/IronPython_Main/Runtime/Tests/LinqDlrTests/testenv/perl/lib/pod/perlguts.pod

=head1 NAME



perlguts - Introduction to the Perl API



=head1 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.



=head1 Variables



=head2 Datatypes



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



    SV  Scalar Value

    AV  Array Value

    HV  Hash Value



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



=head2 What is an "IV"?



Perl uses a special typedef IV which is a simple 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.)



=head2 Working with SVs



An SV can be created and loaded with one command.  There are four types of

values that can be loaded: an integer value (IV), a double (NV),

a string (PV), and another scalar (SV).



The six routines are:



    SV*  newSViv(IV);

    SV*  newSVnv(double);

    SV*  newSVpv(const char*, int);

    SV*  newSVpvn(const char*, int);

    SV*  newSVpvf(const char*, ...);

    SV*  newSVsv(SV*);



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



    void  sv_setiv(SV*, IV);

    void  sv_setuv(SV*, UV);

    void  sv_setnv(SV*, double);

    void  sv_setpv(SV*, const char*);

    void  sv_setpvn(SV*, const char*, int)

    void  sv_setpvf(SV*, const char*, ...);

    void  sv_setpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool);

    void  sv_setsv(SV*, SV*);



Notice that you can choose to specify the length of the string to be

assigned by using C<sv_setpvn>, C<newSVpvn>, or C<newSVpv>, or you may

allow Perl to calculate the length by using C<sv_setpv> or by specifying

0 as the second argument to C<newSVpv>.  Be warned, though, that Perl will

determine the string's length by using C<strlen>, which depends on the

string terminating with a NUL character.



The arguments of C<sv_setpvf> are processed like C<sprintf>, and the

formatted output becomes the value.



C<sv_setpvfn> is an analogue of C<vsprintf>, but it allows you to specify

either a pointer to a variable argument list or the address and length of

an array of SVs.  The last argument points to a boolean; on return, if that

boolean is true, then locale-specific information has been used to format

the string, and the string's contents are therefore untrustworthy (see

L<perlsec>).  This pointer may be NULL if that information is not

important.  Note that this function requires you to specify the length of

the format.



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 C<sv_set*()> functions are not generic enough to operate on values

that have "magic".  See L<Magic Virtual Tables> later in this document.



All SVs that contain strings should be terminated with a NUL character.

If it is not NUL-terminated there is a risk of

core dumps and corruptions from code which passes the string to C

functions or system calls which expect a NUL-terminated string.

Perl's own functions typically add a trailing NUL for this reason.

Nevertheless, you should be very careful when you pass a string stored

in an SV to a C function or system call.



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



    SvIV(SV*)

    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 C<SvPV> macro, the length of the string returned is placed into the

variable C<len> (this is a macro, so you do I<not> use C<&len>).  If you do

not care what the length of the data is, use the C<SvPV_nolen> macro.

Historically the C<SvPV> macro with the global variable C<PL_na> has been

used in this case.  But that can be quite inefficient because C<PL_na> must

be accessed in thread-local storage in threaded Perl.  In any case, remember

that Perl allows arbitrary strings of data that may both contain NULs and

might not be terminated by a NUL.



Also remember that C doesn't allow you to safely say C<foo(SvPV(s, len),

len);>. It might work with your compiler, but it won't work for everyone.

Break this sort of statement up into separate assignments:



	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 C<sv_grow>.  Note that C<SvGROW> can only increase, not

decrease, the allocated memory of an SV and that it does not automatically

add a byte for the a trailing NUL (perl's own string functions typically do

C<SvGROW(sv, len + 1)>).



If you have an SV and want to know what kind of data Perl thinks is stored

in it, you can use the following macros to check the type of SV you have.



    SvIOK(SV*)

    SvNOK(SV*)

    SvPOK(SV*)



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

the following macros:



    SvCUR(SV*)

    SvCUR_set(SV*, I32 val)



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

with the macro:



    SvEND(SV*)



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



If you want to append something to the end of string stored in an C<SV*>,

you can use the following functions:



    void  sv_catpv(SV*, const char*);

    void  sv_catpvn(SV*, const char*, STRLEN);

    void  sv_catpvf(SV*, const char*, ...);

    void  sv_catpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool);

    void  sv_catsv(SV*, SV*);



The first function calculates the length of the string to be appended by

using C<strlen>.  In the second, you specify the length of the string

yourself.  The third function processes its arguments like C<sprintf> and

appends the formatted output.  The fourth function works like C<vsprintf>.

You can specify the address and length of an array of SVs instead of the

va_list argument. The fifth function extends the string stored in the first

SV with the string stored in the second SV.  It also forces the second SV

to be interpreted as a string.



The C<sv_cat*()> functions are not generic enough to operate on values that

have "magic".  See L<Magic Virtual Tables> later in this document.



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

by using the following:



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



This returns NULL if the variable does not exist.



If you want to know if this variable (or any other SV) is actually C<defined>,

you can call:



    SvOK(SV*)



The scalar C<undef> value is stored in an SV instance called C<PL_sv_undef>.  Its

address can be used whenever an C<SV*> is needed.



There are also the two values C<PL_sv_yes> and C<PL_sv_no>, which contain Boolean

TRUE and FALSE values, respectively.  Like C<PL_sv_undef>, their addresses can

be used whenever an C<SV*> is needed.



Do not be fooled into thinking that C<(SV *) 0> is the same as C<&PL_sv_undef>.

Take this code:



    SV* sv = (SV*) 0;

    if (I-am-to-return-a-real-value) {

            sv = sv_2mortal(newSViv(42));

    }

    sv_setsv(ST(0), sv);



This code tries to return a new SV (which contains the value 42) if it should

return a real value, or undef otherwise.  Instead it has returned a NULL

pointer which, somewhere down the line, will cause a segmentation violation,

bus error, or just weird results.  Change the zero to C<&PL_sv_undef> in the first

line and all will be well.



To free an SV that you've created, call C<SvREFCNT_dec(SV*)>.  Normally this

call is not necessary (see L<Reference Counts and Mortality>).



=head2 Offsets



Perl provides the function C<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, C<sv_chop> sets the flag C<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 C<SvPVX>) forward that

many bytes, and adjusts C<SvCUR> and C<SvLEN>. 



Hence, at this point, the start of the buffer that we allocated lives

at C<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="12345"; $a=~s/.//; Dump($a)'

  SV = PVIV(0x8128450) at 0x81340f0

    REFCNT = 1

    FLAGS = (POK,OOK,pPOK)

    IV = 1  (OFFSET)

    PV = 0x8135781 ( "1" . ) "2345"\0

    CUR = 4

    LEN = 5



Here the number of bytes chopped off (1) is put into IV, and

C<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 C<SvCUR> and C<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

C<AvARRAY> points to the first element in the array that is visible from

Perl, C<AvALLOC> points to the real start of the C array. These are

usually the same, but a C<shift> operation can be carried out by

increasing C<AvARRAY> by one and decreasing C<AvFILL> and C<AvLEN>.

Again, the location of the real start of the C array only comes into

play when freeing the array. See C<av_shift> in F<av.c>.



=head2 What's Really Stored in an SV?



Recall that the usual method of determining the type of scalar you have is

to use C<Sv*OK> macros.  Because a scalar can be both a number and a string,

usually these macros will always return TRUE and calling the C<Sv*V>

macros will do the appropriate conversion of string to integer/double or

integer/double to string.



If you I<really> need to know if you have an integer, double, or string

pointer in an SV, you can use the following three macros instead:



    SvIOKp(SV*)

    SvNOKp(SV*)

    SvPOKp(SV*)



These will tell you if you truly have an integer, double, or string pointer

stored in your SV.  The "p" stands for private.



