Linux Knowledge Base and Tutorial

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 com ments to the author below.

Variables

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. 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, respec tively. (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. 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*); "STRLEN" is an integer type (Size_t, usually defined as size_t in config.h) guaranteed to be large enough to rep 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 "sv_setpvn", "newSVpvn", or "newSVpv", or you may allow Perl to calculate the length by using "sv_setpv" or by specifying 0 as the second argu ment to "newSVpv". Be warned, though, that Perl will determine the string's length by using "strlen", which depends on the string terminating with a NUL character. The arguments of "sv_setpvf" are processed like "sprintf", and the formatted output becomes the value. "sv_vsetpvfn" is an analogue of "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 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 "sv_set*()" functions are not generic enough to oper ate on values that have "magic". See "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) might not be terminated by a NUL. Also remember that C doesn't allow you to safely say "foo(SvPV(s, len), len);". It might work with your com piler, 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 "sv_grow". Note that "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 "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 "SvPOK()" is true. be appended by using "strlen". In the second, you specify the length of the string yourself. The third function processes its arguments like "sprintf" and appends the formatted output. The fourth function works like "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 "sv_cat*()" functions are not generic enough to oper ate on values that have "magic". See "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 "defined", you can call: SvOK(SV*) The scalar "undef" value is stored in an SV instance called "PL_sv_undef". Its address can be used whenever an "SV*" is needed. However, you have to be careful when using &PL_sv_undef as a value in AVs or HVs (see "AVs, HVs and undefined values"). There are also the two values "PL_sv_yes" and "PL_sv_no", which contain boolean TRUE and FALSE values, respectively. Like "PL_sv_undef", their addresses can be used whenever an "SV*" is needed. Do not be fooled into thinking that "(SV *) 0" is the same as &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 oth erwise. Instead it has returned a NULL pointer which, somewhere down the line, will cause a segmentation viola tion, bus error, or just weird results. Change the zero to &PL_sv_undef in the first line and all will be well. by means of a little hack: instead of actually removing the characters, "sv_chop" sets the flag "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 "SvPVX") forward that many bytes, and adjusts "SvCUR" and "SvLEN". Hence, at this point, the start of the buffer that we allocated lives at "SvPVX(sv) - SvIV(sv)" in memory and the PV pointer is pointing into the middle of this allo cated 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 "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 "SvCUR" and "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 begin ning of the array; while "AvARRAY" points to the first element in the array that is visible from Perl, "AvALLOC" points to the real start of the C array. These are usually the same, but a "shift" operation can be carried out by increasing "AvARRAY" by one and decreasing "AvFILL" and "AvLEN". Again, the location of the real start of the C array only comes into play when freeing the array. See "av_shift" in av.c. What's Really Stored in an SV? Recall that the usual method of determining the type of scalar you have is to use "Sv*OK" macros. Because a scalar can be both a number and a string, usually these macros will always return TRUE and calling the "Sv*V" macros will do the appropriate conversion of string to integer/double or integer/double to string. If you really need to know if you have an integer, double, the data should be accessed via the FETCH routine rather than directly, so SvIOK is false. Another is when numeric conversion has occured and precision has been lost: only the 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 "Sv*V" macros. 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 popu lates it with SVs: AV* av_make(I32 num, SV **ptr); The second argument points to an array containing "num" "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 "av_unshift". This routine adds "num" elements at the front of the array with the "undef" value. You must then use "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 "av_len" function returns the highest index value in array (just like $#array in Perl). If the array is empty, -1 is returned. The "av_fetch" function returns the value at index "key", but if "lval" is non-zero, then "av_fetch" will store an undef value at that index. The "av_store" function stores the value "val" at index "key", and does not increment the reference count of "val". Thus the the array plus the array itself. The "av_extend" function extends the array so that it contains at least "key+1" elements. If "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 "Understanding the Magic of Tied Hashes and Arrays" for more information on how to use the array access func tions on tied arrays. 