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perlguts - Perl's Internal Functions
This document attempts to describe some of the internal functions of the
Perl executable. It is far from complete and probably contains many
errors. Please refer any questions or comments to the author below.
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 integer type that is
guaranteed to be large enough to hold a pointer (as well as an integer).
Perl also uses two special typedefs, I32 and I16, which will always be at
least 32-bits and 16-bits long, respectively.
Working with SVs [Toc] [Back]
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 five routines are:
SV* newSViv(IV);
SV* newSVnv(double);
SV* newSVpv(char*, int);
SV* newSVpvf(const char*, ...);
SV* newSVsv(SV*);
To change the value of an *already-existing* SV, there are six routines:
void sv_setiv(SV*, IV);
void sv_setnv(SV*, double);
void sv_setpv(SV*, char*);
void sv_setpvn(SV*, char*, int)
void sv_setpvf(SV*, const char*, ...);
void sv_setsv(SV*, SV*);
Notice that you can choose to specify the length of the string to be
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assigned by using sv_setpvn 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.
All SVs that will contain strings should, but need not, be terminated
with a NUL character. If it is not NUL-terminated there is a risk of
core dumps and corruptions from code which passes the string to C
functions or system calls which expect a NUL-terminated string. Perl's
own functions typically add a trailing NUL for this reason.
Nevertheless, you should be very careful when you pass a string stored in
an SV to a C function or system call.
To access the actual value that an SV points to, you can use the macros:
SvIV(SV*)
SvNV(SV*)
SvPV(SV*, STRLEN len)
which will automatically coerce the actual scalar type into an IV,
double, or string.
In the SvPV macro, the length of the string returned is placed into the
variable len (this is a macro, so you do not use &len). If you do not
care what the length of the data is, use the global variable na.
Remember, however, that Perl allows arbitrary strings of data that may
both contain NULs and might not be terminated by a NUL.
If you want to know if the scalar value is TRUE, you can use:
SvTRUE(SV*)
Although Perl will automatically grow strings for you, if you need to
force Perl to allocate more memory for your SV, you can use the macro
SvGROW(SV*, STRLEN newlen)
which will determine if more memory needs to be allocated. If so, it
will call the function sv_grow. Note that SvGROW can only increase, not
decrease, the allocated memory of an SV and that it does not
automatically add a byte for the a trailing NUL (perl's own string
functions typically do SvGROW(sv, len + 1)).
If you have an SV and want to know what kind of data Perl thinks is
stored in it, you can use the following macros to check the type of SV
you have.
SvIOK(SV*)
SvNOK(SV*)
SvPOK(SV*)
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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)
You can also get a pointer to the end of the string stored in the SV with
the macro:
SvEND(SV*)
But note that these last three macros are valid only if SvPOK() is true.
If you want to append something to the end of string stored in an SV*,
you can use the following functions:
void sv_catpv(SV*, char*);
void sv_catpvn(SV*, char*, int);
void sv_catpvf(SV*, const char*, ...);
void sv_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 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.
If you know the name of a scalar variable, you can get a pointer to its
SV by using the following:
SV* perl_get_sv("package::varname", FALSE);
This returns NULL if the variable does not exist.
If you want to know if this variable (or any other SV) is actually
defined, you can call:
SvOK(SV*)
The scalar undef value is stored in an SV instance called sv_undef. Its
address can be used whenever an SV* is needed.
There are also the two values sv_yes and sv_no, which contain Boolean
TRUE and FALSE values, respectively. Like 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 &sv_undef.
Take this code:
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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
&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 the section on Reference Counts and
Mortality).
What's Really Stored in an SV?
Recall that the usual method of determining the type of scalar you have
is to use 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.
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.
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Once the AV has been created, the following operations are possible on
AVs:
void av_push(AV*, SV*);
SV* av_pop(AV*);
SV* av_shift(AV*);
void av_unshift(AV*, I32 num);
These should be familiar operations, with the exception of av_unshift.
