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=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(const char*, int);
    SV*  newSVpvn(const char*, int);
    SV*  newSVpvf(const char*, ...);
    SV*  newSVsv(SV*);

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

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

Notice that you can choose to specify the length of the string to be
assigned by using 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)
    SvPV_nolen(SV*)

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 C<SvPV_nolen> macro.
Historically the C<SvPV> macro with the global variable C<PL_na> has been
used in this case.  But that 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*, const char*);
    void  sv_catpvn(SV*, const char*, STRLEN);
    void  sv_catpvf(SV*, const char*, ...);
    void  sv_catpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool);
    void  sv_catsv(SV*, SV*);

The first function calculates the length of the string to be appended by
using 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*, const char* key, U32 klen, SV* val, U32 hash);
    SV**  hv_fetch(HV*, const 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*, const char* key, U32 klen);
    SV*   hv_delete(HV*, const 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++;
    hash = hash + (hash >> 5);                  /* after 5.6 */

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

See 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, const 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, const char* classname, IV iv);
        SV* sv_setref_nv(SV* rv, const 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, const 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, const char* classname, PV iv, STRLEN length);

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

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

Tests whether the SV is a reference to a blessed object.

        int  sv_isobject(SV* sv);

Tests whether the SV is derived from the specified class. SV can be either
a reference to a blessed object or a string containing a class name. This
is the function implementing the C<UNIVERSAL::isa> functionality.

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

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

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

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

The first function takes a literal string, the second uses the string stored
in the SV.  Remember that a stash is just a hash table, so you get back an
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, const 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, const 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(const char*, I32);
    I32  perl_call_method(const char*, I32);
    I32  perl_call_argv(const 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 The Perl API

WARNING: This information is subject to radical changes prior to
the Perl 5.6 release.  Use with caution.

=head2 Background and PERL_IMPLICIT_CONTEXT

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

Four macros control the way Perl is built: PERL_IMPLICIT_CONTEXT
(build for multiple interpreters?), MULTIPLICITY (we pass around an
C interpreter structure as the first argument), USE_THREADS (we pass
around a thread as the first argument), and PERL_OBJECT (we build a
C++ class for the interpreter so the Perl API implementation has a
C<this> object).  If PERL_IMPLICIT_CONTEXT is not defined, then
subroutines take no first argument.

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

First problem: deciding which functions will be C++ public methods and
which will be private.  Those functions whose names begin C<Perl_> are
public, and those whose names begin C<S_> are protected (think "S" for
"Secret").  You can't call them from C++, and should not call them
from C.  If you find yourself calling an C<S_> function, consider your
code broken (even though it works, it may not do so forever).

Some functions have no prefix (e.g., restore_rsfp in toke.c).  These
are not parts of the object or pseudo-structure because you need to
pass pointers to them to other subroutines.

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

  STATIC void
  S_incline(pTHX_ char *s)

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

A public function (i.e. part of the API) begins like this:

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

C<pTHX_> is one of a number of macros (in perl.h) that hide the
details of the interpreter's context.  THX stands for "thread", "this",
or "thingy", as the case may be.  (And no, George Lucas is not involved. :-)
The first character could be 'p' for a B<p>rototype, 'a' for B<a>rgument,
or 'd' for B<d>eclaration.

When Perl is built without PERL_IMPLICIT_CONTEXT, there is no first
argument containing the interpreter's context.  The trailing underscore
in the pTHX_ macro indicates that the macro expansion needs a comma
after the context argument because other arguments follow it.  If
PERL_IMPLICIT_CONTEXT is not defined, pTHX_ will be ignored, and the
subroutine is not prototyped to take an argument.  The form of the
macro without the trailing underscore is used when there are no
explicit arguments.

When an core function calls another, it must pass the context.  This
is normally hidden via macros.  Consider C<sv_setsv>.  It expands
something like this:

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

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

    sv_setsv(foo, bar);

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

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

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

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

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

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

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

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

You can ignore [pad]THX[xo] when browsing the Perl headers/sources.
Those are strictly for use within the core.  Extensions and embedders
need only be aware of [pad]THX.

=head2 How do I use all this in extensions?

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

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

        sv_setsv(asv, bsv);

in your extesion will translate to this:

        Perl_sv_setsv(GetPerlInterpreter(), asv, bsv);

when PERL_IMPLICIT_CONTEXT is in effect, or to this otherwise:

        Perl_sv_setsv(asv, bsv);

You have to do nothing new in your extension to get this; since
the Perl library provides GetPerlInterpreter(), it will all just
work.

The second, more efficient way is to use the following template for
your Foo.xs:

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

        static my_private_function(int arg1, int arg2);

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

        [... etc ...]

        MODULE = Foo            PACKAGE = Foo

        /* typical XSUB */

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

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

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


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

        /* pTHX_ only needed for functions that call Perl API */
        static my_private_function(pTHX_ int arg1, int arg2);

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

        [... etc ...]

        MODULE = Foo            PACKAGE = Foo

        /* typical XSUB */

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

This implementation never has to fetch the context using a function
call, since it is always passed as an extra argument.  Depending on
your needs for simplicity or efficiency, you may mix the previous
two approaches freely.

Never say C<pTHX,> yourself--always use the form of the macro with the
underscore for functions that take explicit arguments, or the form
without the argument for functions with no explicit arguments.

=head2 Future Plans and PERL_IMPLICIT_SYS

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

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

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

=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 may be disabled in a future release.

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, const 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, const char* name)
        GV*     gv_fetchmethod_autoload (HV* stash, const 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 (const 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, const 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, const 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, const 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, const 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 *, const 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
either declare a local variable and use that instead or to use the C<SvPV_nolen>
macro.

=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, Perl will compute the length using
strlen().  For efficiency, consider using C<newSVpvn> instead.

        SV*     newSVpv (const 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.  Note that if C<len> is zero, Perl will create a zero length 
string.  You are responsible for ensuring that the source string is at least
C<len> bytes long.

        SV*     newSVpvn (const 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, const 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 (const 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 (const char* methname, I32 flags)

=item perl_call_pv

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

        I32     perl_call_pv (const 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 (const 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 (const char* name, I32 create)

=item perl_get_cv

Returns the CV of the specified Perl subroutine.  If C<create> is set and
the Perl subroutine does not exist then it will be declared (which has
the same effect as saying C<sub name;>).  If C<create> is not
set and the subroutine does not exist then NULL is returned.

        CV*     perl_get_cv (const 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 (const 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 (const 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 (const 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 (const 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 (const 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, const char* ptr)

=item sv_catpv_mg

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

        void    sv_catpvn (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, const char* ptr, STRLEN len)

=item sv_catpvn_mg

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

        void    sv_catpvn_mg (SV* sv, const 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, const 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, const 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, const 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 SvPV_nolen

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_nolen (SV* sv)

=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
<okamoto@corp.hp.com>.  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 <roehrich@cray.com>.