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=head1 NAME perlguts - Introduction to the Perl API =head1 DESCRIPTION This document attempts to describe how to use the Perl API, as well as containing some info on the basic workings of the Perl core. It is far from complete and probably contains many errors. Please refer any questions or comments to the author below. =head1 Variables =head2 Datatypes Perl has three typedefs that handle Perl's three main data types: SV Scalar Value AV Array Value HV Hash Value Each typedef has specific routines that manipulate the various data types. =head2 What is an "IV"? Perl uses a special typedef IV which is a simple signed integer type that is guaranteed to be large enough to hold a pointer (as well as an integer). Additionally, there is the UV, which is simply an unsigned IV. Perl also uses two special typedefs, I32 and I16, which will always be at least 32-bits and 16-bits long, respectively. (Again, there are U32 and U16, as well.) =head2 Working with SVs An SV can be created and loaded with one command. There are four types of values that can be loaded: an integer value (IV), a double (NV), a string, (PV), and another scalar (SV). The six routines are: SV* newSViv(IV); SV* newSVnv(double); SV* newSVpv(const char*, int); SV* newSVpvn(const char*, int); SV* newSVpvf(const char*, ...); SV* newSVsv(SV*); To change the value of an *already-existing* SV, there are seven routines: void sv_setiv(SV*, IV); void sv_setuv(SV*, UV); void sv_setnv(SV*, double); void sv_setpv(SV*, const char*); void sv_setpvn(SV*, const char*, int) void sv_setpvf(SV*, const char*, ...); void sv_setpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool); void sv_setsv(SV*, SV*); Notice that you can choose to specify the length of the string to be assigned by using C<sv_setpvn>, C<newSVpvn>, or C<newSVpv>, or you may allow Perl to calculate the length by using C<sv_setpv> or by specifying 0 as the second argument to C<newSVpv>. Be warned, though, that Perl will determine the string's length by using C<strlen>, which depends on the string terminating with a NUL character. The arguments of C<sv_setpvf> are processed like C<sprintf>, and the formatted output becomes the value. C<sv_setpvfn> is an analogue of C<vsprintf>, but it allows you to specify either a pointer to a variable argument list or the address and length of an array of SVs. The last argument points to a boolean; on return, if that boolean is true, then locale-specific information has been used to format the string, and the string's contents are therefore untrustworthy (see L<perlsec>). This pointer may be NULL if that information is not important. Note that this function requires you to specify the length of the format. The C<sv_set*()> functions are not generic enough to operate on values that have "magic". See L<Magic Virtual Tables> later in this document. All SVs that contain strings should be terminated with a NUL character. If it is not NUL-terminated there is a risk of core dumps and corruptions from code which passes the string to C functions or system calls which expect a NUL-terminated string. Perl's own functions typically add a trailing NUL for this reason. Nevertheless, you should be very careful when you pass a string stored in an SV to a C function or system call. To access the actual value that an SV points to, you can use the macros: SvIV(SV*) SvUV(SV*) SvNV(SV*) SvPV(SV*, STRLEN len) SvPV_nolen(SV*) which will automatically coerce the actual scalar type into an IV, UV, double, or string. In the C<SvPV> macro, the length of the string returned is placed into the variable C<len> (this is a macro, so you do I<not> use C<&len>). If you do not care what the length of the data is, use the C<SvPV_nolen> macro. Historically the C<SvPV> macro with the global variable C<PL_na> has been used in this case. But that can be quite inefficient because C<PL_na> must be accessed in thread-local storage in threaded Perl. In any case, remember that Perl allows arbitrary strings of data that may both contain NULs and might not be terminated by a NUL. Also remember that C doesn't allow you to safely say C<foo(SvPV(s, len), len);>. It might work with your compiler, but it won't work for everyone. Break this sort of statement up into separate assignments: SV *s; STRLEN len; char * ptr; ptr = SvPV(s, len); foo(ptr, len); If you want to know if the scalar value is TRUE, you can use: SvTRUE(SV*) Although Perl will automatically grow strings for you, if you need to force Perl to allocate more memory for your SV, you can use the macro SvGROW(SV*, STRLEN newlen) which will determine if more memory needs to be allocated. If so, it will call the function C<sv_grow>. Note that C<SvGROW> can only increase, not decrease, the allocated memory of an SV and that it does not automatically add a byte for the a trailing NUL (perl's own string functions typically do C<SvGROW(sv, len + 1)>). If you have an SV and want to know what kind of data Perl thinks is stored in it, you can use the following macros to check the type of SV you have. SvIOK(SV*) SvNOK(SV*) SvPOK(SV*) You can get and set the current length of the string stored in an SV with the following macros: SvCUR(SV*) SvCUR_set(SV*, I32 val) You can also get a pointer to the end of the string stored in the SV with the macro: SvEND(SV*) But note that these last three macros are valid only if C<SvPOK()> is true. If you want to append something to the end of string stored in an C<SV*>, you can use the following functions: void sv_catpv(SV*, const char*); void sv_catpvn(SV*, const char*, STRLEN); void sv_catpvf(SV*, const char*, ...); void sv_catpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool); void sv_catsv(SV*, SV*); The first function calculates the length of the string to be appended by using C<strlen>. In the second, you specify the length of the string yourself. The third function processes its arguments like C<sprintf> and appends the formatted output. The fourth function works like C<vsprintf>. You can specify the address and length of an array of SVs instead of the va_list argument. The fifth function extends the string stored in the first SV with the string stored in the second SV. It also forces the second SV to be interpreted as a string. The C<sv_cat*()> functions are not generic enough to operate on values that have "magic". See L<Magic Virtual Tables> later in this document. If you know the name of a scalar variable, you can get a pointer to its SV by using the following: SV* get_sv("package::varname", FALSE); This returns NULL if the variable does not exist. If you want to know if this variable (or any other SV) is actually C<defined>, you can call: SvOK(SV*) The scalar C<undef> value is stored in an SV instance called C<PL_sv_undef>. Its address can be used whenever an C<SV*> is needed. There are also the two values C<PL_sv_yes> and C<PL_sv_no>, which contain Boolean TRUE and FALSE values, respectively. Like C<PL_sv_undef>, their addresses can be used whenever an C<SV*> is needed. Do not be fooled into thinking that C<(SV *) 0> is the same as C<&PL_sv_undef>. Take this code: SV* sv = (SV*) 0; if (I-am-to-return-a-real-value) { sv = sv_2mortal(newSViv(42)); } sv_setsv(ST(0), sv); This code tries to return a new SV (which contains the value 42) if it should return a real value, or undef otherwise. Instead it has returned a NULL pointer which, somewhere down the line, will cause a segmentation violation, bus error, or just weird results. Change the zero to C<&PL_sv_undef> in the first line and all will be well. To free an SV that you've created, call C<SvREFCNT_dec(SV*)>. Normally this call is not necessary (see L<Reference Counts and Mortality>). =head2 What's Really Stored in an SV? Recall that the usual method of determining the type of scalar you have is to use C<Sv*OK> macros. Because a scalar can be both a number and a string, usually these macros will always return TRUE and calling the C<Sv*V> macros will do the appropriate conversion of string to integer/double or integer/double to