perlguts - Perl's Internal Functions This document attempts to describe some of the internal functions of the Perl executable. It is far from complete and probably contains many errors. Please refer any questions or comments to the author below.

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 integer type that is guaranteed to be large enough to hold a pointer (as well as an integer). Perl also uses two special typedefs, I32 and I16, which will always be at least 32-bits and 16-bits long, respectively.

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 five routines are: SV* newSViv(IV); SV* newSVnv(double); SV* newSVpv(char*, int); SV* newSVpvf(const char*, ...); SV* newSVsv(SV*); To change the value of an *already-existing* SV, there are six routines: void sv_setiv(SV*, IV); void sv_setnv(SV*, double); void sv_setpv(SV*, char*); void sv_setpvn(SV*, char*, int) void sv_setpvf(SV*, const char*, ...); void sv_setsv(SV*, SV*); Notice that you can choose to specify the length of the string to be assigned by using sv_setpvn or newSVpv, or you may allow Perl to calculate the length by using sv_setpv or by specifying 0 as the second argument 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. All SVs that will contain strings should, but need not, 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*) SvNV(SV*) SvPV(SV*, STRLEN len) which will automatically coerce the actual scalar type into an IV, double, or string. In the SvPV macro, the length of the string returned is placed into the variable len (this is a macro, so you do not use &len). If you do not care what the length of the data is, use the global variable na. Remember, however, that Perl allows arbitrary strings of data that may both contain NULs and might not be terminated by a NUL. 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. If you want to append something to the end of string stored in an SV*, you can use the following functions: void sv_catpv(SV*, char*); void sv_catpvn(SV*, char*, int); void sv_catpvf(SV*, const char*, ...); void sv_catsv(SV*, SV*); The first function calculates the length of the string to 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 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. If you know the name of a scalar variable, you can get a pointer to its SV by using the following: SV* perl_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 sv_undef. Its address can be used whenever an SV* is needed. There are also the two values sv_yes and sv_no, which contain Boolean TRUE and FALSE values, respectively. Like 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 &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 &sv_undef in the first line and all will be well. To free an SV that you've created, call SvREFCNT_dec(SV*). Normally this call is not necessary (see the section on Reference Counts and Mortality).

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, 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 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 populates 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 caller is responsible for taking care of that, and if av_store returns NULL, the caller will have to decrement the reference count to avoid a memory leak. Note that av_fetch and av_store both return SV**'s, not SV*'s as their return value. void av_clear(AV*); void av_undef(AV*); void av_extend(AV*, I32 key); The av_clear function deletes all the elements in the AV* array, but does not actually delete the array itself. The av_undef function will delete all the elements in the array plus the array itself. The av_extend function extends the array so that it contains key elements. If key is less than the current 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* perl_get_av("package::varname", FALSE); This returns NULL if the variable does not exist. See the section on Understanding the Magic of Tied Hashes and Arrays for more information on how to use the array access functions 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*, char* key, U32 klen, SV* val, U32 hash); SV** hv_fetch(HV*, 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*, char* key, U32 klen); SV* hv_delete(HV*, char* key, U32 klen, I32 flags); If flags does not include the G_DISCARD flag then 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, hv_clear deletes all the entries in the hash table but does not actually delete the hash table. The 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 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 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* perl_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: i = klen; hash = 0; s = key; while (i--) hash = hash * 33 + *s++; See the section on Understanding the Magic of Tied Hashes and Arrays for more information on how to use the hash access functions on tied hashes.

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 SV* keys, which simplifies writing of extension code that deals with hash structures. These functions also allow passing of SV* keys to tie functions without forcing you to stringify the keys (unlike the previous set of functions). 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 the section on API LISTING later in this document 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 the section on API LISTING later in this document 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 increment 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 reference count of val to avoid a memory leak.

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 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 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.

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 sv argument must be a reference. The stash argument specifies which class the reference will belong to. See the section on 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, char* classname); Copies integer or double into an SV whose reference is rv. SV is blessed if classname is non-null. SV* sv_setref_iv(SV* rv, char* classname, IV iv); SV* sv_setref_nv(SV* rv, 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, 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, char* classname, PV iv, int length); int sv_isa(SV* sv, char* name); int sv_isobject(SV* sv);

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* perl_get_sv("package::varname", TRUE); AV* perl_get_av("package::varname", TRUE); HV* perl_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 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.

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 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 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 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 newRV_noinc instead of 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 the perlcall manpage and the perlxs manpage for more details on these macros. "Mortalization" then is at its simplest a deferred 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 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 SV*) to the sv_2mortal or sv_mortalcopy routines.

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 File Handle Directory Handle Format Subroutine There is a single stash called "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 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(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 package whose symbol table you want. The default package is called main. If you have multiply nested packages, pass their names to gv_stash*v, separated by :: 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 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 the perlref manpage.

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 "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 = perl_get_sv("dberror", TRUE); sv_setiv(sv, (IV) dberror); sv_setpv(sv, dberror_list[dberror]); SvIOK_on(sv); If the order of sv_setiv and sv_setpv had been reversed, then the macro SvPOK_on would need to be called instead of SvIOK_on.

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 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, 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 set the SVt_PVMG flag for the 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 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 Table" section below. The how argument is also stored in the mg_type field. The obj argument is stored in the mg_obj field of the MAGIC structure. If it is not the same as the sv argument, the reference count of the obj object is incremented. If it is the same, or if the how argument is "#", or if it is a NULL pointer, then obj is merely stored, without the reference count being incremented. There is also a function to add magic to an HV: void hv_magic(HV *hv, GV *gv, int how); This simply calls sv_magic and coerces the gv argument 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 a 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 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 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 vtbl_sv (which corresponds to an 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 magic_get is called. All the various routines for the various magical types begin with magic_. 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 MAGIC's mg_ptr field points to a 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 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. 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.

