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PERLGUTS
!!!PERLGUTS
NAME
DESCRIPTION
Variables
Subroutines
Compiled code
Examining internal data structures with the dump functions
How multiple interpreters and concurrency are supported
Internal Functions
Unicode Support
AUTHORS
SEE ALSO
----
!!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 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.


STRLEN is an integer type (Size_t, usually
defined as size_t in config.h) guaranteed to be large enough
to represent the size of any string that perl can
handle.


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


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


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


    SvIV(SV*)
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 ). 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'' 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(
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 . 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  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'').


__Offsets__


Perl provides the function sv_chop to efficiently
remove characters from the beginning of a string; you give
it an SV and a pointer to somewhere inside
the the PV , and it discards everything
before the pointer. The efficiency comes by means of a
little hack: instead of actually removing the characters,
sv_chop sets the flag OOK (offset
OK ) to signal to other functions that the
offset hack is in effect, and it puts the number of bytes
chopped off into the IV field of the
SV . It then moves the PV
pointer (called SvPVX) forward that many bytes, and
adjusts SvCUR and SvLEN.


Hence, at this point, the start of the buffer that we
allocated lives at SvPVX(sv) - SvIV(sv) in memory
and the PV pointer is pointing into the
middle of this allocated storage.


This is best demonstrated by example:


  % ./perl -Ilib -MDevel::Peek -le '$a=
Here the number of bytes chopped off (1) is put into IV , and Devel::Peek::Dump helpfully reminds us that this is an offset. The portion of the string between the ``real'' and the ``fake'' beginnings is shown in parentheses, and the values of SvCUR and SvLEN reflect the fake beginning, not the real one.


Something similar to the offset hack is perfomed on AVs to
enable efficient shifting and splicing off the beginning of
the array; while AvARRAY points to the first
element in the array that is visible from Perl,
AvALLOC points to the real start of the C array.
These are usually the same, but a shift operation
can be carried out by increasing AvARRAY by one and
decreasing AvFILL and AvLEN. Again, the
location of the real start of the C array only comes into
play when freeing the array. See av_shift in
''av.c''.


__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(
This returns NULL if the variable does not exist.


See ``Understanding the Magic of Tied Hashes and Arrays''
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(
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
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''
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 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 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'' 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)


__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(
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
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
perlcall and 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
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(
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
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)
D        vtbl_regdata        Regex match position data (@+ and @- vars)
d        vtbl_regdatum       Regex match position data element
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 %_
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   =
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
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'' 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(
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'' in 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 in that it is also a mechanism for doing
a delayed SvREFCNT_dec. However, while
sv_2mortal extends the lifetime of sv
until the beginning of the next statement,
SAVEFREESV extends it until the end of the
enclosing scope. These lifetimes can be wildly
different.


Also compare SAVEMORTALIZESV.


SAVEMORTALIZESV(SV *sv)


Just like SAVEFREESV, but mortalizes sv at
the end of the current scope instead of decrementing its
reference count. This usually has the effect of keeping
sv alive until the statement that called the
currently live scope has finished executing.


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 *.


SV* save_scalar(GV *gv)


Equivalent to Perl code local $gv.


AV* save_ary(GV *gv)


HV* 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.


SV* save_svref(SV **sptr)


Similar to save_scalar, but will reinstate a SV
*.


void save_aptr(AV **aptr)


void save_hptr(HV **hptr)


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. However, see ``Putting a C value on Perl stack''


For more information, consult perlxs and
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 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 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 (''target''s) which
are (as a corollary) not constantly
freed/created.


Each of the targets is created only once (but see
``Scratchpads and recursion'' below), and when an opcode
needs to put an integer, a double, or a string on stack, it
just sets the corresponding parts of its ''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].


Because the target is reused, you must be careful when
pushing multiple values on the stack. The following code
will not do what you think:


    XPUSHi(10);
XPUSHi(20);
This translates as TARG to 10, push a pointer to TARG onto the stack; set TARG to 20, push a pointer to TARG onto the stackTARG, which we have set to 20. If you need to push multiple different values, use XPUSHs, which bypasses TARG.


