perlguts - Introduction to the Perl API
This document attempts to describe how to use the Perl
API, as well as to provide some info on the basic workings
of the Perl core. It is far from complete and probably
contains many errors. Please refer any questions or comments
to the author below.
Datatypes
Perl has three typedefs that handle Perl's three main data
types:
SV Scalar Value
AV Array Value
HV Hash Value
Each typedef has specific routines that manipulate the
various data types.
What is an "IV"?
Perl uses a special typedef IV which is a simple 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.
Working with SVs [Toc] [Back]
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*, STRLEN);
SV* newSVpvn(const char*, STRLEN);
SV* newSVpvf(const char*, ...);
SV* newSVsv(SV*);
"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.
In the unlikely case of a SV requiring 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 *sv = newSV(0); /* no storage allocated */
SV *sv = newSV(10); /* 10 (+1) bytes of uninitialised
storage allocated */
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*, STRLEN)
void sv_setpvf(SV*, const char*, ...);
void sv_vsetpvfn(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_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.
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*)
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);
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*, 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*);
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);
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. Make
sure that you don't try to compare a random sv with
&PL_sv_undef. For example when interfacing Perl code,
it'll work correctly for:
foo(undef);
But won't work when called as:
$x = undef;
foo($x);
So to repeat always use SvOK() to check whether an sv is
defined.
Also you have to be careful when using &PL_sv_undef as a
value in AVs or HVs (see "AVs, HVs and undefined values").
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").
Offsets
Perl provides the function "sv_chop" to efficiently remove
characters from the beginning of a string; you give it an
SV and a pointer to somewhere inside the 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"
CUR = 4
LEN = 5
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.
What's Really Stored in an SV?
Recall that the usual method of determining the type of
scalar you have is to use "Sv*OK" macros. Because a
scalar can be both a number and a string, usually these
macros will always return TRUE and calling the "Sv*V"
macros will do the appropriate conversion of string to
integer/double or integer/double to string.
If you really need to know if you have an integer, double,
or string pointer in an SV, you can use the following
three macros instead:
SvIOKp(SV*)
SvNOKp(SV*)
SvPOKp(SV*)
These will tell you if you truly have an integer, double,
or string pointer stored in your SV. The "p" stands for
private.
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.
Working with AVs [Toc] [Back]
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 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);
This returns NULL if the variable does not exist.
See "Understanding the Magic of Tied Hashes and Arrays"
for more information on how to use the array access functions
on tied arrays.
Working with HVs [Toc] [Back]
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*, const char* key, U32 klen, SV*
val, U32 hash);
SV** hv_fetch(HV*, const char* key, U32 klen, I32
lval);
The "klen" parameter is the length of the key being passed
in (Note that you cannot pass 0 in as a value of "klen" to
tell Perl to measure the length of the key). The "val"
argument contains the SV pointer to the scalar being
stored, and "hash" is the precomputed hash value (zero if
you want "hv_store" to calculate it for you). The "lval"
parameter indicates whether this fetch is actually a part
of a store operation, in which case a new undefined value
will be added to the HV with the supplied key and
"hv_fetch" will return as if the value had already
existed.
Remember that "hv_store" and "hv_fetch" return "SV**"'s
and not just "SV*". To access the scalar value, you must
first dereference the return value. However, you should
check to make sure that the return value is not NULL
before dereferencing it.
These two functions check if a hash table entry exists,
and deletes it.
bool hv_exists(HV*, const char* key, U32 klen);
SV* hv_delete(HV*, const char* key, U32 klen, I32
flags);
If "flags" does not include the "G_DISCARD" flag then
"hv_delete" will create and return a mortal copy of the
deleted value.
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 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 */
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);
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 */
The last step was added in version 5.6 to improve distribution
of lower bits in the resulting hash value.
See "Understanding the Magic of Tied Hashes and Arrays"
for more information on how to use the hash access functions
on tied hashes.
Hash API Extensions [Toc] [Back]
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 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)
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.
AVs, HVs and undefined values
Sometimes you have to store undefined values in AVs or
HVs. Although this may be a rare case, it can be tricky.
That's because you're used to using &PL_sv_undef if you
need an undefined SV.
For example, intuition tells you that this XS code:
AV *av = newAV();
av_store( av, 0, &PL_sv_undef );
is equivalent to this Perl code:
my @av;
$av[0] = undef;
Unfortunately, this isn't true. AVs use &PL_sv_undef as a
marker for indicating that an array element has not yet
been initialized. Thus, "exists $av[0]" would be true for
the above Perl code, but false for the array generated by
the XS code.
