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perlguts - Introduction to the Perl API
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perlguts - Introduction to the Perl API
This document attempts to describe how to use the Perl API, as well as
containing some info on the basic workings of the Perl core. It is far
from complete and probably contains many errors. Please refer any
questions or comments to the author below.
Perl has three typedefs that handle Perl's three main data types:
SV Scalar Value
AV Array Value
HV Hash Value
Each typedef has specific routines that manipulate the various data types.
Perl uses a special typedef IV which is a simple 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.)
An SV can be created and loaded with one command. There are four types of
values that can be loaded: an integer value (IV), a double (NV),
a string (PV), and another scalar (SV).
The six routines are:
SV* newSViv(IV);
SV* newSVnv(double);
SV* newSVpv(const char*, int);
SV* newSVpvn(const char*, int);
SV* newSVpvf(const char*, ...);
SV* newSVsv(SV*);
To change the value of an *already-existing* SV, there are seven routines:
void sv_setiv(SV*, IV);
void sv_setuv(SV*, UV);
void sv_setnv(SV*, double);
void sv_setpv(SV*, const char*);
void sv_setpvn(SV*, const char*, int)
void sv_setpvf(SV*, const char*, ...);
void sv_setpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool);
void sv_setsv(SV*, SV*);
Notice that you can choose to specify the length of the string to be
assigned by using sv_setpvn, newSVpvn, or newSVpv, or you may
allow Perl to calculate the length by using sv_setpv or by specifying
0 as the second argument to newSVpv. Be warned, though, that Perl will
determine the string's length by using strlen, which depends on the
string terminating with a NUL character.
The arguments of sv_setpvf are processed like sprintf, and the
formatted output becomes the value.
sv_setpvfn is an analogue of vsprintf, but it allows you to specify
either a pointer to a variable argument list or the address and length of
an array of SVs. The last argument points to a boolean; on return, if that
boolean is true, then locale-specific information has been used to format
the string, and the string's contents are therefore untrustworthy (see
the perlsec manpage). This pointer may be NULL if that information is not
important. Note that this function requires you to specify the length of
the format.
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*)
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_catpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool);
void sv_catsv(SV*, SV*);
The first function calculates the length of the string to be appended by
using strlen. In the second, you specify the length of the string
yourself. The third function processes its arguments like sprintf and
appends the formatted output. The fourth function works like vsprintf.
You can specify the address and length of an array of SVs instead of the
va_list argument. The fifth function extends the string stored in the first
SV with the string stored in the second SV. It also forces the second SV
to be interpreted as a string.
The sv_cat*() functions are not generic enough to operate on values that
have ``magic''. See Magic Virtual Tables later in this document.
If you know the name of a scalar variable, you can get a pointer to its SV
by using the following:
SV* get_sv("package::varname", FALSE);
This returns NULL if the variable does not exist.
If you want to know if this variable (or any other SV) is actually defined,
you can call:
SvOK(SV*)
The scalar undef value is stored in an SV instance called PL_sv_undef. Its
address can be used whenever an SV* is needed.
There are also the two values PL_sv_yes and PL_sv_no, which contain Boolean
TRUE and FALSE values, respectively. Like PL_sv_undef, their addresses can
be used whenever an SV* is needed.
Do not be fooled into thinking that (SV *) 0 is the same as &PL_sv_undef.
Take this code:
SV* sv = (SV*) 0;
if (I-am-to-return-a-real-value) {
sv = sv_2mortal(newSViv(42));
}
sv_setsv(ST(0), sv);
This code tries to return a new SV (which contains the value 42) if it should
return a real value, or undef otherwise. Instead it has returned a NULL
pointer which, somewhere down the line, will cause a segmentation violation,
bus error, or just weird results. Change the zero to &PL_sv_undef in the first
line and all will be well.
To free an SV that you've created, call SvREFCNT_dec(SV*). Normally this
call is not necessary (see Reference Counts and Mortality).