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 "klen" parameter is the length of the key being passed in (Note that you cannot pass 0 in as a value of "klen" to tell Perl to measure the length of the key). The "val" argument contains the SV pointer to the scalar being stored, and "hash" is the precomputed hash value (zero if you want "hv_store" to calculate it for you). The "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 "hv_fetch" will return as if the value had already existed. Remember that "hv_store" and "hv_fetch" return "SV**"'s and not just "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); 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 "SV*". However, once you have an "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", FALSE); This returns NULL if the variable does not exist. The hash algorithm is defined in the "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 distri bution of lower bits in the resulting hash value. See "Understanding the Magic of Tied Hashes and Arrays" for more information on how to use the hash access func tions on tied hashes. Hash API Extensions Beginning with version 5.004, the following functions are also supported: They also return and accept whole hash entries ("HE*"), making their use more efficient (since the hash number for a particular string doesn't have to be recomputed every time). See 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 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 "SV*"s: HeKEY(HE* he) HeKLEN(HE* he) Note that both "hv_store" and "hv_store_ent" do not incre ment the reference count of the stored "val", which is the caller's responsibility. If these functions return a NULL value, the caller will usually have to decrement the ref erence count of "val" to avoid a memory leak. 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 &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 &PL_sv_undef as a marker for indicating that an array element has not yet been initialized. Thus, "exists $av[0]" would be true for the above Perl code, but false for the array generated by the XS code. You can run into similar problems when you store &PL_sv_true or &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 vari ables &PL_sv_undef, &PL_sv_true and &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 &PL_sv_undef, but rather cre ate a new undefined value using the "newSV" function, for example: av_store( av, 42, newSV(0) ); hv_store( hv, "foo", 3, newSV(0), 0 ); 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 func tions: SV* newRV_inc((SV*) thing); SV* newRV_noinc((SV*) thing); The "thing" argument can be any of an "SV*", "AV*", or "HV*". The functions are identical except that "newRV_inc" increments the reference count of the "thing", while "newRV_noinc" does not. For historical reasons, "newRV" is a synonym for "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 "SV*" to either an "AV*" or "HV*", if required. To determine if an SV is a reference, you can use the fol lowing macro: SvROK(SV*) To discover what type of value the reference refers to, use the following macro and then check the return value. SVt_PVGV Glob (possible a file handle) SVt_PVMG Blessed or Magical Scalar See the sv.h header file for more details. Blessed References and Class Objects References are also used to support object-oriented pro gramming. 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 follow ing function: SV* sv_bless(SV* sv, HV* stash); The "sv" argument must be a reference value. The "stash" argument specifies which class the reference will belong to. See "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 "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 "rv". SV is blessed if "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 (the address, not the string!) into an SV whose reference is rv. SV is blessed if "classname" is non-null. SV* sv_setref_pv(SV* rv, const char* classname, PV iv); Copies string into an SV whose reference is "rv". Set length to 0 to let Perl calculate the string length. SV is blessed if "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. 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)) { ... } 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 "TRUE" argument to enable certain extra features. Those bits are: GV_ADDMULTI Marks the variable as multiply defined, thus prevent ing 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 cre ated in the current package. Reference Counts and Mortality Perl uses a reference count-driven garbage collection mechanism. SVs, AVs, or HVs (xV for short in the follow ing) 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 fied argument. As a side effect, it increments the argu ment's reference count. If this is not what you want, use "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 "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 "newRV_inc" has been incremented to two. Now you return the reference from the XSUB rou tine 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 hap pens. 