X-Git-Url: https://perl5.git.perl.org/perl5.git/blobdiff_plain/038fcae385eee134ba22a23fbbf09eafafbe7927..e69061dc352aead76c31e0d932d915ea7ce26512:/pod/perlguts.pod diff --git a/pod/perlguts.pod b/pod/perlguts.pod index 78a1dfc..d8f0527 100644 --- a/pod/perlguts.pod +++ b/pod/perlguts.pod @@ -126,11 +126,11 @@ Also remember that C doesn't allow you to safely say C. 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); + 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: @@ -191,7 +191,7 @@ have "magic". See L later in this document. If you know the name of a scalar variable, you can get a pointer to its SV by using the following: - SV* get_sv("package::varname", FALSE); + SV* get_sv("package::varname", 0); This returns NULL if the variable does not exist. @@ -201,9 +201,22 @@ you can call: SvOK(SV*) The scalar C value is stored in an SV instance called C. -Its address can be used whenever an C is needed. -However, you have to be careful when using C<&PL_sv_undef> as a value in AVs -or HVs (see L). + +Its address can be used whenever an C 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). There are also the two values C and C, which contain boolean TRUE and FALSE values, respectively. Like C, their @@ -265,7 +278,7 @@ efficient shifting and splicing off the beginning of the array; while C points to the first element in the array that is visible from Perl, C points to the real start of the C array. These are usually the same, but a C operation can be carried out by -increasing C by one and decreasing C and C. +increasing C by one and decreasing C and C. Again, the location of the real start of the C array only comes into play when freeing the array. See C in F. @@ -291,7 +304,7 @@ The 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 occured and precision has been lost: only the +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. @@ -354,7 +367,7 @@ then nothing is done. If you know the name of an array variable, you can get a pointer to its AV by using the following: - AV* get_av("package::varname", FALSE); + AV* get_av("package::varname", 0); This returns NULL if the variable does not exist. @@ -429,7 +442,7 @@ specified below. If you know the name of a hash variable, you can get a pointer to its HV by using the following: - HV* get_hv("package::varname", FALSE); + HV* get_hv("package::varname", 0); This returns NULL if the variable does not exist. @@ -526,14 +539,14 @@ 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 function. -You can run into similar problems when you store C<&PL_sv_true> or -C<&PL_sv_false> into AVs or HVs. Trying to modify such elements +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_true> and C<&PL_sv_false> with AVs and +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 @@ -587,7 +600,7 @@ The most useful types that will be returned are: SVt_PVGV Glob (possible a file handle) SVt_PVMG Blessed or Magical Scalar - See the sv.h header file for more details. +See the F header file for more details. =head2 Blessed References and Class Objects @@ -606,41 +619,45 @@ L for information on converting class names into stashes. /* Still under construction */ -Upgrades rv to reference if not already one. Creates new SV for rv to -point to. If C is non-null, the SV is blessed into the specified -class. SV is returned. +The following function upgrades rv to reference if not already one. +Creates a new SV for rv to point to. If C is non-null, the SV +is blessed into the specified class. SV is returned. SV* newSVrv(SV* rv, const char* classname); -Copies integer, unsigned integer or double into an SV whose reference is C. SV is blessed -if C is non-null. +The following three functions copy integer, unsigned integer or double +into an SV whose reference is C. SV is blessed if C 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); -Copies the pointer value (I) into an SV whose -reference is rv. SV is blessed if C is non-null. +The following function copies the pointer value (I) into an SV whose reference is rv. SV is blessed if C +is non-null. - SV* sv_setref_pv(SV* rv, const char* classname, PV iv); + SV* sv_setref_pv(SV* rv, const char* classname, void* pv); -Copies string into an SV whose reference is C. Set length to 0 to let -Perl calculate the string length. SV is blessed if C is non-null. +The following function copies string into an SV whose reference is C. +Set length to 0 to let Perl calculate the string length. SV is blessed if +C is non-null. - SV* sv_setref_pvn(SV* rv, const char* classname, PV iv, STRLEN length); + SV* sv_setref_pvn(SV* rv, const