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Perl has three typedefs that handle Perl's three main data types:
SV Scalar Value
AV Array Value
HV Hash Value
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Each typedef has specific routines that manipulate the various data types.
Perl uses a special typedef IV which is a simple signed integer type that is guaranteed to
be large enough to hold a pointer (as well as an integer). Additionally, there is the UV,
which is simply an unsigned IV.
Perl also uses two special typedefs, I32 and I16, which will always be at least 32-bits and
16-bits long, respectively. (Again, there are U32 and U16, as well.) They will usually be
exactly 32 and 16 bits long, but on Crays they will both be 64 bits.
An SV can be created and loaded with one command. There are five types of values that can
be loaded: an integer value (IV), an unsigned integer value (UV), a double (NV), a string
(PV), and another scalar (SV).
The seven routines are:
SV* newSViv(IV);
SV* newSVuv(UV);
SV* newSVnv(double);
SV* newSVpv(const char*, int);
SV* newSVpvn(const char*, int);
SV* newSVpvf(const char*, ...);
SV* newSVsv(SV*);
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If you require more complex initialisation you can create an empty SV with newSV(len). If len
is 0 an empty SV of type NULL is returned, else an SV of type PV is returned with len + 1 (for
the NUL) bytes of storage allocated, accessible via SvPVX. In both cases the SV has value
undef.
SV* newSV(0); /* no storage allocated */
SV* newSV(10); /* 10 (+1) bytes of uninitialised storage allocated */
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To change the value of an *already-existing* SV, there are eight routines:
void sv_setiv(SV*, IV);
void sv_setuv(SV*, UV);
void sv_setnv(SV*, double);
void sv_setpv(SV*, const char*);
void sv_setpvn(SV*, const char*, int)
void sv_setpvf(SV*, const char*, ...);
void sv_vsetpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool *);
void sv_setsv(SV*, SV*);
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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_vsetpvfn is an analogue of vsprintf, but it allows you to
specify either a pointer to a variable argument list or the address and length of an array of
SVs. The last argument points to a boolean; on return, if that boolean is true, then
locale-specific information has been used to format the string, and the string's contents are
therefore untrustworthy (see perlsec).
This pointer may be NULL if that information is not important. Note that this function
requires you to specify the length of the format.
STRLEN is an integer type (Size_t, usually defined as size_t in config.h) guaranteed to be
large enough to represent the size of any string that perl can handle.
The sv_set*() functions are not generic enough to operate on values that have
"magic". See Magic Virtual Tables later in this
document.
All SVs that contain strings should be terminated with a NUL character. If it is not NUL-terminated
there is a risk of core dumps and corruptions from code which passes the string to C functions
or system calls which expect a NUL-terminated string. Perl's own functions typically add a
trailing NUL for this reason. Nevertheless, you should be very careful when you pass a string
stored in an SV to a C function or system call.
To access the actual value that an SV points to, you can use the macros:
SvIV(SV*)
SvUV(SV*)
SvNV(SV*)
SvPV(SV*, STRLEN len)
SvPV_nolen(SV*)
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which will automatically coerce the actual scalar type into an IV, UV, double, or string.
In the SvPV macro, the length of the string returned is placed into the
variable len (this is a macro, so you do not use &len).
If you do not care what the length of the data is, use the SvPV_nolen macro.
Historically the SvPV macro with the global variable PL_na has been
used in this case. But that can be quite inefficient because PL_na must be
accessed in thread-local storage in threaded Perl. In any case, remember that Perl allows
arbitrary strings of data that may both contain NULs and might not be terminated by a NUL.
Also remember that C doesn't allow you to safely say foo(SvPV(s, len), len);.
