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-=head1 NAME
-
-perlguts - Introduction to the Perl API
-
-=head1 DESCRIPTION
-
-This document attempts to describe how to use the Perl API, as well as
-to provide 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.) They will usually be exactly 32 and 16 bits long, but on Crays
-they will both be 64 bits.
-
-=head2 Working with SVs
-
-An SV can be created and loaded with one command. There are five types of
-values that can be loaded: an integer value (IV), an unsigned integer
-value (UV), a double (NV), a string (PV), and another scalar (SV).
-
-The seven routines are:
-
- SV* newSViv(IV);
- SV* newSVuv(UV);
- SV* newSVnv(double);
- SV* newSVpv(const char*, STRLEN);
- SV* newSVpvn(const char*, STRLEN);
- SV* newSVpvf(const char*, ...);
- SV* newSVsv(SV*);
-
-C<STRLEN> is an integer type (Size_t, usually defined as size_t in
-F<config.h>) guaranteed to be large enough to represent the size of
-any string that perl can handle.
-
-In the unlikely case of a SV requiring more complex initialisation, you
-can create an empty SV with newSV(len). If C<len> is 0 an empty SV of
-type NULL is returned, else an SV of type PV is returned with len + 1 (for
-the NUL) bytes of storage allocated, accessible via SvPVX. In both cases
-the SV has the undef value.
-
- SV *sv = newSV(0); /* no storage allocated */
- SV *sv = newSV(10); /* 10 (+1) bytes of uninitialised storage
- * allocated */
-
-To change the value of an I<already-existing> SV, there are eight 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*, STRLEN)
- void sv_setpvf(SV*, const char*, ...);
- void sv_vsetpvfn(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, and not otherwise containing
-NULs.
-
-The arguments of C<sv_setpvf> are processed like C<sprintf>, and the
-formatted output becomes the value.
-
-C<sv_vsetpvfn> 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.
-
-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 space for the trailing NUL byte (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_vcatpvfn(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", 0);
-
-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. Make sure that
-you don't try to compare a random sv with C<&PL_sv_undef>. For example
-when interfacing Perl code, it'll work correctly for:
-
- foo(undef);
-
-But won't work when called as:
-
- $x = undef;
- foo($x);
-
-So to repeat always use SvOK() to check whether an sv is defined.
-
-Also you have to be careful when using C<&PL_sv_undef> as a value in
-AVs or HVs (see L<AVs, HVs and undefined values>).
-
-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 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 performed 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<AvMAX>.
-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.
-
-There are various ways in which the private and public flags may differ.
-For example, a tied SV may have a valid underlying value in the IV slot
-(so SvIOKp is true), but the data should be accessed via the FETCH
-routine rather than directly, so SvIOK is false. Another is when
-numeric conversion has occurred and precision has been lost: only the
-private flag is set on 'lossy' values. So when an NV is converted to an
-IV with loss, SvIOKp, SvNOKp and SvNOK will be set, while SvIOK wont be.
-
-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 it:
-
- 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 an 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.
-
-A few more:
-
- 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", 0);
-
-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 it:
-
- 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.
-
-The first of these two functions checks if a hash table entry exists, and the
-second 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 a 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 an 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", 0);
-
-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 AVs, HVs and undefined values
-
-Sometimes you have to store undefined values in AVs or HVs. Although
-this may be a rare case, it can be tricky. That's because you're
-used to using C<&PL_sv_undef> if you need an undefined SV.
-
-For example, intuition tells you that this XS code:
-
- AV *av = newAV();
- av_store( av, 0, &PL_sv_undef );
-
-is equivalent to this Perl code:
-
- my @av;
- $av[0] = undef;
-
-Unfortunately, this isn't true. AVs use C<&PL_sv_undef> as a marker
-for indicating that an array element has not yet been initialized.
-Thus, C<exists $av[0]> would be true for the above Perl code, but
-false for the array generated by the XS code.
-
-Other problems can occur when storing C<&PL_sv_undef> in HVs:
-
- hv_store( hv, "key", 3, &PL_sv_undef, 0 );
-
-This will indeed make the value C<undef>, but if you try to modify
-the value of C<key>, you'll get the following error:
-
- Modification of non-creatable hash value attempted
-
-In perl 5.8.0, C<&PL_sv_undef> was also used to mark placeholders
-in restricted hashes. This caused such hash entries not to appear
-when iterating over the hash or when checking for the keys
-with the C<hv_exists> function.
-
-You can run into similar problems when you store C<&PL_sv_yes> or
-C<&PL_sv_no> into AVs or HVs. Trying to modify such elements
-will give you the following error:
-
- Modification of a read-only value attempted
-
-To make a long story short, you can use the special variables
-C<&PL_sv_undef>, C<&PL_sv_yes> and C<&PL_sv_no> with AVs and
-HVs, but you have to make sure you know what you're doing.
-
-Generally, if you want to store an undefined value in an AV
-or HV, you should not use C<&PL_sv_undef>, but rather create a
-new undefined value using the C<newSV> function, for example:
-
- av_store( av, 42, newSV(0) );
- hv_store( hv, "foo", 3, newSV(0), 0 );
-
-=head2 References
-
-References are a special type of scalar that point to other data types
-(including other 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 (possibly a file handle)
- SVt_PVMG Blessed or Magical Scalar
-
-See the F<sv.h> header file for more details.