char* classname, char* pv, STRLEN length); -Tests whether the SV is blessed into the specified class. It does not -check inheritance relationships. +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); -Tests whether the SV is a reference to a blessed object. +The following function tests whether the SV is a reference to a blessed object. int sv_isobject(SV* sv); -Tests whether the SV is derived from the specified class. SV can be either -a reference to a blessed object or a string containing a class name. This -is the function implementing the C functionality. +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 functionality. bool sv_derived_from(SV* sv, const char* name); @@ -654,15 +671,15 @@ to write: To create a new Perl variable with an undef value which can be accessed from your Perl script, use the following routines, depending on the variable type. - SV* get_sv("package::varname", TRUE); - AV* get_av("package::varname", TRUE); - HV* get_hv("package::varname", TRUE); + 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 TRUE as the second parameter. The new variable can now +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 argument to enable certain extra features. Those bits are: +C argument to enable certain extra features. Those bits are: =over @@ -773,7 +790,7 @@ to survive its use on the stack you need not do any mortalization. If you are not sure then doing an C and C, or making a C is safer. -The mortal routines are not just for SVs -- AVs and HVs can be +The mortal routines are not just for SVs; AVs and HVs can be made mortal by passing their address (type-casted to C) to the C or C routines. @@ -801,12 +818,12 @@ in the stash C in C's stash. To get the stash pointer for a particular package, use the function: - HV* gv_stashpv(const char* name, I32 create) - HV* gv_stashsv(SV*, I32 create) + 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. The C flag will create a new package if it is set. +C. The C flag will create a new package if it is set to GV_ADD. The name that C wants is the name of the package whose symbol table you want. The default package is called C
. If you have multiply nested @@ -865,7 +882,7 @@ following code: extern int dberror; extern char *dberror_list; - SV* sv = get_sv("dberror", TRUE); + SV* sv = get_sv("dberror", GV_ADD); sv_setiv(sv, (IV) dberror); sv_setpv(sv, dberror_list[dberror]); SvIOK_on(sv); @@ -888,9 +905,9 @@ linked list of C's, typedef'ed to C. U16 mg_private; char mg_type; U8 mg_flags; + I32 mg_len; SV* mg_obj; char* mg_ptr; - I32 mg_len; }; Note this is current as of patchlevel 0, and could change at any time. @@ -913,8 +930,11 @@ SV. The C and C arguments are used to associate a string with the magic, typically the name of a variable. C is stored in the -C field and if C is non-null and C E= 0 a malloc'd -copy of the name is stored in C field. +C field and if C is non-null then either a C copy of +C or C itself is stored in the C field, depending on +whether C is greater than zero or equal to zero respectively. As a +special case, if C<(name && namlen == HEf_SVKEY)> then C is assumed +to contain an C and is stored as-is with its REFCNT incremented. The sv_magic function uses C to determine which, if any, predefined "Magic Virtual Table" should be assigned to the C field. @@ -931,6 +951,9 @@ count of the C object is incremented. If it is the same, or if the C argument is C, or if it is a NULL pointer, then C is merely stored, without the reference count being incremented. +See also C in L for a more flexible way to add magic +to an SV. + There is also a function to add magic to an C: void hv_magic(HV *hv, GV *gv, int how); @@ -939,11 +962,17 @@ This simply calls C and coerces the C argument into an C. To remove the magic from an SV, call the function sv_unmagic: - void sv_unmagic(SV *sv, int type); + int sv_unmagic(SV *sv, int type); The C argument should be equal to the C value when the C was initially made magical. +However, note that C removes all magic of a certain C from the +C. If you want to remove only certain magic of a C based on the magic +virtual table, use C instead: + + int sv_unmagicext(SV *sv, int type, MGVTBL *vtbl); + =head2 Magic Virtual Tables The C field in the C structure is a pointer to an @@ -951,7 +980,8 @@ C, 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 has five pointers to the following routine types: +The C 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); @@ -959,19 +989,28 @@ The C has five pointers to the following routine types: 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 and there are -currently 19 types (or 21 with overloading turned on). These different -structures contain pointers to various routines that perform additional -actions depending on which function is being called. +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_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 (which corresponds to an C of C) contains: @@ -983,52 +1022,71 @@ called. All the various routines for the various magical types begin with C. 