It might work with your compiler, but it won't work for everyone. Break this sort of statement
up into separate assignments:
SV *s;
STRLEN len;
char * ptr;
ptr = SvPV(s, len);
foo(ptr, len);
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If you want to know if the scalar value is TRUE, you can use:
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)
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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*)
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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)
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You can also get a pointer to the end of the string stored in the SV with the macro:
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*, const char*);
void sv_catpvn(SV*, const char*, STRLEN);
void sv_catpvf(SV*, const char*, ...);
void sv_vcatpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool);
void sv_catsv(SV*, SV*);
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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* get_sv("package::varname", FALSE);
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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:
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);
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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).
Perl provides the function sv_chop to efficiently remove characters from the
beginning of a string; you give it an SV and a pointer to somewhere inside the PV, and it
discards everything before the pointer. The efficiency comes by means of a little hack:
instead of actually removing the characters, sv_chop sets the flag OOK
(offset OK) to signal to other functions that the offset hack is in effect, and it puts the
number of bytes chopped off into the IV field of the SV. It then moves the PV pointer (called SvPVX)
forward that many bytes, and adjusts SvCUR and SvLEN.
Hence, at this point, the start of the buffer that we allocated lives at SvPVX(sv) -
SvIV(sv) in memory and the PV pointer is pointing into the middle of this allocated
storage.
This is best demonstrated by example:
% ./perl -Ilib -MDevel::Peek -le '$a="12345"; $a=~s/.//; Dump($a)'
SV = PVIV(0x8128450) at 0x81340f0
REFCNT = 1
FLAGS = (POK,OOK,pPOK)
IV = 1 (OFFSET)
PV = 0x8135781 ( "1" . ) "2345"\0
CUR = 4
LEN = 5
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Here the number of bytes chopped off (1) is put into IV, and Devel::Peek::Dump
helpfully reminds us that this is an offset. The portion of the string between the
"real" and the "fake" beginnings is shown in parentheses, and the values
of SvCUR and SvLEN reflect the fake beginning, not the real one.
Something similar to the offset hack is performed on AVs to enable efficient shifting and
splicing off the beginning of the array; while AvARRAY points to the first
element in the array that is visible from Perl, AvALLOC points to the real start
of the C array. These are usually the same, but a shift operation can be carried
out by increasing AvARRAY by one and decreasing AvFILL and AvLEN.
Again, the location of the real start of the C array only comes into play when freeing the
array. See av_shift in av.c.
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*)
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These will tell you if you truly have an integer, double, or string pointer stored in your
SV. The "p" stands for private.
The are various ways in which the private and public flags may differ. For example, a tied
SV may have a valid underlying value in the IV slot (so SvIOKp is true), but the data should
be accessed via the FETCH routine rather than directly, so SvIOK is false. Another is when
numeric conversion has occured and precision has been lost: only the private flag is set on 'lossy'
values. So when an NV is converted to an IV with loss, SvIOKp, SvNOKp and SvNOK will be set,
while SvIOK wont be.
In general, though, it's best to use the Sv*V macros.
There are two ways to create and load an AV. The first method creates an empty AV:
The second method both creates the AV and initially populates it with SVs:
AV* av_make(I32 num, SV **ptr);
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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);
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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);
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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);
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The av_clear function deletes all the elements in the AV* array, but does not
actually delete the array itself. The av_undef function will delete all the
elements in the array plus the array itself. The av_extend function extends the
array so that it contains at least key+1 elements. If key+1 is less
than the currently allocated length of the array, then nothing is done.
If you know the name of an array variable, you can get a pointer to its AV by using the
following:
AV* get_av("package::varname", FALSE);
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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:
Once the HV has been created, the following operations are possible on HVs:
SV** hv_store(HV*, const char* key, U32 klen, SV* val, U32 hash);
SV** hv_fetch(HV*, const char* key, U32 klen, I32 lval);
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The klen parameter is the length of the key being passed in (Note that you
cannot pass 0 in as a value of klen to tell Perl to measure the length of the
key). The val argument contains the SV pointer to the scalar being stored, and hash
is the precomputed hash value (zero if you want hv_store to calculate it for
you). The lval parameter indicates whether this fetch is actually a part of a
store operation, in which case a new undefined value will be added to the HV with the supplied
key and hv_fetch will return as if the value had already existed.