-
-=head2 Blessed References and Class Objects
-
-References are also used to support object-oriented programming. In perl's
-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 value. 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 */
-
-The following function upgrades rv to reference if not already one.
-Creates a 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);
-
-The following three functions copy integer, unsigned 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_uv(SV* rv, const char* classname, UV uv);
- SV* sv_setref_nv(SV* rv, const char* classname, NV iv);
-
-The following function 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, void* pv);
-
-The following function copies a 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, char* pv,
- STRLEN length);
-
-The following function tests whether the SV is blessed into the specified
-class. It does not check inheritance relationships.
-
- int sv_isa(SV* sv, const char* name);
-
-The following function tests whether the SV is a reference to a blessed object.
-
- int sv_isobject(SV* sv);
-
-The following function 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", GV_ADD);
- AV* get_av("package::varname", GV_ADD);
- HV* get_hv("package::varname", GV_ADD);
-
-Notice the use of GV_ADD 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<GV_ADD> argument to enable certain extra features. Those bits are:
-
-=over
-
-=item GV_ADDMULTI
-
-Marks the variable as multiply defined, thus preventing the:
-
- Name <varname> used only once: possible typo
-
-warning.
-
-=item GV_ADDWARN
-
-Issues the warning:
-
- Had to create <varname> unexpectedly
-
-if the variable did not exist before the function was called.
-
-=back
-
-If you do not specify a package name, the variable is created in the current
-package.
-
-=head2 Reference Counts and Mortality
-
-Perl uses a 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.
-
-"Mortal" SVs are mainly used for SVs that are placed on perl's stack.
-For example an SV which is created just to pass a number to a called sub
-is made mortal to have it cleaned up automatically when it's popped off
-the stack. Similarly, results returned by XSUBs (which are pushed on the
-stack) are often made mortal.
-
-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 (with no value), 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.
-Because C<sv_newmortal> gives the new SV no value, it must normally be given one
-via C<sv_setpv>, C<sv_setiv>, etc. :
-
- SV *tmp = sv_newmortal();
- sv_setiv(tmp, an_integer);
-
-As that is multiple C statements it is quite common so see this idiom instead:
-
- SV *tmp = sv_2mortal(newSViv(an_integer));
-
-
-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. Thinking of "Mortalization"
-as deferred C<SvREFCNT_dec> should help to minimize such problems.
-For example if you are passing an SV which you I<know> has a high enough REFCNT
-to survive its use on the stack you need not do any mortalization.
-If you are not sure then doing an C<SvREFCNT_inc> and C<sv_2mortal>, or
-making a C<sv_mortalcopy> is safer.
-
-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 B<stash> is a hash that contains all variables that are defined
-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 C<PL_defstash> that holds the items that exist
-in the C<main> package. To get at the items in other packages, append the
-string "::" to the package name. The items in the C<Foo> package are in
-the stash C<Foo::> in PL_defstash. The items in the C<Bar::Baz> package are
-in the stash C<Baz::> in C<Bar::>'s stash.
-
-To get the stash pointer for a particular package, use the function:
-
- HV* gv_stashpv(const char* name, I32 flags)
- HV* gv_stashsv(SV*, I32 flags)
-
-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<flags> flag will create a new package if it is set to GV_ADD.
-
-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", GV_ADD);
- 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;
- I32 mg_len;
- SV* mg_obj;
- char* mg_ptr;
- };
-
-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
-convert C<sv> to type C<SVt_PVMG>. Perl then continues by adding new magic
-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 then either a C<savepvn> copy of
-C<name> or C<name> itself is stored in the C<mg_ptr> field, depending on
-whether C<namlen> is greater than zero or equal to zero respectively. As a
-special case, if C<(name && namlen == HEf_SVKEY)> then C<name> is assumed
-to contain an C<SV*> and is stored as-is with its REFCNT incremented.
-
-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 L<Magic Virtual Tables> section below. The C<how> argument is also
-stored in the C<mg_type> field. The value of C<how> should be chosen
-from the set of macros C<PERL_MAGIC_foo> found in F<perl.h>. Note that before
-these macros were added, Perl internals used to directly use character
-literals, so you may occasionally come across old code or documentation
-referring to 'U' magic rather than C<PERL_MAGIC_uvar> for example.
-
-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 C<PERL_MAGIC_arylen>, or if it is a NULL pointer,
-then C<obj> is merely stored, without the reference count being incremented.
-
-See also C<sv_magicext> in L<perlapi> for a more flexible way to add magic
-to an SV.
-
-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:
-
- int 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.
-
-However, note that C<sv_unmagic> removes all magic of a certain C<type> from the
-C<SV>. If you want to remove only certain magic of a C<type> based on the magic
-virtual table, use C<sv_unmagicext> instead:
-
- int sv_unmagicext(SV *sv, int type, MGVTBL *vtbl);
-
-=head2 Magic Virtual Tables
-
-The C<mg_virtual> field in the C<MAGIC> structure is a pointer to an
-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 (or sometimes eight) 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);
-
- int (*svt_copy)(SV *sv, MAGIC* mg, SV *nsv,
- const char *name, I32 namlen);
- int (*svt_dup)(MAGIC *mg, CLONE_PARAMS *param);
- int (*svt_local)(SV *nsv, MAGIC *mg);
-
-
-This MGVTBL structure is set at compile-time in F<perl.h> and there are
-currently 32 types. 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 before 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.
