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perlguts ()
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    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 comments to the
         author below.
    
    
    

    Variables

         Datatypes
    
         Perl has three typedefs that handle Perl's three main data
         types:
    
             SV  Scalar Value
             AV  Array Value
             HV  Hash Value
    
         Each typedef has specific routines that manipulate the
         various data types.
    
         What is an "IV"?
    
         Perl uses a special typedef IV which is a simple signed
         integer type that is guaranteed to be large enough to hold a
         pointer (as well as an integer).  Additionally, there is the
         UV, which is simply an unsigned IV.
    
         Perl also uses two special typedefs, I32 and I16, which will
         always be at least 32-bits and 16-bits long, respectively.
         (Again, there are U32 and U16, as well.)
    
         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:
             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 argument
         to `newSVpv'.  Be warned, though, that Perl will determine
         the string's length by using `strlen', which depends on the
         string terminating with a NUL character.
    
         The arguments of `sv_setpvf' are processed like `sprintf',
         and the formatted output becomes the value.
    
         `sv_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.
    
         The `sv_set*()' functions are not generic enough to operate
         on values that have "magic".  See the Magic Virtual Tables
         entry elsewhere in this document 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)
             SvPV_nolen(SV*)
    
         which will automatically coerce the actual scalar type into
         an IV, UV, double, or string.
    
         In the `SvPV' macro, the length of the string returned is
         placed into the variable `len' (this is a macro, so you do
         not use `&len').  If you do not care what the length of the
         data is, use the `SvPV_nolen' macro.  Historically the
         `SvPV' macro with the global variable `PL_na' has been used
         in this case.  But that can be quite inefficient because
         `PL_na' must be accessed in thread-local storage in threaded
         Perl.  In any case, remember that Perl allows arbitrary
         strings of data that may both contain NULs and might not be
         terminated by a NUL.
    
         Also remember that C doesn't allow you to safely say
         `foo(SvPV(s, len), len);'. It might work with your compiler,
         but it won't work for everyone.  Break this sort of
         statement up into separate assignments:
    
                 SV *s;
                 STRLEN len;
                 char * ptr;
                 ptr = SvPV(s, len);
                 foo(ptr, len);
    
         If you want to know if the scalar value is TRUE, you can
         use:
    
             SvTRUE(SV*)
    
         Although Perl will automatically grow strings for you, if
         you need to force Perl to allocate more memory for your SV,
         you can use the macro
    
             SvGROW(SV*, STRLEN newlen)
    
         which will determine if more memory needs to be allocated.
         If so, it will call the function `sv_grow'.  Note that
         `SvGROW' can only increase, not decrease, the allocated
         memory of an SV and that it does not automatically add a
         byte for the a trailing NUL (perl's own string functions
         typically do `SvGROW(sv, len + 1)').
    
         If you have an SV and want to know what kind of data Perl
         thinks is stored in it, you can use the following macros to
         check the type of SV you have.
    
             SvIOK(SV*)
             SvNOK(SV*)
             SvPOK(SV*)
    
         You can get and set the current length of the string stored
         in an SV with the following macros:
    
             SvCUR(SV*)
             SvCUR_set(SV*, I32 val)
    
         You can also get a pointer to the end of the string stored
         in the SV with the macro:
    
             SvEND(SV*)
    
         But note that these last three macros are valid only if
         `SvPOK()' is true.
    
         If you want to append something to the end of string stored
         in an `SV*', you can use the following functions:
    
             void  sv_catpv(SV*, const char*);
             void  sv_catpvn(SV*, const char*, STRLEN);
             void  sv_catpvf(SV*, const char*, ...);
             void  sv_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 operate
         on values that have "magic".  See the Magic Virtual Tables
         entry elsewhere in this document 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.
         There are also the two values `PL_sv_yes' and `PL_sv_no',
         which contain Boolean TRUE and FALSE values, respectively.
         Like `PL_sv_undef', their addresses can be used whenever an
         `SV*' is needed.
    
         Do not be fooled into thinking that `(SV *) 0' is the same
         as `&PL_sv_undef'.  Take this code:
    
             SV* sv = (SV*) 0;
             if (I-am-to-return-a-real-value) {
                     sv = sv_2mortal(newSViv(42));
             }
             sv_setsv(ST(0), sv);
    
         This code tries to return a new SV (which contains the value
         42) if it should return a real value, or undef otherwise.
         Instead it has returned a NULL pointer which, somewhere down
         the line, will cause a segmentation violation, bus error, or
         just weird results.  Change the zero to `&PL_sv_undef' in
         the first line and all will be well.
    
         To free an SV that you've created, call `SvREFCNT_dec(SV*)'.
         Normally this call is not necessary (see the Reference
         Counts and Mortality entry elsewhere in this document).
    