Perl provides the function sv_chop to efficiently remove characters
from the beginning of a string; you give it an SV and a pointer to
somewhere inside the the PV, and it discards everything before the
pointer. The efficiency comes by means of a little hack: instead of
actually removing the characters, sv_chop sets the flag OOK
(offset OK) to signal to other functions that the offset hack is in
effect, and it puts the number of bytes chopped off into the IV field
of the SV. It then moves the PV pointer (called SvPVX) forward that
many bytes, and adjusts SvCUR and SvLEN.
Hence, at this point, the start of the buffer that we allocated lives
at SvPVX(sv) - SvIV(sv) in memory and the PV pointer is pointing
into the middle of this allocated storage.
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
Here the number of bytes chopped off (1) is put into IV, and
Devel::Peek::Dump helpfully reminds us that this is an offset. The
portion of the string between the ``real'' and the ``fake'' beginnings is
shown in parentheses, and the values of SvCUR and SvLEN reflect
the fake beginning, not the real one.
Something similar to the offset hack is perfomed on AVs to enable
efficient shifting and splicing off the beginning of the array; while
AvARRAY points to the first element in the array that is visible from
Perl, AvALLOC points to the real start of the C array. These are
usually the same, but a shift operation can be carried out by
increasing AvARRAY by one and decreasing AvFILL and AvLEN.
Again, the location of the real start of the C array only comes into
play when freeing the array. See av_shift in av.c.
Recall that the usual method of determining the type of scalar you have is
to use Sv*OK macros. Because a scalar can be both a number and a string,
usually these macros will always return TRUE and calling the Sv*V
macros will do the appropriate conversion of string to integer/double or
integer/double to string.
If you really need to know if you have an integer, double, or string
pointer in an SV, you can use the following three macros instead:
SvIOKp(SV*)
SvNOKp(SV*)
SvPOKp(SV*)
These will tell you if you truly have an integer, double, or string pointer
stored in your SV. The ``p'' stands for private.
In general, though, it's best to use the Sv*V macros.
There are two ways to create and load an AV. The first method creates an
empty AV:
AV* newAV();
The second method both creates the AV and initially populates it with SVs:
AV* av_make(I32 num, SV **ptr);
The second argument points to an array containing num SV*'s. Once the
AV has been created, the SVs can be destroyed, if so desired.
Once the AV has been created, the following operations are possible on AVs:
void av_push(AV*, SV*);
SV* av_pop(AV*);
SV* av_shift(AV*);
void av_unshift(AV*, I32 num);
These should be familiar operations, with the exception of av_unshift.
This routine adds num elements at the front of the array with the undef
value. You must then use av_store (described below) to assign values
to these new elements.
Here are some other functions:
I32 av_len(AV*);
SV** av_fetch(AV*, I32 key, I32 lval);
SV** av_store(AV*, I32 key, SV* val);
The av_len function returns the highest index value in array (just
like $#array in Perl). If the array is empty, -1 is returned. The
av_fetch function returns the value at index key, but if lval
is non-zero, then av_fetch will store an undef value at that index.
The av_store function stores the value val at index key, and does
not increment the reference count of val. Thus the caller is responsible
for taking care of that, and if av_store returns NULL, the caller will
have to decrement the reference count to avoid a memory leak. Note that
av_fetch and av_store both return SV**'s, not SV*'s as their
return value.
void av_clear(AV*);
void av_undef(AV*);
void av_extend(AV*, I32 key);
The av_clear function deletes all the elements in the AV* array, but
does not actually delete the array itself. The av_undef function will
delete all the elements in the array plus the array itself. The
av_extend function extends the array so that it contains 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.
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 a SV pointer to the value of the HE
structure */
SV* hv_iternextsv(HV*, char** key, I32* retlen);
/* This convenience routine combines hv_iternext,
hv_iterkey, and hv_iterval. The key and retlen
arguments are return values for the key and its
length. The value is returned in the SV* argument */
If you know the name of a hash variable, you can get a pointer to its HV
by using the following:
HV* 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.
