perlguts - Perl's Internal Functions
This document attempts to describe some of the internal functions of the Perl executable. It is far from complete and probably contains many errors. Please refer any questions or comments to the author below.
Perl has three typedefs that handle Perl's three main data types:
SV Scalar Value
AV Array Value
HV Hash Value
Each typedef has specific routines that manipulate the various data types.
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.
An SV can be created and loaded with one command. There are four types of values that can be loaded: an integer value (IV), a double (NV), a string, (PV), and another scalar (SV).
The six routines are:
SV* newSViv(IV);
SV* newSVnv(double);
SV* newSVpv(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 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 untrustworty (see
the perlsec manpage). This pointer may be
NULL if that information is not important. Note that
this function requires you to specify the length of the format.
The sv_set*() functions are not generic enough to 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 to, you can use the macros:
SvIV(SV*)
SvNV(SV*)
SvPV(SV*, STRLEN len)
which will automatically coerce the actual scalar type into an IV, double, or string.
In the SvPV macro, the length of the string returned is placed into the variable len (this is a macro, so you do not use &len). If you do not care what the length of the data is, use the global
variable PL_na. Remember, however, that Perl allows arbitrary strings of data that may both contain NULs and might not be terminated by a
NUL.
If you want to know if the scalar value is TRUE, you can use:
SvTRUE(SV*)
Although Perl will automatically grow strings for you, if you need to force Perl to allocate more memory for your SV, you can use the macro
SvGROW(SV*, STRLEN newlen)
which will determine if more memory needs to be allocated. If so, it will call the function sv_grow. Note that SvGROW can only increase, not decrease, the allocated memory of an SV and that it does not automatically add a byte for the a trailing NUL (perl's own string functions typically do SvGROW(sv, len + 1)).
If you have an SV and want to know what kind of data Perl thinks is stored in it, you can use the following macros to check the type of SV you have.
SvIOK(SV*)
SvNOK(SV*)
SvPOK(SV*)
You can get and set the current length of the string stored in an SV with the following macros:
SvCUR(SV*)
SvCUR_set(SV*, I32 val)
You can also get a pointer to the end of the string stored in the SV with the macro:
SvEND(SV*)
But note that these last three macros are valid only if SvPOK() is true.
If you want to append something to the end of string stored in an SV*, you can use the following functions:
void sv_catpv(SV*, char*);
void sv_catpvn(SV*, char*, int);
void sv_catpvf(SV*, const char*, ...);
void sv_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 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 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 &PL_sv_undef. Take this code:
SV* sv = (SV*) 0;
if (I-am-to-return-a-real-value) {
sv = sv_2mortal(newSViv(42));
}
sv_setsv(ST(0), sv);
This code tries to return a new SV (which contains the value 42) if it should return a real value, or undef otherwise. Instead it has returned a NULL pointer which, somewhere down the line, will cause a segmentation violation, bus error, or just weird results. Change the zero to &PL_sv_undef in the first line and all will be well.
To free an SV that you've created, call SvREFCNT_dec(SV*). Normally this call is not necessary (see Reference Counts and Mortality).
Recall that the usual method of determining the type of scalar you have is
to use Sv*OK macros. Because a scalar can be both a number and a string, usually these macros will always return
TRUE and calling the
Sv*V
macros will do the appropriate conversion of string to integer/double or
integer/double to string.
If you really need to know if you have an integer, double, or string pointer in an SV, you can use the following three macros instead:
SvIOKp(SV*)
SvNOKp(SV*)
SvPOKp(SV*)
These will tell you if you truly have an integer, double, or string pointer stored in your SV. The ``p'' stands for private.
In general, though, it's best to use the Sv*V macros.
There are two ways to create and load an AV. The first method creates an empty AV:
AV* newAV();
The second method both creates the AV and initially populates it with SVs:
AV* av_make(I32 num, SV **ptr);
The second argument points to an array containing num SV*'s. Once the
AV has been created, the SVs can be destroyed, if so
desired.
Once the AV has been created, the following operations are possible on AVs:
void av_push(AV*, SV*);
SV* av_pop(AV*);
SV* av_shift(AV*);
void av_unshift(AV*, I32 num);
These should be familiar operations, with the exception of av_unshift. This routine adds num elements at the front of the array with the undef
value. You must then use av_store (described below) to assign values to these new elements.
Here are some other functions:
I32 av_len(AV*);
SV** av_fetch(AV*, I32 key, I32 lval);
SV** av_store(AV*, I32 key, SV* val);
The av_len function returns the highest index value in array (just like $#array in
Perl). If the array is empty, -1 is returned. The
av_fetch function returns the value at index key, but if lval
is non-zero, then av_fetch will store an undef value at that index. The av_store function stores the value val at index key, and does not increment the reference count of val. Thus the caller is responsible for taking care of that, and if av_store returns
NULL, the caller will have to decrement the reference
count to avoid a memory leak. Note that
av_fetch and av_store both return SV**'s, not SV*'s as their return value.
void av_clear(AV*);
void av_undef(AV*);
void av_extend(AV*, I32 key);
The av_clear function deletes all the elements in the
AV* array, but does not actually delete the array
itself. The av_undef function will delete all the elements in the array plus the array itself.
The
av_extend function extends the array so that it contains key
elements. If key is less than the current length of the array, then nothing is done.
If you know the name of an array variable, you can get a pointer to its AV by using the following:
AV* perl_get_av("package::varname", FALSE);
This returns NULL if the variable does not exist.
See Understanding the Magic of Tied Hashes and Arrays for more information on how to use the array access functions on tied arrays.
To create an HV, you use the following routine:
HV* newHV();
Once the HV has been created, the following operations are possible on HVs:
SV** hv_store(HV*, char* key, U32 klen, SV* val, U32 hash);
SV** hv_fetch(HV*, char* key, U32 klen, I32 lval);
The klen parameter is the length of the key being passed in (Note that you cannot
pass 0 in as a value of klen to tell Perl to measure the length of the key). The val argument contains the
SV pointer to the scalar being stored, and hash is the precomputed hash value (zero if you want hv_store to calculate it for you). The lval parameter indicates whether this fetch is actually a part of a store operation, in which case a new undefined value will be added to the
HV with the supplied key and
hv_fetch will return as if the value had already existed.
Remember that hv_store and hv_fetch return SV**'s and not just SV*. To access the scalar value, you must first dereference the return value. However, you should check to make sure that the return value is not NULL before dereferencing it.
These two functions check if a hash table entry exists, and deletes it.
bool hv_exists(HV*, char* key, U32 klen);
SV* hv_delete(HV*, char* key, U32 klen, I32 flags);
If flags does not include the G_DISCARD flag then hv_delete will create and return a mortal copy of the deleted value.
And more miscellaneous functions:
void hv_clear(HV*);
void hv_undef(HV*);
Like their AV counterparts, hv_clear deletes all the entries in the hash table but does not actually delete the hash table. The hv_undef deletes both the entries and the hash table itself.
Perl keeps the actual data in linked list of structures with a typedef of
HE. These contain the actual key and value pointers
(plus extra administrative overhead). The key is a string pointer; the
value is an SV*. However, once you have an HE*, to get the actual key and value, use the routines specified below.
