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File: perl.info,  Node: perlguts,  Next: perlcall,  Prev: perlxstut,  Up: Top

Introduction to the Perl API
****************************

NAME
====

   perlguts - Introduction to the Perl API

DESCRIPTION
===========

   This document attempts to describe how to use the Perl API, as well as
containing some info on the basic workings of the Perl core. It is far
from complete and probably contains many errors. Please refer any
questions or comments to the author below.

Variables
=========

Datatypes
---------

   Perl has three typedefs that handle Perl's three main data types:

     SV  Scalar Value
     AV  Array Value
     HV  Hash Value

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

What is an "IV"?
----------------

   Perl uses a special typedef IV which is a simple signed integer type
that is guaranteed to be large enough to hold a pointer (as well as an
integer).  Additionally, there is the UV, which is simply an unsigned IV.

   Perl also uses two special typedefs, I32 and I16, which will always be
at least 32-bits and 16-bits long, respectively. (Again, there are U32 and
U16, as well.)

Working with SVs
----------------

   An SV can be created and loaded with one command.  There are four types
of values that can be loaded: an integer value (IV), a double (NV), a
string, (PV), and another scalar (SV).

   The six routines are:

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

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

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

   Notice that you can choose to specify the length of the string to be
assigned by using sv_setpvn, newSVpvn, or newSVpv, or you may allow Perl
to calculate the length by using sv_setpv or by specifying 0 as the second
argument to newSVpv.  Be warned, though, that Perl will determine the
string's length by using strlen, which depends on the string terminating
with a NUL character.

   The arguments of sv_setpvf are processed like sprintf, and the
formatted output becomes the value.

   `sv_setpvfn' is an analogue of vsprintf, but it allows you to specify
either a pointer to a variable argument list or the address and length of
an array of SVs.  The last argument points to a boolean; on return, if that
boolean is true, then locale-specific information has been used to format
the string, and the string's contents are therefore untrustworthy (see
*Note Perlsec: perlsec,).  This pointer may be NULL if that information is
not important.  Note that this function requires you to specify the length
of the format.

   The `sv_set*()' functions are not generic enough to operate on values
that have "magic".  See `Magic Virtual Tables' in this node later in this
document.

   All SVs that contain strings should be terminated with a NUL character.
If it is not NUL-terminated there is a risk of core dumps and corruptions
from code which passes the string to C functions or system calls which
expect a NUL-terminated string.  Perl's own functions typically add a
trailing NUL for this reason.  Nevertheless, you should be very careful
when you pass a string stored in an SV to a C function or system call.

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

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

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

   In the SvPV macro, the length of the string returned is placed into the
variable len (this is a macro, so you do not use `&len').  If you do not
care what the length of the data is, use the SvPV_nolen macro.
Historically the SvPV macro with the global variable PL_na has been used
in this case.  But that can be quite inefficient because PL_na must be
accessed in thread-local storage in threaded Perl.  In any case, remember
that Perl allows arbitrary strings of data that may both contain NULs and
might not be terminated by a NUL.

   Also remember that C doesn't allow you to safely say `foo(SvPV(s, len),
len);'. It might work with your compiler, but it won't work for everyone.
Break this sort of statement up into separate assignments:

     SV *s;
     STRLEN len;
     char * ptr;
     ptr = SvPV(s, len);
     foo(ptr, len);

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

     SvTRUE(SV*)

   Although Perl will automatically grow strings for you, if you need to
force Perl to allocate more memory for your SV, you can use the macro

     SvGROW(SV*, STRLEN newlen)

   which will determine if more memory needs to be allocated.  If so, it
will call the function sv_grow.  Note that SvGROW can only increase, not
decrease, the allocated memory of an SV and that it does not automatically
add a byte for the a trailing NUL (perl's own string functions typically do
`SvGROW(sv, len + 1)').

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

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

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

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

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

     SvEND(SV*)

   But note that these last three macros are valid only if `SvPOK()' is
true.

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

     void  sv_catpv(SV*, const char*);
     void  sv_catpvn(SV*, const char*, STRLEN);
     void  sv_catpvf(SV*, const char*, ...);
     void  sv_catpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool);
     void  sv_catsv(SV*, SV*);

   The first function calculates the length of the string to be appended by
using strlen.  In the second, you specify the length of the string
yourself.  The third function processes its arguments like sprintf and
appends the formatted output.  The fourth function works like vsprintf.
You can specify the address and length of an array of SVs instead of the
va_list argument. The fifth function extends the string stored in the first
SV with the string stored in the second SV.  It also forces the second SV
to be interpreted as a string.

   The `sv_cat*()' functions are not generic enough to operate on values
that have "magic".  See `Magic Virtual Tables' in this node later in this
document.

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

     SV*  get_sv("package::varname", FALSE);

   This returns NULL if the variable does not exist.

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

     SvOK(SV*)

   The scalar undef value is stored in an SV instance called PL_sv_undef.
Its address can be used whenever an `SV*' is needed.