This routine adds num elements at the front of the array with the undef
value. You must then use av_store (described below) to assign values to
these new elements.
Here are some other functions:
I32 av_len(AV*);
SV** av_fetch(AV*, I32 key, I32 lval);
SV** av_store(AV*, I32 key, SV* val);
The av_len function returns the highest index value in array (just like
$#array in Perl). If the array is empty, -1 is returned. The av_fetch
function returns the value at index key, but if lval is non-zero, then
av_fetch will store an undef value at that index. The av_store function
stores the value val at index key, and does not increment the reference
count of val. Thus the caller is responsible for taking care of that,
and if av_store returns NULL, the caller will have to decrement the
reference count to avoid a memory leak. Note that av_fetch and av_store
both return SV**'s, not SV*'s as their return value.
void av_clear(AV*);
void av_undef(AV*);
void av_extend(AV*, I32 key);
The av_clear function deletes all the elements in the AV* array, but does
not actually delete the array itself. The av_undef function will delete
all the elements in the array plus the array itself. The av_extend
function extends the array so that it contains key elements. If key is
less than the current length of the array, then nothing is done.
If you know the name of an array variable, you can get a pointer to its
AV by using the following:
AV* perl_get_av("package::varname", FALSE);
This returns NULL if the variable does not exist.
See the section on Understanding the Magic of Tied Hashes and Arrays for
more information on how to use the array access functions on tied arrays.
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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*, char* key, U32 klen, SV* val, U32 hash);
SV** hv_fetch(HV*, char* key, U32 klen, I32 lval);
The klen parameter is the length of the key being passed in (Note that
you cannot pass 0 in as a value of klen to tell Perl to measure the
length of the key). The val argument contains the SV pointer to the
scalar being stored, and hash is the precomputed hash value (zero if you
want hv_store to calculate it for you). The lval parameter indicates
whether this fetch is actually a part of a store operation, in which case
a new undefined value will be added to the HV with the supplied key and
hv_fetch will return as if the value had already existed.
Remember that hv_store and hv_fetch return SV**'s and not just SV*. To
access the scalar value, you must first dereference the return value.
However, you should check to make sure that the return value is not NULL
before dereferencing it.
These two functions check if a hash table entry exists, and deletes it.
bool hv_exists(HV*, char* key, U32 klen);
SV* hv_delete(HV*, char* key, U32 klen, I32 flags);
If flags does not include the G_DISCARD flag then hv_delete will create
and return a mortal copy of the deleted value.
And more miscellaneous functions:
void hv_clear(HV*);
void hv_undef(HV*);
Like their AV counterparts, hv_clear deletes all the entries in the hash
table but does not actually delete the hash table. The hv_undef deletes
both the entries and the hash table itself.
Perl keeps the actual data in linked list of structures with a typedef of
HE. These contain the actual key and value pointers (plus extra
administrative overhead). The key is a string pointer; the value is an
SV*. However, once you have an HE*, to get the actual key and value, use
the routines specified below.
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I32 hv_iterinit(HV*);
/* Prepares starting point to traverse hash table */
HE* hv_iternext(HV*);
/* Get the next entry, and return a pointer to a
structure that has both the key and value */
char* hv_iterkey(HE* entry, I32* retlen);
/* Get the key from an HE structure and also return
the length of the key string */
SV* hv_iterval(HV*, HE* entry);
/* Return a SV pointer to the value of the HE
structure */
SV* hv_iternextsv(HV*, char** key, I32* retlen);
/* This convenience routine combines hv_iternext,
hv_iterkey, and hv_iterval. The key and retlen
arguments are return values for the key and its
length. The value is returned in the SV* argument */
If you know the name of a hash variable, you can get a pointer to its HV
by using the following:
HV* perl_get_hv("package::varname", FALSE);
This returns NULL if the variable does not exist.