I32 hv_iterinit(HV*);
/* Prepares starting point to traverse hash table */
HE* hv_iternext(HV*);
/* Get the next entry, and return a pointer to a
structure that has both the key and value */
char* hv_iterkey(HE* entry, I32* retlen);
/* Get the key from an HE structure and also return
the length of the key string */
SV* hv_iterval(HV*, HE* entry);
/* Return a SV pointer to the value of the HE
structure */
SV* hv_iternextsv(HV*, char** key, I32* retlen);
/* This convenience routine combines hv_iternext,
hv_iterkey, and hv_iterval. The key and retlen
arguments are return values for the key and its
length. The value is returned in the SV* argument */
If you know the name of a hash variable, you can get a pointer to its HV by using the following:
HV* perl_get_hv("package::varname", FALSE);
This returns NULL if the variable does not exist.
The hash algorithm is defined in the PERL_HASH(hash, key, klen) macro:
i = klen;
hash = 0;
s = key;
while (i--)
hash = hash * 33 + *s++;
See Understanding the Magic of Tied Hashes and Arrays for more information on how to use the hash access functions on tied hashes.
Beginning with version 5.004, the following functions are also supported:
HE* hv_fetch_ent (HV* tb, SV* key, I32 lval, U32 hash);
HE* hv_store_ent (HV* tb, SV* key, SV* val, U32 hash);
bool hv_exists_ent (HV* tb, SV* key, U32 hash);
SV* hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash);
SV* hv_iterkeysv (HE* entry);
Note that these functions take SV* keys, which simplifies writing of extension code that deals with hash structures. These functions also allow passing of SV* keys to tie functions without forcing you to stringify the keys (unlike the previous set of functions).
They also return and accept whole hash entries (HE*), making their use more efficient (since the hash number for a particular
string doesn't have to be recomputed every time). See API LISTING later in this document for detailed descriptions.
The following macros must always be used to access the contents of hash entries. Note that the arguments to these macros must be simple variables, since they may get evaluated more than once. See API LISTING later in this document for detailed descriptions of these macros.
HePV(HE* he, STRLEN len)
HeVAL(HE* he)
HeHASH(HE* he)
HeSVKEY(HE* he)
HeSVKEY_force(HE* he)
HeSVKEY_set(HE* he, SV* sv)
These two lower level macros are defined, but must only be used when dealing with keys that are not SV*s:
HeKEY(HE* he)
HeKLEN(HE* he)
Note that both hv_store and hv_store_ent do not increment the reference count of the stored val, which is the caller's responsibility. If these functions return a
NULL value, the caller will usually have to decrement
the reference count of val to avoid a memory leak.
References are a special type of scalar that point to other data types (including references).
To create a reference, use either of the following functions:
SV* newRV_inc((SV*) thing);
SV* newRV_noinc((SV*) thing);
The thing argument can be any of an SV*, AV*, or HV*. The functions are identical except that newRV_inc increments the reference count of the thing, while newRV_noinc does not. For historical reasons, newRV is a synonym for newRV_inc.
Once you have a reference, you can use the following macro to dereference the reference:
SvRV(SV*)
then call the appropriate routines, casting the returned SV* to either an AV* or HV*, if required.
To determine if an SV is a reference, you can use the following macro:
SvROK(SV*)
To discover what type of value the reference refers to, use the following macro and then check the return value.
SvTYPE(SvRV(SV*))
The most useful types that will be returned are:
SVt_IV Scalar
SVt_NV Scalar
SVt_PV Scalar
SVt_RV Scalar
SVt_PVAV Array
SVt_PVHV Hash
SVt_PVCV Code
SVt_PVGV Glob (possible a file handle)
SVt_PVMG Blessed or Magical Scalar
See the sv.h header file for more details.
References are also used to support object-oriented programming. In the OO lexicon, an object is simply a reference that has been blessed into a package (or class). Once blessed, the programmer may now use the reference to access the various methods in the class.
A reference can be blessed into a package with the following function:
SV* sv_bless(SV* sv, HV* stash);
The sv argument must be a reference. The stash argument specifies which class the reference will belong to. See
Stashes and Globs for information on converting class names into stashes.
/* Still under construction */
Upgrades rv to reference if not already one. Creates new
SV for rv to point to. If classname is non-null, the
SV is blessed into the specified class.
SV is returned.
SV* newSVrv(SV* rv, char* classname);
Copies integer or double into an
SV whose reference is rv.
SV is blessed if classname is non-null.
SV* sv_setref_iv(SV* rv, char* classname, IV iv);
SV* sv_setref_nv(SV* rv, char* classname, NV iv);
Copies the pointer value (the address, not the string!) into an
SV whose reference is rv.
SV is blessed if
classname is non-null.
SV* sv_setref_pv(SV* rv, char* classname, PV iv);
Copies string into an
SV whose reference is rv. Set length to 0 to let Perl calculate the string length.
SV is blessed if classname is non-null.
SV* sv_setref_pvn(SV* rv, char* classname, PV iv, int length);
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
UNIVERSAL::isa functionality.
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)) { ... }
To create a new Perl variable with an undef value which can be accessed from your Perl script, use the following routines, depending on the variable type.
SV* perl_get_sv("package::varname", TRUE);
AV* perl_get_av("package::varname", TRUE);
HV* perl_get_hv("package::varname", TRUE);
Notice the use of TRUE as the second parameter. The new variable can now be set, using the routines appropriate to the data type.
There are additional macros whose values may be bitwise OR'ed with the
TRUE argument to enable certain extra features. Those bits are:
GV_ADDMULTI Marks the variable as multiply defined, thus preventing the
"Name <varname> used only once: possible typo" warning.
GV_ADDWARN Issues the warning "Had to create <varname> unexpectedly" if
the variable did not exist before the function was called.
If you do not specify a package name, the variable is created in the current package.
Perl uses an reference count-driven garbage collection mechanism. SVs, AVs, or HVs (xV for short in the following) start their life with a reference count of 1. If the reference count of an xV ever drops to 0, then it will be destroyed and its memory made available for reuse.
This normally doesn't happen at the Perl level unless a variable is undef'ed or the last variable holding a reference to it is changed or overwritten. At the internal level, however, reference counts can be manipulated with the following macros:
int SvREFCNT(SV* sv);
SV* SvREFCNT_inc(SV* sv);
void SvREFCNT_dec(SV* sv);
However, there is one other function which manipulates the reference count of its argument. The newRV_inc function, you will recall, creates a reference to the specified argument. As a side effect, it increments the argument's reference count. If this is not what you want, use newRV_noinc instead.
For example, imagine you want to return a reference from an XSUB function. Inside the XSUB routine, you create an SV which initially has a reference count of one. Then you call newRV_inc, passing it the just-created SV. This returns the reference as a new SV, but the reference count of the SV you passed to newRV_inc has been incremented to two. Now you return the reference from the XSUB routine and forget about the SV. But Perl hasn't! Whenever the returned reference is destroyed, the reference count of the original SV is decreased to one and nothing happens. The SV will hang around without any way to access it until Perl itself terminates. This is a memory leak.
The correct procedure, then, is to use newRV_noinc instead of newRV_inc. Then, if and when the last reference is destroyed, the reference count of the SV will go to zero and it will be destroyed, stopping any memory leak.
There are some convenience functions available that can help with the destruction of xVs. These functions introduce the concept of ``mortality''. An xV that is mortal has had its reference count marked to be decremented, but not actually decremented, until ``a short time later''. Generally the term ``short time later'' means a single Perl statement, such as a call to an XSUB function. The actual determinant for when mortal xVs have their reference count decremented depends on two macros, SAVETMPS and FREETMPS. See the perlcall manpage and the perlxs manpage for more details on these macros.