-
- svt_copy copy tied variable magic to a tied element
- svt_dup duplicate a magic structure during thread cloning
- svt_local copy magic to local value during 'local'
-
-For instance, the MGVTBL structure called C<vtbl_sv> (which corresponds
-to an C<mg_type> of C<PERL_MAGIC_sv>) contains:
-
- { magic_get, magic_set, magic_len, 0, 0 }
-
-Thus, when an SV is determined to be magical and of type C<PERL_MAGIC_sv>,
-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 last three slots are a recent addition, and for source code
-compatibility they are only checked for if one of the three flags
-MGf_COPY, MGf_DUP or MGf_LOCAL is set in mg_flags. This means that most
-code can continue declaring a vtable as a 5-element value. These three are
-currently used exclusively by the threading code, and are highly subject
-to change.
-
-The current kinds of Magic Virtual Tables are:
-
-=for comment
-This table is generated by regen/mg_vtable.pl. Any changes made here
-will be lost.
-
-=for mg_vtable.pl begin
-
- mg_type
- (old-style char and macro) MGVTBL Type of magic
- -------------------------- ------ -------------
- \0 PERL_MAGIC_sv vtbl_sv Special scalar variable
- # PERL_MAGIC_arylen vtbl_arylen Array length ($#ary)
- % PERL_MAGIC_rhash (none) extra data for restricted
- hashes
- . PERL_MAGIC_pos vtbl_pos pos() lvalue
- : PERL_MAGIC_symtab (none) extra data for symbol
- tables
- < PERL_MAGIC_backref vtbl_backref for weak ref data
- @ PERL_MAGIC_arylen_p (none) to move arylen out of
- XPVAV
- A PERL_MAGIC_overload vtbl_amagic %OVERLOAD hash
- a PERL_MAGIC_overload_elem vtbl_amagicelem %OVERLOAD hash element
- B PERL_MAGIC_bm vtbl_regexp Boyer-Moore
- (fast string search)
- c PERL_MAGIC_overload_table vtbl_ovrld Holds overload table
- (AMT) on stash
- D PERL_MAGIC_regdata vtbl_regdata Regex match position data
- (@+ and @- vars)
- d PERL_MAGIC_regdatum vtbl_regdatum Regex match position data
- element
- E PERL_MAGIC_env vtbl_env %ENV hash
- e PERL_MAGIC_envelem vtbl_envelem %ENV hash element
- f PERL_MAGIC_fm vtbl_regdata Formline
- ('compiled' format)
- G PERL_MAGIC_study vtbl_regexp study()ed string
- g PERL_MAGIC_regex_global vtbl_mglob m//g target
- H PERL_MAGIC_hints vtbl_hints %^H hash
- h PERL_MAGIC_hintselem vtbl_hintselem %^H hash element
- I PERL_MAGIC_isa vtbl_isa @ISA array
- i PERL_MAGIC_isaelem vtbl_isaelem @ISA array element
- k PERL_MAGIC_nkeys vtbl_nkeys scalar(keys()) lvalue
- L PERL_MAGIC_dbfile (none) Debugger %_<filename
- l PERL_MAGIC_dbline vtbl_dbline Debugger %_<filename
- element
- N PERL_MAGIC_shared (none) Shared between threads
- n PERL_MAGIC_shared_scalar (none) Shared between threads
- o PERL_MAGIC_collxfrm vtbl_collxfrm Locale transformation
- P PERL_MAGIC_tied vtbl_pack Tied array or hash
- p PERL_MAGIC_tiedelem vtbl_packelem Tied array or hash element
- q PERL_MAGIC_tiedscalar vtbl_packelem Tied scalar or handle
- r PERL_MAGIC_qr vtbl_regexp precompiled qr// regex
- S PERL_MAGIC_sig (none) %SIG hash
- s PERL_MAGIC_sigelem vtbl_sigelem %SIG hash element
- t PERL_MAGIC_taint vtbl_taint Taintedness
- U PERL_MAGIC_uvar vtbl_uvar Available for use by
- extensions
- u PERL_MAGIC_uvar_elem (none) Reserved for use by
- extensions
- V PERL_MAGIC_vstring vtbl_vstring SV was vstring literal
- v PERL_MAGIC_vec vtbl_vec vec() lvalue
- w PERL_MAGIC_utf8 vtbl_utf8 Cached UTF-8 information
- x PERL_MAGIC_substr vtbl_substr substr() lvalue
- y PERL_MAGIC_defelem vtbl_defelem Shadow "foreach" iterator
- variable / smart parameter
- vivification
- ] PERL_MAGIC_checkcall (none) inlining/mutation of call
- to this CV
- ~ PERL_MAGIC_ext (none) Available for use by
- extensions
-
-=for mg_vtable.pl end
-
-When an uppercase and lowercase letter both exist in the table, then the
-uppercase letter is typically 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. Some internals code makes use of this case
-relationship. However, 'v' and 'V' (vec and v-string) are in no way related.
-
-The C<PERL_MAGIC_ext> and C<PERL_MAGIC_uvar> magic types are defined
-specifically for use by extensions and will not be used by perl itself.
-Extensions can use C<PERL_MAGIC_ext> 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, C<PERL_MAGIC_uvar> 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)(pTHX_ IV, SV*);
- I32 (*uf_set)(pTHX_ 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 C<PERL_MAGIC_uvar>
-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, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf));
-
-Attaching C<PERL_MAGIC_uvar> to arrays is permissible but has no effect.