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 the perlapi manpage 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
the perlapi manpage for detailed descriptions of these macros.
HePV(HE* he, STRLEN len)
HeVAL(HE* he)
HeHASH(HE* he)
HeSVKEY(HE* he)
HeSVKEY_force(HE* he)
HeSVKEY_set(HE* he, SV* sv)
These two lower level macros are defined, but must only be used when
dealing with keys that are not SV*s:
HeKEY(HE* he)
HeKLEN(HE* he)
Note that both hv_store and hv_store_ent do not increment the
reference count of the stored val, which is the caller's responsibility.
If these functions return a NULL value, the caller will usually have to
decrement the reference count of val to avoid a memory leak.
References are a special type of scalar that point to other data types
(including references).
To create a reference, use either of the following functions:
SV* newRV_inc((SV*) thing);
SV* newRV_noinc((SV*) thing);
The thing argument can be any of an SV*, AV*, or HV*. The
functions are identical except that newRV_inc increments the reference
count of the thing, while newRV_noinc does not. For historical
reasons, newRV is a synonym for newRV_inc.
Once you have a reference, you can use the following macro to dereference
the reference:
SvRV(SV*)
then call the appropriate routines, casting the returned SV* to either an
AV* or HV*, if required.
To determine if an SV is a reference, you can use the following macro:
SvROK(SV*)
To discover what type of value the reference refers to, use the following
macro and then check the return value.
SvTYPE(SvRV(SV*))
The most useful types that will be returned are:
SVt_IV Scalar
SVt_NV Scalar
SVt_PV Scalar
SVt_RV Scalar
SVt_PVAV Array
SVt_PVHV Hash
SVt_PVCV Code
SVt_PVGV Glob (possible a file handle)
SVt_PVMG Blessed or Magical Scalar
See the sv.h header file for more details.
References are also used to support object-oriented programming. In the
OO lexicon, an object is simply a reference that has been blessed into a
package (or class). Once blessed, the programmer may now use the reference
to access the various methods in the class.
A reference can be blessed into a package with the following function:
SV* sv_bless(SV* sv, HV* stash);
The sv argument must be a reference. The stash argument specifies
which class the reference will belong to. See
Stashes and Globs for information on converting class names into stashes.
/* Still under construction */
Upgrades rv to reference if not already one. Creates new SV for rv to
point to. If classname is non-null, the SV is blessed into the specified
class. SV is returned.
SV* newSVrv(SV* rv, const char* classname);
Copies integer or double into an SV whose reference is rv. SV is blessed
if classname is non-null.
SV* sv_setref_iv(SV* rv, const char* classname, IV iv);
SV* sv_setref_nv(SV* rv, const char* classname, NV iv);
Copies the pointer value (the address, not the string!) into an SV whose
reference is rv. SV is blessed if classname is non-null.
SV* sv_setref_pv(SV* rv, const char* classname, PV iv);
Copies string into an SV whose reference is rv. Set length to 0 to let
Perl calculate the string length. SV is blessed if classname is non-null.
SV* sv_setref_pvn(SV* rv, const char* classname, PV iv, STRLEN length);
Tests whether the SV is blessed into the specified class. It does not
check inheritance relationships.
int sv_isa(SV* sv, const char* name);
Tests whether the SV is a reference to a blessed object.
int sv_isobject(SV* sv);
Tests whether the SV is derived from the specified class. SV can be either
a reference to a blessed object or a string containing a class name. This
is the function implementing the UNIVERSAL::isa functionality.
bool sv_derived_from(SV* sv, const char* name);
To check if you've got an object derived from a specific class you have
to write:
if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }
To create a new Perl variable with an undef value which can be accessed from
your Perl script, use the following routines, depending on the variable type.
SV* 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.
Perl uses an reference count-driven garbage collection mechanism. SVs,
AVs, or HVs (xV for short in the following) start their life with a
reference count of 1. If the reference count of an xV ever drops to 0,
then it will be destroyed and its memory made available for reuse.