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 "newRV_noinc" instead of "newRV_inc". Then, if and when the last refer ence 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 intro duce 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". Gener ally 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 perlcall and perlxs for more details on these macros. "Mortalization" then is at its simplest a deferred "SvRE FCNT_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 You should be careful about creating mortal variables. Strange things can happen if you make the same value mor tal within multiple contexts, or if you make a variable mortal multiple times. Thinking of "Mortalization" as deferred "SvREFCNT_dec" should help to minimize such prob lems. For example if you are passing an SV which you 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 "SvREFCNT_inc" and "sv_2mortal", or making a "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 "SV*") to the "sv_2mortal" or "sv_mortalcopy" routines. Stashes and Globs A stash is a hash that contains all variables that are defined within a package. Each key of the stash is a sym bol 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 "HV*". The "create" flag will create a new package if it is set. The name that "gv_stash*v" wants is the name of the pack 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 "SV*", must be a reference, and the second argument is a stash. The returned "SV*" can now be used in the same way as any other SV. For more information on references and blessings, consult perlref. 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 $! contains either the numeric value of "errno" or its string equivalent from either "strerror" or "sys_errlist[]". To force multiple data values into an SV, you must do two things: use the "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 "sv_set*v" routine you called first. This is because every "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 "dber ror" 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); 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 "struct magic"'s, typedef'ed to "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. 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 "sv" argument is a pointer to the SV that is to acquire a new magical feature. If "sv" is not already magical, Perl uses the "SvUPGRADE" macro to convert "sv" to type "SVt_PVMG". Perl then con tinues 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 overrid den, and multiple instances of the same type of magic can be associated with an SV. The "name" and "namlen" arguments are used to associate a string with the magic, typically the name of a variable. "namlen" is stored in the "mg_len" field and if "name" is non-null and "namlen" >= 0 a malloc'd copy of the name is stored in "mg_ptr" field. The sv_magic function uses "how" to determine which, if any, predefined "Magic Virtual Table" should be assigned to the "mg_virtual" field. See the "Magic Virtual Tables" section below. The "how" argument is also stored in the "mg_type" field. The value of "how" should be chosen from the set of macros "PERL_MAGIC_foo" found in perl.h. Note that before these macros were added, Perl internals used to directly use character literals, so you may occasion ally come across old code or documentation referring to 'U' magic rather than "PERL_MAGIC_uvar" for example. into an "SV". To remove the magic from an SV, call the function sv_unmagic: void sv_unmagic(SV *sv, int type); The "type" argument should be equal to the "how" value when the "SV" was initially made magical. Magic Virtual Tables The "mg_virtual" field in the "MAGIC" structure is a pointer to an "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 vari able. The "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 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 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. For instance, the MGVTBL structure called "vtbl_sv" (which corresponds to an "mg_type" of "PERL_MAGIC_sv") contains: { magic_get, magic_set, magic_len, 0, 0 } Thus, when an SV is determined to be magical and of type "PERL_MAGIC_sv", if a get operation is being performed, the routine "magic_get" is called. All the various rou tines for the various magical types begin with "magic_". NOTE: the magic routines are not considered part of the Perl API, and may not be exported by the Perl library. on stash B PERL_MAGIC_bm vtbl_bm Boyer-Moore (fast string search) D PERL_MAGIC_regdata vtbl_regdata Regex match position data (@+ and @- vars) d PERL_MAGIC_regdatum vtbl_regdatum Regex match position data element E PERL_MAGIC_env vtbl_env %ENV hash e PERL_MAGIC_envelem vtbl_envelem %ENV hash element f PERL_MAGIC_fm vtbl_fm Formline ('compiled' format) g PERL_MAGIC_regex_global vtbl_mglob m//g target / study()ed string I PERL_MAGIC_isa vtbl_isa @ISA array i PERL_MAGIC_isaelem vtbl_isaelem @ISA array element k PERL_MAGIC_nkeys vtbl_nkeys scalar(keys()) lvalue L PERL_MAGIC_dbfile (none) Debugger %_<filename l PERL_MAGIC_dbline vtbl_dbline Debugger %_<filename element m PERL_MAGIC_mutex vtbl_mutex ??? 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_glob vtbl_glob GV (typeglob) # PERL_MAGIC_arylen vtbl_arylen Array length ($#ary) . PERL_MAGIC_pos vtbl_pos pos() lvalue < PERL_MAGIC_backref vtbl_backref ??? ~ PERL_MAGIC_ext (none) Available for use by extensions When an uppercase and lowercase letter both exist in the table, then the uppercase letter is typically used to rep resent 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 "PERL_MAGIC_ext" and "PERL_MAGIC_uvar" magic types are defined specifically for use by extensions and will not be used by perl itself. Extensions can use "PERL_MAGIC_ext" magic to 'attach' private information to variables (typi cally objects). This is especially useful because there is no way for normal perl code to corrupt this private When the SV is read from or written to, the "uf_val" or "uf_set" function will be called with "uf_index" as the first arg and a pointer to the SV as the second. A simple example of how to add "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)); Note that because multiple extensions may be using "PERL_MAGIC_ext" or "PERL_MAGIC_uvar" magic, it is impor tant 