pod/perlipc.pod patch

[perl5.git] / pod / perlguts.pod

=head1 NAME

perlguts - Perl's Internal Functions

=head1 DESCRIPTION

This document attempts to describe some of the internal functions of the
Perl executable.  It is far from complete and probably contains many errors.
Please refer any questions or comments to the author below.

=head1 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 large enough to hold either an
integer or a pointer.

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 four routines are:

    SV*  newSViv(IV);
    SV*  newSVnv(double);
    SV*  newSVpv(char*, int);
    SV*  newSVsv(SV*);

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

    void  sv_setiv(SV*, IV);
    void  sv_setnv(SV*, double);
    void  sv_setpvn(SV*, char*, int)
    void  sv_setpv(SV*, char*);
    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> 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.

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

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

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

In the C<SvPV> macro, the length of the string returned is placed into the
variable C<len> (this is a macro, so you do I<not> use C<&len>).  If you do not
care what the length of the data is, use the global variable C<na>.  Remember,
however, that Perl allows arbitrary strings of data that may both contain
NULs and not be terminated by a NUL.

If you simply 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.

If you have an SV and want to know what kind of data Perl thinks is stored
in it, you can use the following macros to check the type of SV you have.

    SvIOK(SV*)
    SvNOK(SV*)
    SvPOK(SV*)

You can get and set the current length of the string stored in an SV with
the following macros:

    SvCUR(SV*)
    SvCUR_set(SV*, I32 val)

You can also get a pointer to the end of the string stored in the SV
with the macro:

    SvEND(SV*)

But note that these last three macros are valid only if C<SvPOK()> is true.

If you want to append something to the end of string stored in an C<SV*>,
you can use the following functions:

    void  sv_catpv(SV*, char*);
    void  sv_catpvn(SV*, char*, int);
    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 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.

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("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<sv_undef>.  Its
address can be used whenever an C<SV*> is needed.

There are also the two values C<sv_yes> and C<sv_no>, which contain Boolean
TRUE and FALSE values, respectively.  Like C<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<&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 plain weird results.  Change the zero to C<&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 the section on L<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.  Since 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 just use the C<Sv*V> macros.

=head2 Working with AVs

There are two ways to create and load an AV.  The first method just 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*); /* Returns highest index value in array */

    SV**  av_fetch(AV*, I32 key, I32 lval);
            /* Fetches value at key offset, but it stores an undef value
               at the offset if lval is non-zero */
    SV**  av_store(AV*, I32 key, SV* val);
            /* Stores val at offset key */

Take note that C<av_fetch> and C<av_store> return C<SV**>s, not C<SV*>s.

    void  av_clear(AV*);
            /* Clear out all elements, but leave the array */
    void  av_undef(AV*);
            /* Undefines the array, removing all elements */
    void  av_extend(AV*, I32 key);
            /* Extend the array to a total of key elements */

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("varname", FALSE);

This returns NULL if the variable does not exist.

=head2 Working with HVs

To create an HV, you use the following routine:

    HV*  newHV();

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

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

The C<klen> parameter is the length of the key being passed in.  The C<val>
argument contains the SV pointer to the scalar being stored, and C<hash> is
the pre-computed 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.

Remember that C<hv_store> and C<hv_fetch> return C<SV**>s and not just
C<SV*>.  In order to access the scalar value, you must first dereference
the return value.  However, you should check to make sure that the return
value is not NULL before dereferencing it.

These two functions check if a hash table entry exists, and deletes it.

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

And more miscellaneous functions:

    void   hv_clear(HV*);
            /* Clears all entries in hash table */
    void   hv_undef(HV*);
            /* Undefines the hash table */

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("varname", FALSE);

This returns NULL if the variable does not exist.

The hash algorithm, for those who are interested, is:

    i = klen;
    hash = 0;
    s = key;
    while (i--)
	hash = hash * 33 + *s++;

=head2 References

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

To create a reference, use the following command:

    SV* newRV((SV*) thing);

The C<thing> argument can be any of an C<SV*>, C<AV*>, or C<HV*>.  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 actually discover what the reference refers to, you must use the following
macro and then check the value returned.

    SvTYPE(SvRV(SV*))

The most useful types that will be returned are:

    SVt_IV    Scalar
    SVt_NV    Scalar
    SVt_PV    Scalar
    SVt_PVAV  Array
    SVt_PVHV  Hash
    SVt_PVCV  Code
    SVt_PVMG  Blessed Scalar

=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 the section on L<Stashes>
for information on converting class names into stashes.