string. If you I<really> need to know if you have an integer, double, or string pointer in an SV, you can use the following three macros instead: SvIOKp(SV*) SvNOKp(SV*) SvPOKp(SV*) These will tell you if you truly have an integer, double, or string pointer stored in your SV. The "p" stands for private. In general, though, it's best to use the C<Sv*V> macros. =head2 Working with AVs There are two ways to create and load an AV. The first method creates an empty AV: AV* newAV(); The second method both creates the AV and initially populates it with SVs: AV* av_make(I32 num, SV **ptr); The second argument points to an array containing C<num> C<SV*>'s. Once the AV has been created, the SVs can be destroyed, if so desired. Once the AV has been created, the following operations are possible on AVs: void av_push(AV*, SV*); SV* av_pop(AV*); SV* av_shift(AV*); void av_unshift(AV*, I32 num); These should be familiar operations, with the exception of C<av_unshift>. This routine adds C<num> elements at the front of the array with the C<undef> value. You must then use C<av_store> (described below) to assign values to these new elements. Here are some other functions: I32 av_len(AV*); SV** av_fetch(AV*, I32 key, I32 lval); SV** av_store(AV*, I32 key, SV* val); The C<av_len> function returns the highest index value in array (just like $#array in Perl). If the array is empty, -1 is returned. The C<av_fetch> function returns the value at index C<key>, but if C<lval> is non-zero, then C<av_fetch> will store an undef value at that index. The C<av_store> function stores the value C<val> at index C<key>, and does not increment the reference count of C<val>. Thus the caller is responsible for taking care of that, and if C<av_store> returns NULL, the caller will have to decrement the reference count to avoid a memory leak. Note that C<av_fetch> and C<av_store> both return C<SV**>'s, not C<SV*>'s as their return value. void av_clear(AV*); void av_undef(AV*); void av_extend(AV*, I32 key); The C<av_clear> function deletes all the elements in the AV* array, but does not actually delete the array itself. The C<av_undef> function will delete all the elements in the array plus the array itself. The C<av_extend> function extends the array so that it contains at least C<key+1> elements. If C<key+1> is less than the currently allocated length of the array, then nothing is done. If you know the name of an array variable, you can get a pointer to its AV by using the following: AV* get_av("package::varname", FALSE); This returns NULL if the variable does not exist. See L<Understanding the Magic of Tied Hashes and Arrays> for more information on how to use the array access functions on tied arrays. =head2 Working with HVs To create an HV, you use the following routine: HV* newHV(); Once the HV has been created, the following operations are possible on HVs: SV** hv_store(HV*, const char* key, U32 klen, SV* val, U32 hash); SV** hv_fetch(HV*, const char* key, U32 klen, I32 lval); The C<klen> parameter is the length of the key being passed in (Note that you cannot pass 0 in as a value of C<klen> to tell Perl to measure the length of the key). The C<val> argument contains the SV pointer to the scalar being stored, and C<hash> is the precomputed hash value (zero if you want C<hv_store> to calculate it for you). The C<lval> parameter indicates whether this fetch is actually a part of a store operation, in which case a new undefined value will be added to the HV with the supplied key and C<hv_fetch> will return as if the value had already existed. Remember that C<hv_store> and C<hv_fetch> return C<SV**>'s and not just C<SV*>. To access the scalar value, you must first dereference the return value. However, you should check to make sure that the return value is not NULL before dereferencing it. These two functions check if a hash table entry exists, and deletes it. bool hv_exists(HV*, const char* key, U32 klen); SV* hv_delete(HV*, const char* key, U32 klen, I32 flags); If C<flags> does not include the C<G_DISCARD> flag then C<hv_delete> will create and return a mortal copy of the deleted value. And more miscellaneous functions: void hv_clear(HV*); void hv_undef(HV*); Like their AV counterparts, C<hv_clear> deletes all the entries in the hash table but does not actually delete the hash table. The C<hv_undef> deletes both the entries and the hash table itself. Perl keeps the actual data in linked list of structures with a typedef of HE. These contain the actual key and value pointers (plus extra administrative overhead). The key is a string pointer; the value is an C<SV*>. However, once you have an C<HE*>, to get the actual key and value, use the routines specified below. I32 hv_iterinit(HV*); /* Prepares starting point to traverse hash table */ HE* hv_iternext(HV*); /* Get the next entry, and return a pointer to a structure that has both the key and value */ char* hv_iterkey(HE* entry, I32* retlen); /* Get the key from an HE structure and also return the length of the key string */ SV* hv_iterval(HV*, HE* entry); /* Return a SV pointer to the value of the HE structure */ SV* hv_iternextsv(HV*, char** key, I32* retlen); /* This convenience routine combines hv_iternext, hv_iterkey, and hv_iterval. The key and retlen arguments are return values for the key and its length. The value is returned in the SV* argument */ If you know the name of a hash variable, you can get a pointer to its HV by using the following: HV* get_hv("package::varname", FALSE); This returns NULL if the variable does not exist. The hash algorithm is defined in the C<PERL_HASH(hash, key, klen)> macro: hash = 0; while (klen--) hash = (hash * 33) + *key++; hash = hash + (hash >> 5); /* after 5.6 */ The last step was added in version 5.6 to improve distribution of lower bits in the resulting hash value. See L<Understanding the Magic of Tied Hashes and Arrays> for more information on how to use the hash access functions on tied hashes. =head2 Hash API Extensions Beginning with version 5.004, the following functions are also supported: HE* hv_fetch_ent (HV* tb, SV* key, I32 lval, U32 hash); HE* hv_store_ent (HV* tb, SV* key, SV* val, U32 hash); bool hv_exists_ent (HV* tb, SV* key, U32 hash); SV* hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash); SV* hv_iterkeysv (HE* entry); Note that these functions take C<SV*> keys, which simplifies writing of extension code that deals with hash structures. These functions also allow passing of C<SV*> keys to C<tie> functions without forcing you to stringify the keys (unlike the previous set of functions). They also return and accept whole hash entries (C<HE*>), making their use more efficient (since the hash number for a particular string doesn't have to be recomputed every time). See L<perlapi> for detailed descriptions. The following macros must always be used to access the contents of hash entries. Note that the arguments to these macros must be simple variables, since they may get evaluated more than once. See L<perlapi> for detailed descriptions of these macros. HePV(HE* he, STRLEN len) HeVAL(HE* he) HeHASH(HE* he) HeSVKEY(HE* he) HeSVKEY_force(HE* he) HeSVKEY_set(HE* he, SV* sv) These two lower level macros are defined, but must only be used when dealing with keys that are not C<SV*>s: HeKEY(HE* he) HeKLEN(HE* he) Note that both C<hv_store> and C<hv_store_ent> do not increment the reference count of the stored C<val>, which is the caller's responsibility. If these functions return a NULL value, the caller will usually have to decrement the reference count of C<val> to avoid a memory leak. =head2 References References are a special type of scalar that point to other data types (including references). To create a reference, use either of the following functions: SV* newRV_inc((SV*) thing); SV* newRV_noinc((SV*) thing); The C<thing> argument can be any of an C<SV*>, C<AV*>, or C<HV*>. The functions are identical except that C<newRV_inc> increments the reference count of the C<thing>, while C<newRV_noinc> does not. For historical reasons, C<newRV> is a synonym