PERLGUTS
NAME DESCRIPTION Variables Subroutines Compiled code Examining internal data structures with the dump functions How multiple interpreters and concurrency are supported Internal Functions Unicode Support AUTHORS SEE ALSO
perlguts - Introduction to the Perl API
This document attempts to describe how to use the Perl API , as well as containing some info on the basic workings of the Perl core. It is far from complete and probably contains many errors. Please refer any questions or comments to the author below.
Datatypes
Perl has three typedefs that handle Perl's three main data
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, respectively. (Again, there are U32 and U16, as well.)
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 ).
SV* newSVnv(double); SV* newSVpv(const char*, int); SV* newSVpvn(const char*, int); SV* newSVpvf(const char*, ...); SV* newSVsv(SV*);
void sv_setuv(SV*, UV); void sv_setnv(SV*, double); void sv_setpv(SV*, const char*); void sv_setpvn(SV*, const char*, int) void sv_setpvf(SV*, const char*, ...); void sv_setpvfn(SV*, const char*, STRLEN, va_list , SV **, I32, bool); void sv_setsv(SV, SV*); Notice that you can choose to specify the length of the string to be assigned by using 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 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.
sv_setpvfn 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.
STRLEN is an integer type (Size_t, usually defined as size_t in config.h) guaranteed to be large enough to represent the size of any string that perl can handle.
The sv_set*() functions are not generic enough to operate 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
SvUV(SV*) SvNV(SV*) SvPV(SV*, STRLEN len) SvPV_nolen(SV*) which will automatically coerce the actual scalar type into an IV , UV , double, or string.
In the SvPV macro, the length of the string returned is placed into the variable len (this is a macro, so you do not use ). If you do not care what the length of the data is, use the SvPV_nolen macro. Historically the SvPV macro with the global variable PL_na has been used in this case. But that can be quite inefficient because PL_na must be accessed in thread-local storage in threaded Perl. In any case, remember that Perl allows arbitrary strings of data that may both contain NULs and might not be terminated by a NUL .
Also remember that C doesn't allow you to safely say foo(SvPV(s, len), len);. It might work with your compiler, but it won't work for everyone. Break this sort of
SV *s;
STRLEN len; char * ptr; ptr = SvPV(s, len); foo(ptr, len);
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*)
SvCUR_set(SV*, I32 val)
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
void sv_catpvn(SV*, const char*, STRLEN); void sv_catpvf(SV*, const char*, ...); void sv_catpvfn(SV*, const char*, STRLEN, va_list , SV **, I32, bool); void sv_catsv(SV, SV*); The first function calculates the length of the string to be appended by using 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 operate 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
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
The scalar undef value is stored in an SV instance called PL_sv_undef. Its address can be used whenever an SV* is needed.
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 . Take this
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 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 ``Reference Counts and Mortality'').
Offsets
Perl provides the function sv_chop to efficiently remove characters from the beginning of a string; you give it an SV and a pointer to somewhere inside the the PV , and it discards everything before the pointer. The efficiency comes 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 allocated storage.
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 perfomed on AVs to enable efficient shifting and splicing off the beginning 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, or string pointer in an SV , you can
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
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
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.
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 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
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 functions on tied arrays.
Working with HVs
To create an HV , you use the following
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); If flags does not include the G_DISCARD flag then hv_delete will create and return a mortal copy of the deleted value.
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 */
This returns NULL if the variable does not exist.
The hash algorithm is defined in the PERL_HASH(hash,
while (klen--) hash = (hash * 33) + *key++; hash = hash + (hash The last step was added in version 5.6 to improve distribution 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 functions on tied hashes.
Hash API Extensions
Beginning with version 5.004, the following functions are
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 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)
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
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
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
To discover what type of value the reference refers to, use the following macro and then check the return value.
SvTYPE(SvRV(SV*))
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
The sv argument must be a reference. 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 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_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. It does not check inheritance relationships.
int sv_isa(SV* sv, const char* name);
Tests whether the SV is a reference to a blessed object.
int sv_isobject(SV* sv);
Tests whether the SV is derived from the specified class. SV can be either a reference to a blessed object or a string containing a class name. This is the function implementing the UNIVERSAL::isa functionality.
bool sv_derived_from(SV* sv, const char* name);
if (sv_isobject(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* get_sv(
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
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
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 perlcall and perlxs 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.