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 "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 "sv_set*()" and "sv_cat*()" functions described earlier do not invoke 'set' magic on their tar gets. This must be done by the user either by calling the "SvSETMAGIC()" macro after calling these functions, or by using one of the "sv_set*_mg()" or "sv_cat*_mg()" func tions. Similarly, generic C code must call the "SvGET MAGIC()" macro to invoke any 'get' magic if they use an SV obtained from external sources in functions that don't handle magic. See perlapi for a description of these functions. For example, calls to the "sv_cat*()" func tions typically need to be followed by "SvSETMAGIC()", but they don't need a prior "SvGETMAGIC()" since their imple mentation handles 'get' magic. Finding Magic MAGIC* mg_find(SV*, int type); /* Finds the magic pointer of that type */ This routine returns a pointer to the "MAGIC" structure stored in the SV. If the SV does not have that magical feature, "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); array and hash access functions requires understanding a few caveats. Some of these caveats are actually consid ered 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 car ries 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 "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, (GV*)tie, PERL_MAGIC_tied); RETVAL = newRV_noinc(hash); OUTPUT: RETVAL The "av_store" function, when given a tied array argument, merely copies the magic of the array onto the value to be "stored", using "mg_copy". It may also return NULL, indi cating that the value did not actually need to be stored in the array. [MAYCHANGE] After a call to "av_store" on a tied array, the caller will usually need to call "mg_set(val)" to actually invoke the perl level "STORE" method on the TIEARRAY object. If "av_store" did return NULL, a call to "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 "hv_store" and "hv_store_ent" functions as well. the case of tied arrays and hashes. They merely call "mg_copy" to attach magic to the values that were meant to be "stored" or "fetched". Later calls to "mg_get" and "mg_set" actually do the job of invoking the TIE methods on the underlying objects. Thus the magic mechanism cur rently 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 pro vide 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 syn tax. The use of this sugar imposes some overhead (typi cally 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 them selves substantial, but if they are only a few statements long, the overhead will not be insignificant. Localizing changes Perl has a very handy construction { local $var = 2; ... } This construction is 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: "goto", "return", "die"/"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 pseudo-block, and arrange for some changes "SAVEINT(int i)" "SAVEIV(IV i)" "SAVEI32(I32 i)" "SAVELONG(long i)" These macros arrange things to restore the value of integer variable "i" at the end of enclosing pseudo- block. SAVESPTR(s) SAVEPPTR(p) These macros arrange things to restore the value of pointers "s" and "p". "s" must be a pointer of a type which survives conversion to "SV*" and back, "p" should be able to survive conversion to "char*" and back. "SAVEFREESV(SV *sv)" The refcount of "sv" would be decremented at the end of pseudo-block. This is similar to "sv_2mortal" in that it is also a mechanism for doing a delayed "SvRE FCNT_dec". However, while "sv_2mortal" extends the lifetime of "sv" until the beginning of the next statement, "SAVEFREESV" extends it until the end of the enclosing scope. These lifetimes can be wildly different. Also compare "SAVEMORTALIZESV". "SAVEMORTALIZESV(SV *sv)" Just like "SAVEFREESV", but mortalizes "sv" at the end of the current scope instead of decrementing its ref erence count. This usually has the effect of keeping "sv" alive until the statement that called the cur rently live scope has finished executing. "SAVEFREEOP(OP *op)" The "OP *" is op_free()ed at the end of pseudo-block. SAVEFREEPV(p) The chunk of memory which is pointed to by "p" is Safefree()ed at the end of pseudo-block. "SAVECLEARSV(SV *sv)" Clears a slot in the current scratchpad which corre sponds to "sv" at the end of pseudo-block. "SAVEDELETE(HV *hv, char *key, I32 length)" The key "key" of "hv" is deleted at the end of pseudo- block. The string pointed to by "key" is Safefree()ed. If one has a key in short-lived storage, the corre sponding string may be reallocated like this: The following API list contains functions, thus one needs to provide pointers to the modifiable data explicitly (either C pointers, or Perlish "GV *"s). Where the above macros take "int", a similar function takes "int *". "SV* save_scalar(GV *gv)" Equivalent to Perl code "local $gv". "AV* save_ary(GV *gv)" "HV* save_hash(GV *gv)" Similar to "save_scalar", but localize @gv and %gv. "void save_item(SV *item)" Duplicates the current value of "SV", on the exit from the current "ENTER"/"LEAVE" pseudo-block will restore the value of "SV" using the stored value. "void save_list(SV **sarg, I32 maxsarg)" A variant of "save_item" which takes multiple argu ments via an array "sarg" of "SV*" of length "maxsarg". "SV* save_svref(SV **sptr)" Similar to "save_scalar", but will reinstate an "SV *". "void save_aptr(AV **aptr)" "void save_hptr(HV **hptr)" Similar to "save_svref", but localize "AV *" and "HV *". The "Alias" module implements localization of the basic types within the caller's scope. People who are inter ested in how to localize things in the containing scope should take a look there too.

