windows-xp/Source/XPSP1/NT/tools/x86/perl/lib/pod/perlguts.pod


								=head1 NAME


								perlguts - Perl's Internal Functions


								=head1 DESCRIPTION


								This document attempts to describe some of the internal functions of the

								Perl executable.  It is far from complete and probably contains many errors.

								Please refer any questions or comments to the author below.


								=head1 Variables


								=head2 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.


								=head2 What is an "IV"?


								Perl uses a special typedef IV which is a simple integer type that is

								guaranteed to be large enough to hold a pointer (as well as an integer).


								Perl also uses two special typedefs, I32 and I16, which will always be at

								least 32-bits and 16-bits long, respectively.


								=head2 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(char*, int);

								    SV*  newSVpvn(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 C<sv_setpvn>, C<newSVpvn>, or C<newSVpv>, or you may

								allow Perl to calculate the length by using C<sv_setpv> or by specifying

								0 as the second argument to C<newSVpv>.  Be warned, though, that Perl will

								determine the string's length by using C<strlen>, which depends on the

								string terminating with a NUL character.


								The arguments of C<sv_setpvf> are processed like C<sprintf>, and the

								formatted output becomes the value.


								C<sv_setpvfn> is an analogue of C<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

								L<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 C<sv_set*()> functions are not generic enough to operate on values

								that have "magic".  See L<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*)

								    SvNV(SV*)

								    SvPV(SV*, STRLEN len)


								which will automatically coerce the actual scalar type into an IV, double,

								or string.


								In the C<SvPV> macro, the length of the string returned is placed into the

								variable C<len> (this is a macro, so you do I<not> use C<&len>).  If you do not

								care what the length of the data is, use the global variable C<PL_na> or a

								local variable of type C<STRLEN>. However using C<PL_na> can be quite

								inefficient because C<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 C<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:


									STRLEN len;

									char * ptr;

									ptr = SvPV(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 C<sv_grow>.  Note that C<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

								C<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 C<SvPOK()> is true.


								If you want to append something to the end of string stored in an C<SV*>,

								you can use the following functions:


								    void  sv_catpv(SV*, char*);

								    void  sv_catpvn(SV*, 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 C<strlen>.  In the second, you specify the length of the string

								yourself.  The third function processes its arguments like C<sprintf> and

								appends the formatted output.  The fourth function works like C<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 C<sv_cat*()> functions are not generic enough to operate on values that

								have "magic".  See L<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*  perl_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 C<defined>,

								you can call:


								    SvOK(SV*)


								The scalar C<undef> value is stored in an SV instance called C<PL_sv_undef>.  Its

								address can be used whenever an C<SV*> is needed.


								There are also the two values C<PL_sv_yes> and C<PL_sv_no>, which contain Boolean

								TRUE and FALSE values, respectively.  Like C<PL_sv_undef>, their addresses can

								be used whenever an C<SV*> is needed.


								Do not be fooled into thinking that C<(SV *) 0> is the same as C<&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 C<&PL_sv_undef> in the first

								line and all will be well.


								To free an SV that you've created, call C<SvREFCNT_dec(SV*)>.  Normally this

								call is not necessary (see L<Reference Counts and Mortality>).


								=head2 What's Really Stored in an SV?


								Recall that the usual method of determining the type of scalar you have is

								to use C<Sv*OK> macros.  Because a scalar can be both a number and a string,

								usually these macros will always return TRUE and calling the C<Sv*V>

								macros will do the appropriate conversion of string to integer/double or

								integer/double to string.


								If you I<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 C<Sv*V> macros.


								=head2 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 C<num> C<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 C<av_unshift>.

								This routine adds C<num> elements at the front of the array with the C<undef>

								value.  You must then use C<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 C<av_len> function returns the highest index value in array (just

								like $#array in Perl).  If the array is empty, -1 is returned.  The

								C<av_fetch> function returns the value at index C<key>, but if C<lval>

								is non-zero, then C<av_fetch> will store an undef value at that index.

								The C<av_store> function stores the value C<val> at index C<key>, and does

								not increment the reference count of C<val>.  Thus the caller is responsible

								for taking care of that, and if C<av_store> returns NULL, the caller will

								have to decrement the reference count to avoid a memory leak.  Note that

								C<av_fetch> and C<av_store> both return C<SV**>'s, not C<SV*>'s as their

								return value.


								    void  av_clear(AV*);

								    void  av_undef(AV*);

								    void  av_extend(AV*, I32 key);


								The C<av_clear> function deletes all the elements in the AV* array, but

								does not actually delete the array itself.  The C<av_undef> function will

								delete all the elements in the array plus the array itself.  The

								C<av_extend> function extends the array so that it contains at least C<key+1>

								elements.  If C<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*  perl_get_av("package::varname", FALSE);


								This returns NULL if the variable does not exist.


								See L<Understanding the Magic of Tied Hashes and Arrays> for more

								information on how to use the array access functions on tied arrays.


								=head2 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*, char* key, U32 klen, SV* val, U32 hash);

								    SV**  hv_fetch(HV*, char* key, U32 klen, I32 lval);


								The C<klen> parameter is the length of the key being passed in (Note that

								you cannot pass 0 in as a value of C<klen> to tell Perl to measure the

								length of the key).  The C<val> argument contains the SV pointer to the

								scalar being stored, and C<hash> is the precomputed hash value (zero if

								you want C<hv_store> to calculate it for you).  The C<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 C<hv_fetch> will return as if the value had already existed.


								Remember that C<hv_store> and C<hv_fetch> return C<SV**>'s and not just

								C<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*, char* key, U32 klen);

								    SV*   hv_delete(HV*, char* key, U32 klen, I32 flags);


								If C<flags> does not include the C<G_DISCARD> flag then C<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, C<hv_clear> deletes all the entries in the hash

								table but does not actually delete the hash table.  The C<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 C<SV*>.  However,

								once you have an C<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*  perl_get_hv("package::varname", FALSE);


								This returns NULL if the variable does not exist.


								The hash algorithm is defined in the C<PERL_HASH(hash, key, klen)> macro:


								    hash = 0;

								    while (klen--)

									hash = (hash * 33) + *key++;


								See L<Understanding the Magic of Tied Hashes and Arrays> for more

								information on how to use the hash access functions on tied hashes.


								=head2 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 C<SV*> keys, which simplifies writing

								of extension code that deals with hash structures.  These functions

								also allow passing of C<SV*> keys to C<tie> functions without forcing

								you to stringify the keys (unlike the previous set of functions).


								They also return and accept whole hash entries (C<HE*>), making their

								use more efficient (since the hash number for a particular string

								doesn't have to be recomputed every time).  See L<API LISTING> later in

								this document 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

								L<API LISTING> later in this document 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 C<SV*>s:


								    HeKEY(HE* he)

								    HeKLEN(HE* he)


								Note that both C<hv_store> and C<hv_store_ent> do not increment the

								reference count of the stored C<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 C<val> to avoid a memory leak.


								=head2 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 C<thing> argument can be any of an C<SV*>, C<AV*>, or C<HV*>.  The

								functions are identical except that C<newRV_inc> increments the reference

								count of the C<thing>, while C<newRV_noinc> does not.  For historical

								reasons, C<newRV> is a synonym for C<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 C<SV*> to either an

								C<AV*> or C<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.


								=head2 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 C<sv> argument must be a reference.  The C<stash> argument specifies

								which class the reference will belong to.  See

								L<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 C<classname> is non-null, the SV is blessed into the specified

								class.  SV is returned.


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


								Copies integer or double into an SV whose reference is C<rv>.  SV is blessed

								if C<classname> is non-null.


									SV* sv_setref_iv(SV* rv, char* classname, IV iv);

									SV* sv_setref_nv(SV* rv, char* classname, NV iv);


								Copies the pointer value (I<the address, not the string!>) into an SV whose

								reference is rv.  SV is blessed if C<classname> is non-null.


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


								Copies string into an SV whose reference is C<rv>.  Set length to 0 to let

								Perl calculate the string length.  SV is blessed if C<classname> is non-null.


									SV* sv_setref_pvn(SV* rv, 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, 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 C<UNIVERSAL::isa> functionality.


									bool sv_derived_from(SV* sv, 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)) { ... }


								=head2 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*  perl_get_sv("package::varname", TRUE);

								    AV*  perl_get_av("package::varname", TRUE);

								    HV*  perl_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

								C<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.


								=head2 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 C<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 C<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 C<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 C<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 C<newRV_noinc> instead of

								C<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 L<perlcall> and L<perlxs> for more details on these macros.


								"Mortalization" then is at its simplest a deferred C<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 C<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 C<SV*>) to the

								C<sv_2mortal> or C<sv_mortalcopy> routines.


								=head2 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(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

								C<HV*>.  The C<create> flag will create a new package if it is set.


								The name that C<gv_stash*v> wants is the name of the package whose symbol table

								you want.  The default package is called C<main>.  If you have multiply nested

								packages, pass their names to C<gv_stash*v>, separated by C<::> 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 C<SV*>, must be a reference, and the second

								argument is a stash.  The returned C<SV*> can now be used in the same way

								as any other SV.


								For more information on references and blessings, consult L<perlref>.


								=head2 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 C<$!> contains either the numeric value of C<errno>

								or its string equivalent from either C<strerror> or C<sys_errlist[]>.


								To force multiple data values into an SV, you must do two things: use the

								C<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 C<sv_set*v> routine

								you called first.  This is because every C<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 = perl_get_sv("dberror", TRUE);

								    sv_setiv(sv, (IV) dberror);

								    sv_setpv(sv, dberror_list[dberror]);

								    SvIOK_on(sv);


								If the order of C<sv_setiv> and C<sv_setpv> had been reversed, then the

								macro C<SvPOK_on> would need to be called instead of C<SvIOK_on>.


