perlguts
(1)
Name
perlguts - Introduction to the Perl API
Synopsis
Please see following description for synopsis
Description
Perl Programmers Reference Guide PERLGUTS(1)
NAME
perlguts - Introduction to the Perl API
DESCRIPTION
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.
Variables
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
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
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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
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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)
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which will determine if more memory needs to be allocated.
If so, it will call the function "sv_grow". Note that
"SvGROW" can only increase, not decrease, the allocated
memory of an SV and that it does not automatically add a
byte for the a trailing NUL (perl's own string functions
typically do "SvGROW(sv, len + 1)").
If you have an SV and want to know what kind of data Perl
thinks is stored in it, you can use the following macros to
check the type of SV you have.
SvIOK(SV*)
SvNOK(SV*)
SvPOK(SV*)
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.
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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", 0);
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
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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"\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 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 "AvMAX". 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.
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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 occurred 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
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:
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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", 0);
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.
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Working with HVs
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
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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", 0);
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
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);
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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:
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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
References are a special type of scalar that point to other
data types (including references).
To create a reference, use either of the following
functions:
SV* newRV_inc((SV*) thing);
SV* newRV_noinc((SV*) thing);
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The "thing" argument can be any of an "SV*", "AV*", or
"HV*". The functions are identical except that "newRV_inc"
increments the reference count of the "thing", while
"newRV_noinc" does not. For historical reasons, "newRV" is
a synonym for "newRV_inc".
Once you have a reference, you can use the following macro
to dereference the reference:
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
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.
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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)) { ... }
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Creating New Variables
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", GV_ADD);
AV* get_av("package::varname", GV_ADD);
HV* get_hv("package::varname", GV_ADD);
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
Perl uses a reference count-driven garbage collection
mechanism. SVs, AVs, or HVs (xV for short in the following)
start their life with a reference count of 1. If the
reference count of an xV ever drops to 0, then it will be
destroyed and its memory made available for reuse.
This normally doesn't happen at the Perl level unless a
variable is undef'ed or the last variable holding a
reference to it is changed or overwritten. At the internal
level, however, reference counts can be manipulated with the
following macros:
int SvREFCNT(SV* sv);
SV* SvREFCNT_inc(SV* sv);
void SvREFCNT_dec(SV* sv);
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However, there is one other function which manipulates the
reference count of its argument. The "newRV_inc" function,
you will recall, creates a reference to the specified
argument. As a side effect, it increments the argument's
reference count. If this is not what you want, use
"newRV_noinc" instead.
For example, imagine you want to return a reference from an
XSUB function. Inside the XSUB routine, you create an SV
which initially has a reference count of one. Then you call
"newRV_inc", passing it the just-created SV. This returns
the reference as a new SV, but the reference count of the SV
you passed to "newRV_inc" has been incremented to two. Now
you return the reference from the XSUB routine and forget
about the SV. But Perl hasn't! Whenever the returned
reference is destroyed, the reference count of the original
SV is decreased to one and nothing happens. The SV will
hang around without any way to access it until Perl itself
terminates. This is a memory leak.
The correct procedure, then, is to use "newRV_noinc" instead
of "newRV_inc". Then, if and when the last reference is
destroyed, the reference count of the SV will go to zero and
it will be destroyed, stopping any memory leak.
There are some convenience functions available that can help
with the destruction of xVs. These functions introduce the
concept of "mortality". An xV that is mortal has had its
reference count marked to be decremented, but not actually
decremented, until "a short time later". Generally the term
"short time later" means a single Perl statement, such as a
call to an XSUB function. The actual determinant for when
mortal xVs have their reference count decremented depends on
two macros, SAVETMPS and FREETMPS. See perlcall and perlxs
for more details on these macros.
"Mortalization" then is at its simplest a deferred
"SvREFCNT_dec". However, if you mortalize a variable twice,
the reference count will later be decremented twice.
"Mortal" SVs are mainly used for SVs that are placed on
perl's stack. For example an SV which is created just to
pass a number to a called sub is made mortal to have it
cleaned up automatically when 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*)
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The first call creates a mortal SV (with no value), the
second converts an existing SV to a mortal SV (and thus
defers a call to "SvREFCNT_dec"), and the third creates a
mortal copy of an existing SV. Because "sv_newmortal" gives
the new SV no value,it must normally be given one via
"sv_setpv", "sv_setiv", etc. :
SV *tmp = sv_newmortal();
sv_setiv(tmp, an_integer);
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
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.
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To get the stash pointer for a particular package, use the
function:
HV* gv_stashpv(const char* name, I32 flags)
HV* gv_stashsv(SV*, I32 flags)
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 "flags" flag
will create a new package if it is set to GV_ADD.
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
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
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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", GV_ADD);
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
[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;
I32 mg_len;
SV* mg_obj;
char* mg_ptr;
};
Note this is current as of patchlevel 0, and could change at
any time.
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Assigning Magic
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 then either a "savepvn" copy of "name" or "name"
itself is stored in the "mg_ptr" field, depending on whether
"namlen" is greater than zero or equal to zero respectively.
As a special case, if "(name && namlen == HEf_SVKEY)" then
"name" is assumed to contain an "SV*" and is stored as-is
with its REFCNT incremented.
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.
See also "sv_magicext" in perlapi for a more flexible way to
add magic to an SV.
There is also a function to add magic to an "HV":
void hv_magic(HV *hv, GV *gv, int how);
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This simply calls "sv_magic" and coerces the "gv" argument
into an "SV".
To remove the magic from an SV, call the function
sv_unmagic:
void sv_unmagic(SV *sv, int type);
The "type" argument should be equal to the "how" value when
the "SV" was initially made magical.
Magic Virtual Tables
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 (or sometimes eight) 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);
int (*svt_copy)(SV *sv, MAGIC* mg, SV *nsv, const char *name, I32 namlen);
int (*svt_dup)(MAGIC *mg, CLONE_PARAMS *param);
int (*svt_local)(SV *nsv, MAGIC *mg);
This MGVTBL structure is set at compile-time in perl.h and
there are currently 32 types. 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.
svt_copy copy tied variable magic to a tied element
svt_dup duplicate a magic structure during thread cloning
svt_local copy magic to local value during 'local'
For instance, the MGVTBL structure called "vtbl_sv" (which
corresponds to an "mg_type" of "PERL_MAGIC_sv") contains:
{ magic_get, magic_set, magic_len, 0, 0 }
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Thus, when an SV is determined to be magical and of type
"PERL_MAGIC_sv", if a get operation is being performed, the
routine "magic_get" is called. All the various routines for
the various magical types begin with "magic_". NOTE: the
magic routines are not considered part of the Perl API, and
may not be exported by the Perl library.