On a related note, if you do use (X)PUSH[[npi], then
you're going to need a dTARG in your variable
declarations so that the *PUSH* macros can make use
of the local variable TARG.


__Scratchpads__


The question remains on when the SVs which are
''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 SVs_PADMY set, and
''target''s have SVs_PADTMP set.


The correspondence between OPs and ''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.


__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 ''target''s 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 ---
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  ===
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.


Each of these nodes represents an op, a fundamental
operation inside the Perl core. The code which implements
each operation can be found in the ''pp*.c'' files; the
function which implements the op with type gvsv is
pp_gvsv, and so on. As the tree above shows,
different ops have different numbers of children:
add is a binary operator, as one would expect, and
so has two children. To accommodate the various different
numbers of children, there are various types of op data
structure, and they link together in different
ways.


The simplest type of op structure is OP: this has
no children. Unary operators, UNOPs, have one
child, and this is pointed to by the op_first
field. Binary operators (BINOPs) have not only an
op_first field but also an op_last field.
The most complex type of op is a LISTOP, which has
any number of children. In this case, the first child is
pointed to by op_first and the last child by
op_last. The children in between can be found by
iteratively following the op_sibling pointer from
the first child to the last.


There are also two other op types: a PMOP holds a
regular expression, and has no children, and a LOOP
may or may not have children. If the op_children
field is non-zero, it behaves like a LISTOP. To
complicate matters, if a UNOP is actually a
null op after optimization (see ``Compile pass 2:
context propagation'') it will still have children in
accordance with its former type.


__Compile pass 1: check routines__


The tree is created by the 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.
!!Examining internal data structures with the dump functions


To aid debugging, the source file ''dump.c'' contains a
number of functions which produce formatted output of
internal data structures.


The most commonly used of these functions is
Perl_sv_dump; it's used for dumping SVs, AVs, HVs,
and CVs. The Devel::Peek module calls
sv_dump to produce debugging output from
Perl-space, so users of that module should already be
familiar with its format.


Perl_op_dump can be used to dump an OP
structure or any of its derivatives, and produces output
similiar to perl -Dx; in fact,
Perl_dump_eval will dump the main root of the code
being evaluated, exactly like -Dx.


Other useful functions are Perl_dump_sub, which
turns a GV into an op tree,
Perl_dump_packsubs which calls
Perl_dump_sub on all the subroutines in a package
like so: (Thankfully, these are all xsubs, so there is no op
tree)


    (gdb) print Perl_dump_packsubs(PL_defstash)
    SUB attributes::bootstrap = (xsub 0x811fedc 0)
    SUB UNIVERSAL::can = (xsub 0x811f50c 0)
    SUB UNIVERSAL::isa = (xsub 0x811f304 0)
    SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0)
    SUB !DynaLoader::boot_!DynaLoader = (xsub 0x805b188 0)
and Perl_dump_all, which dumps all the subroutines in the stash and the op tree of the main root.
!!How multiple interpreters and concurrency are supported


__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 (now deprecated)
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 . (See ``Internal Functions''.) The
easiest way to be __sure__ a function is part of the
API is to find its entry in perlapi. If it
exists in perlapi, it's part of the API . If
it doesn't, and you think it should be (i.e., you need it
for your extension), send mail via perlbug explaining why
you think it should be.


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 __p__rototype, 'a' for __a__rgument, or 'd' for __d__eclaration, so we have pTHX, aTHX and dTHX, and their variants.


When Perl is built without options that set
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 into 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-
Under PERL_OBJECT in extensions (aka PERL_CAPI ), or under MULTIPLICITY/USE_THREADS with PERL_IMPLICIT_CONTEXT in both core and extensions, it will become:


    Perl_sv_setsv(aTHX_ foo, bar);     # the canonical Perl
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.