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, char* key, STRLEN klen); This routine checks to see what types of magic sv has. If the mg_type field is an uppercase letter, then the mg_obj is copied to nsv, but the mg_type field is changed to be the lowercase letter.

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 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, indicating 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. av_fetch and the corresponding hash functions hv_fetch and hv_fetch_ent actually return an undefined mortal value whose magic has been initialized using 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 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 mg_set() on the return value after possibly assigning a suitable value to it using 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 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 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.

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 to be automatically undone at the end of it, either explicit, or via a non-local exit (via die()). A block-like construct is created by a pair of ENTER/LEAVE macros (see the section on EXAMPLE/"Returning a Scalar in the perlcall manpage). 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 ENTER/LEAVE pair. Inside such a pseudo-block the following service is available: These macros arrange things to restore the value of integer variable i at the end of enclosing pseudo-block. 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. The refcount of sv would be decremented at the end of pseudo-block. This is similar to sv_2mortal, which should (?) be used instead. The OP * is op_free()ed at the end of pseudo-block. The chunk of memory which is pointed to by p is Safefree()ed at the end of pseudo-block. Clears a slot in the current scratchpad which corresponds to sv at the end of pseudo-block. 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 corresponding string may be reallocated like this: SAVEDELETE(defstash, savepv(tmpbuf), strlen(tmpbuf)); At the end of pseudo-block the function f is called with the only argument (of type void*) p. The current offset on the Perl internal stack (cf. SP) is restored at the end of pseudo-block. 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 *. Equivalent to Perl code local $gv. Similar to save_scalar, but localize @gv and %gv. 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. A variant of save_item which takes multiple arguments via an array sarg of SV* of length maxsarg. Similar to save_scalar, but will reinstate a SV *. 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 interested in how to localize things in the containing scope should take a look there too.

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 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. 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 sp is 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 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 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 EXTEND to extend the stack. For more information, consult the perlxs manpage and the perlxstut manpage.

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 perl_call_sv(SV*, I32); I32 perl_call_pv(char*, I32); I32 perl_call_method(char*, I32); I32 perl_call_argv(char*, I32, register char**); The routine most often used is perl_call_sv. The 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. When using any of these routines (except perl_call_argv), the programmer must manipulate the Perl stack. These include the following macros and functions: dSP PUSHMARK() PUTBACK SPAGAIN ENTER SAVETMPS FREETMPS LEAVE XPUSH*() POP*() For a detailed description of calling conventions from C to Perl, consult the perlcall manpage.

Memory Allocation
It is suggested that you use 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 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 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 argument. Unlike the New and Newc macros, the Newz macro calls 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 Renew and Renewc match those of New and 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 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 "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 the perlapio manpage.

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 (targets) which are (as a corollary) not constantly freed/created. Each of the targets is created only once (but see the section on 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].

Scratchpads
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 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 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 element is the scratchpad AV. Why do we need an extra level of indirection? The answer is recursion, and maybe (sometime soon) 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. (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 undefs, but they are already marked with correct flags.

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 reflect 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 $a = $b + $c it is different: some nodes 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.

Examining the tree
If you have your perl compiled for debugging (usually done with -D optimize=-g on 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 correspond to {} 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 ===> marks, thus it is 3 4 5 6 (node 6 is not included into above listing), i.e., gvsv gvsv add whatever.

Compile pass 1: check routines
The tree is created by the 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 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 constructed except for the execution-order thread. Since at this time there is 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 ck_*. They are usually called from new*OP subroutines (or convert) (which in turn are called from perly.y).

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 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.

Compile pass 2: context propagation
When a context for a part of compile tree is known, it is propagated down through the tree. Aat 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).