								=head2 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 C<struct magic>'s, typedef'ed to C<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.


								=head2 Assigning Magic


								Perl adds magic to an SV using the sv_magic function:


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


								The C<sv> argument is a pointer to the SV that is to acquire a new magical

								feature.


								If C<sv> is not already magical, Perl uses the C<SvUPGRADE> macro to

								set the C<SVt_PVMG> flag for the C<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 C<name> and C<namlen> arguments are used to associate a string with

								the magic, typically the name of a variable. C<namlen> is stored in the

								C<mg_len> field and if C<name> is non-null and C<namlen> >= 0 a malloc'd

								copy of the name is stored in C<mg_ptr> field.


								The sv_magic function uses C<how> to determine which, if any, predefined

								"Magic Virtual Table" should be assigned to the C<mg_virtual> field.

								See the "Magic Virtual Table" section below.  The C<how> argument is also

								stored in the C<mg_type> field.


								The C<obj> argument is stored in the C<mg_obj> field of the C<MAGIC>

								structure.  If it is not the same as the C<sv> argument, the reference

								count of the C<obj> object is incremented.  If it is the same, or if

								the C<how> argument is "#", or if it is a NULL pointer, then C<obj> is

								merely stored, without the reference count being incremented.


								There is also a function to add magic to an C<HV>:


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


								This simply calls C<sv_magic> and coerces the C<gv> argument into an C<SV>.


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


								    void sv_unmagic(SV *sv, int type);


								The C<type> argument should be equal to the C<how> value when the C<SV>

								was initially made magical.


								=head2 Magic Virtual Tables


								The C<mg_virtual> field in the C<MAGIC> structure is a pointer to a

								C<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 C<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 C<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 C<vtbl_sv> (which corresponds

								to an C<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 C<magic_get> is called.  All

								the various routines for the various magical types begin with C<magic_>.


								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 C<MAGIC>'s

								C<mg_ptr> field points to a C<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 C<uf_val> or C<uf_set>

								function will be called with C<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 C<sv_set*()> and C<sv_cat*()> functions described

								earlier do B<not> invoke 'set' magic on their targets.  This must

								be done by the user either by calling the C<SvSETMAGIC()> macro after

								calling these functions, or by using one of the C<sv_set*_mg()> or

								C<sv_cat*_mg()> functions.  Similarly, generic C code must call the

								C<SvGETMAGIC()> macro to invoke any 'get' magic if they use an SV

								obtained from external sources in functions that don't handle magic.

								L<API LISTING> later in this document identifies such functions.

								For example, calls to the C<sv_cat*()> functions typically need to be

								followed by C<SvSETMAGIC()>, but they don't need a prior C<SvGETMAGIC()>

								since their implementation handles 'get' magic.


								=head2 Finding Magic


								    MAGIC* mg_find(SV*, int type); /* Finds the magic pointer of that type */


								This routine returns a pointer to the C<MAGIC> structure stored in the SV.

								If the SV does not have that magical feature, C<NULL> is returned.  Also,

								if the SV is not of type SVt_PVMG, Perl may core dump.


								    int mg_copy(SV* sv, SV* nsv, char* key, STRLEN klen);


								This routine checks to see what types of magic C<sv> has.  If the mg_type

								field is an uppercase letter, then the mg_obj is copied to C<nsv>, but

								the mg_type field is changed to be the lowercase letter.


								=head2 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 L<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, tie, 'P');

								        RETVAL = newRV_noinc(hash);

								    OUTPUT:

								        RETVAL


								The C<av_store> function, when given a tied array argument, merely

								copies the magic of the array onto the value to be "stored", using

								C<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

								C<av_store> on a tied array, the caller will usually need to call

								C<mg_set(val)> to actually invoke the perl level "STORE" method on the

								TIEARRAY object.  If C<av_store> did return NULL, a call to

								C<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

								C<hv_store> and C<hv_store_ent> functions as well.


								C<av_fetch> and the corresponding hash functions C<hv_fetch> and

								C<hv_fetch_ent> actually return an undefined mortal value whose magic

								has been initialized using C<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 C<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 C<mg_set()> on the return value after possibly assigning

								a suitable value to it using C<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 C<mg_copy> to attach magic to the values that were meant to be

								"stored" or "fetched".  Later calls to C<mg_get> and C<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.


								=head2 Localizing changes


								Perl has a very handy construction


								  {

								    local $var = 2;

								    ...

								  }


								This construction is I<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: C<goto>, C<return>, C<die>/C<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

								I<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 I<block>-like construct is created by a pair of

								C<ENTER>/C<LEAVE> macros (see L<perlcall/"Returning a Scalar">).

								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 C<ENTER>/C<LEAVE> pair.


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


								=over


								=item C<SAVEINT(int i)>


								=item C<SAVEIV(IV i)>


								=item C<SAVEI32(I32 i)>


								=item C<SAVELONG(long i)>


								These macros arrange things to restore the value of integer variable

								C<i> at the end of enclosing I<pseudo-block>.


								=item C<SAVESPTR(s)>


								=item C<SAVEPPTR(p)>


								These macros arrange things to restore the value of pointers C<s> and

								C<p>. C<s> must be a pointer of a type which survives conversion to

								C<SV*> and back, C<p> should be able to survive conversion to C<char*>

								and back.


								=item C<SAVEFREESV(SV *sv)>


								The refcount of C<sv> would be decremented at the end of

								I<pseudo-block>. This is similar to C<sv_2mortal>, which should (?) be

								used instead.


								=item C<SAVEFREEOP(OP *op)>


								The C<OP *> is op_free()ed at the end of I<pseudo-block>.


								=item C<SAVEFREEPV(p)>


								The chunk of memory which is pointed to by C<p> is Safefree()ed at the

								end of I<pseudo-block>.


								=item C<SAVECLEARSV(SV *sv)>


								Clears a slot in the current scratchpad which corresponds to C<sv> at

								the end of I<pseudo-block>.


								=item C<SAVEDELETE(HV *hv, char *key, I32 length)>


								The key C<key> of C<hv> is deleted at the end of I<pseudo-block>. The

								string pointed to by C<key> is Safefree()ed.  If one has a I<key> in

								short-lived storage, the corresponding string may be reallocated like

								this:


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


								=item C<SAVEDESTRUCTOR(f,p)>


								At the end of I<pseudo-block> the function C<f> is called with the

								only argument (of type C<void*>) C<p>.


								=item C<SAVESTACK_POS()>


								The current offset on the Perl internal stack (cf. C<SP>) is restored

								at the end of I<pseudo-block>.


								=back


								The following API list contains functions, thus one needs to

								provide pointers to the modifiable data explicitly (either C pointers,

								or Perlish C<GV *>s).  Where the above macros take C<int>, a similar

								function takes C<int *>.


								=over


								=item C<SV* save_scalar(GV *gv)>


								Equivalent to Perl code C<local $gv>.


								=item C<AV* save_ary(GV *gv)>


								=item C<HV* save_hash(GV *gv)>


								Similar to C<save_scalar>, but localize C<@gv> and C<%gv>.


								=item C<void save_item(SV *item)>


								Duplicates the current value of C<SV>, on the exit from the current

								C<ENTER>/C<LEAVE> I<pseudo-block> will restore the value of C<SV>

								using the stored value.


								=item C<void save_list(SV **sarg, I32 maxsarg)>


								A variant of C<save_item> which takes multiple arguments via an array

								C<sarg> of C<SV*> of length C<maxsarg>.


								=item C<SV* save_svref(SV **sptr)>


								Similar to C<save_scalar>, but will reinstate a C<SV *>.


								=item C<void save_aptr(AV **aptr)>


								=item C<void save_hptr(HV **hptr)>


								Similar to C<save_svref>, but localize C<AV *> and C<HV *>.


								=back


								The C<Alias> module implements localization of the basic types within the

								I<caller's scope>.  People who are interested in how to localize things in

								the containing scope should take a look there too.


								=head1 Subroutines


								=head2 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 C<ST(n)> macro, which returns

								the C<n>'th stack argument.  Argument 0 is the first argument passed in the

								Perl subroutine call.  These arguments are C<SV*>, and can be used anywhere

								an C<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 C<SP> is the macro that represents the local copy of the stack pointer,

								and C<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 C<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 C<EXTEND> to extend the stack.


								For more information, consult L<perlxs> and L<perlxstut>.


								=head2 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  perl_call_sv(SV*, I32);

								    I32  perl_call_pv(char*, I32);

								    I32  perl_call_method(char*, I32);

								    I32  perl_call_argv(char*, I32, register char**);


								The routine most often used is C<perl_call_sv>.  The C<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.


								When using any of these routines (except C<perl_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 L<perlcall>.


								=head2 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 C<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 C<pointer> should be the name of a variable that will

								point to the newly allocated memory.


								The third and fourth arguments C<number> and C<type> specify how many of

								the specified type of data structure should be allocated.  The argument

								C<type> is passed to C<sizeof>.  The final argument to C<Newc>, C<cast>,

								should be used if the C<pointer> argument is different from the C<type>

								argument.


								Unlike the C<New> and C<Newc> macros, the C<Newz> macro calls C<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 C<Renew> and C<Renewc>

								match those of C<New> and C<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 C<source> and C<dest> arguments point to the source and

								destination starting points.  Perl will move, copy, or zero out C<number>

								instances of the size of the C<type> data structure (using the C<sizeof>

								function).


								=head2 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 L<perlapio>.


								=head2 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 (I<target>s) which are (as a corollary)

								not constantly freed/created.


								Each of the targets is created only once (but see

								L<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 I<target> and puts the I<target> on stack.


								The macro to put this target on stack is C<PUSHTARG>, and it is

								directly used in some opcodes, as well as indirectly in zillions of

								others, which use it via C<(X)PUSH[pni]>.