The last three slots are a recent addition, and for source
code compatibility they are only checked for if one of the
three flags MGf_COPY, MGf_DUP or MGf_LOCAL is set in
mg_flags. This means that most code can continue declaring a
vtable as a 5-element value. These three are currently used
exclusively by the threading code, and are highly subject to
change.
The current kinds of Magic Virtual Tables are:
mg_type
(old-style char and macro) MGVTBL Type of magic
-------------------------- ------ -------------
\0 PERL_MAGIC_sv vtbl_sv Special scalar variable
A PERL_MAGIC_overload vtbl_amagic %OVERLOAD hash
a PERL_MAGIC_overload_elem vtbl_amagicelem %OVERLOAD hash element
c PERL_MAGIC_overload_table (none) Holds overload table (AMT)
on stash
B PERL_MAGIC_bm vtbl_bm Boyer-Moore (fast string search)
D PERL_MAGIC_regdata vtbl_regdata Regex match position data
(@+ and @- vars)
d PERL_MAGIC_regdatum vtbl_regdatum Regex match position data
element
E PERL_MAGIC_env vtbl_env %ENV hash
e PERL_MAGIC_envelem vtbl_envelem %ENV hash element
f PERL_MAGIC_fm vtbl_fm Formline ('compiled' format)
g PERL_MAGIC_regex_global vtbl_mglob m//g target / study()ed string
H PERL_MAGIC_hints vtbl_hints %^H hash
h PERL_MAGIC_hintselem vtbl_hintselem %^H hash element
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
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
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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_arylen vtbl_arylen Array length ($#ary)
. PERL_MAGIC_pos vtbl_pos pos() lvalue
< PERL_MAGIC_backref vtbl_backref back pointer to a weak ref
~ PERL_MAGIC_ext (none) Available for use by extensions
: PERL_MAGIC_symtab (none) hash used as symbol table
% PERL_MAGIC_rhash (none) hash used as restricted hash
@ PERL_MAGIC_arylen_p vtbl_arylen_p pointer to $#a from @a
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.
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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));
Attaching "PERL_MAGIC_uvar" to arrays is permissible but has
no effect.
For hashes there is a specialized hook that gives control
over hash keys (but not values). This hook calls
"PERL_MAGIC_uvar" 'get' magic if the "set" function in the
"ufuncs" structure is NULL. The hook is activated whenever
the hash is accessed with a key specified as an "SV" through
the functions "hv_store_ent", "hv_fetch_ent",
"hv_delete_ent", and "hv_exists_ent". Accessing the key as
a string through the functions without the "..._ent" suffix
circumvents the hook. See "Guts" in Hash::Util::Fieldhash
for a detailed description.
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
MAGIC* mg_find(SV*, int type); /* Finds the magic pointer of that type */
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This routine returns a pointer to the "MAGIC" structure
stored in the SV. If the SV does not have that magical
feature, "NULL" is returned. Also, if the SV is not of type
SVt_PVMG, Perl may core dump.
int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen);
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
"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", GV_ADD);
sv_bless(tie, stash);
hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
RETVAL = newRV_noinc(hash);
OUTPUT:
RETVAL
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The "av_store" function, when given a tied array argument,
merely copies the magic of the array onto the value to be
"stored", using "mg_copy". It may also return NULL,
indicating that the value did not actually need to be stored
in the array. [MAYCHANGE] After a call to "av_store" on a
tied array, the caller will usually need to call
"mg_set(val)" to actually invoke the perl level "STORE"
method on the TIEARRAY object. If "av_store" did return
NULL, a call to "SvREFCNT_dec(val)" will also be usually
necessary to avoid a memory leak. [/MAYCHANGE]
The previous paragraph is applicable verbatim to tied hash
access using the "hv_store" and "hv_store_ent" functions as
well.
"av_fetch" and the corresponding hash functions "hv_fetch"
and "hv_fetch_ent" actually return an undefined mortal value
whose magic has been initialized using "mg_copy". Note the
value so returned does not need to be deallocated, as it is
already mortal. [MAYCHANGE] But you will need to call
"mg_get()" on the returned value in order to actually invoke
the perl level "FETCH" method on the underlying TIE object.
Similarly, you may also call "mg_set()" on the return value
after possibly assigning a suitable value to it using
"sv_setsv", which will invoke the "STORE" method on the TIE
object. [/MAYCHANGE]
[MAYCHANGE] In other words, the array or hash fetch/store
functions don't really fetch and store actual values in the
case of tied arrays and hashes. They merely call "mg_copy"
to attach magic to the values that were meant to be "stored"
or "fetched". Later calls to "mg_get" and "mg_set" actually
do the job of invoking the TIE methods on the underlying
objects. Thus the magic mechanism currently implements a
kind of lazy access to arrays and hashes.
Currently (as of perl version 5.004), use of the hash and
array access functions requires the user to be aware of
whether they are operating on "normal" hashes and arrays, or
on their tied variants. The API may be changed to provide
more transparent access to both tied and normal data types
in future versions. [/MAYCHANGE]
You would do well to understand that the TIEARRAY and
TIEHASH interfaces are mere sugar to invoke some perl method
calls while using the uniform hash and array syntax. The
use of this sugar imposes some overhead (typically about two
to four extra opcodes per FETCH/STORE operation, in addition
to the creation of all the mortal variables required to
invoke the methods). This overhead will be comparatively
small if the TIE methods are themselves substantial, but if
they are only a few statements long, the overhead will not
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be insignificant.
Localizing changes
Perl has a very handy construction
{
local $var = 2;
...
}
This construction is approximately equivalent to
{
my $oldvar = $var;
$var = 2;
...