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: mg_type - (old-style char and macro) MGVTBL Type of magic - -------------------------- ------ ---------------------------- - \0 PERL_MAGIC_sv vtbl_sv Special scalar variable - A PERL_MAGIC_overload vtbl_amagic %OVERLOAD hash + (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 - c PERL_MAGIC_overload_table (none) Holds overload table (AMT) - on stash - B PERL_MAGIC_bm vtbl_bm Boyer-Moore (fast string search) - 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_fm Formline ('compiled' format) - g PERL_MAGIC_regex_global vtbl_mglob m//g target / study()ed string - 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 %_ 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 'get' magic +if the "set" function in the C structure is NULL. The hook +is activated whenever the hash is accessed with a key specified as +an C through the functions C, C, +C, and C. Accessing the key as a string +through the functions without the C<..._ent> suffix circumvents the +hook. See L for a detailed description. + Note that because multiple extensions may be using C or C 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 magic, it may also be appropriate to add an I32 -'signature' at the top of the private data area and check that. +For C magic, it is usually a good idea to define an +C, even if all its fields will be C<0>, so that individual +C pointers can be identified as a particular kind of magic +using their magic virtual table. C 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 and C functions described earlier do B invoke 'set' magic on their targets. This must @@ -1091,11 +1172,18 @@ since their implementation handles 'get' magic. =head2 Finding Magic - MAGIC* mg_find(SV*, int type); /* Finds the magic pointer of that type */ + MAGIC *mg_find(SV *sv, int type); /* Finds the magic pointer of that type */ + +This routine returns a pointer to a C structure stored in the SV. +If the SV does not have that magical feature, C is returned. If the +SV has multiple instances of that magical feature, the first one will be +returned. C can be used to find a C structure of an SV +based on both it's magic type and it's magic virtual table: -This routine returns a pointer to the C structure stored in the SV. -If the SV does not have that magical feature, C is returned. Also, -if the SV is not of type SVt_PVMG, Perl may core dump. + MAGIC *mg_findext(SV *sv, int type, MGVTBL *vtbl); + +Also, if the SV passed to C or C is not of type +SVt_PVMG, Perl may core dump. int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen); @@ -1135,7 +1223,7 @@ to do this. CODE: hash = newHV(); tie = newRV_noinc((SV*)newHV()); - stash = gv_stashpv("MyTie", TRUE); + stash = gv_stashpv("MyTie", GV_ADD); sv_bless(tie, stash); hv_magic(hash, (GV*)tie, PERL_MAGIC_tied); RETVAL = newRV_noinc(hash); @@ -1381,11 +1469,14 @@ and C 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 macro. The pushed values will often need to be "mortal" (See -L). +L): 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))) - PUSHs(sv_2mortal(newSVnv(3.141592))) + /* 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, the two values will be assigned as in: @@ -1401,8 +1492,9 @@ This macro automatically adjust the stack for you, if needed. Thus, you do not need to call C to extend the stack. Despite their suggestions in earlier versions of this document the macros -C, C and C are I suited to XSUBs which return -multiple results, see L. +C<(X)PUSH[iunp]> are I suited to XSUBs which return multiple results. +For that, either stick to the C<(X)PUSHs> macros shown above, or use the new +C macros instead; see L. For more information, consult L and L. @@ -1465,25 +1557,20 @@ platforms, it may cause spurious malloc or free errors. The following three macros are used to initially allocate memory : - New(x, pointer, number, type); - Newc(x, pointer, number, type, cast); - Newz(x, pointer, number, type); - -The first argument C was a "magic cookie" that was used to keep track -of who called the macro, to help when debugging memory problems. However, -the