVs can be
       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
       the array plus the array itself.  The "av_extend" function
       extends the array so that it contains at least "key+1"
       elements.  If "key+1" is less than the currently allocated
       length of the array, then nothing is done.

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

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

       This returns NULL if the variable does not exist.

       See "Understanding the Magic of Tied Hashes and Arrays"
       for more information on how to use the array access func­
       tions 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);


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

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

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

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

       This returns NULL if the variable does not exist.

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

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

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

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

       Hash API Extensions

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

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

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

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

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

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

       Note that both "hv_store" and "hv_store_ent" do not 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.

       AVs, HVs and undefined values

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

       For example, intuition tells you that this XS code:

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

       is equivalent to this Perl code:

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

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

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

           Modification of a read-only value attempted

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

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

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

       References

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

       To create a reference, use either of the following 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.
           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 perl's OO lexicon, an object is simply a
       reference that has been blessed into a package (or class).
       Once blessed, the programmer may now use the reference to
       access the various methods in the class.

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

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

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

       /* Still under construction */

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

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

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

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

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

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

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

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

       Tests whether the SV is blessed into the specified class.

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

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

       Creating New Variables

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

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

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

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

       GV_ADDMULTI
           Marks the variable as multiply defined, thus prevent­
           ing 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 a 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
       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.

       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 perlcall and perlxs 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.

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

       To create a mortal variable, use the functions:

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

       The first call creates a mortal SV (with no value), the

       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. Thinking of "Mortalization" as
       deferred "SvREFCNT_dec" should help to minimize such prob­
       lems.  For example if you are passing an SV which you know
       has high enough REFCNT to survive its use on the stack you
       need not do any mortalization.  If you are not sure then
       doing an "SvREFCNT_inc" and "sv_2mortal", or making a
       "sv_mortalcopy" is safer.

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

       Stashes and Globs

       A stash is a hash that contains all variables that are
       defined within a package.  Each key of the stash is a sym­
       bol 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
       If you need to bless or re-bless an object you can use the
       following function:

           SV*  sv_bless(SV*, HV* stash)

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

       For more information on references and blessings, consult
       perlref.

       Double-Typed SVs

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

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

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

               SvIOK_on
               SvNOK_on
               SvPOK_on
               SvROK_on

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

       For example, to create a new Perl variable called "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);

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

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

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

       Assigning Magic

       Perl adds magic to an SV using the sv_magic function:

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

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

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

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

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

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

           void sv_unmagic(SV *sv, int type);

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

       Magic Virtual Tables

       The "mg_virtual" field in the "MAGIC" structure is a
       pointer to an "MGVTBL", which is a structure of function
       pointers and stands for "Magic Virtual Table" to handle
       the various operations that might be applied to that 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 before the value of the SV is retrieved.
           svt_set             Do something after the SV is assigned a value.
           svt_len             Report on the SV's length.
           svt_clear           Clear something the SV represents.
           svt_free            Free any extra storage associated with the SV.

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

           { magic_get, magic_set, magic_len, 0, 0 }

       Thus, when an SV is determined to be magical and of type
       "PERL_MAGIC_sv", if a get operation is being performed,
       the routine "magic_get" is called.  All the various rou­
       tines 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.
                                                       on stash
           B  PERL_MAGIC_bm             vtbl_bm        Boyer-Moore (fast string search)
           D  PERL_MAGIC_regdata        vtbl_regdata   Regex match position data
                                                       (@+ and @- vars)
           d  PERL_MAGIC_regdatum       vtbl_regdatum  Regex match position data
                                                       element
           E  PERL_MAGIC_env            vtbl_env       %ENV hash
           e  PERL_MAGIC_envelem        vtbl_envelem   %ENV hash element
           f  PERL_MAGIC_fm             vtbl_fm        Formline ('compiled' format)
           g  PERL_MAGIC_regex_global   vtbl_mglob     m//g target / study()ed string
           I  PERL_MAGIC_isa            vtbl_isa       @ISA array
           i  PERL_MAGIC_isaelem        vtbl_isaelem   @ISA array element
           k  PERL_MAGIC_nkeys          vtbl_nkeys     scalar(keys()) lvalue
           L  PERL_MAGIC_dbfile         (none)         Debugger %_<filename
           l  PERL_MAGIC_dbline         vtbl_dbline    Debugger %_<filename element
           m  PERL_MAGIC_mutex          vtbl_mutex     ???
           o  PERL_MAGIC_collxfrm       vtbl_collxfrm  Locale collate transformation
           P  PERL_MAGIC_tied           vtbl_pack      Tied array or hash
           p  PERL_MAGIC_tiedelem       vtbl_packelem  Tied array or hash element
           q  PERL_MAGIC_tiedscalar     vtbl_packelem  Tied scalar or handle
           r  PERL_MAGIC_qr             vtbl_qr        precompiled qr// regex
           S  PERL_MAGIC_sig            vtbl_sig       %SIG hash
           s  PERL_MAGIC_sigelem        vtbl_sigelem   %SIG hash element
           t  PERL_MAGIC_taint          vtbl_taint     Taintedness
           U  PERL_MAGIC_uvar           vtbl_uvar      Available for use by extensions
           v  PERL_MAGIC_vec            vtbl_vec       vec() lvalue
           V  PERL_MAGIC_vstring        (none)         v-string scalars
           w  PERL_MAGIC_utf8           vtbl_utf8      UTF-8 length+offset cache
           x  PERL_MAGIC_substr         vtbl_substr    substr() lvalue
           y  PERL_MAGIC_defelem        vtbl_defelem   Shadow "foreach" iterator
                                                       variable / smart parameter
                                                       vivification
           *  PERL_MAGIC_glob           vtbl_glob      GV (typeglob)
           #  PERL_MAGIC_arylen         vtbl_arylen    Array length ($#ary)
           .  PERL_MAGIC_pos            vtbl_pos       pos() lvalue
           <  PERL_MAGIC_backref        vtbl_backref   ???
           ~  PERL_MAGIC_ext            (none)         Available for use by extensions