for C<newRV_inc>. Once you have a reference, you can use the following macro to dereference the reference: SvRV(SV*) then call the appropriate routines, casting the returned C<SV*> to either an C<AV*> or C<HV*>, if required. To determine if an SV is a reference, you can use the following macro: SvROK(SV*) To discover what type of value the reference refers to, use the following macro and then check the return value. SvTYPE(SvRV(SV*)) The most useful types that will be returned are: SVt_IV Scalar SVt_NV Scalar SVt_PV Scalar SVt_RV Scalar SVt_PVAV Array SVt_PVHV Hash SVt_PVCV Code SVt_PVGV Glob (possible a file handle) SVt_PVMG Blessed or Magical Scalar See the sv.h header file for more details. =head2 Blessed References and Class Objects References are also used to support object-oriented programming. In the OO lexicon, an object is simply a reference that has been blessed into a package (or class). Once blessed, the programmer may now use the reference to access the various methods in the class. A reference can be blessed into a package with the following function: SV* sv_bless(SV* sv, HV* stash); The C<sv> argument must be a reference. The C<stash> argument specifies which class the reference will belong to. See L<Stashes and Globs> for information on converting class names into stashes. /* Still under construction */ Upgrades rv to reference if not already one. Creates new SV for rv to point to. If C<classname> is non-null, the SV is blessed into the specified class. SV is returned. SV* newSVrv(SV* rv, const char* classname); Copies integer or double into an SV whose reference is C<rv>. SV is blessed if C<classname> is non-null. SV* sv_setref_iv(SV* rv, const char* classname, IV iv); SV* sv_setref_nv(SV* rv, const char* classname, NV iv); Copies the pointer value (I<the address, not the string!>) into an SV whose reference is rv. SV is blessed if C<classname> is non-null. SV* sv_setref_pv(SV* rv, const char* classname, PV iv); Copies string into an SV whose reference is C<rv>. Set length to 0 to let Perl calculate the string length. SV is blessed if C<classname> is non-null. SV* sv_setref_pvn(SV* rv, const char* classname, PV iv, STRLEN length); Tests whether the SV is blessed into the specified class. It does not check inheritance relationships. int sv_isa(SV* sv, const char* name); Tests whether the SV is a reference to a blessed object. int sv_isobject(SV* sv); Tests whether the SV is derived from the specified class. SV can be either a reference to a blessed object or a string containing a class name. This is the function implementing the C<UNIVERSAL::isa> functionality. bool sv_derived_from(SV* sv, const char* name); To check if you've got an object derived from a specific class you have to write: if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... } =head2 Creating New Variables To create a new Perl variable with an undef value which can be accessed from your Perl script, use the following routines, depending on the variable type. SV* get_sv("package::varname", TRUE); AV* get_av("package::varname", TRUE); HV* get_hv("package::varname", TRUE); Notice the use of TRUE as the second parameter. The new variable can now be set, using the routines appropriate to the data type. There are additional macros whose values may be bitwise OR'ed with the C<TRUE> argument to enable certain extra features. Those bits are: GV_ADDMULTI Marks the variable as multiply defined, thus preventing the "Name <varname> used only once: possible typo" warning. GV_ADDWARN Issues the warning "Had to create <varname> unexpectedly" if the variable did not exist before the function was called. If you do not specify a package name, the variable is created in the current package. =head2 Reference Counts and Mortality Perl uses an reference count-driven garbage collection mechanism. SVs, AVs, or HVs (xV for short in the following) start their life with a reference count of 1. If the reference count of an xV ever drops to 0, then it will be destroyed and its memory made available for reuse. This normally doesn't happen at the Perl level unless a variable is undef'ed or the last variable holding a reference to it is changed or overwritten. At the internal level, however, reference counts can be manipulated with the following macros: int SvREFCNT(SV* sv); SV* SvREFCNT_inc(SV* sv); void SvREFCNT_dec(SV* sv); However, there is one other function which manipulates the reference count of its argument. The C<newRV_inc> function, you will recall, creates a reference to the specified argument. As a side effect, it increments the argument's reference count. If this is not what you want, use C<newRV_noinc> instead. For example, imagine you want to return a reference from an XSUB function. Inside the XSUB routine, you create an SV which initially has a reference count of one. Then you call C<newRV_inc>, passing it the just-created SV. This returns the reference as a new SV, but the reference count of the SV you passed to C<newRV_inc> has been incremented to two. Now you return the reference from the XSUB routine and forget about the SV. But Perl hasn't! Whenever the returned reference is destroyed, the reference count of the original SV is decreased to one and nothing happens. The SV will hang around without any way to access it until Perl itself terminates. This is a memory leak. The correct procedure, then, is to use C<newRV_noinc> instead of C<newRV_inc>. Then, if and when the last reference is destroyed, the reference count of the SV will go to zero and it will be destroyed, stopping any memory leak. There are some convenience functions available that can help with the destruction of xVs. These functions introduce the concept of "mortality". An xV that is mortal has had its reference count marked to be decremented, but not actually decremented, until "a short time later". Generally the term "short time later" means a single Perl statement, such as a call to an XSUB function. The actual determinant for when mortal xVs have their reference count decremented depends on two macros, SAVETMPS and FREETMPS. See L<perlcall> and L<perlxs> for more details on these macros. "Mortalization" then is at its simplest a deferred C<SvREFCNT_dec>. However, if you mortalize a variable twice, the reference count will later be decremented twice. You should be careful about creating mortal variables. Strange things can happen if you make the same value mortal within multiple contexts, or if you make a variable mortal multiple times. To create a mortal variable, use the functions: SV* sv_newmortal() SV* sv_2mortal(SV*) SV* sv_mortalcopy(SV*) The first call creates a mortal SV, the second converts an existing SV to a mortal SV (and thus defers a call to C<SvREFCNT_dec>), and the third creates a mortal copy of an existing SV. The mortal routines are not just for SVs -- AVs and HVs can be made mortal by passing their address (type-casted to C<SV*>) to the C<sv_2mortal> or C<sv_mortalcopy> routines. =head2 Stashes and Globs A "stash" is a hash that contains all of the different objects that are contained within a package. Each key of the stash is a symbol name (shared by all the different types of objects that have the same name), and each value in the hash table is a GV (Glob Value). This GV in turn contains references to the various objects of that name, including (but not limited to) the following: Scalar Value Array Value Hash Value I/O Handle Format Subroutine There is a single stash called "PL_defstash" that holds the items that exist in the "main" package. To get at the items in other packages, append the string "::" to the package name. The items in the "Foo" package are in the stash "Foo::" in PL_defstash. The items in the "Bar::Baz" package are in the stash "Baz::" in "Bar::"'s stash. To get the stash pointer for a particular package, use the function: HV* gv_stashpv(const char* name, I32 create) HV* gv_stashsv(SV*, I32 create) The first function takes a literal string, the second uses the string stored in the SV. Remember that a stash is just a hash table, so you get back an C<HV*>. The C<create> flag will create a new package if it is set. The name that C<gv_stash*v> wants is the name of the package whose symbol table you want. The default package is called C<main>. If you have multiply nested packages, pass their names to C<gv_stash*v>, separated by C<::> as in the Perl language itself. Alternately, if you have an SV that is a blessed reference, you can find out the stash pointer by using: HV* SvSTASH(SvRV(SV*)); then use the following to get the package name itself: char* HvNAME(HV* stash); If you need to bless or re-bless an object you can use the following function: SV* sv_bless(SV*, HV* stash) where the first argument, an C<SV*>, must be a reference, and the second argument is a stash. The returned C<SV*> can now be used in the same way as any other SV. For more information on references and blessings, consult L<perlref>. =head2 Double-Typed SVs Scalar variables normally contain only one type of value, an integer, double, pointer, or reference. Perl will automatically convert the actual scalar data from the stored type into the requested type. Some scalar variables contain more than one type of scalar data. For example, the variable C<$!