current code makes no use of this feature (most Perl developers now -use run-time memory checkers), so this argument can be any number. + Newx(pointer, number, type); + Newxc(pointer, number, type, cast); + Newxz(pointer, number, type); -The second argument C should be the name of a variable that will +The first argument C should be the name of a variable that will point to the newly allocated memory. -The third and fourth arguments C and C specify how many of +The second and third arguments C and C specify how many of the specified type of data structure should be allocated. The argument -C is passed to C. The final argument to C, C, +C is passed to C. The final argument to C, C, should be used if the C argument is different from the C argument. -Unlike the C and C macros, the C macro calls C +Unlike the C and C macros, the C macro calls C to zero out all the newly allocated memory. =head3 Reallocation @@ -1536,7 +1623,7 @@ corresponding parts of its I and puts the I on stack. The macro to put this target on stack is C, and it is directly used in some opcodes, as well as indirectly in zillions of -others, which use it via C<(X)PUSH[pni]>. +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: @@ -1548,21 +1635,38 @@ This translates as "set C to 10, push a pointer to C onto the stack; set C to 20, push a pointer to C 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, which we have set -to 20. If you need to push multiple different values, use C, -which bypasses C. +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 macros, +none of which make use of C. The C<(X)PUSHs> macros simply push an +SV* on the stack, which, as noted under L, +will often need to be "mortal". The new C 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 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[npi]>, then you're going to +On a related note, if you do use C<(X)PUSH[iunp]>, then you're going to need a C in your variable declarations so that the C<*PUSH*> -macros can make use of the local variable C. +macros can make use of the local variable C. See also C +and C. =head2 Scratchpads The question remains on when the SVs which are Is 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. +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 @@ -1756,9 +1860,41 @@ of free()ing (i.e. their type is changed to OP_NULL). After the compile tree for a subroutine (or for an C or a file) is created, an additional pass over the code is performed. This pass is neither top-down or bottom-up, but in the execution order (with -additional complications for conditionals). These optimizations are -done in the subroutine peep(). Optimizations performed at this stage -are subject to the same restrictions as in the pass 2. +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. By default, C just +calls the function pointed to by the global variable C. +By default, that performs some basic op fixups and optimisations along +the execution-order op chain, and recursively calls C 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 @@ -1777,6 +1913,76 @@ file, add the line: 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. 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 called at the start of +compiling every lexical scope. The available hooks are: + +=over 4 + +=item C + +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, +the second starts at the C<{> and has C. Both end at the +C<}>, so calls to C and C
 will match. Anything
+pushed onto the save stack by this hook will be popped just before the
+scope ends (between the C and C hooks, in fact).
+
+=item C
+
+This is called at the end of a lexical scope, just before unwinding the
+stack. I 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
+
+This is called at the end of a lexical scope, just after unwinding the
+stack. I is as above. Note that it is possible for calls to C
+and C to nest, if there is something on the save stack that
+calls string eval.
+
+=item C
+
+This is called just before starting to compile an C, C, C or C, after the eval has been set up. I is the
+OP that requested the eval, and will normally be an C,
+C or C.
+
+=back
+
+Once you have your hook functions, you need a C 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 macro, which will also set
+flags indicating which entries are valid. If you do need to allocate
+your C 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 and C 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
 pairs that
+didn't have a matching C.
+
 =head1 Examining internal data structures with the C functions
 