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
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
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
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.
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
extern char *dberror_list;
SV* sv = get_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
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
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 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
This simply calls sv_magic and coerces the gv argument into an SV.
To remove the magic from an SV , call the
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
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.
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_. NOTE: the magic routines are not considered part of the Perl API , and may not be exported by the Perl library.
------- ------ ---------------------------- 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) D vtbl_regdata Regex match position data (@+ and @- vars) d vtbl_regdatum Regex match position data element 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 %_ 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
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. A simple example of how to add 'U' magic is shown below. Note that the ufuncs structure is copied by sv_magic, so you can safely allocate it on the stack.
void
Umagic(sv) SV *sv; PREINIT: struct ufuncs uf; CODE: uf.uf_val = Note that because multiple extensions may be using '' or 'U' magic, it is important for extensions to take extra care to avoid conflict. Typically only using the magic on objects blessed into the same class as the extension is sufficient. For '' magic, it may also be appropriate to add an I32 'signature' at the top of the private data area and check that.
Also note that the sv_set*() and sv_cat*() functions described earlier do not invoke 'set' magic on their targets. 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() functions. Similarly, generic C code must call the SvGETMAGIC() 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*() functions typically need to be followed by SvSETMAGIC(), but they don't need a prior SvGETMAGIC() since their implementation 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);
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 perl tie function associates a variable with an object that implements the various GET , SET etc methods. To perform the equivalent of the perl tie function from an XSUB , you must mimic this behaviour. The code below carries out the necessary steps - firstly it creates a new hash, and then creates a second hash which it blesses into the class which will implement the tie methods. Lastly it ties the two hashes together, and returns a reference to the new tied hash. Note that the code below does NOT call the TIEHASH method in the !MyTie? class - see ``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( 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 ``Returning a Scalar'' in perlcall). 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:
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 SvREFCNT_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 reference count. This usually has the effect of keeping sv alive until the statement that called the currently 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 corresponds 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 corresponding string may be
SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void
At the end of pseudo-block the function f is called with the only argument p.
SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void
At the end of pseudo-block the function f is called with the implicit context argument (if any), and p.
SAVESTACK_POS()
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 *.
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 arguments via an array sarg of SV* of length maxsarg.
SV* save_svref(SV **sptr)
Similar to save_scalar, but will reinstate a 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 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
where SP is the macro that represents the local copy of 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
PUSHn(double) PUSHp(char*, I32) PUSHs(SV*)
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. However, 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
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* argument contains either the name of the Perl subroutine to be called, or a reference to the subroutine. The second argument consists of flags that control the context in which the subroutine is called, whether or not the subroutine is being passed arguments, how errors should be trapped, and how to treat return values.
All four routines return the number of arguments that the subroutine returned on the Perl stack.
These routines used to be called perl_call_sv etc., before Perl v5.6.0, but those names are now deprecated; macros of the same name are provided for compatibility.
When using any of these routines (except call_argv), the programmer must manipulate the Perl stack. These include the following macros and
SP PUSHMARK() PUTBACK SPAGAIN ENTER SAVETMPS FREETMPS LEAVE XPUSH*() POP*() For a detailed description of calling conventions from C to Perl, consult perlcall.
Memory Allocation
All memory meant to be used with the Perl API functions should be manipulated using the macros described in this section. The macros provide the necessary transparency between differences in the actual malloc implementation that is used within perl.
It is suggested that you enable the version of malloc that is distributed with Perl. It keeps pools of various sizes of unallocated memory in order to satisfy allocation requests more quickly. However, on some platforms, it may cause spurious malloc or free errors.
New(x, pointer, number, type);
Newc(x, pointer, number, type, cast); Newz(x, pointer, number, type); These three macros are used to initially allocate memory.
The first argument 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 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. 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 ``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
XPUSHi(20); This translates as TARG to 10, push a pointer to TARG onto the stack; set TARG to 20, push a pointer to TARG onto the stackTARG, which we have set to 20. If you need to push multiple different values, use XPUSHs, which bypasses TARG.
On a related note, if you do use (X)PUSH[npi?, then you're going to need a dTARG in your variable declarations so that the PUSH macros can make use of the local variable TARG.