$var = $oldvar;
}
The biggest difference is that the first construction would
reinstate the initial value of $var, irrespective of how
control exits the block: "goto", "return", "die"/"eval",
etc. It is a little bit more efficient as well.
There is a way to achieve a similar task from C via Perl
API: create a pseudo-block, and arrange for some changes to
be automatically undone at the end of it, either explicit,
or via a non-local exit (via die()). A block-like construct
is created by a pair of "ENTER"/"LEAVE" macros (see
"Returning a Scalar" in perlcall). 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
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which survives conversion to "SV*" and back, "p" should
be able to survive conversion to "char*" and back.
"SAVEFREESV(SV *sv)"
The refcount of "sv" would be decremented at the end of
pseudo-block. This is similar to "sv_2mortal" in that
it is also a mechanism for doing a delayed
"SvREFCNT_dec". However, while "sv_2mortal" extends the
lifetime of "sv" until the beginning of the next
statement, "SAVEFREESV" extends it until the end of the
enclosing scope. These lifetimes can be wildly
different.
Also compare "SAVEMORTALIZESV".
"SAVEMORTALIZESV(SV *sv)"
Just like "SAVEFREESV", but mortalizes "sv" at the end
of the current scope instead of decrementing its
reference count. This usually has the effect of keeping
"sv" alive until the statement that called the currently
live scope has finished executing.
"SAVEFREEOP(OP *op)"
The "OP *" is op_free()ed at the end of pseudo-block.
SAVEFREEPV(p)
The chunk of memory which is pointed to by "p" is
Safefree()ed at the end of pseudo-block.
"SAVECLEARSV(SV *sv)"
Clears a slot in the current scratchpad which
corresponds to "sv" at the end of pseudo-block.
"SAVEDELETE(HV *hv, char *key, I32 length)"
The key "key" of "hv" is deleted at the end of pseudo-
block. The string pointed to by "key" is Safefree()ed.
If one has a key in short-lived storage, the
corresponding string may be reallocated like this:
SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));
"SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)"
At the end of pseudo-block the function "f" is called
with the only argument "p".
"SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)"
At the end of pseudo-block the function "f" is called
with the implicit context argument (if any), and "p".
"SAVESTACK_POS()"
The current offset on the Perl internal stack (cf. "SP")
is restored at the end of pseudo-block.
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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. It doesn't
handle magic. Use "save_scalar" if magic is affected.
"void save_list(SV **sarg, I32 maxsarg)"
A variant of "save_item" which takes multiple arguments
via an array "sarg" of "SV*" of length "maxsarg".
"SV* save_svref(SV **sptr)"
Similar to "save_scalar", but will reinstate an "SV *".
"void save_aptr(AV **aptr)"
"void save_hptr(HV **hptr)"
Similar to "save_svref", but localize "AV *" and "HV *".
The "Alias" module implements localization of the basic
types within the caller's scope. People who are interested
in how to localize things in the containing scope should
take a look there too.
Subroutines
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
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returns two, the local time zone's standard and summer time
abbreviations.
To handle this situation, the PPCODE directive is used and
the stack is extended using the macro:
EXTEND(SP, num);
where "SP" is the macro that represents the local copy of
the stack pointer, and "num" is the number of elements the
stack should be extended by.
Now that there is room on the stack, values can be pushed on
it using "PUSHs" macro. The pushed values will often need to
be "mortal" (See "Reference Counts and Mortality"):
PUSHs(sv_2mortal(newSViv(an_integer)))
PUSHs(sv_2mortal(newSVuv(an_unsigned_integer)))
PUSHs(sv_2mortal(newSVnv(a_double)))
PUSHs(sv_2mortal(newSVpv("Some String",0)))
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 macro:
XPUSHs(SV*)
This macro automatically adjust the stack for you, if
needed. Thus, you do not need to call "EXTEND" to extend
the stack.
Despite their suggestions in earlier versions of this
document the macros "(X)PUSH[iunp]" are not suited to XSUBs
which return multiple results. For that, either stick to
the "(X)PUSHs" macros shown above, or use the new
"m(X)PUSH[iunp]" macros instead; see "Putting a C value on
Perl stack".
For more information, consult perlxs and perlxstut.
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 call_sv(SV*, I32);
I32 call_pv(const char*, I32);
I32 call_method(const char*, I32);
I32 call_argv(const char*, I32, register char**);
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The routine most often used is "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.
These routines used to be called "perl_call_sv", etc.,
before Perl v5.6.0, but those names are now deprecated;
macros of the same name are provided for compatibility.
When using any of these routines (except "call_argv"), the
programmer must manipulate the Perl stack. These include
the following macros and functions:
dSP
SP
PUSHMARK()
PUTBACK
SPAGAIN
ENTER
SAVETMPS
FREETMPS
LEAVE
XPUSH*()
POP*()
For a detailed description of calling conventions from C to
Perl, consult perlcall.
Memory Allocation
Allocation
All memory meant to be used with the Perl API functions
should be manipulated using the macros described in this
section. The macros provide the necessary transparency
between differences in the actual malloc implementation that
is used within perl.
It is suggested that you enable 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.
The following three macros are used to initially allocate
memory :
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Newx(pointer, number, type);
Newxc(pointer, number, type, cast);
Newxz(pointer, number, type);
The first argument "pointer" should be the name of a
variable that will point to the newly allocated memory.
The second and third 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 "Newxc", "cast", should be used if the
"pointer" argument is different from the "type" argument.
Unlike the "Newx" and "Newxc" macros, the "Newxz" macro
calls "memzero" to zero out all the newly allocated memory.
Reallocation
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.
Moving
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
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.
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For a complete description of the PerlIO abstraction,
consult perlapio.
Putting a C value on Perl stack
A lot of opcodes (this is an elementary operation in the
internal perl stack machine) put an SV* on the stack.
However, as an optimization the corresponding SV is
(usually) not recreated each time. The opcodes reuse
specially assigned SVs (targets) which are (as a corollary)
not constantly freed/created.
Each of the targets is created only once (but see
"Scratchpads and recursion" below), and when an opcode needs
to put an integer, a double, or a string on stack, it just
sets the corresponding parts of its target and puts the
target on stack.
The macro to put this target on stack is "PUSHTARG", and it
is directly used in some opcodes, as well as indirectly in
zillions of others, which use it via "(X)PUSH[iunp]".