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



=head2 Working with AVs



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

empty AV:



    AV*  newAV();



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



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



The second argument points to an array containing C<num> C<SV*>'s.  Once the

AV has been created, the SVs can be destroyed, if so desired.



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



    void  av_push(AV*, SV*);

    SV*   av_pop(AV*);

    SV*   av_shift(AV*);

    void  av_unshift(AV*, I32 num);



These should be familiar operations, with the exception of C<av_unshift>.

This routine adds C<num> elements at the front of the array with the C<undef>

value.  You must then use C<av_store> (described below) to assign values

to these new elements.



Here are some other functions:



    I32   av_len(AV*);

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

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



The C<av_len> function returns the highest index value in array (just

like $#array in Perl).  If the array is empty, -1 is returned.  The

C<av_fetch> function returns the value at index C<key>, but if C<lval>

is non-zero, then C<av_fetch> will store an undef value at that index.

The C<av_store> function stores the value C<val> at index C<key>, and does

not increment the reference count of C<val>.  Thus the caller is responsible

for taking care of that, and if C<av_store> returns NULL, the caller will

have to decrement the reference count to avoid a memory leak.  Note that

C<av_fetch> and C<av_store> both return C<SV**>'s, not C<SV*>'s as their

return value.



    void  av_clear(AV*);

    void  av_undef(AV*);

    void  av_extend(AV*, I32 key);



The C<av_clear> function deletes all the elements in the AV* array, but

does not actually delete the array itself.  The C<av_undef> function will

delete all the elements in the array plus the array itself.  The

C<av_extend> function extends the array so that it contains at least C<key+1>

elements.  If C<key+1> is less than the currently allocated length of the array,

then nothing is done.



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

by using the following:



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



This returns NULL if the variable does not exist.



See L<Understanding the Magic of Tied Hashes and Arrays> for more

information on how to use the array access functions on tied arrays.



=head2 Working with HVs



To create an HV, you use the following routine:



    HV*  newHV();



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



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

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



The C<klen> parameter is the length of the key being passed in (Note that

you cannot pass 0 in as a value of C<klen> to tell Perl to measure the

length of the key).  The C<val> argument contains the SV pointer to the

scalar being stored, and C<hash> is the precomputed hash value (zero if

you want C<hv_store> to calculate it for you).  The C<lval> parameter

indicates whether this fetch is actually a part of a store operation, in

which case a new undefined value will be added to the HV with the supplied

key and C<hv_fetch> will return as if the value had already existed.



Remember that C<hv_store> and C<hv_fetch> return C<SV**>'s and not just

C<SV*>.  To access the scalar value, you must first dereference the return

value.  However, you should check to make sure that the return value is

not NULL before dereferencing it.



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



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

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



If C<flags> does not include the C<G_DISCARD> flag then C<hv_delete> will

create and return a mortal copy of the deleted value.



And more miscellaneous functions:



    void   hv_clear(HV*);

    void   hv_undef(HV*);



Like their AV counterparts, C<hv_clear> deletes all the entries in the hash

table but does not actually delete the hash table.  The C<hv_undef> deletes

both the entries and the hash table itself.



Perl keeps the actual data in linked list of structures with a typedef of HE.

These contain the actual key and value pointers (plus extra administrative

overhead).  The key is a string pointer; the value is an C<SV*>.  However,

once you have an C<HE*>, to get the actual key and value, use the routines

specified below.



    I32    hv_iterinit(HV*);

            /* Prepares starting point to traverse hash table */

    HE*    hv_iternext(HV*);

            /* Get the next entry, and return a pointer to a

               structure that has both the key and value */

    char*  hv_iterkey(HE* entry, I32* retlen);

            /* Get the key from an HE structure and also return

               the length of the key string */

    SV*    hv_iterval(HV*, HE* entry);

            /* Return a SV pointer to the value of the HE

               structure */

    SV*    hv_iternextsv(HV*, char** key, I32* retlen);

            /* This convenience routine combines hv_iternext,

	       hv_iterkey, and hv_iterval.  The key and retlen

	       arguments are return values for the key and its

	       length.  The value is returned in the SV* argument */



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

by using the following:



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



This returns NULL if the variable does not exist.



The hash algorithm is defined in the C<PERL_HASH(hash, key, klen)> macro:



    hash = 0;

    while (klen--)

	hash = (hash * 33) + *key++;

    hash = hash + (hash >> 5);			/* after 5.6 */



The last step was added in version 5.6 to improve distribution of

lower bits in the resulting hash value.



See L<Understanding the Magic of Tied Hashes and Arrays> for more

information on how to use the hash access functions on tied hashes.



=head2 Hash API Extensions



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



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

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



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

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



    SV*     hv_iterkeysv  (HE* entry);



Note that these functions take C<SV*> keys, which simplifies writing

of extension code that deals with hash structures.  These functions

also allow passing of C<SV*> keys to C<tie> functions without forcing

you to stringify the keys (unlike the previous set of functions).



They also return and accept whole hash entries (C<HE*>), making their

use more efficient (since the hash number for a particular string

doesn't have to be recomputed every time).  See L<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

L<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 C<SV*>s:



    HeKEY(HE* he)

    HeKLEN(HE* he)



Note that both C<hv_store> and C<hv_store_ent> do not increment the

reference count of the stored C<val>, which is the caller's responsibility.

If these functions return a NULL value, the caller will usually have to

decrement the reference count of C<val> to avoid a memory leak.



=head2 References



References are a special type of scalar that point to other data types

(including references).



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



    SV* newRV_inc((SV*) thing);

    SV* newRV_noinc((SV*) thing);



The C<thing> argument can be any of an C<SV*>, C<AV*>, or C<HV*>.  The

functions are identical except that C<newRV_inc> increments the reference

count of the C<thing>, while C<newRV_noinc> does not.  For historical

reasons, C<newRV> is a synonym for C<newRV_inc>.



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

the reference:



    SvRV(SV*)



then call the appropriate routines, casting the returned C<SV*> to either an

C<AV*> or C<HV*>, if required.



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



    SvROK(SV*)



To discover what type of value the reference refers to, use the following

macro and then check the return value.



    SvTYPE(SvRV(SV*))



The most useful types that will be returned are:



    SVt_IV    Scalar

    SVt_NV    Scalar

    SVt_PV    Scalar

    SVt_RV    Scalar

    SVt_PVAV  Array

    SVt_PVHV  Hash

    SVt_PVCV  Code

    SVt_PVGV  Glob (possible a file handle)

    SVt_PVMG  Blessed or Magical Scalar



    See the sv.h header file for more details.



=head2 Blessed References and Class Objects



References are also used to support object-oriented programming.  In the

OO lexicon, an object is simply a reference that has been blessed into a

package (or class).  Once blessed, the programmer may now use the reference

to access the various methods in the class.



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



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



The C<sv> argument must be a reference.  The C<stash> argument specifies

which class the reference will belong to.  See

L<Stashes and Globs> for information on converting class names into stashes.



/* Still under construction */



Upgrades rv to reference if not already one.  Creates new SV for rv to

point to.  If C<classname> is non-null, the SV is blessed into the specified

class.  SV is returned.



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



Copies integer or double into an SV whose reference is C<rv>.  SV is blessed

if C<classname> is non-null.



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

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



Copies the pointer value (I<the address, not the string!>) into an SV whose

reference is rv.  SV is blessed if C<classname> is non-null.