-
-For hashes there is a specialized hook that gives control over hash
-keys (but not values). This hook calls C<PERL_MAGIC_uvar> 'get' magic
-if the "set" function in the C<ufuncs> structure is NULL. The hook
-is activated whenever the hash is accessed with a key specified as
-an C<SV> through the functions C<hv_store_ent>, C<hv_fetch_ent>,
-C<hv_delete_ent>, and C<hv_exists_ent>. Accessing the key as a string
-through the functions without the C<..._ent> suffix circumvents the
-hook. See L<Hash::Util::FieldHash/GUTS> for a detailed description.
-
-Note that because multiple extensions may be using C<PERL_MAGIC_ext>
-or C<PERL_MAGIC_uvar> 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 C<PERL_MAGIC_ext> magic, it is usually a good idea to define an
-C<MGVTBL>, even if all its fields will be C<0>, so that individual
-C<MAGIC> pointers can be identified as a particular kind of magic
-using their magic virtual table. C<mg_findext> provides an easy way
-to do that:
-
- STATIC MGVTBL my_vtbl = { 0, 0, 0, 0, 0, 0, 0, 0 };
-
- MAGIC *mg;
- if ((mg = mg_findext(sv, PERL_MAGIC_ext, &my_vtbl))) {
- /* this is really ours, not another module's PERL_MAGIC_ext */
- my_priv_data_t *priv = (my_priv_data_t *)mg->mg_ptr;
- ...
- }
-
-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 *sv, int type); /* Finds the magic pointer of that
- * type */
-
-This routine returns a pointer to a C<MAGIC> structure stored in the SV.
-If the SV does not have that magical feature, C<NULL> is returned. If the
-SV has multiple instances of that magical feature, the first one will be
-returned. C<mg_findext> can be used to find a C<MAGIC> structure of an SV
-based on both its magic type and its magic virtual table:
-
- MAGIC *mg_findext(SV *sv, int type, MGVTBL *vtbl);
-
-Also, if the SV passed to C<mg_find> or C<mg_findext> 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 C<PERL_MAGIC_tied>
-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", GV_ADD);
- sv_bless(tie, stash);
- hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
- 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. It doesn't handle magic. Use C<save_scalar> if
-magic is affected.
-
-=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 an 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 C<PUSHs>
-macro. The pushed values will often need to be "mortal" (See
-L</Reference Counts and Mortality>):
-
- PUSHs(sv_2mortal(newSViv(an_integer)))
- PUSHs(sv_2mortal(newSVuv(an_unsigned_integer)))
- PUSHs(sv_2mortal(newSVnv(a_double)))
- PUSHs(sv_2mortal(newSVpv("Some String",0)))
- /* Although the last example is better written as the more
- * efficient: */
- PUSHs(newSVpvs_flags("Some String", SVs_TEMP))
-
-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 macro:
-
- XPUSHs(SV*)
-
-This macro automatically adjusts the stack for you, if needed. Thus, you
-do not need to call C<EXTEND> to extend the stack.
-
-Despite their suggestions in earlier versions of this document the macros
-C<(X)PUSH[iunp]> are I<not> suited to XSUBs which return multiple results.
-For that, either stick to the C<(X)PUSHs> macros shown above, or use the new
-C<m(X)PUSH[iunp]> macros instead; see L</Putting a C value on Perl stack>.
-
-For more information, consult L<perlxs> and L<perlxstut>.
-
-=head2 Autoloading with XSUBs
-
-If an AUTOLOAD routine is an XSUB, as with Perl subroutines, Perl puts the
-fully-qualified name of the autoloaded subroutine in the $AUTOLOAD variable
-of the XSUB's package.
-
-But it also puts the same information in certain fields of the XSUB itself:
-
- HV *stash = CvSTASH(cv);
- const char *subname = SvPVX(cv);
- STRLEN name_length = SvCUR(cv); /* in bytes */
- U32 is_utf8 = SvUTF8(cv);
-
-C<SvPVX(cv)> contains just the sub name itself, not including the package.
-For an AUTOLOAD routine in UNIVERSAL or one of its superclasses,
-C<CvSTASH(cv)> returns NULL during a method call on a nonexistent package.
-
-B<Note>: Setting $AUTOLOAD stopped working in 5.6.1, which did not support
-XS AUTOLOAD subs at all. Perl 5.8.0 introduced the use of fields in the
-XSUB itself. Perl 5.16.0 restored the setting of $AUTOLOAD. If you need
-to support 5.8-5.14, use the XSUB's fields.
-
-=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
-
-=head3 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.
-
-The following three macros are used to initially allocate memory :
-
- Newx(pointer, number, type);
- Newxc(pointer, number, type, cast);
- Newxz(pointer, number, type);
-
-The first argument C<pointer> should be the name of a variable that will
-point to the newly allocated memory.
-
-The second and third 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<Newxc>, C<cast>,
-should be used if the C<pointer> argument is different from the C<type>
-argument.
-
-Unlike the C<Newx> and C<Newxc> macros, the C<Newxz> macro calls C<memzero>
-to zero out all the newly allocated memory.
-
-=head3 Reallocation
-
- 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.
-
-=head3 Moving
-
- 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 have 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[iunp]>.
-
-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 then you should either use
-the C<(X)PUSHs> macros, or else use the new C<m(X)PUSH[iunp]> macros,
-none of which make use of C<TARG>. The C<(X)PUSHs> macros simply push an
-SV* on the stack, which, as noted under L</XSUBs and the Argument Stack>,
-will often need to be "mortal". The new C<m(X)PUSH[iunp]> macros make
-this a little easier to achieve by creating a new mortal for you (via
-C<(X)PUSHmortal>), pushing that onto the stack (extending it if necessary
-in the case of the C<mXPUSH[iunp]> macros), and then setting its value.