         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:
             AV*  newAV();
    
         The second method both creates the AV and initially
         populates it with SVs:
    
             AV*  av_make(I32 num, SV **ptr);
    
         The second argument points to an array containing `num'
         `SV*''s.  Once the AV has been created, the SVs can be
         destroyed, if so desired.
    
         Once the AV has been created, the following operations are
         possible on AVs:
    
             void  av_push(AV*, SV*);
             SV*   av_pop(AV*);
             SV*   av_shift(AV*);
             void  av_unshift(AV*, I32 num);
    
         These should be familiar operations, with the exception of
         `av_unshift'.  This routine adds `num' elements at the front
         of the array with the `undef' value.  You must then use
         `av_store' (described below) to assign values to these new
         elements.
    
         Here are some other functions:
    
             I32   av_len(AV*);
             SV**  av_fetch(AV*, I32 key, I32 lval);
             SV**  av_store(AV*, I32 key, SV* val);
    
         The `av_len' function returns the highest index value in
         array (just like $#array in Perl).  If the array is empty,
         -1 is returned.  The `av_fetch' function returns the value
         at index `key', but if `lval' is non-zero, then `av_fetch'
         will store an undef value at that index.  The `av_store'
         function stores the value `val' at index `key', and does not
         increment the reference count of `val'.  Thus the caller is
         responsible for taking care of that, and if `av_store'
         returns NULL, the caller will have to decrement the
         reference count to avoid a memory leak.  Note that
         `av_fetch' and `av_store' both return `SV**''s, not `SV*''s
         as their return value.
    
             void  av_clear(AV*);
             void  av_undef(AV*);
             void  av_extend(AV*, I32 key);
    
         The `av_clear' function deletes all the elements in the AV*
         array, but does not actually delete the array itself.  The
         `av_undef' function will delete all the elements in the
         array plus the array itself.  The `av_extend' function
         extends the array so that it contains at least `key+1'
         elements.  If `key+1' is less than the currently allocated
         length of the array, then nothing is done.
    
         If you know the name of an array variable, you can get a
         pointer to its AV by using the following:
    
             AV*  get_av("package::varname", FALSE);
    
         This returns NULL if the variable does not exist.
    
         See 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:
    
             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
         distribution 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.
    
         Hash API Extensions
    
         Beginning with version 5.004, the following functions are
         also supported:
    
             HE*     hv_fetch_ent  (HV* tb, SV* key, I32 lval, U32 hash);
             HE*     hv_store_ent  (HV* tb, SV* key, SV* val, U32 hash);
    
             bool    hv_exists_ent (HV* tb, SV* key, U32 hash);
             SV*     hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash);
    
             SV*     hv_iterkeysv  (HE* entry);
    
         Note that these functions take `SV*' keys, which simplifies
         writing of extension code that deals with hash structures.
         These functions also allow passing of `SV*' keys to `tie'
         functions without forcing you to stringify the keys (unlike
         the previous set of functions).
    
         They also return and accept whole hash entries (`HE*'),
         making their use more efficient (since the hash number for a
         particular string doesn't have to be recomputed every time).
         See 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
         increment the reference count of the stored `val', which is
         the caller's responsibility.  If these functions return a
         NULL value, the caller will usually have to decrement the
         reference count of `val' to avoid a memory leak.
    
         References
    
         References are a special type of scalar that point to other
         data types (including references).
    
         To create a reference, use either of the following
         functions:
    
             SV* newRV_inc((SV*) thing);
             SV* newRV_noinc((SV*) thing);
    
         The `thing' argument can be any of an `SV*', `AV*', or
         `HV*'.  The functions are identical except that `newRV_inc'
         increments the reference count of the `thing', while
         `newRV_noinc' does not.  For historical reasons, `newRV' is
         a synonym for `newRV_inc'.
    
         Once you have a reference, you can use the following macro
         to dereference the reference:
    
             SvRV(SV*)
    
         then call the appropriate routines, casting the returned
         `SV*' to either an `AV*' or `HV*', if required.
    
         To determine if an SV is a reference, you can use the
         following macro:
    
             SvROK(SV*)
    
         To discover what type of value the reference refers to, use
         the following macro and then check the return value.
    
             SvTYPE(SvRV(SV*))
    
         The most useful types that will be returned are:
    
             SVt_IV    Scalar
             SVt_NV    Scalar
             SVt_PV    Scalar
             SVt_RV    Scalar
             SVt_PVAV  Array
             SVt_PVHV  Hash
             SVt_PVCV  Code
             SVt_PVGV  Glob (possible a file handle)
             SVt_PVMG  Blessed or Magical Scalar
    
             See the sv.h header file for more details.
    