   There are also the two values PL_sv_yes and PL_sv_no, which contain
Boolean TRUE and FALSE values, respectively.  Like PL_sv_undef, their
addresses can be used whenever an `SV*' is needed.

   Do not be fooled into thinking that `(SV *) 0' is the same as
`&PL_sv_undef'.  Take this code:

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

   This code tries to return a new SV (which contains the value 42) if it
should return a real value, or undef otherwise.  Instead it has returned a
NULL pointer which, somewhere down the line, will cause a segmentation
violation, bus error, or just weird results.  Change the zero to
`&PL_sv_undef' in the first line and all will be well.

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

What's Really Stored in an SV?
------------------------------

   Recall that the usual method of determining the type of scalar you have
is to use `Sv*OK' macros.  Because a scalar can be both a number and a
string, usually these macros will always return TRUE and calling the `Sv*V'
macros will do the appropriate conversion of string to integer/double or
integer/double to string.

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

     SvIOKp(SV*)
     SvNOKp(SV*)
     SvPOKp(SV*)

   These will tell you if you truly have an integer, double, or string
pointer stored in your SV.  The "p" stands for private.

   In general, though, it's best to use the `Sv*V' macros.

Working with AVs
----------------

   There are two ways to create and load an AV.  The first method creates
an empty AV:

     AV*  newAV();

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

     AV*  av_make(I32 num, SV **ptr);

   The second argument points to an array containing num `SV*''s.  Once the
AV has been created, the SVs can be destroyed, if so desired.

   Once the AV has been created, the following operations are possible on
AVs:

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

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

   Here are some other functions:

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

   The av_len function returns the highest index value in array (just like
$#array in Perl).  If the array is empty, -1 is returned.  The av_fetch
function returns the value at index key, but if `lval' is non-zero, then
av_fetch will store an undef value at that index.  The av_store function
stores the value val at index key, and does not increment the reference
count of val.  Thus the caller is responsible for taking care of that, and
if av_store returns NULL, the caller will have to decrement the reference
count to avoid a memory leak.  Note that av_fetch and av_store both return
`SV**''s, not `SV*''s as their return value.

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

   The av_clear function deletes all the elements in the AV* array, but
does not actually delete the array itself.  The av_undef function will
delete all the elements in the array plus the array itself.  The av_extend
function extends the array so that it contains at least `key+1' elements.
If `key+1' is less than the currently allocated length of the array, then
nothing is done.

   If you know the name of an array variable, you can get a pointer to its
AV by using the following:

     AV*  get_av("package::varname", FALSE);

   This returns NULL if the variable does not exist.

   See `Understanding the Magic of Tied Hashes and Arrays' in this node
for more information on how to use the array access functions on tied
arrays.

Working with HVs
----------------

   To create an HV, you use the following routine:

     HV*  newHV();

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

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

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

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

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

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

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

   And more miscellaneous functions:

     void   hv_clear(HV*);
     void   hv_undef(HV*);

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

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

     I32    hv_iterinit(HV*);
             /* Prepares starting point to traverse hash table */
     HE*    hv_iternext(HV*);
             /* Get the next entry, and return a pointer to a
                structure that has both the key and value */
     char*  hv_iterkey(HE* entry, I32* retlen);
             /* Get the key from an HE structure and also return
                the length of the key string */
     SV*    hv_iterval(HV*, HE* entry);
             /* Return a SV pointer to the value of the HE
                structure */
     SV*    hv_iternextsv(HV*, char** key, I32* retlen);
             /* This convenience routine combines hv_iternext,
     	       hv_iterkey, and hv_iterval.  The key and retlen
     	       arguments are return values for the key and its
     	       length.  The value is returned in the SV* argument */

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

     HV*  get_hv("package::varname", FALSE);

   This returns NULL if the variable does not exist.

   The hash algorithm is defined in the `PERL_HASH(hash, key, klen)' macro:

     hash = 0;
     while (klen--)
     	hash = (hash * 33) + *key++;
     hash = hash + (hash >> 5);			/* after 5.6 */

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

   See `Understanding the Magic of Tied Hashes and Arrays' in this node
for more information on how to use the hash access functions on tied
hashes.

Hash API Extensions
-------------------

   Beginning with version 5.004, the following functions are also
supported:

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

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

     SV*     hv_iterkeysv  (HE* entry);

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

   They also return and accept whole hash entries (`HE*'), making their
use more efficient (since the hash number for a particular string doesn't
have to be recomputed every time).  See *Note Perlapi: perlapi, 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 *Note
Perlapi: perlapi, for detailed descriptions of these macros.

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

   These two lower level macros are defined, but must only be used when
dealing with keys that are not `SV*'s:

     HeKEY(HE* he)
     HeKLEN(HE* he)

   Note that both hv_store and hv_store_ent do not increment the reference
count of the stored val, which is the caller's responsibility.  If these
functions return a NULL value, the caller will usually have to decrement
the reference count of val to avoid a memory leak.

References
----------

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

   To create a reference, use either of the following functions:

     SV* newRV_inc((SV*) thing);
     SV* newRV_noinc((SV*) thing);

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

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

     SvRV(SV*)

   then call the appropriate routines, casting the returned `SV*' to
either an `AV*' or `HV*', if required.