Because the target is reused, you must be careful when
pushing multiple values on the stack. The following code
will not do what you think:
XPUSHi(10);
XPUSHi(20);
This translates as "set "TARG" to 10, push a pointer to
"TARG" onto the stack; set "TARG" to 20, push a pointer to
"TARG" onto the stack". At the end of the operation, the
stack does not contain the values 10 and 20, but actually
contains two pointers to "TARG", which we have set to 20.
If you need to push multiple different values then you
should either use the "(X)PUSHs" macros, or else use the new
"m(X)PUSH[iunp]" macros, none of which make use of "TARG".
The "(X)PUSHs" macros simply push an SV* on the stack,
which, as noted under "XSUBs and the Argument Stack", will
often need to be "mortal". The new "m(X)PUSH[iunp]" macros
make this a little easier to achieve by creating a new
mortal for you (via "(X)PUSHmortal"), pushing that onto the
stack (extending it if necessary in the case of the
"mXPUSH[iunp]" macros), and then setting its value. Thus,
instead of writing this to "fix" the example above:
XPUSHs(sv_2mortal(newSViv(10)))
XPUSHs(sv_2mortal(newSViv(20)))
you can simply write:
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mXPUSHi(10)
mXPUSHi(20)
On a related note, if you do use "(X)PUSH[iunp]", then
you're going to need a "dTARG" in your variable declarations
so that the "*PUSH*" macros can make use of the local
variable "TARG". See also "dTARGET" and "dXSTARG".
Scratchpads
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
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 threads. Both these can
create several execution pointers going into the same
subroutine. For the subroutine-child not write over the
temporaries for the subroutine-parent (lifespan of which
covers the call to the child), the parent and the child
should have different scratchpads. (And the lexicals should
be separate anyway!)
So each subroutine is born with an array of scratchpads (of
length 1). On each entry to the subroutine it is checked
that the current depth of the recursion is not more than the
length of this array, and if it is, new scratchpad is
created and pushed into the array.
The targets on this scratchpad are "undef"s, but they are
already marked with correct flags.
Compiled code
Code tree
Here we describe the internal form your code is converted to
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by Perl. Start with a simple example:
$a = $b + $c;
This is converted to a tree similar to this one:
assign-to
/ \
+ $a
/ \
$b $c
(but slightly more complicated). This tree reflects the way
Perl parsed your code, but has nothing to do with the
execution order. There is an additional "thread" going
through the nodes of the tree which shows the order of
execution of the nodes. In our simplified example above it
looks like:
$b ---> $c ---> + ---> $a ---> assign-to
But with the actual compile tree for "$a = $b + $c" it is
different: some nodes optimized away. As a corollary,
though the actual tree contains more nodes than our
simplified example, the execution order is the same as in
our example.
Examining the tree
If you have your perl compiled for debugging (usually done
with "-DDEBUGGING" on the "Configure" command line), you may
examine the compiled tree by specifying "-Dx" on the Perl
command line. The output takes several lines per node, and
for "$b+$c" it looks like this:
5 TYPE = add ===> 6
TARG = 1
FLAGS = (SCALAR,KIDS)
{
TYPE = null ===> (4)
(was rv2sv)
FLAGS = (SCALAR,KIDS)
{
3 TYPE = gvsv ===> 4
FLAGS = (SCALAR)
GV = main::b
}
}
{
TYPE = null ===> (5)
(was rv2sv)
FLAGS = (SCALAR,KIDS)
{
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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".
Each of these nodes represents an op, a fundamental
operation inside the Perl core. The code which implements
each operation can be found in the pp*.c files; the function
which implements the op with type "gvsv" is "pp_gvsv", and
so on. As the tree above shows, different ops have different
numbers of children: "add" is a binary operator, as one
would expect, and so has two children. To accommodate the
various different numbers of children, there are various
types of op data structure, and they link together in
different ways.
The simplest type of op structure is "OP": this has no
children. Unary operators, "UNOP"s, have one child, and this
is pointed to by the "op_first" field. Binary operators
("BINOP"s) have not only an "op_first" field but also an
"op_last" field. The most complex type of op is a "LISTOP",
which has any number of children. In this case, the first
child is pointed to by "op_first" and the last child by
"op_last". The children in between can be found by
iteratively following the "op_sibling" pointer from the
first child to the last.
There are also two other op types: a "PMOP" holds a regular
expression, and has no children, and a "LOOP" may or may not
have children. If the "op_children" field is non-zero, it
behaves like a "LISTOP". To complicate matters, if a "UNOP"
is actually a "null" op after optimization (see "Compile
pass 2: context propagation") it will still have children in
accordance with its former type.
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Another way to examine the tree is to use a compiler back-
end module, such as B::Concise.
Compile pass 1: check routines
The tree is created by the compiler while yacc code feeds it
the constructions it recognizes. Since yacc works bottom-up,
so does the first pass of perl compilation.
What makes this pass interesting for perl developers is that
some optimization may be performed on this pass. This is
optimization by so-called "check routines". The
correspondence between node names and corresponding check
routines is described in opcode.pl (do not forget to run
"make regen_headers" if you modify this file).
A check routine is called when the node is fully constructed
except for the execution-order thread. Since at this time
there are no back-links to the currently constructed node,
one can do most any operation to the top-level node,
including freeing it and/or creating new nodes above/below
it.
The check routine returns the node which should be inserted
into the tree (if the top-level node was not modified, check
routine returns its argument).
By convention, check routines have names "ck_*". They are
usually called from "new*OP" subroutines (or "convert")
(which in turn are called from perly.y).
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. At this time the context
can have 5 values (instead of 2 for runtime context): void,
boolean, scalar, list, and lvalue. In contrast with the
pass 1 this pass is processed from top to bottom: a node's
context determines the context for its children.
Additional context-dependent optimizations are performed at
this time. Since at this moment the compile tree contains
back-references (via "thread" pointers), nodes cannot be
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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
performed. This pass is neither top-down or bottom-up, but
in the execution order (with additional complications for
conditionals). These optimizations are done in the
subroutine peep(). Optimizations performed at this stage
are subject to the same restrictions as in the pass 2.