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 compilications 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. This is a listing of functions, macros, flags, and variables that may be useful to extension writers or that may be found while reading other extensions. Same as av_len. Clears an array, making it empty. Does not free the memory used by the array itself. void av_clear _((AV* ar)); Pre-extend an array. The key is the index to which the array should be extended. void av_extend _((AV* ar, I32 key)); Returns the SV at the specified index in the array. The key is the index. If lval is set then the fetch will be part of a store. Check that the return value is non-null before dereferencing it to a SV*. See the section on Understanding the Magic of Tied Hashes and Arrays for more information on how to use this function on tied arrays. SV** av_fetch _((AV* ar, I32 key, I32 lval)); Returns the highest index in the array. Returns -1 if the array is empty. I32 av_len _((AV* ar)); Creates a new AV and populates it with a list of SVs. The SVs are copied into the array, so they may be freed after the call to av_make. The new AV will have a reference count of 1. AV* av_make _((I32 size, SV** svp)); Pops an SV off the end of the array. Returns &sv_undef if the array is empty. SV* av_pop _((AV* ar)); Pushes an SV onto the end of the array. The array will grow automatically to accommodate the addition. void av_push _((AV* ar, SV* val)); Shifts an SV off the beginning of the array. SV* av_shift _((AV* ar)); Stores an SV in an array. The array index is specified as key. The return value will be NULL if the operation failed or if the value did not need to be actually stored within the array (as in the case of tied arrays). Otherwise it can be dereferenced to get the original SV*. Note that the caller is responsible for suitably incrementing the reference count of val before the call, and decrementing it if the function returned NULL. See the section on Understanding the Magic of Tied Hashes and Arrays for more information on how to use this function on tied arrays. SV** av_store _((AV* ar, I32 key, SV* val)); Undefines the array. Frees the memory used by the array itself. void av_undef _((AV* ar)); Unshift the given number of undef values onto the beginning of the array. The array will grow automatically to accommodate the addition. You must then use av_store to assign values to these new elements. void av_unshift _((AV* ar, I32 num)); Variable which is setup by xsubpp to indicate the class name for a C++ XS constructor. This is always a char*. See THIS and the section on Using XS With C++ in the perlxs manpage. The XSUB-writer's interface to the C memcpy function. The s is the source, d is the destination, n is the number of items, and t is the type. May fail on overlapping copies. See also Move. (void) Copy( s, d, n, t ); This is the XSUB-writer's interface to Perl's die function. Use this function the same way you use the C printf function. See warn. Returns the stash of the CV. HV * CvSTASH( SV* sv ) When Perl is run in debugging mode, with the -d switch, this SV is a boolean which indicates whether subs are being single-stepped. Single-stepping is automatically turned on after every step. This is the C variable which corresponds to Perl's $DB::single variable. See DBsub. When Perl is run in debugging mode, with the -d switch, this GV contains the SV which holds the name of the sub being debugged. This is the C variable which corresponds to Perl's $DB::sub variable. See DBsingle. The sub name can be found by SvPV( GvSV( DBsub ), na ) Trace variable used when Perl is run in debugging mode, with the -d switch. This is the C variable which corresponds to Perl's $DB::trace variable. See DBsingle. Declare a stack marker variable, mark, for the XSUB. See MARK and dORIGMARK. Saves the original stack mark for the XSUB. See ORIGMARK. The C variable which corresponds to Perl's $^W warning variable. Declares a stack pointer variable, sp, for the XSUB. See SP. Sets up stack and mark pointers for an XSUB, calling dSP and dMARK. This is usually handled automatically by xsubpp. Declares the items variable to indicate the number of items on the stack. Sets up the ix variable for an XSUB which has aliases. This is usually handled automatically by xsubpp. Opening bracket on a callback. See LEAVE and the perlcall manpage. ENTER; Used to extend the argument stack for an XSUB's return values. EXTEND( sp, int x ); Closing bracket for temporaries on a callback. See SAVETMPS and the perlcall manpage. FREETMPS; Used to indicate array context. See GIMME_V, GIMME and the perlcall manpage. Indicates that arguments returned from a callback should be discarded. See the perlcall manpage. Used to force a Perl eval wrapper around a callback. See the perlcall manpage. A backward-compatible version of GIMME_V which can only return G_SCALAR or G_ARRAY; in a void context, it returns G_SCALAR. The XSUB-writer's equivalent to Perl's wantarray. Returns G_VOID, G_SCALAR or G_ARRAY for void, scalar or array context, respectively. Indicates that no arguments are being sent to a callback. See the perlcall manpage. Used to indicate scalar context. See GIMME_V, GIMME, and the perlcall manpage. Used to indicate void context. See GIMME_V and the perlcall manpage. Returns the glob with the given name and a defined subroutine or NULL. The glob lives in the given stash, or in the stashes accessable via @ISA and @<UNIVERSAL>. The argument level should be either 0 or -1. If level==0, as a side-effect creates a glob with the given name in the given stash which in the case of success contains an alias for the subroutine, and sets up caching info for this glob. Similarly for all the searched stashes. This function grants "SUPER" token as a postfix of the stash name. The GV returned from gv_fetchmeth may be a method cache entry, which is not visible to Perl code. So when calling perl_call_sv, you should not use the GV directly; instead, you should use the method's CV, which can be obtained from the GV with the GvCV macro. GV* gv_fetchmeth _((HV* stash, char* name, STRLEN len, I32 level)); Returns the glob which contains