``Mortalization'' then is at its simplest a deferred SvREFCNT_dec. However, if you mortalize a variable twice, the reference count will later be decremented twice.
You should be careful about creating mortal variables. Strange things can happen if you make the same value mortal within multiple contexts, or if you make a variable mortal multiple times.
To create a mortal variable, use the functions:
SV* sv_newmortal()
SV* sv_2mortal(SV*)
SV* sv_mortalcopy(SV*)
The first call creates a mortal SV, the second converts an existing SV to a mortal SV (and thus defers a call to SvREFCNT_dec), and the third creates a mortal copy of an existing SV.
The mortal routines are not just for SVs -- AVs and HVs can be made mortal by passing their address (type-casted to SV*) to the sv_2mortal or sv_mortalcopy routines.
A ``stash'' is a hash that contains all of the different objects that are contained within a package. Each key of the stash is a symbol name (shared by all the different types of objects that have the same name), and each value in the hash table is a GV (Glob Value). This GV in turn contains references to the various objects of that name, including (but not limited to) the following:
Scalar Value
Array Value
Hash Value
I/O Handle
Format
Subroutine
There is a single stash called ``PL_defstash'' that holds the items that exist in the ``main'' package. To get at the items in other packages, append the string ``::'' to the package name. The items in the ``Foo'' package are in the stash ``Foo::'' in PL_defstash. The items in the ``Bar::Baz'' package are in the stash ``Baz::'' in ``Bar::'''s stash.
To get the stash pointer for a particular package, use the function:
HV* gv_stashpv(char* name, I32 create)
HV* gv_stashsv(SV*, I32 create)
The first function takes a literal string, the second uses the string stored in the
SV. Remember that a stash is just a hash table, so you get back an
HV*. The create flag will create a new package if it is set.
The name that gv_stash*v wants is the name of the package whose symbol table you want. The default
package is called main. If you have multiply nested packages, pass their names to gv_stash*v, separated by :: as in the Perl language itself.
Alternately, if you have an SV that is a blessed reference, you can find out the stash pointer by using:
HV* SvSTASH(SvRV(SV*));
then use the following to get the package name itself:
char* HvNAME(HV* stash);
If you need to bless or re-bless an object you can use the following function:
SV* sv_bless(SV*, HV* stash)
where the first argument, an SV*, must be a reference, and the second argument is a stash. The returned SV* can now be used in the same way as any other SV.
For more information on references and blessings, consult the perlref manpage.
Scalar variables normally contain only one type of value, an integer, double, pointer, or reference. Perl will automatically convert the actual scalar data from the stored type into the requested type.
Some scalar variables contain more than one type of scalar data. For
example, the variable $! contains either the numeric value of errno
or its string equivalent from either strerror or sys_errlist[].
To force multiple data values into an
SV, you must do two things: use the
sv_set*v routines to add the additional scalar type, then set a flag so that Perl
will believe it contains more than one type of data. The four macros to set
the flags are:
SvIOK_on
SvNOK_on
SvPOK_on
SvROK_on
The particular macro you must use depends on which sv_set*v routine you called first. This is because every sv_set*v routine turns on only the bit for the particular type of data being set,
and turns off all the rest.
For example, to create a new Perl variable called ``dberror'' that contains both the numeric and descriptive string error values, you could use the following code:
extern int dberror;
extern char *dberror_list;
SV* sv = perl_get_sv("dberror", TRUE);
sv_setiv(sv, (IV) dberror);
sv_setpv(sv, dberror_list[dberror]);
SvIOK_on(sv);
If the order of sv_setiv and sv_setpv had been reversed, then the macro SvPOK_on would need to be called instead of SvIOK_on.
[This section still under construction. Ignore everything here. Post no bills. Everything not permitted is forbidden.]
Any
SV may be magical, that is, it has special features that a normal
SV does not have. These features are stored in the
SV structure in a linked list of
struct magic's, typedef'ed to MAGIC.
struct magic {
MAGIC* mg_moremagic;
MGVTBL* mg_virtual;
U16 mg_private;
char mg_type;
U8 mg_flags;
SV* mg_obj;
char* mg_ptr;
I32 mg_len;
};
Note this is current as of patchlevel 0, and could change at any time.
Perl adds magic to an SV using the sv_magic function:
void sv_magic(SV* sv, SV* obj, int how, char* name, I32 namlen);
The sv argument is a pointer to the
SV that is to acquire a new magical feature.
If sv is not already magical, Perl uses the SvUPGRADE macro to set the SVt_PVMG flag for the sv. Perl then continues by adding it to the beginning of the linked list of magical features. Any prior entry of the same type of magic is deleted. Note that this can be overridden, and multiple instances of the same type of magic can be associated with an
SV.
The name and namlen arguments are used to associate a string with the magic, typically the name
of a variable. namlen is stored in the
mg_len field and if name is non-null and namlen >= 0 a malloc'd copy of the name is stored in mg_ptr field.
The sv_magic function uses how to determine which, if any, predefined ``Magic Virtual Table'' should be
assigned to the mg_virtual field. See the ``Magic Virtual Table'' section below. The how argument is also stored in the mg_type field.
The obj argument is stored in the mg_obj field of the MAGIC
structure. If it is not the same as the sv argument, the reference count of the obj object is incremented. If it is the same, or if the how argument is ``#'', or if it is a
NULL pointer, then obj is merely stored, without the reference count being incremented.
There is also a function to add magic to an HV:
void hv_magic(HV *hv, GV *gv, int how);
This simply calls sv_magic and coerces the gv argument into an SV.
To remove the magic from an SV, call the function sv_unmagic:
void sv_unmagic(SV *sv, int type);
The type argument should be equal to the how value when the SV
was initially made magical.
The mg_virtual field in the MAGIC structure is a pointer to a
MGVTBL, which is a structure of function pointers and stands for ``Magic Virtual
Table'' to handle the various operations that might be applied to that
variable.
The MGVTBL has five pointers to the following routine types:
int (*svt_get)(SV* sv, MAGIC* mg);
int (*svt_set)(SV* sv, MAGIC* mg);
U32 (*svt_len)(SV* sv, MAGIC* mg);
int (*svt_clear)(SV* sv, MAGIC* mg);
int (*svt_free)(SV* sv, MAGIC* mg);
This
MGVTBL structure is set at compile-time in perl.h and there are currently 19 types (or 21 with overloading turned on). These
different structures contain pointers to various routines that perform
additional actions depending on which function is being called.
Function pointer Action taken
---------------- ------------
svt_get Do something after the value of the SV is retrieved.
svt_set Do something after the SV is assigned a value.
svt_len Report on the SV's length.
svt_clear Clear something the SV represents.
svt_free Free any extra storage associated with the SV.
For instance, the
MGVTBL structure called vtbl_sv (which corresponds to an mg_type of '\0') contains:
{ magic_get, magic_set, magic_len, 0, 0 }
Thus, when an
SV is determined to be magical and of type '\0', if a
get operation is being performed, the routine magic_get is called. All the various routines for the various magical types begin
with magic_.