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

     SvROK(SV*)

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

     SvTYPE(SvRV(SV*))

   The most useful types that will be returned are:

     SVt_IV    Scalar
     SVt_NV    Scalar
     SVt_PV    Scalar
     SVt_RV    Scalar
     SVt_PVAV  Array
     SVt_PVHV  Hash
     SVt_PVCV  Code
     SVt_PVGV  Glob (possible a file handle)
     SVt_PVMG  Blessed or Magical Scalar

     See the sv.h header file for more details.

Blessed References and Class Objects
------------------------------------

   References are also used to support object-oriented programming.  In the
OO lexicon, an object is simply a reference that has been blessed into a
package (or class).  Once blessed, the programmer may now use the reference
to access the various methods in the class.

   A reference can be blessed into a package with the following function:

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

   The sv argument must be a reference.  The stash argument specifies
which class the reference will belong to.  See `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 `classname' is non-null, the SV is blessed into the specified
class.  SV is returned.

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

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

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

   Copies the pointer value (*the address, not the string!*) into an SV
whose reference is rv.  SV is blessed if `classname' is non-null.

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

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

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

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

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

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

     int  sv_isobject(SV* sv);

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

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

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

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

Creating New Variables
----------------------

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

     SV*  get_sv("package::varname", TRUE);
     AV*  get_av("package::varname", TRUE);
     HV*  get_hv("package::varname", TRUE);

   Notice the use of TRUE as the second parameter.  The new variable can
now be set, using the routines appropriate to the data type.

   There are additional macros whose values may be bitwise OR'ed with the
TRUE argument to enable certain extra features.  Those bits are:

     GV_ADDMULTI	Marks the variable as multiply defined, thus preventing the
     		"Name <varname> used only once: possible typo" warning.
     GV_ADDWARN	Issues the warning "Had to create <varname> unexpectedly" if
     		the variable did not exist before the function was called.

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

Reference Counts and Mortality
------------------------------

   Perl uses an reference count-driven garbage collection mechanism. SVs,
AVs, or HVs (xV for short in the following) start their life with a
reference count of 1.  If the reference count of an xV ever drops to 0,
then it will be destroyed and its memory made available for reuse.

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

     int SvREFCNT(SV* sv);
     SV* SvREFCNT_inc(SV* sv);
     void SvREFCNT_dec(SV* sv);

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

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

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

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

     SV*  sv_newmortal()
     SV*  sv_2mortal(SV*)
     SV*  sv_mortalcopy(SV*)

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

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

Stashes and Globs
-----------------

   A "stash" is a hash that contains all of the different objects that are
contained within a package.  Each key of the stash is a symbol name
(shared by all the different types of objects that have the same name),
and each value in the hash table is a GV (Glob Value).  This GV in turn
contains references to the various objects of that name, including (but
not limited to) the following:

     Scalar Value
     Array Value
     Hash Value
     I/O Handle
     Format
     Subroutine

   There is a single stash called "PL_defstash" that holds the items that
exist in the "main" package.  To get at the items in other packages,
append the string "::" to the package name.  The items in the "Foo"
package are in the stash "Foo::" in PL_defstash.  The items in the
"Bar::Baz" package are in the stash "Baz::" in "Bar::"'s stash.

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

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

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

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

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

     HV*  SvSTASH(SvRV(SV*));

   then use the following to get the package name itself:

     char*  HvNAME(HV* stash);

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

     SV*  sv_bless(SV*, HV* stash)

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

   For more information on references and blessings, consult *Note
Perlref: perlref,.

Double-Typed SVs
----------------

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

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

   To force multiple data values into an SV, you must do two things: use
the `sv_set*v' routines to add the additional scalar type, then set a flag
so that Perl will believe it contains more than one type of data.  The
four macros to set the flags are:

     SvIOK_on
     SvNOK_on
     SvPOK_on
     SvROK_on

   The particular macro you must use depends on which `sv_set*v' routine
you called first.  This is because every `sv_set*v' routine turns on only
the bit for the particular type of data being set, and turns off all the
rest.

   For example, to create a new Perl variable called "dberror" that
contains both the numeric and descriptive string error values, you could
use the following code:

     extern int  dberror;
     extern char *dberror_list;

     SV* sv = get_sv("dberror", TRUE);
     sv_setiv(sv, (IV) dberror);
     sv_setpv(sv, dberror_list[dberror]);
     SvIOK_on(sv);

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

Magic Variables
---------------

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

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

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

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

Assigning Magic
---------------

   Perl adds magic to an SV using the sv_magic function:

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

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

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

   The name and `namlen' arguments are used to associate a string with the
magic, typically the name of a variable. `namlen' is stored in the
`mg_len' field and if name is non-null and `namlen' >= 0 a malloc'd copy
of the name is stored in `mg_ptr' field.