-Thus, instead of writing this to "fix" the example above:
-
- XPUSHs(sv_2mortal(newSViv(10)))
- XPUSHs(sv_2mortal(newSViv(20)))
-
-you can simply write:
-
- mXPUSHi(10)
- mXPUSHi(20)
-
-On a related note, if you do use C<(X)PUSH[iunp]>, 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>. See also C<dTARGET>
-and C<dXSTARG>.
-
-=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 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 + $c> it is different:
-some nodes I<optimized away>. As a corollary, though the actual tree
-contains more nodes than our simplified example, the execution order
-is the same as in our example.
-
-=head2 Examining the tree
-
-If you have your perl compiled for debugging (usually done with
-C<-DDEBUGGING> on the C<Configure> command line), you may examine the
-compiled tree by specifying C<-Dx> on the Perl command line. The
-output takes several lines per node, and for C<$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.
-
-Another way to examine the tree is to use a compiler back-end module, such
-as L<B::Concise>.
-
-=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). Optimizations performed
-at this stage are subject to the same restrictions as in the pass 2.
-
-Peephole optimizations are done by calling the function pointed to
-by the global variable C<PL_peepp>. By default, C<PL_peepp> just
-calls the function pointed to by the global variable C<PL_rpeepp>.
-By default, that performs some basic op fixups and optimisations along
-the execution-order op chain, and recursively calls C<PL_rpeepp> for
-each side chain of ops (resulting from conditionals). Extensions may
-provide additional optimisations or fixups, hooking into either the
-per-subroutine or recursive stage, like this:
-
- static peep_t prev_peepp;
- static void my_peep(pTHX_ OP *o)
- {
- /* custom per-subroutine optimisation goes here */
- prev_peepp(o);
- /* custom per-subroutine optimisation may also go here */
- }
- BOOT:
- prev_peepp = PL_peepp;
- PL_peepp = my_peep;
-
- static peep_t prev_rpeepp;
- static void my_rpeep(pTHX_ OP *o)
- {
- OP *orig_o = o;
- for(; o; o = o->op_next) {
- /* custom per-op optimisation goes here */
- }
- prev_rpeepp(orig_o);
- }
- BOOT:
- prev_rpeepp = PL_rpeepp;
- PL_rpeepp = my_rpeep;
-
-=head2 Pluggable runops
-
-The compile tree is executed in a runops function. There are two runops
-functions, in F<run.c> and in F<dump.c>. C<Perl_runops_debug> is used
-with DEBUGGING and C<Perl_runops_standard> is used otherwise. For fine
-control over the execution of the compile tree it is possible to provide
-your own runops function.
-
-It's probably best to copy one of the existing runops functions and
-change it to suit your needs. Then, in the BOOT section of your XS
-file, add the line:
-
- PL_runops = my_runops;
-
-This function should be as efficient as possible to keep your programs
-running as fast as possible.
-
-=head2 Compile-time scope hooks
-
-As of perl 5.14 it is possible to hook into the compile-time lexical
-scope mechanism using C<Perl_blockhook_register>. This is used like
-this:
-
- STATIC void my_start_hook(pTHX_ int full);
- STATIC BHK my_hooks;
-
- BOOT:
- BhkENTRY_set(&my_hooks, bhk_start, my_start_hook);
- Perl_blockhook_register(aTHX_ &my_hooks);
-
-This will arrange to have C<my_start_hook> called at the start of
-compiling every lexical scope. The available hooks are:
-
-=over 4
-
-=item C<void bhk_start(pTHX_ int full)>
-
-This is called just after starting a new lexical scope. Note that Perl
-code like
-
- if ($x) { ... }
-
-creates two scopes: the first starts at the C<(> and has C<full == 1>,
-the second starts at the C<{> and has C<full == 0>. Both end at the
-C<}>, so calls to C<start> and C<pre/post_end> will match. Anything
-pushed onto the save stack by this hook will be popped just before the
-scope ends (between the C<pre_> and C<post_end> hooks, in fact).
-
-=item C<void bhk_pre_end(pTHX_ OP **o)>
-
-This is called at the end of a lexical scope, just before unwinding the
-stack. I<o> is the root of the optree representing the scope; it is a
-double pointer so you can replace the OP if you need to.
-
-=item C<void bhk_post_end(pTHX_ OP **o)>
-
-This is called at the end of a lexical scope, just after unwinding the
-stack. I<o> is as above. Note that it is possible for calls to C<pre_>
-and C<post_end> to nest, if there is something on the save stack that
-calls string eval.
-
-=item C<void bhk_eval(pTHX_ OP *const o)>
-
-This is called just before starting to compile an C<eval STRING>, C<do
-FILE>, C<require> or C<use>, after the eval has been set up. I<o> is the
-OP that requested the eval, and will normally be an C<OP_ENTEREVAL>,
-C<OP_DOFILE> or C<OP_REQUIRE>.
-
-=back
-
-Once you have your hook functions, you need a C<BHK> structure to put
-them in. It's best to allocate it statically, since there is no way to
-free it once it's registered. The function pointers should be inserted
-into this structure using the C<BhkENTRY_set> macro, which will also set
-flags indicating which entries are valid. If you do need to allocate
-your C<BHK> dynamically for some reason, be sure to zero it before you
-start.