         Blessed References and Class Objects
    
         References are also used to support object-oriented
         programming.  In the OO lexicon, an object is simply a
         reference that has been blessed into a package (or class).
         Once blessed, the programmer may now use the reference to
         access the various methods in the class.
    
         A reference can be blessed into a package with the following
         function:
    
             SV* sv_bless(SV* sv, HV* stash);
    
         The `sv' argument must be a reference.  The `stash' argument
         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:
    
                 if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }
    
    
         Creating New Variables
    
         To create a new Perl variable with an undef value which can
         be accessed from your Perl script, use the following
         routines, depending on the variable type.
    
             SV*  get_sv("package::varname", TRUE);
             AV*  get_av("package::varname", TRUE);
             HV*  get_hv("package::varname", TRUE);
    
         Notice the use of TRUE as the second parameter.  The new
         variable can now be set, using the routines appropriate to
         the data type.
    
         There are additional macros whose values may be bitwise
         OR'ed with the `TRUE' argument to enable certain extra
         features.  Those bits are:
    
             GV_ADDMULTI Marks the variable as multiply defined, thus preventing the
                         "Name <varname> used only once: possible typo" warning.
             GV_ADDWARN  Issues the warning "Had to create <varname> unexpectedly" if
                         the variable did not exist before the function was called.
    
         If you do not specify a package name, the variable is
         created in the current package.
    
         Reference Counts and Mortality
    
         Perl uses an reference count-driven garbage collection
         mechanism. SVs, AVs, or HVs (xV for short in the following)
         start their life with a reference count of 1.  If the
         reference count of an xV ever drops to 0, then it will be
         destroyed and its memory made available for reuse.
    
         This normally doesn't happen at the Perl level unless a
         variable is undef'ed or the last variable holding a
         reference to it is changed or overwritten.  At the internal
         level, however, reference counts can be manipulated with the
         following macros:
    
             int SvREFCNT(SV* sv);
             SV* SvREFCNT_inc(SV* sv);
             void SvREFCNT_dec(SV* sv);
    
         However, there is one other function which manipulates the
         reference count of its argument.  The `newRV_inc' function,
         you will recall, creates a reference to the specified
         argument.  As a side effect, it increments the argument's
         reference count.  If this is not what you want, use
         `newRV_noinc' instead.
    
         For example, imagine you want to return a reference from an
         XSUB function.  Inside the XSUB routine, you create an SV
         which initially has a reference count of one.  Then you call
         `newRV_inc', passing it the just-created SV.  This returns
         the reference as a new SV, but the reference count of the SV
         you passed to `newRV_inc' has been incremented to two.  Now
         you return the reference from the XSUB routine and forget
         about the SV.  But Perl hasn't!  Whenever the returned
         reference is destroyed, the reference count of the original
         SV is decreased to one and nothing happens.  The SV will
         hang around without any way to access it until Perl itself
         terminates.  This is a memory leak.
    
         The correct procedure, then, is to use `newRV_noinc' instead
         of `newRV_inc'.  Then, if and when the last reference is
         destroyed, the reference count of the SV will go to zero and
         it will be destroyed, stopping any memory leak.
    
         There are some convenience functions available that can help
         with the destruction of xVs.  These functions introduce the
         concept of "mortality".  An xV that is mortal has had its
         reference count marked to be decremented, but not actually
         decremented, until "a short time later".  Generally the term
         "short time later" means a single Perl statement, such as a
         call to an XSUB function.  The actual determinant for when
         mortal xVs have their reference count decremented depends on
         two macros, SAVETMPS and FREETMPS.  See the perlcall manpage
         and the perlxs manpage for more details on these macros.
    
         "Mortalization" then is at its simplest a deferred
         `SvREFCNT_dec'.  However, if you mortalize a variable twice,
         the reference count will later be decremented twice.
    
         You should be careful about creating mortal variables.
         Strange things can happen if you make the same value mortal
         within multiple contexts, or if you make a variable mortal
         multiple times.
    
         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.
    
         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 package
         whose symbol table you want.  The default package is called
         `main'.  If you have multiply nested packages, pass their
         names to `gv_stash*v', separated by `::' as in the Perl
         language itself.
    
         Alternately, if you have an SV that is a blessed reference,
         you can find out the stash pointer by using:
    
             HV*  SvSTASH(SvRV(SV*));
    
         then use the following to get the package name itself:
    
             char*  HvNAME(HV* stash);
    
         If you need to bless or re-bless an object you can use the
         following function:
    
             SV*  sv_bless(SV*, HV* stash)
    
         where the first argument, an `SV*', must be a reference, and
         the second argument is a stash.  The returned `SV*' can now
         be used in the same way as any other SV.
    