The hash algorithm is defined in the PERL_HASH(hash, key, klen) macro:
i = klen;
hash = 0;
s = key;
while (i--)
hash = hash * 33 + *s++;
See the section on 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).
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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 section on API LISTING later
in this document for detailed descriptions.
The following macros must always be used to access the contents of hash
entries. Note that the arguments to these macros must be simple
variables, since they may get evaluated more than once. See the section
on API LISTING later in this document for detailed descriptions of these
macros.
HePV(HE* he, STRLEN len)
HeVAL(HE* he)
HeHASH(HE* he)
HeSVKEY(HE* he)
HeSVKEY_force(HE* he)
HeSVKEY_set(HE* he, SV* sv)
These two lower level macros are defined, but must only be used when
dealing with keys that are not SV*s:
HeKEY(HE* he)
HeKLEN(HE* he)
Note that both hv_store and hv_store_ent do not increment the reference
count of the stored val, which is the caller's responsibility. If these
functions return a NULL value, the caller will usually have to decrement
the reference count of val to avoid a memory leak.
References [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.
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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
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 the section on Stashes and Globs
for information on converting class names into stashes.
/* Still under construction */
Upgrades rv to reference if not already one. Creates new SV for rv to
point to. If classname is non-null, the SV is blessed into the specified
class. SV is returned.
SV* newSVrv(SV* rv, char* classname);
Copies integer or double into an SV whose reference is rv. SV is blessed
if classname is non-null.
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SV* sv_setref_iv(SV* rv, char* classname, IV iv);
SV* sv_setref_nv(SV* rv, char* classname, NV iv);
Copies the pointer value (the address, not the string!) into an SV whose
reference is rv. SV is blessed if classname is non-null.
SV* sv_setref_pv(SV* rv, char* classname, PV iv);
Copies string into an SV whose reference is rv. Set length to 0 to let
Perl calculate the string length. SV is blessed if classname is nonnull.
SV* sv_setref_pvn(SV* rv, char* classname, PV iv, int length);
int sv_isa(SV* sv, char* name);
int sv_isobject(SV* sv);
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* perl_get_sv("package::varname", TRUE);
AV* perl_get_av("package::varname", TRUE);
HV* perl_get_hv("package::varname", TRUE);
Notice the use of TRUE as the second parameter. The new variable can now
be set, using the routines appropriate to the data type.
There are additional macros whose values may be bitwise OR'ed with the
TRUE argument to enable certain extra features. Those bits are:
GV_ADDMULTI Marks the variable as multiply defined, thus preventing the
"Name <varname> used only once: possible typo" warning.
GV_ADDWARN Issues the warning "Had to create <varname> unexpectedly" if
the variable did not exist before the function was called.
If you do not specify a package name, the variable is created in the
current package.
Reference Counts and Mortality [Toc] [Back]
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
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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 justcreated
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*)
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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.
Stashes and Globs [Toc] [Back]
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
File Handle
Directory Handle
Format
Subroutine
There is a single stash called "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 defstash. The items in the "Bar::Baz" package are
in the stash "Baz::" in "Bar::"'s stash.
To get the stash pointer for a particular package, use the function:
HV* gv_stashpv(char* name, I32 create)
HV* gv_stashsv(SV*, I32 create)
The first function takes a literal string, the second uses the string
stored in the SV. Remember that a stash is just a hash table, so you get
back an HV*. The create flag will create a new package if it is set.
The name that gv_stash*v wants is the name of the package whose symbol
table you want. The default package is called main. If you have
multiply nested packages, pass their names to gv_stash*v, separated by ::
as in the Perl language itself.
Alternately, if you have an SV that is a blessed reference, you can find
out the stash pointer by using:
HV* SvSTASH(SvRV(SV*));
then use the following to get the package name itself:
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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.