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

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

   There is also a function to add magic to an HV:

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

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

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

     void sv_unmagic(SV *sv, int type);

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

Magic Virtual Tables
--------------------

   The `mg_virtual' field in the MAGIC structure is a pointer to a
`MGVTBL', which is a structure of function pointers and stands for "Magic
Virtual Table" to handle the various operations that might be applied to
that variable.

   The `MGVTBL' has five pointers to the following routine types:

     int  (*svt_get)(SV* sv, MAGIC* mg);
     int  (*svt_set)(SV* sv, MAGIC* mg);
     U32  (*svt_len)(SV* sv, MAGIC* mg);
     int  (*svt_clear)(SV* sv, MAGIC* mg);
     int  (*svt_free)(SV* sv, MAGIC* mg);

   This MGVTBL structure is set at compile-time in `perl.h' and there are
currently 19 types (or 21 with overloading turned on).  These different
structures contain pointers to various routines that perform additional
actions depending on which function is being called.

     Function pointer    Action taken
     ----------------    ------------
     svt_get             Do something after the value of the SV is retrieved.
     svt_set             Do something after the SV is assigned a value.
     svt_len             Report on the SV's length.
     svt_clear		Clear something the SV represents.
     svt_free            Free any extra storage associated with the SV.

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

     { magic_get, magic_set, magic_len, 0, 0 }

   Thus, when an SV is determined to be magical and of type '\0', if a get
operation is being performed, the routine `magic_get' is called.  All the
various routines for the various magical types begin with `magic_'.  NOTE:
the magic routines are not considered part of the Perl API, and may not be
exported by the Perl library.

   The current kinds of Magic Virtual Tables are:

     mg_type  MGVTBL              Type of magic
     -------  ------              ----------------------------
     \0       vtbl_sv             Special scalar variable
     A        vtbl_amagic         %OVERLOAD hash
     a        vtbl_amagicelem     %OVERLOAD hash element
     c        (none)              Holds overload table (AMT) on stash
     B        vtbl_bm             Boyer-Moore (fast string search)
     E        vtbl_env            %ENV hash
     e        vtbl_envelem        %ENV hash element
     f        vtbl_fm             Formline ('compiled' format)
     g        vtbl_mglob          m//g target / study()ed string
     I        vtbl_isa            @ISA array
     i        vtbl_isaelem        @ISA array element
     k        vtbl_nkeys          scalar(keys()) lvalue
     L        (none)              Debugger %_<filename
     l        vtbl_dbline         Debugger %_<filename element
     o        vtbl_collxfrm       Locale transformation
     P        vtbl_pack           Tied array or hash
     p        vtbl_packelem       Tied array or hash element
     q        vtbl_packelem       Tied scalar or handle
     S        vtbl_sig            %SIG hash
     s        vtbl_sigelem        %SIG hash element
     t        vtbl_taint          Taintedness
     U        vtbl_uvar           Available for use by extensions
     v        vtbl_vec            vec() lvalue
     x        vtbl_substr         substr() lvalue
     y        vtbl_defelem        Shadow "foreach" iterator variable /
                                   smart parameter vivification
     *        vtbl_glob           GV (typeglob)
     #        vtbl_arylen         Array length ($#ary)
     .        vtbl_pos            pos() lvalue
     ~        (none)              Available for use by extensions

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

   The '~' and 'U' magic types are defined specifically for use by
extensions and will not be used by perl itself.  Extensions can use '~'
magic to 'attach' private information to variables (typically objects).
This is especially useful because there is no way for normal perl code to
corrupt this private information (unlike using extra elements of a hash
object).

   Similarly, 'U' magic can be used much like tie() to call a C function
any time a scalar's value is used or changed.  The MAGIC's `mg_ptr' field
points to a `ufuncs' structure:

     struct ufuncs {
         I32 (*uf_val)(IV, SV*);
         I32 (*uf_set)(IV, SV*);
         IV uf_index;
     };

   When the SV is read from or written to, the `uf_val' or `uf_set'
function will be called with `uf_index' as the first arg and a pointer to
the SV as the second.  A simple example of how to add 'U' magic is shown
below.  Note that the ufuncs structure is copied by sv_magic, so you can
safely allocate it on the stack.

     void
     Umagic(sv)
         SV *sv;
     PREINIT:
         struct ufuncs uf;
     CODE:
         uf.uf_val   = &my_get_fn;
         uf.uf_set   = &my_set_fn;
         uf.uf_index = 0;
         sv_magic(sv, 0, 'U', (char*)&uf, sizeof(uf));

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

   Also note that the `sv_set*()' and `sv_cat*()' functions described
earlier do not invoke 'set' magic on their targets.  This must be done by
the user either by calling the `SvSETMAGIC()' macro after calling these
functions, or by using one of the `sv_set*_mg()' or `sv_cat*_mg()'
functions.  Similarly, generic C code must call the `SvGETMAGIC()' macro
to invoke any 'get' magic if they use an SV obtained from external sources
in functions that don't handle magic.  See *Note Perlapi: perlapi, for a
description of these functions.  For example, calls to the `sv_cat*()'
functions typically need to be followed by `SvSETMAGIC()', but they don't
need a prior `SvGETMAGIC()' since their implementation handles 'get' magic.