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



Copies string into an SV whose reference is C<rv>.  Set length to 0 to let

Perl calculate the string length.  SV is blessed if C<classname> is non-null.



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



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

check inheritance relationships.



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



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



	int  sv_isobject(SV* sv);



Tests whether the SV is derived from the specified class. SV can be either

a reference to a blessed object or a string containing a class name. This

is the function implementing the C<UNIVERSAL::isa> functionality.



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



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

to write:



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



=head2 Creating New Variables



To create a new Perl variable with an undef value which can be accessed from

your Perl script, use the following routines, depending on the variable type.



    SV*  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

C<TRUE> argument to enable certain extra features.  Those bits are:



    GV_ADDMULTI	Marks the variable as multiply defined, thus preventing the

		"Name <varname> used only once: possible typo" warning.

    GV_ADDWARN	Issues the warning "Had to create <varname> unexpectedly" if

		the variable did not exist before the function was called.



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

package.



=head2 Reference Counts and Mortality



Perl uses an reference count-driven garbage collection mechanism. SVs,

AVs, or HVs (xV for short in the following) start their life with a

reference count of 1.  If the reference count of an xV ever drops to 0,

then it will be destroyed and its memory made available for reuse.



This normally doesn't happen at the Perl level unless a variable is

undef'ed or the last variable holding a reference to it is changed or

overwritten.  At the internal level, however, reference counts can be

manipulated with the following macros:



    int SvREFCNT(SV* sv);

    SV* SvREFCNT_inc(SV* sv);

    void SvREFCNT_dec(SV* sv);



However, there is one other function which manipulates the reference

count of its argument.  The C<newRV_inc> function, you will recall,

creates a reference to the specified argument.  As a side effect,

it increments the argument's reference count.  If this is not what

you want, use C<newRV_noinc> instead.



For example, imagine you want to return a reference from an XSUB function.

Inside the XSUB routine, you create an SV which initially has a reference

count of one.  Then you call C<newRV_inc>, passing it the just-created SV.

This returns the reference as a new SV, but the reference count of the

SV you passed to C<newRV_inc> has been incremented to two.  Now you

return the reference from the XSUB routine and forget about the SV.

But Perl hasn't!  Whenever the returned reference is destroyed, the

reference count of the original SV is decreased to one and nothing happens.

The SV will hang around without any way to access it until Perl itself

terminates.  This is a memory leak.



The correct procedure, then, is to use C<newRV_noinc> instead of

C<newRV_inc>.  Then, if and when the last reference is destroyed,

the reference count of the SV will go to zero and it will be destroyed,

stopping any memory leak.



There are some convenience functions available that can help with the

destruction of xVs.  These functions introduce the concept of "mortality".

An xV that is mortal has had its reference count marked to be decremented,

but not actually decremented, until "a short time later".  Generally the

term "short time later" means a single Perl statement, such as a call to

an XSUB function.  The actual determinant for when mortal xVs have their

reference count decremented depends on two macros, SAVETMPS and FREETMPS.

See L<perlcall> and L<perlxs> for more details on these macros.



"Mortalization" then is at its simplest a deferred C<SvREFCNT_dec>.

However, if you mortalize a variable twice, the reference count will

later be decremented twice.



You should be careful about creating mortal variables.  Strange things

can happen if you make the same value mortal within multiple contexts,

or if you make a variable mortal multiple times.



To create a mortal variable, use the functions:



    SV*  sv_newmortal()

    SV*  sv_2mortal(SV*)

    SV*  sv_mortalcopy(SV*)



The first call creates a mortal SV, the second converts an existing

SV to a mortal SV (and thus defers a call to C<SvREFCNT_dec>), and the

third creates a mortal copy of an existing SV.



The mortal routines are not just for SVs -- AVs and HVs can be

made mortal by passing their address (type-casted to C<SV*>) to the

C<sv_2mortal> or C<sv_mortalcopy> routines.



=head2 Stashes and Globs



A "stash" is a hash that contains all of the different objects that

are contained within a package.  Each key of the stash is a symbol

name (shared by all the different types of objects that have the same

name), and each value in the hash table is a GV (Glob Value).  This GV

in turn contains references to the various objects of that name,

including (but not limited to) the following:



    Scalar Value

    Array Value

    Hash Value

    I/O Handle

    Format

    Subroutine



There is a single stash called "PL_defstash" that holds the items that exist

in the "main" package.  To get at the items in other packages, append the

string "::" to the package name.  The items in the "Foo" package are in

the stash "Foo::" in PL_defstash.  The items in the "Bar::Baz" package are

in the stash "Baz::" in "Bar::"'s stash.



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



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

    HV*  gv_stashsv(SV*, I32 create)



The first function takes a literal string, the second uses the string stored

in the SV.  Remember that a stash is just a hash table, so you get back an

C<HV*>.  The C<create> flag will create a new package if it is set.



The name that C<gv_stash*v> wants is the name of the package whose symbol table

you want.  The default package is called C<main>.  If you have multiply nested

packages, pass their names to C<gv_stash*v>, separated by C<::> as in the Perl

language itself.



Alternately, if you have an SV that is a blessed reference, you can find

out the stash pointer by using:



    HV*  SvSTASH(SvRV(SV*));



then use the following to get the package name itself:



    char*  HvNAME(HV* stash);



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

function:



    SV*  sv_bless(SV*, HV* stash)



where the first argument, an C<SV*>, must be a reference, and the second

argument is a stash.  The returned C<SV*> can now be used in the same way

as any other SV.



For more information on references and blessings, consult L<perlref>.



=head2 Double-Typed SVs



Scalar variables normally contain only one type of value, an integer,

double, pointer, or reference.  Perl will automatically convert the

actual scalar data from the stored type into the requested type.



Some scalar variables contain more than one type of scalar data.  For

example, the variable C<$!> contains either the numeric value of C<errno>

or its string equivalent from either C<strerror> or C<sys_errlist[]>.



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

C<sv_set*v> routines to add the additional scalar type, then set a flag

so that Perl will believe it contains more than one type of data.  The

four macros to set the flags are:



	SvIOK_on

	SvNOK_on

	SvPOK_on

	SvROK_on



The particular macro you must use depends on which C<sv_set*v> routine

you called first.  This is because every C<sv_set*v> routine turns on

only the bit for the particular type of data being set, and turns off

all the rest.



For example, to create a new Perl variable called "dberror" that contains

both the numeric and descriptive string error values, you could use the

following code:



    extern int  dberror;

    extern char *dberror_list;



    SV* sv = get_sv("dberror", TRUE);

    sv_setiv(sv, (IV) dberror);

    sv_setpv(sv, dberror_list[dberror]);

    SvIOK_on(sv);



If the order of C<sv_setiv> and C<sv_setpv> had been reversed, then the

macro C<SvPOK_on> would need to be called instead of C<SvIOK_on>.



=head2 Magic Variables



[This section still under construction.  Ignore everything here.  Post no

bills.  Everything not permitted is forbidden.]



Any SV may be magical, that is, it has special features that a normal

SV does not have.  These features are stored in the SV structure in a

linked list of C<struct magic>'s, typedef'ed to C<MAGIC>.



    struct magic {

        MAGIC*      mg_moremagic;

        MGVTBL*     mg_virtual;

        U16         mg_private;

        char        mg_type;

        U8          mg_flags;

        SV*         mg_obj;

        char*       mg_ptr;

        I32         mg_len;

    };



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



=head2 Assigning Magic



Perl adds magic to an SV using the sv_magic function:



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



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

feature.