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 = perl_get_sv("dberror", TRUE);
sv_setiv(sv, (IV) dberror);
sv_setpv(sv, dberror_list[dberror]);
SvIOK_on(sv);
If the order of sv_setiv and sv_setpv had been reversed, then the macro
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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, 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.
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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.
Magic Virtual Tables [Toc] [Back]
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:
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{ magic_get, magic_set, magic_len, 0, 0 }
Thus, when an SV is determined to be magical and of type '\0', if a get
operation is being performed, the routine magic_get is called. All the
various routines for the various magical types begin with magic_.
The current kinds of Magic Virtual Tables are:
mg_type MGVTBL Type of magic
------- ------ ----------------------------
\0 vtbl_sv Special scalar variable
A vtbl_amagic %OVERLOAD hash
a vtbl_amagicelem %OVERLOAD hash element
c (none) Holds overload table (AMT) on stash
B vtbl_bm Boyer-Moore (fast string search)
E vtbl_env %ENV hash
e vtbl_envelem %ENV hash element
f vtbl_fm Formline ('compiled' format)
g vtbl_mglob m//g target / study()ed string
I vtbl_isa @ISA array
i vtbl_isaelem @ISA array element
k vtbl_nkeys scalar(keys()) lvalue
L (none) Debugger %_<filename
l vtbl_dbline Debugger %_<filename element
o vtbl_collxfrm Locale transformation
P vtbl_pack Tied array or hash
p vtbl_packelem Tied array or hash element
q vtbl_packelem Tied scalar or handle
S vtbl_sig %SIG hash
s vtbl_sigelem %SIG hash element
t vtbl_taint Taintedness
U vtbl_uvar Available for use by extensions
v vtbl_vec vec() lvalue
x vtbl_substr substr() lvalue
y vtbl_defelem Shadow "foreach" iterator variable /
smart parameter vivification
* vtbl_glob GV (typeglob)
# vtbl_arylen Array length ($#ary)
. vtbl_pos pos() lvalue
~ (none) Available for use by extensions
When an uppercase and lowercase letter both exist in the table, then the
uppercase letter is used to represent some kind of composite type (a list
or a hash), and the lowercase letter is used to represent an element of
that composite type.
The '~' and 'U' magic types are defined specifically for use by
extensions and will not be used by perl itself. Extensions can use '~'
magic to 'attach' private information to variables (typically objects).
This is especially useful because there is no way for normal perl code to
corrupt this private information (unlike using extra elements of a hash
object).
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Similarly, 'U' magic can be used much like tie() to call a C function any
time a scalar's value is used or changed. The MAGIC's mg_ptr field
points to a ufuncs structure:
struct ufuncs {
I32 (*uf_val)(IV, SV*);
I32 (*uf_set)(IV, SV*);
IV uf_index;
};
When the SV is read from or written to, the uf_val or uf_set function
will be called with uf_index as the first arg and a pointer to the SV as
the second.
Note that because multiple extensions may be using '~' or 'U' magic, it
is important for extensions to take extra care to avoid conflict.
Typically only using the magic on objects blessed into the same class as
the extension is sufficient. For '~' magic, it may also be appropriate
to add an I32 'signature' at the top of the private data area and check
that.
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, 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
Tied hashes and arrays are magical beasts of the 'P' magic type.
WARNING: As of the 5.004 release, proper usage of the array and hash
access functions requires understanding a few caveats. Some of these
caveats are actually considered bugs in the API, to be fixed in later
releases, and are bracketed with [MAYCHANGE] below. If you find yourself
actually applying such information in this section, be aware that the
behavior may change in the future, umm, without warning.
The av_store function, when given a tied array argument, merely copies
the magic of the array onto the value to be "stored", using mg_copy. It
may also return NULL, indicating that the value did not actually need to
be stored in the array. [MAYCHANGE] After a call to av_store on a tied
array, the caller will usually need to call mg_set(val) to actually
invoke the perl level "STORE" method on the TIEARRAY object. If av_store
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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
Perl has a very handy construction
{
local $var = 2;
...