Pluggable runops
The compile tree is executed in a runops function. There
are two runops functions, in run.c and in dump.c.
"Perl_runops_debug" is used with DEBUGGING and
"Perl_runops_standard" is used otherwise. For fine control
over the execution of the compile tree it is possible to
provide your own runops function.
It's probably best to copy one of the existing runops
functions and change it to suit your needs. Then, in the
BOOT section of your XS file, add the line:
PL_runops = my_runops;
This function should be as efficient as possible to keep
your programs running as fast as possible.
Examining internal data structures with the "dump" functions
To aid debugging, the source file dump.c contains a number
of functions which produce formatted output of internal data
structures.
The most commonly used of these functions is "Perl_sv_dump";
it's used for dumping SVs, AVs, HVs, and CVs. The
"Devel::Peek" module calls "sv_dump" to produce debugging
output from Perl-space, so users of that module should
already be familiar with its format.
"Perl_op_dump" can be used to dump an "OP" structure or any
of its derivatives, and produces output similar to "perl
-Dx"; in fact, "Perl_dump_eval" will dump the main root of
the code being evaluated, exactly like "-Dx".
Other useful functions are "Perl_dump_sub", which turns a
"GV" into an op tree, "Perl_dump_packsubs" which calls
"Perl_dump_sub" on all the subroutines in a package like so:
(Thankfully, these are all xsubs, so there is no op tree)
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(gdb) print Perl_dump_packsubs(PL_defstash)
SUB attributes::bootstrap = (xsub 0x811fedc 0)
SUB UNIVERSAL::can = (xsub 0x811f50c 0)
SUB UNIVERSAL::isa = (xsub 0x811f304 0)
SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0)
SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0)
and "Perl_dump_all", which dumps all the subroutines in the
stash and the op tree of the main root.
How multiple interpreters and concurrency are supported
Background and PERL_IMPLICIT_CONTEXT
The Perl interpreter can be regarded as a closed box: it has
an API for feeding it code or otherwise making it do things,
but it also has functions for its own use. This smells a
lot like an object, and there are ways for you to build Perl
so that you can have multiple interpreters, with one
interpreter represented either as a C structure, or inside a
thread-specific structure. These structures contain all the
context, the state of that interpreter.
One macro controls the major Perl build flavor:
MULTIPLICITY. The MULTIPLICITY build has a C structure that
packages all the interpreter state. With multiplicity-
enabled perls, PERL_IMPLICIT_CONTEXT is also normally
defined, and enables the support for passing in a "hidden"
first argument that represents all three data structures.
MULTIPLICITY makes multi-threaded perls possible (with the
ithreads threading model, related to the macro
USE_ITHREADS.)
Two other "encapsulation" macros are the PERL_GLOBAL_STRUCT
and PERL_GLOBAL_STRUCT_PRIVATE (the latter turns on the
former, and the former turns on MULTIPLICITY.) The
PERL_GLOBAL_STRUCT causes all the internal variables of Perl
to be wrapped inside a single global struct, struct
perl_vars, accessible as (globals) &PL_Vars or PL_VarsPtr or
the function Perl_GetVars(). The
PERL_GLOBAL_STRUCT_PRIVATE goes one step further, there is
still a single struct (allocated in main() either from heap
or from stack) but there are no global data symbols pointing
to it. In either case the global struct should be
initialised as the very first thing in main() using
Perl_init_global_struct() and correspondingly tear it down
after perl_free() using Perl_free_global_struct(), please
see miniperlmain.c for usage details. You may also need to
use "dVAR" in your coding to "declare the global variables"
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when you are using them. dTHX does this for you
automatically.
To see whether you have non-const data you can use a BSD-
compatible "nm":
nm libperl.a | grep -v ' [TURtr] '
If this displays any "D" or "d" symbols, you have non-const
data.
For backward compatibility reasons defining just
PERL_GLOBAL_STRUCT doesn't actually hide all symbols inside
a big global struct: some PerlIO_xxx vtables are left
visible. The PERL_GLOBAL_STRUCT_PRIVATE then hides
everything (see how the PERLIO_FUNCS_DECL is used).
All this obviously requires a way for the Perl internal
functions to be either subroutines taking some kind of
structure as the first argument, or subroutines taking
nothing as the first argument. To enable these two very
different ways of building the interpreter, the Perl source
(as it does in so many other situations) makes heavy use of
macros and subroutine naming conventions.
First problem: deciding which functions will be public API
functions and which will be private. All functions whose
names begin "S_" are private (think "S" for "secret" or
"static"). All other functions begin with "Perl_", but just
because a function begins with "Perl_" does not mean it is
part of the API. (See "Internal Functions".) The easiest way
to be sure a function is part of the API is to find its
entry in perlapi. If it exists in perlapi, it's part of the
API. If it doesn't, and you think it should be (i.e., you
need it for your extension), send mail via perlbug
explaining why you think it should be.
Second problem: there must be a syntax so that the same
subroutine declarations and calls can pass a structure as
their first argument, or pass nothing. To solve this, the
subroutines are named and declared in a particular way.
Here's a typical start of a static function used within the
Perl guts:
STATIC void
S_incline(pTHX_ char *s)
STATIC becomes "static" in C, and may be #define'd to
nothing in some configurations in future.
A public function (i.e. part of the internal API, but not
necessarily sanctioned for use in extensions) begins like
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this:
void
Perl_sv_setiv(pTHX_ SV* dsv, IV num)
"pTHX_" is one of a number of macros (in perl.h) that hide
the details of the interpreter's context. THX stands for
"thread", "this", or "thingy", as the case may be. (And no,
George Lucas is not involved. :-) The first character could
be 'p' for a prototype, 'a' for argument, or 'd' for
declaration, so we have "pTHX", "aTHX" and "dTHX", and their
variants.
When Perl is built without options that set
PERL_IMPLICIT_CONTEXT, there is no first argument containing
the interpreter's context. The trailing underscore in the
pTHX_ macro indicates that the macro expansion needs a comma
after the context argument because other arguments follow
it. If PERL_IMPLICIT_CONTEXT is not defined, pTHX_ will be
ignored, and the subroutine is not prototyped to take the
extra argument. The form of the macro without the trailing
underscore is used when there are no additional explicit
arguments.