-
-Once registered, there is no mechanism to switch these hooks off, so if
-that is necessary you will need to do this yourself. An entry in C<%^H>
-is probably the best way, so the effect is lexically scoped; however it
-is also possible to use the C<BhkDISABLE> and C<BhkENABLE> macros to
-temporarily switch entries on and off. You should also be aware that
-generally speaking at least one scope will have opened before your
-extension is loaded, so you will see some C<pre/post_end> pairs that
-didn't have a matching C<start>.
-
-=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 similar 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 structure,
-or inside a thread-specific structure. These structures contain all
-the context, the state of that interpreter.
-
-One macro controls the major Perl build flavor: MULTIPLICITY. The
-MULTIPLICITY build has a C structure that packages all the interpreter
-state. With multiplicity-enabled perls, PERL_IMPLICIT_CONTEXT is also
-normally defined, and enables the support for passing in a "hidden" first
-argument that represents all three data structures. MULTIPLICITY makes
-multi-threaded perls possible (with the ithreads threading model, related
-to the macro USE_ITHREADS.)
-
-Two other "encapsulation" macros are the PERL_GLOBAL_STRUCT and
-PERL_GLOBAL_STRUCT_PRIVATE (the latter turns on the former, and the
-former turns on MULTIPLICITY.) The PERL_GLOBAL_STRUCT causes all the
-internal variables of Perl to be wrapped inside a single global struct,
-struct perl_vars, accessible as (globals) &PL_Vars or PL_VarsPtr or
-the function Perl_GetVars(). The PERL_GLOBAL_STRUCT_PRIVATE goes
-one step further, there is still a single struct (allocated in main()
-either from heap or from stack) but there are no global data symbols
-pointing to it. In either case the global struct should be initialised
-as the very first thing in main() using Perl_init_global_struct() and
-correspondingly tear it down after perl_free() using Perl_free_global_struct(),
-please see F<miniperlmain.c> for usage details. You may also need
-to use C<dVAR> in your coding to "declare the global variables"
-when you are using them. dTHX does this for you automatically.
-
-To see whether you have non-const data you can use a BSD-compatible C<nm>:
-
- nm libperl.a | grep -v ' [TURtr] '
-
-If this displays any C<D> or C<d> symbols, you have non-const data.
-
-For backward compatibility reasons defining just PERL_GLOBAL_STRUCT
-doesn't actually hide all symbols inside a big global struct: some
-PerlIO_xxx vtables are left visible. The PERL_GLOBAL_STRUCT_PRIVATE
-then hides everything (see how the PERLIO_FUNCS_DECL is used).
-
-All this obviously requires a way for the Perl internal functions to be
-either subroutines taking some kind of structure as the first
-argument, or subroutines taking nothing as the first argument. To
-enable these two 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 may be #define'd to nothing in some
-configurations in the future.
-
-A public function (i.e. part of the internal API, but not necessarily
-sanctioned for use in extensions) begins like this:
-
- void
- Perl_sv_setiv(pTHX_ SV* dsv, IV num)
-
-C<pTHX_> is one of a number of macros (in F<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_setiv>. It expands into
-something like this:
-
- #ifdef PERL_IMPLICIT_CONTEXT
- #define sv_setiv(a,b) Perl_sv_setiv(aTHX_ a, b)
- /* can't do this for vararg functions, see below */
- #else
- #define sv_setiv Perl_sv_setiv
- #endif
-
-This works well, and means that XS authors can gleefully write:
-
- sv_setiv(foo, bar);
-
-and still have it work under all the modes Perl could have been
-compiled with.
-
-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]THXx 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 F<XSUB.h> is #included, it redefines the aTHX
-and aTHX_ macros to call a function that will return the context.
-Thus, something like:
-
- sv_setiv(sv, num);
-
-in your extension will translate to this when PERL_IMPLICIT_CONTEXT is
-in effect:
-
- Perl_sv_setiv(Perl_get_context(), sv, num);
-
-or to this otherwise:
-
- Perl_sv_setiv(sv, num);
-
-You don't have to do anything 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 void my_private_function(int arg1, int arg2);
-
- STATIC void
- 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 void my_private_function(pTHX_ int arg1, int arg2);
-
- STATIC void
- 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.
-
-If one is compiling Perl with the C<-DPERL_GLOBAL_STRUCT> the C<dVAR>
-definition is needed if the Perl global variables (see F<perlvars.h>
-or F<globvar.sym>) are accessed in the function and C<dTHX> is not
-used (the C<dTHX> includes the C<dVAR> if necessary). One notices
-the need for C<dVAR> only with the said compile-time define, because
-otherwise the Perl global variables are visible as-is.
-
-=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 USE_ITHREADS on
-Windows.
-
-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 F<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 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 (C<PERL_CORE> defined), 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>. Note that extension code should I<not> set
-C<PERL_CORE>; this exposes the full perl internals, and is likely to cause
-breakage of the XS in each new perl release.
-
-The file F<embed.h> is generated automatically from
-F<embed.pl> and F<embed.fnc>. 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 in F<embed.fnc> 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. All such functions should also
-have 'd', very few do not.
-
-=item p
-
-This function has a C<Perl_> prefix; i.e. 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. Some functions have 'd' but not 'A'; docs are good.
-
-=back
-
-Other available flags are:
-
-=over 3
-
-=item s
-
-This is a static function and is defined as C<STATIC S_whatever>, and
-usually called within the sources as C<whatever(...)>.