}
This construction is approximately equivalent to
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{
my $oldvar = $var;
$var = 2;
...
$var = $oldvar;
}
The biggest difference is that the first construction would reinstate the
initial value of $var, irrespective of how control exits the block: goto,
return, die/eval etc. It is a little bit more efficient as well.
There is a way to achieve a similar task from C via Perl API: create a
pseudo-block, and arrange for some changes to be automatically undone at
the end of it, either explicit, or via a non-local exit (via die()). A
block-like construct is created by a pair of ENTER/LEAVE macros (see the
section on EXAMPLE/"Returning a Scalar in the perlcall manpage). 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, which should (?) be used instead.
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.
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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(defstash, savepv(tmpbuf), strlen(tmpbuf));
SAVEDESTRUCTOR(f,p)
At the end of pseudo-block the function f is called with the only
argument (of type void*) 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 a 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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XSUBs and the Argument Stack
The XSUB mechanism is a simple way for Perl programs to access C
subroutines. An XSUB routine will have a stack that contains the
arguments from the Perl program, and a way to map from the Perl data
structures to a C equivalent.
The stack arguments are accessible through the ST(n) macro, which returns
the n'th stack argument. Argument 0 is the first argument passed in the
Perl subroutine call. These arguments are SV*, and can be used anywhere
an SV* is used.
Most of the time, output from the C routine can be handled through use of
the RETVAL and OUTPUT directives. However, there are some cases where
the argument stack is not already long enough to handle all the return
values. An example is the POSIX tzname() call, which takes no arguments,
but returns two, the local time zone's standard and summer time
abbreviations.
To handle this situation, the PPCODE directive is used and the stack is
extended using the macro:
EXTEND(sp, num);
where sp is the stack pointer, and num is the number of elements the
stack should be extended by.
Now that there is room on the stack, values can be pushed on it using the
macros to push IVs, doubles, strings, and SV pointers respectively:
PUSHi(IV)
PUSHn(double)
PUSHp(char*, I32)
PUSHs(SV*)
And now the Perl program calling tzname, the two values will be assigned
as in:
($standard_abbrev, $summer_abbrev) = POSIX::tzname;
An alternate (and possibly simpler) method to pushing values on the stack
is to use the macros:
XPUSHi(IV)
XPUSHn(double)
XPUSHp(char*, I32)
XPUSHs(SV*)
These macros automatically adjust the stack for you, if needed. Thus,
you do not need to call EXTEND to extend the stack.
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For more information, consult the perlxs manpage and the perlxstut
manpage.
Calling Perl Routines from within C Programs
There are four routines that can be used to call a Perl subroutine from
within a C program. These four are:
I32 perl_call_sv(SV*, I32);
I32 perl_call_pv(char*, I32);
I32 perl_call_method(char*, I32);
I32 perl_call_argv(char*, I32, register char**);
The routine most often used is perl_call_sv. The SV* argument contains
either the name of the Perl subroutine to be called, or a reference to
the subroutine. The second argument consists of flags that control the
context in which the subroutine is called, whether or not the subroutine
is being passed arguments, how errors should be trapped, and how to treat
return values.
All four routines return the number of arguments that the subroutine
returned on the Perl stack.
When using any of these routines (except perl_call_argv), the programmer
must manipulate the Perl stack. These include the following macros and
functions:
dSP
PUSHMARK()
PUTBACK
SPAGAIN
ENTER
SAVETMPS
FREETMPS
LEAVE
XPUSH*()
POP*()
For a detailed description of calling conventions from C to Perl, consult
the perlcall manpage.
Memory Allocation [Toc] [Back]
It is suggested that you use the version of malloc that is distributed
with Perl. It keeps pools of various sizes of unallocated memory in
order to satisfy allocation requests more quickly. However, on some
platforms, it may cause spurious malloc or free errors.