the subroutine to call to invoke the method on the stash. In fact in the presense of autoloading this may be the glob for "AUTOLOAD". In this case the corresponding variable $AUTOLOAD is already setup. The third parameter of gv_fetchmethod_autoload determines whether AUTOLOAD lookup is performed if the given method is not present: non-zero means yes, look for AUTOLOAD; zero means no, don't look for AUTOLOAD. Calling gv_fetchmethod is equivalent to calling gv_fetchmethod_autoload with a non-zero autoload parameter. These functions grant "SUPER" token as a prefix of the method name. Note that if you want to keep the returned glob for a long time, you need to check for it being "AUTOLOAD", since at the later time the call may load a different subroutine due to $AUTOLOAD changing its value. Use the glob created via a side effect to do this. These functions have the same side-effects and as gv_fetchmeth with level==0. name should be writable if contains ':' or '\''. The warning against passing the GV returned by gv_fetchmeth to perl_call_sv apply equally to these functions. GV* gv_fetchmethod _((HV* stash, char* name)); GV* gv_fetchmethod_autoload _((HV* stash, char* name, I32 autoload)); Returns a pointer to the stash for a specified package. If create is set then the package will be created if it does not already exist. If create is not set and the package does not exist then NULL is returned. HV* gv_stashpv _((char* name, I32 create)); Returns a pointer to the stash for a specified package. See gv_stashpv. HV* gv_stashsv _((SV* sv, I32 create)); Return the SV from the GV. This flag, used in the length slot of hash entries and magic structures, specifies the structure contains a SV* pointer where a char* pointer is to be expected. (For information only--not to be used). Returns the computed hash (type U32) stored in the hash entry. HeHASH(HE* he) Returns the actual pointer stored in the key slot of the hash entry. The pointer may be either char* or SV*, depending on the value of HeKLEN(). Can be assigned to. The HePV() or HeSVKEY() macros are usually preferable for finding the value of a key. HeKEY(HE* he) If this is negative, and amounts to HEf_SVKEY, it indicates the entry holds an SV* key. Otherwise, holds the actual length of the key. Can be assigned to. The HePV() macro is usually preferable for finding key lengths. HeKLEN(HE* he) Returns the key slot of the hash entry as a char* value, doing any necessary dereferencing of possibly SV* keys. The length of the string is placed in len (this is a macro, so do not use &len). If you do not care about what the length of the key is, you may use the global variable na. Remember though, that hash keys in perl are free to contain embedded nulls, so using strlen() or similar is not a good way to find the length of hash keys. This is very similar to the SvPV() macro described elsewhere in this document. HePV(HE* he, STRLEN len) Returns the key as an SV*, or Nullsv if the hash entry does not contain an SV* key. HeSVKEY(HE* he) Returns the key as an SV*. Will create and return a temporary mortal SV* if the hash entry contains only a char* key. HeSVKEY_force(HE* he) Sets the key to a given SV*, taking care to set the appropriate flags to indicate the presence of an SV* key, and returns the same SV*. HeSVKEY_set(HE* he, SV* sv) Returns the value slot (type SV*) stored in the hash entry. HeVAL(HE* he) Clears a hash, making it empty. void hv_clear _((HV* tb)); Releases a hash entry, such as while iterating though the hash, but delays actual freeing of key and value until the end of the current statement (or thereabouts) with sv_2mortal. See hv_iternext and hv_free_ent. void hv_delayfree_ent _((HV* hv, HE* entry)); Deletes a key/value pair in the hash. The value SV is removed from the hash and returned to the caller. The klen is the length of the key. The flags value will normally be zero; if set to G_DISCARD then NULL will be returned. SV* hv_delete _((HV* tb, char* key, U32 klen, I32 flags)); Deletes a key/value pair in the hash. The value SV is removed from the hash and returned to the caller. The flags value will normally be zero; if set to G_DISCARD then NULL will be returned. hash can be a valid precomputed hash value, or 0 to ask for it to be computed. SV* hv_delete_ent _((HV* tb, SV* key, I32 flags, U32 hash)); Returns a boolean indicating whether the specified hash key exists. The klen is the length of the key. bool hv_exists _((HV* tb, char* key, U32 klen)); Returns a boolean indicating whether the specified hash key exists. hash can be a valid precomputed hash value, or 0 to ask for it to be computed. bool hv_exists_ent _((HV* tb, SV* key, U32 hash)); Returns the SV which corresponds to the specified key in the hash. The klen is the length of the key. If lval is set then the fetch will be part of a store. Check that the return value is non-null before dereferencing it to a SV*. See the section on Understanding the Magic of Tied Hashes and Arrays for more information on how to use this function on tied hashes. SV** hv_fetch _((HV* tb, char* key, U32 klen, I32 lval)); Returns the hash entry which corresponds to the specified key in the hash. hash must be a valid precomputed hash number for the given key, or 0 if you want the function to compute it. IF lval is set then the fetch will be part of a store. Make sure the return value is non-null before accessing it. The return value when tb is a tied hash is a pointer to a static location, so be sure to make a copy of the structure if you need to store it somewhere. See the section on Understanding the Magic of Tied Hashes and Arrays for more information on how to use this function on tied hashes. HE* hv_fetch_ent _((HV* tb, SV* key, I32 lval, U32 hash)); Releases a hash entry, such as while iterating though the hash. See hv_iternext and hv_delayfree_ent. void hv_free_ent _((HV* hv, HE* entry)); Prepares a starting point to traverse a hash table. I32 hv_iterinit _((HV* tb)); Note that hv_iterinit currently returns the number of buckets in the hash and not the number of keys (as indicated in the Advanced Perl Programming book). This may change in future. Use the HvKEYS(hv) macro to find