Remember that hv_store and hv_fetch return SV**'s
and not just SV*. To access the scalar value, you must first dereference the
return value. However, you should check to make sure that the return value is not NULL before
dereferencing it.
These two functions check if a hash table entry exists, and deletes it.
bool hv_exists(HV*, const char* key, U32 klen);
SV* hv_delete(HV*, const char* key, U32 klen, I32 flags);
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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*);
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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 an SV pointer to the value of the HE
structure */
SV* hv_iternextsv(HV*, char** key, I32* retlen);
/* This convenience routine combines hv_iternext,
hv_iterkey, and hv_iterval. The key and retlen
arguments are return values for the key and its
length. The value is returned in the SV* argument */
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If you know the name of a hash variable, you can get a pointer to its HV by using the
following:
HV* get_hv("package::varname", FALSE);
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This returns NULL if the variable does not exist.
The hash algorithm is defined in the PERL_HASH(hash, key, klen) macro:
hash = 0;
while (klen--)
hash = (hash * 33) + *key++;
hash = hash + (hash >> 5); /* after 5.6 */
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The last step was added in version 5.6 to improve distribution of lower bits in the
resulting hash value.
See Understanding the Magic of
Tied Hashes and Arrays for more information on how to use the hash access functions on
tied hashes.
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);
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Note that these functions take SV* keys, which simplifies writing of extension
code that deals with hash structures. These functions also allow passing of SV*
keys to tie functions without forcing you to stringify the keys (unlike the
previous set of functions).
They also return and accept whole hash entries (HE*), making their use more
efficient (since the hash number for a particular string doesn't have to be recomputed every
time). See perlapi for
detailed descriptions.
The following macros must always be used to access the contents of hash entries. Note that
the arguments to these macros must be simple variables, since they may get evaluated more than
once. See perlapi for detailed
descriptions of these macros.
HePV(HE* he, STRLEN len)
HeVAL(HE* he)
HeHASH(HE* he)
HeSVKEY(HE* he)
HeSVKEY_force(HE* he)
HeSVKEY_set(HE* he, SV* sv)
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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)
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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);
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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:
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:
To discover what type of value the reference refers to, use the following macro and then
check the return value.
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.
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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);
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The sv argument must be a reference. The stash argument specifies
which class the reference will belong to. See Stashes and Globs
for information on converting class names into stashes.
/* Still under construction */
Upgrades rv to reference if not already one. Creates new SV for rv to point to. If classname
is non-null, the SV is blessed into the specified class. SV is returned.
SV* newSVrv(SV* rv, const char* classname);
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Copies integer, unsigned integer or double into an SV whose reference is rv.
SV is blessed if classname is non-null.
SV* sv_setref_iv(SV* rv, const char* classname, IV iv);
SV* sv_setref_uv(SV* rv, const char* classname, UV uv);
SV* sv_setref_nv(SV* rv, const char* classname, NV iv);
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Copies the pointer value (the address, not the string!) into an SV whose reference
is rv. SV is blessed if classname is non-null.
SV* sv_setref_pv(SV* rv, const char* classname, PV iv);
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Copies string into an SV whose reference is rv. Set length to 0 to let Perl
calculate the string length. SV is blessed if classname is non-null.
SV* sv_setref_pvn(SV* rv, const char* classname, PV iv, STRLEN length);
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Tests whether the SV is blessed into the specified class. It does not check inheritance
relationships.
int sv_isa(SV* sv, const char* name);
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Tests whether the SV is a reference to a blessed object.
Tests whether the SV is derived from the specified class. SV can be either a reference to a
blessed object or a string containing a class name. This is the function implementing the UNIVERSAL::isa
functionality.
bool sv_derived_from(SV* sv, const char* name);
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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)) { ... }
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To create a new Perl variable with an undef value which can be accessed from your Perl
script, use the following routines, depending on the variable type.