New(x, pointer, number, type);
Newc(x, pointer, number, type, cast);
Newz(x, pointer, number, type);
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These three macros are used to initially allocate memory.
The first argument x was a "magic cookie" that was used to keep track of
who called the macro, to help when debugging memory problems. However,
the current code makes no use of this feature (most Perl developers now
use run-time memory checkers), so this argument can be any number.
The second argument pointer should be the name of a variable that will
point to the newly allocated memory.
The third and fourth arguments number and type specify how many of the
specified type of data structure should be allocated. The argument type
is passed to sizeof. The final argument to Newc, cast, should be used if
the pointer argument is different from the type argument.
Unlike the New and Newc macros, the Newz macro calls memzero to zero out
all the newly allocated memory.
Renew(pointer, number, type);
Renewc(pointer, number, type, cast);
Safefree(pointer)
These three macros are used to change a memory buffer size or to free a
piece of memory no longer needed. The arguments to Renew and Renewc
match those of New and Newc with the exception of not needing the "magic
cookie" argument.
Move(source, dest, number, type);
Copy(source, dest, number, type);
Zero(dest, number, type);
These three macros are used to move, copy, or zero out previously
allocated memory. The source and dest arguments point to the source and
destination starting points. Perl will move, copy, or zero out number
instances of the size of the type data structure (using the sizeof
function).
PerlIO [Toc] [Back]
The most recent development releases of Perl has been experimenting with
removing Perl's dependency on the "normal" standard I/O suite and
allowing other stdio implementations to be used. This involves creating
a new abstraction layer that then calls whichever implementation of stdio
Perl was compiled with. All XSUBs should now use the functions in the
PerlIO abstraction layer and not make any assumptions about what kind of
stdio is being used.
For a complete description of the PerlIO abstraction, consult the
perlapio manpage.
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Putting a C value on Perl stack [Toc] [Back]
A lot of opcodes (this is an elementary operation in the internal perl
stack machine) put an SV* on the stack. However, as an optimization the
corresponding SV is (usually) not recreated each time. The opcodes reuse
specially assigned SVs (targets) which are (as a corollary) not
constantly freed/created.
Each of the targets is created only once (but see the section on
Scratchpads and recursion below), and when an opcode needs to put an
integer, a double, or a string on stack, it just sets the corresponding
parts of its target and puts the target on stack.
The macro to put this target on stack is PUSHTARG, and it is directly
used in some opcodes, as well as indirectly in zillions of others, which
use it via (X)PUSH[pni].
Scratchpads [Toc] [Back]
The question remains on when the SVs which are targets for opcodes are
created. The answer is that they are created when the current unit -- a
subroutine or a file (for opcodes for statements outside of subroutines)
-- is compiled. During this time a special anonymous Perl array is
created, which is called a scratchpad for the current unit.
A scratchpad keeps SVs which are lexicals for the current unit and are
targets for opcodes. One can deduce that an SV lives on a scratchpad by
looking on its flags: lexicals have SVs_PADMY set, and targets have
SVs_PADTMP set.
The correspondence between OPs and targets is not 1-to-1. Different OPs
in the compile tree of the unit can use the same target, if this would
not conflict with the expected life of the temporary.
Scratchpads and recursion [Toc] [Back]
In fact it is not 100% true that a compiled unit contains a pointer to
the scratchpad AV. In fact it contains a pointer to an AV of (initially)
one element, and this element is the scratchpad AV. Why do we need an
extra level of indirection?
The answer is recursion, and maybe (sometime soon) threads. Both these
can create several execution pointers going into the same subroutine. For
the subroutine-child not write over the temporaries for the subroutineparent
(lifespan of which covers the call to the child), the parent and
the child should have different scratchpads. (And the lexicals should be
separate anyway!)
So each subroutine is born with an array of scratchpads (of length 1).
On each entry to the subroutine it is checked that the current depth of
the recursion is not more than the length of this array, and if it is,
new scratchpad is created and pushed into the array.