The current kinds of Magic Virtual Tables are:
mg_type MGVTBL Type of magic
------- ------ ----------------------------
\0 vtbl_sv Special scalar variable
A vtbl_amagic %OVERLOAD hash
a vtbl_amagicelem %OVERLOAD hash element
c (none) Holds overload table (AMT) on stash
B vtbl_bm Boyer-Moore (fast string search)
E vtbl_env %ENV hash
e vtbl_envelem %ENV hash element
f vtbl_fm Formline ('compiled' format)
g vtbl_mglob m//g target / study()ed string
I vtbl_isa @ISA array
i vtbl_isaelem @ISA array element
k vtbl_nkeys scalar(keys()) lvalue
L (none) Debugger %_<filename
l vtbl_dbline Debugger %_<filename element
o vtbl_collxfrm Locale transformation
P vtbl_pack Tied array or hash
p vtbl_packelem Tied array or hash element
q vtbl_packelem Tied scalar or handle
S vtbl_sig %SIG hash
s vtbl_sigelem %SIG hash element
t vtbl_taint Taintedness
U vtbl_uvar Available for use by extensions
v vtbl_vec vec() lvalue
x vtbl_substr substr() lvalue
y vtbl_defelem Shadow "foreach" iterator variable /
smart parameter vivification
* vtbl_glob GV (typeglob)
# vtbl_arylen Array length ($#ary)
. vtbl_pos pos() lvalue
~ (none) Available for use by extensions
When an uppercase and lowercase letter both exist in the table, then the uppercase letter is used to represent some kind of composite type (a list or a hash), and the lowercase letter is used to represent an element of that composite type.
The '~' and 'U' magic types are defined specifically for use by extensions and will not be used by perl itself. Extensions can use '~' magic to 'attach' private information to variables (typically objects). This is especially useful because there is no way for normal perl code to corrupt this private information (unlike using extra elements of a hash object).
Similarly,
'U' magic can be used much like
tie() to call a
C function any time a scalar's value is used or changed. The
MAGIC's
mg_ptr field points to a ufuncs structure:
struct ufuncs {
I32 (*uf_val)(IV, SV*);
I32 (*uf_set)(IV, SV*);
IV uf_index;
};
When the
SV is read from or written to, the uf_val or uf_set
function will be called with uf_index as the first arg and a pointer to the
SV as the second.
Note that because multiple extensions may be using '~' or 'U' magic, it is important for extensions to take extra care to avoid conflict. Typically only using the magic on objects blessed into the same class as the extension is sufficient. For '~' magic, it may also be appropriate to add an I32 'signature' at the top of the private data area and check that.
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.
API LISTING later in this document identifies such 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.
MAGIC* mg_find(SV*, int type); /* Finds the magic pointer of that type */
This routine returns a pointer to the MAGIC structure stored in the
SV. If the
SV does not have that magical feature,
NULL is returned. Also, if the
SV is not of type SVt_PVMG, Perl may core dump.
int mg_copy(SV* sv, SV* nsv, char* key, STRLEN klen);
This routine checks to see what types of magic sv has. If the mg_type field is an uppercase letter, then the mg_obj is copied
to nsv, but the mg_type field is changed to be the lowercase letter.
Tied hashes and arrays are magical beasts of the 'P' magic type.
WARNING: As of the 5.004 release, proper usage of the array and hash access functions requires understanding a few caveats. Some of these caveats are actually considered bugs in the API, to be fixed in later releases, and are bracketed with [MAYCHANGE] below. If you find yourself actually applying such information in this section, be aware that the behavior may change in the future, umm, without warning.
The av_store function, when given a tied array argument, merely copies the magic of the array onto the value to be ``stored'', using mg_copy. It may also return NULL, indicating that the value did not actually need to be stored in the array. [MAYCHANGE] After a call to av_store on a tied array, the caller will usually need to call mg_set(val) to actually invoke the perl level ``STORE'' method on the TIEARRAY object. If av_store did return NULL, a call to SvREFCNT_dec(val) will also be usually necessary to avoid a memory leak. [/MAYCHANGE]
The previous paragraph is applicable verbatim to tied hash access using the hv_store and hv_store_ent functions as well.
av_fetch and the corresponding hash functions hv_fetch and hv_fetch_ent actually return an undefined mortal value whose magic has been initialized using mg_copy. Note the value so returned does not need to be deallocated, as it is already mortal. [MAYCHANGE] But you will need to call mg_get() on the returned value in order to actually invoke the perl level ``FETCH'' method on the underlying TIE object. Similarly, you may also call mg_set() on the return value after possibly assigning a suitable value to it using sv_setsv, which will invoke the ``STORE'' method on the TIE object. [/MAYCHANGE]
[MAYCHANGE] In other words, the array or hash fetch/store functions don't really fetch and store actual values in the case of tied arrays and hashes. They merely call mg_copy to attach magic to the values that were meant to be ``stored'' or ``fetched''. Later calls to mg_get and mg_set actually do the job of invoking the TIE methods on the underlying objects. Thus the magic mechanism currently implements a kind of lazy access to arrays and hashes.
Currently (as of perl version 5.004), use of the hash and array access functions requires the user to be aware of whether they are operating on ``normal'' hashes and arrays, or on their tied variants. The API may be changed to provide more transparent access to both tied and normal data types in future versions. [/MAYCHANGE]
You would do well to understand that the TIEARRAY and TIEHASH interfaces are mere sugar to invoke some perl method calls while using the uniform hash and array syntax. The use of this sugar imposes some overhead (typically about two to four extra opcodes per FETCH/STORE operation, in addition to the creation of all the mortal variables required to invoke the methods). This overhead will be comparatively small if the TIE methods are themselves substantial, but if they are only a few statements long, the overhead will not be insignificant.
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 EXAMPLE/"Returning a Scalar). Such a construct may be created specially for some important localized task, or an existing one (like boundaries of enclosing Perl subroutine/block, or an existing pair for freeing TMPs) may be used. (In the second case the overhead of additional localization must be almost negligible.) Note that any
XSUB is automatically enclosed in an
ENTER/LEAVE pair.
Inside such a pseudo-block the following service is available:
i at the end of enclosing pseudo-block.
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.
sv would be decremented at the end of
pseudo-block. This is similar to sv_2mortal, which should (?) be used instead.
OP * is
op_free()ed at the end of pseudo-block.
p is
Safefree()ed at the end
of pseudo-block.
sv at the end of pseudo-block.
key of hv is deleted at the end of pseudo-block. The string pointed to by key is
Safefree()ed. If one has
a key in short-lived storage, the corresponding string may be reallocated like
this:
SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));
f is called with the only argument (of type void*) p.
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 *.
local $gv.
save_scalar, but localize @gv and %gv.
save_item which takes multiple arguments via an array
sarg of SV* of length maxsarg.
save_scalar, but will reinstate a SV *.
save_svref, but localize AV * and HV *.
The Alias module implements localization of the basic types within the
caller's scope. People who are interested in how to localize things in the containing
scope should take a look there too.
The XSUB mechanism is a simple way for Perl programs to access C subroutines. An XSUB routine will have a stack that contains the arguments from the Perl program, and a way to map from the Perl data structures to a C equivalent.
The stack arguments are accessible through the ST(n) macro, which returns the n'th stack argument. Argument 0 is the first argument passed in the Perl
subroutine call. These arguments are SV*, and can be used anywhere an SV* is used.
Most of the time, output from the
C routine can be handled through use of the
RETVAL and
OUTPUT directives. However, there are some cases where the argument stack is not already long enough to handle all the return values. An example is the
POSIX
tzname() call, which takes no arguments, but returns two, the local time zone's standard and summer time abbreviations.
To handle this situation, the PPCODE directive is used and the stack is extended using the macro:
EXTEND(SP, num);
where SP is the macro that represents the local copy of the stack pointer, and num is the number of elements the stack should be extended by.