 To aid debugging, the source file F contains a number of
@@ -1824,13 +2030,39 @@ 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.
 
-Two macros control the major Perl build flavors: MULTIPLICITY and
-USE_5005THREADS.  The MULTIPLICITY build has a C structure
-that packages all the interpreter state, and there is a similar thread-specific
-data structure under USE_5005THREADS.  In both cases,
-PERL_IMPLICIT_CONTEXT is also normally defined, and enables the
-support for passing in a "hidden" first argument that represents all three
-data structures.
+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 for usage details.  You may also need
+to use C 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 libperl.a | grep -v ' [TURtr] '
+
+If this displays any C or C 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
@@ -1867,7 +2099,7 @@ sanctioned for use in extensions) begins like this:
   void
   Perl_sv_setiv(pTHX_ SV* dsv, IV num)
 
-C is one of a number of macros (in perl.h) that hide the
+C is one of a number of macros (in F) 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
rototype, 'a' for Brgument,
@@ -1936,7 +2168,7 @@ built with PERL_IMPLICIT_CONTEXT enabled.
 
 There are three ways to do this.  First, the easy but inefficient way,
 which is also the default, in order to maintain source compatibility
-with extensions: whenever XSUB.h is #included, it redefines the aTHX
+with extensions: whenever F is #included, it redefines the aTHX
 and aTHX_ macros to call a function that will return the context.
 Thus, something like:
 
@@ -1963,9 +2195,9 @@ your Foo.xs:
         #include "perl.h"
         #include "XSUB.h"
 
-        static my_private_function(int arg1, int arg2);
+        STATIC void my_private_function(int arg1, int arg2);
 
-        static SV *
+        STATIC void
         my_private_function(int arg1, int arg2)
         {
             dTHX;       /* fetch context */
@@ -2003,9 +2235,9 @@ the Perl guts:
         #include "XSUB.h"
 
         /* pTHX_ only needed for functions that call Perl API */
-        static my_private_function(pTHX_ int arg1, int arg2);
+        STATIC void my_private_function(pTHX_ int arg1, int arg2);
 
-        static SV *
+        STATIC void
         my_private_function(pTHX_ int arg1, int arg2)
         {
             /* dTHX; not needed here, because THX is an argument */
@@ -2033,6 +2265,13 @@ Never add a comma after C 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
+definition is needed if the Perl global variables (see F
+or F) are accessed in the function and C is not
+used (the C includes the C if necessary).  One notices
+the need for C 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
@@ -2059,14 +2298,14 @@ 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
-and USE_5005THREADS on Windows (see inside iperlsys.h).
+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 win32/perllib.c) for the default perl executable, but for a
+calls (see F) 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.
@@ -2083,9 +2322,13 @@ functions or functions used in a program in which Perl is embedded.
 Similarly, all global variables begin with C. (By convention,
 static functions start with C.)
 
-Inside the Perl core, you can get at the functions either with or
-without the C prefix, thanks to a bunch of defines that live in
-F. This header file is generated automatically from
+Inside the Perl core (C defined), you can get at the functions
+either with or without the C prefix, thanks to a bunch of defines
+that live in F. Note that extension code should I set
+C; this exposes the full perl internals, and is likely to cause
+breakage of the XS in each new perl release.
+
+The file F is generated automatically from
 F and F. F 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
@@ -2102,16 +2345,18 @@ after that are the arguments. The first column is a set of flags:
 
 =item A
 
-This function is a part of the public API.
+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 prefix; ie, it is defined as C
+This function has a C prefix; i.e. it is defined as
+C.
 
 =item d
 
 This function has documentation using the C feature which we'll
-look at in a second.
+look at in a second.  Some functions have 'd' but not 'A'; docs are good.
 
 =back
 
@@ -2121,13 +2366,14 @@ Other available flags are:
 
 =item s
 
-This is a static function and is defined as C, and usually
-called within the sources as C.
+This is a static function and is defined as C, and
+usually called within the sources as C.
 
 =item n
 
-This does not use C and C to pass interpreter context. (See
-L.)
+This does not need a interpreter context, so the definition has no
+C, and it follows that callers don't use C.  (See
+L.)
 
 =item r
 
@@ -2171,6 +2417,10 @@ This function is visible to extensions included in the Perl core.
 Binary backward compatibility; this function is a macro but also has
 a C implementation (which is exported).
 
+=item others
+
+See the comments at the top of C for others.
+
 =back
 
 If you edit F or F, you will need to run
@@ -2221,6 +2471,39 @@ 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 before including F 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 function.
+
+The advantage of using the above macros is that you don't have
+to setup an extra function for C, and that using these
+macros is faster than using C.
+
 =head2 Source Documentation
 
 There's an effort going on to document the internal functions and
@@ -2244,6 +2527,26 @@ source, like this:
 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 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 generates a C header file F that can also
+be run as a Perl script. To generate F, 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.
+
 =head1 Unicode Support
 
 Perl 5.6.0 introduced Unicode support. It's important for porters and XS
@@ -2271,8 +2574,8 @@ 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, instead of just
-one. You can learn more about Unicode at http://www.unicode.org/
+a variable number of bytes to represent a character. You can learn more
+about Unicode and Perl's Unicode model in L.
 
 =head2 How can I recognise a UTF-8 string?
 
@@ -2283,16 +2586,17 @@ C. Unfortunately, the non-Unicode string C
 has that byte sequence as well. So you can't tell just by looking - this
 is what makes Unicode input an interesting problem.
 