Finding Magic
-------------

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

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

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

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

Understanding the Magic of Tied Hashes and Arrays
-------------------------------------------------

   Tied hashes and arrays are magical beasts of the 'P' magic type.

   WARNING: As of the 5.004 release, proper usage of the array and hash
access functions requires understanding a few caveats.  Some of these
caveats are actually considered bugs in the API, to be fixed in later
releases, and are bracketed with [MAYCHANGE] below. If you find yourself
actually applying such information in this section, be aware that the
behavior may change in the future, umm, without warning.

   The perl tie function associates a variable with an object that
implements the various GET, SET etc methods.  To perform the equivalent of
the perl tie function from an XSUB, you must mimic this behaviour.  The
code below carries out the necessary steps - firstly it creates a new
hash, and then creates a second hash which it blesses into the class which
will implement the tie methods. Lastly it ties the two hashes together,
and returns a reference to the new tied hash.  Note that the code below
does NOT call the TIEHASH method in the MyTie class - see `Calling Perl
Routines from within C Programs' in this node for details on how to do
this.

     SV*
     mytie()
     PREINIT:
         HV *hash;
         HV *stash;
         SV *tie;
     CODE:
         hash = newHV();
         tie = newRV_noinc((SV*)newHV());
         stash = gv_stashpv("MyTie", TRUE);
         sv_bless(tie, stash);
         hv_magic(hash, tie, 'P');
         RETVAL = newRV_noinc(hash);
     OUTPUT:
         RETVAL

   The av_store function, when given a tied array argument, merely copies
the magic of the array onto the value to be "stored", using mg_copy.  It
may also return NULL, indicating that the value did not actually need to
be stored in the array.  [MAYCHANGE] After a call to av_store on a tied
array, the caller will usually need to call `mg_set(val)' to actually
invoke the perl level "STORE" method on the TIEARRAY object.  If av_store
did return NULL, a call to `SvREFCNT_dec(val)' will also be usually
necessary to avoid a memory leak. [/MAYCHANGE]

   The previous paragraph is applicable verbatim to tied hash access using
the hv_store and hv_store_ent functions as well.

   av_fetch and the corresponding hash functions hv_fetch and hv_fetch_ent
actually return an undefined mortal value whose magic has been initialized
using mg_copy.  Note the value so returned does not need to be
deallocated, as it is already mortal.  [MAYCHANGE] But you will need to
call `mg_get()' on the returned value in order to actually invoke the perl
level "FETCH" method on the underlying TIE object.  Similarly, you may
also call `mg_set()' on the return value after possibly assigning a
suitable value to it using sv_setsv,  which will invoke the "STORE" method
on the TIE object. [/MAYCHANGE]

   [MAYCHANGE] In other words, the array or hash fetch/store functions
don't really fetch and store actual values in the case of tied arrays and
hashes.  They merely call mg_copy to attach magic to the values that were
meant to be "stored" or "fetched".  Later calls to mg_get and mg_set
actually do the job of invoking the TIE methods on the underlying objects.
Thus the magic mechanism currently implements a kind of lazy access to
arrays and hashes.

   Currently (as of perl version 5.004), use of the hash and array access
functions requires the user to be aware of whether they are operating on
"normal" hashes and arrays, or on their tied variants.  The API may be
changed to provide more transparent access to both tied and normal data
types in future versions.  [/MAYCHANGE]

   You would do well to understand that the TIEARRAY and TIEHASH interfaces
are mere sugar to invoke some perl method calls while using the uniform
hash and array syntax.  The use of this sugar imposes some overhead
(typically about two to four extra opcodes per FETCH/STORE operation, in
addition to the creation of all the mortal variables required to invoke
the methods).  This overhead will be comparatively small if the TIE
methods are themselves substantial, but if they are only a few statements
long, the overhead will not be insignificant.

Localizing changes
------------------

   Perl has a very handy construction

     {
       local $var = 2;
       ...
     }

   This construction is *approximately* equivalent to

     {
       my $oldvar = $var;
       $var = 2;
       ...
       $var = $oldvar;
     }

   The biggest difference is that the first construction would reinstate
the initial value of $var, irrespective of how control exits the block:
goto, return, die/eval etc. It is a little bit more efficient as well.

   There is a way to achieve a similar task from C via Perl API: create a
*pseudo-block*, and arrange for some changes to be automatically undone at
the end of it, either explicit, or via a non-local exit (via die()). A
block-like construct is created by a pair of ENTER/LEAVE macros (see
`"Returning a Scalar"', *Note Perlcall: perlcall,).  Such a construct may
be created specially for some important localized task, or an existing one
(like boundaries of enclosing Perl subroutine/block, or an existing pair
for freeing TMPs) may be used. (In the second case the overhead of
additional localization must be almost negligible.) Note that any XSUB is
automatically enclosed in an ENTER/LEAVE pair.

   Inside such a *pseudo-block* the following service is available:

`SAVEINT(int i)'
`SAVEIV(IV i)'

`SAVEI32(I32 i)'
`SAVELONG(long i)'
     These macros arrange things to restore the value of integer variable
     i at the end of enclosing *pseudo-block*.