Now that there is room on the stack, values can be pushed on it using the macros to push IVs, doubles, strings, and SV pointers respectively:
PUSHi(IV)
PUSHn(double)
PUSHp(char*, I32)
PUSHs(SV*)
And now the Perl program calling tzname, the two values will be assigned as in:
($standard_abbrev, $summer_abbrev) = POSIX::tzname;
An alternate (and possibly simpler) method to pushing values on the stack is to use the macros:
XPUSHi(IV)
XPUSHn(double)
XPUSHp(char*, I32)
XPUSHs(SV*)
These macros automatically adjust the stack for you, if needed. Thus, you do not need to call EXTEND to extend the stack.
For more information, consult the perlxs manpage and the perlxstut manpage.
There are four routines that can be used to call a Perl subroutine from within a C program. These four are:
I32 perl_call_sv(SV*, I32);
I32 perl_call_pv(char*, I32);
I32 perl_call_method(char*, I32);
I32 perl_call_argv(char*, I32, register char**);
The routine most often used is perl_call_sv. The SV* argument contains either the name of the Perl subroutine to be called, or a reference to the subroutine. The second argument consists of flags that control the context in which the subroutine is called, whether or not the subroutine is being passed arguments, how errors should be trapped, and how to treat return values.
All four routines return the number of arguments that the subroutine returned on the Perl stack.
When using any of these routines (except perl_call_argv), the programmer must manipulate the Perl stack. These include the following macros and functions:
dSP
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.
It is suggested that you use the version of malloc that is distributed with Perl. It keeps pools of various sizes of unallocated memory in order to satisfy allocation requests more quickly. However, on some platforms, it may cause spurious malloc or free errors.
New(x, pointer, number, type);
Newc(x, pointer, number, type, cast);
Newz(x, pointer, number, type);
These three macros are used to initially allocate memory.
The first argument x was a ``magic cookie'' that was used to keep track of who called the macro,
to help when debugging memory problems. However, the current code makes no
use of this feature (most Perl developers now use run-time memory
checkers), so this argument can be any number.
The second argument pointer should be the name of a variable that will point to the newly allocated
memory.
The third and fourth arguments number and type specify how many of the specified type of data structure should be
allocated. The argument
type is passed to sizeof. The final argument to Newc, cast, should be used if the pointer argument is different from the type
argument.
Unlike the New and Newc macros, the Newz macro calls memzero
to zero out all the newly allocated memory.
Renew(pointer, number, type);
Renewc(pointer, number, type, cast);
Safefree(pointer)
These three macros are used to change a memory buffer size or to free a piece of memory no longer needed. The arguments to Renew and Renewc match those of New and Newc with the exception of not needing the ``magic cookie'' argument.
Move(source, dest, number, type);
Copy(source, dest, number, type);
Zero(dest, number, type);
These three macros are used to move, copy, or zero out previously allocated
memory. The source and dest arguments point to the source and destination starting points. Perl will
move, copy, or zero out number
instances of the size of the type data structure (using the sizeof
function).
The most recent development releases of Perl has been experimenting with removing Perl's dependency on the ``normal'' standard I/O suite and allowing other stdio implementations to be used. This involves creating a new abstraction layer that then calls whichever implementation of stdio Perl was compiled with. All XSUBs should now use the functions in the PerlIO abstraction layer and not make any assumptions about what kind of stdio is being used.
For a complete description of the PerlIO abstraction, consult the perlapio manpage.
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].
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.
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.
Here we describe the internal form your code is converted to by Perl. Start with a simple example:
$a = $b + $c;
This is converted to a tree similar to this one:
assign-to
/ \
+ $a
/ \
$b $c
(but slightly more complicated). This tree reflects the way Perl parsed your code, but has nothing to do with the execution order. There is an additional ``thread'' going through the nodes of the tree which shows the order of execution of the nodes. In our simplified example above it looks like:
$b ---> $c ---> + ---> $a ---> assign-to
But with the actual compile tree for $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.
If you have your perl compiled for debugging (usually done with -D
optimize=-g on Configure command line), you may examine the compiled tree by specifying -Dx on the Perl command line. The output takes several lines per node, and for $b+$c it looks like this:
5 TYPE = add ===> 6
TARG = 1
FLAGS = (SCALAR,KIDS)
{
TYPE = null ===> (4)
(was rv2sv)
FLAGS = (SCALAR,KIDS)
{
3 TYPE = gvsv ===> 4
FLAGS = (SCALAR)
GV = main::b
}
}
{
TYPE = null ===> (5)
(was rv2sv)
FLAGS = (SCALAR,KIDS)
{
4 TYPE = gvsv ===> 5
FLAGS = (SCALAR)
GV = main::c
}
}
This tree has 5 nodes (one per TYPE specifier), only 3 of them are not optimized away (one per number in the
left column). The immediate children of the given node correspond to {} pairs on the same level of indentation, thus this listing corresponds to
the tree:
add
/ \
null null
| |
gvsv gvsv
The execution order is indicated by ===> marks, thus it is 3
4 5 6 (node 6 is not included into above listing), i.e.,
gvsv gvsv add whatever.
The tree is created by the pseudo-compiler while yacc code feeds it the constructions it recognizes. Since yacc works bottom-up, so does the first pass of perl compilation.
What makes this pass interesting for perl developers is that some
optimization may be performed on this pass. This is optimization by
so-called check routines. The correspondence between node names and corresponding check routines is
described in opcode.pl (do not forget to run make regen_headers if you modify this file).
A check routine is called when the node is fully constructed except for the execution-order thread. Since at this time there 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).
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.
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).
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.
This is a listing of functions, macros, flags, and variables that may be useful to extension writers or that may be found while reading other extensions.
Note that all Perl
API global variables must be referenced with the PL_
prefix. Some macros are provided for compatibility with the older,
unadorned names, but this support will be removed in a future release.
It is strongly recommended that all Perl
API functions that don't begin with perl be referenced with an explicit Perl_ prefix.
The sort order of the listing is case insensitive, with any occurrences of '_' ignored for the the purpose of sorting.
void av_clear (AV* ar)
key is the index to which the array should be extended.
void av_extend (AV* ar, I32 key)
key is the index. If lval is set then the fetch will be part of a store. Check that the return value
is non-null before dereferencing it to a SV*.
See Understanding the Magic of Tied Hashes and Arrays for more information on how to use this function on tied arrays.
SV** av_fetch (AV* ar, I32 key, I32 lval)
I32 av_len (AV* ar)
AV* av_make (I32 size, SV** svp)
SV* av_pop (AV* ar)
void av_push (AV* ar, SV* val)
SV* av_shift (AV* ar)
key. The return value will be
NULL if the operation failed or if the value did not
need to be actually stored within the array (as in the case of tied
arrays). Otherwise it can be dereferenced to get the original SV*. Note that the caller is responsible for suitably incrementing the
reference count of val
before the call, and decrementing it if the function returned
NULL.
See Understanding the Magic of Tied Hashes and Arrays for more information on how to use this function on tied arrays.