								=head2 Scratchpads


								The question remains on when the SVs which are I<target>s 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 C<SVs_PADMY> set, and

								I<target>s have C<SVs_PADTMP> set.


								The correspondence between OPs and I<target>s 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.


								=head2 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 B<recursion>, and maybe (sometime soon) B<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. (I<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 I<target>s on this scratchpad are C<undef>s, but they are already

								marked with correct flags.


								=head1 Compiled code


								=head2 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 C<$a = $b + $c> it is different:

								some nodes I<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.


								=head2 Examining the tree


								If you have your perl compiled for debugging (usually done with C<-D

								optimize=-g> on C<Configure> command line), you may examine the

								compiled tree by specifying C<-Dx> on the Perl command line.  The

								output takes several lines per node, and for C<$b+$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 C<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 C<{}> 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 C<===E<gt>> marks, thus it is C<3

								4 5 6> (node C<6> is not included into above listing), i.e.,

								C<gvsv gvsv add whatever>.


								=head2 Compile pass 1: check routines


								The tree is created by the I<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 I<check routines>.  The correspondence between node names

								and corresponding check routines is described in F<opcode.pl> (do not

								forget to run C<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 C<ck_*>. They are usually

								called from C<new*OP> subroutines (or C<convert>) (which in turn are

								called from F<perly.y>).


								=head2 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 I<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.


								=head2 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).


								=head2 Compile pass 3: peephole optimization


								After the compile tree for a subroutine (or for an C<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.


								=head1 API LISTING


								This is a listing of functions, macros, flags, and variables that may be

								useful to extension writers or that may be found while reading other

								extensions.


								Note that all Perl API global variables must be referenced with the C<PL_>

								prefix.  Some macros are provided for compatibility with the older,

								unadorned names, but this support will be removed in a future release.


								It is strongly recommended that all Perl API functions that don't begin

								with C<perl> be referenced with an explicit C<Perl_> prefix.


								The sort order of the listing is case insensitive, with any

								occurrences of '_' ignored for the purpose of sorting.


								=over 8


								=item av_clear


								Clears an array, making it empty.  Does not free the memory used by the

								array itself.


									void	av_clear (AV* ar)


								=item av_extend


								Pre-extend an array.  The C<key> is the index to which the array should be

								extended.


									void	av_extend (AV* ar, I32 key)


								=item av_fetch


								Returns the SV at the specified index in the array.  The C<key> is the

								index.  If C<lval> is set then the fetch will be part of a store.  Check

								that the return value is non-null before dereferencing it to a C<SV*>.


								See L<Understanding the Magic of Tied Hashes and Arrays> for more

								information on how to use this function on tied arrays.


									SV**	av_fetch (AV* ar, I32 key, I32 lval)


								=item AvFILL


								Same as C<av_len()>.  Deprecated, use C<av_len()> instead.


								=item av_len


								Returns the highest index in the array.  Returns -1 if the array is empty.


									I32	av_len (AV* ar)


								=item av_make


								Creates a new AV and populates it with a list of SVs.  The SVs are copied

								into the array, so they may be freed after the call to av_make.  The new AV

								will have a reference count of 1.


									AV*	av_make (I32 size, SV** svp)


								=item av_pop


								Pops an SV off the end of the array.  Returns C<&PL_sv_undef> if the array is

								empty.


									SV*	av_pop (AV* ar)


								=item av_push


								Pushes an SV onto the end of the array.  The array will grow automatically

								to accommodate the addition.


									void	av_push (AV* ar, SV* val)


								=item av_shift


								Shifts an SV off the beginning of the array.


									SV*	av_shift (AV* ar)


								=item av_store


								Stores an SV in an array.  The array index is specified as C<key>.  The

								return value will be NULL if the operation failed or if the value did not

								need to be actually stored within the array (as in the case of tied arrays).

								Otherwise it can be dereferenced to get the original C<SV*>.  Note that the

								caller is responsible for suitably incrementing the reference count of C<val>

								before the call, and decrementing it if the function returned NULL.


								See L<Understanding the Magic of Tied Hashes and Arrays> for more

								information on how to use this function on tied arrays.


									SV**	av_store (AV* ar, I32 key, SV* val)


								=item av_undef


								Undefines the array.  Frees the memory used by the array itself.


									void	av_undef (AV* ar)


								=item av_unshift


								Unshift the given number of C<undef> values onto the beginning of the

								array.  The array will grow automatically to accommodate the addition.

								You must then use C<av_store> to assign values to these new elements.


									void	av_unshift (AV* ar, I32 num)


								=item CLASS


								Variable which is setup by C<xsubpp> to indicate the class name for a C++ XS

								constructor.  This is always a C<char*>.  See C<THIS> and

								L<perlxs/"Using XS With C++">.


								=item Copy


								The XSUB-writer's interface to the C C<memcpy> function.  The C<s> is the

								source, C<d> is the destination, C<n> is the number of items, and C<t> is

								the type.  May fail on overlapping copies.  See also C<Move>.


									void	Copy( s, d, n, t )


								=item croak


								This is the XSUB-writer's interface to Perl's C<die> function.  Use this

								function the same way you use the C C<printf> function.  See C<warn>.


								=item CvSTASH


								Returns the stash of the CV.


									HV*	CvSTASH( SV* sv )


								=item PL_DBsingle


								When Perl is run in debugging mode, with the B<-d> switch, this SV is a

								boolean which indicates whether subs are being single-stepped.

								Single-stepping is automatically turned on after every step.  This is the C

								variable which corresponds to Perl's $DB::single variable.  See C<PL_DBsub>.


								=item PL_DBsub


								When Perl is run in debugging mode, with the B<-d> switch, this GV contains

								the SV which holds the name of the sub being debugged.  This is the C

								variable which corresponds to Perl's $DB::sub variable.  See C<PL_DBsingle>.

								The sub name can be found by


									SvPV( GvSV( PL_DBsub ), len )


								=item PL_DBtrace


								Trace variable used when Perl is run in debugging mode, with the B<-d>

								switch.  This is the C variable which corresponds to Perl's $DB::trace

								variable.  See C<PL_DBsingle>.


								=item dMARK


								Declare a stack marker variable, C<mark>, for the XSUB.  See C<MARK> and

								C<dORIGMARK>.


								=item dORIGMARK


								Saves the original stack mark for the XSUB.  See C<ORIGMARK>.


								=item PL_dowarn


								The C variable which corresponds to Perl's $^W warning variable.


								=item dSP


								Declares a local copy of perl's stack pointer for the XSUB, available via

								the C<SP> macro.  See C<SP>.


								=item dXSARGS


								Sets up stack and mark pointers for an XSUB, calling dSP and dMARK.  This is

								usually handled automatically by C<xsubpp>.  Declares the C<items> variable

								to indicate the number of items on the stack.


								=item dXSI32


								Sets up the C<ix> variable for an XSUB which has aliases.  This is usually

								handled automatically by C<xsubpp>.


								=item do_binmode


								Switches filehandle to binmode.  C<iotype> is what C<IoTYPE(io)> would

								contain.


									do_binmode(fp, iotype, TRUE);


								=item ENTER


								Opening bracket on a callback.  See C<LEAVE> and L<perlcall>.


									ENTER;


								=item EXTEND


								Used to extend the argument stack for an XSUB's return values.


									EXTEND( sp, int x )


								=item fbm_compile


								Analyses the string in order to make fast searches on it using fbm_instr() --

								the Boyer-Moore algorithm.


									void	fbm_compile(SV* sv, U32 flags)


								=item fbm_instr


								Returns the location of the SV in the string delimited by C<str> and

								C<strend>.  It returns C<Nullch> if the string can't be found.  The

								C<sv> does not have to be fbm_compiled, but the search will not be as

								fast then.


									char*	fbm_instr(char *str, char *strend, SV *sv, U32 flags)


								=item FREETMPS


								Closing bracket for temporaries on a callback.  See C<SAVETMPS> and

								L<perlcall>.


									FREETMPS;


								=item G_ARRAY


								Used to indicate array context.  See C<GIMME_V>, C<GIMME> and L<perlcall>.


								=item G_DISCARD


								Indicates that arguments returned from a callback should be discarded.  See

								L<perlcall>.


								=item G_EVAL


								Used to force a Perl C<eval> wrapper around a callback.  See L<perlcall>.


								=item GIMME


								A backward-compatible version of C<GIMME_V> which can only return

								C<G_SCALAR> or C<G_ARRAY>; in a void context, it returns C<G_SCALAR>.


								=item GIMME_V


								The XSUB-writer's equivalent to Perl's C<wantarray>.  Returns

								C<G_VOID>, C<G_SCALAR> or C<G_ARRAY> for void, scalar or array

								context, respectively.


								=item G_NOARGS


								Indicates that no arguments are being sent to a callback.  See L<perlcall>.


								=item G_SCALAR


								Used to indicate scalar context.  See C<GIMME_V>, C<GIMME>, and L<perlcall>.


								=item gv_fetchmeth


								Returns the glob with the given C<name> and a defined subroutine or

								C<NULL>.  The glob lives in the given C<stash>, or in the stashes

								accessible via @ISA and @UNIVERSAL.


								The argument C<level> should be either 0 or -1.  If C<level==0>, as a

								side-effect creates a glob with the given C<name> in the given

								C<stash> which in the case of success contains an alias for the

								subroutine, and sets up caching info for this glob.  Similarly for all

								the searched stashes.


								This function grants C<"SUPER"> token as a postfix of the stash name.


								The GV returned from C<gv_fetchmeth> may be a method cache entry,

								which is not visible to Perl code.  So when calling C<perl_call_sv>,

								you should not use the GV directly; instead, you should use the

								method's CV, which can be obtained from the GV with the C<GvCV> macro.