/* Still under construction */

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

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

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

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

Copies pointer (I<not a string!>) into an SV whose reference is rv.
SV is blessed if classname is non-null.

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

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

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

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

=head1 Creating New Variables

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

    SV*  perl_get_sv("varname", TRUE);
    AV*  perl_get_av("varname", TRUE);
    HV*  perl_get_hv("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 bits that may be OR'ed with the TRUE argument to enable
certain extra features.  Those bits are:

    0x02  Marks the variable as multiply defined, thus preventing the
	  "Identifier <varname> used only once: possible typo" warning.
    0x04  Issues a "Had to create <varname> unexpectedly" warning if
	  the variable didn't actually exist.  This is useful if
	  you expected the variable to already exist and want to propagate
	  this warning back to the user.

If the C<varname> argument does not contain a package specifier, it is
created in the current package.

=head1 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 timezone'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 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>.

=head1 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 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/EXAMPLE/"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<SAVEI16(I16 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(p)>

=item C<SAVEPPTR(s)>

These macros arrange things to restore the value of pointers C<s> and
C<p>. C<p> must be a pointer of a type which survives conversion to
C<SV*> and back, C<s> 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(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):

=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 *>.

=item C<void save_nogv(GV *gv)>

Will postpone destruction of a I<stub> glob.

=back

=head1 Mortality

Perl uses an reference count-driven garbage collection mechanism. SV's,
AV's, or HV's (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 they will be destroyed and their memory made available for reuse.

This normally doesn't happen at the Perl level unless a variable is
undef'ed.  At the internal level, however, reference counts can be
manipulated with the following macros:

    int SvREFCNT(SV* sv);
    void 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> function, as you should recall,
creates a reference to the specified argument.  As a side effect, it
increments the argument's reference count, which is ok in most
circumstances.  But imagine you want to return a reference from an XS
function.  You create a new SV which initially has a reference count
of 1.  Then you call C<newRV>, passing the just-created SV.  This returns
the reference as a new SV, but the reference count of the SV you passed
to C<newRV> has been incremented to 2.  Now you return the reference 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 1
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 call C<SvREFCNT_dec> on the SV after
C<newRV> has returned.  Then, if and when the reference is destroyed,
the reference count of the SV will go to 0 and also be destroyed, stopping
any memory leak.

There are some convenience functions available that can help with this
process.  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 the "current context" is left.
Generally the "current context" means a single Perl statement, such as
a call to an XSUB function.

"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.  Doing the latter can
cause a variable to become invalid prematurely.

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, the third creates a mortal copy of an existing SV (possibly
destroying it in the process).

The mortal routines are not just for SVs -- AVs and HVs can be made mortal
by passing their address (and casting them to C<SV*>) to the C<sv_2mortal> or
C<sv_mortalcopy> routines.

I<From Ilya:>
Beware that the sv_2mortal() call is eventually equivalent to
svREFCNT_dec(). A value can happily be mortal in two different contexts,
and it will be svREFCNT_dec()ed twice, once on exit from these
contexts. It can also be mortal twice in the same context. This means
that you should be very careful to make a value mortal exactly as many
times as it is needed. The value that go to the Perl stack I<should>
be mortal.


=head1 Stashes

A stash is a hash table (associative array) 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 called a
GV (for 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
    File Handle
    Directory Handle
    Format
    Subroutine

Perl stores various stashes in a separate GV structure (for global
variable) but represents them with an HV structure.  The keys in this
larger GV are the various package names; the values are the C<GV*>s
which are stashes.  It may help to think of a stash purely as an HV,
and that the term "GV" means the global variable hash.

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

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

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

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

Alternately, if you have an SV that is a blessed reference, you can find
out the stash pointer by using:

    HV*  SvSTASH(SvRV(SV*));

then use the following to get the package name itself:

    char*  HvNAME(HV* stash);

If you need to return a blessed value to your Perl script, 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>.

=head1 Magic

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

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

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

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

=head2 Assigning Magic

Perl adds magic to an SV using the sv_magic function:

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

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

If C<sv> is not already magical, Perl uses the C<SvUPGRADE> macro to
set the C<SVt_PVMG> flag for the C<sv>.  Perl then continues by adding
it to the beginning of the linked list of magical features.  Any prior
entry of the same type of magic is deleted.  Note that this can be
overridden, and multiple instances of the same type of magic can be
associated with an SV.