This normally doesn't happen at the Perl level unless a variable is
undef'ed or the last variable holding a reference to it is changed or
overwritten. At the internal level, however, reference counts can be
manipulated with the following macros:
int SvREFCNT(SV* sv);
SV* SvREFCNT_inc(SV* sv);
void SvREFCNT_dec(SV* sv);
However, there is one other function which manipulates the reference
count of its argument. The newRV_inc function, you will recall,
creates a reference to the specified argument. As a side effect,
it increments the argument's reference count. If this is not what
you want, use newRV_noinc instead.
For example, imagine you want to return a reference from an XSUB function.
Inside the XSUB routine, you create an SV which initially has a reference
count of one. Then you call newRV_inc, passing it the just-created SV.
This returns the reference as a new SV, but the reference count of the
SV you passed to newRV_inc has been incremented to two. Now you
return the reference from the XSUB routine and forget about the SV.
But Perl hasn't! Whenever the returned reference is destroyed, the
reference count of the original SV is decreased to one and nothing happens.
The SV will hang around without any way to access it until Perl itself
terminates. This is a memory leak.
The correct procedure, then, is to use newRV_noinc instead of
newRV_inc. Then, if and when the last reference is destroyed,
the reference count of the SV will go to zero and it will be destroyed,
stopping any memory leak.
There are some convenience functions available that can help with the
destruction of xVs. These functions introduce the concept of ``mortality''.
An xV that is mortal has had its reference count marked to be decremented,
but not actually decremented, until ``a short time later''. Generally the
term ``short time later'' means a single Perl statement, such as a call to
an XSUB function. The actual determinant for when mortal xVs have their
reference count decremented depends on two macros, SAVETMPS and FREETMPS.
See the perlcall manpage and the perlxs manpage for more details on these macros.
``Mortalization'' then is at its simplest a deferred SvREFCNT_dec.
However, if you mortalize a variable twice, the reference count will
later be decremented twice.
You should be careful about creating mortal variables. Strange things
can happen if you make the same value mortal within multiple contexts,
or if you make a variable mortal multiple times.
To create a mortal variable, use the functions:
SV* sv_newmortal()
SV* sv_2mortal(SV*)
SV* sv_mortalcopy(SV*)
The first call creates a mortal SV, the second converts an existing
SV to a mortal SV (and thus defers a call to SvREFCNT_dec), and the
third creates a mortal copy of an existing SV.
The mortal routines are not just for SVs -- AVs and HVs can be
made mortal by passing their address (type-casted to SV*) to the
sv_2mortal or sv_mortalcopy routines.
A ``stash'' is a hash that contains all of the different objects that
are contained within a package. Each key of the stash is a symbol
name (shared by all the different types of objects that have the same
name), and each value in the hash table is a GV (Glob Value). This GV
in turn contains references to the various objects of that name,
including (but not limited to) the following:
Scalar Value
Array Value
Hash Value
I/O Handle
Format
Subroutine
There is a single stash called ``PL_defstash'' that holds the items that exist
in the ``main'' package. To get at the items in other packages, append the
string ``::'' to the package name. The items in the ``Foo'' package are in
the stash ``Foo::'' in PL_defstash. The items in the ``Bar::Baz'' package are
in the stash ``Baz::'' in ``Bar::'''s stash.
To get the stash pointer for a particular package, use the function:
HV* gv_stashpv(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 the perlref manpage.
Scalar variables normally contain only one type of value, an integer,
double, pointer, or reference. Perl will automatically convert the
actual scalar data from the stored type into the requested type.
Some scalar variables contain more than one type of scalar data. For
example, the variable $! contains either the numeric value of errno
or its string equivalent from either strerror or sys_errlist[].
To force multiple data values into an SV, you must do two things: use the
sv_set*v routines to add the additional scalar type, then set a flag
so that Perl will believe it contains more than one type of data. The
four macros to set the flags are:
SvIOK_on
SvNOK_on
SvPOK_on
SvROK_on
The particular macro you must use depends on which sv_set*v routine
you called first. This is because every sv_set*v routine turns on
only the bit for the particular type of data being set, and turns off
all the rest.