SV* get_sv("package::varname", TRUE);
AV* get_av("package::varname", TRUE);
HV* get_hv("package::varname", TRUE);
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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
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warning.
- GV_ADDWARN
-
Issues the warning:
Had to create <varname> unexpectedly
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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 a 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);
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However, there is one other function which manipulates the reference count of its argument.
The newRV_inc function, you will recall, creates a reference to the specified
argument. As a side effect, it increments the argument's reference count. If this is not what
you want, use newRV_noinc instead.
For example, imagine you want to return a reference from an XSUB function. Inside the XSUB
routine, you create an SV which initially has a reference count of one. Then you call newRV_inc,
passing it the just-created SV. This returns the reference as a new SV, but the reference
count of the SV you passed to newRV_inc has been incremented to two. Now you
return the reference from the XSUB routine and forget about the SV. But Perl hasn't! Whenever
the returned reference is destroyed, the reference count of the original SV is decreased to
one and nothing happens. The SV will hang around without any way to access it until Perl
itself terminates. This is a memory leak.
The correct procedure, then, is to use newRV_noinc instead of newRV_inc.
Then, if and when the last reference is destroyed, the reference count of the SV will go to
zero and it will be destroyed, stopping any memory leak.
There are some convenience functions available that can help with the destruction of xVs.
These functions introduce the concept of "mortality". An xV that is mortal has had
its reference count marked to be decremented, but not actually decremented, until "a
short time later". Generally the term "short time later" means a single Perl
statement, such as a call to an XSUB function. The actual determinant for when mortal xVs have
their reference count decremented depends on two macros, SAVETMPS and FREETMPS. See perlcall and perlxs for more details on these
macros.
"Mortalization" then is at its simplest a deferred SvREFCNT_dec.
However, if you mortalize a variable twice, the reference count will later be decremented
twice.
"Mortal" SVs are mainly used for SVs that are placed on perl's stack. For example
an SV which is created just to pass a number to a called sub is made mortal to have it cleaned
up automatically when stack is popped. Similarly results returned by XSUBs (which go in the
stack) are often made mortal.
To create a mortal variable, use the functions:
SV* sv_newmortal()
SV* sv_2mortal(SV*)
SV* sv_mortalcopy(SV*)
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The first call creates a mortal SV (with no value), 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. Because sv_newmortal gives the new SV no value,it
must normally be given one via sv_setpv, sv_setiv, etc. :
SV *tmp = sv_newmortal();
sv_setiv(tmp, an_integer);
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As that is multiple C statements it is quite common so see this idiom instead:
SV *tmp = sv_2mortal(newSViv(an_integer));
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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. Thinking of "Mortalization" as deferred SvREFCNT_dec should help
to minimize such problems. For example if you are passing an SV which you know has high
enough REFCNT to survive its use on the stack you need not do any mortalization. If you are
not sure then doing an SvREFCNT_inc and sv_2mortal, or making a sv_mortalcopy
is safer.
The mortal routines are not just for SVs -- AVs and HVs can be made mortal by passing their
address (type-casted to SV*) to the sv_2mortal or sv_mortalcopy
routines.
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
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There is a single stash called "PL_defstash" that holds the items that exist in
the "main" package. To get at the items in other packages, append the string
"::" to the package name. The items in the "Foo" package are in the stash
"Foo::" in PL_defstash. The items in the "Bar::Baz" package are in the
stash "Baz::" in "Bar::"'s stash.
To get the stash pointer for a particular package, use the function:
HV* gv_stashpv(const char* name, I32 create)
HV* gv_stashsv(SV*, I32 create)
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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:
then use the following to get the package name itself:
If you need to bless or re-bless an object you can use the following function:
SV* sv_bless(SV*, HV* stash)
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where the first argument, an SV*, must be a reference, and the second argument
is a stash. The returned SV* can now be used in the same way as any other SV.