If C<sv> is not already magical, Perl uses the C<SvUPGRADE> macro to

set the C<SVt_PVMG> flag for the C<sv>.  Perl then continues by adding

it to the beginning of the linked list of magical features.  Any prior

entry of the same type of magic is deleted.  Note that this can be

overridden, and multiple instances of the same type of magic can be

associated with an SV.



The C<name> and C<namlen> arguments are used to associate a string with

the magic, typically the name of a variable. C<namlen> is stored in the

C<mg_len> field and if C<name> is non-null and C<namlen> >= 0 a malloc'd

copy of the name is stored in C<mg_ptr> field.



The sv_magic function uses C<how> to determine which, if any, predefined

"Magic Virtual Table" should be assigned to the C<mg_virtual> field.

See the "Magic Virtual Table" section below.  The C<how> argument is also

stored in the C<mg_type> field.



The C<obj> argument is stored in the C<mg_obj> field of the C<MAGIC>

structure.  If it is not the same as the C<sv> argument, the reference

count of the C<obj> object is incremented.  If it is the same, or if

the C<how> argument is "#", or if it is a NULL pointer, then C<obj> is

merely stored, without the reference count being incremented.



There is also a function to add magic to an C<HV>:



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



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



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



    void sv_unmagic(SV *sv, int type);



The C<type> argument should be equal to the C<how> value when the C<SV>

was initially made magical.



=head2 Magic Virtual Tables



The C<mg_virtual> field in the C<MAGIC> structure is a pointer to a

C<MGVTBL>, which is a structure of function pointers and stands for

"Magic Virtual Table" to handle the various operations that might be

applied to that variable.



The C<MGVTBL> has five pointers to the following routine types:



    int  (*svt_get)(SV* sv, MAGIC* mg);

    int  (*svt_set)(SV* sv, MAGIC* mg);

    U32  (*svt_len)(SV* sv, MAGIC* mg);

    int  (*svt_clear)(SV* sv, MAGIC* mg);

    int  (*svt_free)(SV* sv, MAGIC* mg);



This MGVTBL structure is set at compile-time in C<perl.h> and there are

currently 19 types (or 21 with overloading turned on).  These different

structures contain pointers to various routines that perform additional

actions depending on which function is being called.



    Function pointer    Action taken

    ----------------    ------------

    svt_get             Do something after the value of the SV is retrieved.

    svt_set             Do something after the SV is assigned a value.

    svt_len             Report on the SV's length.

    svt_clear		Clear something the SV represents.

    svt_free            Free any extra storage associated with the SV.



For instance, the MGVTBL structure called C<vtbl_sv> (which corresponds

to an C<mg_type> of '\0') contains:



    { magic_get, magic_set, magic_len, 0, 0 }



Thus, when an SV is determined to be magical and of type '\0', if a get

operation is being performed, the routine C<magic_get> is called.  All

the various routines for the various magical types begin with C<magic_>.

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 %_<filename 

    l        vtbl_dbline         Debugger %_<filename element

    o        vtbl_collxfrm       Locale transformation

    P        vtbl_pack           Tied array or hash

    p        vtbl_packelem       Tied array or hash element

    q        vtbl_packelem       Tied scalar or handle

    S        vtbl_sig            %SIG hash

    s        vtbl_sigelem        %SIG hash element

    t        vtbl_taint          Taintedness

    U        vtbl_uvar           Available for use by extensions

    v        vtbl_vec            vec() lvalue

    x        vtbl_substr         substr() lvalue

    y        vtbl_defelem        Shadow "foreach" iterator variable /

                                  smart parameter vivification

    *        vtbl_glob           GV (typeglob)

    #        vtbl_arylen         Array length ($#ary)

    .        vtbl_pos            pos() lvalue

    ~        (none)              Available for use by extensions



When an uppercase and lowercase letter both exist in the table, then the

uppercase letter is used to represent some kind of composite type (a list

or a hash), and the lowercase letter is used to represent an element of

that composite type.



The '~' and 'U' magic types are defined specifically for use by

extensions and will not be used by perl itself.  Extensions can use

'~' magic to 'attach' private information to variables (typically

objects).  This is especially useful because there is no way for

normal perl code to corrupt this private information (unlike using

extra elements of a hash object).



Similarly, 'U' magic can be used much like tie() to call a C function

any time a scalar's value is used or changed.  The C<MAGIC>'s

C<mg_ptr> field points to a C<ufuncs> structure:



    struct ufuncs {

        I32 (*uf_val)(IV, SV*);

        I32 (*uf_set)(IV, SV*);

        IV uf_index;

    };



When the SV is read from or written to, the C<uf_val> or C<uf_set>

function will be called with C<uf_index> as the first arg and a

pointer to the SV as the second.  A simple example of how to add 'U'

magic is shown below.  Note that the ufuncs structure is copied by

sv_magic, so you can safely allocate it on the stack.



    void

    Umagic(sv)

        SV *sv;

    PREINIT:

        struct ufuncs uf;

    CODE:

        uf.uf_val   = &my_get_fn;

        uf.uf_set   = &my_set_fn;

        uf.uf_index = 0;

        sv_magic(sv, 0, 'U', (char*)&uf, sizeof(uf));



Note that because multiple extensions may be using '~' or 'U' magic,

it is important for extensions to take extra care to avoid conflict.

Typically only using the magic on objects blessed into the same class

as the extension is sufficient.  For '~' magic, it may also be

appropriate to add an I32 'signature' at the top of the private data

area and check that.



Also note that the C<sv_set*()> and C<sv_cat*()> functions described

earlier do B<not> invoke 'set' magic on their targets.  This must

be done by the user either by calling the C<SvSETMAGIC()> macro after

calling these functions, or by using one of the C<sv_set*_mg()> or

C<sv_cat*_mg()> functions.  Similarly, generic C code must call the

C<SvGETMAGIC()> macro to invoke any 'get' magic if they use an SV

obtained from external sources in functions that don't handle magic.

See L<perlapi> for a description of these functions.

For example, calls to the C<sv_cat*()> functions typically need to be

followed by C<SvSETMAGIC()>, but they don't need a prior C<SvGETMAGIC()>

since their implementation handles 'get' magic.



=head2 Finding Magic



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



This routine returns a pointer to the C<MAGIC> structure stored in the SV.

If the SV does not have that magical feature, C<NULL> is returned.  Also,

if the SV is not of type SVt_PVMG, Perl may core dump.



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



This routine checks to see what types of magic C<sv> has.  If the mg_type

field is an uppercase letter, then the mg_obj is copied to C<nsv>, but

the mg_type field is changed to be the lowercase letter.



=head2 Understanding the Magic of Tied Hashes and Arrays



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



WARNING: As of the 5.004 release, proper usage of the array and hash

access functions requires understanding a few caveats.  Some

of these caveats are actually considered bugs in the API, to be fixed

in later releases, and are bracketed with [MAYCHANGE] below. If

you find yourself actually applying such information in this section, be

aware that the behavior may change in the future, umm, without warning.



The perl tie function associates a variable with an object that implements

the various GET, SET etc methods.  To perform the equivalent of the perl

tie function from an XSUB, you must mimic this behaviour.  The code below

carries out the necessary steps - firstly it creates a new hash, and then

creates a second hash which it blesses into the class which will implement

the tie methods. Lastly it ties the two hashes together, and returns a

reference to the new tied hash.  Note that the code below does NOT call the

TIEHASH method in the MyTie class -

see L<Calling Perl Routines from within C Programs> for details on how

to do this.



