@node perlguts, perlcall, perlxstut, Top
@unnumberedsec NAME
perlguts - Perl@'s Internal Functions

@unnumberedsec 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.

@unnumberedsec Variables
@unnumberedsubsec Datatypes
Perl has three typedefs that handle Perl@'s three main data types:


@example
SV  Scalar Value
AV  Array Value
HV  Hash Value
@end example

Each typedef has specific routines that manipulate the various data types.

@unnumberedsubsec 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.

@unnumberedsubsec 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 five routines are:


@example
SV*  newSViv(IV);
SV*  newSVnv(double);
SV*  newSVpv(char*, int);
SV*  newSVpvf(const char*, ...);
SV*  newSVsv(SV*);
@end example

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


@example
void  sv_setiv(SV*, IV);
void  sv_setnv(SV*, double);
void  sv_setpv(SV*, char*);
void  sv_setpvn(SV*, char*, int)
void  sv_setpvf(SV*, const char*, ...);
void  sv_setsv(SV*, SV*);
@end example

Notice that you can choose to specify the length of the string to be
assigned by using sv_setpvn or newSVpv, or you may allow Perl to
calculate the length by using sv_setpv or by specifying 0 as the second
argument to newSVpv.  Be warned, though, that Perl will determine the
string@'s length by using @code{strlen}, which depends on the string terminating
with a NUL character.  The arguments of sv_setpvf are processed like
sprintf, and the formatted output becomes the value.

All SVs that will contain strings should, but need not, 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:


@example
SvIV(SV*)
SvNV(SV*)
SvPV(SV*, STRLEN len)
@end example

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

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

If you want to know if the scalar value is TRUE, you can use:


@example
SvTRUE(SV*)
@end example

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


@example
SvGROW(SV*, STRLEN newlen)
@end example

which will determine if more memory needs to be allocated.  If so, it will
call the function sv_grow.  Note that SvGROW can only increase, not
decrease, the allocated memory of an SV and that it does not automatically
add a byte for the a trailing NUL (perl@'s own string functions typically do
@code{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.


@example
SvIOK(SV*)
SvNOK(SV*)
SvPOK(SV*)
@end example

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


@example
SvCUR(SV*)
SvCUR_set(SV*, I32 val)
@end example

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


@example
SvEND(SV*)
@end example

But note that these last three macros are valid only if @code{SvPOK()} is true.

If you want to append something to the end of string stored in an @code{SV*},
you can use the following functions:


@example
void  sv_catpv(SV*, char*);
void  sv_catpvn(SV*, char*, int);
void  sv_catpvf(SV*, const char*, ...);
void  sv_catsv(SV*, SV*);
@end example

The first function calculates the length of the string to be appended by
using @code{strlen}.  In the second, you specify the length of the string
yourself.  The third function processes its arguments like sprintf and
appends the formatted output.  The fourth function 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:


@example
SV*  perl_get_sv("package::varname", FALSE);
@end example

This returns NULL if the variable does not exist.

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


@example
SvOK(SV*)
@end example

The scalar undef value is stored in an SV instance called sv_undef.  Its
address can be used whenever an @code{SV*} is needed.

There are also the two values sv_yes and sv_no, which contain Boolean
TRUE and FALSE values, respectively.  Like sv_undef, their addresses can
be used whenever an @code{SV*} is needed.

Do not be fooled into thinking that @code{(SV *) 0} is the same as @code{&sv_undef}.
Take this code:


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

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 @code{&sv_undef} in the first
line and all will be well.

To free an SV that you@'ve created, call @code{SvREFCNT_dec(SV*)}.  Normally this
call is not necessary (see @samp{Reference Counts and Mortality} in this node).

@unnumberedsubsec What@'s Really Stored in an SV?
Recall that the usual method of determining the type of scalar you have is
to use @code{Sv*OK} macros.  Because a scalar can be both a number and a string,
usually these macros will always return TRUE and calling the @code{Sv*V}
macros will do the appropriate conversion of string to integer/double or
integer/double to string.

If you @emph{really} need to know if you have an integer, double, or string
pointer in an SV, you can use the following three macros instead:


@example
SvIOKp(SV*)
SvNOKp(SV*)
SvPOKp(SV*)
@end example

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 @code{Sv*V} macros.

@unnumberedsubsec Working with AVs
There are two ways to create and load an AV.  The first method creates an
empty AV:


@example
AV*  newAV();
@end example

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


@example
AV*  av_make(I32 num, SV **ptr);
@end example

The second argument points to an array containing @code{num} @code{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:


@example
void  av_push(AV*, SV*);
SV*   av_pop(AV*);
SV*   av_shift(AV*);
void  av_unshift(AV*, I32 num);
@end example

These should be familiar operations, with the exception of av_unshift.
This routine adds @code{num} elements at the front of the array with the undef
value.  You must then use av_store (described below) to assign values
to these new elements.

Here are some other functions:


@example
I32   av_len(AV*);
SV**  av_fetch(AV*, I32 key, I32 lval);
SV**  av_store(AV*, I32 key, SV* val);
@end example

The av_len function returns the highest index value in array (just
like $#array in Perl).  If the array is empty, -1 is returned.  The
av_fetch function returns the value at index @code{key}, but if @code{lval}
is non-zero, then av_fetch will store an undef value at that index.
The av_store function stores the value @code{val} at index @code{key}.
note that av_fetch and av_store both return @code{SV**}@'s, not @code{SV*}@'s
as their return value.


@example
void  av_clear(AV*);
void  av_undef(AV*);
void  av_extend(AV*, I32 key);
@end example

The av_clear function deletes all the elements in the AV* array, but
does not actually delete the array itself.  The av_undef function will
delete all the elements in the array plus the array itself.  The
av_extend function extends the array so that it contains @code{key}
elements.  If @code{key} is less than the current 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:


@example
AV*  perl_get_av("package::varname", FALSE);
@end example

This returns NULL if the variable does not exist.

@unnumberedsubsec Working with HVs
To create an HV, you use the following routine:


@example
HV*  newHV();
@end example

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


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

The @code{klen} parameter is the length of the key being passed in (Note that
you cannot pass 0 in as a value of @code{klen} to tell Perl to measure the
length of the key).  The @code{val} argument contains the SV pointer to the
scalar being stored, and @code{hash} is the precomputed hash value (zero if
you want hv_store to calculate it for you).  The @code{lval} parameter
indicates whether this fetch is actually a part of a store operation, in
which case a new undefined value will be added to the HV with the supplied
key and hv_fetch will return as if the value had already existed.

Remember that hv_store and hv_fetch return @code{SV**}@'s and not just
@code{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.


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

If @code{flags} does not include the G_DISCARD flag then hv_delete will
create and return a mortal copy of the deleted value.

And more miscellaneous functions:


@example
void   hv_clear(HV*);
void   hv_undef(HV*);
@end example

Like their AV counterparts, hv_clear deletes all the entries in the hash
table but does not actually delete the hash table.  The hv_undef deletes
both the entries and the hash table itself.

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


@example
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 */
@end example

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


@example
HV*  perl_get_hv("package::varname", FALSE);
@end example

This returns NULL if the variable does not exist.

The hash algorithm is defined in the @code{PERL_HASH(hash, key, klen)} macro:


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

@unnumberedsubsec Hash API Extensions
Beginning with version 5.004, the following functions are also supported:


@example
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);
@end example

Note that these functions take @code{SV*} keys, which simplifies writing
of extension code that deals with hash structures.  These functions
also allow passing of @code{SV*} keys to tie functions without forcing
you to stringify the keys (unlike the previous set of functions).

They also return and accept whole hash entries (@code{HE*}), making their
use more efficient (since the hash number for a particular string
doesn@'t have to be recomputed every time).  See @samp{API LISTING} in this node 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
@samp{API LISTING} in this node later in this document for detailed descriptions of these
macros.