SV** av_store (AV* ar, I32 key, SV* val)
void av_undef (AV* ar)
void av_unshift (AV* ar, I32 num)
xsubpp to indicate the class name for a
C++
XS constructor. This is always a
char*. See THIS and
Using XS With C++.
memcpy function. The s is the source, d is the destination, n is the number of items, and t is the type. May fail on overlapping copies. See also Move.
void Copy( s, d, n, t )
HV* CvSTASH( SV* sv )
SvPV( GvSV( PL_DBsub ), PL_na )
mark, for the
XSUB. See MARK and
dORIGMARK.
xsubpp. Declares the items variable to indicate the number of items on the stack.
xsubpp.
iotype is what IoTYPE(io) would contain.
do_binmode(fp, iotype, TRUE);
ENTER;
EXTEND( sp, int x )
fbm_instr() -- the
Boyer-Moore algorithm.
void fbm_compile(SV* sv, U32 flags)
str and
strend. It returns Nullch if the string can't be found. The
sv does not have to be fbm_compiled, but the search will not be as fast then.
char* fbm_instr(char *str, char *strend, SV *sv, U32 flags)
FREETMPS;
name and a defined subroutine or
NULL. The glob lives in the given stash, or in the stashes accessible via @ISA and
@UNIVERSAL.
The argument level should be either 0 or -1. If level==0, as a side-effect creates a glob with the given name in the given
stash which in the case of success contains an alias for the subroutine, and sets
up caching info for this glob. Similarly for all the searched stashes.
This function grants "SUPER" token as a postfix of the stash name.
The
GV returned from gv_fetchmeth may be a method cache entry, which is not visible to Perl code. So when
calling perl_call_sv, you should not use the
GV directly; instead, you should use the method's
CV, which can be obtained from the
GV with the
GvCV macro.
GV* gv_fetchmeth (HV* stash, char* name, STRLEN len, I32 level)
stash. In fact in the presense of autoloading this may be the glob for
``AUTOLOAD''. In this case the corresponding variable
$AUTOLOAD is already setup.
The third parameter of gv_fetchmethod_autoload determines whether
AUTOLOAD lookup is performed if the given method is not present: non-zero means yes, look for
AUTOLOAD; zero means no, don't look for
AUTOLOAD. Calling
gv_fetchmethod is equivalent to calling gv_fetchmethod_autoload with a non-zero autoload parameter.
These functions grant "SUPER" token as a prefix of the method name.
Note that if you want to keep the returned glob for a long time, you need to check for it being
``AUTOLOAD'', since at the later time the call may load a different subroutine due to $AUTOLOAD changing its value. Use the glob created via a side effect to do this.
These functions have the same side-effects and as gv_fetchmeth with
level==0. name should be writable if contains ':' or '\''. The warning against passing the
GV returned by gv_fetchmeth to
perl_call_sv apply equally to these functions.
GV* gv_fetchmethod (HV* stash, char* name)
GV* gv_fetchmethod_autoload (HV* stash, char* name, I32 autoload)
create is set then the package will be created if it does not already exist. If create
is not set and the package does not exist then
NULL is returned.
HV* gv_stashpv (char* name, I32 create)
HV* gv_stashsv (SV* sv, I32 create)
char* pointer is to be expected. (For information only--not to be used).
U32 HeHASH(HE* he)
char* or SV*, depending on the value of
HeKLEN(). Can be assigned to. The HePV() or HeSVKEY() macros are usually preferable for finding the value of a key.
char* HeKEY(HE* he)
int HeKLEN(HE* he)
char* value, doing any necessary dereferencing of possibly SV* keys. The length of the string is placed in len (this is a macro, so do not use
&len). If you do not care about what the length of the key is, you may use the
global variable PL_na. Remember though, that hash keys in perl are free to contain embedded
nulls, so using strlen()
or similar is not a good way to find the length of hash keys. This is very
similar to the SvPV() macro described elsewhere in this document.
char* HePV(HE* he, STRLEN len)
HeSVKEY(HE* he)
char* key.
HeSVKEY_force(HE* he)
HeSVKEY_set(HE* he, SV* sv)
HeVAL(HE* he)
void hv_clear (HV* tb)
void hv_delayfree_ent (HV* hv, HE* entry)
klen is the length of the key. The
flags value will normally be zero; if set to
G_DISCARD then
NULL will be returned.
SV* hv_delete (HV* tb, char* key, U32 klen, I32 flags)
flags value will normally be zero; if set to
G_DISCARD then
NULL will be returned.
hash can be a valid precomputed hash value, or 0 to ask for it to be computed.
SV* hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash)
klen is the length of the key.
bool hv_exists (HV* tb, char* key, U32 klen)
hash
can be a valid precomputed hash value, or 0 to ask for it to be computed.
bool hv_exists_ent (HV* tb, SV* key, U32 hash)
klen is the length of the key. If lval is set then the fetch will be part of a store. Check that the return value
is non-null before dereferencing it to a SV*.
See Understanding the Magic of Tied Hashes and Arrays for more information on how to use this function on tied hashes.
SV** hv_fetch (HV* tb, char* key, U32 klen, I32 lval)
hash must be a valid precomputed hash number for the given key, or 0 if you want the function to compute it.
IF lval is set then the fetch will be part of a store. Make sure the return value
is non-null before accessing it. The return value when tb is a tied hash is a pointer to a static location, so be sure to make a copy
of the structure if you need to store it somewhere.
See Understanding the Magic of Tied Hashes and Arrays for more information on how to use this function on tied hashes.
HE* hv_fetch_ent (HV* tb, SV* key, I32 lval, U32 hash)
void hv_free_ent (HV* hv, HE* entry)
I32 hv_iterinit (HV* tb)
Returns the number of keys in the hash (i.e. the same as HvKEYS(tb)). The return value is currently only meaningful for hashes without tie
magic.
NOTE: Before version 5.004_65, hv_iterinit used to return the number of hash buckets that happen to be in use. If you
still need that esoteric value, you can get it through the macro HvFILL(tb).
char* hv_iterkey (HE* entry, I32* retlen)
SV* hv_iterkeysv (HE* entry)
HE* hv_iternext (HV* tb)
SV* hv_iternextsv (HV* hv, char** key, I32* retlen)
SV* hv_iterval (HV* tb, HE* entry)
void hv_magic (HV* hv, GV* gv, int how)
char* HvNAME (HV* stash)
key and klen is the length of the key. The hash parameter is the precomputed hash value; if it is zero then Perl will compute it. The return value will be
NULL if the operation failed or if the value did not need to be actually stored within the hash (as in the case of tied hashes). Otherwise it can be dereferenced to get the original
SV*. Note that the caller is responsible for suitably incrementing the
reference count of val
before the call, and decrementing it if the function returned
NULL.
See Understanding the Magic of Tied Hashes and Arrays for more information on how to use this function on tied hashes.
SV** hv_store (HV* tb, char* key, U32 klen, SV* val, U32 hash)
val in a hash. The hash key is specified as key. The hash
parameter is the precomputed hash value; if it is zero then Perl will compute it. The return value is the new hash entry so created. It will be
NULL if the operation failed or if the value did not need to be actually stored within the hash (as in the case of tied hashes). Otherwise the contents of the return value can be accessed using the
He??? macros described here. Note that the caller is responsible for suitably
incrementing the reference count of val before the call, and decrementing it if the function returned
NULL.
See Understanding the Magic of Tied Hashes and Arrays for more information on how to use this function on tied hashes.
HE* hv_store_ent (HV* tb, SV* key, SV* val, U32 hash)
void hv_undef (HV* tb)
char is an ascii alphanumeric character or digit.
int isALNUM (char c)
char is an ascii alphabetic character.
int isALPHA (char c)
char is an ascii digit.
int isDIGIT (char c)
char is a lowercase character.
int isLOWER (char c)
char is whitespace.
int isSPACE (char c)
char is an uppercase character.
int isUPPER (char c)
xsubpp to indicate the number of items on the stack. See Variable-length Parameter Lists.
xsubpp to indicate which of an XSUB's aliases was used to invoke it. See The ALIAS: Keyword.