`SAVESPTR(s)'
`SAVEPPTR(p)'
     These macros arrange things to restore the value of pointers s and p.
     s must be a pointer of a type which survives conversion to `SV*' and
     back, p should be able to survive conversion to `char*' and back.

`SAVEFREESV(SV *sv)'
     The refcount of sv would be decremented at the end of *pseudo-block*.
     This is similar to sv_2mortal, which should (?) be used instead.

`SAVEFREEOP(OP *op)'
     The `OP *' is op_free()ed at the end of *pseudo-block*.

`SAVEFREEPV(p)'
     The chunk of memory which is pointed to by p is Safefree()ed at the
     end of *pseudo-block*.

`SAVECLEARSV(SV *sv)'
     Clears a slot in the current scratchpad which corresponds to sv at
     the end of *pseudo-block*.

`SAVEDELETE(HV *hv, char *key, I32 length)'
     The key key of hv is deleted at the end of *pseudo-block*. The string
     pointed to by key is Safefree()ed.  If one has a key in short-lived
     storage, the corresponding string may be reallocated like this:

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

`SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)'
     At the end of *pseudo-block* the function f is called with the only
     argument p.

`SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)'
     At the end of *pseudo-block* the function f is called with the
     implicit context argument (if any), and p.

`SAVESTACK_POS()'
     The current offset on the Perl internal stack (cf. SP) is restored at
     the end of *pseudo-block*.

   The following API list contains functions, thus one needs to provide
pointers to the modifiable data explicitly (either C pointers, or Perlish
`GV *'s).  Where the above macros take int, a similar function takes `int
*'.

   * save_scalar(GV *gv)

     Equivalent to Perl code `local $gv'.

   * save_ary(GV *gv)

   * save_hash(GV *gv)

     Similar to `save_scalar', but localize `@gv' and `%gv'.

   * `void save_item(SV *item)' Duplicates the current value of SV, on the
     exit from the current ENTER/LEAVE *pseudo-block* will restore the
     value of SV using the stored value.

   * `void save_list(SV **sarg, I32 maxsarg)' A variant of `save_item'
     which takes multiple arguments via an array `sarg' of `SV*' of length
     `maxsarg'.

   * save_svref(SV **sptr)

     Similar to `save_scalar', but will reinstate a `SV *'.

   * `void save_aptr(AV **aptr)' Similar to `save_svref', but localize `AV
     *' and `HV *'.

   The Alias module implements localization of the basic types within the
*caller's scope*.  People who are interested in how to localize things in
the containing scope should take a look there too.

Subroutines
===========

XSUBs and the Argument Stack
----------------------------

   The XSUB mechanism is a simple way for Perl programs to access C
subroutines.  An XSUB routine will have a stack that contains the
arguments from the Perl program, and a way to map from the Perl data
structures to a C equivalent.

   The stack arguments are accessible through the `ST(n)' macro, which
returns the n'th stack argument.  Argument 0 is the first argument passed
in the Perl subroutine call.  These arguments are `SV*', and can be used
anywhere an `SV*' is used.

   Most of the time, output from the C routine can be handled through use
of the RETVAL and OUTPUT directives.  However, there are some cases where
the argument stack is not already long enough to handle all the return
values.  An example is the POSIX tzname() call, which takes no arguments,
but returns two, the local time zone's standard and summer time
abbreviations.

   To handle this situation, the PPCODE directive is used and the stack is
extended using the macro:

     EXTEND(SP, num);

   where SP is the macro that represents the local copy of the stack
pointer, and num is the number of elements the stack should be extended by.

   Now that there is room on the stack, values can be pushed on it using
the macros to push IVs, doubles, strings, and SV pointers respectively:

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

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

     ($standard_abbrev, $summer_abbrev) = POSIX::tzname;

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

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

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

   For more information, consult *Note Perlxs: perlxs, and *Note
Perlxstut: perlxstut,.

Calling Perl Routines from within C Programs
--------------------------------------------

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

     I32  call_sv(SV*, I32);
     I32  call_pv(const char*, I32);
     I32  call_method(const char*, I32);
     I32  call_argv(const char*, I32, register char**);

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

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

   These routines used to be called `perl_call_sv' etc., before Perl
v5.6.0, but those names are now deprecated; macros of the same name are
provided for compatibility.

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

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

   For a detailed description of calling conventions from C to Perl,
consult *Note Perlcall: perlcall,.

Memory Allocation
-----------------

   All memory meant to be used with the Perl API functions should be
manipulated using the macros described in this section.  The macros
provide the necessary transparency between differences in the actual
malloc implementation that is used within perl.

   It is suggested that you enable the version of malloc that is
distributed with Perl.  It keeps pools of various sizes of unallocated
memory in order to satisfy allocation requests more quickly.  However, on
some platforms, it may cause spurious malloc or free errors.

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

   These three macros are used to initially allocate memory.

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

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

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

   Unlike the New and Newc macros, the Newz macro calls `memzero' to zero
out all the newly allocated memory.