the number of keys in a hash. Returns the key from the current position of the hash iterator. See hv_iterinit. char* hv_iterkey _((HE* entry, I32* retlen)); Returns the key as an SV* from the current position of the hash iterator. The return value will always be a mortal copy of the key. Also see hv_iterinit. SV* hv_iterkeysv _((HE* entry)); Returns entries from a hash iterator. See hv_iterinit. HE* hv_iternext _((HV* tb)); Performs an hv_iternext, hv_iterkey, and hv_iterval in one operation. SV * hv_iternextsv _((HV* hv, char** key, I32* retlen)); Returns the value from the current position of the hash iterator. See hv_iterkey. SV* hv_iterval _((HV* tb, HE* entry)); Adds magic to a hash. See sv_magic. void hv_magic _((HV* hv, GV* gv, int how)); Returns the package name of a stash. See SvSTASH, CvSTASH. char *HvNAME (HV* stash) Stores an SV in a hash. The hash key is specified as key and klen is the length of the key. The hash parameter is the precomputed hash value; if it is zero then Perl will compute it. The return value will be NULL if the operation failed or if the value did not need to be actually stored within the hash (as in the case of tied hashes). Otherwise it can be dereferenced to get the original SV*. Note that the caller is responsible for suitably incrementing the reference count of val before the call, and decrementing it if the function returned NULL. See the section on Understanding the Magic of Tied Hashes and Arrays for more information on how to use this function on tied hashes. SV** hv_store _((HV* tb, char* key, U32 klen, SV* val, U32 hash)); Stores val in a hash. The hash key is specified as key. The hash parameter is the precomputed hash value; if it is zero then Perl will compute it. The return value is the new hash entry so created. It will be NULL if the operation failed or if the value did not need to be actually stored within the hash (as in the case of tied hashes). Otherwise the contents of the return value can be accessed using the He??? macros described here. Note that the caller is responsible for suitably incrementing the reference count of val before the call, and decrementing it if the function returned NULL. See the section on Understanding the Magic of Tied Hashes and Arrays for more information on how to use this function on tied hashes. HE* hv_store_ent _((HV* tb, SV* key, SV* val, U32 hash)); Undefines the hash. void hv_undef _((HV* tb)); Returns a boolean indicating whether the C char is an ascii alphanumeric character or digit. int isALNUM (char c) Returns a boolean indicating whether the C char is an ascii alphabetic character. int isALPHA (char c) Returns a boolean indicating whether the C char is an ascii digit. int isDIGIT (char c) Returns a boolean indicating whether the C char is a lowercase character. int isLOWER (char c) Returns a boolean indicating whether the C char is whitespace. int isSPACE (char c) Returns a boolean indicating whether the C char is an uppercase character. int isUPPER (char c) Variable which is setup by xsubpp to indicate the number of items on the stack. See the section on Variable-length Parameter Lists in the perlxs manpage. Variable which is setup by xsubpp to indicate which of an XSUB's aliases was used to invoke it. See the section on The ALIAS: Keyword in the perlxs manpage. Closing bracket on a callback. See ENTER and the perlcall manpage. LEAVE; Stack marker variable for the XSUB. See dMARK. Clear something magical that the SV represents. See sv_magic. int mg_clear _((SV* sv)); Copies the magic from one SV to another. See sv_magic. int mg_copy _((SV *, SV *, char *, STRLEN)); Finds the magic pointer for type matching the SV. See sv_magic. MAGIC* mg_find _((SV* sv, int type)); Free any magic storage used by the SV. See sv_magic. int mg_free _((SV* sv)); Do magic after a value is retrieved from the SV. See sv_magic. int mg_get _((SV* sv)); Report on the SV's length. See sv_magic. U32 mg_len _((SV* sv)); Turns on the magical status of an SV. See sv_magic. void mg_magical _((SV* sv)); Do magic after a value is assigned to the SV. See sv_magic. int mg_set _((SV* sv)); The XSUB-writer's interface to the C memmove function. The s is the source, d is the destination, n is the number of items, and t is the type. Can do overlapping moves. See also Copy. (void) Move( s, d, n, t ); A variable which may be used with SvPV to tell Perl to calculate the string length. The XSUB-writer's interface to the C malloc function. void * New( x, void *ptr, int size, type ) The XSUB-writer's interface to the C malloc function, with cast. void * Newc( x, void *ptr, int size, type, cast ) The XSUB-writer's interface to the C malloc function. The allocated memory is zeroed with memzero. void * Newz( x, void *ptr, int size, type ) Creates a new AV. The reference count is set to 1. AV* newAV _((void)); Creates a new HV. The reference count is set to 1. HV* newHV _((void)); Creates an RV wrapper for an SV. The reference count for the original SV is incremented. SV* newRV_inc _((SV* ref)); For historical reasons, "newRV" is a synonym for "newRV_inc". Creates an RV wrapper for an SV. The reference count for the original SV is not incremented. SV* newRV_noinc _((SV* ref)); Creates a new SV. The len parameter indicates the number of bytes of preallocated string space the SV should have. The reference count for the new SV is set to 1. SV* newSV _((STRLEN len)); Creates a new SV and copies an integer into it. The reference count for the SV is set to 1. SV* newSViv _((IV i)); Creates a new SV and copies a double into it. The reference count for the SV is set to 1. SV* newSVnv _((NV i)); Creates a new SV and copies a string into it. The reference count for the SV is set to 1. If len is zero then Perl will compute the length. SV* newSVpv _((char* s, STRLEN len)); Creates a new SV for the RV, rv, to point to. If rv is not an RV then it will be upgraded to one. If classname is non-null then the new SV will be blessed in the specified package. The new SV is returned and its reference count is 1. SV* newSVrv _((SV* rv, char* classname)); Creates a new SV which is an exact duplicate of the original SV. SV* newSVsv _((SV* old)); Used