For example, to create a new Perl variable called ``dberror'' that contains
both the numeric and descriptive string error values, you could use the
following code:
extern int dberror;
extern char *dberror_list;
SV* sv = get_sv("dberror", TRUE);
sv_setiv(sv, (IV) dberror);
sv_setpv(sv, dberror_list[dberror]);
SvIOK_on(sv);
If the order of sv_setiv and sv_setpv had been reversed, then the
macro SvPOK_on would need to be called instead of SvIOK_on.
[This section still under construction. Ignore everything here. Post no
bills. Everything not permitted is forbidden.]
Any SV may be magical, that is, it has special features that a normal
SV does not have. These features are stored in the SV structure in a
linked list of struct magic's, typedef'ed to MAGIC.
struct magic {
MAGIC* mg_moremagic;
MGVTBL* mg_virtual;
U16 mg_private;
char mg_type;
U8 mg_flags;
SV* mg_obj;
char* mg_ptr;
I32 mg_len;
};
Note this is current as of patchlevel 0, and could change at any time.
Perl adds magic to an SV using the sv_magic function:
void sv_magic(SV* sv, SV* obj, int how, 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
set the SVt_PVMG flag for the sv. Perl then continues by adding
it to the beginning of the linked list of magical features. Any prior
entry of the same type of magic is deleted. Note that this can be
overridden, and multiple instances of the same type of magic can be
associated with an SV.
The name and namlen arguments are used to associate a string with
the magic, typically the name of a variable. namlen is stored in the
mg_len field and if name is non-null and namlen >= 0 a malloc'd
copy of the name is stored in mg_ptr field.
The sv_magic function uses how to determine which, if any, predefined
``Magic Virtual Table'' should be assigned to the mg_virtual field.
See the ``Magic Virtual Table'' section below. The how argument is also
stored in the mg_type field.
The obj argument is stored in the mg_obj field of the MAGIC
structure. If it is not the same as the sv argument, the reference
count of the obj object is incremented. If it is the same, or if
the how argument is ``#'', or if it is a NULL pointer, then obj is
merely stored, without the reference count being incremented.
There is also a function to add magic to an HV:
void hv_magic(HV *hv, GV *gv, int how);
This simply calls sv_magic and coerces the gv argument into an SV.
To remove the magic from an SV, call the function sv_unmagic:
void sv_unmagic(SV *sv, int type);
The type argument should be equal to the how value when the SV
was initially made magical.
The mg_virtual field in the MAGIC structure is a pointer to a
MGVTBL, which is a structure of function pointers and stands for
``Magic Virtual Table'' to handle the various operations that might be
applied to that variable.
The MGVTBL has five pointers to the following routine types:
int (*svt_get)(SV* sv, MAGIC* mg);
int (*svt_set)(SV* sv, MAGIC* mg);
U32 (*svt_len)(SV* sv, MAGIC* mg);
int (*svt_clear)(SV* sv, MAGIC* mg);
int (*svt_free)(SV* sv, MAGIC* mg);
This MGVTBL structure is set at compile-time in perl.h and there are
currently 19 types (or 21 with overloading turned on). These different
structures contain pointers to various routines that perform additional
actions depending on which function is being called.
Function pointer Action taken
---------------- ------------
svt_get Do something after the value of the SV is retrieved.
svt_set Do something after the SV is assigned a value.
svt_len Report on the SV's length.
svt_clear Clear something the SV represents.
svt_free Free any extra storage associated with the SV.
For instance, the MGVTBL structure called vtbl_sv (which corresponds
to an mg_type of '\0') contains:
{ magic_get, magic_set, magic_len, 0, 0 }
Thus, when an SV is determined to be magical and of type '\0', if a get
operation is being performed, the routine magic_get is called. All
the various routines for the various magical types begin with magic_.