Other problems can occur when storing &PL_sv_undef in HVs:
hv_store( hv, "key", 3, &PL_sv_undef, 0 );
This will indeed make the value "undef", but if you try to
modify the value of "key", you'll get the following error:
Modification of non-creatable hash value attempted
In perl 5.8.0, &PL_sv_undef was also used to mark placeholders
in restricted hashes. This caused such hash
entries not to appear when iterating over the hash or when
checking for the keys with the "hv_exists" function.
You can run into similar problems when you store
&PL_sv_true or &PL_sv_false into AVs or HVs. Trying to
modify such elements will give you the following error:
Modification of a read-only value attempted
To make a long story short, you can use the special variables
&PL_sv_undef, &PL_sv_true and &PL_sv_false with AVs
and HVs, but you have to make sure you know what you're
doing.
Generally, if you want to store an undefined value in an
AV or HV, you should not use &PL_sv_undef, but rather create
a new undefined value using the "newSV" function, for
example:
av_store( av, 42, newSV(0) );
hv_store( hv, "foo", 3, newSV(0), 0 );
References [Toc] [Back]
References are a special type of scalar that point to
other data types (including references).
To create a reference, use either of the following functions:
SV* newRV_inc((SV*) thing);
SV* newRV_noinc((SV*) thing);
The "thing" argument can be any of an "SV*", "AV*", or
"HV*". The functions are identical except that
"newRV_inc" increments the reference count of the "thing",
while "newRV_noinc" does not. For historical reasons,
"newRV" is a synonym for "newRV_inc".
Once you have a reference, you can use the following macro
to dereference the reference:
SvRV(SV*)
then call the appropriate routines, casting the returned
"SV*" to either an "AV*" or "HV*", if required.
To determine if an SV is a reference, you can use the following
macro:
SvROK(SV*)
To discover what type of value the reference refers to,
use the following macro and then check the return value.
SvTYPE(SvRV(SV*))
The most useful types that will be returned are:
SVt_IV Scalar
SVt_NV Scalar
SVt_PV Scalar
SVt_RV Scalar
SVt_PVAV Array
SVt_PVHV Hash
SVt_PVCV Code
SVt_PVGV Glob (possible a file handle)
SVt_PVMG Blessed or Magical Scalar
See the sv.h header file for more details.
Blessed References and Class Objects [Toc] [Back]
References are also used to support object-oriented programming.
In perl's 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 value. The "stash"
argument specifies which class the reference will belong
to. See "Stashes and Globs" for information on converting
class names into stashes.
/* Still under construction */
Upgrades rv to reference if not already one. Creates new
SV for rv to point to. If "classname" is non-null, the SV
is blessed into the specified class. SV is returned.
SV* newSVrv(SV* rv, const char* classname);
Copies integer, 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);
Copies the pointer value (the address, not the string!)
into an SV whose reference is rv. SV is blessed if
"classname" is non-null.
SV* sv_setref_pv(SV* rv, const char* classname, PV
iv);
Copies string into an SV whose reference is "rv". Set
length to 0 to let Perl calculate the string length. SV
is blessed if "classname" is non-null.
SV* sv_setref_pvn(SV* rv, const char* classname,
PV iv, STRLEN length);
Tests whether the SV is blessed into the specified class.
It does not check inheritance relationships.
int sv_isa(SV* sv, const char* name);
Tests whether the SV is a reference to a blessed object.
int sv_isobject(SV* sv);
Tests whether the SV is derived from the specified class.
SV can be either a reference to a blessed object or a
string containing a class name. This is the function
implementing the "UNIVERSAL::isa" functionality.
bool sv_derived_from(SV* sv, const char* name);
To check if you've got an object derived from a specific
class you have to write:
if (sv_isobject(sv) && sv_derived_from(sv, class))
{ ... }
Creating New Variables [Toc] [Back]
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);
Notice the use of TRUE as the second parameter. The new
variable can now be set, using the routines appropriate to
the data type.
There are additional macros whose values may be bitwise
OR'ed with the "TRUE" argument to enable certain extra
features. Those bits are:
GV_ADDMULTI
Marks the variable as multiply defined, thus preventing
the:
Name <varname> used only once: possible typo
warning.
GV_ADDWARN
Issues the warning:
Had to create <varname> unexpectedly
if the variable did not exist before the function was
called.
If you do not specify a package name, the variable is created
in the current package.
Reference Counts and Mortality [Toc] [Back]
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);
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 it's popped off the stack.
Similarly, results returned by XSUBs (which are pushed on
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*)
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);
As that is multiple C statements it is quite common so see
this idiom instead:
SV *tmp = sv_2mortal(newSViv(an_integer));
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.
Stashes and Globs [Toc] [Back]
A stash is a hash that contains all variables that are
defined 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(const 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
perlref.