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

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


@example
HeKEY(HE* he)
HeKLEN(HE* he)
@end example

@unnumberedsubsec 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:


@example
SV* newRV_inc((SV*) thing);
SV* newRV_noinc((SV*) thing);
@end example

The @code{thing} argument can be any of an @code{SV*}, @code{AV*}, or @code{HV*}.  The
functions are identical except that newRV_inc increments the reference
count of the @code{thing}, while newRV_noinc does not.  For historical
reasons, @code{newRV} is a synonym for newRV_inc.

Once you have a reference, you can use the following macro to dereference
the reference:


@example
SvRV(SV*)
@end example

then call the appropriate routines, casting the returned @code{SV*} to either an
@code{AV*} or @code{HV*}, if required.

To determine if an SV is a reference, you can use the following macro:


@example
SvROK(SV*)
@end example

To discover what type of value the reference refers to, use the following
macro and then check the return value.


@example
SvTYPE(SvRV(SV*))
@end example

The most useful types that will be returned are:


@example
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.
@end example

@unnumberedsubsec 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:


@example
SV* sv_bless(SV* sv, HV* stash);
@end example

The @code{sv} argument must be a reference.  The @code{stash} argument specifies
which class the reference will belong to.  See
@samp{Stashes and Globs} in this node 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 @code{classname} is non-null, the SV is blessed into the specified
class.  SV is returned.


@example
SV* newSVrv(SV* rv, char* classname);
@end example

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


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

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


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

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


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

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

@unnumberedsubsec 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.


@example
SV*  perl_get_sv("package::varname", TRUE);
AV*  perl_get_av("package::varname", TRUE);
HV*  perl_get_hv("package::varname", TRUE);
@end example

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
@code{TRUE} argument to enable certain extra features.  Those bits are:


@example
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.
@end example

If you do not specify a package name, the variable is created in the current
package.

@unnumberedsubsec 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:


@example
int SvREFCNT(SV* sv);
SV* SvREFCNT_inc(SV* sv);
void SvREFCNT_dec(SV* sv);
@end example

However, there is one other function which manipulates the reference
count of its argument.  The newRV_inc function, you will recall,
creates a reference to the specified argument.  As a side effect,
it increments the argument@'s reference count.  If this is not what
you want, use newRV_noinc instead.

For example, imagine you want to return a reference from an XSUB function.
Inside the XSUB routine, you create an SV which initially has a reference
count of one.  Then you call newRV_inc, passing it the just-created SV.
This returns the reference as a new SV, but the reference count of the
SV you passed to newRV_inc has been incremented to two.  Now you
return the reference from the XSUB routine and forget about the SV.
But Perl hasn@'t!  Whenever the returned reference is destroyed, the
reference count of the original SV is decreased to one and nothing happens.
The SV will hang around without any way to access it until Perl itself
terminates.  This is a memory leak.

The correct procedure, then, is to use newRV_noinc instead of
newRV_inc.  Then, if and when the last reference is destroyed,
the reference count of the SV will go to zero and it will be destroyed,
stopping any memory leak.

There are some convenience functions available that can help with the
destruction of xVs.  These functions introduce the concept of "mortality".
An xV that is mortal has had its reference count marked to be decremented,
but not actually decremented, until "a short time later".  Generally the
term "short time later" means a single Perl statement, such as a call to
an XSUB function.  The actual determinant for when mortal xVs have their
reference count decremented depends on two macros, SAVETMPS and FREETMPS.
See @xref{perlcall,Perlcall}, and @xref{perlxs,Perlxs}, for more details on these macros.

"Mortalization" then is at its simplest a deferred SvREFCNT_dec.
However, if you mortalize a variable twice, the reference count will
later be decremented twice.

You should be careful about creating mortal variables.  Strange things
can happen if you make the same value mortal within multiple contexts,
or if you make a variable mortal multiple times.

To create a mortal variable, use the functions:


@example
SV*  sv_newmortal()
SV*  sv_2mortal(SV*)
SV*  sv_mortalcopy(SV*)
@end example

The first call creates a mortal SV, the second converts an existing
SV to a mortal SV (and thus defers a call to SvREFCNT_dec), and the
third creates a mortal copy of an existing SV.

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

@unnumberedsubsec 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:


@example
Scalar Value
Array Value
Hash Value
File Handle
Directory Handle
Format
Subroutine
@end example

There is a single stash called "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 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:


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

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
@code{HV*}.  The @code{create} flag will create a new package if it is set.

The name that @code{gv_stash*v} wants is the name of the package whose symbol table
you want.  The default package is called @code{main}.  If you have multiply nested
packages, pass their names to @code{gv_stash*v}, separated by @code{::} 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:


@example
HV*  SvSTASH(SvRV(SV*));
@end example

then use the following to get the package name itself:


@example
char*  HvNAME(HV* stash);
@end example

If you need to bless or re-bless an object you can use the following
function:


@example
SV*  sv_bless(SV*, HV* stash)
@end example

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

For more information on references and blessings, consult @xref{perlref,Perlref}.

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

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

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


@example
SvIOK_on
SvNOK_on
SvPOK_on
SvROK_on
@end example

The particular macro you must use depends on which @code{sv_set*v} routine
you called first.  This is because every @code{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:


@example
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);
@end example

If the order of sv_setiv and sv_setpv had been reversed, then the
macro SvPOK_on would need to be called instead of SvIOK_on.

@unnumberedsubsec 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 @code{struct magic}@'s, typedef@'ed to MAGIC.


@example
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;
@};
@end example

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

@unnumberedsubsec Assigning Magic
Perl adds magic to an SV using the sv_magic function:


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

The @code{sv} argument is a pointer to the SV that is to acquire a new magical
feature.

If @code{sv} is not already magical, Perl uses the SvUPGRADE macro to
set the SVt_PVMG flag for the @code{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 @code{name} and @code{namlen} arguments are used to associate a string with
the magic, typically the name of a variable. @code{namlen} is stored in the
mg_len field and if @code{name} is non-null and @code{namlen} >= 0 a malloc@'d
copy of the name is stored in @code{mg_ptr} field.

The sv_magic function uses how to determine which, if any, predefined
"Magic Virtual Table" should be assigned to the @code{mg_virtual} field.
See the "Magic Virtual Table" section below.  The how argument is also
stored in the @code{mg_type} field.

The @code{obj} argument is stored in the @code{mg_obj} field of the MAGIC
structure.  If it is not the same as the @code{sv} argument, the reference
count of the @code{obj} object is incremented.  If it is the same, or if
the how argument is "#", or if it is a null pointer, then @code{obj} is
merely stored, without the reference count being incremented.

There is also a function to add magic to an @code{HV}:


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

This simply calls sv_magic and coerces the @code{gv} argument into an @code{SV}.

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


@example
void sv_unmagic(SV *sv, int type);
@end example

The @code{type} argument should be equal to the how value when the @code{SV}
was initially made magical.

@unnumberedsubsec Magic Virtual Tables
The @code{mg_virtual} field in the MAGIC structure is a pointer to a
@code{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 @code{MGVTBL} has five pointers to the following routine types:


@example
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);
@end example

This MGVTBL structure is set at compile-time in @code{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.


@example
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.
@end example

For instance, the MGVTBL structure called @code{vtbl_sv} (which corresponds
to an @code{mg_type} of @'\0@') contains:


@example
@{ magic_get, magic_set, magic_len, 0, 0 @}
@end example

Thus, when an SV is determined to be magical and of type @'\0@', if a get
operation is being performed, the routine @code{magic_get} is called.  All
the various routines for the various magical types begin with @code{magic_}.

The current kinds of Magic Virtual Tables are:


@example
mg_type  MGVTBL              Type of magical
-------  ------              ----------------------------
\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 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            Signal Hash
s        vtbl_sigelem        Signal Hash element
t        vtbl_taint          Taintedness
U        vtbl_uvar               ???
v        vtbl_vec                Vector
x        vtbl_substr         Substring???
y        vtbl_itervar        Shadow "foreach" iterator variable
*        vtbl_glob           GV???
#        vtbl_arylen         Array Length
.        vtbl_pos                $. scalar variable
~        None                Used by certain extensions
@end example

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 @'~@' magic type is 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).