    SV*

    mytie()

    PREINIT:

        HV *hash;

        HV *stash;

        SV *tie;

    CODE:

        hash = newHV();

        tie = newRV_noinc((SV*)newHV());

        stash = gv_stashpv("MyTie", TRUE);

        sv_bless(tie, stash);

        hv_magic(hash, tie, 'P');

        RETVAL = newRV_noinc(hash);

    OUTPUT:

        RETVAL



The C<av_store> function, when given a tied array argument, merely

copies the magic of the array onto the value to be "stored", using

C<mg_copy>.  It may also return NULL, indicating that the value did not

actually need to be stored in the array.  [MAYCHANGE] After a call to

C<av_store> on a tied array, the caller will usually need to call

C<mg_set(val)> to actually invoke the perl level "STORE" method on the

TIEARRAY object.  If C<av_store> did return NULL, a call to

C<SvREFCNT_dec(val)> will also be usually necessary to avoid a memory

leak. [/MAYCHANGE]



The previous paragraph is applicable verbatim to tied hash access using the

C<hv_store> and C<hv_store_ent> functions as well.



C<av_fetch> and the corresponding hash functions C<hv_fetch> and

C<hv_fetch_ent> actually return an undefined mortal value whose magic

has been initialized using C<mg_copy>.  Note the value so returned does not

need to be deallocated, as it is already mortal.  [MAYCHANGE] But you will

need to call C<mg_get()> on the returned value in order to actually invoke

the perl level "FETCH" method on the underlying TIE object.  Similarly,

you may also call C<mg_set()> on the return value after possibly assigning

a suitable value to it using C<sv_setsv>,  which will invoke the "STORE"

method on the TIE object. [/MAYCHANGE]



[MAYCHANGE]

In other words, the array or hash fetch/store functions don't really

fetch and store actual values in the case of tied arrays and hashes.  They

merely call C<mg_copy> to attach magic to the values that were meant to be

"stored" or "fetched".  Later calls to C<mg_get> and C<mg_set> actually

do the job of invoking the TIE methods on the underlying objects.  Thus

the magic mechanism currently implements a kind of lazy access to arrays

and hashes.



Currently (as of perl version 5.004), use of the hash and array access

functions requires the user to be aware of whether they are operating on

"normal" hashes and arrays, or on their tied variants.  The API may be

changed to provide more transparent access to both tied and normal data

types in future versions.

[/MAYCHANGE]



You would do well to understand that the TIEARRAY and TIEHASH interfaces

are mere sugar to invoke some perl method calls while using the uniform hash

and array syntax.  The use of this sugar imposes some overhead (typically

about two to four extra opcodes per FETCH/STORE operation, in addition to

the creation of all the mortal variables required to invoke the methods).

This overhead will be comparatively small if the TIE methods are themselves

substantial, but if they are only a few statements long, the overhead

will not be insignificant.



=head2 Localizing changes



Perl has a very handy construction



  {

    local $var = 2;

    ...

  }



This construction is I<approximately> equivalent to



  {

    my $oldvar = $var;

    $var = 2;

    ...

    $var = $oldvar;

  }



The biggest difference is that the first construction would

reinstate the initial value of $var, irrespective of how control exits

the block: C<goto>, C<return>, C<die>/C<eval> etc. It is a little bit

more efficient as well.



There is a way to achieve a similar task from C via Perl API: create a

I<pseudo-block>, and arrange for some changes to be automatically

undone at the end of it, either explicit, or via a non-local exit (via

die()). A I<block>-like construct is created by a pair of

C<ENTER>/C<LEAVE> macros (see L<perlcall/"Returning a Scalar">).

Such a construct may be created specially for some important localized

task, or an existing one (like boundaries of enclosing Perl

subroutine/block, or an existing pair for freeing TMPs) may be

used. (In the second case the overhead of additional localization must

be almost negligible.) Note that any XSUB is automatically enclosed in

an C<ENTER>/C<LEAVE> pair.



Inside such a I<pseudo-block> the following service is available:



=over 4



=item C<SAVEINT(int i)>



=item C<SAVEIV(IV i)>



=item C<SAVEI32(I32 i)>



=item C<SAVELONG(long i)>



These macros arrange things to restore the value of integer variable

C<i> at the end of enclosing I<pseudo-block>.



=item C<SAVESPTR(s)>



=item C<SAVEPPTR(p)>



These macros arrange things to restore the value of pointers C<s> and

C<p>. C<s> must be a pointer of a type which survives conversion to

C<SV*> and back, C<p> should be able to survive conversion to C<char*>

and back.



=item C<SAVEFREESV(SV *sv)>



The refcount of C<sv> would be decremented at the end of

I<pseudo-block>.  This is similar to C<sv_2mortal> in that it is also a

mechanism for doing a delayed C<SvREFCNT_dec>.  However, while C<sv_2mortal>

extends the lifetime of C<sv> until the beginning of the next statement,

C<SAVEFREESV> extends it until the end of the enclosing scope.  These

lifetimes can be wildly different.



Also compare C<SAVEMORTALIZESV>.



=item C<SAVEMORTALIZESV(SV *sv)>



Just like C<SAVEFREESV>, but mortalizes C<sv> at the end of the current

scope instead of decrementing its reference count.  This usually has the

effect of keeping C<sv> alive until the statement that called the currently

live scope has finished executing.



=item C<SAVEFREEOP(OP *op)>



The C<OP *> is op_free()ed at the end of I<pseudo-block>.



=item C<SAVEFREEPV(p)>



The chunk of memory which is pointed to by C<p> is Safefree()ed at the

end of I<pseudo-block>.



=item C<SAVECLEARSV(SV *sv)>



Clears a slot in the current scratchpad which corresponds to C<sv> at

the end of I<pseudo-block>.



=item C<SAVEDELETE(HV *hv, char *key, I32 length)>



The key C<key> of C<hv> is deleted at the end of I<pseudo-block>. The

string pointed to by C<key> is Safefree()ed.  If one has a I<key> in

short-lived storage, the corresponding string may be reallocated like

this:



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



=item C<SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)>



At the end of I<pseudo-block> the function C<f> is called with the

only argument C<p>.



=item C<SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)>



At the end of I<pseudo-block> the function C<f> is called with the

implicit context argument (if any), and C<p>.



=item C<SAVESTACK_POS()>



The current offset on the Perl internal stack (cf. C<SP>) is restored

at the end of I<pseudo-block>.



=back



The following API list contains functions, thus one needs to

provide pointers to the modifiable data explicitly (either C pointers,

or Perlish C<GV *>s).  Where the above macros take C<int>, a similar 

function takes C<int *>.



=over 4



=item C<SV* save_scalar(GV *gv)>



Equivalent to Perl code C<local $gv>.



=item C<AV* save_ary(GV *gv)>



=item C<HV* save_hash(GV *gv)>



Similar to C<save_scalar>, but localize C<@gv> and C<%gv>.



=item C<void save_item(SV *item)>



Duplicates the current value of C<SV>, on the exit from the current

C<ENTER>/C<LEAVE> I<pseudo-block> will restore the value of C<SV>

using the stored value.



=item C<void save_list(SV **sarg, I32 maxsarg)>



A variant of C<save_item> which takes multiple arguments via an array

C<sarg> of C<SV*> of length C<maxsarg>.



=item C<SV* save_svref(SV **sptr)>



Similar to C<save_scalar>, but will reinstate a C<SV *>.



=item C<void save_aptr(AV **aptr)>



=item C<void save_hptr(HV **hptr)>



Similar to C<save_svref>, but localize C<AV *> and C<HV *>.



=back



The C<Alias> module implements localization of the basic types within the

I<caller's scope>.  People who are interested in how to localize things in

the containing scope should take a look there too.