     Renew(pointer, number, type);
     Renewc(pointer, number, type, cast);
     Safefree(pointer)

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

     Move(source, dest, number, type);
     Copy(source, dest, number, type);
     Zero(dest, number, type);

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

PerlIO
------

   The most recent development releases of Perl has been experimenting with
removing Perl's dependency on the "normal" standard I/O suite and allowing
other stdio implementations to be used.  This involves creating a new
abstraction layer that then calls whichever implementation of stdio Perl
was compiled with.  All XSUBs should now use the functions in the PerlIO
abstraction layer and not make any assumptions about what kind of stdio is
being used.

   For a complete description of the PerlIO abstraction, consult *Note
Perlapio: perlapio,.

Putting a C value on Perl stack
-------------------------------

   A lot of opcodes (this is an elementary operation in the internal perl
stack machine) put an SV* on the stack. However, as an optimization the
corresponding SV is (usually) not recreated each time. The opcodes reuse
specially assigned SVs (targets) which are (as a corollary) not constantly
freed/created.

   Each of the targets is created only once (but see `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 target and puts the target on stack.

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

Scratchpads
-----------

   The question remains on when the SVs which are targets for opcodes are
created. The answer is that they are created when the current unit - a
subroutine or a file (for opcodes for statements outside of subroutines) -
is compiled. During this time a special anonymous Perl array is created,
which is called a scratchpad for the current unit.

   A scratchpad keeps SVs which are lexicals for the current unit and are
targets for opcodes. One can deduce that an SV lives on a scratchpad by
looking on its flags: lexicals have `SVs_PADMY' set, and targets have
`SVs_PADTMP' set.

   The correspondence between OPs and targets is not 1-to-1. Different OPs
in the compile tree of the unit can use the same target, if this would not
conflict with the expected life of the temporary.

Scratchpads and recursion
-------------------------

   In fact it is not 100% true that a compiled unit contains a pointer to
the scratchpad AV. In fact it contains a pointer to an AV of (initially)
one element, and this element is the scratchpad AV. Why do we need an
extra level of indirection?

   The answer is *recursion*, and maybe (sometime soon) threads. Both
these can create several execution pointers going into the same
subroutine. For the subroutine-child not write over the temporaries for
the subroutine-parent (lifespan of which covers the call to the child),
the parent and the child should have different scratchpads. (And the
lexicals should be separate anyway!)

   So each subroutine is born with an array of scratchpads (of length 1).
On each entry to the subroutine it is checked that the current depth of
the recursion is not more than the length of this array, and if it is, new
scratchpad is created and pushed into the array.

   The targets on this scratchpad are undefs, but they are already marked
with correct flags.

Compiled code
=============

Code tree
---------

   Here we describe the internal form your code is converted to by Perl.
Start with a simple example:

     $a = $b + $c;

   This is converted to a tree similar to this one:

     assign-to
                /           \
               +             $a
             /   \
           $b     $c

   (but slightly more complicated).  This tree reflects the way Perl
parsed your code, but has nothing to do with the execution order.  There
is an additional "thread" going through the nodes of the tree which shows
the order of execution of the nodes.  In our simplified example above it
looks like:

     $b ---> $c ---> + ---> $a ---> assign-to

   But with the actual compile tree for `$a = $b + $c' it is different:
some nodes *optimized away*.  As a corollary, though the actual tree
contains more nodes than our simplified example, the execution order is
the same as in our example.

Examining the tree
------------------

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

     5           TYPE = add  ===> 6
                 TARG = 1
                 FLAGS = (SCALAR,KIDS)
                 {
                     TYPE = null  ===> (4)
                       (was rv2sv)
                     FLAGS = (SCALAR,KIDS)
                     {
     3                   TYPE = gvsv  ===> 4
                         FLAGS = (SCALAR)
                         GV = main::b
                     }
                 }
                 {
                     TYPE = null  ===> (5)
                       (was rv2sv)
                     FLAGS = (SCALAR,KIDS)
                     {
     4                   TYPE = gvsv  ===> 5
                         FLAGS = (SCALAR)
                         GV = main::c
                     }
                 }

   This tree has 5 nodes (one per TYPE specifier), only 3 of them are not
optimized away (one per number in the left column).  The immediate
children of the given node correspond to `{}' pairs on the same level of
indentation, thus this listing corresponds to the tree:

     add
                      /     \
                    null    null
                     |       |
                    gvsv    gvsv

   The execution order is indicated by `===>' marks, thus it is `3 4 5 6'
(node 6 is not included into above listing), i.e., `gvsv gvsv add
whatever'.

Compile pass 1: check routines
------------------------------

   The tree is created by the *pseudo-compiler* while yacc code feeds it
the constructions it recognizes. Since yacc works bottom-up, so does the
first pass of perl compilation.

   What makes this pass interesting for perl developers is that some
optimization may be performed on this pass.  This is optimization by
so-called *check routines*.  The correspondence between node names and
corresponding check routines is described in `opcode.pl' (do not forget to
run `make regen_headers' if you modify this file).