Note that because multiple extensions may be using ~ magic it is
important for extensions to take extra care with it.  Typically only
using it on objects blessed into the same class as the extension
is sufficient.  It may also be appropriate to add an I32 @'signature@'
at the top of the private data area and check that.

@unnumberedsubsec Finding Magic

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

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


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

This routine checks to see what types of magic @code{sv} has.  If the mg_type
field is an uppercase letter, then the mg_obj is copied to @code{nsv}, but
the mg_type field is changed to be the lowercase letter.

@unnumberedsec Subroutines
@unnumberedsubsec 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 @code{ST(n)} macro, which returns
the n@'th stack argument.  Argument 0 is the first argument passed in the
Perl subroutine call.  These arguments are @code{SV*}, and can be used anywhere
an @code{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:


@example
EXTEND(sp, num);
@end example

where @code{sp} is the stack pointer, and @code{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:


@example
PUSHi(IV)
PUSHn(double)
PUSHp(char*, I32)
PUSHs(SV*)
@end example

And now the Perl program calling @code{tzname}, the two values will be assigned
as in:


@example
($standard_abbrev, $summer_abbrev) = POSIX::tzname;
@end example

An alternate (and possibly simpler) method to pushing values on the stack is
to use the macros:


@example
XPUSHi(IV)
XPUSHn(double)
XPUSHp(char*, I32)
XPUSHs(SV*)
@end example

These macros automatically adjust the stack for you, if needed.  Thus, you
do not need to call EXTEND to extend the stack.

For more information, consult @xref{perlxs,Perlxs}, and @xref{perlxstut,Perlxstut},.

@unnumberedsubsec 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:


@example
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**);
@end example

The routine most often used is perl_call_sv.  The @code{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 perl_call_argv), the programmer
must manipulate the Perl stack.  These include the following macros and
functions:


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

For a detailed description of calling conventions from C to Perl,
consult @xref{perlcall,Perlcall}.

@unnumberedsubsec Memory Allocation
It is 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 satisfy allocation requests more quickly.  However, on some
platforms, it may cause spurious malloc or free errors.


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

These three macros are used to initially allocate memory.

The first argument x was a "magic cookie" that was used to keep track
of who called the macro, to help when debugging memory problems.  However,
the current code makes no use of this feature (most Perl developers now
use run-time memory checkers), so this argument can be any number.

The second argument @code{pointer} should be the name of a variable that will
point to the newly allocated memory.

The third and fourth arguments @code{number} and @code{type} specify how many of
the specified type of data structure should be allocated.  The argument
@code{type} is passed to @code{sizeof}.  The final argument to Newc, @code{cast},
should be used if the @code{pointer} argument is different from the @code{type}
argument.

Unlike the New and Newc macros, the Newz macro calls @code{memzero}
to zero out all the newly allocated memory.


@example
Renew(pointer, number, type);
Renewc(pointer, number, type, cast);
Safefree(pointer)
@end example

These three macros are used to change a memory buffer size or to free a
piece of memory no longer needed.  The arguments to Renew and Renewc
match those of New and Newc with the exception of not needing the
"magic cookie" argument.


@example
Move(source, dest, number, type);
Copy(source, dest, number, type);
Zero(dest, number, type);
@end example

These three macros are used to move, copy, or zero out previously allocated
memory.  The @code{source} and @code{dest} arguments point to the source and
destination starting points.  Perl will move, copy, or zero out @code{number}
instances of the size of the @code{type} data structure (using the @code{sizeof}
function).

@unnumberedsubsec 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 @xref{perlapio,Perlapio}.

@unnumberedsubsec 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 (@emph{target}s) which are (as a corollary)
not constantly freed/created.

Each of the targets is created only once (but see
@samp{Scratchpads and recursion} in this node 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 @emph{target} and puts the @emph{target} on stack.

The macro to put this target on stack is @code{PUSHTARG}, and it is
directly used in some opcodes, as well as indirectly in zillions of
others, which use it via @code{(X)PUSH[pni]}.

@unnumberedsubsec Scratchpads
The question remains on when the SVs which are @emph{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 @code{SVs_PADMY} set, and
@emph{target}s have @code{SVs_PADTMP} set.

The correspondence between OPs and @emph{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.

@unnumberedsubsec 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 @strong{recursion}, and maybe (sometime soon) @strong{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. (@emph{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 @emph{target}s on this scratchpad are undefs, but they are already
marked with correct flags.

@unnumberedsec Compiled code
@unnumberedsubsec Code tree
Here we describe the internal form your code is converted to by
Perl. Start with a simple example:


@example
$a = $b + $c;
@end example

This is converted to a tree similar to this one:


@example
assign-to
           /           \
          +             $a
        /   \
      $b     $c
@end example

(but slightly more complicated).  This tree reflect 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:


@example
$b ---> $c ---> + ---> $a ---> assign-to
@end example

But with the actual compile tree for @code{$a = $b + $c} it is different:
some nodes @emph{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.

@unnumberedsubsec Examining the tree
If you have your perl compiled for debugging (usually done with @code{-D
optimize=-g} on @code{Configure} command line), you may examine the
compiled tree by specifying @code{-Dx} on the Perl command line.  The
output takes several lines per node, and for @code{$b+$c} it looks like
this:


@example
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
                @}
            @}
@end example

This tree has 5 nodes (one per @code{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 @code{@{@}} pairs on the same level
of indentation, thus this listing corresponds to the tree:


@example
add
                 /     \
               null    null
                |       |
               gvsv    gvsv
@end example

The execution order is indicated by C<===>> marks, thus it is @code{3
4 5 6} (node 6 is not included into above listing), i.e.,
@code{gvsv gvsv add whatever}.

@unnumberedsubsec Compile pass 1: check routines
The tree is created by the @emph{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 @emph{check routines}.  The correspondence between node names
and corresponding check routines is described in @file{opcode.pl} (do not
forget to run @code{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 is 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 @code{ck_*}. They are usually
called from @code{new*OP} subroutines (or @code{convert}) (which in turn are
called from @file{perly.y}).

@unnumberedsubsec 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 @emph{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.

@unnumberedsubsec Compile pass 2: context propagation
When a context for a part of compile tree is known, it is propagated
down through the tree.  Aat 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).

@unnumberedsubsec Compile pass 3: peephole optimization
After the compile tree for a subroutine (or for an eval or a file)
is created, an additional pass over the code is performed. This pass
is neither top-down or bottom-up, but in the execution order (with
additional compilications 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.

@unnumberedsec 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.

@table @asis
@item AvFILL
See av_len.

@item av_clear
Clears an array, making it empty.  Does not free the memory used by the
array itself.


@example
void    av_clear _((AV* ar));
@end example

@item av_extend
Pre-extend an array.  The @code{key} is the index to which the array should be
extended.


@example
void    av_extend _((AV* ar, I32 key));
@end example

@item av_fetch
Returns the SV at the specified index in the array.  The @code{key} is the
index.  If @code{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 @code{SV*}.


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

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


@example
I32     av_len _((AV* ar));
@end example

@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.


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

@item av_pop
Pops an SV off the end of the array.  Returns @code{&sv_undef} if the array is
empty.


@example
SV*     av_pop _((AV* ar));
@end example

@item av_push
Pushes an SV onto the end of the array.  The array will grow automatically
to accommodate the addition.


@example
void    av_push _((AV* ar, SV* val));
@end example

@item av_shift
Shifts an SV off the beginning of the array.


@example
SV*     av_shift _((AV* ar));
@end example

@item av_store
Stores an SV in an array.  The array index is specified as @code{key}.  The
return value will be null if the operation failed, otherwise it can be
dereferenced to get the original @code{SV*}.


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

@item av_undef
Undefines the array.  Frees the memory used by the array itself.