When a core function calls another, it must pass the
context. This is normally hidden via macros. Consider
"sv_setiv". It expands into something like this:
#ifdef PERL_IMPLICIT_CONTEXT
#define sv_setiv(a,b) Perl_sv_setiv(aTHX_ a, b)
/* can't do this for vararg functions, see below */
#else
#define sv_setiv Perl_sv_setiv
#endif
This works well, and means that XS authors can gleefully
write:
sv_setiv(foo, bar);
and still have it work under all the modes Perl could have
been compiled with.
This doesn't work so cleanly for varargs functions, though,
as macros imply that the number of arguments is known in
advance. Instead we either need to spell them out fully,
passing "aTHX_" as the first argument (the Perl core tends
to do this with functions like Perl_warner), or use a
context-free version.
The context-free version of Perl_warner is called
Perl_warner_nocontext, and does not take the extra argument.
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Instead it does dTHX; to get the context from thread-local
storage. We "#define warner Perl_warner_nocontext" so that
extensions get source compatibility at the expense of
performance. (Passing an arg is cheaper than grabbing it
from thread-local storage.)
You can ignore [pad]THXx when browsing the Perl
headers/sources. Those are strictly for use within the
core. Extensions and embedders need only be aware of
[pad]THX.
So what happened to dTHR?
"dTHR" was introduced in perl 5.005 to support the older
thread model. The older thread model now uses the "THX"
mechanism to pass context pointers around, so "dTHR" is not
useful any more. Perl 5.6.0 and later still have it for
backward source compatibility, but it is defined to be a no-
op.
How do I use all this in extensions?
When Perl is built with PERL_IMPLICIT_CONTEXT, extensions
that call any functions in the Perl API will need to pass
the initial context argument somehow. The kicker is that
you will need to write it in such a way that the extension
still compiles when Perl hasn't been built with
PERL_IMPLICIT_CONTEXT enabled.
There are three ways to do this. First, the easy but
inefficient way, which is also the default, in order to
maintain source compatibility with extensions: whenever
XSUB.h is #included, it redefines the aTHX and aTHX_ macros
to call a function that will return the context. Thus,
something like:
sv_setiv(sv, num);
in your extension will translate to this when
PERL_IMPLICIT_CONTEXT is in effect:
Perl_sv_setiv(Perl_get_context(), sv, num);
or to this otherwise:
Perl_sv_setiv(sv, num);
You have to do nothing new in your extension to get this;
since the Perl library provides Perl_get_context(), it will
all just work.
The second, more efficient way is to use the following
template for your Foo.xs:
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#define PERL_NO_GET_CONTEXT /* we want efficiency */
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
STATIC void my_private_function(int arg1, int arg2);
STATIC void
my_private_function(int arg1, int arg2)
{
dTHX; /* fetch context */
... call many Perl API functions ...
}
[... etc ...]
MODULE = Foo PACKAGE = Foo
/* typical XSUB */
void
my_xsub(arg)
int arg
CODE:
my_private_function(arg, 10);
Note that the only two changes from the normal way of
writing an extension is the addition of a "#define
PERL_NO_GET_CONTEXT" before including the Perl headers,
followed by a "dTHX;" declaration at the start of every
function that will call the Perl API. (You'll know which
functions need this, because the C compiler will complain
that there's an undeclared identifier in those functions.)
No changes are needed for the XSUBs themselves, because the
XS() macro is correctly defined to pass in the implicit
context if needed.
The third, even more efficient way is to ape how it is done
within the Perl guts:
#define PERL_NO_GET_CONTEXT /* we want efficiency */
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
/* pTHX_ only needed for functions that call Perl API */
STATIC void my_private_function(pTHX_ int arg1, int arg2);
STATIC void
my_private_function(pTHX_ int arg1, int arg2)
{
/* dTHX; not needed here, because THX is an argument */
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... call Perl API functions ...
}
[... etc ...]
MODULE = Foo PACKAGE = Foo
/* typical XSUB */
void
my_xsub(arg)
int arg
CODE:
my_private_function(aTHX_ arg, 10);
This implementation never has to fetch the context using a
function call, since it is always passed as an extra
argument. Depending on your needs for simplicity or
efficiency, you may mix the previous two approaches freely.
Never add a comma after "pTHX" yourself--always use the form
of the macro with the underscore for functions that take
explicit arguments, or the form without the argument for
functions with no explicit arguments.
If one is compiling Perl with the "-DPERL_GLOBAL_STRUCT" the
"dVAR" definition is needed if the Perl global variables
(see perlvars.h or globvar.sym) are accessed in the function
and "dTHX" is not used (the "dTHX" includes the "dVAR" if
necessary). One notices the need for "dVAR" only with the
said compile-time define, because otherwise the Perl global
variables are visible as-is.
Should I do anything special if I call perl from multiple
threads?
If you create interpreters in one thread and then proceed to
call them in another, you need to make sure perl's own
Thread Local Storage (TLS) slot is initialized correctly in
each of those threads.
The "perl_alloc" and "perl_clone" API functions will
automatically set the TLS slot to the interpreter they
created, so that there is no need to do anything special if
the interpreter is always accessed in the same thread that
created it, and that thread did not create or call any other
interpreters afterwards. If that is not the case, you have
to set the TLS slot of the thread before calling any
functions in the Perl API on that particular interpreter.
This is done by calling the "PERL_SET_CONTEXT" macro in that
thread as the first thing you do:
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/* do this before doing anything else with some_perl */
PERL_SET_CONTEXT(some_perl);
... other Perl API calls on some_perl go here ...
Future Plans and PERL_IMPLICIT_SYS
Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up
everything that the interpreter knows about itself and pass
it around, so too are there plans to allow the interpreter
to bundle up everything it knows about the environment it's
running on. This is enabled with the PERL_IMPLICIT_SYS
macro. Currently it only works with USE_ITHREADS on
Windows.
This allows the ability to provide an extra pointer (called
the "host" environment) for all the system calls. This
makes it possible for all the system stuff to maintain their
own state, broken down into seven C structures. These are
thin wrappers around the usual system calls (see
win32/perllib.c) for the default perl executable, but for a
more ambitious host (like the one that would do fork()
emulation) all the extra work needed to pretend that
different interpreters are actually different "processes",
would be done here.