-
-=item n
-
-This does not need an interpreter context, so the definition has no
-C<pTHX>, and it follows that callers don't use C<aTHX>. (See
-L</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 x
-
-This function isn't exported out of the Perl core.
-
-=item m
-
-This is implemented as a macro.
-
-=item X
-
-This function is explicitly exported.
-
-=item E
-
-This function is visible to extensions included in the Perl core.
-
-=item b
-
-Binary backward compatibility; this function is a macro but also has
-a C<Perl_> implementation (which is exported).
-
-=item others
-
-See the comments at the top of C<embed.fnc> for others.
-
-=back
-
-If you edit F<embed.pl> or F<embed.fnc>, 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 Exception Handling
-
-There are a couple of macros to do very basic exception handling in XS
-modules. You have to define C<NO_XSLOCKS> before including F<XSUB.h> to
-be able to use these macros:
-
- #define NO_XSLOCKS
- #include "XSUB.h"
-
-You can use these macros if you call code that may croak, but you need
-to do some cleanup before giving control back to Perl. For example:
-
- dXCPT; /* set up necessary variables */
-
- XCPT_TRY_START {
- code_that_may_croak();
- } XCPT_TRY_END
-
- XCPT_CATCH
- {
- /* do cleanup here */
- XCPT_RETHROW;
- }
-
-Note that you always have to rethrow an exception that has been
-caught. Using these macros, it is not possible to just catch the
-exception and ignore it. If you have to ignore the exception, you
-have to use the C<call_*> function.
-
-The advantage of using the above macros is that you don't have
-to setup an extra function for C<call_*>, and that using these
-macros is faster than using C<call_*>.
-
-=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.
-
-=head2 Backwards compatibility
-
-The Perl API changes over time. New functions are added or the interfaces
-of existing functions are changed. The C<Devel::PPPort> module tries to
-provide compatibility code for some of these changes, so XS writers don't
-have to code it themselves when supporting multiple versions of Perl.
-
-C<Devel::PPPort> generates a C header file F<ppport.h> that can also
-be run as a Perl script. To generate F<ppport.h>, run:
-
- perl -MDevel::PPPort -eDevel::PPPort::WriteFile
-
-Besides checking existing XS code, the script can also be used to retrieve
-compatibility information for various API calls using the C<--api-info>
-command line switch. For example:
-
- % perl ppport.h --api-info=sv_magicext
-
-For details, see C<perldoc ppport.h>.
-
-=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 UTF-8. UTF-8 uses
-a variable number of bytes to represent a character. You can learn more
-about Unicode and Perl's Unicode model in L<perlunicode>.
-
-=head2 How can I recognise a UTF-8 string?
-
-You can't. This is because UTF-8 data is stored in bytes just like
-non-UTF-8 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.
-
-In general, you either have to know what you're dealing with, or you
-have to guess. The API function C<is_utf8_string> can help; it'll tell
-you if a string contains only valid UTF-8 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 UTF-8.
-
-=head2 How does UTF-8 represent Unicode characters?
-
-As mentioned above, UTF-8 uses a variable number of bytes to store a
-character. Characters with values 0...127 are stored in one byte, just
-like good ol' ASCII. Character 128 is stored as C<v194.128>; 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 UTF-8 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 UTF-8 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 UTF-8 character will have the high bit set,
-so you can test if you need to do something special with this
-character like this (the UTF8_IS_INVARIANT() is a macro that tests
-whether the byte can be encoded as a single byte even in UTF-8):
-
- U8 *utf;
- U8 *utf_end; /* 1 beyond buffer pointed to by utf */
- UV uv; /* Note: a UV, not a U8, not a char */
- STRLEN len; /* length of character in bytes */
-
- if (!UTF8_IS_INVARIANT(*utf))
- /* Must treat this as UTF-8 */
- uv = utf8_to_uvchr_buf(utf, utf_end, &len);
- else
- /* OK to treat this character as a byte */
- uv = *utf;
-
-You can also see in that example that we use C<utf8_to_uvchr_buf> to get the
-value of the character; the inverse function C<uvchr_to_utf8> is available
-for putting a UV into UTF-8:
-
- if (!UTF8_IS_INVARIANT(uv))
- /* Must treat this as UTF8 */
- utf8 = uvchr_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 UTF-8 and non-UTF-8
-characters. You may not skip over UTF-8 characters in this case. If you
-do this, you'll lose the ability to match hi-bit non-UTF-8 characters;
-for instance, if your UTF-8 string contains C<v196.172>, and you skip
-that character, you can never match a C<chr(200)> in a non-UTF-8 string.
-So don't do that!
-
-=head2 How does Perl store UTF-8 strings?
-
-Currently, Perl deals with Unicode strings and non-Unicode strings
-slightly differently. A flag in the SV, C<SVf_UTF8>, indicates that the
-string is internally encoded as UTF-8. Without it, the byte value is the
-codepoint number and vice versa (in other words, the string is encoded
-as iso-8859-1, but C<use feature 'unicode_strings'> is needed to get iso-8859-1
-semantics). 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 as UTF-8, and contains a byte sequence that could be UTF-8 -
-especially when combining non-UTF-8 and UTF-8 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 UTF-8 data, so that it can handle the string
-appropriately.
-
-Since just passing an SV to an XS function and copying the data of
-the SV is not enough to copy the UTF8 flags, even less right is just
-passing a C<char *> to an XS function.