Subroutines

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 equiva lent. The stack arguments are accessible through the ST(n) macro, which returns the "n"'th stack argument. Argument 0 is the first argument passed in the Perl subroutine call. These arguments are "SV*", and can be used anywhere an "SV*" is used. the stack pointer, and "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 "PUSHs" macro. The pushed values will often need to be "mortal" (See "Reference Counts and Mortal ity"). PUSHs(sv_2mortal(newSViv(an_integer))) PUSHs(sv_2mortal(newSVpv("Some String",0))) PUSHs(sv_2mortal(newSVnv(3.141592))) And now the Perl program calling "tzname", the two values will be assigned as in: ($standard_abbrev, $summer_abbrev) = POSIX::tzname; An alternate (and possibly simpler) method to pushing val ues 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 "EXTEND" to extend the stack. Despite their suggestions in earlier versions of this doc ument the macros "PUSHi", "PUSHn" and "PUSHp" are not suited to XSUBs which return multiple results, see "Putting a C value on Perl stack". For more information, consult perlxs and perlxstut. 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 "call_sv". The "SV*" argu ment 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 subrou tine 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 SPAGAIN ENTER SAVETMPS FREETMPS LEAVE XPUSH*() POP*() For a detailed description of calling conventions from C to Perl, consult perlcall. Memory Allocation 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 : New(x, pointer, number, type); Newc(x, pointer, number, type, cast); Newz(x, pointer, number, type); The first argument "x" was a "magic cookie" that was used to keep track of who called the macro, to help when debug ging 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 "pointer" should be the name of a variable that will point to the newly allocated memory. The third and fourth arguments "number" and "type" specify how many of the specified type of data structure should be allocated. The argument "type" is passed to "sizeof". The final argument to "Newc", "cast", should be used if the "pointer" argument is different from the "type" argu ment. Unlike the "New" and "Newc" macros, the "Newz" macro calls "memzero" to zero out all the newly allocated memory. 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 "source" and "dest" arguments point to the source and destination starting points. Perl will move, copy, or zero out "number" instances of the size of the "type" data structure (using the "sizeof" function). PerlIO The most recent development releases of Perl has been experimenting with removing Perl's dependency on the "nor mal" standard I/O suite and allowing other stdio implemen tations to be used. This involves creating a new abstrac tion 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, con sult perlapio. 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. How ever, as an optimization the corresponding SV is (usually) not recreated each time. The opcodes reuse specially assigned SVs (targets) which are (as a corollary) not con stantly freed/created. Each of the targets is created only once (but see "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 target and puts the target on stack. The macro to put this target on stack is "PUSHTARG", and it is directly used in some opcodes, as well as indirectly in zillions of others, which use it via "(X)PUSH[pni]". Because the target is reused, you must be careful when pushing multiple values on the stack. The following code will not do what you think: XPUSHi(10); XPUSHi(20); The question remains on when the SVs which are targets 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 com piled. 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 "SVs_PADMY" set, and targets have "SVs_PADTMP" set. The correspondence between OPs and targets 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. 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 ele ment is the scratchpad AV. Why do we need an extra level of indirection? The answer is recursion, and maybe threads. Both these can create several execution pointers going into the same sub routine. For the subroutine-child not write over the tem poraries for the subroutine-parent (lifespan of which cov ers the call to the child), the parent and the child should have different scratchpads. (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 targets on this scratchpad are "undef"s, but they are already marked with correct flags.