by xsubpp to hook up XSUBs as Perl subs. Used by xsubpp to hook up XSUBs as Perl subs. Adds Perl prototypes to the subs. Null AV pointer. Null character pointer. Null CV pointer. Null HV pointer. Null SV pointer. The original stack mark for the XSUB. See dORIGMARK. Allocates a new Perl interpreter. See the perlembed manpage. Performs a callback to the specified Perl sub. See the perlcall manpage. I32 perl_call_argv _((char* subname, I32 flags, char** argv)); Performs a callback to the specified Perl method. The blessed object must be on the stack. See the perlcall manpage. I32 perl_call_method _((char* methname, I32 flags)); Performs a callback to the specified Perl sub. See the perlcall manpage. I32 perl_call_pv _((char* subname, I32 flags)); Performs a callback to the Perl sub whose name is in the SV. See the perlcall manpage. I32 perl_call_sv _((SV* sv, I32 flags)); Initializes a new Perl interpreter. See the perlembed manpage. Shuts down a Perl interpreter. See the perlembed manpage. Tells Perl to eval the string in the SV. I32 perl_eval_sv _((SV* sv, I32 flags)); Tells Perl to eval the given string and return an SV* result. SV* perl_eval_pv _((char* p, I32 croak_on_error)); Releases a Perl interpreter. See the perlembed manpage. Returns the AV of the specified Perl array. If create is set and the Perl variable does not exist then it will be created. If create is not set and the variable does not exist then NULL is returned. AV* perl_get_av _((char* name, I32 create)); Returns the CV of the specified Perl sub. If create is set and the Perl variable does not exist then it will be created. If create is not set and the variable does not exist then NULL is returned. CV* perl_get_cv _((char* name, I32 create)); Returns the HV of the specified Perl hash. If create is set and the Perl variable does not exist then it will be created. If create is not set and the variable does not exist then NULL is returned. HV* perl_get_hv _((char* name, I32 create)); Returns the SV of the specified Perl scalar. If create is set and the Perl variable does not exist then it will be created. If create is not set and the variable does not exist then NULL is returned. SV* perl_get_sv _((char* name, I32 create)); Tells a Perl interpreter to parse a Perl script. See the perlembed manpage. Tells Perl to require a module. void perl_require_pv _((char* pv)); Tells a Perl interpreter to run. See the perlembed manpage. Pops an integer off the stack. int POPi(); Pops a long off the stack. long POPl(); Pops a string off the stack. char * POPp(); Pops a double off the stack. double POPn(); Pops an SV off the stack. SV* POPs(); Opening bracket for arguments on a callback. See PUTBACK and the perlcall manpage. PUSHMARK(p) Push an integer onto the stack. The stack must have room for this element. See XPUSHi. PUSHi(int d) Push a double onto the stack. The stack must have room for this element. See XPUSHn. PUSHn(double d) Push a string onto the stack. The stack must have room for this element. The len indicates the length of the string. See XPUSHp. PUSHp(char *c, int len ) Push an SV onto the stack. The stack must have room for this element. See XPUSHs. PUSHs(sv) Closing bracket for XSUB arguments. This is usually handled by xsubpp. See PUSHMARK and the perlcall manpage for other uses. PUTBACK; The XSUB-writer's interface to the C realloc function. void * Renew( void *ptr, int size, type ) The XSUB-writer's interface to the C realloc function, with cast. void * Renewc( void *ptr, int size, type, cast ) Variable which is setup by xsubpp to hold the return value for an XSUB. This is always the proper type for the XSUB. See the section on The RETVAL Variable in the perlxs manpage. The XSUB-writer's interface to the C free function. The XSUB-writer's interface to the C malloc function. The XSUB-writer's interface to the C realloc function. Copy a string to a safe spot. This does not use an SV. char* savepv _((char* sv)); Copy a string to a safe spot. The len indicates number of bytes to copy. This does not use an SV. char* savepvn _((char* sv, I32 len)); Opening bracket for temporaries on a callback. See FREETMPS and the perlcall manpage. SAVETMPS; Stack pointer. This is usually handled by xsubpp. See dSP and SPAGAIN. Refetch the stack pointer. Used after a callback. See the perlcall manpage. SPAGAIN; Used to access elements on the XSUB's stack. SV* ST(int x) Test two strings to see if they are equal. Returns true or false. int strEQ( char *s1, char *s2 ) Test two strings to see if the first, s1, is greater than or equal to the second, s2. Returns true or false. int strGE( char *s1, char *s2 ) Test two strings to see if the first, s1, is greater than the second, s2. Returns true or false. int strGT( char *s1, char *s2 ) Test two strings to see if the first, s1, is less than or equal to the second, s2. Returns true or false. int strLE( char *s1, char *s2 ) Test two strings to see if the first, s1, is less than the second, s2. Returns true or false. int strLT( char *s1, char *s2 ) Test two strings to see if they are different. Returns true or false. int strNE( char *s1, char *s2 ) Test two strings to see if they are equal. The len parameter indicates the number of bytes to compare. Returns true or false. int strnEQ( char *s1, char *s2 ) Test two strings to see if they are different. The len parameter indicates the number of bytes to compare. Returns true or false. int strnNE( char *s1, char *s2, int len ) Marks an SV as mortal. The SV will be destroyed when the current context ends. SV* sv_2mortal _((SV* sv)); Blesses an SV into a specified package. The SV must be an RV. The package must be designated by its stash (see gv_stashpv()). The reference count of the SV is unaffected. SV* sv_bless _((SV* sv, HV* stash)); Concatenates the string onto the end of the string which is in the SV. void sv_catpv _((SV* sv, char* ptr)); Concatenates the string onto the end of the string which is in the SV. The len indicates number of bytes to copy. void sv_catpvn _((SV* sv, char* ptr, STRLEN len)); Processes its arguments like sprintf and appends the formatted output to an SV. void sv_catpvf _((SV* sv, const char* pat, ...)); Concatenates