@example
void    av_undef _((AV* ar));
@end example

@item av_unshift
Unshift the given number of undef values onto the beginning of the
array.  The array will grow automatically to accommodate the addition.
You must then use av_store to assign values to these new elements.


@example
void    av_unshift _((AV* ar, I32 num));
@end example

@item CLASS
Variable which is setup by xsubpp to indicate the class name for a C++ XS
constructor.  This is always a @code{char*}.  See THIS and
@samp{"Using XS With C++"}, @xref{perlxs,Perlxs}.

@item Copy
The XSUB-writer@'s interface to the C @code{memcpy} function.  The s is the
source, d is the destination, n is the number of items, and t is
the type.  May fail on overlapping copies.  See also Move.


@example
(void) Copy( s, d, n, t );
@end example

@item croak
This is the XSUB-writer@'s interface to Perl@'s die function.  Use this
function the same way you use the C printf function.  See warn.

@item CvSTASH
Returns the stash of the CV.


@example
HV * CvSTASH( SV* sv )
@end example

@item DBsingle
When Perl is run in debugging mode, with the -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 DBsub.

@item DBsub
When Perl is run in debugging mode, with the -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 DBsingle.
The sub name can be found by


@example
SvPV( GvSV( DBsub ), na )
@end example

@item DBtrace
Trace variable used when Perl is run in debugging mode, with the -d
switch.  This is the C variable which corresponds to Perl@'s $DB::trace
variable.  See DBsingle.

@item dMARK
Declare a stack marker variable, @code{mark}, for the XSUB.  See MARK and
dORIGMARK.

@item dORIGMARK
Saves the original stack mark for the XSUB.  See ORIGMARK.

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

@item dSP
Declares a stack pointer variable, @code{sp}, for the XSUB.  See SP.

@item dXSARGS
Sets up stack and mark pointers for an XSUB, calling dSP and dMARK.  This is
usually handled automatically by xsubpp.  Declares the items variable
to indicate the number of items on the stack.

@item dXSI32
Sets up the ix variable for an XSUB which has aliases.  This is usually
handled automatically by xsubpp.

@item dXSI32
Sets up the ix variable for an XSUB which has aliases.  This is usually
handled automatically by xsubpp.

@item ENTER
Opening bracket on a callback.  See LEAVE and @xref{perlcall,Perlcall}.


@example
ENTER;
@end example

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


@example
EXTEND( sp, int x );
@end example

@item FREETMPS
Closing bracket for temporaries on a callback.  See SAVETMPS and
@xref{perlcall,Perlcall}.


@example
FREETMPS;
@end example

@item G_ARRAY
Used to indicate array context.  See GIMME_V, GIMME and @xref{perlcall,Perlcall}.

@item G_DISCARD
Indicates that arguments returned from a callback should be discarded.  See
@xref{perlcall,Perlcall}.

@item G_EVAL
Used to force a Perl eval wrapper around a callback.  See @xref{perlcall,Perlcall}.

@item GIMME
A backward-compatible version of GIMME_V which can only return
G_SCALAR or G_ARRAY; in a void context, it returns G_SCALAR.

@item GIMME_V
The XSUB-writer@'s equivalent to Perl@'s wantarray.  Returns
G_VOID, G_SCALAR or G_ARRAY for void, scalar or array
context, respectively.

@item G_NOARGS
Indicates that no arguments are being sent to a callback.  See @xref{perlcall,Perlcall}.

@item G_SCALAR
Used to indicate scalar context.  See GIMME_V, GIMME, and @xref{perlcall,Perlcall}.

@item G_VOID
Used to indicate void context.  See GIMME_V and @xref{perlcall,Perlcall}.

@item gv_fetchmeth
Returns the glob with the given @code{name} and a defined subroutine or
NULL.  The glob lives in the given @code{stash}, or in the stashes
accessable via @@ISA and @@<UNIVERSAL>.

The argument @code{level} should be either 0 or -1.  If @code{level==0}, as a
side-effect creates a glob with the given @code{name} in the given
@code{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 @code{"SUPER"} token as a postfix of the stash name.

The GV returned from gv_fetchmeth may be a method cache entry,
which is not visible to Perl code.  So when calling 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 @code{GvCV} macro.


@example
GV*     gv_fetchmeth _((HV* stash, char* name, STRLEN len, I32 level));
@end example

@item gv_fetchmethod
@itemx gv_fetchmethod_autoload
Returns the glob which contains the subroutine to call to invoke the
method on the @code{stash}.  In fact in the presense of autoloading this may
be the glob for "AUTOLOAD".  In this case the corresponding variable
$AUTOLOAD is already setup.

The third parameter of 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
gv_fetchmethod is equivalent to calling gv_fetchmethod_autoload with a
non-zero @code{autoload} parameter.

These functions grant @code{"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 gv_fetchmeth with
@code{level==0}.  @code{name} should be writable if contains @code{@':@'} or @code{@'\@'@'}.
The warning against passing the GV returned by gv_fetchmeth to
perl_call_sv apply equally to these functions.


@example
GV*     gv_fetchmethod _((HV* stash, char* name));
GV*     gv_fetchmethod_autoload _((HV* stash, char* name,
                                   I32 autoload));
@end example

@item gv_stashpv
Returns a pointer to the stash for a specified package.  If @code{create} is set
then the package will be created if it does not already exist.  If @code{create}
is not set and the package does not exist then NULL is returned.


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

@item gv_stashsv
Returns a pointer to the stash for a specified package.  See gv_stashpv.


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

@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 @code{SV*} pointer where a
@code{char*} pointer is to be expected. (For information only--not to be used).

@item HeHASH
Returns the computed hash (type @code{U32}) stored in the hash entry.


@example
HeHASH(HE* he)
@end example

@item HeKEY
Returns the actual pointer stored in the key slot of the hash entry.
The pointer may be either @code{char*} or @code{SV*}, depending on the value of
@code{HeKLEN()}.  Can be assigned to.  The @code{HePV()} or @code{HeSVKEY()} macros
are usually preferable for finding the value of a key.


@example
HeKEY(HE* he)
@end example

@item HeKLEN
If this is negative, and amounts to HEf_SVKEY, it indicates the entry
holds an @code{SV*} key.  Otherwise, holds the actual length of the key.
Can be assigned to. The @code{HePV()} macro is usually preferable for finding
key lengths.


@example
HeKLEN(HE* he)
@end example

@item HePV
Returns the key slot of the hash entry as a @code{char*} value, doing any
necessary dereferencing of possibly @code{SV*} keys.  The length of
the string is placed in @code{len} (this is a macro, so do @emph{not} use
@code{&len}).  If you do not care about what the length of the key is,
you may use the global variable na.  Remember though, that hash
keys in perl are free to contain embedded nulls, so using @code{strlen()}
or similar is not a good way to find the length of hash keys.
This is very similar to the @code{SvPV()} macro described elsewhere in
this document.


@example
HePV(HE* he, STRLEN len)
@end example

@item HeSVKEY
Returns the key as an @code{SV*}, or Nullsv if the hash entry
does not contain an @code{SV*} key.


@example
HeSVKEY(HE* he)
@end example

@item HeSVKEY_force
Returns the key as an @code{SV*}.  Will create and return a temporary
mortal @code{SV*} if the hash entry contains only a @code{char*} key.


@example
HeSVKEY_force(HE* he)
@end example

@item HeSVKEY_set
Sets the key to a given @code{SV*}, taking care to set the appropriate flags
to indicate the presence of an @code{SV*} key, and returns the same @code{SV*}.


@example
HeSVKEY_set(HE* he, SV* sv)
@end example

@item HeVAL
Returns the value slot (type @code{SV*}) stored in the hash entry.


@example
HeVAL(HE* he)
@end example

@item hv_clear
Clears a hash, making it empty.


@example
void    hv_clear _((HV* tb));
@end example

@item hv_delayfree_ent
Releases a hash entry, such as while iterating though the hash, but
delays actual freeing of key and value until the end of the current
statement (or thereabouts) with sv_2mortal.  See hv_iternext
and hv_free_ent.