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
assign-to
/ \
/ \ $b $c
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
several lines per node, and for $b+$c it looks like
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.
Each of these nodes represents an op, a fundamental operation inside the Perl core. The code which implements each operation can be found in the pp*.c files; the function which implements the op with type gvsv is pp_gvsv, and so on. As the tree above shows, different ops have different numbers of children: add is a binary operator, as one would expect, and so has two children. To accommodate the various different numbers of children, there are various types of op data structure, and they link together in different ways.
The simplest type of op structure is OP: this has no children. Unary operators, UNOPs, have one child, and this is pointed to by the op_first field. Binary operators (BINOPs) 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 regular 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 matters, if a UNOP is actually a null op after optimization (see ``Compile pass 2: context propagation'') it will still have children in accordance with its former type.
Compile pass 1: check routines
The tree is created by the 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 are no back-links to the currently constructed node, one can do most any operation to the top-level node, including freeing it and/or creating new nodes above/below it.
The check routine returns the node which should be inserted into the tree (if the top-level node was not modified, check routine returns its argument).
By convention, check routines have names 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. At this time the context can have 5 values (instead of 2 for runtime context): void, boolean, scalar, list, and lvalue. In contrast with the pass 1 this pass is processed from top to bottom: a node's context determines the context for its children.
Additional context-dependent optimizations are performed at this time. Since at this moment the compile tree contains back-references (via ``thread pointers), nodes cannot be free()d now. To allow optimized-away nodes at this stage, such nodes are null()ified instead of free()''ing (i.e. their type is changed to OP_NULL ).
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 subroutine peep(). Optimizations performed at this stage are subject to the same restrictions as in the pass 2.
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.
Perl_op_dump can be used to dump an OP structure or any of its derivatives, and produces output similiar to perl -Dx; in fact, Perl_dump_eval will dump the main root of the code being evaluated, exactly like -Dx.
Other useful functions are Perl_dump_sub, which turns a GV into an op tree, Perl_dump_packsubs which calls Perl_dump_sub on all the subroutines in a package like so: (Thankfully, these are all xsubs, so there is no op tree)
(gdb) print Perl_dump_packsubs(PL_defstash) SUB attributes::bootstrap = (xsub 0x811fedc 0) SUB UNIVERSAL::can = (xsub 0x811f50c 0) SUB UNIVERSAL::isa = (xsub 0x811f304 0) SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0) SUB !DynaLoader?::boot_!DynaLoader? = (xsub 0x805b188 0)
and Perl_dump_all, which dumps all the subroutines in the stash and the op tree of the main root.
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
The thread, the C structure, or the C ++ object will contain all the context, the state of that interpreter.
Three macros control the major Perl build flavors: MULTIPLICITY , USE_THREADS and PERL_OBJECT . The MULTIPLICITY build has a C structure that packages all the interpreter state, there is a similar thread-specific data structure under USE_THREADS , and the (now deprecated) PERL_OBJECT build has a C ++ class to maintain interpreter state. In all three cases, PERL_IMPLICIT_CONTEXT is also normally defined, and enables the support for passing in a ``hidden'' first argument that represents all three data structures.
All this obviously requires a way for the Perl internal functions to be C ++ methods, subroutines taking some kind of structure as the first argument, or subroutines taking nothing as the first argument. To enable these three very different ways of building the interpreter, the Perl source (as it does in so many other situations) makes heavy use of macros and subroutine naming conventions.
First problem: deciding which functions will be public API functions and which will be private. All functions whose names begin 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 perlbug explaining why you think it should be.
Second problem: there must be a syntax so that the same subroutine declarations and calls can pass a structure as their first argument, or pass nothing. To solve this, the subroutines are named and declared in a particular way. Here's a typical start of a static function used within the
S_incline(pTHX_ char *s) STATIC becomes ``static'' in C, and is #define'd to nothing in C ++ .
A public function (i.e. part of the internal API , but not necessarily sanctioned for use
Perl_sv_setsv(pTHX_ SV* dsv, SV* ssv) 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 containing the interpreter's context. The trailing underscore in the pTHX_ macro indicates that the macro expansion needs a comma after the context argument because other arguments follow it. If PERL_IMPLICIT_CONTEXT is not defined, pTHX_ will be ignored, and the subroutine is not prototyped to take the extra argument. The form of the macro without the trailing underscore is used when there are no additional explicit arguments.