the string from SV ssv onto the end of the string in SV dsv. void sv_catsv _((SV* dsv, SV* ssv)); Compares the strings in two SVs. Returns -1, 0, or 1 indicating whether the string in sv1 is less than, equal to, or greater than the string in sv2. I32 sv_cmp _((SV* sv1, SV* sv2)); Returns the length of the string which is in the SV. See SvLEN. int SvCUR (SV* sv) Set the length of the string which is in the SV. See SvCUR. SvCUR_set (SV* sv, int val ) Auto-decrement of the value in the SV. void sv_dec _((SV* sv)); Returns a pointer to the last character in the string which is in the SV. See SvCUR. Access the character as *SvEND(sv) Returns a boolean indicating whether the strings in the two SVs are identical. I32 sv_eq _((SV* sv1, SV* sv2)); Expands the character buffer in the SV. Calls sv_grow to perform the expansion if necessary. Returns a pointer to the character buffer. char * SvGROW( SV* sv, int len ) Expands the character buffer in the SV. This will use sv_unref and will upgrade the SV to SVt_PV. Returns a pointer to the character buffer. Use SvGROW. Auto-increment of the value in the SV. void sv_inc _((SV* sv)); Returns a boolean indicating whether the SV contains an integer. int SvIOK (SV* SV) Unsets the IV status of an SV. SvIOK_off (SV* sv) Tells an SV that it is an integer. SvIOK_on (SV* sv) Tells an SV that it is an integer and disables all other OK bits. SvIOK_on (SV* sv) Returns a boolean indicating whether the SV contains an integer. Checks the private setting. Use SvIOK. int SvIOKp (SV* SV) Returns a boolean indicating whether the SV is blessed into the specified class. This does not know how to check for subtype, so it doesn't work in an inheritance relationship. int sv_isa _((SV* sv, char* name)); Returns the integer which is in the SV. int SvIV (SV* sv) Returns a boolean indicating whether the SV is an RV pointing to a blessed object. If the SV is not an RV, or if the object is not blessed, then this will return false. int sv_isobject _((SV* sv)); Returns the integer which is stored in the SV. int SvIVX (SV* sv); Returns the size of the string buffer in the SV. See SvCUR. int SvLEN (SV* sv) Returns the length of the string in the SV. Use SvCUR. STRLEN sv_len _((SV* sv)); Adds magic to an SV. void sv_magic _((SV* sv, SV* obj, int how, char* name, I32 namlen)); Creates a new SV which is a copy of the original SV. The new SV is marked as mortal. SV* sv_mortalcopy _((SV* oldsv)); Returns a boolean indicating whether the value is an SV. int SvOK (SV* sv) Creates a new SV which is mortal. The reference count of the SV is set to 1. SV* sv_newmortal _((void)); This is the false SV. See sv_yes. Always refer to this as &sv_no. Returns a boolean indicating whether the SV contains a number, integer or double. int SvNIOK (SV* SV) Unsets the NV/IV status of an SV. SvNIOK_off (SV* sv) Returns a boolean indicating whether the SV contains a number, integer or double. Checks the private setting. Use SvNIOK. int SvNIOKp (SV* SV) Returns a boolean indicating whether the SV contains a double. int SvNOK (SV* SV) Unsets the NV status of an SV. SvNOK_off (SV* sv) Tells an SV that it is a double. SvNOK_on (SV* sv) Tells an SV that it is a double and disables all other OK bits. SvNOK_on (SV* sv) Returns a boolean indicating whether the SV contains a double. Checks the private setting. Use SvNOK. int SvNOKp (SV* SV) Returns the double which is stored in the SV. double SvNV (SV* sv); Returns the double which is stored in the SV. double SvNVX (SV* sv); Returns a boolean indicating whether the SV contains a character string. int SvPOK (SV* SV) Unsets the PV status of an SV. SvPOK_off (SV* sv) Tells an SV that it is a string. SvPOK_on (SV* sv) Tells an SV that it is a string and disables all other OK bits. SvPOK_on (SV* sv) Returns a boolean indicating whether the SV contains a character string. Checks the private setting. Use SvPOK. int SvPOKp (SV* SV) Returns a pointer to the string in the SV, or a stringified form of the SV if the SV does not contain a string. If len is na then Perl will handle the length on its own. char * SvPV (SV* sv, int len ) Returns a pointer to the string in the SV. The SV must contain a string. char * SvPVX (SV* sv) Returns the value of the object's reference count. int SvREFCNT (SV* sv); Decrements the reference count of the given SV. void SvREFCNT_dec (SV* sv) Increments the reference count of the given SV. void SvREFCNT_inc (SV* sv) Tests if the SV is an RV. int SvROK (SV* sv) Unsets the RV status of an SV. SvROK_off (SV* sv) Tells an SV that it is an RV. SvROK_on (SV* sv) Dereferences an RV to return the SV. SV* SvRV (SV* sv); Taints an SV if tainting is enabled SvTAINT (SV* sv); Checks to see if an SV is tainted. Returns TRUE if it is, FALSE if not. SvTAINTED (SV* sv); Untaints an SV. Be very careful with this routine, as it short-circuits some of Perl's fundamental security features. XS module authors should not use this function unless they fully understand all the implications of unconditionally untainting the value. Untainting should be done in the standard perl fashion, via a carefully crafted regexp, rather than directly untainting variables. SvTAINTED_off (SV* sv); Marks an SV as tainted. SvTAINTED_on (SV* sv); Copies an integer into the given SV. void sv_setiv _((SV* sv, IV num)); Copies a double into the given SV. void sv_setnv _((SV* sv, double num)); Copies a string into an SV. The string must be null-terminated. void sv_setpv _((SV* sv, char* ptr)); Copies a string into an SV. The len parameter indicates the number of bytes to be copied. void sv_setpvn _((SV* sv, char* ptr, STRLEN len)); Processes its arguments like sprintf and sets an SV to the formatted output. void sv_setpvf _((SV* sv, const char* pat, ...)); Copies an integer into a new SV, optionally blessing the SV. The rv argument will be upgraded to an RV. That RV will be modified to point to the new SV. The classname argument indicates the package for the blessing. Set classname to Nullch to avoid the blessing. The new SV will be returned and will have a reference count of 1. SV* sv_setref_iv _((SV *rv, char *classname, IV iv)); Copies a double into a new SV, optionally