> contains either the numeric value of C<errno> or its string equivalent from either C<strerror> or C<sys_errlist[]>. To force multiple data values into an SV, you must do two things: use the C<sv_set*v> routines to add the additional scalar type, then set a flag so that Perl will believe it contains more than one type of data. The four macros to set the flags are: SvIOK_on SvNOK_on SvPOK_on SvROK_on The particular macro you must use depends on which C<sv_set*v> routine you called first. This is because every C<sv_set*v> routine turns on only the bit for the particular type of data being set, and turns off all the rest. For example, to create a new Perl variable called "dberror" that contains both the numeric and descriptive string error values, you could use the following code: extern int dberror; extern char *dberror_list; SV* sv = get_sv("dberror", TRUE); sv_setiv(sv, (IV) dberror); sv_setpv(sv, dberror_list[dberror]); SvIOK_on(sv); If the order of C<sv_setiv> and C<sv_setpv> had been reversed, then the macro C<SvPOK_on> would need to be called instead of C<SvIOK_on>. =head2 Magic Variables [This section still under construction. Ignore everything here. Post no bills. Everything not permitted is forbidden.] Any SV may be magical, that is, it has special features that a normal SV does not have. These features are stored in the SV structure in a linked list of C<struct magic>'s, typedef'ed to C<MAGIC>. struct magic { MAGIC* mg_moremagic; MGVTBL* mg_virtual; U16 mg_private; char mg_type; U8 mg_flags; SV* mg_obj; char* mg_ptr; I32 mg_len; }; Note this is current as of patchlevel 0, and could change at any time. =head2 Assigning Magic Perl adds magic to an SV using the sv_magic function: void sv_magic(SV* sv, SV* obj, int how, const char* name, I32 namlen); The C<sv> argument is a pointer to the SV that is to acquire a new magical feature. If C<sv> is not already magical, Perl uses the C<SvUPGRADE> macro to set the C<SVt_PVMG> flag for the C<sv>. Perl then continues by adding it to the beginning of the linked list of magical features. Any prior entry of the same type of magic is deleted. Note that this can be overridden, and multiple instances of the same type of magic can be associated with an SV. The C<name> and C<namlen> arguments are used to associate a string with the magic, typically the name of a variable. C<namlen> is stored in the C<mg_len> field and if C<name> is non-null and C<namlen> >= 0 a malloc'd copy of the name is stored in C<mg_ptr> field. The sv_magic function uses C<how> to determine which, if any, predefined "Magic Virtual Table" should be assigned to the C<mg_virtual> field. See the "Magic Virtual Table" section below. The C<how> argument is also stored in the C<mg_type> field. The C<obj> argument is stored in the C<mg_obj> field of the C<MAGIC> structure. If it is not the same as the C<sv> argument, the reference count of the C<obj> object is incremented. If it is the same, or if the C<how> argument is "#", or if it is a NULL pointer, then C<obj> is merely stored, without the reference count being incremented. There is also a function to add magic to an C<HV>: void hv_magic(HV *hv, GV *gv, int how); This simply calls C<sv_magic> and coerces the C<gv> argument into an C<SV>. To remove the magic from an SV, call the function sv_unmagic: void sv_unmagic(SV *sv, int type); The C<type> argument should be equal to the C<how> value when the C<SV> was initially made magical. =head2 Magic Virtual Tables The C<mg_virtual> field in the C<MAGIC> structure is a pointer to a C<MGVTBL>, which is a structure of function pointers and stands for "Magic Virtual Table" to handle the various operations that might be applied to that variable. The C<MGVTBL> has five pointers to the following routine types: int (*svt_get)(SV* sv, MAGIC* mg); int (*svt_set)(SV* sv, MAGIC* mg); U32 (*svt_len)(SV* sv, MAGIC* mg); int (*svt_clear)(SV* sv, MAGIC* mg); int (*svt_free)(SV* sv, MAGIC* mg); This MGVTBL structure is set at compile-time in C<perl.h> and there are currently 19 types (or 21 with overloading turned on). These different structures contain pointers to various routines that perform additional actions depending on which function is being called. Function pointer Action taken ---------------- ------------ svt_get Do something after the value of the SV is retrieved. svt_set Do something after the SV is assigned a value. svt_len Report on the SV's length. svt_clear Clear something the SV represents. svt_free Free any extra storage associated with the SV. For instance, the MGVTBL structure called C<vtbl_sv> (which corresponds to an C<mg_type> of '\0') contains: { magic_get, magic_set, magic_len, 0, 0 } Thus, when an SV is determined to be magical and of type '\0', if a get operation is being performed, the routine C<magic_get> is called. All the various routines for the various magical types begin with C<magic_>. NOTE: the magic routines are not considered part of the Perl API, and may not be exported by the Perl library. The current kinds of Magic Virtual Tables are: mg_type MGVTBL Type of magic ------- ------ ---------------------------- \0 vtbl_sv Special scalar variable A vtbl_amagic %OVERLOAD hash a vtbl_amagicelem %OVERLOAD hash element c (none) Holds overload table (AMT) on stash B vtbl_bm Boyer-Moore (fast string search) E vtbl_env %ENV hash e vtbl_envelem %ENV hash element f vtbl_fm Formline ('compiled' format) g vtbl_mglob m//g target / study()ed string I vtbl_isa @ISA array i vtbl_isaelem @ISA array element k vtbl_nkeys scalar(keys()) lvalue L (none) Debugger %_<filename l vtbl_dbline Debugger %_<filename element o vtbl_collxfrm Locale transformation P vtbl_pack Tied array or hash p vtbl_packelem Tied array or hash element q vtbl_packelem Tied scalar or handle S vtbl_sig %SIG hash s vtbl_sigelem %SIG hash element t vtbl_taint Taintedness U vtbl_uvar Available for use by extensions v vtbl_vec vec() lvalue x vtbl_substr substr() lvalue y vtbl_defelem Shadow "foreach" iterator variable / smart parameter vivification * vtbl_glob GV (typeglob) # vtbl_arylen Array length ($#ary) . vtbl_pos pos() lvalue ~ (none) Available for use by extensions When an uppercase and lowercase letter both exist in the table, then the uppercase letter is used to represent some kind of composite type (a list or a hash), and the lowercase letter is used to represent an element of that composite type. The '~' and 'U' magic types are defined specifically for use by extensions and will not be used by perl itself. Extensions can use '~' magic to 'attach' private information to variables (typically objects). This is especially useful because there is no way for normal perl code to corrupt this private information (unlike using extra elements of a hash object). Similarly, 'U' magic can be used much like tie() to call a C function any time a scalar's value is used or