Compiled code

Code tree Here we describe the internal form your code is converted to by Perl. Start with a simple example: $a = $b + $c; $b ---> $c ---> + ---> $a ---> assign-to But with the actual compile tree for "$a = $b + $c" it is different: some nodes optimized away. As a corollary, though the actual tree contains more nodes than our sim plified example, the execution order is the same as in our example. Examining the tree If you have your perl compiled for debugging (usually done with "-DDEBUGGING" on the "Configure" command line), you may examine the compiled tree by specifying "-Dx" on the Perl command line. The output takes several lines per node, and for "$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 "TYPE" specifier), only 3 of them are not optimized away (one per number in the left column). The immediate children of the given node corre spond to "{}" pairs on the same level of indentation, thus this listing corresponds to the tree: add / \ null null | | gvsv gvsv different ways. The simplest type of op structure is "OP": this has no children. Unary operators, "UNOP"s, have one child, and this is pointed to by the "op_first" field. Binary opera tors ("BINOP"s) have not only an "op_first" field but also an "op_last" field. The most complex type of op is a "LISTOP", which has any number of children. In this case, the first child is pointed to by "op_first" and the last child by "op_last". The children in between can be found by iteratively following the "op_sibling" pointer from the first child to the last. There are also two other op types: a "PMOP" holds a regu lar expression, and has no children, and a "LOOP" may or may not have children. If the "op_children" field is non-zero, it behaves like a "LISTOP". To complicate mat ters, if a "UNOP" is actually a "null" op after optimiza tion (see "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 B::Concise. Compile pass 1: check routines The tree is created by the compiler while yacc code feeds it the constructions it recognizes. Since yacc works bot tom-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 opcode.pl (do not forget to run "make regen_headers" if you modify this file). A check routine is called when the node is fully con structed except for the execution-order thread. Since at this time there are no back-links to the currently con structed 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 mod ified, check routine returns its argument). By convention, check routines have names "ck_*". They are usually called from "new*OP" subroutines (or "convert") (which in turn are called from perly.y). When a context for a part of compile tree is known, it is propagated down through the tree. At this time the con text 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 con tains back-references (via "thread" pointers), nodes can not 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). Compile pass 3: peephole optimization After the compile tree for a subroutine (or for an "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 sub routine peep(). Optimizations performed at this stage are subject to the same restrictions as in the pass 2. Pluggable runops The compile tree is executed in a runops function. There are two runops functions in run.c. "Perl_runops_debug" is used with DEBUGGING and "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.