__So what happened to dTHR?__


dTHR was introduced in perl 5.005 to support the
older thread model. The older thread model now uses the
THX mechanism to pass context pointers around, so
dTHR is not useful any more. Perl 5.6.0 and later
still have it for backward source compatibility, but it is
defined to be a no-op.


__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 extension 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
        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
        /* 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.


__Should I do anything special if I call perl from multiple
threads?__


If you create interpreters in one thread and then proceed to
call them in another, you need to make sure perl's own
Thread Local Storage ( TLS ) slot is
initialized correctly in each of those threads.


The perl_alloc and perl_clone
API functions will automatically set the
TLS slot to the interpreter they created, so
that there is no need to do anything special if the
interpreter is always accessed in the same thread that
created it, and that thread did not create or call any other
interpreters afterwards. If that is not the case, you have
to set the TLS slot of the thread before
calling any functions in the Perl API on that
particular interpreter. This is done by calling the
PERL_SET_CONTEXT macro in that thread as the first
thing you do:


        /* do this before doing anything else with some_perl */
PERL_SET_CONTEXT(some_perl);
        ... other Perl API calls on some_perl go here ...


__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 and
USE_THREADS on Windows (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.
!!Internal Functions


All of Perl's internal functions which will be exposed to
the outside world are be prefixed by Perl_ so that
they will not conflict with XS functions or
functions used in a program in which Perl is embedded.
Similarly, all global variables begin with PL_. (By
convention, static functions start with
S_)


Inside the Perl core, you can get at the functions either
with or without the Perl_ prefix, thanks to a bunch
of defines that live in ''embed.h''. This header file is
generated automatically from ''embed.pl''.
''embed.pl'' also creates the prototyping header files
for the internal functions, generates the documentation and
a lot of other bits and pieces. It's important that when you
add a new function to the core or change an existing one,
you change the data in the table at the end of
''embed.pl'' as well. Here's a sample entry from that
table:


    Apd SV**   av_fetch   AV* arI32 keyI32 lval
The second column is the return type, the third column the name. Columns after that are the arguments. The first column is a set of flags:


A


This function is a part of the public API
.


p


This function has a Perl_ prefix; ie, it is defined
as Perl_av_fetch


d


This function has documentation using the apidoc
feature which we'll look at in a second.


Other available flags are:


s


This is a static function and is defined as
S_whatever, and usually called within the sources
as whatever(...).


n


This does not use aTHX_ and pTHX to pass
interpreter context. (See ``Background and
PERL_IMPLICIT_CONTEXT '' in
perlguts.)


r


This function never returns; croak, exit
and friends.


f


This function takes a variable number of arguments,
printf style. The argument list should end with
..., like this:


    Afprd   void   croak          const char* pat...


M


This function is part of the experimental development
API , and may change or disappear without
notice.


o


This function should not have a compatibility macro to
define, say, Perl_parse to parse. It must
be called as Perl_parse.


j


This function is not a member of CPerlObj. If you
don't know what this means, don't use it.


x


This function isn't exported out of the Perl
core.


If you edit ''embed.pl'', you will need to run make
regen_headers to force a rebuild of ''embed.h'' and
other auto-generated files.


__Formatted Printing of IVs, UVs, and NVs__


If you are printing IVs, UVs, or NVS instead
of the stdio(3) style formatting codes like
%d, %ld, %f, you should use the
following macros for portability


        IVdf            IV in decimal
UVuf            UV in decimal
UVof            UV in octal
UVxf            UV in hexadecimal
NVef            NV %e-like
NVff            NV %f-like
NVgf            NV %g-like
These will take care of 64-bit integers and long doubles. For example:


        printf(
The IVdf will expand to whatever is the correct format for the IVs.


If you are printing addresses of pointers, use UVxf combined
with ''PTR2UV ()'', do not use
%lx or %p.


__Pointer-To-Integer and
Integer-To-Pointer__


Because pointer size does not necessarily equal integer
size, use the follow macros to do it right.