								        GV*     gv_fetchmeth (HV* stash, char* name, STRLEN len, I32 level)


								=item gv_fetchmethod


								=item gv_fetchmethod_autoload


								Returns the glob which contains the subroutine to call to invoke the

								method on the C<stash>.  In fact in the presence of autoloading this may

								be the glob for "AUTOLOAD".  In this case the corresponding variable

								$AUTOLOAD is already setup.


								The third parameter of C<gv_fetchmethod_autoload> determines whether AUTOLOAD

								lookup is performed if the given method is not present: non-zero means

								yes, look for AUTOLOAD; zero means no, don't look for AUTOLOAD.  Calling

								C<gv_fetchmethod> is equivalent to calling C<gv_fetchmethod_autoload> with a

								non-zero C<autoload> parameter.


								These functions grant C<"SUPER"> token as a prefix of the method name.


								Note that if you want to keep the returned glob for a long time, you

								need to check for it being "AUTOLOAD", since at the later time the call

								may load a different subroutine due to $AUTOLOAD changing its value.

								Use the glob created via a side effect to do this.


								These functions have the same side-effects and as C<gv_fetchmeth> with

								C<level==0>.  C<name> should be writable if contains C<':'> or C<'\''>.

								The warning against passing the GV returned by C<gv_fetchmeth> to

								C<perl_call_sv> apply equally to these functions.


								        GV*     gv_fetchmethod (HV* stash, char* name)

								        GV*     gv_fetchmethod_autoload (HV* stash, char* name, I32 autoload)


								=item G_VOID


								Used to indicate void context.  See C<GIMME_V> and L<perlcall>.


								=item gv_stashpv


								Returns a pointer to the stash for a specified package.  If C<create> is set

								then the package will be created if it does not already exist.  If C<create>

								is not set and the package does not exist then NULL is returned.


									HV*	gv_stashpv (char* name, I32 create)


								=item gv_stashsv


								Returns a pointer to the stash for a specified package.  See C<gv_stashpv>.


									HV*	gv_stashsv (SV* sv, I32 create)


								=item GvSV


								Return the SV from the GV.


								=item HEf_SVKEY


								This flag, used in the length slot of hash entries and magic

								structures, specifies the structure contains a C<SV*> pointer where a

								C<char*> pointer is to be expected. (For information only--not to be used).


								=item HeHASH


								Returns the computed hash stored in the hash entry.


									U32	HeHASH(HE* he)


								=item HeKEY


								Returns the actual pointer stored in the key slot of the hash entry.

								The pointer may be either C<char*> or C<SV*>, depending on the value of

								C<HeKLEN()>.  Can be assigned to.  The C<HePV()> or C<HeSVKEY()> macros

								are usually preferable for finding the value of a key.


									char*	HeKEY(HE* he)


								=item HeKLEN


								If this is negative, and amounts to C<HEf_SVKEY>, it indicates the entry

								holds an C<SV*> key.  Otherwise, holds the actual length of the key.

								Can be assigned to. The C<HePV()> macro is usually preferable for finding

								key lengths.


									int	HeKLEN(HE* he)


								=item HePV


								Returns the key slot of the hash entry as a C<char*> value, doing any

								necessary dereferencing of possibly C<SV*> keys.  The length of

								the string is placed in C<len> (this is a macro, so do I<not> use

								C<&len>).  If you do not care about what the length of the key is,

								you may use the global variable C<PL_na>, though this is rather less

								efficient than using a local variable.  Remember though, that hash

								keys in perl are free to contain embedded nulls, so using C<strlen()>

								or similar is not a good way to find the length of hash keys.

								This is very similar to the C<SvPV()> macro described elsewhere in

								this document.


									char*	HePV(HE* he, STRLEN len)


								=item HeSVKEY


								Returns the key as an C<SV*>, or C<Nullsv> if the hash entry

								does not contain an C<SV*> key.


									HeSVKEY(HE* he)


								=item HeSVKEY_force


								Returns the key as an C<SV*>.  Will create and return a temporary

								mortal C<SV*> if the hash entry contains only a C<char*> key.


									HeSVKEY_force(HE* he)


								=item HeSVKEY_set


								Sets the key to a given C<SV*>, taking care to set the appropriate flags

								to indicate the presence of an C<SV*> key, and returns the same C<SV*>.


									HeSVKEY_set(HE* he, SV* sv)


								=item HeVAL


								Returns the value slot (type C<SV*>) stored in the hash entry.


									HeVAL(HE* he)


								=item hv_clear


								Clears a hash, making it empty.


									void	hv_clear (HV* tb)


								=item hv_delete


								Deletes a key/value pair in the hash.  The value SV is removed from the hash

								and returned to the caller.  The C<klen> is the length of the key.  The

								C<flags> value will normally be zero; if set to G_DISCARD then NULL will be

								returned.


									SV*	hv_delete (HV* tb, char* key, U32 klen, I32 flags)


								=item hv_delete_ent


								Deletes a key/value pair in the hash.  The value SV is removed from the hash

								and returned to the caller.  The C<flags> value will normally be zero; if set

								to G_DISCARD then NULL will be returned.  C<hash> can be a valid precomputed

								hash value, or 0 to ask for it to be computed.


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


								=item hv_exists


								Returns a boolean indicating whether the specified hash key exists.  The

								C<klen> is the length of the key.


									bool	hv_exists (HV* tb, char* key, U32 klen)


								=item hv_exists_ent


								Returns a boolean indicating whether the specified hash key exists. C<hash>

								can be a valid precomputed hash value, or 0 to ask for it to be computed.


									bool    hv_exists_ent (HV* tb, SV* key, U32 hash)


								=item hv_fetch


								Returns the SV which corresponds to the specified key in the hash.  The

								C<klen> is the length of the key.  If C<lval> is set then the fetch will be

								part of a store.  Check that the return value is non-null before

								dereferencing it to a C<SV*>.


								See L<Understanding the Magic of Tied Hashes and Arrays> for more

								information on how to use this function on tied hashes.


									SV**	hv_fetch (HV* tb, char* key, U32 klen, I32 lval)


								=item hv_fetch_ent


								Returns the hash entry which corresponds to the specified key in the hash.

								C<hash> must be a valid precomputed hash number for the given C<key>, or

								0 if you want the function to compute it.  IF C<lval> is set then the

								fetch will be part of a store.  Make sure the return value is non-null

								before accessing it.  The return value when C<tb> is a tied hash

								is a pointer to a static location, so be sure to make a copy of the

								structure if you need to store it somewhere.


								See L<Understanding the Magic of Tied Hashes and Arrays> for more

								information on how to use this function on tied hashes.


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


								=item hv_iterinit


								Prepares a starting point to traverse a hash table.


									I32	hv_iterinit (HV* tb)


								Returns the number of keys in the hash (i.e. the same as C<HvKEYS(tb)>).

								The return value is currently only meaningful for hashes without tie

								magic.


								NOTE: Before version 5.004_65, C<hv_iterinit> used to return the number

								of hash buckets that happen to be in use.  If you still need that

								esoteric value, you can get it through the macro C<HvFILL(tb)>.


								=item hv_iterkey


								Returns the key from the current position of the hash iterator.  See

								C<hv_iterinit>.


									char*	hv_iterkey (HE* entry, I32* retlen)


								=item hv_iterkeysv


								Returns the key as an C<SV*> from the current position of the hash

								iterator.  The return value will always be a mortal copy of the

								key.  Also see C<hv_iterinit>.


									SV*     hv_iterkeysv  (HE* entry)


								=item hv_iternext


								Returns entries from a hash iterator.  See C<hv_iterinit>.


									HE*	hv_iternext (HV* tb)


								=item hv_iternextsv


								Performs an C<hv_iternext>, C<hv_iterkey>, and C<hv_iterval> in one

								operation.


									SV*	hv_iternextsv (HV* hv, char** key, I32* retlen)


								=item hv_iterval


								Returns the value from the current position of the hash iterator.  See

								C<hv_iterkey>.


									SV*	hv_iterval (HV* tb, HE* entry)


								=item hv_magic


								Adds magic to a hash.  See C<sv_magic>.


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


								=item HvNAME


								Returns the package name of a stash.  See C<SvSTASH>, C<CvSTASH>.


									char*	HvNAME (HV* stash)


								=item hv_store


								Stores an SV in a hash.  The hash key is specified as C<key> and C<klen> is

								the length of the key.  The C<hash> parameter is the precomputed hash

								value; if it is zero then Perl will compute it.  The return value will be

								NULL if the operation failed or if the value did not need to be actually

								stored within the hash (as in the case of tied hashes).  Otherwise it can

								be dereferenced to get the original C<SV*>.  Note that the caller is

								responsible for suitably incrementing the reference count of C<val>

								before the call, and decrementing it if the function returned NULL.


								See L<Understanding the Magic of Tied Hashes and Arrays> for more

								information on how to use this function on tied hashes.


									SV**	hv_store (HV* tb, char* key, U32 klen, SV* val, U32 hash)


								=item hv_store_ent


								Stores C<val> in a hash.  The hash key is specified as C<key>.  The C<hash>

								parameter is the precomputed hash value; if it is zero then Perl will

								compute it.  The return value is the new hash entry so created.  It will be

								NULL if the operation failed or if the value did not need to be actually

								stored within the hash (as in the case of tied hashes).  Otherwise the

								contents of the return value can be accessed using the C<He???> macros

								described here.  Note that the caller is responsible for suitably

								incrementing the reference count of C<val> before the call, and decrementing

								it if the function returned NULL.


								See L<Understanding the Magic of Tied Hashes and Arrays> for more

								information on how to use this function on tied hashes.