LEAVE;
int looks_like_number(SV*)
int mg_clear (SV* sv)
int mg_copy (SV *, SV *, char *, STRLEN)
MAGIC* mg_find (SV* sv, int type)
int mg_free (SV* sv)
int mg_get (SV* sv)
U32 mg_len (SV* sv)
void mg_magical (SV* sv)
int mg_set (SV* sv)
memmove function. The s is the source, d is the destination, n is the number of items, and t is the type. Can do overlapping moves. See also Copy.
void Move( s, d, n, t )
malloc function.
void* New( x, void *ptr, int size, type )
AV* newAV (void)
malloc function, with cast.
void* Newc( x, void *ptr, int size, type, cast )
sub FOO () { 123 }
which is eligible for inlining at compile-time.
void newCONSTSUB(HV* stash, char* name, SV* sv)
HV* newHV (void)
SV* newRV_inc (SV* ref)
For historical reasons, ``newRV'' is a synonym for ``newRV_inc''.
SV* newRV_noinc (SV* ref)
len parameter indicates the number of bytes of preallocated string space the
SV should have. An extra byte for a tailing
NUL is also reserved. (SvPOK is not set for the
SV even if string space is allocated.) The reference count for the new
SV is set to 1.
id is an integer id between 0 and 1299 (used to identify leaks).
SV* NEWSV (int id, STRLEN len)
SV* newSViv (IV i)
SV* newSVnv (NV i)
len is zero then Perl will compute the length.
SV* newSVpv (char* s, STRLEN len)
SV* newSVpvf(const char* pat, ...);
len is zero then Perl will create a zero length string.
SV* newSVpvn (char* s, STRLEN len)
rv, to point to. If rv is not an
RV then it will be upgraded to one. If classname is non-null then the new
SV will be blessed in the specified package. The new
SV is returned and its reference count is 1.
SV* newSVrv (SV* rv, char* classname)
SV* newSVsv (SV* old)
xsubpp to hook up XSUBs as Perl subs.
xsubpp to hook up XSUBs as Perl subs. Adds Perl prototypes to the subs.
malloc function. The allocated memory is zeroed with memzero.
void* Newz( x, void *ptr, int size, type )
I32 perl_call_argv (char* subname, I32 flags, char** argv)
I32 perl_call_method (char* methname, I32 flags)
I32 perl_call_pv (char* subname, I32 flags)
I32 perl_call_sv (SV* sv, I32 flags)
I32 perl_eval_sv (SV* sv, I32 flags)
SV* perl_eval_pv (char* p, I32 croak_on_error)
create is set and the Perl variable does not exist then it will be created. If create is not set and the variable does not exist then
NULL is returned.
AV* perl_get_av (char* name, I32 create)
create is set and the Perl variable does not exist then it will be created. If create is not set and the variable does not exist then
NULL is returned.
CV* perl_get_cv (char* name, I32 create)
create is set and the Perl variable does not exist then it will be created. If create is not set and the variable does not exist then
NULL is returned.
HV* perl_get_hv (char* name, I32 create)
create is set and the Perl variable does not exist then it will be created. If create is not set and the variable does not exist then
NULL is returned.
SV* perl_get_sv (char* name, I32 create)
void perl_require_pv (char* pv)
int POPi()
long POPl()
char* POPp()
double POPn()
SV* POPs()
PUSHMARK(p)
void PUSHi(int d)
void PUSHn(double d)
len indicates the length of the string. Handles 'set' magic. See
XPUSHp.
void PUSHp(char *c, int len )
void PUSHs(sv)
void PUSHu(unsigned int d)
xsubpp. See PUSHMARK and the perlcall manpage for other uses.
PUTBACK;
realloc function.
void* Renew( void *ptr, int size, type )
realloc function, with cast.
void* Renewc( void *ptr, int size, type, cast )
xsubpp to hold the return value for an
XSUB. This is always the proper type for the
XSUB. See
The RETVAL Variable.
free function.
malloc function.
realloc function.
char* savepv (char* sv)
len indicates number of bytes to copy. This does not use an
SV.
char* savepvn (char* sv, I32 len)
SAVETMPS;
xsubpp. See dSP and
SPAGAIN.
SPAGAIN;
SV* ST(int x)
int strEQ( char *s1, char *s2 )
s1, is greater than or equal to the second, s2. Returns true or false.
int strGE( char *s1, char *s2 )
s1, is greater than the second,
s2. Returns true or false.
int strGT( char *s1, char *s2 )
s1, is less than or equal to the second, s2. Returns true or false.
int strLE( char *s1, char *s2 )
s1, is less than the second,
s2. Returns true or false.
int strLT( char *s1, char *s2 )
int strNE( char *s1, char *s2 )
len parameter indicates the number of bytes to compare. Returns true or false.
int strnEQ( char *s1, char *s2 )
len parameter indicates the number of bytes to compare. Returns true or false.
int strnNE( char *s1, char *s2, int len )
SV* sv_2mortal (SV* sv)
SV* sv_bless (SV* sv, HV* stash)
void sv_catpv (SV* sv, char* ptr)
void sv_catpvn (SV* sv, char* ptr)
len indicates number of bytes to copy. Handles 'get' magic, but not 'set'
magic. See sv_catpvn_mg.
void sv_catpvn (SV* sv, char* ptr, STRLEN len)
void sv_catpvn_mg (SV* sv, char* ptr, STRLEN len)
void sv_catpvf (SV* sv, const char* pat, ...)
void sv_catpvf_mg (SV* sv, const char* pat, ...)
ssv onto the end of the string in
SV
dsv. Handles 'get' magic, but not 'set' magic. See sv_catsv_mg.
void sv_catsv (SV* dsv, SV* ssv)
void sv_catsv_mg (SV* dsv, SV* ssv)
SvPOK(sv) must be true and
the ptr must be a pointer to somewhere inside the string buffer. The ptr becomes the first character of the adjusted string.
void sv_chop(SV* sv, char *ptr)
sv1 is less than, equal to, or greater than the string in
sv2.
I32 sv_cmp (SV* sv1, SV* sv2)
int SvCUR (SV* sv)
void SvCUR_set (SV* sv, int val )
void sv_dec (SV* sv)
int sv_derived_from(SV* sv, char* class)
UNIVERSAL::isa. It works for class names as well as for objects.
bool sv_derived_from _((SV* sv, char* name));
char* SvEND(sv)
I32 sv_eq (SV* sv1, SV* sv2)
void SvGETMAGIC( SV *sv )
char* SvGROW( SV* sv, int len )
void sv_inc (SV* sv)
substr() function.
void sv_insert(SV *sv, STRLEN offset, STRLEN len,
char *str, STRLEN strlen)
int SvIOK (SV* SV)
void SvIOK_off (SV* sv)
void SvIOK_on (SV* sv)
void SvIOK_only (SV* sv)
int SvIOKp (SV* SV)
int sv_isa (SV* sv, char* name)
int sv_isobject (SV* sv)
int SvIV (SV* sv)
int SvIVX (SV* sv)
int SvLEN (SV* sv)
STRLEN sv_len (SV* sv)
void sv_magic (SV* sv, SV* obj, int how, char* name, I32 namlen)
SV* sv_mortalcopy (SV* oldsv)
SV* sv_newmortal (void)
int SvNIOK (SV* SV)
void SvNIOK_off (SV* sv)
int SvNIOKp (SV* SV)
false
SV. See PL_sv_yes. Always refer to this as &PL_sv_no.