changed. The C<MAGIC>'s C<mg_ptr> field points to a C<ufuncs> structure: struct ufuncs { I32 (*uf_val)(IV, SV*); I32 (*uf_set)(IV, SV*); IV uf_index; }; When the SV is read from or written to, the C<uf_val> or C<uf_set> function will be called with C<uf_index> as the first arg and a pointer to the SV as the second. A simple example of how to add 'U' magic is shown below. Note that the ufuncs structure is copied by sv_magic, so you can safely allocate it on the stack. void Umagic(sv) SV *sv; PREINIT: struct ufuncs uf; CODE: uf.uf_val = &my_get_fn; uf.uf_set = &my_set_fn; uf.uf_index = 0; sv_magic(sv, 0, 'U', (char*)&uf, sizeof(uf)); Note that because multiple extensions may be using '~' or 'U' magic, it is important for extensions to take extra care to avoid conflict. Typically only using the magic on objects blessed into the same class as the extension is sufficient. For '~' magic, it may also be appropriate to add an I32 'signature' at the top of the private data area and check that. Also note that the C<sv_set*()> and C<sv_cat*()> functions described earlier do B<not> invoke 'set' magic on their targets. This must be done by the user either by calling the C<SvSETMAGIC()> macro after calling these functions, or by using one of the C<sv_set*_mg()> or C<sv_cat*_mg()> functions. Similarly, generic C code must call the C<SvGETMAGIC()> macro to invoke any 'get' magic if they use an SV obtained from external sources in functions that don't handle magic. See L<perlapi> for a description of these functions. For example, calls to the C<sv_cat*()> functions typically need to be followed by C<SvSETMAGIC()>, but they don't need a prior C<SvGETMAGIC()> since their implementation handles 'get' magic. =head2 Finding Magic MAGIC* mg_find(SV*, int type); /* Finds the magic pointer of that type */ This routine returns a pointer to the C<MAGIC> structure stored in the SV. If the SV does not have that magical feature, C<NULL> is returned. Also, if the SV is not of type SVt_PVMG, Perl may core dump. int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen); This routine checks to see what types of magic C<sv> has. If the mg_type field is an uppercase letter, then the mg_obj is copied to C<nsv>, but the mg_type field is changed to be the lowercase letter. =head2 Understanding the Magic of Tied Hashes and Arrays Tied hashes and arrays are magical beasts of the 'P' magic type. WARNING: As of the 5.004 release, proper usage of the array and hash access functions requires understanding a few caveats. Some of these caveats are actually considered bugs in the API, to be fixed in later releases, and are bracketed with [MAYCHANGE] below. If you find yourself actually applying such information in this section, be aware that the behavior may change in the future, umm, without warning. The perl tie function associates a variable with an object that implements the various GET, SET etc methods. To perform the equivalent of the perl tie function from an XSUB, you must mimic this behaviour. The code below carries out the necessary steps - firstly it creates a new hash, and then creates a second hash which it blesses into the class which will implement the tie methods. Lastly it ties the two hashes together, and returns a reference to the new tied hash. Note that the code below does NOT call the TIEHASH method in the MyTie class - see L<Calling Perl Routines from within C Programs> for details on how to do this. SV* mytie() PREINIT: HV *hash; HV *stash; SV *tie; CODE: hash = newHV(); tie = newRV_noinc((SV*)newHV()); stash = gv_stashpv("MyTie", TRUE); sv_bless(tie, stash); hv_magic(hash, tie, 'P'); RETVAL = newRV_noinc(hash); OUTPUT: RETVAL The C<av_store> function, when given a tied array argument, merely copies the magic of the array onto the value to be "stored", using C<mg_copy>. It may also return NULL, indicating that the value did not actually need to be stored in the array. [MAYCHANGE] After a call to C<av_store> on a tied array, the caller will usually need to call C<mg_set(val)> to actually invoke the perl level "STORE" method on the TIEARRAY object. If C<av_store> did return NULL, a call to C<SvREFCNT_dec(val)> will also be usually necessary to avoid a memory leak. [/MAYCHANGE] The previous paragraph is applicable verbatim to tied hash access using the C<hv_store> and C<hv_store_ent> functions as well. C<av_fetch> and the corresponding hash functions C<hv_fetch> and C<hv_fetch_ent> actually return an undefined mortal value whose magic has been initialized using C<mg_copy>. Note the value so returned does not need to be deallocated, as it is already mortal. [MAYCHANGE] But you will need to call C<mg_get()> on the returned value in order to actually invoke the perl level "FETCH" method on the underlying TIE object. Similarly, you may also call C<mg_set()> on the return value after possibly assigning a suitable value to it using C<sv_setsv>, which will invoke the "STORE" method on the TIE object. [/MAYCHANGE] [MAYCHANGE] In other words, the array or hash fetch/store functions don't really fetch and store actual values in the case of tied arrays and hashes. They merely call C<mg_copy> to attach magic to the values that were meant to be "stored" or "fetched". Later calls to C<mg_get> and C<mg_set> actually do the job of invoking the TIE methods on the underlying objects. Thus the magic mechanism currently implements a kind of lazy access to arrays and hashes. Currently (as of perl version 5.004), use of the hash and array access functions requires the user to be aware of whether they are operating on "normal" hashes and arrays, or on their tied variants. The API may be changed to provide more transparent access to both tied and normal data types in future versions. [/MAYCHANGE] You would do well to understand that the TIEARRAY and TIEHASH interfaces are mere sugar to invoke some perl method calls while using the uniform hash and array syntax. The use of this sugar imposes some overhead (typically about two to four extra opcodes per FETCH/STORE operation, in addition to the creation of all the mortal variables required to invoke the methods). This overhead will be comparatively small if the TIE methods are themselves substantial, but if they are only a few statements long, the overhead will not be insignificant. =head2 Localizing changes Perl has a very handy construction { local $var = 2; ... } This construction is I<approximately> equivalent to { my $oldvar = $var; $var = 2; ... $var = $oldvar; } The biggest difference is that the first construction would reinstate the initial value of $var, irrespective of how control exits the block: C<goto>, C<return>, C<die>/C<eval> etc. It is a little bit more efficient as well. There is a way to achieve a similar task from C via Perl API: create a I<pseudo-block>, and arrange for some changes to be automatically undone at the end of it, either explicit, or via a non-local exit (via die()). A I<block>-like construct is created by a pair of C<ENTER>/C<LEAVE> macros (see L<perlcall/"Returning a Scalar">). Such a construct may be created specially for some important localized task, or an existing one (like boundaries of enclosing Perl subroutine/block, or an existing pair for freeing TMPs) may be used. (In the second case the overhead of additional localization must be almost negligible.) Note that any XSUB is automatically enclosed in an C<ENTER>/C<LEAVE> pair. Inside such a I<pseudo-block> the following service is available: =over =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 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. C<s> must be a pointer of a type which survives conversion to C<SV*> and back, C 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>, which should (?) be used instead. =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 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. =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. =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 =item C<SV* save_scalar(GV *gv)> Equivalent to Perl code C<local $gv>. =item C<AV* save_ary(GV *gv)> =item C<HV* save_hash(GV *gv)> Similar to C<save_scalar>, but localize C<@gv> and C<%gv>. =item C<void save_item(SV *item)> Duplicates the current value of C<SV>, on the exit from the current C<ENTER>/C<LEAVE> I<pseudo-block> will restore the value of C<SV> using the stored value. =item C<void save_list(SV **sarg, I32 maxsarg)> A variant of C<save_item> which takes multiple arguments via an array C<sarg> of C<SV*> of length C<maxsarg>. =item C<SV* save_svref(SV **sptr)> Similar to C<save_scalar>, but will reinstate a C<SV *>. =item C<void save_aptr(AV **aptr)> =item C<void save_hptr(HV **hptr)> Similar to C<save_svref>, but localize C<AV *> and C<HV *>. =back The C<Alias> module implements localization of the basic types within the I<caller's scope>. People who are interested in how to localize things in the containing scope should take a look there too. =head1 Subroutines =head2 XSUBs and the Argument Stack The XSUB mechanism is a simple way for Perl programs to access C subroutines. An XSUB routine will have a stack that contains the arguments from the Perl program, and a way to map from the Perl data structures to a C equivalent. The stack arguments are accessible through the C<ST(n)> macro, which returns the C<n>'th stack argument. Argument 0 is the first argument passed in the Perl subroutine call. These arguments are C<SV*>, and can be used anywhere an C<SV*> is used. Most of the time, output from the C routine can be handled through use of the RETVAL and OUTPUT directives. However, there are some cases where the argument stack is not already long enough to handle all the return values. An example is the POSIX tzname() call, which takes no arguments, but returns two, the local time zone's standard and summer time abbreviations. To handle this situation, the PPCODE directive is used and the stack is extended using the macro: EXTEND(SP, num); where C<SP> is the macro that represents the local copy of the stack pointer, and C<num> is the number of elements the stack should be extended by. Now that there is room on the stack, values can be pushed on it using the macros to push IVs, doubles, strings, and SV pointers respectively: PUSHi(IV) PUSHn(double) PUSHp(char*, I32) PUSHs(SV*) And now the Perl program calling C<tzname>, the two values will be assigned as in: ($standard_abbrev, $summer_abbrev) = POSIX::tzname; An alternate (and possibly simpler) method to pushing values on the stack is to use the macros: XPUSHi(IV) XPUSHn(double) XPUSHp(char*, I32) XPUSHs(SV*) These macros automatically adjust the stack for you, if needed. Thus, you do not need to call C<EXTEND> to extend the stack. For more information, consult L<perlxs> and L<perlxstut>. =head2 Calling Perl Routines from within C Programs There are four routines that can be used to call a Perl subroutine from within a C program. These four are: I32 call_sv(SV*, I32); I32 call_pv(const char*, I32); I32 call_method(const char*, I32); I32 call_argv(const char*, I32, register char**); The routine most often used is C<call_sv>. The C<SV*> argument contains either the name of the Perl subroutine to be called, or a reference to the subroutine. The second argument consists of flags that control the context in which the subroutine is called, whether or not the subroutine is being passed arguments, how errors should be trapped, and how to treat return values. All four routines return the number of arguments that the subroutine returned on the Perl stack. These routines used to be called C<perl_call_sv> etc., before Perl v5.6.0, but those names are now deprecated; macros of the same name are provided for compatibility. When using any of these routines (except C<call_argv>), the programmer must manipulate the Perl stack. These include the following macros and functions: dSP SP PUSHMARK() PUTBACK SPAGAIN ENTER SAVETMPS FREETMPS LEAVE XPUSH*() POP*() For a detailed description of calling conventions from C to Perl, consult L<perlcall>. =head2 Memory Allocation All memory meant to be used with the Perl API functions should be manipulated using the macros described in this section. The macros provide the necessary transparency between differences in the actual malloc implementation that is used within perl. It is suggested that you enable the version of malloc that is distributed with Perl. It keeps pools of various sizes of unallocated memory in order to satisfy allocation requests more quickly. However, on some platforms, it may cause spurious malloc or free errors. New(x, pointer, number, type); Newc(x, pointer, number, type, cast); Newz(x, pointer, number, type); These three macros are used to initially allocate memory. The first argument C<x> was a "magic cookie" that was used to keep track of who called the macro, to help when debugging memory problems. However, the current code makes no use of this feature (most Perl developers now use run-time memory checkers), so this argument can be any number. The second argument C<pointer> should be the name of a variable that will point to the newly allocated memory. The third and fourth arguments C<number> and C<type> specify how many of the specified type of data structure should be allocated. The argument C<type> is passed to C<sizeof>. The final argument to C<Newc>, C<cast>, should be used if the C<pointer> argument is different from the C<type> argument. Unlike the C<New> and C<Newc> macros, the C<Newz> macro calls C<memzero> to zero out all the newly allocated memory. Renew(pointer, number, type); Renewc(pointer, number, type, cast); Safefree(pointer) These three macros are used to change a memory buffer size or to free a piece of memory no longer needed. The arguments to C<Renew> and C<Renewc> match those of C<New> and C<Newc> with the exception of not needing the "magic cookie" argument. Move(source, dest, number, type); Copy(source, dest, number, type); Zero(dest, number, type); These three macros are used to move, copy, or zero out previously allocated memory. The C<source> and C<dest> arguments point to the source and destination starting points. Perl will move, copy, or zero out C<number> instances of the size of the C<type> data structure (using the C<sizeof> function). =head2 PerlIO The most recent development releases of Perl has been experimenting with removing Perl's dependency on the "normal" standard I/O suite and allowing other stdio implementations to be used. This involves creating a new abstraction layer that then calls whichever implementation of stdio Perl was compiled with. All XSUBs should now use the functions in the PerlIO abstraction layer and not make any assumptions about what kind of stdio is being used. For a complete description of the PerlIO abstraction, consult L<perlapio>. =head2 Putting a C value on Perl stack A lot of opcodes (this is an elementary operation in the internal perl stack machine) put an SV* on the stack. However, as an optimization the corresponding SV is (usually) not recreated each time. The opcodes reuse specially assigned SVs (I<target>s) which are (as a corollary) not constantly freed/created. Each of the targets is created only once (but see L<Scratchpads and recursion> below), and when an opcode needs to put an integer, a double, or a string on stack, it just sets the corresponding parts of its I<target> and puts the I<target> on stack. The macro to put this target on stack is C<PUSHTARG>, and it is directly used in some opcodes, as well as indirectly in zillions of others, which use it via C<(X)PUSH[pni]>. =head2 Scratchpads The question remains on when the SVs which are I<target>s for opcodes are created. The answer is that they are created when the current unit -- a subroutine or a file (for opcodes for statements outside of subroutines) -- is compiled. During this time a special anonymous Perl array is created, which is called a scratchpad for the current unit. A scratchpad keeps SVs which are lexicals for the current unit and are targets for opcodes. One can deduce that an SV lives on a scratchpad by looking on its flags: lexicals have C<SVs_PADMY> set, and