The C<name> and C<namlem> arguments are used to associate a string with
the magic, typically the name of a variable. C<namlem> is stored in the
C<mg_len> field and if C<name> is non-null and C<namlem> >= 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 magicalness
    -------  ------              -------------------
    \0       vtbl_sv             Regexp???
    A        vtbl_amagic         Operator Overloading
    a        vtbl_amagicelem     Operator Overloading
    c        0                   Used in Operator Overloading
    B        vtbl_bm             Boyer-Moore???
    E        vtbl_env            %ENV hash
    e        vtbl_envelem        %ENV hash element
    g        vtbl_mglob          Regexp /g flag???
    I        vtbl_isa            @ISA array
    i        vtbl_isaelem        @ISA array element
    L        0 (but sets RMAGICAL)     Perl Module/Debugger???
    l        vtbl_dbline         Debugger?
    o        vtbl_collxfrm       Locale Collation
    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            Signal Hash
    s        vtbl_sigelem        Signal Hash element
    t        vtbl_taint          Taintedness
    U        vtbl_uvar	         ???
    v        vtbl_vec	         Vector
    x        vtbl_substr         Substring???
    *        vtbl_glob           GV???
    #        vtbl_arylen         Array Length
    .        vtbl_pos	         $. scalar variable
    ~        Reserved for extensions, but multiple extensions may clash

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

=head2 Finding Magic

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

This routine returns a pointer to the C<MAGIC> structure stored in the SV.
If the SV does not have that magical feature, C<NULL> is returned.  Also,
if the SV is not of type SVt_PVMG, Perl may core-dump.

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

This routine checks to see what types of magic C<sv> has.  If the mg_type
field is an upper-case letter, then the mg_obj is copied to C<nsv>, but
the mg_type field is changed to be the lower-case letter.

=head1 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>.

=head1 Calling Perl Routines from within C Programs

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

    I32  perl_call_sv(SV*, I32);
    I32  perl_call_pv(char*, I32);
    I32  perl_call_method(char*, I32);
    I32  perl_call_argv(char*, I32, register char**);

The routine most often used is C<perl_call_sv>.  The C<SV*> argument
contains either the name of the Perl subroutine to be called, or a
reference to the subroutine.  The second argument consists of flags
that control the context in which the subroutine is called, whether
or not the subroutine is being passed arguments, how errors should be
trapped, and how to treat return values.

All four routines return the number of arguments that the subroutine returned
on the Perl stack.

When using any of these routines (except C<perl_call_argv>), the programmer
must manipulate the Perl stack.  These include the following macros and
functions:

    dSP
    PUSHMARK()
    PUTBACK
    SPAGAIN
    ENTER
    SAVETMPS
    FREETMPS
    LEAVE
    XPUSH*()
    POP*()

For more information, consult L<perlcall>.

=head1 Memory Allocation

It is strongly suggested that you use the version of malloc that is distributed
with Perl.  It keeps pools of various sizes of unallocated memory in order to
more quickly satisfy allocation requests.
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 (Larry has switched to using a run-time memory checker),
so this argument can be any number.

The second argument C<pointer> 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).

=head1 Scratchpads

=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.

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 recursions

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 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.

=over 8

=item AvFILL

See C<av_len>.

=item av_clear

Clears an array, making it empty.

	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*>.

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

=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 refcount of 1.

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

=item av_pop

Pops an SV off the end of the array.  Returns C<&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, otherwise it can be
dereferenced to get the original C<SV*>.

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

=item av_undef

Undefines the array.

	void	av_undef _((AV* ar));

=item av_unshift

Unshift an SV onto the beginning of the array.  The array will grow
automatically to accommodate the addition.

	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.

	(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 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<DBsub>.

=item 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<DBsingle>.
The sub name can be found by

	SvPV( GvSV( DBsub ), na )

=item 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<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 dowarn

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

=item dSP

Declares a stack pointer variable, C<sp>, for the XSUB.  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 dXSI32

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

=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 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> 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

The XSUB-writer's equivalent to Perl's C<wantarray>.  Returns C<G_SCALAR> or
C<G_ARRAY> for scalar or array context.

=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> and L<perlcall>.

=item gv_stashpv

Returns a pointer to the stash for a specified package.  If C<create> is set
then the package will be created if it does not already exist.  If C<create>
is not set and the package does not exist then NULL is returned.