   A check routine is called when the node is fully constructed except for
the execution-order thread.  Since at this time there are no back-links to
the currently constructed node, one can do most any operation to the
top-level node, including freeing it and/or creating new nodes above/below
it.

   The check routine returns the node which should be inserted into the
tree (if the top-level node was not modified, check routine returns its
argument).

   By convention, check routines have names `ck_*'. They are usually
called from `new*OP' subroutines (or convert) (which in turn are called
from `perly.y').

Compile pass 1a: constant folding
---------------------------------

   Immediately after the check routine is called the returned node is
checked for being compile-time executable.  If it is (the value is judged
to be constant) it is immediately executed, and a constant node with the
"return value" of the corresponding subtree is substituted instead.  The
subtree is deleted.

   If constant folding was not performed, the execution-order thread is
created.

Compile pass 2: context propagation
-----------------------------------

   When a context for a part of compile tree is known, it is propagated
down through the tree.  At this time the context can have 5 values
(instead of 2 for runtime context): void, boolean, scalar, list, and
lvalue.  In contrast with the pass 1 this pass is processed from top to
bottom: a node's context determines the context for its children.

   Additional context-dependent optimizations are performed at this time.
Since at this moment the compile tree contains back-references (via
"thread" pointers), nodes cannot be free()d now.  To allow optimized-away
nodes at this stage, such nodes are null()ified instead of free()ing (i.e.
their type is changed to OP_NULL).

Compile pass 3: peephole optimization
-------------------------------------

   After the compile tree for a subroutine (or for an eval or a file) is
created, an additional pass over the code is performed. This pass is
neither top-down or bottom-up, but in the execution order (with additional
complications for conditionals).  These optimizations are done in the
subroutine peep().  Optimizations performed at this stage are subject to
the same restrictions as in the pass 2.

How multiple interpreters and concurrency are supported
=======================================================

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

Background and PERL_IMPLICIT_CONTEXT
------------------------------------

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

   Three macros control the major Perl build flavors: MULTIPLICITY,
USE_THREADS and PERL_OBJECT.  The MULTIPLICITY build has a C structure
that packages all the interpreter state, there is a similar thread-specific
data structure under USE_THREADS, and the PERL_OBJECT build has a C++
class to maintain interpreter state.  In all three cases,
PERL_IMPLICIT_CONTEXT is also normally defined, and enables the support
for passing in a "hidden" first argument that represents all three data
structures.

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

   First problem: deciding which functions will be public API functions and
which will be private.  All functions whose names begin `S_' are private
(think "S" for "secret" or "static").  All other functions begin with
"Perl_", but just because a function begins with "Perl_" does not mean it
is part of the API. The easiest way to be *sure* a function is part of the
API is to find its entry in *Note Perlapi: perlapi,.  If it exists in
*Note Perlapi: perlapi,, it's part of the API.  If it doesn't, and you
think it should be (i.e., you need it fo r your extension), send mail via
`perlbug' in this node explaining why you think it should be.

   (*Note Perlapi: perlapi, itself is generated by embed.pl, a Perl script
that generates significant portions of the Perl source code.  It has a
list of almost all the functions defined by the Perl interpreter along
with their calling characteristics and some flags.  Functions that are
part of the public API are marked with an 'A' in its flags.)

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

     STATIC void
     S_incline(pTHX_ char *s)

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

   A public function (i.e. part of the internal API, but not necessarily
sanctioned for use in extensions) begins like this:

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

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

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

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

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

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

     sv_setsv(foo, bar);

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

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

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

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

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

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

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

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

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

How do I use all this in extensions?
------------------------------------

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

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

     sv_setsv(asv, bsv);

   in your extesion will translate to this when PERL_IMPLICIT_CONTEXT is
in effect:

     Perl_sv_setsv(Perl_get_context(), asv, bsv);

   or to this otherwise:

     Perl_sv_setsv(asv, bsv);

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

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

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

     static my_private_function(int arg1, int arg2);

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

     [... etc ...]

     MODULE = Foo		PACKAGE = Foo

     /* typical XSUB */

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

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

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

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

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

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

     [... etc ...]

     MODULE = Foo		PACKAGE = Foo

     /* typical XSUB */

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

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

   Never add a comma after `pTHX' yourself-always use the form of the
macro with the underscore for functions that take explicit arguments, or
the form without the argument for functions with no explicit arguments.

Future Plans and PERL_IMPLICIT_SYS
----------------------------------

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

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

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

AUTHORS
=======

   Until May 1997, this document was maintained by Jeff Okamoto
<okamoto@corp.hp.com>.  It is now maintained as part of Perl itself by the
Perl 5 Porters <perl5-porters@perl.org>.

   With lots of help and suggestions from Dean Roehrich, Malcolm Beattie,
Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil Bowers,
Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer, Stephen
McCamant, and Gurusamy Sarathy.

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

   Modifications to autogenerate the API listing (*Note Perlapi: perlapi,)
by Benjamin Stuhl.

SEE ALSO
========

   perlapi(1), perlintern(1), perlxs(1), perlembed(1)