@example
void    hv_delayfree_ent _((HV* hv, HE* entry));
@end example

@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 @code{klen} is the length of the key.  The
@code{flags} value will normally be zero; if set to G_DISCARD then null will be
returned.


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

@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 @code{flags} value will normally be zero; if set
to G_DISCARD then null will be returned.  @code{hash} can be a valid precomputed
hash value, or 0 to ask for it to be computed.


@example
SV*     hv_delete_ent _((HV* tb, SV* key, I32 flags, U32 hash));
@end example

@item hv_exists
Returns a boolean indicating whether the specified hash key exists.  The
@code{klen} is the length of the key.


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

@item hv_exists_ent
Returns a boolean indicating whether the specified hash key exists. @code{hash}
can be a valid precomputed hash value, or 0 to ask for it to be computed.


@example
bool    hv_exists_ent _((HV* tb, SV* key, U32 hash));
@end example

@item hv_fetch
Returns the SV which corresponds to the specified key in the hash.  The
@code{klen} is the length of the key.  If @code{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 @code{SV*}.


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

@item hv_fetch_ent
Returns the hash entry which corresponds to the specified key in the hash.
@code{hash} must be a valid precomputed hash number for the given @code{key}, or
0 if you want the function to compute it.  IF @code{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 @code{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.


@example
HE*     hv_fetch_ent  _((HV* tb, SV* key, I32 lval, U32 hash));
@end example

@item hv_free_ent
Releases a hash entry, such as while iterating though the hash.  See
hv_iternext and hv_delayfree_ent.


@example
void    hv_free_ent _((HV* hv, HE* entry));
@end example

@item hv_iterinit
Prepares a starting point to traverse a hash table.


@example
I32     hv_iterinit _((HV* tb));
@end example

@item hv_iterkey
Returns the key from the current position of the hash iterator.  See
hv_iterinit.


@example
char*   hv_iterkey _((HE* entry, I32* retlen));
@end example

@item hv_iterkeysv
Returns the key as an @code{SV*} from the current position of the hash
iterator.  The return value will always be a mortal copy of the
key.  Also see hv_iterinit.


@example
SV*     hv_iterkeysv  _((HE* entry));
@end example

@item hv_iternext
Returns entries from a hash iterator.  See hv_iterinit.


@example
HE*     hv_iternext _((HV* tb));
@end example

@item hv_iternextsv
Performs an hv_iternext, hv_iterkey, and hv_iterval in one
operation.


@example
SV *    hv_iternextsv _((HV* hv, char** key, I32* retlen));
@end example

@item hv_iterval
Returns the value from the current position of the hash iterator.  See
hv_iterkey.


@example
SV*     hv_iterval _((HV* tb, HE* entry));
@end example

@item hv_magic
Adds magic to a hash.  See sv_magic.


@example
void    hv_magic _((HV* hv, GV* gv, int how));
@end example

@item HvNAME
Returns the package name of a stash.  See SvSTASH, CvSTASH.


@example
char *HvNAME (HV* stash)
@end example

@item hv_store
Stores an SV in a hash.  The hash key is specified as @code{key} and @code{klen} is
the length of the key.  The @code{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, otherwise it can be dereferenced to get the
original @code{SV*}.


@example
SV**    hv_store _((HV* tb, char* key, U32 klen, SV* val, U32 hash));
@end example

@item hv_store_ent
Stores @code{val} in a hash.  The hash key is specified as @code{key}.  The @code{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 entry was stored in a tied hash.
Otherwise the contents of the return value can be accessed using the
@code{He???} macros described here.


@example
HE*     hv_store_ent  _((HV* tb, SV* key, SV* val, U32 hash));
@end example

@item hv_undef
Undefines the hash.


@example
void    hv_undef _((HV* tb));
@end example

@item isALNUM
Returns a boolean indicating whether the C @code{char} is an ascii alphanumeric
character or digit.


@example
int isALNUM (char c)
@end example

@item isALPHA
Returns a boolean indicating whether the C @code{char} is an ascii alphabetic
character.


@example
int isALPHA (char c)
@end example

@item isDIGIT
Returns a boolean indicating whether the C @code{char} is an ascii digit.


@example
int isDIGIT (char c)
@end example

@item isLOWER
Returns a boolean indicating whether the C @code{char} is a lowercase character.


@example
int isLOWER (char c)
@end example

@item isSPACE
Returns a boolean indicating whether the C @code{char} is whitespace.


@example
int isSPACE (char c)
@end example

@item isUPPER
Returns a boolean indicating whether the C @code{char} is an uppercase character.


@example
int isUPPER (char c)
@end example

@item items
Variable which is setup by xsubpp to indicate the number of items on the
stack.  See @samp{"Variable-length Parameter Lists"}, @xref{perlxs,Perlxs}.

@item ix
Variable which is setup by xsubpp to indicate which of an XSUB@'s aliases
was used to invoke it.  See @samp{"The ALIAS: Keyword"}, @xref{perlxs,Perlxs}.

@item LEAVE
Closing bracket on a callback.  See ENTER and @xref{perlcall,Perlcall}.


@example
LEAVE;
@end example

@item MARK
Stack marker variable for the XSUB.  See dMARK.

@item mg_clear
Clear something magical that the SV represents.  See sv_magic.


@example
int     mg_clear _((SV* sv));
@end example

@item mg_copy
Copies the magic from one SV to another.  See sv_magic.


@example
int     mg_copy _((SV *, SV *, char *, STRLEN));
@end example

@item mg_find
Finds the magic pointer for type matching the SV.  See sv_magic.


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

@item mg_free
Free any magic storage used by the SV.  See sv_magic.


@example
int     mg_free _((SV* sv));
@end example

@item mg_get
Do magic after a value is retrieved from the SV.  See sv_magic.


@example
int     mg_get _((SV* sv));
@end example

@item mg_len
Report on the SV@'s length.  See sv_magic.


@example
U32     mg_len _((SV* sv));
@end example

@item mg_magical
Turns on the magical status of an SV.  See sv_magic.


@example
void    mg_magical _((SV* sv));
@end example

@item mg_set
Do magic after a value is assigned to the SV.  See sv_magic.


@example
int     mg_set _((SV* sv));
@end example

@item Move
The XSUB-writer@'s interface to the C @code{memmove} function.  The s is the
source, d is the destination, n is the number of items, and t is
the type.  Can do overlapping moves.  See also Copy.


@example
(void) Move( s, d, n, t );
@end example

@item na
A variable which may be used with SvPV to tell Perl to calculate the
string length.

@item New
The XSUB-writer@'s interface to the C @code{malloc} function.


@example
void * New( x, void *ptr, int size, type )
@end example

@item Newc
The XSUB-writer@'s interface to the C @code{malloc} function, with cast.


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

@item Newz
The XSUB-writer@'s interface to the C @code{malloc} function.  The allocated
memory is zeroed with @code{memzero}.


@example
void * Newz( x, void *ptr, int size, type )
@end example

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


@example
AV*     newAV _((void));
@end example

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


@example
HV*     newHV _((void));
@end example

@item newRV_inc
Creates an RV wrapper for an SV.  The reference count for the original SV is
incremented.


@example
SV*     newRV_inc _((SV* ref));
@end example

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 @strong{not} incremented.


@example
SV*     newRV_noinc _((SV* ref));
@end example

@item newSV
Creates a new SV.  The @code{len} parameter indicates the number of bytes of
preallocated string space the SV should have.  The reference count for the
new SV is set to 1.


@example
SV*     newSV _((STRLEN len));
@end example

@item newSViv
Creates a new SV and copies an integer into it.  The reference count for the
SV is set to 1.


@example
SV*     newSViv _((IV i));
@end example

@item newSVnv
Creates a new SV and copies a double into it.  The reference count for the
SV is set to 1.