blessing the SV. The rv argument will be upgraded to an RV. That RV will be modified to point to the new SV. The classname argument indicates the package for the blessing. Set classname to Nullch to avoid the blessing. The new SV will be returned and will have a reference count of 1. SV* sv_setref_nv _((SV *rv, char *classname, double nv)); Copies a pointer into a new SV, optionally blessing the SV. The rv argument will be upgraded to an RV. That RV will be modified to point to the new SV. If the pv argument is NULL then sv_undef will be placed into the SV. The classname argument indicates the package for the blessing. Set classname to Nullch to avoid the blessing. The new SV will be returned and will have a reference count of 1. SV* sv_setref_pv _((SV *rv, char *classname, void* pv)); Do not use with integral Perl types such as HV, AV, SV, CV, because those objects will become corrupted by the pointer copy process. Note that sv_setref_pvn copies the string while this copies the pointer. Copies a string into a new SV, optionally blessing the SV. The length of the string must be specified with n. The rv argument will be upgraded to an RV. That RV will be modified to point to the new SV. The classname argument indicates the package for the blessing. Set classname to Nullch to avoid the blessing. The new SV will be returned and will have a reference count of 1. SV* sv_setref_pvn _((SV *rv, char *classname, char* pv, I32 n)); Note that sv_setref_pv copies the pointer while this copies the string. Copies the contents of the source SV ssv into the destination SV dsv. The source SV may be destroyed if it is mortal. void sv_setsv _((SV* dsv, SV* ssv)); Returns the stash of the SV. HV * SvSTASH (SV* sv) Integer type flag for scalars. See svtype. Pointer type flag for scalars. See svtype. Type flag for arrays. See svtype. Type flag for code refs. See svtype. Type flag for hashes. See svtype. Type flag for blessed scalars. See svtype. Double type flag for scalars. See svtype. Returns a boolean indicating whether Perl would evaluate the SV as true or false, defined or undefined. int SvTRUE (SV* sv) Returns the type of the SV. See svtype. svtype SvTYPE (SV* sv) An enum of flags for Perl types. These are found in the file sv.h in the svtype enum. Test these flags with the SvTYPE macro. Used to upgrade an SV to a more complex form. Uses sv_upgrade to perform the upgrade if necessary. See svtype. bool SvUPGRADE _((SV* sv, svtype mt)); Upgrade an SV to a more complex form. Use SvUPGRADE. See svtype. This is the undef SV. Always refer to this as &sv_undef. Unsets the RV status of the SV, and decrements the reference count of whatever was being referenced by the RV. This can almost be thought of as a reversal of newSVrv. See SvROK_off. void sv_unref _((SV* sv)); Tells an SV to use ptr to find its string value. Normally the string is stored inside the SV but sv_usepvn allows the SV to use an outside string. The ptr should point to memory that was allocated by malloc. The string length, len, must be supplied. This function will realloc the memory pointed to by ptr, so that pointer should not be freed or used by the programmer after giving it to sv_usepvn. void sv_usepvn _((SV* sv, char* ptr, STRLEN len)); This is the true SV. See sv_no. Always refer to this as &sv_yes. Variable which is setup by xsubpp to designate the object in a C++ XSUB. This is always the proper type for the C++ object. See CLASS and the section on Using XS With C++ in the perlxs manpage. Converts the specified character to lowercase. int toLOWER (char c) Converts the specified character to uppercase. int toUPPER (char c) This is the XSUB-writer's interface to Perl's warn function. Use this function the same way you use the C printf function. See croak(). Push an integer onto the stack, extending the stack if necessary. See PUSHi. XPUSHi(int d) Push a double onto the stack, extending the stack if necessary. See PUSHn. XPUSHn(double d) Push a string onto the stack, extending the stack if necessary. The len indicates the length of the string. See PUSHp. XPUSHp(char *c, int len) Push an SV onto the stack, extending the stack if necessary. See PUSHs. XPUSHs(sv) Macro to declare an XSUB and its C parameter list. This is handled by xsubpp. Return from XSUB, indicating number of items on the stack. This is usually handled by xsubpp. XSRETURN(int x); Return an empty list from an XSUB immediately. XSRETURN_EMPTY; Return an integer from an XSUB immediately. Uses XST_mIV. XSRETURN_IV(IV v); Return &sv_no from an XSUB immediately. Uses XST_mNO. XSRETURN_NO; Return an double from an XSUB immediately. Uses XST_mNV. XSRETURN_NV(NV v); Return a copy of a string from an XSUB immediately. Uses XST_mPV. XSRETURN_PV(char *v); Return &sv_undef from an XSUB immediately. Uses XST_mUNDEF. XSRETURN_UNDEF; Return &sv_yes from an XSUB immediately. Uses XST_mYES. XSRETURN_YES; Place an integer into the specified position i on the stack. The value is stored in a new mortal SV. XST_mIV( int i, IV v ); Place a double into the specified position i on the stack. The value is stored in a new mortal SV. XST_mNV( int i, NV v ); Place &sv_no into the specified position i on the stack. XST_mNO( int i ); Place a copy of a string into the specified position i on the stack. The value is stored in a new mortal SV. XST_mPV( int i, char *v ); Place &sv_undef into the specified position i on the stack. XST_mUNDEF( int i ); Place &sv_yes into the specified position i on the stack. XST_mYES( int i ); The version identifier for an XS module. This is usually handled automatically by ExtUtils::MakeMaker. See XS_VERSION_BOOTCHECK. Macro to verify that a PM module's $VERSION variable matches the XS module's XS_VERSION variable. This is usually handled automatically by xsubpp. See the section on The VERSIONCHECK: Keyword in the perlxs manpage. The XSUB-writer's interface to the C memzero function. The d is the destination, n is the number of items, and t is the type. (void) Zero( d, n, t ); Jeff Okamoto <okamoto@corp.hp.com> 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, and Stephen McCamant. API Listing by Dean Roehrich <roehrich@cray.com>. Version 31.8: 1997/5/17