=head1 Subroutines



=head2 XSUBs and the Argument Stack



The XSUB mechanism is a simple way for Perl programs to access C subroutines.

An XSUB routine will have a stack that contains the arguments from the Perl

program, and a way to map from the Perl data structures to a C equivalent.



The stack arguments are accessible through the C<ST(n)> macro, which returns

the C<n>'th stack argument.  Argument 0 is the first argument passed in the

Perl subroutine call.  These arguments are C<SV*>, and can be used anywhere

an C<SV*> is used.



Most of the time, output from the C routine can be handled through use of

the RETVAL and OUTPUT directives.  However, there are some cases where the

argument stack is not already long enough to handle all the return values.

An example is the POSIX tzname() call, which takes no arguments, but returns

two, the local time zone's standard and summer time abbreviations.



To handle this situation, the PPCODE directive is used and the stack is

extended using the macro:



    EXTEND(SP, num);



where C<SP> is the macro that represents the local copy of the stack pointer,

and C<num> is the number of elements the stack should be extended by.



Now that there is room on the stack, values can be pushed on it using the

macros to push IVs, doubles, strings, and SV pointers respectively:



    PUSHi(IV)

    PUSHn(double)

    PUSHp(char*, I32)

    PUSHs(SV*)



And now the Perl program calling C<tzname>, the two values will be assigned

as in:



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



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

to use the macros:



    XPUSHi(IV)

    XPUSHn(double)

    XPUSHp(char*, I32)

    XPUSHs(SV*)



These macros automatically adjust the stack for you, if needed.  Thus, you

do not need to call C<EXTEND> to extend the stack.

However, see L</Putting a C value on Perl stack>



For more information, consult L<perlxs> and L<perlxstut>.



=head2 Calling Perl Routines from within C Programs



There are four routines that can be used to call a Perl subroutine from

within a C program.  These four are:



    I32  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 C<call_sv>.  The C<SV*> argument

contains either the name of the Perl subroutine to be called, or a

reference to the subroutine.  The second argument consists of flags

that control the context in which the subroutine is called, whether

or not the subroutine is being passed arguments, how errors should be

trapped, and how to treat return values.



All four routines return the number of arguments that the subroutine returned

on the Perl stack.



These routines used to be called C<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 C<call_argv>), the programmer

must manipulate the Perl stack.  These include the following macros and

functions:



    dSP

    SP

    PUSHMARK()

    PUTBACK

    SPAGAIN

    ENTER

    SAVETMPS

    FREETMPS

    LEAVE

    XPUSH*()

    POP*()



For a detailed description of calling conventions from C to Perl,

consult L<perlcall>.



=head2 Memory Allocation



All memory meant to be used with the Perl API functions should be manipulated

using the macros described in this section.  The macros provide the necessary

transparency between differences in the actual malloc implementation that is

used within perl.



It is suggested that you enable the version of malloc that is distributed

with Perl.  It keeps pools of various sizes of unallocated memory in

order to satisfy allocation requests more quickly.  However, on some

platforms, it may cause spurious malloc or free errors.



    New(x, pointer, number, type);

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

    Newz(x, pointer, number, type);



These three macros are used to initially allocate memory.



The first argument C<x> was a "magic cookie" that was used to keep track

of who called the macro, to help when debugging memory problems.  However,

the current code makes no use of this feature (most Perl developers now

use run-time memory checkers), so this argument can be any number.



The second argument C<pointer> should be the name of a variable that will

point to the newly allocated memory.



The third and fourth arguments C<number> and C<type> specify how many of

the specified type of data structure should be allocated.  The argument

C<type> is passed to C<sizeof>.  The final argument to C<Newc>, C<cast>,

should be used if the C<pointer> argument is different from the C<type>

argument.



Unlike the C<New> and C<Newc> macros, the C<Newz> macro calls C<memzero>

to zero out all the newly allocated memory.



    Renew(pointer, number, type);

    Renewc(pointer, number, type, cast);

    Safefree(pointer)



These three macros are used to change a memory buffer size or to free a

piece of memory no longer needed.  The arguments to C<Renew> and C<Renewc>

match those of C<New> and C<Newc> with the exception of not needing the

"magic cookie" argument.



    Move(source, dest, number, type);

    Copy(source, dest, number, type);

    Zero(dest, number, type);



These three macros are used to move, copy, or zero out previously allocated

memory.  The C<source> and C<dest> arguments point to the source and

destination starting points.  Perl will move, copy, or zero out C<number>

instances of the size of the C<type> data structure (using the C<sizeof>

function).



=head2 PerlIO



The most recent development releases of Perl has been experimenting with

removing Perl's dependency on the "normal" standard I/O suite and allowing

other stdio implementations to be used.  This involves creating a new

abstraction layer that then calls whichever implementation of stdio Perl

was compiled with.  All XSUBs should now use the functions in the PerlIO

abstraction layer and not make any assumptions about what kind of stdio

is being used.



For a complete description of the PerlIO abstraction, consult L<perlapio>.



=head2 Putting a C value on Perl stack



A lot of opcodes (this is an elementary operation in the internal perl

stack machine) put an SV* on the stack. However, as an optimization

the corresponding SV is (usually) not recreated each time. The opcodes

reuse specially assigned SVs (I<target>s) which are (as a corollary)

not constantly freed/created.



Each of the targets is created only once (but see

L<Scratchpads and recursion> below), and when an opcode needs to put

an integer, a double, or a string on stack, it just sets the

corresponding parts of its I<target> and puts the I<target> on stack.



The macro to put this target on stack is C<PUSHTARG>, and it is

directly used in some opcodes, as well as indirectly in zillions of

others, which use it via C<(X)PUSH[pni]>.



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 "set C<TARG> to 10, push a pointer to C<TARG> onto

the stack; set C<TARG> to 20, push a pointer to C<TARG> onto the stack".

At the end of the operation, the stack does not contain the values 10

and 20, but actually contains two pointers to C<TARG>, which we have set

to 20. If you need to push multiple different values, use C<XPUSHs>,

which bypasses C<TARG>.



On a related note, if you do use C<(X)PUSH[npi]>, then you're going to

need a C<dTARG> in your variable declarations so that the C<*PUSH*>

macros can make use of the local variable C<TARG>. 



=head2 Scratchpads



The question remains on when the SVs which are I<target>s for opcodes

are created. The answer is that they are created when the current unit --

a subroutine or a file (for opcodes for statements outside of

subroutines) -- is compiled. During this time a special anonymous Perl

array is created, which is called a scratchpad for the current

unit.



A scratchpad keeps SVs which are lexicals for the current unit and are

targets for opcodes. One can deduce that an SV lives on a scratchpad

by looking on its flags: lexicals have C<SVs_PADMY> set, and

I<target>s have C<SVs_PADTMP> set.



The correspondence between OPs and I<target>s is not 1-to-1. Different

OPs in the compile tree of the unit can use the same target, if this

would not conflict with the expected life of the temporary.



=head2 Scratchpads and recursion



In fact it is not 100% true that a compiled unit contains a pointer to

the scratchpad AV. In fact it contains a pointer to an AV of

(initially) one element, and this element is the scratchpad AV. Why do

we need an extra level of indirection?



The answer is B<recursion>, and maybe (sometime soon) B<threads>. Both

these can create several execution pointers going into the same

subroutine. For the subroutine-child not write over the temporaries

for the subroutine-parent (lifespan of which covers the call to the

child), the parent and the child should have different

scratchpads. (I<And> the lexicals should be separate anyway!)



So each subroutine is born with an array of scratchpads (of length 1).