         For more information on references and blessings, consult
         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 "dberror"
         that contains both the numeric and descriptive string error
         values, you could use the following code:
    
             extern int  dberror;
             extern char *dberror_list;
    
             SV* sv = get_sv("dberror", TRUE);
             sv_setiv(sv, (IV) dberror);
             sv_setpv(sv, dberror_list[dberror]);
             SvIOK_on(sv);
    
         If the order of `sv_setiv' and `sv_setpv' had been reversed,
         then the macro `SvPOK_on' would need to be called instead of
         `SvIOK_on'.
    
         Magic Variables
    
         [This section still under construction.  Ignore everything
         here.  Post no bills.  Everything not permitted is
         forbidden.]
    
         Any SV may be magical, that is, it has special features that
         a normal SV does not have.  These features are stored in the
         SV structure in a linked list of `struct magic''s,
         typedef'ed to `MAGIC'.
    
             struct magic {
                 MAGIC*      mg_moremagic;
                 MGVTBL*     mg_virtual;
                 U16         mg_private;
                 char        mg_type;
                 U8          mg_flags;
                 SV*         mg_obj;
                 char*       mg_ptr;
                 I32         mg_len;
             };
    
         Note this is current as of patchlevel 0, and could change at
         any time.
    
         Assigning Magic
    
         Perl adds magic to an SV using the sv_magic function:
    
             void sv_magic(SV* sv, SV* obj, int how, const char* name, I32 namlen);
    
         The `sv' argument is a pointer to the SV that is to acquire
         a new 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
         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 Table" section
         below.  The `how' argument is also stored in the `mg_type'
         field.
    
         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 variable.
    
         The `MGVTBL' has five pointers to the following routine
         types:
    
    
    
             int  (*svt_get)(SV* sv, MAGIC* mg);
             int  (*svt_set)(SV* sv, MAGIC* mg);
             U32  (*svt_len)(SV* sv, MAGIC* mg);
             int  (*svt_clear)(SV* sv, MAGIC* mg);
             int  (*svt_free)(SV* sv, MAGIC* mg);
    
         This MGVTBL structure is set at compile-time in `perl.h' and
         there are currently 19 types (or 21 with overloading turned
         on).  These different structures contain pointers to various
         routines that perform additional actions depending on which
         function is being called.
    
             Function pointer    Action taken
             ----------------    ------------
             svt_get             Do something 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
         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)
             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 lowercase
         letter is used to represent an element of that composite
         type.
    
         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 information
         to variables (typically objects).  This is especially useful
         because there is no way for normal perl code to corrupt this
         private information (unlike using extra elements of a hash
         object).
    
         Similarly, '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' structure:
    
    
             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
         targets.  This must be done by the user either by calling
         the `SvSETMAGIC()' macro after calling these functions, or
         by using one of the `sv_set*_mg()' or `sv_cat*_mg()'
         functions.  Similarly, generic C code must call the
         `SvGETMAGIC()' macro to invoke any 'get' magic if they use
         an SV obtained from external sources in functions that don't
         handle magic.  See the perlapi manpage for a description of
         these functions.  For example, calls to the `sv_cat*()'
         functions typically need to be followed by `SvSETMAGIC()',
         but they don't need a prior `SvGETMAGIC()' since their
         implementation handles 'get' magic.
    
         Finding Magic
    
             MAGIC* mg_find(SV*, int type); /* Finds the magic pointer of that type */
    
         This routine returns a pointer to the `MAGIC' structure
         stored in the SV.  If the SV does not have that magical
         feature, `NULL' is returned.  Also, if the SV is not of type
         SVt_PVMG, Perl may core dump.
    
             int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen);
    
         This routine checks to see what types of magic `sv' has.  If
         the mg_type field is an uppercase letter, then the mg_obj is
         copied to `nsv', but the mg_type field is changed to be the
         lowercase letter.
    
         Understanding the Magic of Tied Hashes and Arrays
    
         Tied hashes and arrays are magical beasts of the '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 considered bugs
         in the API, to be fixed in later releases, and are bracketed
         with [MAYCHANGE] below. If you find yourself actually
         applying such information in this section, be aware that the
         behavior may change in the future, umm, without warning.
    