	HV*	gv_stashpv _((char* name, I32 create));

=item gv_stashsv

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

	HV*	gv_stashsv _((SV* sv, I32 create));

=item GvSV

Return the SV from the GV.

=item he_free

Releases a hash entry from an iterator.  See C<hv_iternext>.

=item hv_clear

Clears a hash, making it empty.

	void	hv_clear _((HV* tb));

=item hv_delete

Deletes a key/value pair in the hash.  The value SV is removed from the hash
and returned to the caller.  The C<klen> is the length of the key.  The
C<flags> value will normally be zero; if set to G_DISCARD then null will be
returned.

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

=item hv_exists

Returns a boolean indicating whether the specified hash key exists.  The
C<klen> is the length of the key.

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

=item hv_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*>.

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

=item hv_iterinit

Prepares a starting point to traverse a hash table.

	I32	hv_iterinit _((HV* 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_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 pre-computed hash
value; if it is zero then Perl will compute it.  The return value will be
null if the operation failed, otherwise it can be dereferenced to get the
original C<SV*>.

	SV**	hv_store _((HV* tb, char* key, U32 klen, 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.

	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 MARK

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

=item mg_clear

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

	int	mg_clear _((SV* sv));

=item mg_copy

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

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

=item mg_find

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

	MAGIC*	mg_find _((SV* sv, int type));

=item mg_free

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

	int	mg_free _((SV* sv));

=item mg_get

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

	int	mg_get _((SV* sv));

=item mg_len

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

	U32	mg_len _((SV* sv));

=item mg_magical

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

	void	mg_magical _((SV* sv));

=item mg_set

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

	int	mg_set _((SV* sv));

=item 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.

	(void) Move( s, d, n, t );

=item na

A variable which may be used with C<SvPV> to tell Perl to calculate the
string length.

=item New

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

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

=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 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 newAV

Creates a new AV.  The refcount is set to 1.

	AV*	newAV _((void));

=item newHV

Creates a new HV.  The refcount is set to 1.

	HV*	newHV _((void));

=item newRV

Creates an RV wrapper for an SV.  The refcount for the original SV is
incremented.

	SV*	newRV _((SV* ref));

=item newSV

Creates a new SV.  The C<len> parameter indicates the number of bytes of
pre-allocated string space the SV should have.  The refcount for the new SV
is set to 1.

	SV*	newSV _((STRLEN len));

=item newSViv

Creates a new SV and copies an integer into it.  The refcount for the SV is
set to 1.

	SV*	newSViv _((IV i));

=item newSVnv

Creates a new SV and copies a double into it.  The refcount for the SV is
set to 1.

	SV*	newSVnv _((NV i));

=item newSVpv

Creates a new SV and copies a string into it.  The refcount for the SV is
set to 1.  If C<len> is zero then Perl will compute the length.

	SV*	newSVpv _((char* s, STRLEN len));

=item 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
refcount is 1.

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

=item newSVsv

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

	SV*	newSVsv _((SV* old));

=item newXS

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

=item newXSproto

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

=item Nullav

Null AV pointer.

=item Nullch

Null character pointer.

=item Nullcv

Null CV pointer.

=item Nullhv

Null HV pointer.

=item Nullsv

Null SV pointer.

=item ORIGMARK

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

=item perl_alloc

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

=item perl_call_argv

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

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

=item perl_call_method

Performs a callback to the specified Perl method.  The blessed object must
be on the stack.  See L<perlcall>.

	I32	perl_call_method _((char* methname, I32 flags));

=item perl_call_pv

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

	I32	perl_call_pv _((char* subname, I32 flags));

=item perl_call_sv

Performs a callback to the Perl sub whose name is in the SV.  See
L<perlcall>.

	I32	perl_call_sv _((SV* sv, I32 flags));

=item perl_construct

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

=item perl_destruct

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

=item perl_eval_sv

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

	I32	perl_eval_sv _((SV* sv, I32 flags));

=item perl_free

Releases a Perl interpreter.  See L<perlembed>.

=item perl_get_av

Returns the AV of the specified Perl array.  If C<create> is set and the
Perl variable does not exist then it will be created.  If C<create> is not
set and the variable does not exist then null is returned.

	AV*	perl_get_av _((char* name, I32 create));

=item perl_get_cv

Returns the CV of the specified Perl sub.  If C<create> is set and the Perl
variable does not exist then it will be created.  If C<create> is not
set and the variable does not exist then null is returned.