									HE*     hv_store_ent  (HV* tb, SV* key, SV* val, U32 hash)


								=item hv_undef


								Undefines the hash.


									void	hv_undef (HV* tb)


								=item isALNUM


								Returns a boolean indicating whether the C C<char> is an ascii alphanumeric

								character or digit.


									int	isALNUM (char c)


								=item isALPHA


								Returns a boolean indicating whether the C C<char> is an ascii alphabetic

								character.


									int	isALPHA (char c)


								=item isDIGIT


								Returns a boolean indicating whether the C C<char> is an ascii digit.


									int	isDIGIT (char c)


								=item isLOWER


								Returns a boolean indicating whether the C C<char> is a lowercase character.


									int	isLOWER (char c)


								=item isSPACE


								Returns a boolean indicating whether the C C<char> is whitespace.


									int	isSPACE (char c)


								=item isUPPER


								Returns a boolean indicating whether the C C<char> is an uppercase character.


									int	isUPPER (char c)


								=item items


								Variable which is setup by C<xsubpp> to indicate the number of items on the

								stack.  See L<perlxs/"Variable-length Parameter Lists">.


								=item ix


								Variable which is setup by C<xsubpp> to indicate which of an XSUB's aliases

								was used to invoke it.  See L<perlxs/"The ALIAS: Keyword">.


								=item LEAVE


								Closing bracket on a callback.  See C<ENTER> and L<perlcall>.


									LEAVE;


								=item looks_like_number


								Test if an the content of an SV looks like a number (or is a number).


									int	looks_like_number(SV*)


								=item MARK


								Stack marker variable for the XSUB.  See C<dMARK>.


								=item mg_clear


								Clear something magical that the SV represents.  See C<sv_magic>.


									int	mg_clear (SV* sv)


								=item mg_copy


								Copies the magic from one SV to another.  See C<sv_magic>.


									int	mg_copy (SV *, SV *, char *, STRLEN)


								=item mg_find


								Finds the magic pointer for type matching the SV.  See C<sv_magic>.


									MAGIC*	mg_find (SV* sv, int type)


								=item mg_free


								Free any magic storage used by the SV.  See C<sv_magic>.


									int	mg_free (SV* sv)


								=item mg_get


								Do magic after a value is retrieved from the SV.  See C<sv_magic>.


									int	mg_get (SV* sv)


								=item mg_len


								Report on the SV's length.  See C<sv_magic>.


									U32	mg_len (SV* sv)


								=item mg_magical


								Turns on the magical status of an SV.  See C<sv_magic>.


									void	mg_magical (SV* sv)


								=item mg_set


								Do magic after a value is assigned to the SV.  See C<sv_magic>.


									int	mg_set (SV* sv)


								=item modglobal


								C<modglobal> is a general purpose, interpreter global HV for use by

								extensions that need to keep information on a per-interpreter basis.

								In a pinch, it can also be used as a symbol table for extensions

								to share data among each other.  It is a good idea to use keys

								prefixed by the package name of the extension that owns the data.


								=item Move


								The XSUB-writer's interface to the C C<memmove> function.  The C<s> is the

								source, C<d> is the destination, C<n> is the number of items, and C<t> is

								the type.  Can do overlapping moves.  See also C<Copy>.


									void	Move( s, d, n, t )


								=item PL_na


								A convenience variable which is typically used with C<SvPV> when one doesn't

								care about the length of the string.  It is usually more efficient to

								declare a local variable and use that instead.


								=item New


								The XSUB-writer's interface to the C C<malloc> function.


									void*	New( x, void *ptr, int size, type )


								=item newAV


								Creates a new AV.  The reference count is set to 1.


									AV*	newAV (void)


								=item Newc


								The XSUB-writer's interface to the C C<malloc> function, with cast.


									void*	Newc( x, void *ptr, int size, type, cast )


								=item newCONSTSUB


								Creates a constant sub equivalent to Perl C<sub FOO () { 123 }>

								which is eligible for inlining at compile-time.


									void	newCONSTSUB(HV* stash, char* name, SV* sv)


								=item newHV


								Creates a new HV.  The reference count is set to 1.


									HV*	newHV (void)


								=item newRV_inc


								Creates an RV wrapper for an SV.  The reference count for the original SV is

								incremented.


									SV*	newRV_inc (SV* ref)


								For historical reasons, "newRV" is a synonym for "newRV_inc".


								=item newRV_noinc


								Creates an RV wrapper for an SV.  The reference count for the original

								SV is B<not> incremented.


									SV*     newRV_noinc (SV* ref)


								=item NEWSV


								Creates a new SV.  A non-zero C<len> parameter indicates the number of

								bytes of preallocated string space the SV should have.  An extra byte

								for a tailing NUL is also reserved.  (SvPOK is not set for the SV even

								if string space is allocated.)  The reference count for the new SV is

								set to 1.  C<id> is an integer id between 0 and 1299 (used to identify

								leaks).


									SV*	NEWSV (int id, STRLEN len)


								=item newSViv


								Creates a new SV and copies an integer into it.  The reference count for the

								SV is set to 1.


									SV*	newSViv (IV i)


								=item newSVnv


								Creates a new SV and copies a double into it.  The reference count for the

								SV is set to 1.


									SV*	newSVnv (NV i)


								=item newSVpv


								Creates a new SV and copies a string into it.  The reference count for the

								SV is set to 1.  If C<len> is zero then Perl will compute the length.


									SV*	newSVpv (char* s, STRLEN len)


								=item newSVpvf


								Creates a new SV an initialize it with the string formatted like

								C<sprintf>.


									SV*     newSVpvf(const char* pat, ...);


								=item newSVpvn


								Creates a new SV and copies a string into it.  The reference count for the

								SV is set to 1.  If C<len> is zero then Perl will create a zero length

								string.


									SV*	newSVpvn (char* s, STRLEN len)


								=item newSVrv


								Creates a new SV for the RV, C<rv>, to point to.  If C<rv> is not an RV then

								it will be upgraded to one.  If C<classname> is non-null then the new SV will

								be blessed in the specified package.  The new SV is returned and its

								reference count is 1.


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


								=item newSVsv


								Creates a new SV which is an exact duplicate of the original SV.


									SV*	newSVsv (SV* old)


								=item newXS


								Used by C<xsubpp> to hook up XSUBs as Perl subs.


								=item newXSproto


								Used by C<xsubpp> to hook up XSUBs as Perl subs.  Adds Perl prototypes to

								the subs.


								=item Newz


								The XSUB-writer's interface to the C C<malloc> function.  The allocated

								memory is zeroed with C<memzero>.


									void*	Newz( x, void *ptr, int size, type )


								=item Nullav


								Null AV pointer.


								=item Nullch


								Null character pointer.


								=item Nullcv


								Null CV pointer.


								=item Nullhv


								Null HV pointer.


								=item Nullsv


								Null SV pointer.


								=item ORIGMARK


								The original stack mark for the XSUB.  See C<dORIGMARK>.


								=item perl_alloc


								Allocates a new Perl interpreter.  See L<perlembed>.


								=item perl_call_argv


								Performs a callback to the specified Perl sub.  See L<perlcall>.


									I32	perl_call_argv (char* subname, I32 flags, char** argv)


								=item perl_call_method


								Performs a callback to the specified Perl method.  The blessed object must

								be on the stack.  See L<perlcall>.


									I32	perl_call_method (char* methname, I32 flags)


								=item perl_call_pv


								Performs a callback to the specified Perl sub.  See L<perlcall>.


									I32	perl_call_pv (char* subname, I32 flags)


								=item perl_call_sv


								Performs a callback to the Perl sub whose name is in the SV.  See

								L<perlcall>.


									I32	perl_call_sv (SV* sv, I32 flags)


								=item perl_construct


								Initializes a new Perl interpreter.  See L<perlembed>.


								=item perl_destruct


								Shuts down a Perl interpreter.  See L<perlembed>.


								=item perl_eval_sv


								Tells Perl to C<eval> the string in the SV.


									I32	perl_eval_sv (SV* sv, I32 flags)


								=item perl_eval_pv


								Tells Perl to C<eval> the given string and return an SV* result.


									SV*	perl_eval_pv (char* p, I32 croak_on_error)


								=item perl_free


								Releases a Perl interpreter.  See L<perlembed>.


								=item perl_get_av


								Returns the AV of the specified Perl array.  If C<create> is set and the

								Perl variable does not exist then it will be created.  If C<create> is not

								set and the variable does not exist then NULL is returned.


									AV*	perl_get_av (char* name, I32 create)


								=item perl_get_cv


								Returns the CV of the specified Perl sub.  If C<create> is set and the Perl

								variable does not exist then it will be created.  If C<create> is not

								set and the variable does not exist then NULL is returned.


									CV*	perl_get_cv (char* name, I32 create)


								=item perl_get_hv


								Returns the HV of the specified Perl hash.  If C<create> is set and the Perl

								variable does not exist then it will be created.  If C<create> is not

								set and the variable does not exist then NULL is returned.


									HV*	perl_get_hv (char* name, I32 create)


								=item perl_get_sv


								Returns the SV of the specified Perl scalar.  If C<create> is set and the

								Perl variable does not exist then it will be created.  If C<create> is not

								set and the variable does not exist then NULL is returned.


									SV*	perl_get_sv (char* name, I32 create)


								=item perl_parse


								Tells a Perl interpreter to parse a Perl script.  See L<perlembed>.


								=item perl_require_pv


								Tells Perl to C<require> a module.


									void	perl_require_pv (char* pv)


								=item perl_run


								Tells a Perl interpreter to run.  See L<perlembed>.


								=item POPi


								Pops an integer off the stack.


									int	POPi()


								=item POPl


								Pops a long off the stack.


									long	POPl()


								=item POPp


								Pops a string off the stack.


									char*	POPp()


								=item POPn


								Pops a double off the stack.


									double	POPn()


								=item POPs


								Pops an SV off the stack.


									SV*	POPs()


								=item PUSHMARK


								Opening bracket for arguments on a callback.  See C<PUTBACK> and L<perlcall>.