@example
SV*     newSVnv _((NV i));
@end example

@item newSVpv
Creates a new SV and copies a string into it.  The reference count for the
SV is set to 1.  If @code{len} is zero then Perl will compute the length.


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

@item newSVrv
Creates a new SV for the RV, @code{rv}, to point to.  If @code{rv} is not an RV then
it will be upgraded to one.  If @code{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.


@example
SV*     newSVrv _((SV* rv, char* classname));
@end example

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


@example
SV*     newSVsv _((SV* old));
@end example

@item newXS
Used by xsubpp to hook up XSUBs as Perl subs.

@item newXSproto
Used by 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 dORIGMARK.

@item perl_alloc
Allocates a new Perl interpreter.  See @xref{perlembed,Perlembed}.

@item perl_call_argv
Performs a callback to the specified Perl sub.  See @xref{perlcall,Perlcall}.


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

@item perl_call_method
Performs a callback to the specified Perl method.  The blessed object must
be on the stack.  See @xref{perlcall,Perlcall}.


@example
I32     perl_call_method _((char* methname, I32 flags));
@end example

@item perl_call_pv
Performs a callback to the specified Perl sub.  See @xref{perlcall,Perlcall}.


@example
I32     perl_call_pv _((char* subname, I32 flags));
@end example

@item perl_call_sv
Performs a callback to the Perl sub whose name is in the SV.  See
@xref{perlcall,Perlcall}.


@example
I32     perl_call_sv _((SV* sv, I32 flags));
@end example

@item perl_construct
Initializes a new Perl interpreter.  See @xref{perlembed,Perlembed}.

@item perl_destruct
Shuts down a Perl interpreter.  See @xref{perlembed,Perlembed}.

@item perl_eval_sv
Tells Perl to eval the string in the SV.


@example
I32     perl_eval_sv _((SV* sv, I32 flags));
@end example

@item perl_eval_pv
Tells Perl to eval the given string and return an SV* result.


@example
SV*     perl_eval_pv _((char* p, I32 croak_on_error));
@end example

@item perl_free
Releases a Perl interpreter.  See @xref{perlembed,Perlembed}.

@item perl_get_av
Returns the AV of the specified Perl array.  If @code{create} is set and the
Perl variable does not exist then it will be created.  If @code{create} is not
set and the variable does not exist then null is returned.


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

@item perl_get_cv
Returns the CV of the specified Perl sub.  If @code{create} is set and the Perl
variable does not exist then it will be created.  If @code{create} is not
set and the variable does not exist then null is returned.


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

@item perl_get_hv
Returns the HV of the specified Perl hash.  If @code{create} is set and the Perl
variable does not exist then it will be created.  If @code{create} is not
set and the variable does not exist then null is returned.


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

@item perl_get_sv
Returns the SV of the specified Perl scalar.  If @code{create} is set and the
Perl variable does not exist then it will be created.  If @code{create} is not
set and the variable does not exist then null is returned.


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

@item perl_parse
Tells a Perl interpreter to parse a Perl script.  See @xref{perlembed,Perlembed}.

@item perl_require_pv
Tells Perl to require a module.


@example
void    perl_require_pv _((char* pv));
@end example

@item perl_run
Tells a Perl interpreter to run.  See @xref{perlembed,Perlembed}.

@item POPi
Pops an integer off the stack.


@example
int POPi();
@end example

@item POPl
Pops a long off the stack.


@example
long POPl();
@end example

@item POPp
Pops a string off the stack.


@example
char * POPp();
@end example

@item POPn
Pops a double off the stack.


@example
double POPn();
@end example

@item POPs
Pops an SV off the stack.


@example
SV* POPs();
@end example

@item PUSHMARK
Opening bracket for arguments on a callback.  See PUTBACK and @xref{perlcall,Perlcall}.


@example
PUSHMARK(p)
@end example

@item PUSHi
Push an integer onto the stack.  The stack must have room for this element.
See XPUSHi.


@example
PUSHi(int d)
@end example

@item PUSHn
Push a double onto the stack.  The stack must have room for this element.
See XPUSHn.


@example
PUSHn(double d)
@end example

@item PUSHp
Push a string onto the stack.  The stack must have room for this element.
The @code{len} indicates the length of the string.  See XPUSHp.


@example
PUSHp(char *c, int len )
@end example

@item PUSHs
Push an SV onto the stack.  The stack must have room for this element.  See
XPUSHs.


@example
PUSHs(sv)
@end example

@item PUTBACK
Closing bracket for XSUB arguments.  This is usually handled by xsubpp.
See PUSHMARK and @xref{perlcall,Perlcall}, for other uses.


@example
PUTBACK;
@end example

@item Renew
The XSUB-writer@'s interface to the C @code{realloc} function.


@example
void * Renew( void *ptr, int size, type )
@end example

@item Renewc
The XSUB-writer@'s interface to the C @code{realloc} function, with cast.


@example
void * Renewc( void *ptr, int size, type, cast )
@end example

@item RETVAL
Variable which is setup by xsubpp to hold the return value for an XSUB.
This is always the proper type for the XSUB.
See @samp{"The RETVAL Variable"}, @xref{perlxs,Perlxs}.

@item safefree
The XSUB-writer@'s interface to the C @code{free} function.

@item safemalloc
The XSUB-writer@'s interface to the C @code{malloc} function.

@item saferealloc
The XSUB-writer@'s interface to the C @code{realloc} function.

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


@example
char*   savepv _((char* sv));
@end example

@item savepvn
Copy a string to a safe spot.  The @code{len} indicates number of bytes to
copy.  This does not use an SV.


@example
char*   savepvn _((char* sv, I32 len));
@end example

@item SAVETMPS
Opening bracket for temporaries on a callback.  See FREETMPS and
@xref{perlcall,Perlcall}.


@example
SAVETMPS;
@end example

@item SP
Stack pointer.  This is usually handled by xsubpp.  See dSP and
SPAGAIN.

@item SPAGAIN
Refetch the stack pointer.  Used after a callback.  See @xref{perlcall,Perlcall}.


@example
SPAGAIN;
@end example

@item ST
Used to access elements on the XSUB@'s stack.


@example
SV* ST(int x)
@end example

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


@example
int strEQ( char *s1, char *s2 )
@end example

@item strGE
Test two strings to see if the first, @code{s1}, is greater than or equal to the
second, @code{s2}.  Returns true or false.


@example
int strGE( char *s1, char *s2 )
@end example

@item strGT
Test two strings to see if the first, @code{s1}, is greater than the second,
@code{s2}.  Returns true or false.


@example
int strGT( char *s1, char *s2 )
@end example

@item strLE
Test two strings to see if the first, @code{s1}, is less than or equal to the
second, @code{s2}.  Returns true or false.


@example
int strLE( char *s1, char *s2 )
@end example

@item strLT
Test two strings to see if the first, @code{s1}, is less than the second,
@code{s2}.  Returns true or false.


@example
int strLT( char *s1, char *s2 )
@end example

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


@example
int strNE( char *s1, char *s2 )
@end example

@item strnEQ
Test two strings to see if they are equal.  The @code{len} parameter indicates
the number of bytes to compare.  Returns true or false.


@example
int strnEQ( char *s1, char *s2 )
@end example

@item strnNE
Test two strings to see if they are different.  The @code{len} parameter
indicates the number of bytes to compare.  Returns true or false.


@example
int strnNE( char *s1, char *s2, int len )
@end example

@item sv_2mortal
Marks an SV as mortal.  The SV will be destroyed when the current context
ends.


@example
SV*     sv_2mortal _((SV* sv));
@end example

@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 @code{gv_stashpv()}).  The reference count
of the SV is unaffected.


@example
SV*     sv_bless _((SV* sv, HV* stash));
@end example

@item sv_catpv
Concatenates the string onto the end of the string which is in the SV.


@example
void    sv_catpv _((SV* sv, char* ptr));
@end example

@item sv_catpvn
Concatenates the string onto the end of the string which is in the SV.  The
@code{len} indicates number of bytes to copy.


