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perlman:perlgutsby gods (Initiate) |
| on Aug 25, 1999 at 07:29 UTC ( [id://424]=perlman: print w/replies, xml ) | Need Help?? |
perlgutsCurrent Perl documentation can be found at perldoc.perl.org. Here is our local, out-dated (pre-5.6) version:
NAMEperlguts - Perl's Internal Functions
DESCRIPTIONThis 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.
Variables
DatatypesPerl 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 SVsAn SV can be created and loaded with one command. There are four types of values that can be loaded: an integer value (IV), a double (NV), a string, (PV), and another scalar (SV). The six routines are:
SV* newSViv(IV);
SV* newSVnv(double);
SV* newSVpv(char*, int);
SV* newSVpvn(char*, int);
SV* newSVpvf(const char*, ...);
SV* newSVsv(SV*);
To change the value of an *already-existing* SV, there are seven routines:
void sv_setiv(SV*, IV);
void sv_setuv(SV*, UV);
void sv_setnv(SV*, double);
void sv_setpv(SV*, char*);
void sv_setpvn(SV*, 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 perlman:perlguts, perlman:perlguts, or perlman:perlguts, or you may allow Perl to calculate the length by using perlman:perlguts or by specifying 0 as the second argument to perlman:perlguts. Be warned, though, that Perl will determine the string's length by using The arguments of perlman:perlguts are processed like sprintf, and the formatted output becomes the value.
The All SVs that contain strings should be terminated with a NUL character. If it is not NUL-terminated there is a risk of core dumps and corruptions from code which passes the string to C functions or system calls which expect a NUL-terminated string. Perl's own functions typically add a trailing NUL for this reason. Nevertheless, you should be very careful when you pass a string stored in an SV to a C function or system call. To access the actual value that an SV points to, you can use the macros:
SvIV(SV*)
SvNV(SV*)
SvPV(SV*, STRLEN len)
which will automatically coerce the actual scalar type into an IV, double, or string.
In the perlman:perlguts macro, the length of the string returned is placed into the variable 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 perlman:perlguts. Note that perlman:perlguts 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 perlman:perlguts). 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 perlman:perlguts is true. If you want to append something to the end of string stored in an perlman:perlguts, 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_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
The 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 perlman:perlguts. Its address can be used whenever an perlman:perlguts is needed. There are also the two values perlman:perlguts and perlman:perlguts, which contain Boolean TRUE and FALSE values, respectively. Like perlman:perlguts, their addresses can be used whenever an perlman:perlguts is needed.
Do not be fooled into thinking that
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 perlman:perlguts in the first line and all will be well. To free an SV that you've created, call perlman:perlguts. Normally this call is not necessary (see 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 perlman:perlguts macros. Because a scalar can be both a number and a string, usually these macros will always return
TRUE and calling the
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
Working with AVsThere 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 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 perlman:perlguts. This routine adds 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 perlman:perlguts function returns the highest index value in array (just like $#array in
Perl). If the array is empty, -1 is returned. The
perlman:perlguts function returns the value at index
void av_clear(AV*);
void av_undef(AV*);
void av_extend(AV*, I32 key);
The perlman:perlguts function deletes all the elements in the
AV* array, but does not actually delete the array
itself. The perlman:perlguts function will delete all the elements in the array plus the array itself.
The
perlman:perlguts function extends the array so that it contains 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 Understanding the Magic of Tied Hashes and Arrays for more information on how to use the array access functions on tied arrays.
Working with HVsTo 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 Remember that perlman:perlguts and perlman:perlguts return perlman:perlguts's and not just perlman:perlguts. 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 And more miscellaneous functions:
void hv_clear(HV*);
void hv_undef(HV*);
Like their AV counterparts, perlman:perlguts deletes all the entries in the hash table but does not actually delete the hash table. The perlman:perlguts 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 perlman:perlguts. However, once you have an
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
i = klen;
hash = 0;
s = key;
while (i--)
hash = hash * 33 + *s++;
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 ExtensionsBeginning 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 perlman:perlguts keys, which simplifies writing of extension code that deals with hash structures. These functions also allow passing of perlman:perlguts 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 ( 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 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 perlman:perlgutss:
HeKEY(HE* he)
HeKLEN(HE* he)
Note that both perlman:perlguts and perlman:perlguts do not increment the reference count of the stored
ReferencesReferences 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 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 perlman:perlguts to either an perlman:perlguts or perlman:perlguts, 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 ObjectsReferences 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 /* Still under construction */
Upgrades rv to reference if not already one. Creates new
SV for rv to point to. If
SV* newSVrv(SV* rv, char* classname);
Copies integer or double into an
SV whose reference is
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
SV* sv_setref_pv(SV* rv, char* classname, PV iv);
Copies string into an
SV whose reference is
SV* sv_setref_pvn(SV* rv, char* classname, PV iv, int length); Tests whether the SV is blessed into the specified class. It does not check inheritance relationships.
int sv_isa(SV* sv, 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
bool sv_derived_from(SV* sv, char* name); To check if you've got an object derived from a specific class you have to write:
if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }
Creating New VariablesTo 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
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 MortalityPerl 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 perlman:perlguts 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 perlman:perlguts 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 perlman:perlguts, passing it the just-created SV. This returns the reference as a new SV, but the reference count of the SV you passed to perlman:perlguts 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 perlman:perlguts instead of perlman:perlguts. 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 perlman:perlguts. 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 perlman:perlguts), 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 perlman:perlguts) to the perlman:perlguts or perlman:perlguts routines.