On each entry to the subroutine it is checked that the current

depth of the recursion is not more than the length of this array, and

if it is, new scratchpad is created and pushed into the array.



The I<target>s on this scratchpad are C<undef>s, but they are already

marked with correct flags.



=head1 Compiled code



=head2 Code tree



Here we describe the internal form your code is converted to by

Perl. Start with a simple example:



  $a = $b + $c;



This is converted to a tree similar to this one:



             assign-to

           /           \

          +             $a

        /   \

      $b     $c



(but slightly more complicated).  This tree reflects the way Perl

parsed your code, but has nothing to do with the execution order.

There is an additional "thread" going through the nodes of the tree

which shows the order of execution of the nodes.  In our simplified

example above it looks like:



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



But with the actual compile tree for C<$a = $b + $c> it is different:

some nodes I<optimized away>.  As a corollary, though the actual tree

contains more nodes than our simplified example, the execution order

is the same as in our example.



=head2 Examining the tree



If you have your perl compiled for debugging (usually done with C<-D

optimize=-g> on C<Configure> command line), you may examine the

compiled tree by specifying C<-Dx> on the Perl command line.  The

output takes several lines per node, and for C<$b+$c> it looks like

this:



    5           TYPE = add  ===> 6

                TARG = 1

                FLAGS = (SCALAR,KIDS)

                {

                    TYPE = null  ===> (4)

                      (was rv2sv)

                    FLAGS = (SCALAR,KIDS)

                    {

    3                   TYPE = gvsv  ===> 4

                        FLAGS = (SCALAR)

                        GV = main::b

                    }

                }

                {

                    TYPE = null  ===> (5)

                      (was rv2sv)

                    FLAGS = (SCALAR,KIDS)

                    {

    4                   TYPE = gvsv  ===> 5

                        FLAGS = (SCALAR)

                        GV = main::c

                    }

                }



This tree has 5 nodes (one per C<TYPE> specifier), only 3 of them are

not optimized away (one per number in the left column).  The immediate

children of the given node correspond to C<{}> pairs on the same level

of indentation, thus this listing corresponds to the tree:



                   add

                 /     \

               null    null

                |       |

               gvsv    gvsv



The execution order is indicated by C<===E<gt>> marks, thus it is C<3

4 5 6> (node C<6> is not included into above listing), i.e.,

C<gvsv gvsv add whatever>.



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

F<pp*.c> files; the function which implements the op with type C<gvsv>

is C<pp_gvsv>, and so on. As the tree above shows, different ops have

different numbers of children: C<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 C<OP>: this has no children. Unary

operators, C<UNOP>s, have one child, and this is pointed to by the

C<op_first> field. Binary operators (C<BINOP>s) have not only an

C<op_first> field but also an C<op_last> field. The most complex type of

op is a C<LISTOP>, which has any number of children. In this case, the

first child is pointed to by C<op_first> and the last child by

C<op_last>. The children in between can be found by iteratively

following the C<op_sibling> pointer from the first child to the last.



There are also two other op types: a C<PMOP> holds a regular expression,

and has no children, and a C<LOOP> may or may not have children. If the

C<op_children> field is non-zero, it behaves like a C<LISTOP>. To

complicate matters, if a C<UNOP> is actually a C<null> op after

optimization (see L</Compile pass 2: context propagation>) it will still

have children in accordance with its former type.



=head2 Compile pass 1: check routines



The tree is created by the compiler while I<yacc> code feeds it

the constructions it recognizes. Since I<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 F<opcode.pl> (do not

forget to run C<make regen_headers> if you modify this file).



A check routine is called when the node is fully constructed except

for the execution-order thread.  Since at this time there are no

back-links to the currently constructed node, one can do most any

operation to the top-level node, including freeing it and/or creating

new nodes above/below it.



The check routine returns the node which should be inserted into the

tree (if the top-level node was not modified, check routine returns

its argument).



By convention, check routines have names C<ck_*>. They are usually

called from C<new*OP> subroutines (or C<convert>) (which in turn are

called from F<perly.y>).



=head2 Compile pass 1a: constant folding



Immediately after the check routine is called the returned node is

checked for being compile-time executable.  If it is (the value is

judged to be constant) it is immediately executed, and a I<constant>

node with the "return value" of the corresponding subtree is

substituted instead.  The subtree is deleted.



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

created.



=head2 Compile pass 2: context propagation



When a context for a part of compile tree is known, it is propagated

down through the tree.  At this time the context can have 5 values

(instead of 2 for runtime context): void, boolean, scalar, list, and

lvalue.  In contrast with the pass 1 this pass is processed from top

to bottom: a node's context determines the context for its children.



Additional context-dependent optimizations are performed at this time.

Since at this moment the compile tree contains back-references (via

"thread" pointers), nodes cannot be free()d now.  To allow

optimized-away nodes at this stage, such nodes are null()ified instead

of free()ing (i.e. their type is changed to OP_NULL).



=head2 Compile pass 3: peephole optimization



After the compile tree for a subroutine (or for an C<eval> or a file)

is created, an additional pass over the code is performed. This pass

is neither top-down or bottom-up, but in the execution order (with

additional complications for conditionals).  These optimizations are

done in the subroutine peep().  Optimizations performed at this stage

are subject to the same restrictions as in the pass 2.



=head1 Examining internal data structures with the C<dump> functions



To aid debugging, the source file F<dump.c> contains a number of

functions which produce formatted output of internal data structures.



The most commonly used of these functions is C<Perl_sv_dump>; it's used

for dumping SVs, AVs, HVs, and CVs. The C<Devel::Peek> module calls

C<sv_dump> to produce debugging output from Perl-space, so users of that

module should already be familiar with its format. 



C<Perl_op_dump> can be used to dump an C<OP> structure or any of its

derivatives, and produces output similiar to C<perl -Dx>; in fact,

C<Perl_dump_eval> will dump the main root of the code being evaluated,

exactly like C<-Dx>.



Other useful functions are C<Perl_dump_sub>, which turns a C<GV> into an

op tree, C<Perl_dump_packsubs> which calls C<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 C<Perl_dump_all>, which dumps all the subroutines in the stash and

the op tree of the main root.



=head1 How multiple interpreters and concurrency are supported



=head2 Background and PERL_IMPLICIT_CONTEXT



The Perl interpreter can be regarded as a closed box: it has an API

for feeding it code or otherwise making it do things, but it also has

functions for its own use.  This smells a lot like an object, and

there are ways for you to build Perl so that you can have multiple

interpreters, with one interpreter represented either as a C++ object,

a C structure, or inside a thread.  The thread, the C structure, or

the C++ object will contain all the context, the state of that

interpreter.



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 C<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 L</Internal Functions>.) The easiest way to be B<sure> a 

function is part of the API is to find its entry in L<perlapi>.  

If it exists in L<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 

L<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)



C<pTHX_> is one of a number of macros (in perl.h) that hide the

details of the interpreter's context.  THX stands for "thread", "this",

or "thingy", as the case may be.  (And no, George Lucas is not involved. :-)

The first character could be 'p' for a B<p>rototype, 'a' for B<a>rgument,

or 'd' for B<d>eclaration, so we have C<pTHX>, C<aTHX> and C<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 C<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->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 with PERL_IMPLICIT_CONTEXT in both core

and extensions, it will become:



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

                                       # for all build flavors



This doesn't work so cleanly for varargs functions, though, as macros

imply that the number of arguments is known in advance.  Instead we

either need to spell them out fully, passing C<aTHX_> as the first

argument (the Perl core tends to do this with functions like

Perl_warner), or use a context-free version.