The Perl engine/interpreter and the host are orthogonal
entities. There could be one or more interpreters in a
process, and one or more "hosts", with free association
between them.
Internal Functions
All of Perl's internal functions which will be exposed to
the outside world are prefixed by "Perl_" so that they will
not conflict with XS functions or functions used in a
program in which Perl is embedded. Similarly, all global
variables begin with "PL_". (By convention, static functions
start with "S_".)
Inside the Perl core ("PERL_CORE" defined), you can get at
the functions either with or without the "Perl_" prefix,
thanks to a bunch of defines that live in embed.h. Note that
extension code should not set "PERL_CORE"; this exposes the
full perl internals, and is likely to cause breakage of the
XS in each new perl release.
The file embed.h is generated automatically from embed.pl
and embed.fnc. embed.pl also creates the prototyping header
files for the internal functions, generates the
documentation and a lot of other bits and pieces. It's
important that when you add a new function to the core or
change an existing one, you change the data in the table in
embed.fnc as well. Here's a sample entry from that table:
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Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval
The second column is the return type, the third column the
name. Columns after that are the arguments. The first column
is a set of flags:
A This function is a part of the public API. All such
functions should also have 'd', very few do not.
p This function has a "Perl_" prefix; i.e. it is defined as
"Perl_av_fetch".
d This function has documentation using the "apidoc"
feature which we'll look at in a second. Some functions
have 'd' but not 'A'; docs are good.
Other available flags are:
s This is a static function and is defined as "STATIC
S_whatever", and usually called within the sources as
"whatever(...)".
n This does not need a interpreter context, so the
definition has no "pTHX", and it follows that callers
don't use "aTHX". (See "Background and
PERL_IMPLICIT_CONTEXT" in perlguts.)
r This function never returns; "croak", "exit" and friends.
f This function takes a variable number of arguments,
"printf" style. The argument list should end with "...",
like this:
Afprd |void |croak |const char* pat|...
M This function is part of the experimental development
API, and may change or disappear without notice.
o This function should not have a compatibility macro to
define, say, "Perl_parse" to "parse". It must be called
as "Perl_parse".
x This function isn't exported out of the Perl core.
m This is implemented as a macro.
X This function is explicitly exported.
E This function is visible to extensions included in the
Perl core.
b Binary backward compatibility; this function is a macro
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but also has a "Perl_" implementation (which is
exported).
others
See the comments at the top of "embed.fnc" for others.
If you edit embed.pl or embed.fnc, you will need to run
"make regen_headers" to force a rebuild of embed.h and other
auto-generated files.
Formatted Printing of IVs, UVs, and NVs
If you are printing IVs, UVs, or NVS instead of the stdio(3)
style formatting codes like %d, %ld, %f, you should use the
following macros for portability
IVdf IV in decimal
UVuf UV in decimal
UVof UV in octal
UVxf UV in hexadecimal
NVef NV %e-like
NVff NV %f-like
NVgf NV %g-like
These will take care of 64-bit integers and long doubles.
For example:
printf("IV is %"IVdf"\n", iv);
The IVdf will expand to whatever is the correct format for
the IVs.
If you are printing addresses of pointers, use UVxf combined
with PTR2UV(), do not use %lx or %p.
Pointer-To-Integer and Integer-To-Pointer
Because pointer size does not necessarily equal integer
size, use the follow macros to do it right.
PTR2UV(pointer)
PTR2IV(pointer)
PTR2NV(pointer)
INT2PTR(pointertotype, integer)
For example:
IV iv = ...;
SV *sv = INT2PTR(SV*, iv);
and
AV *av = ...;
UV uv = PTR2UV(av);
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Exception Handling
There are a couple of macros to do very basic exception
handling in XS modules. You have to define "NO_XSLOCKS"
before including XSUB.h to be able to use these macros:
#define NO_XSLOCKS
#include "XSUB.h"
You can use these macros if you call code that may croak,
but you need to do some cleanup before giving control back
to Perl. For example:
dXCPT; /* set up necessary variables */
XCPT_TRY_START {
code_that_may_croak();
} XCPT_TRY_END
XCPT_CATCH
{
/* do cleanup here */
XCPT_RETHROW;
}
Note that you always have to rethrow an exception that has
been caught. Using these macros, it is not possible to just
catch the exception and ignore it. If you have to ignore the
exception, you have to use the "call_*" function.
The advantage of using the above macros is that you don't
have to setup an extra function for "call_*", and that using
these macros is faster than using "call_*".
Source Documentation
There's an effort going on to document the internal
functions and automatically produce reference manuals from
them - perlapi is one such manual which details all the
functions which are available to XS writers. perlintern is
the autogenerated manual for the functions which are not
part of the API and are supposedly for internal use only.
Source documentation is created by putting POD comments into
the C source, like this:
/*
=for apidoc sv_setiv
Copies an integer into the given SV. Does not handle 'set' magic. See
C<sv_setiv_mg>.
=cut
*/
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Please try and supply some documentation if you add
functions to the Perl core.
Backwards compatibility
The Perl API changes over time. New functions are added or
the interfaces of existing functions are changed. The
"Devel::PPPort" module tries to provide compatibility code
for some of these changes, so XS writers don't have to code
it themselves when supporting multiple versions of Perl.
"Devel::PPPort" generates a C header file ppport.h that can
also be run as a Perl script. To generate ppport.h, run:
perl -MDevel::PPPort -eDevel::PPPort::WriteFile
Besides checking existing XS code, the script can also be
used to retrieve compatibility information for various API
calls using the "--api-info" command line switch. For
example:
% perl ppport.h --api-info=sv_magicext
For details, see "perldoc ppport.h".
Unicode Support
Perl 5.6.0 introduced Unicode support. It's important for
porters and XS writers to understand this support and make
sure that the code they write does not corrupt Unicode data.
What is Unicode, anyway?
In the olden, less enlightened times, we all used to use
ASCII. Most of us did, anyway. The big problem with ASCII is
that it's American. Well, no, that's not actually the
problem; the problem is that it's not particularly useful
for people who don't use the Roman alphabet. What used to
happen was that particular languages would stick their own
alphabet in the upper range of the sequence, between 128 and
255. Of course, we then ended up with plenty of variants
that weren't quite ASCII, and the whole point of it being a
standard was lost.