I<target>s have C<SVs_PADTMP> set. The correspondence between OPs and I<target>s is not 1-to-1. Different OPs in the compile tree of the unit can use the same target, if this would not conflict with the expected life of the temporary. =head2 Scratchpads and recursion In fact it is not 100% true that a compiled unit contains a pointer to the scratchpad AV. In fact it contains a pointer to an AV of (initially) one element, and this element is the scratchpad AV. Why do we need an extra level of indirection? The answer is B<recursion>, and maybe (sometime soon) B<threads>. Both these can create several execution pointers going into the same subroutine. For the subroutine-child not write over the temporaries for the subroutine-parent (lifespan of which covers the call to the child), the parent and the child should have different scratchpads. (I<And> the lexicals should be separate anyway!) So each subroutine is born with an array of scratchpads (of length 1). On each entry to the subroutine it is checked that the current depth of the recursion is not more than the length of this array, and if it is, new scratchpad is created and pushed into the array. The I<target>s on this scratchpad are C<undef>s, but they are already marked with correct flags. =head1 Compiled code =head2 Code tree Here we describe the internal form your code is converted to by Perl. Start with a simple example: $a = $b + $c; This is converted to a tree similar to this one: assign-to / \ + $a / \ $b $c (but slightly more complicated). This tree reflects the way Perl parsed your code, but has nothing to do with the execution order. There is an additional "thread" going through the nodes of the tree which shows the order of execution of the nodes. In our simplified example above it looks like: $b ---> $c ---> + ---> $a ---> assign-to But with the actual compile tree for C<$a = $b + $c> it is different: some nodes I<optimized away>. As a corollary, though the actual tree contains more nodes than our simplified example, the execution order is the same as in our example. =head2 Examining the tree If you have your perl compiled for debugging (usually done with C<-D optimize=-g> on C<Configure> command line), you may examine the compiled tree by specifying C<-Dx> on the Perl command line. The output takes several lines per node, and for C<$b+$c> it looks like this: 5 TYPE = add ===> 6 TARG = 1 FLAGS = (SCALAR,KIDS) { TYPE = null ===> (4) (was rv2sv) FLAGS = (SCALAR,KIDS) { 3 TYPE = gvsv ===> 4 FLAGS = (SCALAR) GV = main::b } } { TYPE = null ===> (5) (was rv2sv) FLAGS = (SCALAR,KIDS) { 4 TYPE = gvsv ===> 5 FLAGS = (SCALAR) GV = main::c } } This tree has 5 nodes (one per C<TYPE> specifier), only 3 of them are not optimized away (one per number in the left column). The immediate children of the given node correspond to C<{}> pairs on the same level of indentation, thus this listing corresponds to the tree: add / \ null null | | gvsv gvsv The execution order is indicated by C<===E<gt>> marks, thus it is C<3 4 5 6> (node C<6> is not included into above listing), i.e., C<gvsv gvsv add whatever>. =head2 Compile pass 1: check routines The tree is created by the I<pseudo-compiler> while yacc code feeds it the constructions it recognizes. Since yacc works bottom-up, so does the first pass of perl compilation. What makes this pass interesting for perl developers is that some optimization may be performed on this pass. This is optimization by so-called I<check routines>. The correspondence between node names and corresponding check routines is described in F<opcode.pl> (do not forget to run C<make regen_headers> if you modify this file). A check routine is called when the node is fully constructed except for the execution-order thread. Since at this time there are no back-links to the currently constructed node, one can do most any operation to the top-level node, including freeing it and/or creating new nodes above/below it. The check routine returns the node which should be inserted into the tree (if the top-level node was not modified, check routine returns its argument). By convention, check routines have names C<ck_*>. They are usually called from C<new*OP> subroutines (or C<convert>) (which in turn are called from F<perly.y>). =head2 Compile pass 1a: constant folding Immediately after the check routine is called the returned node is checked for being compile-time executable. If it is (the value is judged to be constant) it is immediately executed, and a I<constant> node with the "return value" of the corresponding subtree is substituted instead. The subtree is deleted. If constant folding was not performed, the execution-order thread is created. =head2 Compile pass 2: context propagation When a context for a part of compile tree is known, it is propagated down through the tree. At this time the context can have 5 values (instead of 2 for runtime context): void, boolean, scalar, list, and lvalue. In contrast with the pass 1 this pass is processed from top to bottom: a node's context determines the context for its children. Additional context-dependent optimizations are performed at this time. Since at this moment the compile tree contains back-references (via "thread" pointers), nodes cannot be free()d now. To allow optimized-away nodes at this stage, such nodes are null()ified instead of free()ing (i.e. their type is changed to OP_NULL). =head2 Compile pass 3: peephole optimization After the compile tree for a subroutine (or for an C<eval> or a file) is created, an additional pass over the code is performed. This pass is neither top-down or bottom-up, but in the execution order (with additional complications for conditionals). These optimizations are done in the subroutine peep(). Optimizations performed at this stage are subject to the same restrictions as in the pass 2. =head1 How multiple interpreters and concurrency are supported WARNING: This information is subject to radical changes prior to the Perl 5.6 release. Use with caution. =head2 Background and PERL_IMPLICIT_CONTEXT The Perl interpreter can be regarded as a closed box: it has an API for feeding it code or otherwise making it do things, but it also has functions for its own use. This smells a lot like an object, and there are ways for you to build Perl so that you can have multiple interpreters, with one interpreter represented either as a C++ object, a C structure, or inside a thread. The thread, the C structure, or the C++ object will contain all the context, the state of that interpreter. Three macros control the major Perl build flavors: MULTIPLICITY, USE_THREADS and PERL_OBJECT. The MULTIPLICITY build has a C structure that packages all the interpreter state, there is a similar thread-specific data structure under USE_THREADS, and the PERL_OBJECT build has a C++ class to maintain interpreter state. In all three cases, PERL_IMPLICIT_CONTEXT is also normally defined, and enables the support for passing in a "hidden" first argument that represents all three data structures. All this obviously requires a way for the Perl internal functions to be C++ methods, subroutines taking some kind of structure as the first argument, or subroutines taking nothing as the first argument. To enable these three very different ways of building the interpreter, the Perl source (as it does in so many other situations) makes heavy use of macros and subroutine naming conventions. First problem: deciding which functions will be public API functions and which will be private. All functions whose names begin C<S_> are private (think "S" for "secret" or "static"). All other functions begin with "Perl_", but just because a function begins with "Perl_" does not mean it is part of the API. 