The context-free version of Perl_warner is called

Perl_warner_nocontext, and does not take the extra argument.  Instead

it does dTHX; to get the context from thread-local storage.  We

C<#define warner Perl_warner_nocontext> so that extensions get source

compatibility at the expense of performance.  (Passing an arg is

cheaper than grabbing it from thread-local storage.)



You can ignore [pad]THX[xo] when browsing the Perl headers/sources.

Those are strictly for use within the core.  Extensions and embedders

need only be aware of [pad]THX.



=head2 So what happened to dTHR?



C<dTHR> was introduced in perl 5.005 to support the older thread model.

The older thread model now uses the C<THX> mechanism to pass context

pointers around, so C<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.



=head2 How do I use all this in extensions?



When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call

any functions in the Perl API will need to pass the initial context

argument somehow.  The kicker is that you will need to write it in

such a way that the extension still compiles when Perl hasn't been

built with PERL_IMPLICIT_CONTEXT enabled.



There are three ways to do this.  First, the easy but inefficient way,

which is also the default, in order to maintain source compatibility

with extensions: whenever XSUB.h is #included, it redefines the aTHX

and aTHX_ macros to call a function that will return the context.

Thus, something like:



        sv_setsv(asv, bsv);



in your 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 "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 C<#define PERL_NO_GET_CONTEXT> before

including the Perl headers, followed by a C<dTHX;> declaration at

the start of every function that will call the Perl API.  (You'll

know which functions need this, because the C compiler will complain

that there's an undeclared identifier in those functions.)  No changes

are needed for the XSUBs themselves, because the XS() macro is

correctly defined to pass in the implicit context if needed.



The third, even more efficient way is to ape how it is done within

the Perl guts:





        #define PERL_NO_GET_CONTEXT     /* we want efficiency */

        #include "EXTERN.h"

        #include "perl.h"

        #include "XSUB.h"



        /* pTHX_ only needed for functions that call Perl API */

        static my_private_function(pTHX_ int arg1, int arg2);



        static SV *

        my_private_function(pTHX_ int arg1, int arg2)

        {

            /* dTHX; not needed here, because THX is an argument */

            ... call Perl API functions ...

        }



        [... etc ...]



        MODULE = Foo            PACKAGE = Foo



        /* typical XSUB */



        void

        my_xsub(arg)

                int arg

            CODE:

                my_private_function(aTHX_ arg, 10);



This implementation never has to fetch the context using a function

call, since it is always passed as an extra argument.  Depending on

your needs for simplicity or efficiency, you may mix the previous

two approaches freely.



Never add a comma after C<pTHX> yourself--always use the form of the

macro with the underscore for functions that take explicit arguments,

or the form without the argument for functions with no explicit arguments.



=head2 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 C<perl_alloc> and C<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 C<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 ...



=head2 Future Plans and PERL_IMPLICIT_SYS



Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything

that the interpreter knows about itself and pass it around, so too are

there plans to allow the interpreter to bundle up everything it knows

about the environment it's running on.  This is enabled with the

PERL_IMPLICIT_SYS macro.  Currently it only works with PERL_OBJECT

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.



=head1 Internal Functions



All of Perl's internal functions which will be exposed to the outside

world are be prefixed by C<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 C<PL_>. (By convention,

static functions start with C<S_>)



Inside the Perl core, you can get at the functions either with or

without the C<Perl_> prefix, thanks to a bunch of defines that live in

F<embed.h>. This header file is generated automatically from

F<embed.pl>. F<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 F<embed.pl> as well. Here's a sample entry from that table:



    Apd |SV**   |av_fetch   |AV* ar|I32 key|I32 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:



=over 3



=item A



This function is a part of the public API.



=item p



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



=item d



This function has documentation using the C<apidoc> feature which we'll

look at in a second.



=back



Other available flags are:



=over 3



=item s



This is a static function and is defined as C<S_whatever>, and usually

called within the sources as C<whatever(...)>.



=item n



This does not use C<aTHX_> and C<pTHX> to pass interpreter context. (See

L<perlguts/Background and PERL_IMPLICIT_CONTEXT>.)



=item r



This function never returns; C<croak>, C<exit> and friends.



=item f



This function takes a variable number of arguments, C<printf> style.

The argument list should end with C<...>, like this:



    Afprd   |void   |croak          |const char* pat|...



=item M



This function is part of the experimental development API, and may change 

or disappear without notice.



=item o



This function should not have a compatibility macro to define, say,

C<Perl_parse> to C<parse>. It must be called as C<Perl_parse>.



=item j



This function is not a member of C<CPerlObj>. If you don't know

what this means, don't use it.



=item x



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



=back



If you edit F<embed.pl>, you will need to run C<make regen_headers> to

force a rebuild of F<embed.h> and other auto-generated files.



=head2 Formatted Printing of IVs, UVs, and NVs



If you are printing IVs, UVs, or NVS instead of the stdio(3) style

formatting codes like C<%d>, C<%ld>, C<%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("IV is %"IVdf"\n", iv);



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.



=head2 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);



=head2 Source Documentation



There's an effort going on to document the internal functions and

automatically produce reference manuals from them - L<perlapi> is one

such manual which details all the functions which are available to XS

writers. L<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<sv_setiv_mg>.



 =cut

 */



Please try and supply some documentation if you add functions to the

Perl core.



=head1 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.



=head2 What B<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/



=head2 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, (C<0xC8> for you hex types)

capital E with a grave accent, is represented by the two bytes

C<v196.172>. Unfortunately, the non-Unicode string C<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 C<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, C<is_utf8_char> will tell you

whether the current character in a string is valid UTF8.



=head2 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 C<v194.129>; this

continues up to character 191, which is C<v194.191>. Now we've run out of

bits (191 is binary C<10111111>) so we move on; 192 is C<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 C<UTF8SKIP> macro:



    char *utf = "\305\233\340\240\201";

    I32 len;



    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

C<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 & 0x80)

        /* Must treat this as UTF8 */

        uv = utf8_to_uv(utf);

    else

        /* OK to treat this character as a byte */

        uv = *utf;



You can also see in that example that we use C<utf8_to_uv> to get the

value of the character; the inverse function C<uv_to_utf8> is available

for putting a UV into UTF8:



    if (uv > 0x80)

        /* Must treat this as UTF8 */

        utf8 = uv_to_utf8(utf8, uv);

    else

        /* OK to treat this character as a byte */

        *utf8++ = uv;



You B<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 C<v196.172>, and you skip

that character, you can never match a C<chr(200)> in a non-UTF8 string.

So don't do that!



=head2 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, C<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,

C<length>, C<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 C<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 C<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 C<frobnicate> function should be made aware of whether or

not it's dealing with UTF8 data, so that it can handle the string

appropriately.



=head2 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, C<bytes_to_utf8> will give you a UTF8-encoded B<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

C<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.



=head2 Is there anything else I need to know?



Not really. Just remember these things:



=over 3



=item *



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 C<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.



=item *



If a string is UTF8, B<always> use C<utf8_to_uv> to get at the value,

unless C<!(*s & 0x80)> in which case you can use C<*s>.



=item *



When writing to a UTF8 string, B<always> use C<uv_to_utf8>, unless

C<uv < 0x80> in which case you can use C<*s = uv>.



=item *



Mixing UTF8 and non-UTF8 strings is tricky. Use C<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 - C<HALF_UPGRADE> is one of those.



=back



=head1 AUTHORS



Until May 1997, this document was maintained by Jeff Okamoto

<okamoto@corp.hp.com>.  It is now maintained as part of Perl itself

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 (L<perlapi>) by Benjamin

Stuhl.



=head1 SEE ALSO



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