        PTR2UV(pointer)
PTR2IV(pointer)
PTR2NV(pointer)
INT2PTR(pointertotype, integer)
For example:


        IV  iv = ...;
SV *sv = INT2PTR(SV*, iv);
and


        AV *av = ...;
UV  uv = PTR2UV(av);


__Source Documentation__


There's an effort going on to document the internal
functions and automatically produce reference manuals from
them - perlapi is one such manual which details all the
functions which are available to XS writers.
perlintern is the autogenerated manual for the functions
which are not part of the API and are
supposedly for internal use only.


Source documentation is created by putting
POD comments into the C source, like
this:


 /*
=for apidoc sv_setiv
 Copies an integer into the given SV.  Does not handle 'set' magic.  See
C
 =cut
*/
Please try and supply some documentation if you add functions to the Perl core.
!!Unicode Support


Perl 5.6.0 introduced Unicode support. It's important for
porters and XS writers to understand this
support and make sure that the code they write does not
corrupt Unicode data.


__What is Unicode, anyway?__


In the olden, less enlightened times, we all used to use
ASCII . Most of us did, anyway. The big
problem with ASCII is that it's American.
Well, no, that's not actually the problem; the problem is
that it's not particularly useful for people who don't use
the Roman alphabet. What used to happen was that particular
languages would stick their own alphabet in the upper range
of the sequence, between 128 and 255. Of course, we then
ended up with plenty of variants that weren't quite
ASCII , and the whole point of it being a
standard was lost.


Worse still, if you've got a language like Chinese or
Japanese that has hundreds or thousands of characters, then
you really can't fit them into a mere 256, so they had to
forget about ASCII altogether, and build
their own systems using pairs of numbers to refer to one
character.


To fix this, some people formed Unicode, Inc. and produced a
new character set containing all the characters you can
possibly think of and more. There are several ways of
representing these characters, and the one Perl uses is
called UTF8 . UTF8 uses a
variable number of bytes to represent a character, instead
of just one. You can learn more about Unicode at
http://www.unicode.org/


__How can I recognise a UTF8
string?__


You can't. This is because UTF8 data is
stored in bytes just like non-UTF8 data. The Unicode
character 200, (0xC8 for you hex types) capital E
with a grave accent, is represented by the two bytes
v196.172. Unfortunately, the non-Unicode string
chr(196).chr(172) has that byte sequence as well.
So you can't tell just by looking - this is what makes
Unicode input an interesting problem.


The API function is_utf8_string can
help; it'll tell you if a string contains only valid
UTF8 characters. However, it can't do the
work for you. On a character-by-character basis,
is_utf8_char will tell you whether the current
character in a string is valid UTF8
.


__How does UTF8 represent Unicode
characters?__


As mentioned above, UTF8 uses a variable
number of bytes to store a character. Characters with values
1...128 are stored in one byte, just like good ol'
ASCII . Character 129 is stored as
v194.129; this continues up to character 191, which
is v194.191. Now we've run out of bits (191 is
binary 10111111) so we move on; 192 is
v195.128. And so it goes on, moving to three bytes
at character 2048.


Assuming you know you're dealing with a UTF8
string, you can find out how long the first character in it
is with the UTF8SKIP macro:


    char *utf =
    len = UTF8SKIP(utf); /* len is 2 here */
utf += len;
len = UTF8SKIP(utf); /* len is 3 here */
Another way to skip over characters in a UTF8 string is to use utf8_hop, which takes a string and a number of characters to skip over. You're on your own about bounds checking, though, so don't use it lightly.


All bytes in a multi-byte UTF8 character will
have the high bit set, so you can test if you need to do
something special with this character like
this:


    UV uv;
    if (utf
You can also see in that example that we use utf8_to_uv to get the value of the character; the inverse function uv_to_utf8 is available for putting a UV into UTF8:


    if (uv
You __must__ convert characters to UVs using the above functions if you're ever in a situation where you have to match UTF8 and non-UTF8 characters. You may not skip over UTF8 characters in this case. If you do this, you'll lose the ability to match hi-bit non-UTF8 characters; for instance, if your UTF8 string contains v196.172, and you skip that character, you can never match a chr(200) in a non-UTF8 string. So don't do that!