         The perl tie function associates a variable with an object
         that implements the various GET, SET etc methods.  To
         perform the equivalent of the perl tie function from an
         XSUB, you must mimic this behaviour.  The code below carries
         out the necessary steps - firstly it creates a new hash, and
         then creates a second hash which it blesses into the class
         which will implement the tie methods. Lastly it ties the two
         hashes together, and returns a reference to the new tied
         hash.  Note that the code below does NOT call the TIEHASH
         method in the MyTie class - see the Calling Perl Routines
         from within C Programs entry elsewhere in this document 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, 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,
         indicating that the value did not actually need to be stored
         in the array.  [MAYCHANGE] After a call to `av_store' on a
         tied array, the caller will usually need to call
         `mg_set(val)' to actually invoke the perl level "STORE"
         method on the TIEARRAY object.  If `av_store' did return
         NULL, a call to `SvREFCNT_dec(val)' will also be usually
         necessary to avoid a memory leak. [/MAYCHANGE]
    
         The previous paragraph is applicable verbatim to tied hash
         access using the `hv_store' and `hv_store_ent' functions as
         well.
    
         `av_fetch' and the corresponding hash functions `hv_fetch'
         and `hv_fetch_ent' actually return an undefined mortal value
         whose magic has been initialized using `mg_copy'.  Note the
         value so returned does not need to be deallocated, as it is
         already mortal.  [MAYCHANGE] But you will need to call
         `mg_get()' on the returned value in order to actually invoke
         the perl level "FETCH" method on the underlying TIE object.
         Similarly, you may also call `mg_set()' on the return value
         after possibly assigning a suitable value to it using
         `sv_setsv',  which will invoke the "STORE" method on the TIE
         object. [/MAYCHANGE]
    
         [MAYCHANGE] In other words, the array or hash fetch/store
         functions don't really fetch and store actual values in the
         case of tied arrays and hashes.  They merely call `mg_copy'
         to attach magic to the values that were meant to be "stored"
         or "fetched".  Later calls to `mg_get' and `mg_set' actually
         do the job of invoking the TIE methods on the underlying
         objects.  Thus the magic mechanism currently implements a
         kind of lazy access to arrays and hashes.
    
         Currently (as of perl version 5.004), use of the hash and
         array access functions requires the user to be aware of
         whether they are operating on "normal" hashes and arrays, or
         on their tied variants.  The API may be changed to provide
         more transparent access to both tied and normal data types
         in future versions.  [/MAYCHANGE]
    
         You would do well to understand that the TIEARRAY and
         TIEHASH interfaces are mere sugar to invoke some perl method
         calls while using the uniform hash and array syntax.  The
         use of this sugar imposes some overhead (typically about two
         to four extra opcodes per FETCH/STORE operation, in addition
         to the creation of all the mortal variables required to
         invoke the methods).  This overhead will be comparatively
         small if the TIE methods are themselves substantial, but if
         they are only a few statements long, the overhead will not
         be insignificant.
    
         Localizing changes
    
         Perl has a very handy construction
    
           {
             local $var = 2;
             ...
           }
    
         This construction is approximately equivalent to
    
           {
             my $oldvar = $var;
             $var = 2;
             ...
             $var = $oldvar;
           }
    
         The biggest difference is that the first construction would
         reinstate the initial value of $var, irrespective of how
         control exits the block: `goto', `return', `die'/`eval' etc.
         It is a little bit more efficient as well.
    
         There is a way to achieve a similar task from C via Perl
         API: create a pseudo-block, and arrange for some changes to
         be automatically undone at the end of it, either explicit,
         or via a non-local exit (via die()). A block-like construct
         is created by a pair of `ENTER'/`LEAVE' macros (see the
         Returning a Scalar entry in the perlcall manpage).  Such a
         construct may be created specially for some important
         localized task, or an existing one (like boundaries of
         enclosing Perl subroutine/block, or an existing pair for
         freeing TMPs) may be used. (In the second case the overhead
         of additional localization must be almost negligible.) Note
         that any XSUB is automatically enclosed in an
         `ENTER'/`LEAVE' pair.
    
         Inside such a pseudo-block the following service is
         available:
    
         `SAVEINT(int i)'
    
         `SAVEIV(IV i)'
    
         `SAVEI32(I32 i)'
    
         `SAVELONG(long i)'
             These macros arrange things to restore the value of
             integer variable `i' at the end of enclosing pseudo-
             block.
    
         `SAVESPTR(s)'
    
         `SAVEPPTR(p)'
             These macros arrange things to restore the value of
             pointers `s' and `p'. `s' must be a pointer of a type
             which survives conversion to `SV*' and back, `p' should
             be able to survive conversion to `char*' and back.
    
         `SAVEFREESV(SV *sv)'
             The refcount of `sv' would be decremented at the end of
             pseudo-block. This is similar to `sv_2mortal', which
             should (?) be used instead.
    
         `SAVEFREEOP(OP *op)'
             The `OP *' is op_free()ed at the end of pseudo-block.
    