Stashes and GlobsA ``stash'' is a hash that contains all of the different objects that are contained within a package. Each key of the stash is a symbol name (shared by all the different types of objects that have the same name), and each value in the hash table is a GV (Glob Value). This GV in turn contains references to the various objects of that name, including (but not limited to) the following:
Scalar Value
Array Value
Hash Value
I/O Handle
Format
Subroutine
There is a single stash called ``PL_defstash'' that holds the items that exist in the ``main'' package. To get at the items in other packages, append the string ``::'' to the package name. The items in the ``Foo'' package are in the stash ``Foo::'' in PL_defstash. The items in the ``Bar::Baz'' package are in the stash ``Baz::'' in ``Bar::'''s stash. To get the stash pointer for a particular package, use the function:
HV* gv_stashpv(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
perlman:perlguts. The
The name that 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 perlman:perlguts, must be a reference, and the second argument is a stash. The returned perlman:perlguts 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 SVsScalar 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
To force multiple data values into an
SV, you must do two things: use the
SvIOK_on
SvNOK_on
SvPOK_on
SvROK_on
The particular macro you must use depends on which 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 perlman:perlguts and perlman:perlguts had been reversed, then the macro perlman:perlguts would need to be called instead of perlman:perlguts.
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 {
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 MagicPerl adds magic to an SV using the sv_magic function:
void sv_magic(SV* sv, SV* obj, int how, char* name, I32 namlen);
The
If
The
The sv_magic function uses
The There is also a function to add magic to an perlman:perlguts:
void hv_magic(HV *hv, GV *gv, int how);
This simply calls perlman:perlguts and coerces the To remove the magic from an SV, call the function sv_unmagic:
void sv_unmagic(SV *sv, int type);
The
Magic Virtual Tables
The
The
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
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
{ 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 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
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 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
Finding Magic
MAGIC* mg_find(SV*, int type); /* Finds the magic pointer of that type */
This routine returns a pointer to the
int mg_copy(SV* sv, SV* nsv, char* key, STRLEN klen);
This routine checks to see what types of magic
Understanding the Magic of Tied Hashes and ArraysTied 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 perlman:perlguts function, when given a tied array argument, merely copies the magic of the array onto the value to be ``stored'', using perlman:perlguts. It may also return NULL, indicating that the value did not actually need to be stored in the array. [MAYCHANGE] After a call to perlman:perlguts on a tied array, the caller will usually need to call perlman:perlguts to actually invoke the perl level ``STORE'' method on the TIEARRAY object. If perlman:perlguts did return NULL, a call to perlman:perlguts will also be usually necessary to avoid a memory leak. [/MAYCHANGE] The previous paragraph is applicable verbatim to tied hash access using the perlman:perlguts and perlman:perlguts functions as well. perlman:perlguts and the corresponding hash functions perlman:perlguts and perlman:perlguts actually return an undefined mortal value whose magic has been initialized using perlman:perlguts. Note the value so returned does not need to be deallocated, as it is already mortal. [MAYCHANGE] But you will need to call perlman:perlguts on the returned value in order to actually invoke the perl level ``FETCH'' method on the underlying TIE object. Similarly, you may also call perlman:perlguts on the return value after possibly assigning a suitable value to it using perlman:perlguts, 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 perlman:perlguts to attach magic to the values that were meant to be ``stored'' or ``fetched''. Later calls to perlman:perlguts and perlman:perlguts 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 changesPerl 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 $var, irrespective of how control exits the block: goto, return, die/eval etc. It is a little bit more efficient as well.
There is a way to achieve a similar task from
C via Perl
API: create a
pseudo-block, and arrange for some changes to be automatically undone at the end of it, either explicit, or via a non-local exit (via
Inside such a pseudo-block the following service is available:
The following
API list contains functions, thus one needs to provide pointers to the modifiable data explicitly (either
C pointers, or Perlish
The
Subroutines
XSUBs and the Argument StackThe XSUB mechanism is a simple way for Perl programs to access C subroutines. An XSUB routine will have a stack that contains the arguments from the Perl program, and a way to map from the Perl data structures to a C equivalent.
The stack arguments are accessible through the perlman:perlguts macro, which returns the
Most of the time, output from the
C routine can be handled through use of the
RETVAL and
OUTPUT directives. However, there are some cases where the argument stack is not already long enough to handle all the return values. An example is the
POSIX
To handle this situation, the PPCODE directive is used and the stack is extended using the macro:
EXTEND(SP, num);
where perlman:perlguts is the macro that represents the local copy of the stack pointer, and Now that there is room on the stack, values can be pushed on it using the macros to push IVs, doubles, strings, and SV pointers respectively:
PUSHi(IV)
PUSHn(double)
PUSHp(char*, I32)
PUSHs(SV*)
And now the Perl program calling
($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 perlman:perlguts to extend the stack. For more information, consult the perlxs manpage and the perlxstut manpage.
Calling Perl Routines from within C ProgramsThere 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 perlman:perlguts. The perlman:perlguts 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 perlman:perlguts), the programmer must manipulate the Perl stack. These include the following macros and functions:
dSP
SP
PUSHMARK()
PUTBACK
SPAGAIN
ENTER
SAVETMPS
FREETMPS
LEAVE
XPUSH*()
POP*()
For a detailed description of calling conventions from C to Perl, consult the perlcall manpage.
Memory AllocationIt 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
The second argument
The third and fourth arguments
Unlike the perlman:perlguts and perlman:perlguts macros, the perlman:perlguts macro calls
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 perlman:perlguts and perlman:perlguts match those of perlman:perlguts and perlman:perlguts 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 |
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