__How does Perl store UTF8
strings?__


Currently, Perl deals with Unicode strings and non-Unicode
strings slightly differently. If a string has been
identified as being UTF-8 encoded, Perl will
set a flag in the SV , SVf_UTF8. You
can check and manipulate this flag with the following
macros:


    SvUTF8(sv)
SvUTF8_on(sv)
SvUTF8_off(sv)
This flag has an important effect on Perl's treatment of the string: if Unicode data is not properly distinguished, regular expressions, length, substr and other string handling operations will have undesirable results.


The problem comes when you have, for instance, a string that
isn't flagged is UTF8 , and contains a byte
sequence that could be UTF8 - especially when
combining non-UTF8 and UTF8
strings.


Never forget that the SVf_UTF8 flag is separate to
the PV value; you need be sure you don't
accidentally knock it off while you're manipulating SVs.
More specifically, you cannot expect to do
this:


    SV *sv;
SV *nsv;
STRLEN len;
char *p;
    p = SvPV(sv, len);
frobnicate(p);
nsv = newSVpvn(p, len);
The char* string does not tell you the whole story, and you can't copy or reconstruct an SV just by copying the string value. Check if the old SV has the UTF8 flag set, and act accordingly:


    p = SvPV(sv, len);
frobnicate(p);
nsv = newSVpvn(p, len);
if (SvUTF8(sv))
SvUTF8_on(nsv);
In fact, your frobnicate function should be made aware of whether or not it's dealing with UTF8 data, so that it can handle the string appropriately.


__How do I convert a string to UTF8
?__


If you're mixing UTF8 and non-UTF8 strings,
you might find it necessary to upgrade one of the strings to
UTF8 . If you've got an SV ,
the easiest way to do this is:


    sv_utf8_upgrade(sv);
However, you must not do this, for example:


    if (!SvUTF8(left))
sv_utf8_upgrade(left);
If you do this in a binary operator, you will actually change one of the strings that came into the operator, and, while it shouldn't be noticeable by the end user, it can cause problems.


Instead, bytes_to_utf8 will give you a UTF8-encoded
__copy__ of its string argument. This is useful for
having the data available for comparisons and so on, without
harming the original SV . There's also
utf8_to_bytes to go the other way, but naturally,
this will fail if the string contains any characters above
255 that can't be represented in a single byte.


__Is there anything else I need to know?__


Not really. Just remember these things:


There's no way to tell if a string is UTF8 or
not. You can tell if an SV is
UTF8 by looking at is SvUTF8 flag.
Don't forget to set the flag if something should be
UTF8 . Treat the flag as part of the
PV , even though it's not - if you pass on
the PV to somewhere, pass on the flag
too.


If a string is UTF8 , __always__ use
utf8_to_uv to get at the value, unless !(*s
 in which case you can use
*s.


When writing to a UTF8 string, __always__
use uv_to_utf8, unless uv  in
which case you can use *s = uv.


Mixing UTF8 and non-UTF8 strings is tricky.
Use bytes_to_utf8 to get a new string which is
UTF8 encoded. There are tricks you can use to
delay deciding whether you need to use a UTF8
string until you get to a high character -
HALF_UPGRADE is one of those.
!!AUTHORS


Until May 1997, this document was maintained by Jeff Okamoto


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


Modifications to autogenerate the API listing
(perlapi) by Benjamin Stuhl.
!!SEE ALSO


perlapi(1), perlintern(1), perlxs(1),
perlembed(1)
----
3 pages link to perlguts(1):
This page is a man page (or other imported legacy content). We are unable to automatically determine the license status of this page.
Last edited on Tuesday, June 4, 2002 12:22:35 am by "perry"
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