Examining internal data structures with the "dump" functions

To aid debugging, the source file dump.c contains a number of functions which produce formatted output of internal data structures. The most commonly used of these functions is "Perl_sv_dump"; it's used for dumping SVs, AVs, HVs, and CVs. The "Devel::Peek" module calls "sv_dump" to produce debugging output from Perl-space, so users of that module should already be familiar with its format. 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 "Perl_dump_all", which dumps all the subroutines in the stash and the op tree of the main root.

How multiple interpreters and concurrency are supported

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. Two macros control the major Perl build flavors: MULTI PLICITY and USE_5005THREADS. The MULTIPLICITY build has a C structure that packages all the interpreter state, and there is a similar thread-specific data structure under USE_5005THREADS. In both cases, PERL_IMPLICIT_CONTEXT is also normally defined, and enables the support for passing in a "hidden" first argument that represents all three data structures. 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 "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 "Internal Functions".) The easiest way to be sure a function is part of the API is to find its entry in perlapi. If it exists in 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 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) "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 prototype, 'a' for argument, or 'd' for declaration, so we have "pTHX", "aTHX" and "dTHX", and their variants. When Perl is built without options that set PERL_IMPLICIT_CONTEXT, there is no first argument contain ing 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 addi tional explicit arguments. When a core function calls another, it must pass the con text. This is normally hidden via macros. Consider "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 "aTHX_" as the first argument (the Perl core tends to do this with functions like Perl_warner), or use a context-free version. "dTHR" was introduced in perl 5.005 to support the older thread model. The older thread model now uses the "THX" mechanism to pass context pointers around, so "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. 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_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 my_private_function(int arg1, int arg2); static SV * my_private_function(int arg1, int arg2) { my_private_function(arg, 10); Note that the only two changes from the normal way of writing an extension is the addition of a "#define PERL_NO_GET_CONTEXT" before including the Perl headers, followed by a "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 argu ment. Depending on your needs for simplicity or effi ciency, you may mix the previous two approaches freely. Never add a comma after "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. 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 inter preter. This is done by calling the "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 ... 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 inter preter to bundle up everything it knows about the environ ment it's running on. This is enabled with the PERL_IMPLICIT_SYS macro. Currently it only works with USE_ITHREADS and USE_5005THREADS on Windows (see inside iperlsys.h). 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 main tain 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 dif ferent 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.

Internal Functions

All of Perl's internal functions which will be exposed to the outside world are prefixed by "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 "PL_". (By convention, static func tions start with "S_".) Inside the Perl core, you can get at the functions either with or without the "Perl_" prefix, thanks to a bunch of defines that live in embed.h. This header file is gener ated automatically from embed.pl. embed.pl also creates the prototyping header files for the internal functions, generates the documentation and a lot of other bits and as "Perl_av_fetch" d This function has documentation using the "apidoc" fea ture which we'll look at in a second. Other available flags are: s This is a static function and is defined as "S_what ever", and usually called within the sources as "what ever(...)". n This does not use "aTHX_" and "pTHX" to pass inter preter context. (See "Background and PERL_IMPLICIT_CON TEXT" in perlguts.) r This function never returns; "croak", "exit" and friends. f This function takes a variable number of arguments, "printf" style. The argument list should end with "...", like this: Afprd |void |croak |const char* pat|... M This function is part of the experimental development API, and may change or disappear without notice. o This function should not have a compatibility macro to define, say, "Perl_parse" to "parse". It must be called as "Perl_parse". j This function is not a member of "CPerlObj". If you don't know what this means, don't use it. x This function isn't exported out of the Perl core. If you edit embed.pl, you will need to run "make regen_headers" to force a rebuild of embed.h and other auto-generated files. Formatted Printing of IVs, UVs, and NVs If you are printing IVs, UVs, or NVS instead of the stdio(3) style formatting codes like %d, %ld, %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 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); Source Documentation There's an effort going on to document the internal func tions and automatically produce reference manuals from them - perlapi is one such manual which details all the functions which are available to XS writers. 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 func tions to the Perl core.