-
-=head2 How do I convert a string to UTF-8?
-
-If you're mixing UTF-8 and non-UTF-8 strings, it is necessary to upgrade
-one of the strings to UTF-8. 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 in deficient code.
-
-Instead, C<bytes_to_utf8> will give you a UTF-8-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 UTF-8 or not. You can tell if an SV
-is UTF-8 by looking at its C<SvUTF8> flag. Don't forget to set the flag if
-something should be UTF-8. 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 UTF-8, B<always> use C<utf8_to_uvchr_buf> to get at the value,
-unless C<UTF8_IS_INVARIANT(*s)> in which case you can use C<*s>.
-
-=item *
-
-When writing a character C<uv> to a UTF-8 string, B<always> use
-C<uvchr_to_utf8>, unless C<UTF8_IS_INVARIANT(uv))> in which case
-you can use C<*s = uv>.
-
-=item *
-
-Mixing UTF-8 and non-UTF-8 strings is tricky. Use C<bytes_to_utf8> to get
-a new string which is UTF-8 encoded, and then combine them.
-
-=back
-
-=head1 Custom Operators
-
-Custom operator support is a new experimental feature that allows you to
-define your own ops. This is primarily to allow the building of
-interpreters for other languages in the Perl core, but it also allows
-optimizations through the creation of "macro-ops" (ops which perform the
-functions of multiple ops which are usually executed together, such as
-C<gvsv, gvsv, add>.)
-
-This feature is implemented as a new op type, C<OP_CUSTOM>. The Perl
-core does not "know" anything special about this op type, and so it will
-not be involved in any optimizations. This also means that you can
-define your custom ops to be any op structure - unary, binary, list and
-so on - you like.
-
-It's important to know what custom operators won't do for you. They
-won't let you add new syntax to Perl, directly. They won't even let you
-add new keywords, directly. In fact, they won't change the way Perl
-compiles a program at all. You have to do those changes yourself, after
-Perl has compiled the program. You do this either by manipulating the op
-tree using a C<CHECK> block and the C<B::Generate> module, or by adding
-a custom peephole optimizer with the C<optimize> module.
-
-When you do this, you replace ordinary Perl ops with custom ops by
-creating ops with the type C<OP_CUSTOM> and the C<pp_addr> of your own
-PP function. This should be defined in XS code, and should look like
-the PP ops in C<pp_*.c>. You are responsible for ensuring that your op
-takes the appropriate number of values from the stack, and you are
-responsible for adding stack marks if necessary.
-
-You should also "register" your op with the Perl interpreter so that it
-can produce sensible error and warning messages. Since it is possible to
-have multiple custom ops within the one "logical" op type C<OP_CUSTOM>,
-Perl uses the value of C<< o->op_ppaddr >> to determine which custom op
-it is dealing with. You should create an C<XOP> structure for each
-ppaddr you use, set the properties of the custom op with
-C<XopENTRY_set>, and register the structure against the ppaddr using
-C<Perl_custom_op_register>. A trivial example might look like:
-
- static XOP my_xop;
- static OP *my_pp(pTHX);
-
- BOOT:
- XopENTRY_set(&my_xop, xop_name, "myxop");
- XopENTRY_set(&my_xop, xop_desc, "Useless custom op");
- Perl_custom_op_register(aTHX_ my_pp, &my_xop);
-
-The available fields in the structure are:
-
-=over 4
-
-=item xop_name
-
-A short name for your op. This will be included in some error messages,
-and will also be returned as C<< $op->name >> by the L<B|B> module, so
-it will appear in the output of module like L<B::Concise|B::Concise>.
-
-=item xop_desc
-
-A short description of the function of the op.
-
-=item xop_class
-
-Which of the various C<*OP> structures this op uses. This should be one of
-the C<OA_*> constants from F<op.h>, namely
-
-=over 4
-
-=item OA_BASEOP
-
-=item OA_UNOP
-
-=item OA_BINOP
-
-=item OA_LOGOP
-
-=item OA_LISTOP
-
-=item OA_PMOP
-
-=item OA_SVOP
-
-=item OA_PADOP
-
-=item OA_PVOP_OR_SVOP
-
-This should be interpreted as 'C<PVOP>' only. The C<_OR_SVOP> is because
-the only core C<PVOP>, C<OP_TRANS>, can sometimes be a C<SVOP> instead.
-
-=item OA_LOOP
-
-=item OA_COP
-
-=back
-
-The other C<OA_*> constants should not be used.
-
-=item xop_peep
-
-This member is of type C<Perl_cpeep_t>, which expands to C<void
-(*Perl_cpeep_t)(aTHX_ OP *o, OP *oldop)>. If it is set, this function
-will be called from C<Perl_rpeep> when ops of this type are encountered
-by the peephole optimizer. I<o> is the OP that needs optimizing;
-I<oldop> is the previous OP optimized, whose C<op_next> points to I<o>.
-
-=back
-
-C<B::Generate> directly supports the creation of custom ops by name.
-
-=head1 AUTHORS
-
-Until May 1997, this document was maintained by Jeff Okamoto
-E<lt>okamoto@corp.hp.comE<gt>. It is now maintained as part of Perl
-itself by the Perl 5 Porters E<lt>perl5-porters@perl.orgE<gt>.
-
-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.
-
-=head1 SEE ALSO
-
-L<perlapi>, L<perlintern>, L<perlxs>, L<perlembed>