Double-Typed SVs [Toc] [Back]
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 = get_sv("dberror", TRUE);
sv_setiv(sv, (IV) dberror);
sv_setpv(sv, dberror_list[dberror]);
SvIOK_on(sv);
If the order of "sv_setiv" and "sv_setpv" had been
reversed, then the macro "SvPOK_on" would need to be
called instead of "SvIOK_on".
Magic Variables [Toc] [Back]
[This section still under construction. Ignore everything
here. Post no bills. Everything not permitted is forbidden.]
Any SV may be magical, that is, it has special features
that a normal SV does not have. These features are stored
in the SV structure in a linked list of "struct magic"'s,
typedef'ed to "MAGIC".
struct magic {
MAGIC* mg_moremagic;
MGVTBL* mg_virtual;
U16 mg_private;
char mg_type;
U8 mg_flags;
SV* mg_obj;
char* mg_ptr;
I32 mg_len;
};
Note this is current as of patchlevel 0, and could change
at any time.
Assigning Magic [Toc] [Back]
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);
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 Tables"
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 in 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);
This simply calls "sv_magic" and coerces the "gv" argument
into an "SV".
To remove the magic from an SV, call the function
sv_unmagic:
void sv_unmagic(SV *sv, int type);
The "type" argument should be equal to the "how" value
when the "SV" was initially made magical.
Magic Virtual Tables [Toc] [Back]
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);
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.
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 }
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
-------------------------- ------
----------------------------
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 BoyerMoore (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
V PERL_MAGIC_vstring (none) v-string
scalars
w PERL_MAGIC_utf8 vtbl_utf8 UTF-8
length+offset cache
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
When an uppercase and lowercase letter both exist in the
table, then the uppercase letter is typically 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. However, 'v' and 'V' (vec and
v-string) are in no way related.
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;
};
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));
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.
Finding Magic [Toc] [Back]
MAGIC* mg_find(SV*, int type); /* Finds the magic
pointer of that type */
This routine returns a pointer to the "MAGIC" structure
stored in the SV. If the SV does not have that magical
feature, "NULL" is returned. Also, if the SV is not of
type SVt_PVMG, Perl may core dump.
int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN
klen);
This routine checks to see what types of magic "sv" has.
If the mg_type field is an uppercase letter, then the
mg_obj is copied to "nsv", but the mg_type field is
changed to be the lowercase letter.
Understanding the Magic of Tied Hashes and Arrays [Toc] [Back]
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
The "av_store" function, when given a tied array argument,
merely copies the magic of the array onto the value to be
"stored", using "mg_copy". It may also return NULL, indicating
that the value did not actually need to be stored
in the array. [MAYCHANGE] After a call to "av_store" on a
tied array, the caller will usually need to call
"mg_set(val)" to actually invoke the perl level "STORE"
method on the TIEARRAY object. If "av_store" did return
NULL, a call to "SvREFCNT_dec(val)" will also be usually
necessary to avoid a memory leak. [/MAYCHANGE]
The previous paragraph is applicable verbatim to tied hash
access using the "hv_store" and "hv_store_ent" functions
as well.
"av_fetch" and the corresponding hash functions "hv_fetch"
and "hv_fetch_ent" actually return an undefined mortal
value whose magic has been initialized using "mg_copy".
Note the value so returned does not need to be deallocated,
as it is already mortal. [MAYCHANGE] But you will
need to call "mg_get()" on the returned value in order to
actually invoke the perl level "FETCH" method on the
underlying TIE object. Similarly, you may also call
"mg_set()" on the return value after possibly assigning a
suitable value to it using "sv_setsv", which will invoke
the "STORE" method on the TIE object. [/MAYCHANGE]
[MAYCHANGE] In other words, the array or hash fetch/store
functions don't really fetch and store actual values in
the case of tied arrays and hashes. They merely call
"mg_copy" to attach magic to the values that were meant to
be "stored" or "fetched". Later calls to "mg_get" and
"mg_set" actually do the job of invoking the TIE methods
on the underlying objects. Thus the magic mechanism currently
implements a kind of lazy access to arrays and
hashes.
Currently (as of perl version 5.004), use of the hash and
array access functions requires the user to be aware of
whether they are operating on "normal" hashes and arrays,
or on their tied variants. The API may be changed to provide
more transparent access to both tied and normal data
types in future versions. [/MAYCHANGE]
You would do well to understand that the TIEARRAY and
TIEHASH interfaces are mere sugar to invoke some perl
method calls while using the uniform hash and array syntax.
The use of this sugar imposes some overhead (typically
about two to four extra opcodes per FETCH/STORE
operation, in addition to the creation of all the mortal
variables required to invoke the methods). This overhead
will be comparatively small if the TIE methods are themselves
substantial, but if they are only a few statements
long, the overhead will not be insignificant.
Localizing changes [Toc]
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