int SvNOK (SV* SV)
void SvNOK_off (SV* sv)
void SvNOK_on (SV* sv)
void SvNOK_only (SV* sv)
int SvNOKp (SV* SV)
double SvNV (SV* sv)
double SvNVX (SV* sv)
int SvOK (SV* sv)
int SvOOK(SV* sv)
int SvPOK (SV* SV)
void SvPOK_off (SV* sv)
void SvPOK_on (SV* sv)
void SvPOK_only (SV* sv)
int SvPOKp (SV* SV)
len is PL_na then Perl will handle the length on its own. Handles 'get' magic.
char* SvPV (SV* sv, int len )
char* SvPV_force(SV* sv, int len)
char* SvPVX (SV* sv)
int SvREFCNT (SV* sv)
void SvREFCNT_dec (SV* sv)
void SvREFCNT_inc (SV* sv)
int SvROK (SV* sv)
void SvROK_off (SV* sv)
void SvROK_on (SV* sv)
SV* SvRV (SV* sv)
void SvSETMAGIC( SV *sv )
void sv_setiv (SV* sv, IV num)
void sv_setiv_mg (SV* sv, IV num)
void sv_setnv (SV* sv, double num)
void sv_setnv_mg (SV* sv, double num)
void sv_setpv (SV* sv, char* ptr)
void sv_setpv_mg (SV* sv, char* ptr)
void sv_setpviv (SV* sv, IV num)
void sv_setpviv_mg (SV* sv, IV num)
len parameter indicates the number of bytes to be copied. Does not handle 'set'
magic. See sv_setpvn_mg.
void sv_setpvn (SV* sv, char* ptr, STRLEN len)
void sv_setpvn_mg (SV* sv, char* ptr, STRLEN len)
void sv_setpvf (SV* sv, const char* pat, ...)
void sv_setpvf_mg (SV* sv, const char* pat, ...)
rv
argument will be upgraded to an
RV. That
RV will be modified to point to the new
SV. The
classname argument indicates the package for the blessing. Set classname to Nullch to avoid the blessing. The new
SV will be returned and will have a reference count of
1.
SV* sv_setref_iv (SV *rv, char *classname, IV iv)
rv
argument will be upgraded to an
RV. That
RV will be modified to point to the new
SV. The
classname argument indicates the package for the blessing. Set classname to Nullch to avoid the blessing. The new
SV will be returned and will have a reference count of
1.
SV* sv_setref_nv (SV *rv, char *classname, double nv)
rv
argument will be upgraded to an
RV. That
RV will be modified to point to the new
SV. If the
pv argument is
NULL then PL_sv_undef will be placed into the
SV. The classname argument indicates the package for the blessing. Set classname to Nullch to avoid the blessing. The new
SV will be returned and will have a reference count of
1.
SV* sv_setref_pv (SV *rv, char *classname, void* pv)
Do not use with integral Perl types such as HV, AV, SV, CV, because those objects will become corrupted by the pointer copy process.
Note that sv_setref_pvn copies the string while this copies the pointer.
n. The rv argument will be upgraded to an
RV. That
RV will be modified to point to the new
SV. The
classname
argument indicates the package for the blessing. Set classname to
Nullch to avoid the blessing. The new
SV will be returned and will have a reference count of
1.
SV* sv_setref_pvn (SV *rv, char *classname, char* pv, I32 n)
Note that sv_setref_pv copies the pointer while this copies the string.
void SvSetSV (SV* dsv, SV* ssv)
void SvSetSV_nosteal (SV* dsv, SV* ssv)
ssv into the destination
SV dsv. The source
SV may be destroyed if it is mortal. Does not handle
'set' magic. See the macro forms SvSetSV, SvSetSV_nosteal and sv_setsv_mg.
void sv_setsv (SV* dsv, SV* ssv)
void sv_setsv_mg (SV* dsv, SV* ssv)
void sv_setuv (SV* sv, UV num)
void sv_setuv_mg (SV* sv, UV num)
HV* SvSTASH (SV* sv)
void SvTAINT (SV* sv)
int SvTAINTED (SV* sv)
void SvTAINTED_off (SV* sv)
void SvTAINTED_on (SV* sv)
int SvTRUE (SV* sv)
svtype SvTYPE (SV* sv)
void sv_unref (SV* sv)
bool SvUPGRADE (SV* sv, svtype mt)
ptr to find its string value. Normally the string is stored inside the
SV but sv_usepvn allows the
SV to use an outside string. The
ptr should point to memory that was allocated by malloc. The string length, len, must be supplied. This function will realloc the memory pointed to by ptr, so that pointer should not be freed or used by the programmer after
giving it to sv_usepvn. Does not handle 'set' magic. See sv_usepvn_mg.
void sv_usepvn (SV* sv, char* ptr, STRLEN len)
void sv_usepvn_mg (SV* sv, char* ptr, STRLEN len)
vsprintf and appends the formatted output to an
SV. Uses an array of SVs if the
C style variable argument list is missing
(NULL). Indicates if locale information has been used for formatting.
void sv_catpvfn _((SV* sv, const char* pat, STRLEN patlen,
va_list *args, SV **svargs, I32 svmax,
bool *used_locale));
vcatpvfn but copies the text into the
SV instead of appending it.
void sv_setpvfn _((SV* sv, const char* pat, STRLEN patlen,
va_list *args, SV **svargs, I32 svmax,
bool *used_locale));
UV SvUV(SV* sv)
UV SvUVX(SV* sv)
true
SV. See PL_sv_no. Always refer to this as &PL_sv_yes.
xsubpp to designate the object in a
C++
XSUB. This is always the proper type for the
C++ object. See
CLASS and
Using XS With C++.
int toLOWER (char c)
int toUPPER (char c)
XPUSHi(int d)
XPUSHn(double d)
len
indicates the length of the string. Handles 'set' magic. See PUSHp.
XPUSHp(char *c, int len)
XPUSHs(sv)
xsubpp.
xsubpp.
XSRETURN(int x)
XSRETURN_EMPTY;
XSRETURN_IV(IV v)
XSRETURN_NO;
XSRETURN_NV(NV v)
XSRETURN_PV(char *v)
XSRETURN_UNDEF;
XSRETURN_YES;
i on the stack. The value is stored in a new mortal
SV.
XST_mIV( int i, IV v )
i on the stack. The value is stored in a new mortal
SV.
XST_mNV( int i, NV v )
i on the stack.
XST_mNO( int i )
i on the stack. The value is stored in a new mortal
SV.
XST_mPV( int i, char *v )
i on the stack.
XST_mUNDEF( int i )
i on the stack.
XST_mYES( int i )
ExtUtils::MakeMaker. See XS_VERSION_BOOTCHECK.
$VERSION variable matches the
XS module's
XS_VERSION variable. This is usually handled automatically by
xsubpp. See The VERSIONCHECK: Keyword.
memzero function. The d is the destination, n is the number of items, and t is the type.
void Zero( d, n, t )
Until May 1997, this document was maintained by Jeff Okamoto <okamoto@corp.hp.com> It is now maintained as part of Perl itself.
With lots of help and suggestions from Dean Roehrich, Malcolm Beattie, Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil Bowers, Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer, Stephen McCamant, and Gurusamy Sarathy.
API Listing originally by Dean Roehrich <roehrich@cray.com>
If rather than formatting bugs, you encounter substantive content errors in these documents, such as mistakes in the explanations or code, please use the perlbug utility included with the Perl distribution.