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

@item sv_catpvf
Processes its arguments like sprintf and appends the formatted output
to an SV.


@example
void    sv_catpvf _((SV* sv, const char* pat, ...));
@end example

@item sv_catsv
Concatenates the string from SV @code{ssv} onto the end of the string in SV
@code{dsv}.


@example
void    sv_catsv _((SV* dsv, SV* ssv));
@end example

@item sv_cmp
Compares the strings in two SVs.  Returns -1, 0, or 1 indicating whether the
string in @code{sv1} is less than, equal to, or greater than the string in
@code{sv2}.


@example
I32     sv_cmp _((SV* sv1, SV* sv2));
@end example

@item sv_cmp
Compares the strings in two SVs.  Returns -1, 0, or 1 indicating whether the
string in @code{sv1} is less than, equal to, or greater than the string in
@code{sv2}.


@example
I32     sv_cmp _((SV* sv1, SV* sv2));
@end example

@item SvCUR
Returns the length of the string which is in the SV.  See SvLEN.


@example
int SvCUR (SV* sv)
@end example

@item SvCUR_set
Set the length of the string which is in the SV.  See SvCUR.


@example
SvCUR_set (SV* sv, int val )
@end example

@item sv_dec
Auto-decrement of the value in the SV.


@example
void    sv_dec _((SV* sv));
@end example

@item sv_dec
Auto-decrement of the value in the SV.


@example
void    sv_dec _((SV* sv));
@end example

@item SvEND
Returns a pointer to the last character in the string which is in the SV.
See SvCUR.  Access the character as


@example
*SvEND(sv)
@end example

@item sv_eq
Returns a boolean indicating whether the strings in the two SVs are
identical.


@example
I32     sv_eq _((SV* sv1, SV* sv2));
@end example

@item SvGROW
Expands the character buffer in the SV.  Calls sv_grow to perform the
expansion if necessary.  Returns a pointer to the character buffer.


@example
char * SvGROW( SV* sv, int len )
@end example

@item sv_grow
Expands the character buffer in the SV.  This will use sv_unref and will
upgrade the SV to SVt_PV.  Returns a pointer to the character buffer.
Use SvGROW.

@item sv_inc
Auto-increment of the value in the SV.


@example
void    sv_inc _((SV* sv));
@end example

@item SvIOK
Returns a boolean indicating whether the SV contains an integer.


@example
int SvIOK (SV* SV)
@end example

@item SvIOK_off
Unsets the IV status of an SV.


@example
SvIOK_off (SV* sv)
@end example

@item SvIOK_on
Tells an SV that it is an integer.


@example
SvIOK_on (SV* sv)
@end example

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


@example
SvIOK_on (SV* sv)
@end example

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


@example
SvIOK_on (SV* sv)
@end example

@item SvIOKp
Returns a boolean indicating whether the SV contains an integer.  Checks the
@strong{private} setting.  Use SvIOK.


@example
int SvIOKp (SV* SV)
@end example

@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.


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

@item SvIV
Returns the integer which is in the SV.


@example
int SvIV (SV* sv)
@end example

@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.


@example
int     sv_isobject _((SV* sv));
@end example

@item SvIVX
Returns the integer which is stored in the SV.


@example
int  SvIVX (SV* sv);
@end example

@item SvLEN
Returns the size of the string buffer in the SV.  See SvCUR.


@example
int SvLEN (SV* sv)
@end example

@item sv_len
Returns the length of the string in the SV.  Use SvCUR.


@example
STRLEN  sv_len _((SV* sv));
@end example

@item sv_len
Returns the length of the string in the SV.  Use SvCUR.


@example
STRLEN  sv_len _((SV* sv));
@end example

@item sv_magic
Adds magic to an SV.


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

@item sv_mortalcopy
Creates a new SV which is a copy of the original SV.  The new SV is marked
as mortal.


@example
SV*     sv_mortalcopy _((SV* oldsv));
@end example

@item SvOK
Returns a boolean indicating whether the value is an SV.


@example
int SvOK (SV* sv)
@end example

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


@example
SV*     sv_newmortal _((void));
@end example

@item sv_no
This is the @code{false} SV.  See sv_yes.  Always refer to this as @code{&sv_no}.

@item SvNIOK
Returns a boolean indicating whether the SV contains a number, integer or
double.


@example
int SvNIOK (SV* SV)
@end example

@item SvNIOK_off
Unsets the NV/IV status of an SV.


@example
SvNIOK_off (SV* sv)
@end example

@item SvNIOKp
Returns a boolean indicating whether the SV contains a number, integer or
double.  Checks the @strong{private} setting.  Use SvNIOK.


@example
int SvNIOKp (SV* SV)
@end example

@item SvNOK
Returns a boolean indicating whether the SV contains a double.


@example
int SvNOK (SV* SV)
@end example

@item SvNOK_off
Unsets the NV status of an SV.


@example
SvNOK_off (SV* sv)
@end example

@item SvNOK_on
Tells an SV that it is a double.


@example
SvNOK_on (SV* sv)
@end example

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


@example
SvNOK_on (SV* sv)
@end example

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


@example
SvNOK_on (SV* sv)
@end example

@item SvNOKp
Returns a boolean indicating whether the SV contains a double.  Checks the
@strong{private} setting.  Use SvNOK.


@example
int SvNOKp (SV* SV)
@end example

@item SvNV
Returns the double which is stored in the SV.


@example
double SvNV (SV* sv);
@end example

@item SvNVX
Returns the double which is stored in the SV.


@example
double SvNVX (SV* sv);
@end example

@item SvPOK
Returns a boolean indicating whether the SV contains a character string.


@example
int SvPOK (SV* SV)
@end example

@item SvPOK_off
Unsets the PV status of an SV.


@example
SvPOK_off (SV* sv)
@end example

@item SvPOK_on
Tells an SV that it is a string.


@example
SvPOK_on (SV* sv)
@end example

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


@example
SvPOK_on (SV* sv)
@end example

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


@example
SvPOK_on (SV* sv)
@end example

@item SvPOKp
Returns a boolean indicating whether the SV contains a character string.
Checks the @strong{private} setting.  Use SvPOK.


@example
int SvPOKp (SV* SV)
@end example

@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 @code{len} is na then Perl will
handle the length on its own.


@example
char * SvPV (SV* sv, int len )
@end example

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


@example
char * SvPVX (SV* sv)
@end example

@item SvREFCNT
Returns the value of the object@'s reference count.


@example
int SvREFCNT (SV* sv);
@end example

@item SvREFCNT_dec
Decrements the reference count of the given SV.


@example
void SvREFCNT_dec (SV* sv)
@end example

@item SvREFCNT_inc
Increments the reference count of the given SV.


@example
void SvREFCNT_inc (SV* sv)
@end example

@item SvROK
Tests if the SV is an RV.


@example
int SvROK (SV* sv)
@end example

@item SvROK_off
Unsets the RV status of an SV.


@example
SvROK_off (SV* sv)
@end example

@item SvROK_on
Tells an SV that it is an RV.


@example
SvROK_on (SV* sv)
@end example

@item SvRV
Dereferences an RV to return the SV.


@example
SV*     SvRV (SV* sv);
@end example

@item sv_setiv
Copies an integer into the given SV.


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

@item sv_setnv
Copies a double into the given SV.


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

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


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

@item sv_setpvn
Copies a string into an SV.  The @code{len} parameter indicates the number of
bytes to be copied.


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

@item sv_setpvf
Processes its arguments like sprintf and sets an SV to the formatted
output.


@example
void    sv_setpvf _((SV* sv, const char* pat, ...));
@end example

@item sv_setref_iv
Copies an integer into a new SV, optionally blessing the SV.  The @code{rv}
argument will be upgraded to an RV.  That RV will be modified to point to
the new SV.  The @code{classname} argument indicates the package for the
blessing.  Set @code{classname} to Nullch to avoid the blessing.  The new SV
will be returned and will have a reference count of 1.