         `SAVEFREEPV(p)'
             The chunk of memory which is pointed to by `p' is
             Safefree()ed at the end of pseudo-block.
    
         `SAVECLEARSV(SV *sv)'
             Clears a slot in the current scratchpad which
             corresponds to `sv' at the end of pseudo-block.
    
         `SAVEDELETE(HV *hv, char *key, I32 length)'
             The key `key' of `hv' is deleted at the end of pseudo-
             block. The string pointed to by `key' is Safefree()ed.
             If one has a key in short-lived storage, the
             corresponding string may be reallocated like this:
    
               SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));
    
    
         `SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)'
             At the end of pseudo-block the function `f' is called
             with the only argument `p'.
    
         `SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)'
             At the end of pseudo-block the function `f' is called
             with the implicit context argument (if any), and `p'.
    
         `SAVESTACK_POS()'
             The current offset on the Perl internal stack (cf. `SP')
             is restored at the end of pseudo-block.
    
         The following API list contains functions, thus one needs to
         provide pointers to the modifiable data explicitly (either C
         pointers, or 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 arguments
             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 interested
         in how to localize things in the containing scope should
         take a look there too.
    
    
    

    Subroutines

         XSUBs and the Argument Stack
    
         The XSUB mechanism is a simple way for Perl programs to
         access C subroutines.  An XSUB routine will have a stack
         that contains the arguments from the Perl program, and a way
         to map from the Perl data structures to a C equivalent.
    
         The stack arguments are accessible through the `ST(n)'
         macro, which returns the `n''th stack argument.  Argument 0
         is the first argument passed in the Perl subroutine call.
         These arguments are `SV*', and can be used anywhere an `SV*'
         is used.
    
         Most of the time, output from the C routine can be handled
         through use of the RETVAL and OUTPUT directives.  However,
         there are some cases where the argument stack is not already
         long enough to handle all the return values.  An example is
         the POSIX tzname() call, which takes no arguments, but
         returns two, the local time zone's standard and summer time
         abbreviations.
    
         To handle this situation, the PPCODE directive is used and
         the stack is extended using the macro:
    
             EXTEND(SP, num);
    
         where `SP' is the macro that represents the local copy of
         the stack pointer, and `num' is the number of elements the
         stack should be extended by.
    
         Now that there is room on the stack, values can be pushed on
         it using 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 values
         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
         needed.  Thus, you do not need to call `EXTEND' to extend
         the stack.
    
         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*'
         argument contains either the name of the Perl subroutine to
         be called, or a reference to the subroutine.  The second
         argument consists of flags that control the context in which
         the subroutine is called, whether or not the subroutine is
         being passed arguments, how errors should be trapped, and
         how to treat return values.
         All four routines return the number of arguments that the
         subroutine returned on the Perl stack.
    
         These routines used to be called `perl_call_sv' etc., before
         Perl v5.6.0, but those names are now deprecated; macros of
         the same name are provided for compatibility.
    
         When using any of these routines (except `call_argv'), the
         programmer must manipulate the Perl stack.  These include
         the following macros and functions:
    
             dSP
             SP
             PUSHMARK()
             PUTBACK
             SPAGAIN
             ENTER
             SAVETMPS
             FREETMPS
             LEAVE
             XPUSH*()
             POP*()
    
         For a detailed description of calling conventions from C to
         Perl, consult the perlcall manpage.
    
         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 debugging
         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' argument.
    
         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
         arguments 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
         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
         "normal" standard I/O suite and allowing other stdio
         implementations to be used.  This involves creating a new
         abstraction layer that then calls whichever implementation
         of stdio Perl was compiled with.  All XSUBs should now use
         the functions in the PerlIO abstraction layer and not make
         any assumptions about what kind of stdio is being used.
    
         For a complete description of the PerlIO abstraction,
         consult 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.
         However, as an optimization the corresponding SV is
         (usually) not recreated each time. The opcodes reuse
         specially assigned SVs (targets) which are (as a corollary)
         not constantly freed/created.
    
         Each of the targets is created only once (but see the
         Scratchpads and recursion entry elsewhere in this document
         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]'.
    
         Scratchpads
    
         The question remains on when the SVs which are targets for
         opcodes are created. The answer is that they are created
         when the current unit -- a subroutine or a file (for opcodes
         for statements outside of subroutines) -- is compiled.
         During this time a special anonymous Perl array is created,
         which is called a scratchpad for the current unit.
    
         A scratchpad keeps SVs which are lexicals for the current
         unit and are targets for opcodes. One can deduce that an SV
         lives on a scratchpad by looking on its flags: lexicals have
         `SVs_PADMY' set, and targets have `SVs_PADTMP' set.
    