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

       The "PERL_MAGIC_ext" and "PERL_MAGIC_uvar" magic types are
       defined specifically for use by extensions and will not be
       used by perl itself.  Extensions can use "PERL_MAGIC_ext"
       magic to 'attach' private information to variables (typi­
       cally objects).  This is especially useful because there
       is no way for normal perl code to corrupt this private
       When the SV is read from or written to, the "uf_val" or
       "uf_set" function will be called with "uf_index" as the
       first arg and a pointer to the SV as the second.  A simple
       example of how to add "PERL_MAGIC_uvar" magic is shown
       below.  Note that the ufuncs structure is copied by
       sv_magic, so you can safely allocate it on the stack.

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

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

       Also note that the "sv_set*()" and "sv_cat*()" functions
       described earlier do not invoke 'set' magic on their 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 perlapi for a description of these
       functions.  For example, calls to the "sv_cat*()" func­
       tions typically need to be followed by "SvSETMAGIC()", but
       they don't need a prior "SvGETMAGIC()" since their imple­
       mentation 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);

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

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

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

       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
       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

       "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:

       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 an "SV
           *".

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

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


Subroutines

       XSUBs and the Argument Stack

       The XSUB mechanism is a simple way for Perl programs to
       access C 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.

       the stack pointer, and "num" is the number of elements the
       stack should be extended by.

       Now that there is room on the stack, values can be pushed
       on it using "PUSHs" macro. The pushed values will often
       need to be "mortal" (See "Reference Counts and Mortal­
       ity").

           PUSHs(sv_2mortal(newSViv(an_integer)))
           PUSHs(sv_2mortal(newSVpv("Some String",0)))
           PUSHs(sv_2mortal(newSVnv(3.141592)))

       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 macro:

           XPUSHs(SV*)

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

       Despite their suggestions in earlier versions of this doc­
       ument the macros "PUSHi", "PUSHn" and "PUSHp" are not
       suited to XSUBs which return multiple results, see
       "Putting a C value on Perl stack".

       For more information, consult perlxs and perlxstut.

       Calling Perl Routines from within C Programs

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

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

       The routine most often used is "call_sv".  The "SV*" 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
           SPAGAIN
           ENTER
           SAVETMPS
           FREETMPS
           LEAVE
           XPUSH*()
           POP*()

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

       Memory Allocation

       Allocation

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

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

       The following three macros are used to initially allocate
       memory :

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

       The first argument "x" was a "magic cookie" that was used
       to keep track of who called the macro, to help when 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.

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

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

       Putting a C value on Perl stack

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

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


       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 threads. Both these can
       create several execution pointers going into the same sub­
       routine. For the subroutine-child not write over the tem­
       poraries for the subroutine-parent (lifespan of which cov­
       ers the call to the child), the parent and the child
       should have different scratchpads. (And the lexicals
       should be separate anyway!)

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

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


Compiled code

       Code tree

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

         $a = $b + $c;


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

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

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

                          add
                        /     \
                      null    null
                       |       |
                      gvsv    gvsv

       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 opera­
       tors ("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 mat­
       ters, if a "UNOP" is actually a "null" op after optimiza­
       tion (see "Compile pass 2: context propagation") it will
       still have children in accordance with its former type.