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The targets on this scratchpad are undefs, but they are already marked
with correct flags.
Code tree
Here we describe the internal form your code is converted to by Perl.
Start with a simple example:
$a = $b + $c;
This is converted to a tree similar to this one:
assign-to
/ \
+ $a
/ \
$b $c
(but slightly more complicated). This tree reflect the way Perl parsed
your code, but has nothing to do with the execution order. There is an
additional "thread" going through the nodes of the tree which shows the
order of execution of the nodes. In our simplified example above it
looks like:
$b ---> $c ---> + ---> $a ---> assign-to
But with the actual compile tree for $a = $b + $c it is different: some
nodes optimized away. As a corollary, though the actual tree contains
more nodes than our simplified example, the execution order is the same
as in our example.
Examining the tree [Toc] [Back]
If you have your perl compiled for debugging (usually done with -D
optimize=-g on Configure command line), you may examine the compiled tree
by specifying -Dx on the Perl command line. The output takes several
lines per node, and for $b+$c it looks like this:
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5 TYPE = add ===> 6
TARG = 1
FLAGS = (SCALAR,KIDS)
{
TYPE = null ===> (4)
(was rv2sv)
FLAGS = (SCALAR,KIDS)
{
3 TYPE = gvsv ===> 4
FLAGS = (SCALAR)
GV = main::b
}
}
{
TYPE = null ===> (5)
(was rv2sv)
FLAGS = (SCALAR,KIDS)
{
4 TYPE = gvsv ===> 5
FLAGS = (SCALAR)
GV = main::c
}
}
This tree has 5 nodes (one per TYPE specifier), only 3 of them are not
optimized away (one per number in the left column). The immediate
children of the given node correspond to {} pairs on the same level of
indentation, thus this listing corresponds to the tree:
add
/ \
null null
| |
gvsv gvsv
The execution order is indicated by ===> marks, thus it is 3 4 5 6 (node
6 is not included into above listing), i.e., gvsv gvsv add whatever.
Compile pass 1: check routines
The tree is created by the pseudo-compiler while yacc code feeds it the
constructions it recognizes. Since yacc works bottom-up, so does the
first pass of perl compilation.
What makes this pass interesting for perl developers is that some
optimization may be performed on this pass. This is optimization by socalled
check routines. The correspondence between node names and
corresponding check routines is described in opcode.pl (do not forget to
run make regen_headers if you modify this file).
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A check routine is called when the node is fully constructed except for
the execution-order thread. Since at this time there is no back-links to
the currently constructed node, one can do most any operation to the
top-level node, including freeing it and/or creating new nodes
above/below it.
The check routine returns the node which should be inserted into the tree
(if the top-level node was not modified, check routine returns its
argument).
By convention, check routines have names ck_*. They are usually called
from new*OP subroutines (or convert) (which in turn are called from
perly.y).
Compile pass 1a: constant folding
Immediately after the check routine is called the returned node is
checked for being compile-time executable. If it is (the value is judged
to be constant) it is immediately executed, and a constant node with the
"return value" of the corresponding subtree is substituted instead. The
subtree is deleted.
If constant folding was not performed, the execution-order thread is
created.
Compile pass 2: context propagation
When a context for a part of compile tree is known, it is propagated down
through the tree. Aat this time the context can have 5 values (instead
of 2 for runtime context): void, boolean, scalar, list, and lvalue. In
contrast with the pass 1 this pass is processed from top to bottom: a
node's context determines the context for its children.
Additional context-dependent optimizations are performed at this time.
Since at this moment the compile tree contains back-references (via
"thread" pointers), nodes cannot be free()d now. To allow optimized-away
nodes at this stage, such nodes are null()ified instead of free()ing
(i.e. their type is changed to OP_NULL).
Compile pass 3: peephole optimization
After the compile tree for a subroutine (or for an eval or a file) is
created, an additional pass over the code is p
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