         The correspondence between OPs and targets is not 1-to-1.
         Different OPs in the compile tree of the unit can use the
         same target, if this would not conflict with the expected
         life of the temporary.
    
         Scratchpads and recursion
    
         In fact it is not 100% true that a compiled unit contains a
         pointer to the scratchpad AV. In fact it contains a pointer
         to an AV of (initially) one element, and this element is the
         scratchpad AV. Why do we need an extra level of indirection?
    
         The answer is recursion, and maybe (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 parent and the child
         should have different scratchpads. (And the lexicals should
         be separate anyway!)
    
         So each subroutine is born with an array of scratchpads (of
         length 1).  On each entry to the subroutine it is checked
         that the current depth of the recursion is not more than the
         length of this array, and if it is, new scratchpad is
         created and pushed into the array.
    
         The targets on this scratchpad are `undef's, but they are
         already marked with correct flags.
    
    
    

    Compiled code

         Code tree
    
         Here we describe the internal form your code is converted to
         by Perl. Start with a simple example:
    
           $a = $b + $c;
    
         This is converted to a tree similar to this one:
    
                      assign-to
                    /           \
                   +             $a
                 /   \
               $b     $c
    
         (but slightly more complicated).  This tree reflects the way
         Perl parsed your code, but has nothing to do with the
         execution order.  There is an additional "thread" going
         through the nodes of the tree which shows the order of
         execution of the nodes.  In our simplified example above it
         looks like:
    
              $b ---> $c ---> + ---> $a ---> assign-to
    
         But with the actual compile tree for `$a = $b + $c' it is
         different:  some nodes optimized away.  As a corollary,
         though the actual tree contains more nodes than our
         simplified example, the execution order is the same as in
         our example.
    
         Examining the tree
    
         If you have your perl compiled for debugging (usually done
         with `-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:
    
    
    
             5           TYPE = add  ===> 6
                         TARG = 1
                         FLAGS = (SCALAR,KIDS)
                         {
                             TYPE = null  ===> (4)
                               (was rv2sv)
                             FLAGS = (SCALAR,KIDS)
                             {
             3                   TYPE = gvsv  ===> 4
                                 FLAGS = (SCALAR)
                                 GV = main::b
                             }
                         }
                         {
                             TYPE = null  ===> (5)
                               (was rv2sv)
                             FLAGS = (SCALAR,KIDS)
                             {
             4                   TYPE = gvsv  ===> 5
                                 FLAGS = (SCALAR)
                                 GV = main::c
                             }
                         }
    
         This tree has 5 nodes (one per `TYPE' specifier), only 3 of
         them are not optimized away (one per number in the left
         column).  The immediate children of the given node
         correspond to `{}' pairs on the same level of indentation,
         thus this listing corresponds to the tree:
    
                            add
                          /     \
                        null    null
                         |       |
                        gvsv    gvsv
    
         The execution order is indicated by `===>' marks, thus it is
         `3 4 5 6' (node `6' is not included into above listing),
         i.e., `gvsv gvsv add whatever'.
    
         Compile pass 1: check routines
    
         The tree is created by the pseudo-compiler while yacc code
         feeds it the constructions it recognizes. Since yacc works
         bottom-up, so does the first pass of perl compilation.
    
         What makes this pass interesting for perl developers is that
         some optimization may be performed on this pass.  This is
         optimization by so-called check routines.  The
         correspondence between node names and corresponding check
         routines is described in opcode.pl (do not forget to run
         `make regen_headers' if you modify this file).
         A check routine is called when the node is fully constructed
         except for the execution-order thread.  Since at this time
         there are no back-links to the currently constructed node,
         one can do most any operation to the top-level node,
         including freeing it and/or creating new nodes above/below
         it.
    
         The check routine returns the node which should be inserted
         into the tree (if the top-level node was not modified, check
         routine returns its argument).
    
         By convention, check routines have names `ck_*'. They are
         usually called from `new*OP' subroutines (or `convert')
         (which in turn are called from perly.y).
    
         Compile pass 1a: constant folding
    
         Immediately after the check routine is called the returned
         node is checked for being compile-time executable.  If it is
         (the value is judged to be constant) it is immediately
         executed, and a constant node with the "return value" of the
         corresponding subtree is substituted instead.  The subtree
         is deleted.
    
         If constant folding was not performed, the execution-order
         thread is created.
    
         Compile pass 2: context propagation
    
         When a context for a part of compile tree is known, it is
         propagated down through the tree.  At this time the context
         can have 5 values (instead of 2 for runtime context): void,
         boolean, scalar, list, and lvalue.  In contrast with the
         pass 1 this pass is processed from top to bottom: a node's
         context determines the context for its children.
    