When a core function calls another, it must pass the context. This is normally hidden via macros. Consider sv_setsv. It expands into something like
define sv_setsv(a,b) Perl_sv_setsv(aTHX_ a, b) /* can't do this for vararg functions, see below */ else define sv_setsv Perl_sv_setsv endif
and still have it work under all the modes Perl could have been compiled with.
Under PERL_OBJECT in the core, that will
or
pPerl-
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.
The context-free version of Perl_warner is called Perl_warner_nocontext, and does not take the extra argument. Instead it does dTHX; to get the context from thread-local storage. We #define warner Perl_warner_nocontext so that extensions get source compatibility at the expense of performance. (Passing an arg is cheaper than grabbing it from thread-local storage.)
You can ignore [pad?THX[xo? when browsing the Perl headers/sources. Those are strictly for use within the core. Extensions and embedders need only be aware of [pad?THX.
So what happened to dTHR?
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
sv_setsv(asv, bsv);
Perl_sv_setsv(Perl_get_context(), asv, bsv);
Perl_sv_setsv(asv, bsv);
You have to do nothing new in your extension to get this; since the Perl library provides Perl_get_context(), it will all just work.
The second, more efficient way is to use the following
include
static my_private_function(int arg1, int arg2); static SV *
my_private_function(int arg1, int arg2) { dTHX; /* fetch context */ ... call many Perl API functions ... }
[... etc ...? MODULE = Foo PACKAGE = Foo /* typical XSUB */ void
my_xsub(arg) int arg CODE: my_private_function(arg, 10); Note that the only two changes from the normal way of writing an extension is the addition of a #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
include
/* pTHX_ only needed for functions that call Perl API */
static my_private_function(pTHX_ int arg1, int arg2);
static SV *
my_private_function(pTHX_ int arg1, int arg2) { /* dTHX; not needed here, because THX is an argument */ ... call Perl API functions ... }
[... etc ...? MODULE = Foo PACKAGE = Foo /* typical XSUB */ void
my_xsub(arg) int arg CODE: my_private_function(aTHX_ arg, 10); This implementation never has to fetch the context using a function call, since it is always passed as an extra argument. Depending on your needs for simplicity or efficiency, you may mix the previous two approaches freely.
Never add a comma after 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.
Should I do anything special if I call perl from multiple threads?
If you create interpreters in one thread and then proceed to call them in another, you need to make sure perl's own Thread Local Storage ( TLS ) slot is initialized correctly in each of those threads.
The perl_alloc and perl_clone API functions will automatically set the TLS slot to the interpreter they created, so that there is no need to do anything special if the interpreter is always accessed in the same thread that created it, and that thread did not create or call any other interpreters afterwards. If that is not the case, you have to set the TLS slot of the thread before calling any functions in the Perl API on that particular interpreter. This is done by calling the PERL_SET_CONTEXT macro in that thread as the first
/* 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 interpreter to bundle up everything it knows about the environment it's running on. This is enabled with the PERL_IMPLICIT_SYS macro. Currently it only works with PERL_OBJECT and USE_THREADS 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 maintain their own state, broken down into seven C structures. These are thin wrappers around the usual system calls (see win32/perllib.c) for the default perl executable, but for a more ambitious host (like the one that would do fork() emulation) all the extra work needed to pretend that different interpreters are actually different ``processes, would be done here.
The Perl engine/interpreter and the host are orthogonal entities. There could be one or more interpreters in a process, and one or more ``hosts'', with free association between them.
All of Perl's internal functions which will be exposed to the outside world are be 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 functions 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 generated 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 pieces. It's important that when you add a new function to the core or change an existing one, you change the data in the table at the end of embed.pl as well. Here's a sample entry from that
The second column is the return type, the third column the name. Columns after that are the arguments. The first column is a set of flags:
A
This function is a part of the public API .
p
This function has a Perl_ prefix; ie, it is defined as Perl_av_fetch
d
This function has documentation using the apidoc feature which we'll look at in a second.
Other available flags are:
s
This is a static function and is defined as S_whatever, and usually called within the sources as whatever(...).
n
This does not use aTHX_ and pTHX to pass interpreter context. (See ``Background and PERL_IMPLICIT_CONTEXT '' 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
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 NVgf NV %g-like
printf(
The IVdf will expand to whatever is the correct format for the IVs.