For more information on references and blessings, consult perlref.
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
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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 = get_sv("dberror", TRUE);
sv_setiv(sv, (IV) dberror);
sv_setpv(sv, dberror_list[dberror]);
SvIOK_on(sv);
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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;
};
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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, const char* name, I32 namlen);
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The sv argument is a pointer to the SV that is to acquire a new magical
feature.
If sv is not already magical, Perl uses the SvUPGRADE macro to
convert sv to type SVt_PVMG. Perl then continues by adding new magic
to the beginning of the linked list of magical features. Any prior entry of the same type of
magic is deleted. Note that this can be 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 value of how should be chosen from
the set of macros PERL_MAGIC_foo found perl.h. Note that before these macros were
added, Perl internals used to directly use character literals, so you may occasionally come
across old code or documentation referring to 'U' magic rather than PERL_MAGIC_uvar
for example.
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 PERL_MAGIC_arylen,
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);
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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);
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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 an MGVTBL,
which is a structure of function pointers and stands for "Magic Virtual Table" to
handle the various operations that might be applied to that 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);
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This MGVTBL structure is set at compile-time in perl.h and there are currently
19 types (or 21 with overloading turned on). These different structures contain pointers to
various routines that perform additional actions depending on which function is being called.
Function pointer Action taken
---------------- ------------
svt_get Do something before the value of the SV is retrieved.
svt_set Do something after the SV is assigned a value.
svt_len Report on the SV's length.
svt_clear Clear something the SV represents.
svt_free Free any extra storage associated with the SV.
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For instance, the MGVTBL structure called vtbl_sv (which corresponds to an mg_type
of PERL_MAGIC_sv) contains:
{ magic_get, magic_set, magic_len, 0, 0 }
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Thus, when an SV is determined to be magical and of type PERL_MAGIC_sv, if a
get operation is being performed, the routine magic_get is called. All the
various routines for the various magical types begin with magic_. NOTE: the magic
routines are not considered part of the Perl API, and may not be exported by the Perl library.
The current kinds of Magic Virtual Tables are:
mg_type
(old-style char and macro) MGVTBL Type of magic
-------------------------- ------ ----------------------------
\0 PERL_MAGIC_sv vtbl_sv Special scalar variable
A PERL_MAGIC_overload vtbl_amagic %OVERLOAD hash
a PERL_MAGIC_overload_elem vtbl_amagicelem %OVERLOAD hash element
c PERL_MAGIC_overload_table (none) Holds overload table (AMT)
on stash
B PERL_MAGIC_bm vtbl_bm Boyer-Moore (fast string search)
D PERL_MAGIC_regdata vtbl_regdata Regex match position data
(@+ and @- vars)
d PERL_MAGIC_regdatum vtbl_regdatum Regex match position data
element
E PERL_MAGIC_env vtbl_env %ENV hash
e PERL_MAGIC_envelem vtbl_envelem %ENV hash element
f PERL_MAGIC_fm vtbl_fm Formline ('compiled' format)
g PERL_MAGIC_regex_global vtbl_mglob m//g target / study()ed string
I PERL_MAGIC_isa vtbl_isa @ISA array
i PERL_MAGIC_isaelem vtbl_isaelem @ISA array element
k PERL_MAGIC_nkeys vtbl_nkeys scalar(keys()) lvalue
L PERL_MAGIC_dbfile (none) Debugger %_<filename
l PERL_MAGIC_dbline vtbl_dbline Debugger %_<filename element
m PERL_MAGIC_mutex vtbl_mutex ???