	CV*	perl_get_cv _((char* name, I32 create));

=item perl_get_hv

Returns the HV of the specified Perl hash.  If C<create> is set and the Perl
variable does not exist then it will be created.  If C<create> is not
set and the variable does not exist then null is returned.

	HV*	perl_get_hv _((char* name, I32 create));

=item perl_get_sv

Returns the SV of the specified Perl scalar.  If C<create> is set and the
Perl variable does not exist then it will be created.  If C<create> is not
set and the variable does not exist then null is returned.

	SV*	perl_get_sv _((char* name, I32 create));

=item perl_parse

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

=item perl_require_pv

Tells Perl to C<require> a module.

	void	perl_require_pv _((char* pv));

=item perl_run

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

=item POPi

Pops an integer off the stack.

	int POPi();

=item POPl

Pops a long off the stack.

	long POPl();

=item POPp

Pops a string off the stack.

	char * POPp();

=item POPn

Pops a double off the stack.

	double POPn();

=item POPs

Pops an SV off the stack.

	SV* POPs();

=item PUSHMARK

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

	PUSHMARK(p)

=item PUSHi

Push an integer onto the stack.  The stack must have room for this element.
See C<XPUSHi>.

	PUSHi(int d)

=item PUSHn

Push a double onto the stack.  The stack must have room for this element.
See C<XPUSHn>.

	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.  See C<XPUSHp>.

	PUSHp(char *c, int len )

=item PUSHs

Push an SV onto the stack.  The stack must have room for this element.  See
C<XPUSHs>.

	PUSHs(sv)

=item PUTBACK

Closing bracket for XSUB arguments.  This is usually handled by C<xsubpp>.
See C<PUSHMARK> and L<perlcall> for other uses.

	PUTBACK;

=item Renew

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

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

=item Renewc

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

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

=item RETVAL

Variable which is setup by C<xsubpp> to hold the return value for an XSUB.
This is always the proper type for the XSUB.
See L<perlxs/"The RETVAL Variable">.

=item safefree

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

=item safemalloc

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

=item saferealloc

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

=item savepv

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

	char*	savepv _((char* sv));

=item savepvn

Copy a string to a safe spot.  The C<len> indicates number of bytes to
copy.  This does not use an SV.

	char*	savepvn _((char* sv, I32 len));

=item SAVETMPS

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

	SAVETMPS;

=item SP

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

=item SPAGAIN

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

	SPAGAIN;

=item ST

Used to access elements on the XSUB's stack.

	SV* ST(int x)

=item strEQ

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

	int strEQ( char *s1, char *s2 )

=item strGE

Test two strings to see if the first, C<s1>, is greater than or equal to the
second, C<s2>.  Returns true or false.

	int strGE( char *s1, char *s2 )

=item strGT

Test two strings to see if the first, C<s1>, is greater than the second,
C<s2>.  Returns true or false.

	int strGT( char *s1, char *s2 )

=item strLE

Test two strings to see if the first, C<s1>, is less than or equal to the
second, C<s2>.  Returns true or false.

	int strLE( char *s1, char *s2 )

=item strLT

Test two strings to see if the first, C<s1>, is less than the second,
C<s2>.  Returns true or false.

	int strLT( char *s1, char *s2 )

=item strNE

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

	int strNE( char *s1, char *s2 )

=item strnEQ

Test two strings to see if they are equal.  The C<len> parameter indicates
the number of bytes to compare.  Returns true or false.

	int strnEQ( char *s1, char *s2 )

=item strnNE

Test two strings to see if they are different.  The C<len> parameter
indicates the number of bytes to compare.  Returns true or false.

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

=item sv_2mortal

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

	SV*	sv_2mortal _((SV* sv));

=item sv_bless

Blesses an SV into a specified package.  The SV must be an RV.  The package
must be designated by its stash (see C<gv_stashpv()>).  The refcount 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.

	void	sv_catpv _((SV* sv, 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.

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

=item sv_catsv

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

	void	sv_catsv _((SV* dsv, SV* ssv));

=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 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>.

	SvCUR_set (SV* sv, int val )

=item sv_dec

Autodecrement of the value in the SV.

	void	sv_dec _((SV* sv));

=item sv_dec

Autodecrement of the value in the SV.

	void	sv_dec _((SV* sv));

=item SvEND

Returns a pointer to the last character in the string which is in the SV.
See C<SvCUR>.  Access the character as

	*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 SvGROW

Expands the character buffer in the SV.  Calls C<sv_grow> to perform the
expansion if necessary.  Returns a pointer to the character buffer.

	char * SvGROW( SV* sv, int 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

Autoincrement of the value in the SV.

	void	sv_inc _((SV* sv));

=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.