									PUSHMARK(p)


								=item PUSHi


								Push an integer onto the stack.  The stack must have room for this element.

								Handles 'set' magic.  See C<XPUSHi>.


									void	PUSHi(int d)


								=item PUSHn


								Push a double onto the stack.  The stack must have room for this element.

								Handles 'set' magic.  See C<XPUSHn>.


									void	PUSHn(double d)


								=item PUSHp


								Push a string onto the stack.  The stack must have room for this element.

								The C<len> indicates the length of the string.  Handles 'set' magic.  See

								C<XPUSHp>.


									void	PUSHp(char *c, int len )


								=item PUSHs


								Push an SV onto the stack.  The stack must have room for this element.  Does

								not handle 'set' magic.  See C<XPUSHs>.


									void	PUSHs(sv)


								=item PUSHu


								Push an unsigned integer onto the stack.  The stack must have room for

								this element.  See C<XPUSHu>.


									void	PUSHu(unsigned int d)


								=item PUTBACK


								Closing bracket for XSUB arguments.  This is usually handled by C<xsubpp>.

								See C<PUSHMARK> and L<perlcall> for other uses.


									PUTBACK;


								=item Renew


								The XSUB-writer's interface to the C C<realloc> function.


									void*	Renew( void *ptr, int size, type )


								=item Renewc


								The XSUB-writer's interface to the C C<realloc> function, with cast.


									void*	Renewc( void *ptr, int size, type, cast )


								=item RETVAL


								Variable which is setup by C<xsubpp> to hold the return value for an XSUB.

								This is always the proper type for the XSUB.

								See L<perlxs/"The RETVAL Variable">.


								=item safefree


								The XSUB-writer's interface to the C C<free> function.


								=item safemalloc


								The XSUB-writer's interface to the C C<malloc> function.


								=item saferealloc


								The XSUB-writer's interface to the C C<realloc> function.


								=item savepv


								Copy a string to a safe spot.  This does not use an SV.


									char*	savepv (char* sv)


								=item savepvn


								Copy a string to a safe spot.  The C<len> indicates number of bytes to

								copy.  This does not use an SV.


									char*	savepvn (char* sv, I32 len)


								=item SAVETMPS


								Opening bracket for temporaries on a callback.  See C<FREETMPS> and

								L<perlcall>.


									SAVETMPS;


								=item SP


								Stack pointer.  This is usually handled by C<xsubpp>.  See C<dSP> and

								C<SPAGAIN>.


								=item SPAGAIN


								Refetch the stack pointer.  Used after a callback.  See L<perlcall>.


									SPAGAIN;


								=item ST


								Used to access elements on the XSUB's stack.


									SV*	ST(int x)


								=item strEQ


								Test two strings to see if they are equal.  Returns true or false.


									int	strEQ( char *s1, char *s2 )


								=item strGE


								Test two strings to see if the first, C<s1>, is greater than or equal to the

								second, C<s2>.  Returns true or false.


									int	strGE( char *s1, char *s2 )


								=item strGT


								Test two strings to see if the first, C<s1>, is greater than the second,

								C<s2>.  Returns true or false.


									int	strGT( char *s1, char *s2 )


								=item strLE


								Test two strings to see if the first, C<s1>, is less than or equal to the

								second, C<s2>.  Returns true or false.


									int	strLE( char *s1, char *s2 )


								=item strLT


								Test two strings to see if the first, C<s1>, is less than the second,

								C<s2>.  Returns true or false.


									int	strLT( char *s1, char *s2 )


								=item strNE


								Test two strings to see if they are different.  Returns true or false.


									int	strNE( char *s1, char *s2 )


								=item strnEQ


								Test two strings to see if they are equal.  The C<len> parameter indicates

								the number of bytes to compare.  Returns true or false.


									int	strnEQ( char *s1, char *s2 )


								=item strnNE


								Test two strings to see if they are different.  The C<len> parameter

								indicates the number of bytes to compare.  Returns true or false.


									int	strnNE( char *s1, char *s2, int len )


								=item sv_2mortal


								Marks an SV as mortal.  The SV will be destroyed when the current context

								ends.


									SV*	sv_2mortal (SV* sv)


								=item sv_bless


								Blesses an SV into a specified package.  The SV must be an RV.  The package

								must be designated by its stash (see C<gv_stashpv()>).  The reference count

								of the SV is unaffected.


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


								=item sv_catpv


								Concatenates the string onto the end of the string which is in the SV.

								Handles 'get' magic, but not 'set' magic.  See C<sv_catpv_mg>.


									void	sv_catpv (SV* sv, char* ptr)


								=item sv_catpv_mg


								Like C<sv_catpv>, but also handles 'set' magic.


									void	sv_catpv_mg (SV* sv, const char* ptr)


								=item sv_catpvn


								Concatenates the string onto the end of the string which is in the SV.  The

								C<len> indicates number of bytes to copy.  Handles 'get' magic, but not

								'set' magic.  See C<sv_catpvn_mg>.


									void	sv_catpvn (SV* sv, char* ptr, STRLEN len)


								=item sv_catpvn_mg


								Like C<sv_catpvn>, but also handles 'set' magic.


									void	sv_catpvn_mg (SV* sv, char* ptr, STRLEN len)


								=item sv_catpvf


								Processes its arguments like C<sprintf> and appends the formatted output

								to an SV.  Handles 'get' magic, but not 'set' magic.  C<SvSETMAGIC()> must

								typically be called after calling this function to handle 'set' magic.


									void	sv_catpvf (SV* sv, const char* pat, ...)


								=item sv_catpvf_mg


								Like C<sv_catpvf>, but also handles 'set' magic.


									void	sv_catpvf_mg (SV* sv, const char* pat, ...)


								=item sv_catsv


								Concatenates the string from SV C<ssv> onto the end of the string in SV

								C<dsv>.  Handles 'get' magic, but not 'set' magic.  See C<sv_catsv_mg>.


									void	sv_catsv (SV* dsv, SV* ssv)


								=item sv_catsv_mg


								Like C<sv_catsv>, but also handles 'set' magic.


									void	sv_catsv_mg (SV* dsv, SV* ssv)


								=item sv_chop


								Efficient removal of characters from the beginning of the string

								buffer.  SvPOK(sv) must be true and the C<ptr> must be a pointer to

								somewhere inside the string buffer.  The C<ptr> becomes the first

								character of the adjusted string.


									void    sv_chop(SV* sv, char *ptr)


								=item sv_cmp


								Compares the strings in two SVs.  Returns -1, 0, or 1 indicating whether the

								string in C<sv1> is less than, equal to, or greater than the string in

								C<sv2>.


									I32	sv_cmp (SV* sv1, SV* sv2)


								=item SvCUR


								Returns the length of the string which is in the SV.  See C<SvLEN>.


									int	SvCUR (SV* sv)


								=item SvCUR_set


								Set the length of the string which is in the SV.  See C<SvCUR>.


									void	SvCUR_set (SV* sv, int val)


								=item sv_dec


								Auto-decrement of the value in the SV.


									void	sv_dec (SV* sv)


								=item sv_derived_from


								Returns a boolean indicating whether the SV is derived from the specified

								class.  This is the function that implements C<UNIVERSAL::isa>.  It works

								for class names as well as for objects.


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


								=item SvEND


								Returns a pointer to the last character in the string which is in the SV.

								See C<SvCUR>.  Access the character as


									char*	SvEND(sv)


								=item sv_eq


								Returns a boolean indicating whether the strings in the two SVs are

								identical.


									I32	sv_eq (SV* sv1, SV* sv2)


								=item SvGETMAGIC


								Invokes C<mg_get> on an SV if it has 'get' magic.  This macro evaluates

								its argument more than once.


									void	SvGETMAGIC(SV *sv)


								=item SvGROW


								Expands the character buffer in the SV so that it has room for the

								indicated number of bytes (remember to reserve space for an extra

								trailing NUL character).  Calls C<sv_grow> to perform the expansion if

								necessary.  Returns a pointer to the character buffer.


									char*	SvGROW(SV* sv, STRLEN len)


								=item sv_grow


								Expands the character buffer in the SV.  This will use C<sv_unref> and will

								upgrade the SV to C<SVt_PV>.  Returns a pointer to the character buffer.

								Use C<SvGROW>.


								=item sv_inc


								Auto-increment of the value in the SV.


									void	sv_inc (SV* sv)


								=item sv_insert


								Inserts a string at the specified offset/length within the SV.

								Similar to the Perl substr() function.


									void	sv_insert(SV *sv, STRLEN offset, STRLEN len,

											  char *str, STRLEN strlen)


								=item SvIOK


								Returns a boolean indicating whether the SV contains an integer.


									int	SvIOK (SV* SV)


								=item SvIOK_off


								Unsets the IV status of an SV.


									void	SvIOK_off (SV* sv)


								=item SvIOK_on


								Tells an SV that it is an integer.


									void	SvIOK_on (SV* sv)


								=item SvIOK_only


								Tells an SV that it is an integer and disables all other OK bits.


									void	SvIOK_only (SV* sv)


								=item SvIOKp


								Returns a boolean indicating whether the SV contains an integer.  Checks the

								B<private> setting.  Use C<SvIOK>.


									int	SvIOKp (SV* SV)


								=item sv_isa


								Returns a boolean indicating whether the SV is blessed into the specified

								class.  This does not check for subtypes; use C<sv_derived_from> to verify

								an inheritance relationship.


									int	sv_isa (SV* sv, char* name)


								=item sv_isobject


								Returns a boolean indicating whether the SV is an RV pointing to a blessed

								object.  If the SV is not an RV, or if the object is not blessed, then this

								will return false.


									int	sv_isobject (SV* sv)


								=item SvIV


								Coerces the given SV to an integer and returns it.


									int SvIV (SV* sv)


								=item SvIVX


								Returns the integer which is stored in the SV, assuming SvIOK is true.


									int	SvIVX (SV* sv)


								=item SvLEN


								Returns the size of the string buffer in the SV.  See C<SvCUR>.


									int	SvLEN (SV* sv)


								=item sv_len


								Returns the length of the string in the SV.  Use C<SvCUR>.