If you are printing addresses of pointers, use UVxf combined with PTR2UV (), do not use %lx or %p.
Pointer-To-Integer and Integer-To-Pointer
Because pointer size does not necessarily equal integer size, use the follow macros to do it right.
PTR2UV(pointer)
PTR2IV(pointer) PTR2NV(pointer) INT2PTR(pointertotype, integer)
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 functions 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
=for apidoc sv_setiv
Copies an integer into the given SV. Does not handle 'set' magic. See
C
=cut
Please try and supply some documentation if you add functions to the Perl core.
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 ASCII . Most of us did, anyway. The big problem with ASCII is that it's American. Well, no, that's not actually the problem; the problem is that it's not particularly useful for people who don't use the Roman alphabet. What used to happen was that particular languages would stick their own alphabet in the upper range of the sequence, between 128 and 255. Of course, we then ended up with plenty of variants that weren't quite ASCII , and the whole point of it being a standard was lost.
Worse still, if you've got a language like Chinese or Japanese that has hundreds or thousands of characters, then you really can't fit them into a mere 256, so they had to forget about ASCII altogether, and build their own systems using pairs of numbers to refer to one character.
To fix this, some people formed Unicode, Inc. and produced a
new character set containing all the characters you can
possibly think of and more. There are several ways of
representing these characters, and the one Perl uses is
called UTF8 . UTF8 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 UTF8 string?
You can't. This is because UTF8 data is stored in bytes just like non-UTF8 data. The Unicode character 200, (0xC8 for you hex types) capital E with a grave accent, is represented 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 UTF8 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 UTF8 .
How does UTF8 represent Unicode characters?
As mentioned above, UTF8 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 character 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 UTF8 string, you can find out how long the first character in it
utf += len; len = UTF8SKIP(utf); /* len is 3 here */ Another way to skip over characters in a UTF8 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.
All bytes in a multi-byte UTF8 character will have the high bit set, so you can test if you need to do something special with this character like
You must convert characters to UVs using the above functions if you're ever in a situation where you have to match UTF8 and non-UTF8 characters. You may not skip over UTF8 characters in this case. If you do this, you'll lose the ability to match hi-bit non-UTF8 characters; for instance, if your UTF8 string contains v196.172, and you skip that character, you can never match a chr(200) in a non-UTF8 string. So don't do that!
How does Perl store UTF8 strings?
Currently, Perl deals with Unicode strings and non-Unicode strings slightly differently. If a string has been identified as being UTF-8 encoded, Perl will set a flag in the SV , SVf_UTF8. You can check and manipulate this flag with the following
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 UTF8 , and contains a byte sequence that could be UTF8 - especially when combining non-UTF8 and UTF8 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
SV *nsv; STRLEN len; char *p;
p = SvPV(sv, len);
frobnicate(p); nsv = newSVpvn(p, len);
frobnicate(p); 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 UTF8 data, so that it can handle the string appropriately.
How do I convert a string to UTF8 ?
If you're mixing UTF8 and non-UTF8 strings, you might find it necessary to upgrade one of the strings to UTF8 . If you've got an SV ,
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 UTF8-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 represented 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 UTF8 or not. You can tell if an SV is UTF8 by looking at is SvUTF8 flag. Don't forget to set the flag if something should be UTF8 . 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 UTF8 , always use utf8_to_uv to get at the value, unless !(*s
in which case you can use
When writing to a UTF8 string, always use uv_to_utf8, unless uv in which case you can use *s = uv.
Mixing UTF8 and non-UTF8 strings is tricky. Use bytes_to_utf8 to get a new string which is UTF8 encoded. There are tricks you can use to delay deciding whether you need to use a UTF8 string until you get to a high character - HALF_UPGRADE is one of those.
Until May 1997, this document was maintained by Jeff Okamoto
With lots of help and suggestions from Dean Roehrich, Malcolm Beattie, Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil Bowers, Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer, Stephen !McCamant?, and Gurusamy Sarathy.
API Listing originally by Dean Roehrich
Modifications to autogenerate the API listing (perlapi) by Benjamin Stuhl.
perlapi(1), perlintern(1), perlxs(1), perlembed(1)
3 pages link to perlguts(1):