o PERL_MAGIC_collxfrm vtbl_collxfrm Locale collate transformation
P PERL_MAGIC_tied vtbl_pack Tied array or hash
p PERL_MAGIC_tiedelem vtbl_packelem Tied array or hash element
q PERL_MAGIC_tiedscalar vtbl_packelem Tied scalar or handle
r PERL_MAGIC_qr vtbl_qr precompiled qr// regex
S PERL_MAGIC_sig vtbl_sig %SIG hash
s PERL_MAGIC_sigelem vtbl_sigelem %SIG hash element
t PERL_MAGIC_taint vtbl_taint Taintedness
U PERL_MAGIC_uvar vtbl_uvar Available for use by extensions
v PERL_MAGIC_vec vtbl_vec vec() lvalue
x PERL_MAGIC_substr vtbl_substr substr() lvalue
y PERL_MAGIC_defelem vtbl_defelem Shadow "foreach" iterator
variable / smart parameter
vivification
* PERL_MAGIC_glob vtbl_glob GV (typeglob)
# PERL_MAGIC_arylen vtbl_arylen Array length ($#ary)
. PERL_MAGIC_pos vtbl_pos pos() lvalue
< PERL_MAGIC_backref vtbl_backref ???
~ PERL_MAGIC_ext (none) Available for use by extensions
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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. Some internals code makes use of this
case relationship.
The PERL_MAGIC_ext and PERL_MAGIC_uvar magic types are defined
specifically for use by extensions and will not be used by perl itself. Extensions can use PERL_MAGIC_ext
magic to 'attach' private information to variables (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, PERL_MAGIC_uvar 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)(pTHX_ IV, SV*);
I32 (*uf_set)(pTHX_ IV, SV*);
IV uf_index;
};
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When the SV is read from or written to, the uf_val or uf_set
function will be called with uf_index as the first arg and a pointer to the SV as
the second. A simple example of how to add PERL_MAGIC_uvar magic is shown below.
Note that the ufuncs structure is copied by sv_magic, so you can safely allocate it on the
stack.
void
Umagic(sv)
SV *sv;
PREINIT:
struct ufuncs uf;
CODE:
uf.uf_val = &my_get_fn;
uf.uf_set = &my_set_fn;
uf.uf_index = 0;
sv_magic(sv, 0, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf));
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Note that because multiple extensions may be using PERL_MAGIC_ext or PERL_MAGIC_uvar
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 PERL_MAGIC_ext
magic, it may also be appropriate to add an I32 'signature' at the top of the private data
area and check that.
Also note that the sv_set*() and sv_cat*() functions described
earlier do not invoke 'set' magic on their targets. This must be done by the user
either by calling the SvSETMAGIC() macro after calling these functions, or by
using one of the sv_set*_mg() or sv_cat*_mg() functions. Similarly,
generic C code must call the SvGETMAGIC() macro to invoke any 'get' magic if they
use an SV obtained from external sources in functions that don't handle magic. See perlapi for a description of
these functions. For example, calls to the sv_cat*() functions typically need to
be followed by SvSETMAGIC(), but they don't need a prior SvGETMAGIC()
since their implementation handles 'get' magic.
MAGIC* mg_find(SV*, int type); /* Finds the magic pointer of that type */
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This routine returns a pointer to the MAGIC structure stored in the SV. If the
SV does not have that magical feature, NULL is returned. Also, if the SV is not
of type SVt_PVMG, Perl may core dump.
int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen);
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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 PERL_MAGIC_tied magic type.
WARNING: As of the 5.004 release, proper usage of the array and hash access functions
requires understanding a few caveats. Some of these caveats are actually considered bugs in
the API, to be fixed in later releases, and are bracketed with [MAYCHANGE] below. If you find
yourself actually applying such information in this section, be aware that the behavior may
change in the future, umm, without warning.
The perl tie function associates a variable with an object that implements the various GET,
SET, etc methods. To perform the equivalent of the perl tie function from an XSUB, you must
mimic this behaviour. The code below carries out the necessary steps - firstly it creates a
new hash, and then creates a second hash which it blesses into the class which will implement
the tie methods. Lastly it ties the two hashes together, and returns a reference to the new
tied hash. Note that the code below does NOT call the TIEHASH method in the MyTie class - see Calling Perl Routines from within C
Programs for details on how to do this.