									STRLEN	sv_len (SV* sv)


								=item sv_magic


								Adds magic to an SV.


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


								=item sv_mortalcopy


								Creates a new SV which is a copy of the original SV.  The new SV is marked

								as mortal.


									SV*	sv_mortalcopy (SV* oldsv)


								=item sv_newmortal


								Creates a new SV which is mortal.  The reference count of the SV is set to 1.


									SV*	sv_newmortal (void)


								=item SvNIOK


								Returns a boolean indicating whether the SV contains a number, integer or

								double.


									int	SvNIOK (SV* SV)


								=item SvNIOK_off


								Unsets the NV/IV status of an SV.


									void	SvNIOK_off (SV* sv)


								=item SvNIOKp


								Returns a boolean indicating whether the SV contains a number, integer or

								double.  Checks the B<private> setting.  Use C<SvNIOK>.


									int	SvNIOKp (SV* SV)


								=item PL_sv_no


								This is the C<false> SV.  See C<PL_sv_yes>.  Always refer to this as C<&PL_sv_no>.


								=item SvNOK


								Returns a boolean indicating whether the SV contains a double.


									int	SvNOK (SV* SV)


								=item SvNOK_off


								Unsets the NV status of an SV.


									void	SvNOK_off (SV* sv)


								=item SvNOK_on


								Tells an SV that it is a double.


									void	SvNOK_on (SV* sv)


								=item SvNOK_only


								Tells an SV that it is a double and disables all other OK bits.


									void	SvNOK_only (SV* sv)


								=item SvNOKp


								Returns a boolean indicating whether the SV contains a double.  Checks the

								B<private> setting.  Use C<SvNOK>.


									int	SvNOKp (SV* SV)


								=item SvNV


								Coerce the given SV to a double and return it.


									double	SvNV (SV* sv)


								=item SvNVX


								Returns the double which is stored in the SV, assuming SvNOK is true.


									double	SvNVX (SV* sv)


								=item SvOK


								Returns a boolean indicating whether the value is an SV.


									int	SvOK (SV* sv)


								=item SvOOK


								Returns a boolean indicating whether the SvIVX is a valid offset value

								for the SvPVX.  This hack is used internally to speed up removal of

								characters from the beginning of a SvPV.  When SvOOK is true, then the

								start of the allocated string buffer is really (SvPVX - SvIVX).


									int     SvOOK(SV* sv)


								=item SvPOK


								Returns a boolean indicating whether the SV contains a character string.


									int	SvPOK (SV* SV)


								=item SvPOK_off


								Unsets the PV status of an SV.


									void	SvPOK_off (SV* sv)


								=item SvPOK_on


								Tells an SV that it is a string.


									void	SvPOK_on (SV* sv)


								=item SvPOK_only


								Tells an SV that it is a string and disables all other OK bits.


									void	SvPOK_only (SV* sv)


								=item SvPOKp


								Returns a boolean indicating whether the SV contains a character string.

								Checks the B<private> setting.  Use C<SvPOK>.


									int	SvPOKp (SV* SV)


								=item SvPV


								Returns a pointer to the string in the SV, or a stringified form of the SV

								if the SV does not contain a string.  Handles 'get' magic.


									char*	SvPV (SV* sv, STRLEN len)


								=item SvPV_force


								Like <SvPV> but will force the SV into becoming a string (SvPOK).  You

								want force if you are going to update the SvPVX directly.


									char*	SvPV_force(SV* sv, STRLEN len)


								=item SvPVX


								Returns a pointer to the string in the SV.  The SV must contain a string.


									char*	SvPVX (SV* sv)


								=item SvREFCNT


								Returns the value of the object's reference count.


									int	SvREFCNT (SV* sv)


								=item SvREFCNT_dec


								Decrements the reference count of the given SV.


									void	SvREFCNT_dec (SV* sv)


								=item SvREFCNT_inc


								Increments the reference count of the given SV.


									void	SvREFCNT_inc (SV* sv)


								=item SvROK


								Tests if the SV is an RV.


									int	SvROK (SV* sv)


								=item SvROK_off


								Unsets the RV status of an SV.


									void	SvROK_off (SV* sv)


								=item SvROK_on


								Tells an SV that it is an RV.


									void	SvROK_on (SV* sv)


								=item SvRV


								Dereferences an RV to return the SV.


									SV*	SvRV (SV* sv)


								=item SvSETMAGIC


								Invokes C<mg_set> on an SV if it has 'set' magic.  This macro evaluates

								its argument more than once.


									void	SvSETMAGIC( SV *sv )


								=item sv_setiv


								Copies an integer into the given SV.  Does not handle 'set' magic.

								See C<sv_setiv_mg>.


									void	sv_setiv (SV* sv, IV num)


								=item sv_setiv_mg


								Like C<sv_setiv>, but also handles 'set' magic.


									void	sv_setiv_mg (SV* sv, IV num)


								=item sv_setnv


								Copies a double into the given SV.  Does not handle 'set' magic.

								See C<sv_setnv_mg>.


									void	sv_setnv (SV* sv, double num)


								=item sv_setnv_mg


								Like C<sv_setnv>, but also handles 'set' magic.


									void	sv_setnv_mg (SV* sv, double num)


								=item sv_setpv


								Copies a string into an SV.  The string must be null-terminated.

								Does not handle 'set' magic.  See C<sv_setpv_mg>.


									void	sv_setpv (SV* sv, const char* ptr)


								=item sv_setpv_mg


								Like C<sv_setpv>, but also handles 'set' magic.


									void	sv_setpv_mg (SV* sv, const char* ptr)


								=item sv_setpviv


								Copies an integer into the given SV, also updating its string value.

								Does not handle 'set' magic.  See C<sv_setpviv_mg>.


									void	sv_setpviv (SV* sv, IV num)


								=item sv_setpviv_mg


								Like C<sv_setpviv>, but also handles 'set' magic.


									void	sv_setpviv_mg (SV* sv, IV num)


								=item sv_setpvn


								Copies a string into an SV.  The C<len> parameter indicates the number of

								bytes to be copied.  Does not handle 'set' magic.  See C<sv_setpvn_mg>.


									void	sv_setpvn (SV* sv, const char* ptr, STRLEN len)


								=item sv_setpvn_mg


								Like C<sv_setpvn>, but also handles 'set' magic.


									void	sv_setpvn_mg (SV* sv, const char* ptr, STRLEN len)


								=item sv_setpvf


								Processes its arguments like C<sprintf> and sets an SV to the formatted

								output.  Does not handle 'set' magic.  See C<sv_setpvf_mg>.


									void	sv_setpvf (SV* sv, const char* pat, ...)


								=item sv_setpvf_mg


								Like C<sv_setpvf>, but also handles 'set' magic.


									void	sv_setpvf_mg (SV* sv, const char* pat, ...)


								=item sv_setref_iv


								Copies an integer into a new SV, optionally blessing the SV.  The C<rv>

								argument will be upgraded to an RV.  That RV will be modified to point to

								the new SV.  The C<classname> argument indicates the package for the

								blessing.  Set C<classname> to C<Nullch> to avoid the blessing.  The new SV

								will be returned and will have a reference count of 1.


									SV*	sv_setref_iv (SV *rv, char *classname, IV iv)


								=item sv_setref_nv


								Copies a double into a new SV, optionally blessing the SV.  The C<rv>

								argument will be upgraded to an RV.  That RV will be modified to point to

								the new SV.  The C<classname> argument indicates the package for the

								blessing.  Set C<classname> to C<Nullch> to avoid the blessing.  The new SV

								will be returned and will have a reference count of 1.


									SV*	sv_setref_nv (SV *rv, char *classname, double nv)


								=item sv_setref_pv


								Copies a pointer into a new SV, optionally blessing the SV.  The C<rv>

								argument will be upgraded to an RV.  That RV will be modified to point to

								the new SV.  If the C<pv> argument is NULL then C<PL_sv_undef> will be placed

								into the SV.  The C<classname> argument indicates the package for the

								blessing.  Set C<classname> to C<Nullch> to avoid the blessing.  The new SV

								will be returned and will have a reference count of 1.


									SV*	sv_setref_pv (SV *rv, char *classname, void* pv)


								Do not use with integral Perl types such as HV, AV, SV, CV, because those

								objects will become corrupted by the pointer copy process.


								Note that C<sv_setref_pvn> copies the string while this copies the pointer.


								=item sv_setref_pvn


								Copies a string into a new SV, optionally blessing the SV.  The length of the

								string must be specified with C<n>.  The C<rv> argument will be upgraded to

								an RV.  That RV will be modified to point to the new SV.  The C<classname>

								argument indicates the package for the blessing.  Set C<classname> to

								C<Nullch> to avoid the blessing.  The new SV will be returned and will have

								a reference count of 1.


									SV*	sv_setref_pvn (SV *rv, char *classname, char* pv, I32 n)


								Note that C<sv_setref_pv> copies the pointer while this copies the string.


								=item SvSetSV


								Calls C<sv_setsv> if dsv is not the same as ssv.  May evaluate arguments

								more than once.


									void	SvSetSV (SV* dsv, SV* ssv)


								=item SvSetSV_nosteal


								Calls a non-destructive version of C<sv_setsv> if dsv is not the same as ssv.

								May evaluate arguments more than once.


									void	SvSetSV_nosteal (SV* dsv, SV* ssv)


								=item sv_setsv


								Copies the contents of the source SV C<ssv> into the destination SV C<dsv>.

								The source SV may be destroyed if it is mortal.  Does not handle 'set' magic.