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 MGVTBL Type of magic
------- ------ ----------------------------
\0 vtbl_sv Special scalar variable
A vtbl_amagic %OVERLOAD hash
a vtbl_amagicelem %OVERLOAD hash element
c (none) Holds overload table (AMT) on stash
B vtbl_bm Boyer-Moore (fast string search)
D vtbl_regdata Regex match position data (@+ and @- vars)
d vtbl_regdatum Regex match position data element
E vtbl_env %ENV hash
e vtbl_envelem %ENV hash element
f vtbl_fm Formline ('compiled' format)
g vtbl_mglob m//g target / study()ed string
I vtbl_isa @ISA array
i vtbl_isaelem @ISA array element
k vtbl_nkeys scalar(keys()) lvalue
L (none) Debugger %_<filename
l vtbl_dbline Debugger %_<filename element
o vtbl_collxfrm Locale transformation
P vtbl_pack Tied array or hash
p vtbl_packelem Tied array or hash element
q vtbl_packelem Tied scalar or handle
S vtbl_sig %SIG hash
s vtbl_sigelem %SIG hash element
t vtbl_taint Taintedness
U vtbl_uvar Available for use by extensions
v vtbl_vec vec() lvalue
x vtbl_substr substr() lvalue
y vtbl_defelem Shadow "foreach" iterator variable /
smart parameter vivification
* vtbl_glob GV (typeglob)
# vtbl_arylen Array length ($#ary)
. vtbl_pos pos() lvalue
~ (none) Available for use by extensions
When an uppercase and lowercase letter both exist in the table, then the
uppercase letter is used to represent some kind of composite type (a list
or a hash), and the lowercase letter is used to represent an element of
that composite type.
The '~' and 'U' magic types are defined specifically for use by
extensions and will not be used by perl itself. Extensions can use
'~' magic to 'attach' private information to variables (typically
objects). This is especially useful because there is no way for
normal perl code to corrupt this private information (unlike using
extra elements of a hash object).
Similarly, 'U' magic can be used much like tie() to call a C function
any time a scalar's value is used or changed. The MAGIC's
mg_ptr field points to a ufuncs structure:
struct ufuncs {
I32 (*uf_val)(IV, SV*);
I32 (*uf_set)(IV, SV*);
IV uf_index;
};
When the SV is read from or written to, the uf_val or uf_set
function will be called with uf_index as the first arg and a
pointer to the SV as the second. A simple example of how to add 'U'
magic is shown below. Note that the ufuncs structure is copied by
sv_magic, so you can safely allocate it on the stack.
void
Umagic(sv)
SV *sv;
PREINIT:
struct ufuncs uf;
CODE:
uf.uf_val = &my_get_fn;
uf.uf_set = &my_set_fn;
uf.uf_index = 0;
sv_magic(sv, 0, 'U', (char*)&uf, sizeof(uf));
Note that because multiple extensions may be using '~' or 'U' magic,
it is important for extensions to take extra care to avoid conflict.
Typically only using the magic on objects blessed into the same class
as the extension is sufficient. For '~' magic, it may also be
appropriate to add an I32 'signature' at the top of the private data
area and check that.
Also note that the sv_set*() and sv_cat*() functions described
earlier do not invoke 'set' magic on their targets. This must
be done by the user either by calling the SvSETMAGIC() macro after
calling these functions, or by using one of the sv_set*_mg() or
sv_cat*_mg() functions. Similarly, generic C code must call the
SvGETMAGIC() macro to invoke any 'get' magic if they use an SV
obtained from external sources in functions that don't handle magic.
See the perlapi manpage 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 */
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.
Tied hashes and arrays are magical beasts of the 'P' magic type.
WARNING: As of the 5.004 release, proper usage of the array and hash
access functions requires understanding a few caveats. Some
of these caveats are actually considered bugs in the API, to be fixed
in later releases, and are bracketed with [MAYCHANGE] below. If
you find yourself actually applying such information in this section, be
aware that the beha | 
