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diff --git a/Master/tlpkg/tlperl0/lib/pods/perlguts.pod b/Master/tlpkg/tlperl0/lib/pods/perlguts.pod new file mode 100755 index 00000000000..d119937145c --- /dev/null +++ b/Master/tlpkg/tlperl0/lib/pods/perlguts.pod @@ -0,0 +1,2671 @@ +=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 value undef. + + 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. + +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 a byte for the a trailing NUL (perl's own string functions typically do +C<SvGROW(sv, len + 1)>). + +If you have an SV and want to know what kind of data Perl thinks is stored +in it, you can use the following macros to check the type of SV you have. + + SvIOK(SV*) + SvNOK(SV*) + SvPOK(SV*) + +You can get and set the current length of the string stored in an SV with +the following macros: + + SvCUR(SV*) + SvCUR_set(SV*, I32 val) + +You can also get a pointer to the end of the string stored in the SV +with the macro: + + SvEND(SV*) + +But note that these last three macros are valid only if C<SvPOK()> is true. + +If you want to append something to the end of string stored in an C<SV*>, +you can use the following functions: + + void sv_catpv(SV*, const char*); + void sv_catpvn(SV*, const char*, STRLEN); + void sv_catpvf(SV*, const char*, ...); + void sv_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<AvLEN>. +Again, the location of the real start of the C array only comes into +play when freeing the array. See C<av_shift> in F<av.c>. + +=head2 What's Really Stored in an SV? + +Recall that the usual method of determining the type of scalar you have is +to use C<Sv*OK> macros. Because a scalar can be both a number and a string, +usually these macros will always return TRUE and calling the C<Sv*V> +macros will do the appropriate conversion of string to integer/double or +integer/double to string. + +If you I<really> need to know if you have an integer, double, or string +pointer in an SV, you can use the following three macros instead: + + SvIOKp(SV*) + SvNOKp(SV*) + SvPOKp(SV*) + +These will tell you if you truly have an integer, double, or string pointer +stored in your SV. The "p" stands for private. + +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 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 AVs: + + void av_push(AV*, SV*); + SV* av_pop(AV*); + SV* av_shift(AV*); + void av_unshift(AV*, I32 num); + +These should be familiar operations, with the exception of C<av_unshift>. +This routine adds C<num> elements at the front of the array with the C<undef> +value. You must then use C<av_store> (described below) to assign values +to these new elements. + +Here are some other functions: + + I32 av_len(AV*); + SV** av_fetch(AV*, I32 key, I32 lval); + SV** av_store(AV*, I32 key, SV* val); + +The C<av_len> function returns the highest index value in array (just +like $#array in Perl). If the array is empty, -1 is returned. The +C<av_fetch> function returns the value at index C<key>, but if C<lval> +is non-zero, then C<av_fetch> will store an undef value at that index. +The C<av_store> function stores the value C<val> at index C<key>, and does +not increment the reference count of C<val>. Thus the caller is responsible +for taking care of that, and if C<av_store> returns NULL, the caller will +have to decrement the reference count to avoid a memory leak. Note that +C<av_fetch> and C<av_store> both return C<SV**>'s, not C<SV*>'s as their +return value. + + void av_clear(AV*); + void av_undef(AV*); + void av_extend(AV*, I32 key); + +The C<av_clear> function deletes all the elements in the AV* array, but +does not actually delete the array itself. The C<av_undef> function will +delete all the elements in the array plus the array itself. The +C<av_extend> function extends the array so that it contains at least C<key+1> +elements. If C<key+1> is less than the currently allocated length of the array, +then nothing is done. + +If you know the name of an array variable, you can get a pointer to its AV +by using the following: + + AV* get_av("package::varname", 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 HVs: + + SV** hv_store(HV*, const char* key, U32 klen, SV* val, U32 hash); + SV** hv_fetch(HV*, const char* key, U32 klen, I32 lval); + +The C<klen> parameter is the length of the key being passed in (Note that +you cannot pass 0 in as a value of C<klen> to tell Perl to measure the +length of the key). The C<val> argument contains the SV pointer to the +scalar being stored, and C<hash> is the precomputed hash value (zero if +you want C<hv_store> to calculate it for you). The C<lval> parameter +indicates whether this fetch is actually a part of a store operation, in +which case a new undefined value will be added to the HV with the supplied +key and C<hv_fetch> will return as if the value had already existed. + +Remember that C<hv_store> and C<hv_fetch> return C<SV**>'s and not just +C<SV*>. To access the scalar value, you must first dereference the return +value. However, you should check to make sure that the return value is +not NULL before dereferencing it. + +These two functions check if a hash table entry exists, and deletes it. + + bool hv_exists(HV*, const char* key, U32 klen); + SV* hv_delete(HV*, const char* key, U32 klen, I32 flags); + +If C<flags> does not include the C<G_DISCARD> flag then C<hv_delete> will +create and return a mortal copy of the deleted value. + +And more miscellaneous functions: + + void hv_clear(HV*); + void hv_undef(HV*); + +Like their AV counterparts, C<hv_clear> deletes all the entries in the hash +table but does not actually delete the hash table. The C<hv_undef> deletes +both the entries and the hash table itself. + +Perl keeps the actual data in linked list of structures with a typedef of HE. +These contain the actual key and