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. What is Unicode, anyway? In the olden, less enlightened times, we all used to use 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 rep resenting these characters, and the one Perl uses is called UTF-8. UTF-8 uses a variable number of bytes to represent a character, instead of just one. You can learn more about Unicode at http://www.unicode.org/ 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, (0xC8 for you hex types) capital E with a grave accent, is rep resented by the two bytes "v196.172". Unfortunately, the non-Unicode string "chr(196).chr(172)" has that byte sequence as well. So you can't tell just by looking - this is what makes Unicode input an interesting problem. The API function "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, "is_utf8_char" will tell you whether the current character in a string is valid UTF-8. How does UTF-8 represent Unicode characters? As mentioned above, UTF-8 uses a variable number of bytes to store a character. Characters with values 1...128 are stored in one byte, just like good ol' ASCII. Character 129 is stored as "v194.129"; this continues up to charac ter 191, which is "v194.191". Now we've run out of bits (191 is binary 10111111) so we move on; 192 is "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 "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 "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. /* OK to treat this character as a byte */ uv = *utf; You can also see in that example that we use "utf8_to_uv" to get the value of the character; the inverse function "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 must convert characters to UVs using the above func tions 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 "v196.172", and you skip that character, you can never match a "chr(200)" in a non-UTF-8 string. So don't do that! How does Perl store UTF-8 strings? Currently, Perl deals with Unicode strings and non-Unicode strings slightly differently. If a string has been identi fied as being UTF-8 encoded, Perl will set a flag in the SV, "SVf_UTF8". You can check and manipulate this flag with the following macros: SvUTF8(sv) SvUTF8_on(sv) SvUTF8_off(sv) This flag has an important effect on Perl's treatment of the string: if Unicode data is not properly distinguished, regular expressions, "length", "substr" and other string handling operations will have undesirable results. The problem comes when you have, for instance, a string that isn't flagged is 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 "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; nsv = newSVpvn(p, len); if (SvUTF8(sv)) SvUTF8_on(nsv); In fact, your "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 UTF-8 flags, even less right is just passing a "char *" to an XS function. How do I convert a string to UTF-8? If you're mixing UTF-8 and non-UTF-8 strings, you might find it necessary to upgrade one of the strings to UTF-8. If you've got an SV, the easiest way to do this is: sv_utf8_upgrade(sv); However, you must not do this, for example: if (!SvUTF8(left)) sv_utf8_upgrade(left); If you do this in a binary operator, you will actually change one of the strings that came into the operator, and, while it shouldn't be noticeable by the end user, it can cause problems. Instead, "bytes_to_utf8" will give you a UTF-8-encoded 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 "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 repre sented in a single byte. Is there anything else I need to know? Not really. Just remember these things: · 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 "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. · If a string is UTF-8, always use "utf8_to_uv" to get at the value, unless "UTF8_IS_INVARIANT(*s)" in which case you can use *s. 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 func tions of multiple ops which are usually executed together, such as "gvsv, gvsv, add".) This feature is implemented as a new op type, "OP_CUSTOM". The Perl core does not "know" anything special about this op type, and so it will not be involved in any optimiza tions. 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 "CHECK" block and the "B::Generate" module, or by adding a custom peephole optimizer with the "optimize" module. When you do this, you replace ordinary Perl ops with cus tom ops by creating ops with the type "OP_CUSTOM" and the "pp_addr" of your own PP function. This should be defined in XS code, and should look like the PP ops in "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 inter preter 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 "OP_CUSTOM", Perl uses the value of "o->op_ppaddr" as a key into the "PL_cus tom_op_descs" and "PL_custom_op_names" hashes. This means you need to enter a name and description for your op at the appropriate place in the "PL_custom_op_names" and "PL_custom_op_descs" hashes. Forthcoming versions of "B::Generate" (version 1.0 and above) should directly support the creation of custom ops by name; "Opcodes::Custom" will provide functions which make it trivial to "register" custom ops to the Perl interpreter.

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>.

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