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

       Compile pass 1: check routines

       The tree is created by the compiler while yacc code feeds
       it the constructions it recognizes. Since yacc works 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).

       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.

       Pluggable runops

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

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

         PL_runops = my_runops;

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


Examining internal data structures with the "dump" functions

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

       The most commonly used of these functions is
       "Perl_sv_dump"; it's used for dumping SVs, AVs, HVs, and
       CVs. The "Devel::Peek" module calls "sv_dump" to produce
       debugging output from Perl-space, so users of that module
       should already be familiar with its format.
           SUB attributes::bootstrap = (xsub 0x811fedc 0)

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

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

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

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

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


How multiple interpreters and concurrency are supported

       Background and PERL_IMPLICIT_CONTEXT

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

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

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

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

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

         void
         Perl_sv_setiv(pTHX_ SV* dsv, IV num)

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

       When Perl is built without options that set
       PERL_IMPLICIT_CONTEXT, there is no first argument 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_setiv".  It expands into something like this:

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

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

           sv_setiv(foo, bar);

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

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

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

       How do I use all this in extensions?

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

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

               sv_setiv(sv, num);

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

               Perl_sv_setiv(Perl_get_context(), sv, num);

       or to this otherwise:

               Perl_sv_setiv(sv, num);

       You have to do nothing new in your extension to get this;
       since the Perl library provides Perl_get_context(), it
       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)
               {
                       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
       take explicit arguments, or the form without the argument
       for functions with no explicit arguments.
       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
       USE_ITHREADS and USE_5005THREADS on Windows (see inside
       iperlsys.h).

       This allows the ability to provide an extra pointer
       (called the "host" environment) for all the system calls.
       This makes it possible for all the system stuff to 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.


Internal Functions

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

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

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

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

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

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

       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

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

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

       For example:

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

       and

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

       Source Documentation

       There's an effort going on to document the internal func­
       tions and automatically produce reference manuals from
       them - perlapi is one such manual which details all the
       functions which are available to XS writers. perlintern is
       the 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
       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 UTF-8. UTF-8 uses a variable number of bytes to
       represent a character, instead of just one. You can learn
       more about Unicode at http://www.unicode.org/

       How can I recognise a UTF-8 string?

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

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

       How does UTF-8 represent Unicode characters?

       As mentioned above, UTF-8 uses a variable number of bytes
       to store a character. Characters with values 1...128 are
       stored in one byte, just like good ol' ASCII. Character
       129 is stored as "v194.129"; this continues up to 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 UTF-8 string, you
       can find out how long the first character in it is with
       the "UTF8SKIP" macro:

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

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

       Another way to skip over characters in a UTF-8 string is
       to use "utf8_hop", which takes a string and a number of
       characters to skip over. You're on your own about bounds
       checking, though, so don't use it lightly.
               /* OK to treat this character as a byte */
               uv = *utf;

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

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

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

       How does Perl store UTF-8 strings?

       Currently, Perl deals with Unicode strings and non-Unicode
       strings slightly differently. 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 UTF-8, and contains a byte sequence
       that could be UTF-8 - especially when combining non-UTF-8
       and UTF-8 strings.

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

           SV *sv;
           SV *nsv;
           nsv = newSVpvn(p, len);
           if (SvUTF8(sv))
               SvUTF8_on(nsv);

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

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

       How do I convert a string to UTF-8?

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

           sv_utf8_upgrade(sv);

       However, you must not do this, for example:

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

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

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

       Is there anything else I need to know?

       Not really. Just remember these things:

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

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

       allow the building of interpreters for other languages in
       the Perl core, but it also allows optimizations through
       the creation of "macro-ops" (ops which perform the func­
       tions of multiple ops which are usually executed together,
       such as "gvsv, gvsv, add".)

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

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

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

       You should also "register" your op with the Perl inter­
       preter so that it can produce sensible error and warning
       messages. Since it is possible to have multiple custom ops
       within the one "logical" op type "OP_CUSTOM", Perl uses
       the value of "o->op_ppaddr" as a key into the "PL_cus­
       tom_op_descs" and "PL_custom_op_names" hashes. This means
       you need to enter a name and description for your op at
       the appropriate place in the "PL_custom_op_names" and
       "PL_custom_op_descs" hashes.

       Forthcoming versions of "B::Generate" (version 1.0 and
       above) should directly support the creation of custom ops
       by name; "Opcodes::Custom" will provide functions which
       make it trivial to "register" custom ops to the Perl
       interpreter.


AUTHORS

       Until May 1997, this document was maintained by Jeff
       Okamoto <okamoto@corp.hp.com>.  It is now maintained as
       part of Perl itself by the Perl 5 Porters
       <perl5-porters@perl.org>.
  




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