-The API function C 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 will tell you
-whether the current character in a string is valid UTF-8.
+In general, you either have to know what you're dealing with, or you
+have to guess.  The API function C 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
+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 1...128 are stored in one byte, just
-like good ol' ASCII. Character 129 is stored as C; this
+character. Characters with values 0...127 are stored in one byte, just
+like good ol' ASCII. Character 128 is stored as C; this
 continues up to character 191, which is C. Now we've run out of
 bits (191 is binary C<10111111>) so we move on; 192 is C. And
 so it goes on, moving to three bytes at character 2048.
@@ -2349,9 +2653,12 @@ 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. If a string has been identified as being UTF-8
-encoded, Perl will set a flag in the SV, C. You can check and
-manipulate this flag with the following macros:
+slightly differently. A flag in the SV, C, 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 is needed to get iso-8859-1
+semantics). You can check and manipulate this flag with the
+following macros:
 
     SvUTF8(sv)
     SvUTF8_on(sv)
@@ -2363,7 +2670,7 @@ C, C and other string handling operations will have
 undesirable results.
 
 The problem comes when you have, for instance, a string that isn't
-flagged is UTF-8, and contains a byte sequence that could be UTF-8 -
+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 flag is separate to the PV value; you
@@ -2381,7 +2688,7 @@ manipulating SVs. More specifically, you cannot expect to do this:
 
 The C 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 UTF-8 flag set, and act accordingly:
+old SV has the UTF8 flag set, and act accordingly:
 
     p = SvPV(sv, len);
     frobnicate(p);
@@ -2394,14 +2701,14 @@ 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 UTF-8 flags, even less right is just
+the SV is not enough to copy the UTF8 flags, even less right is just
 passing a C 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, you might find it necessary
-to upgrade one of the strings to UTF-8. If you've got an SV, the easiest
-way to do this is:
+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);
 
@@ -2412,7 +2719,7 @@ However, you must not do this, for example:
 
 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.
+by the end user, it can cause problems in deficient code.
 
 Instead, C will give you a UTF-8-encoded B of its
 string argument. This is useful for having the data available for
@@ -2448,9 +2755,7 @@ you can use C<*s = uv>.
 =item *
 
 Mixing UTF-8 and non-UTF-8 strings is tricky. Use C to get
-a new string which is UTF-8 encoded. There are tricks you can use to
-delay deciding whether you need to use a UTF-8 string until you get to a
-high character - C is one of those.
+a new string which is UTF-8 encoded, and then combine them.
 
 =back
 
@@ -2461,7 +2766,7 @@ 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.) 
+C.)
 
 This feature is implemented as a new op type, C. The Perl
 core does not "know" anything special about this op type, and so it will
@@ -2487,15 +2792,81 @@ 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,
-Perl uses the value of C<< o->op_ppaddr >> as a key into the
-C and C hashes. This means you
-need to enter a name and description for your op at the appropriate
-place in the C and C hashes.
+Perl uses the value of C<< o->op_ppaddr >> to determine which custom op
+it is dealing with. You should create an C structure for each
+ppaddr you use, set the properties of the custom op with
+C, and register the structure against the ppaddr using
+C. 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 module, so
+it will appear in the output of module like L.
+
+=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 constants from F, 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' only. The C<_OR_SVOP> is because
+the only core C, C, can sometimes be a C instead.
+
+=item OA_LOOP
+
+=item OA_COP
+
+=back
+
+The other C constants should not be used.
+
+=item xop_peep
+
+This member is of type C, which expands to C. If it is set, this function
+will be called from C when ops of this type are encountered
+by the peephole optimizer. I is the OP that needs optimizing;
+I is the previous OP optimized, whose C points to I.
+
+=back
 
-Forthcoming versions of C (version 1.0 and above) should
-directly support the creation of custom ops by name; C 
-will provide functions which make it trivial to "register" custom ops to
-the Perl interpreter.
+C directly supports the creation of custom ops by name.
 
 =head1 AUTHORS
 
@@ -2510,4 +2881,4 @@ Stephen McCamant, and Gurusamy Sarathy.
 
 =head1 SEE ALSO
 
-perlapi(1), perlintern(1), perlxs(1), perlembed(1)
+L, L, L, L