         Additional context-dependent optimizations are performed at
         this time.  Since at this moment the compile tree contains
         back-references (via "thread" pointers), nodes cannot be
         free()d now.  To allow optimized-away nodes at this stage,
         such nodes are null()ified instead of free()ing (i.e. their
         type is changed to OP_NULL).
    
         Compile pass 3: peephole optimization
    
         After the compile tree for a subroutine (or for an `eval' or
         a file) is created, an additional pass over the code is
         performed. This pass is neither top-down or bottom-up, but
         in the execution order (with additional complications for
         conditionals).  These optimizations are done in the
         subroutine peep().  Optimizations performed at this stage
         are subject to the same restrictions as in the pass 2.
    
    
    

    How multiple interpreters and concurrency are supported

         WARNING: This information is subject to radical changes
         prior to the Perl 5.6 release.  Use with caution.
    
         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 structure,
         or the C++ object will contain all the context, the state of
         that interpreter.
    
         Three macros control the major Perl build flavors:
         MULTIPLICITY, 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 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. 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
         fo r your extension), send mail via the perlbug manpage
         explaining why you think it should be.
    
         (the perlapi manpage itself is generated by embed.pl, a Perl
         script that generates significant portions of the Perl
         source code.  It has a list of almost all the functions
         defined by the Perl interpreter along with their calling
         characteristics and some flags.  Functions that are part of
         the public API are marked with an 'A' in its flags.)
         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.
    
         When Perl is built without PERL_IMPLICIT_CONTEXT, there is
         no first argument containing the interpreter's context.  The
         trailing underscore in the pTHX_ macro indicates that the
         macro expansion needs a comma after the context argument
         because other arguments follow it.  If PERL_IMPLICIT_CONTEXT
         is not defined, pTHX_ will be ignored, and the subroutine is
         not prototyped to take the extra argument.  The form of the
         macro without the trailing underscore is used when there are
         no additional explicit arguments.
    
         When a core function calls another, it must pass the
         context.  This is normally hidden via macros.  Consider
         `sv_setsv'.  It expands 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:
    
    
             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 w/ PERL_IMPLICIT_CONTEXT in both
         core and extensions, it will be:
    
             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 argument.
         Instead it does dTHX; to get the context from thread-local
         storage.  We `#define warner Perl_warner_nocontext' so that
         extensions get source compatibility at the expense of
         performance.  (Passing an arg is cheaper than grabbing it
         from thread-local storage.)
    
         You can ignore [pad]THX[xo] when browsing the Perl
         headers/sources.  Those are strictly for use within the
         core.  Extensions and embedders need only be aware of
         [pad]THX.
    
         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_setsv(asv, bsv);
    
         in your extesion 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 */
    
                 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
         argument.  Depending on your needs for simplicity or
         efficiency, you may mix the previous two approaches freely.
    
         Never add a comma after `pTHX' yourself--always use the form
         of the macro with the underscore for functions that take
         explicit arguments, or the form without the argument for
         functions with no explicit arguments.
    
    
    
         Future Plans and PERL_IMPLICIT_SYS
    
         Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up
         everything that the interpreter knows about itself and pass
         it around, so too are there plans to allow the interpreter
         to bundle up everything it knows about the environment it's
         running on.  This is enabled with the PERL_IMPLICIT_SYS
         macro.  Currently it only works with PERL_OBJECT, but is
         mostly there for MULTIPLICITY and USE_THREADS (see inside
         iperlsys.h).
    
         This allows the ability to provide an extra pointer (called
         the "host" environment) for all the system calls.  This
         makes it possible for all the system stuff to maintain their
         own state, broken down into seven C structures.  These are
         thin wrappers around the usual system calls (see
         win32/perllib.c) for the default perl executable, but for a
         more ambitious host (like the one that would do fork()
         emulation) all the extra work needed to pretend that
         different interpreters are actually different "processes",
         would be done here.
    
         The Perl engine/interpreter and the host are orthogonal
         entities.  There could be one or more interpreters in a
         process, and one or more "hosts", with free association
         between them.
    
    
    

    AUTHORS

         Until May 1997, this document was maintained by Jeff Okamoto
         <okamoto@corp.hp.com>.  It is now maintained as part of Perl
         itself by the Perl 5 Porters <perl5-porters@perl.org>.
    
         With lots of help and suggestions from Dean Roehrich,
         Malcolm Beattie, Andreas Koenig, Paul Hudson, Ilya
         Zakharevich, Paul Marquess, Neil Bowers, Matthew Green, Tim
         Bunce, Spider Boardman, Ulrich Pfeifer, Stephen McCamant,
         and Gurusamy Sarathy.
    
         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)
    
    
    
    


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