value pointers (plus extra administrative +overhead). The key is a string pointer; the value is an C<SV*>. However, +once you have an C<HE*>, to get the actual key and value, use the routines +specified below. + + I32 hv_iterinit(HV*); + /* Prepares starting point to traverse hash table */ + HE* hv_iternext(HV*); + /* Get the next entry, and return a pointer to a + structure that has both the key and value */ + char* hv_iterkey(HE* entry, I32* retlen); + /* Get the key from an HE structure and also return + the length of the key string */ + SV* hv_iterval(HV*, HE* entry); + /* Return 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_true> or +C<&PL_sv_false> 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 +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 references). + +To create a reference, use either of the following functions: + + SV* newRV_inc((SV*) thing); + SV* newRV_noinc((SV*) thing); + +The C<thing> argument can be any of an C<SV*>, C<AV*>, or C<HV*>. The +functions are identical except that C<newRV_inc> increments the reference +count of the C<thing>, while C<newRV_noinc> does not. For historical +reasons, C<newRV> is a synonym for C<newRV_inc>. + +Once you have a reference, you can use the following macro to dereference +the reference: + + SvRV(SV*) + +then call the appropriate routines, casting the returned C<SV*> to either an +C<AV*> or C<HV*>, if required. + +To determine if an SV is a reference, you can use the following macro: + + SvROK(SV*) + +To discover what type of value the reference refers to, use the following +macro and then check the return value. + + SvTYPE(SvRV(SV*)) + +The most useful types that will be returned are: + + SVt_IV Scalar + SVt_NV Scalar + SVt_PV Scalar + SVt_RV Scalar + SVt_PVAV Array + SVt_PVHV Hash + SVt_PVCV Code + SVt_PVGV Glob (possible a file handle) + SVt_PVMG Blessed or Magical Scalar + +See the 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 */ + +Upgrades rv to reference if not already one. Creates new SV for rv to +point to. If C<classname> is non-null, the SV is blessed into the specified +class. SV is returned. + + SV* newSVrv(SV* rv, const char* classname); + +Copies integer, 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); + +Copies the pointer value (I<the address, not the string!>) into an SV whose +reference is rv. SV is blessed if C<classname> is non-null. + + SV* sv_setref_pv(SV* rv, const char* classname, PV iv); + +Copies string into an SV whose reference is C<rv>. Set length to 0 to let +Perl calculate the string length. SV is blessed if C<classname> is non-null. + + SV* sv_setref_pvn(SV* rv, const char* classname, PV iv, STRLEN length); + +Tests whether the SV is blessed into the specified class. It does not +check inheritance relationships. + + int sv_isa(SV* sv, const char* name); + +Tests whether the SV is a reference to a blessed object. + + int sv_isobject(SV* sv); + +Tests whether the SV is derived from the specified class. SV can be either +a reference to a blessed object or a string containing a class name. This +is the function implementing the C<UNIVERSAL::isa> functionality. + + bool sv_derived_from(SV* sv, const char* name); + +To check if you've got an object derived from a specific class you have +to write: + + if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... } + +=head2 Creating New Variables + +To create a new Perl variable with an undef value which can be accessed from +your Perl script, use the following routines, depending on the variable type. + + SV* get_sv("package::varname", 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 +be set, using the routines appropriate to the data type. + +There are additional macros whose values may be bitwise OR'ed with the +C<TRUE> argument to enable certain extra features. Those bits are: + +=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 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: + + void sv_unmagic(SV *sv, int type); + +The C<type> argument should be equal to the C<how> value when the C<SV> +was initially made magical. + +=head2 Magic Virtual Tables + +The C<mg_virtual> field in the C<MAGIC> structure is a pointer to 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, int 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: + + 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 + 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 + H PERL_MAGIC_hints vtbl_sig %^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 + o PERL_MAGIC_collxfrm vtbl_collxfrm Locale collate 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_qr precompiled qr// regex + S PERL_MAGIC_sig vtbl_sig %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 + v PERL_MAGIC_vec vtbl_vec vec() lvalue + V PERL_MAGIC_vstring (none) v-string scalars + w PERL_MAGIC_utf8 vtbl_utf8 UTF-8 length+offset cache + x PERL_MAGIC_substr vtbl_substr substr() lvalue + y PERL_MAGIC_defelem vtbl_defelem Shadow "foreach" iterator + variable / smart parameter + vivification + # PERL_MAGIC_arylen vtbl_arylen Array length ($#ary) + . PERL_MAGIC_pos vtbl_pos pos() lvalue + < PERL_MAGIC_backref vtbl_backref back pointer to a weak ref + ~ PERL_MAGIC_ext (none) Available for use by extensions + : PERL_MAGIC_symtab (none) hash used as symbol table + % PERL_MAGIC_rhash (none) hash used as restricted hash + @ PERL_MAGIC_arylen_p vtbl_arylen_p pointer to $#a from @a + + +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 may also be appropriate to add an I32 +'signature' at the top of the private data area and check that. + +Also note that the C<sv_set*()> and C<sv_cat*()> functions described +earlier do B<not> invoke 'set' magic on their targets. This must +be done by the user either by calling the C<SvSETMAGIC()> macro after +calling these functions, or by using one of the C<sv_set*_mg()> or +C<sv_cat*_mg()> functions. Similarly, generic C code must call the +C<SvGETMAGIC()> macro to invoke any 'get' magic if they use an SV +obtained from external sources in functions that don't handle magic. +See L<perlapi> for a description of these functions. +For example, calls to the C<sv_cat*()> functions typically need to be +followed by C<SvSETMAGIC()>, but they don't need a prior C<SvGETMAGIC()> +since their implementation handles 'get' magic. + +=head2 Finding Magic + + MAGIC* mg_find(SV*, int type); /* Finds the magic pointer of that type */ + +This routine returns a pointer to the C<MAGIC> structure stored in the SV. +If the SV does not have that magical feature, C<NULL> is returned. Also, +if the SV is not of type SVt_PVMG, Perl may core dump. + + int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen); + +This routine checks to see what types of magic C<sv> has. If the mg_type +field is an uppercase letter, then the mg_obj is copied to C<nsv>, but +the mg_type field is changed to be the lowercase letter. + +=head2 Understanding the Magic of Tied Hashes and Arrays + +Tied hashes and arrays are magical beasts of the 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))) + +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 adjust 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 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 has been experimenting with +removing Perl's dependency on the "normal" standard I/O suite and allowing +other stdio implementations to be used. This involves creating a new +abstraction layer that then calls whichever implementation of stdio Perl +was compiled with. All XSUBs should now use the functions in the PerlIO +abstraction layer and not make any assumptions about what kind of stdio +is being used. + +For a complete description of the PerlIO abstraction, consult L<perlapio>. + +=head2 Putting a C value on Perl stack + +A lot of opcodes (this is an elementary operation in the internal perl +stack machine) put an SV* on the stack. However, as an optimization +the corresponding SV is (usually) not recreated each time. The opcodes +reuse specially assigned SVs (I<target>s) which are (as a corollary) +not constantly freed/created. + +Each of the targets is created only once (but see +L<Scratchpads and recursion> below), and when an opcode needs to put +an integer, a double, or a string on stack, it just sets the +corresponding parts of its I<target> and puts the I<target> on stack. + +The macro to put this target on stack is C<PUSHTARG>, and it is +directly used in some opcodes, as well as indirectly in zillions of +others, which use it via C<(X)PUSH[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). These optimizations are +done in the subroutine peep(). Optimizations performed at this stage +are subject to the same restrictions as in the pass 2. + +=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. + +=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 +mutli-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 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 have to do nothing new in your extension to get this; since +the Perl library provides Perl_get_context(), it will all just +work. + +The second, more efficient way is to use the following template for +your Foo.xs: + + #define PERL_NO_GET_CONTEXT /* we want efficiency */ + #include "EXTERN.h" + #include "perl.h" + #include "XSUB.h" + + STATIC 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, you can get at the functions either with or +without the C<Perl_> prefix, thanks to a bunch of defines that live in +F<embed.h>. This header file is generated automatically from +F<embed.pl> 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 a interpreter context, so the definition has no +C<pTHX>, and it follows that callers don't use C<aTHX>. (See +L<perlguts/Background and PERL_IMPLICIT_CONTEXT>.) + +=item r + +This function never returns; C<croak>, C<exit> and friends. + +=item f + +This function takes a variable number of arguments, C<printf> style. +The argument list should end with C<...>, like this: + + Afprd |void |croak |const char* pat|... + +=item M + +This function is part of the experimental development API, and may change +or disappear without notice. + +=item o + +This function should not have a compatibility macro to define, say, +C<Perl_parse> to C<parse>. It must be called as C<Perl_parse>. + +=item 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; + UV uv; /* Note: a UV, not a U8, not a char */ + + if (!UTF8_IS_INVARIANT(*utf)) + /* Must treat this as UTF-8 */ + uv = utf8_to_uv(utf); + else + /* OK to treat this character as a byte */ + uv = *utf; + +You can also see in that example that we use C<utf8_to_uv> to get the +value of the character; the inverse function C<uv_to_utf8> is available +for putting a UV into UTF-8: + + if (!UTF8_IS_INVARIANT(uv)) + /* Must treat this as UTF8 */ + utf8 = uv_to_utf8(utf8, uv); + else + /* OK to treat this character as a byte */ + *utf8++ = uv; + +You B<must> convert characters to UVs using the above functions if +you're ever in a situation where you have to match 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). 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 is 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_uv> 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<uv_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 >> as a key into the +C<PL_custom_op_descs> and C<PL_custom_op_names> hashes. This means you +need to enter a name and description for your op at the appropriate +place in the C<PL_custom_op_names> and C<PL_custom_op_descs> hashes. + +Forthcoming versions of C<B::Generate> (version 1.0 and above) should +directly support 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 + +perlapi(1), perlintern(1), perlxs(1), perlembed(1) |