								See the macro forms C<SvSetSV>, C<SvSetSV_nosteal> and C<sv_setsv_mg>.


									void	sv_setsv (SV* dsv, SV* ssv)


								=item sv_setsv_mg


								Like C<sv_setsv>, but also handles 'set' magic.


									void	sv_setsv_mg (SV* dsv, SV* ssv)


								=item sv_setuv


								Copies an unsigned integer into the given SV.  Does not handle 'set' magic.

								See C<sv_setuv_mg>.


									void	sv_setuv (SV* sv, UV num)


								=item sv_setuv_mg


								Like C<sv_setuv>, but also handles 'set' magic.


									void	sv_setuv_mg (SV* sv, UV num)


								=item SvSTASH


								Returns the stash of the SV.


									HV*	SvSTASH (SV* sv)


								=item SvTAINT


								Taints an SV if tainting is enabled


									void	SvTAINT (SV* sv)


								=item SvTAINTED


								Checks to see if an SV is tainted. Returns TRUE if it is, FALSE if not.


									int	SvTAINTED (SV* sv)


								=item SvTAINTED_off


								Untaints an SV. Be I<very> careful with this routine, as it short-circuits

								some of Perl's fundamental security features. XS module authors should

								not use this function unless they fully understand all the implications

								of unconditionally untainting the value. Untainting should be done in

								the standard perl fashion, via a carefully crafted regexp, rather than

								directly untainting variables.


									void	SvTAINTED_off (SV* sv)


								=item SvTAINTED_on


								Marks an SV as tainted.


									void	SvTAINTED_on (SV* sv)


								=item SVt_IV


								Integer type flag for scalars.  See C<svtype>.


								=item SVt_PV


								Pointer type flag for scalars.  See C<svtype>.


								=item SVt_PVAV


								Type flag for arrays.  See C<svtype>.


								=item SVt_PVCV


								Type flag for code refs.  See C<svtype>.


								=item SVt_PVHV


								Type flag for hashes.  See C<svtype>.


								=item SVt_PVMG


								Type flag for blessed scalars.  See C<svtype>.


								=item SVt_NV


								Double type flag for scalars.  See C<svtype>.


								=item SvTRUE


								Returns a boolean indicating whether Perl would evaluate the SV as true or

								false, defined or undefined.  Does not handle 'get' magic.


									int	SvTRUE (SV* sv)


								=item SvTYPE


								Returns the type of the SV.  See C<svtype>.


									svtype	SvTYPE (SV* sv)


								=item svtype


								An enum of flags for Perl types.  These are found in the file B<sv.h> in the

								C<svtype> enum.  Test these flags with the C<SvTYPE> macro.


								=item PL_sv_undef


								This is the C<undef> SV.  Always refer to this as C<&PL_sv_undef>.


								=item sv_unref


								Unsets the RV status of the SV, and decrements the reference count of

								whatever was being referenced by the RV.  This can almost be thought of

								as a reversal of C<newSVrv>.  See C<SvROK_off>.


									void    sv_unref (SV* sv)


								=item SvUPGRADE


								Used to upgrade an SV to a more complex form.  Uses C<sv_upgrade> to perform

								the upgrade if necessary.  See C<svtype>.


									bool    SvUPGRADE (SV* sv, svtype mt)


								=item sv_upgrade


								Upgrade an SV to a more complex form.  Use C<SvUPGRADE>.  See C<svtype>.


								=item sv_usepvn


								Tells an SV to use C<ptr> to find its string value.  Normally the string is

								stored inside the SV but sv_usepvn allows the SV to use an outside string.

								The C<ptr> should point to memory that was allocated by C<malloc>.  The

								string length, C<len>, must be supplied.  This function will realloc the

								memory pointed to by C<ptr>, so that pointer should not be freed or used by

								the programmer after giving it to sv_usepvn.  Does not handle 'set' magic.

								See C<sv_usepvn_mg>.


									void	sv_usepvn (SV* sv, char* ptr, STRLEN len)


								=item sv_usepvn_mg


								Like C<sv_usepvn>, but also handles 'set' magic.


									void	sv_usepvn_mg (SV* sv, char* ptr, STRLEN len)


								=item sv_vcatpvfn(sv, pat, patlen, args, svargs, svmax, used_locale)


								Processes its arguments like C<vsprintf> and appends the formatted output

								to an SV.  Uses an array of SVs if the C style variable argument list is

								missing (NULL).  Indicates if locale information has been used for formatting.


									void	sv_catpvfn _((SV* sv, const char* pat, STRLEN patlen,

											      va_list *args, SV **svargs, I32 svmax,

											      bool *used_locale));


								=item sv_vsetpvfn(sv, pat, patlen, args, svargs, svmax, used_locale)


								Works like C<vcatpvfn> but copies the text into the SV instead of

								appending it.


									void	sv_setpvfn _((SV* sv, const char* pat, STRLEN patlen,

											      va_list *args, SV **svargs, I32 svmax,

											      bool *used_locale));


								=item SvUV


								Coerces the given SV to an unsigned integer and returns it.


									UV      SvUV(SV* sv)


								=item SvUVX


								Returns the unsigned integer which is stored in the SV, assuming SvIOK is true.


									UV      SvUVX(SV* sv)


								=item PL_sv_yes


								This is the C<true> SV.  See C<PL_sv_no>.  Always refer to this as C<&PL_sv_yes>.


								=item THIS


								Variable which is setup by C<xsubpp> to designate the object in a C++ XSUB.

								This is always the proper type for the C++ object.  See C<CLASS> and

								L<perlxs/"Using XS With C++">.


								=item toLOWER


								Converts the specified character to lowercase.


									int	toLOWER (char c)


								=item toUPPER


								Converts the specified character to uppercase.


									int	toUPPER (char c)


								=item warn


								This is the XSUB-writer's interface to Perl's C<warn> function.  Use this

								function the same way you use the C C<printf> function.  See C<croak()>.


								=item XPUSHi


								Push an integer onto the stack, extending the stack if necessary.  Handles

								'set' magic. See C<PUSHi>.


									XPUSHi(int d)


								=item XPUSHn


								Push a double onto the stack, extending the stack if necessary.  Handles 'set'

								magic.  See C<PUSHn>.


									XPUSHn(double d)


								=item XPUSHp


								Push a string onto the stack, extending the stack if necessary.  The C<len>

								indicates the length of the string.  Handles 'set' magic.  See C<PUSHp>.


									XPUSHp(char *c, int len)


								=item XPUSHs


								Push an SV onto the stack, extending the stack if necessary.  Does not

								handle 'set' magic.  See C<PUSHs>.


									XPUSHs(sv)


								=item XPUSHu


								Push an unsigned integer onto the stack, extending the stack if

								necessary.  See C<PUSHu>.


								=item XS


								Macro to declare an XSUB and its C parameter list.  This is handled by

								C<xsubpp>.


								=item XSRETURN


								Return from XSUB, indicating number of items on the stack.  This is usually

								handled by C<xsubpp>.


									XSRETURN(int x)


								=item XSRETURN_EMPTY


								Return an empty list from an XSUB immediately.


									XSRETURN_EMPTY;


								=item XSRETURN_IV


								Return an integer from an XSUB immediately.  Uses C<XST_mIV>.


									XSRETURN_IV(IV v)


								=item XSRETURN_NO


								Return C<&PL_sv_no> from an XSUB immediately.  Uses C<XST_mNO>.


									XSRETURN_NO;


								=item XSRETURN_NV


								Return an double from an XSUB immediately.  Uses C<XST_mNV>.


									XSRETURN_NV(NV v)


								=item XSRETURN_PV


								Return a copy of a string from an XSUB immediately.  Uses C<XST_mPV>.


									XSRETURN_PV(char *v)


								=item XSRETURN_UNDEF


								Return C<&PL_sv_undef> from an XSUB immediately.  Uses C<XST_mUNDEF>.


									XSRETURN_UNDEF;


								=item XSRETURN_YES


								Return C<&PL_sv_yes> from an XSUB immediately.  Uses C<XST_mYES>.


									XSRETURN_YES;


								=item XST_mIV


								Place an integer into the specified position C<i> on the stack.  The value is

								stored in a new mortal SV.


									XST_mIV( int i, IV v )


								=item XST_mNV


								Place a double into the specified position C<i> on the stack.  The value is

								stored in a new mortal SV.


									XST_mNV( int i, NV v )


								=item XST_mNO


								Place C<&PL_sv_no> into the specified position C<i> on the stack.


									XST_mNO( int i )


								=item XST_mPV


								Place a copy of a string into the specified position C<i> on the stack.  The

								value is stored in a new mortal SV.


									XST_mPV( int i, char *v )


								=item XST_mUNDEF


								Place C<&PL_sv_undef> into the specified position C<i> on the stack.


									XST_mUNDEF( int i )


								=item XST_mYES


								Place C<&PL_sv_yes> into the specified position C<i> on the stack.


									XST_mYES( int i )


								=item XS_VERSION


								The version identifier for an XS module.  This is usually handled

								automatically by C<ExtUtils::MakeMaker>.  See C<XS_VERSION_BOOTCHECK>.


								=item XS_VERSION_BOOTCHECK


								Macro to verify that a PM module's $VERSION variable matches the XS module's

								C<XS_VERSION> variable.  This is usually handled automatically by

								C<xsubpp>.  See L<perlxs/"The VERSIONCHECK: Keyword">.


								=item Zero


								The XSUB-writer's interface to the C C<memzero> function.  The C<d> is the

								destination, C<n> is the number of items, and C<t> is the type.


									void	Zero( d, n, t )


								=back


								=head1 AUTHORS


								Until May 1997, this document was maintained by Jeff Okamoto

								<[email protected]>.  It is now maintained as part of Perl itself.


								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 <[email protected]>.