@example
SV*     sv_setref_iv _((SV *rv, char *classname, IV iv));
@end example

@item sv_setref_nv
Copies a double into a new SV, optionally blessing the SV.  The @code{rv}
argument will be upgraded to an RV.  That RV will be modified to point to
the new SV.  The @code{classname} argument indicates the package for the
blessing.  Set @code{classname} to Nullch to avoid the blessing.  The new SV
will be returned and will have a reference count of 1.


@example
SV*     sv_setref_nv _((SV *rv, char *classname, double nv));
@end example

@item sv_setref_pv
Copies a pointer into a new SV, optionally blessing the SV.  The @code{rv}
argument will be upgraded to an RV.  That RV will be modified to point to
the new SV.  If the @code{pv} argument is NULL then sv_undef will be placed
into the SV.  The @code{classname} argument indicates the package for the
blessing.  Set @code{classname} to Nullch to avoid the blessing.  The new SV
will be returned and will have a reference count of 1.


@example
SV*     sv_setref_pv _((SV *rv, char *classname, void* pv));
@end example

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 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 n.  The @code{rv} argument will be upgraded to
an RV.  That RV will be modified to point to the new SV.  The @code{classname}
argument indicates the package for the blessing.  Set @code{classname} to
Nullch to avoid the blessing.  The new SV will be returned and will have
a reference count of 1.


@example
SV*     sv_setref_pvn _((SV *rv, char *classname, char* pv, I32 n));
@end example

Note that sv_setref_pv copies the pointer while this copies the string.

@item sv_setsv
Copies the contents of the source SV @code{ssv} into the destination SV @code{dsv}.
The source SV may be destroyed if it is mortal.


@example
void    sv_setsv _((SV* dsv, SV* ssv));
@end example

@item SvSTASH
Returns the stash of the SV.


@example
HV * SvSTASH (SV* sv)
@end example

@item SVt_IV
Integer type flag for scalars.  See svtype.

@item SVt_PV
Pointer type flag for scalars.  See svtype.

@item SVt_PVAV
Type flag for arrays.  See svtype.

@item SVt_PVCV
Type flag for code refs.  See svtype.

@item SVt_PVHV
Type flag for hashes.  See svtype.

@item SVt_PVMG
Type flag for blessed scalars.  See svtype.

@item SVt_NV
Double type flag for scalars.  See svtype.

@item SvTRUE
Returns a boolean indicating whether Perl would evaluate the SV as true or
false, defined or undefined.


@example
int SvTRUE (SV* sv)
@end example

@item SvTYPE
Returns the type of the SV.  See svtype.


@example
svtype  SvTYPE (SV* sv)
@end example

@item svtype
An enum of flags for Perl types.  These are found in the file @strong{sv.h} in the
svtype enum.  Test these flags with the SvTYPE macro.

@item SvUPGRADE
Used to upgrade an SV to a more complex form.  Uses sv_upgrade to perform
the upgrade if necessary.  See svtype.


@example
bool    SvUPGRADE _((SV* sv, svtype mt));
@end example

@item sv_upgrade
Upgrade an SV to a more complex form.  Use SvUPGRADE.  See svtype.

@item sv_undef
This is the undef SV.  Always refer to this as @code{&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 newSVrv.  See SvROK_off.


@example
void    sv_unref _((SV* sv));
@end example

@item sv_usepvn
Tells an SV to use @code{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 @code{ptr} should point to memory that was allocated by @code{malloc}.  The
string length, @code{len}, must be supplied.  This function will realloc the
memory pointed to by @code{ptr}, so that pointer should not be freed or used by
the programmer after giving it to sv_usepvn.


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

@item sv_yes
This is the @code{true} SV.  See sv_no.  Always refer to this as @code{&sv_yes}.

@item THIS
Variable which is setup by xsubpp to designate the object in a C++ XSUB.
This is always the proper type for the C++ object.  See CLASS and
@samp{"Using XS With C++"}, @xref{perlxs,Perlxs}.

@item toLOWER
Converts the specified character to lowercase.


@example
int toLOWER (char c)
@end example

@item toUPPER
Converts the specified character to uppercase.


@example
int toUPPER (char c)
@end example

@item warn
This is the XSUB-writer@'s interface to Perl@'s warn function.  Use this
function the same way you use the C printf function.  See @code{croak()}.

@item XPUSHi
Push an integer onto the stack, extending the stack if necessary.  See
PUSHi.


@example
XPUSHi(int d)
@end example

@item XPUSHn
Push a double onto the stack, extending the stack if necessary.  See
PUSHn.


@example
XPUSHn(double d)
@end example

@item XPUSHp
Push a string onto the stack, extending the stack if necessary.  The @code{len}
indicates the length of the string.  See PUSHp.


@example
XPUSHp(char *c, int len)
@end example

@item XPUSHs
Push an SV onto the stack, extending the stack if necessary.  See PUSHs.


@example
XPUSHs(sv)
@end example

@item XS
Macro to declare an XSUB and its C parameter list.  This is handled by
xsubpp.

@item XSRETURN
Return from XSUB, indicating number of items on the stack.  This is usually
handled by xsubpp.


@example
XSRETURN(int x);
@end example

@item XSRETURN_EMPTY
Return an empty list from an XSUB immediately.


@example
XSRETURN_EMPTY;
@end example

@item XSRETURN_IV
Return an integer from an XSUB immediately.  Uses XST_mIV.


@example
XSRETURN_IV(IV v);
@end example

@item XSRETURN_NO
Return @code{&sv_no} from an XSUB immediately.  Uses XST_mNO.


@example
XSRETURN_NO;
@end example

@item XSRETURN_NV
Return an double from an XSUB immediately.  Uses XST_mNV.


@example
XSRETURN_NV(NV v);
@end example

@item XSRETURN_PV
Return a copy of a string from an XSUB immediately.  Uses XST_mPV.


@example
XSRETURN_PV(char *v);
@end example

@item XSRETURN_UNDEF
Return @code{&sv_undef} from an XSUB immediately.  Uses XST_mUNDEF.


@example
XSRETURN_UNDEF;
@end example

@item XSRETURN_YES
Return @code{&sv_yes} from an XSUB immediately.  Uses XST_mYES.


@example
XSRETURN_YES;
@end example

@item XST_mIV
Place an integer into the specified position i on the stack.  The value is
stored in a new mortal SV.


@example
XST_mIV( int i, IV v );
@end example

@item XST_mNV
Place a double into the specified position i on the stack.  The value is
stored in a new mortal SV.


@example
XST_mNV( int i, NV v );
@end example

@item XST_mNO
Place @code{&sv_no} into the specified position i on the stack.


@example
XST_mNO( int i );
@end example

@item XST_mPV
Place a copy of a string into the specified position i on the stack.  The
value is stored in a new mortal SV.


@example
XST_mPV( int i, char *v );
@end example

@item XST_mUNDEF
Place @code{&sv_undef} into the specified position i on the stack.


@example
XST_mUNDEF( int i );
@end example

@item XST_mYES
Place @code{&sv_yes} into the specified position i on the stack.


@example
XST_mYES( int i );
@end example

@item XS_VERSION
The version identifier for an XS module.  This is usually handled
automatically by ExtUtils::MakeMaker.  See XS_VERSION_BOOTCHECK.

@item XS_VERSION_BOOTCHECK
Macro to verify that a PM module@'s $VERSION variable matches the XS module@'s
XS_VERSION variable.  This is usually handled automatically by
xsubpp.  See @samp{"The VERSIONCHECK: Keyword"}, @xref{perlxs,Perlxs}.

@item Zero
The XSUB-writer@'s interface to the C @code{memzero} function.  The d is the
destination, n is the number of items, and t is the type.


@example
(void) Zero( d, n, t );
@end example

@end table
@unnumberedsec EDITOR
Jeff Okamoto <@file{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 <@file{roehrich@@cray.com}>.

@unnumberedsec DATE
Version 31.7: 1997/5/1