	SvIOK_off (SV* sv)

=item SvIOK_on

Tells an SV that it is an integer.

	SvIOK_on (SV* sv)

=item SvIOK_only

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

	SvIOK_on (SV* sv)

=item SvIOK_only

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

	SvIOK_on (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 know how to check for subtype, so it doesn't work in
an inheritance relationship.

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

=item SvIV

Returns the integer which is in the SV.

	int SvIV (SV* sv)

=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 SvIVX

Returns the integer which is stored in the SV.

	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_len

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

	STRLEN	sv_len _((SV* sv));

=item sv_magic

Adds magic to an SV.

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

=item sv_mortalcopy

Creates a new SV which is a copy of the original SV.  The new SV is marked
as mortal.  The old SV may become invalid if it was marked as a temporary.

	SV*	sv_mortalcopy _((SV* oldsv));

=item SvOK

Returns a boolean indicating whether the value is an SV.

	int SvOK (SV* sv)

=item sv_newmortal

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

	SV*	sv_newmortal _((void));

=item sv_no

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

=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.

	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 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.

	SvNOK_off (SV* sv)

=item SvNOK_on

Tells an SV that it is a double.

	SvNOK_on (SV* sv)

=item SvNOK_only

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

	SvNOK_on (SV* sv)

=item SvNOK_only

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

	SvNOK_on (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

Returns the double which is stored in the SV.

	double SvNV (SV* sv);

=item SvNVX

Returns the double which is stored in the SV.

	double SvNVX (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.

	SvPOK_off (SV* sv)

=item SvPOK_on

Tells an SV that it is a string.

	SvPOK_on (SV* sv)

=item SvPOK_only

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

	SvPOK_on (SV* sv)

=item SvPOK_only

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

	SvPOK_on (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.  If C<len> is C<na> then Perl will
handle the length on its own.

	char * SvPV (SV* sv, int len )

=item SvPVX

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

	char * SvPVX (SV* sv)

=item SvREFCNT

Returns the value of the object's refcount.

	int SvREFCNT (SV* sv);

=item SvREFCNT_dec

Decrements the refcount of the given SV.

	void SvREFCNT_dec (SV* sv)

=item SvREFCNT_inc

Increments the refcount 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.

	SvROK_off (SV* sv)

=item SvROK_on

Tells an SV that it is an RV.

	SvROK_on (SV* sv)

=item SvRV

Dereferences an RV to return the SV.

	SV*	SvRV (SV* sv);

=item sv_setiv

Copies an integer into the given SV.

	void	sv_setiv _((SV* sv, IV num));

=item sv_setnv

Copies a double into the given SV.

	void	sv_setnv _((SV* sv, double num));

=item sv_setpv

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

	void	sv_setpv _((SV* sv, char* ptr));

=item sv_setpvn

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

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

=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 refcount 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 refcount 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<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 refcount 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 refcount 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 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 or temporary.

	void	sv_setsv _((SV* dsv, SV* ssv));

=item SvSetSV

A wrapper around C<sv_setsv>. Safe even if C<dst==ssv>.

=item SvSTASH

Returns the stash of the SV.

	HV * SvSTASH (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.

	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 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_undef

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

=item sv_unref

Unsets the RV status of the SV, and decrements the refcount 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 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.

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

=item sv_yes

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

	XPUSHi(int d)

=item XPUSHn

Push a double onto the stack, extending the stack if necessary.  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.  See C<PUSHp>.

	XPUSHp(char *c, int len)

=item XPUSHs

Push an SV onto the stack, extending the stack if necessary.  See C<PUSHs>.

	XPUSHs(sv)

=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<&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<&sv_undef> from an XSUB immediately.  Uses C<XST_mUNDEF>.

	XSRETURN_UNDEF;

=item XSRETURN_YES

Return C<&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<&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<&sv_undef> into the specified position C<i> on the stack.

	XST_mUNDEF( int i );

=item XST_mYES

Place C<&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 AUTHOR

Jeff Okamoto <okamoto@corp.hp.com>

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, and Ulrich Pfeifer.

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

=head1 DATE

Version 23.1: 1996/10/19