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 fo r your extension), send mail via L<perlbug> explaining why you think it should be. (L<perlapi> itself is generated by embed.pl, a Perl script that generates significant portions of the Perl source code. It has a list of almost all the functions defined by the Perl interpreter along with their calling characteristics and some flags. Functions that are part of the public API are marked with an 'A' in its flags.) Second problem: there must be a syntax so that the same subroutine declarations and calls can pass a structure as their first argument, or pass nothing. To solve this, the subroutines are named and declared in a particular way. Here's a typical start of a static function used within the Perl guts: STATIC void S_incline(pTHX_ char *s) STATIC becomes "static" in C, and is #define'd to nothing in C++. A public function (i.e. part of the internal API, but not necessarily sanctioned for use in extensions) begins like this: void Perl_sv_setsv(pTHX_ SV* dsv, SV* ssv) C<pTHX_> is one of a number of macros (in perl.h) that hide the details of the interpreter's context. THX stands for "thread", "this", or "thingy", as the case may be. (And no, George Lucas is not involved. :-) The first character could be 'p' for a Brototype, 'a' for B<a>rgument, or 'd' for B<d>eclaration. When Perl is built without PERL_IMPLICIT_CONTEXT, there is no first argument containing the interpreter's context. The trailing underscore in the pTHX_ macro indicates that the macro expansion needs a comma after the context argument because other arguments follow it. If PERL_IMPLICIT_CONTEXT is not defined, pTHX_ will be ignored, and the subroutine is not prototyped to take the extra argument. The form of the macro without the trailing underscore is used when there are no additional explicit arguments. When a core function calls another, it must pass the context. This is normally hidden via macros. Consider C<sv_setsv>. It expands something like this: ifdef PERL_IMPLICIT_CONTEXT define sv_setsv(a,b) Perl_sv_setsv(aTHX_ a, b) /* can't do this for vararg functions, see below */ else define sv_setsv Perl_sv_setsv endif This works well, and means that XS authors can gleefully write: sv_setsv(foo, bar); and still have it work under all the modes Perl could have been compiled with. Under PERL_OBJECT in the core, that will translate to either: CPerlObj::Perl_sv_setsv(foo,bar); # in CPerlObj functions, # C++ takes care of 'this' or pPerl->Perl_sv_setsv(foo,bar); # in truly static functions, # see objXSUB.h Under PERL_OBJECT in extensions (aka PERL_CAPI), or under MULTIPLICITY/USE_THREADS w/ PERL_IMPLICIT_CONTEXT in both core and extensions, it will be: Perl_sv_setsv(aTHX_ foo, bar); # the canonical Perl "API" # for all build flavors This doesn't work so cleanly for varargs functions, though, as macros imply that the number of arguments is known in advance. Instead we either need to spell them out fully, passing C<aTHX_> as the first argument (the Perl core tends to do this with functions like Perl_warner), or use a context-free version. The context-free version of Perl_warner is called Perl_warner_nocontext, and does not take the extra argument. Instead it does dTHX; to get the context from thread-local storage. We C<#define warner Perl_warner_nocontext> so that extensions get source compatibility at the expense of performance. (Passing an arg is cheaper than grabbing it from thread-local storage.) You can ignore [pad]THX[xo] when browsing the Perl headers/sources. Those are strictly for use within the core. Extensions and embedders need only be aware of [pad]THX. =head2 How do I use all this in extensions? When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call any functions in the Perl API will need to pass the initial context argument somehow. The kicker is that you will need to write it in such a way that the extension still compiles when Perl hasn't been built with PERL_IMPLICIT_CONTEXT enabled. There are three ways to do this. First, the easy but inefficient way, which is also the default, in order to maintain source compatibility with extensions: whenever XSUB.h is #included, it redefines the aTHX and aTHX_ macros to call a function that will return the context. Thus, something like: sv_setsv(asv, bsv); in your extesion will translate to this when PERL_IMPLICIT_CONTEXT is in effect: Perl_sv_setsv(Perl_get_context(), asv, bsv); or to this otherwise: Perl_sv_setsv(asv, bsv); You have to do nothing new in your extension to get this; since the Perl library provides Perl_get_context(), it will all just work. The second, more efficient way is to use the following template for your Foo.xs: #define PERL_NO_GET_CONTEXT /* we want efficiency */ #include "EXTERN.h" #include "perl.h" #include "XSUB.h" static my_private_function(int arg1, int arg2); static SV * my_private_function(int arg1, int arg2) { dTHX; /* fetch context */ ... call many Perl API functions ... } [... etc ...] MODULE = Foo PACKAGE = Foo /* typical XSUB */ void my_xsub(arg) int arg CODE: my_private_function(arg, 10); Note that the only two changes from the normal way of writing an extension is the addition of a C<#define PERL_NO_GET_CONTEXT> before including the Perl headers, followed by a C<dTHX;> declaration at the start of every function that will call the Perl API. (You'll know which functions need this, because the C compiler will complain that there's an undeclared identifier in those functions.) No changes are needed for the XSUBs themselves, because the XS() macro is correctly defined to pass in the implicit context if needed. The third, even more efficient way is to ape how it is done within the Perl guts: #define PERL_NO_GET_CONTEXT /* we want efficiency */ #include "EXTERN.h" #include "perl.h" #include "XSUB.h" /* pTHX_ only needed for functions that call Perl API */ static my_private_function(pTHX_ int arg1, int arg2); static SV * my_private_function(pTHX_ int arg1, int arg2) { /* dTHX; not needed here, because THX is an argument */ ... call Perl API functions ... } [... etc ...] MODULE = Foo PACKAGE = Foo /* typical XSUB */ void my_xsub(arg) int arg CODE: my_private_function(aTHX_ arg, 10); This implementation never has to fetch the context using a function call, since it is always passed as an extra argument. Depending on your needs for simplicity or efficiency, you may mix the previous two approaches freely. Never add a comma after C<pTHX> yourself--always use the form of the macro with the underscore for functions that take explicit arguments, or the form without the argument for functions with no explicit arguments. =head2 Future Plans and PERL_IMPLICIT_SYS Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything that the interpreter knows about itself and pass it around, so too are there plans to allow the interpreter to bundle up everything it knows about the environment it's running on. This is enabled with the PERL_IMPLICIT_SYS macro. Currently it only works with PERL_OBJECT, but is mostly there for MULTIPLICITY and USE_THREADS (see inside iperlsys.h). This allows the ability to provide an extra pointer (called the "host" environment) for all the system calls. This makes it possible for all the system stuff to maintain their own state, broken down into seven C structures. These are thin wrappers around the usual system calls (see win32/perllib.c) for the default perl executable, but for a more ambitious host (like the one that would do fork() emulation) all the extra work needed to pretend that different interpreters are actually different "processes", would be done here. The Perl engine/interpreter and the host are orthogonal entities. There could be one or more interpreters in a process, and one or more "hosts", with free association between them. =head1 AUTHORS Until May 1997, this document was maintained by Jeff Okamoto <okamoto@corp.hp.com>. It is now maintained as part of Perl itself by the Perl 5 Porters <perl5-porters@perl.org>. With lots of help and suggestions from Dean Roehrich, Malcolm Beattie, Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil Bowers, Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer, Stephen McCamant, and Gurusamy Sarathy. API Listing originally by Dean Roehrich <roehrich@cray.com>. Modifications to autogenerate the API listing (L<perlapi>) by Benjamin Stuhl. =head1 SEE ALSO perlapi(1), perlintern(1), perlxs(1), perlembed(1)