SV*
mytie()
PREINIT:
HV *hash;
HV *stash;
SV *tie;
CODE:
hash = newHV();
tie = newRV_noinc((SV*)newHV());
stash = gv_stashpv("MyTie", TRUE);
sv_bless(tie, stash);
hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
RETVAL = newRV_noinc(hash);
OUTPUT:
RETVAL
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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
This construction is approximately equivalent to
{
my $oldvar = $var;
$var = 2;
...
$var = $oldvar;
}
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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 perlcall/"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:
SAVEINT(int i)
-
SAVEIV(IV i)
-
SAVEI32(I32 i)
-
SAVELONG(long i)
- These macros arrange things to restore the value of integer variable
i at
the end of enclosing pseudo-block.
SAVESPTR(s)
-
SAVEPPTR(p)
- These macros arrange things to restore the value of pointers
s and p.
s must be a pointer of a type which survives conversion to SV*
and back, p should be able to survive conversion to char* and
back.
SAVEFREESV(SV *sv)
-
The refcount of sv would be decremented at the end of pseudo-block.
This is similar to sv_2mortal in that it is also a mechanism for doing a
delayed SvREFCNT_dec. However, while sv_2mortal extends the
lifetime of sv until the beginning of the next statement, SAVEFREESV
extends it until the end of the enclosing scope. These lifetimes can be wildly different.
Also compare SAVEMORTALIZESV.
SAVEMORTALIZESV(SV *sv)
- Just like
SAVEFREESV, but mortalizes sv at the end of the
current scope instead of decrementing its reference count. This usually has the effect of
keeping sv alive until the statement that called the currently live scope has
finished executing.
SAVEFREEOP(OP *op)
- The
OP * is op_free()ed at the end of pseudo-block.
SAVEFREEPV(p)
- The chunk of memory which is pointed to by
p is Safefree()ed at the end of pseudo-block.
SAVECLEARSV(SV *sv)
- Clears a slot in the current scratchpad which corresponds to
sv at the end
of pseudo-block.
SAVEDELETE(HV *hv, char
*key, I32 length)
-
The key key of hv is deleted at the end of pseudo-block.
The string pointed to by key is Safefree()ed. If one has a key in
short-lived storage, the corresponding string may be reallocated like this:
SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));
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SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t
f, void *p)
- At the end of pseudo-block the function
f is called with the only
argument p.
SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t
f, void *p)
- At the end of pseudo-block the function
f is called with the
implicit context argument (if any), and p.
SAVESTACK_POS()
- The current offset on the Perl internal stack (cf.
SP) is restored at the
end of pseudo-block.
The following API list contains functions, thus one needs to provide pointers to the
modifiable data explicitly (either C pointers, or Perlish GV *s). Where the above
macros take int, a similar function takes int *.
SV* save_scalar(GV *gv)
- Equivalent to Perl code
local $gv.
AV* save_ary(GV *gv)
-
HV* save_hash(GV *gv)
- Similar to
save_scalar, but localize @gv and %gv.
void save_item(SV *item)
- Duplicates the current value of
SV, on the exit from the current ENTER/LEAVE
pseudo-block will restore the value of SV using the stored value.
void save_list(SV **sarg, I32
maxsarg)
- A variant of
save_item which takes multiple arguments via an array sarg
of SV* of length maxsarg.
SV* save_svref(SV **sptr)
- Similar to
save_scalar, but will reinstate an SV *.
void save_aptr(AV **aptr)
-
void save_hptr(HV **hptr)
- Similar to
save_svref, but localize AV * and HV *.
The Alias module implements localization of the basic types within the caller's
scope. People who are interested in how to localize things in the containing scope should
take a look there too.
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