Worse still, if you've got a language like Chinese or
Japanese that has hundreds or thousands of characters, then
you really can't fit them into a mere 256, so they had to
forget about ASCII altogether, and build their own systems
using pairs of numbers to refer to one character.
To fix this, some people formed Unicode, Inc. and produced a
new character set containing all the characters you can
possibly think of and more. There are several ways of
representing these characters, and the one Perl uses is
called UTF-8. UTF-8 uses a variable number of bytes to
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represent a character. You can learn more about Unicode and
Perl's Unicode model in perlunicode.
How can I recognise a UTF-8 string?
You can't. This is because UTF-8 data is stored in bytes
just like non-UTF-8 data. The Unicode character 200, (0xC8
for you hex types) capital E with a grave accent, is
represented by the two bytes "v196.172". Unfortunately, the
non-Unicode string "chr(196).chr(172)" has that byte
sequence as well. So you can't tell just by looking - this
is what makes Unicode input an interesting problem.
In general, you either have to know what you're dealing
with, or you have to guess. The API function
"is_utf8_string" can help; it'll tell you if a string
contains only valid UTF-8 characters. However, it can't do
the work for you. On a character-by-character basis,
"is_utf8_char" will tell you whether the current character
in a string is valid UTF-8.
How does UTF-8 represent Unicode characters?
As mentioned above, UTF-8 uses a variable number of bytes to
store a character. Characters with values 0...127 are stored
in one byte, just like good ol' ASCII. Character 128 is
stored as "v194.128"; this continues up to character 191,
which is "v194.191". Now we've run out of bits (191 is
binary 10111111) so we move on; 192 is "v195.128". And so it
goes on, moving to three bytes at character 2048.
Assuming you know you're dealing with a UTF-8 string, you
can find out how long the first character in it is with the
"UTF8SKIP" macro:
char *utf = "\305\233\340\240\201";
I32 len;
len = UTF8SKIP(utf); /* len is 2 here */
utf += len;
len = UTF8SKIP(utf); /* len is 3 here */
Another way to skip over characters in a UTF-8 string is to
use "utf8_hop", which takes a string and a number of
characters to skip over. You're on your own about bounds
checking, though, so don't use it lightly.
All bytes in a multi-byte UTF-8 character will have the high
bit set, so you can test if you need to do something special
with this character like this (the UTF8_IS_INVARIANT() is a
macro that tests whether the byte can be encoded as a single
byte even in UTF-8):
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U8 *utf;
UV uv; /* Note: a UV, not a U8, not a char */
if (!UTF8_IS_INVARIANT(*utf))
/* Must treat this as UTF-8 */
uv = utf8_to_uv(utf);
else
/* OK to treat this character as a byte */
uv = *utf;
You can also see in that example that we use "utf8_to_uv" to
get the value of the character; the inverse function
"uv_to_utf8" is available for putting a UV into UTF-8:
if (!UTF8_IS_INVARIANT(uv))
/* Must treat this as UTF8 */
utf8 = uv_to_utf8(utf8, uv);
else
/* OK to treat this character as a byte */
*utf8++ = uv;
You must convert characters to UVs using the above functions
if you're ever in a situation where you have to match UTF-8
and non-UTF-8 characters. You may not skip over UTF-8
characters in this case. If you do this, you'll lose the
ability to match hi-bit non-UTF-8 characters; for instance,
if your UTF-8 string contains "v196.172", and you skip that
character, you can never match a "chr(200)" in a non-UTF-8
string. So don't do that!
How does Perl store UTF-8 strings?
Currently, Perl deals with Unicode strings and non-Unicode
strings slightly differently. A flag in the SV, "SVf_UTF8",
indicates that the string is internally encoded as UTF-8.
Without it, the byte value is the codepoint number and vice
versa (in other words, the string is encoded as iso-8859-1,
but "use feature 'unicode_strings'" is needed to get
iso-8859-1 semantics). You can check and manipulate this
flag with the following macros:
SvUTF8(sv)
SvUTF8_on(sv)
SvUTF8_off(sv)
This flag has an important effect on Perl's treatment of the
string: if Unicode data is not properly distinguished,
regular expressions, "length", "substr" and other string
handling operations will have undesirable results.
The problem comes when you have, for instance, a string that
isn't flagged as UTF-8, and contains a byte sequence that
could be UTF-8 - especially when combining non-UTF-8 and
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UTF-8 strings.
Never forget that the "SVf_UTF8" flag is separate to the PV
value; you need be sure you don't accidentally knock it off
while you're manipulating SVs. More specifically, you cannot
expect to do this:
SV *sv;
SV *nsv;
STRLEN len;
char *p;
p = SvPV(sv, len);
frobnicate(p);
nsv = newSVpvn(p, len);
The "char*" string does not tell you the whole story, and
you can't copy or reconstruct an SV just by copying the
string value. Check if the old SV has the UTF8 flag set, and
act accordingly:
p = SvPV(sv, len);
frobnicate(p);
nsv = newSVpvn(p, len);
if (SvUTF8(sv))
SvUTF8_on(nsv);
In fact, your "frobnicate" function should be made aware of
whether or not it's dealing with UTF-8 data, so that it can
handle the string appropriately.
Since just passing an SV to an XS function and copying the
data of the SV is not enough to copy the UTF8 flags, even
less right is just passing a "char *" to an XS function.
How do I convert a string to UTF-8?
If you're mixing UTF-8 and non-UTF-8 strings, it is
necessary to upgrade one of the strings to UTF-8. If you've
got an SV, the easiest way to do this is:
sv_utf8_upgrade(sv);
However, you must not do this, for example:
if (!SvUTF8(left))
sv_utf8_upgrade(left);
If you do this in a binary operator, you will actually
change one of the strings that came into the operator, and,
while it shouldn't be noticeable by the end user, it can
cause problems in deficient code.
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Instead, "bytes_to_utf8" will give you a UTF-8-encoded copy
of its string argument. This is useful for having the data
available for comparisons and so on, without harming the
original SV. There's also "utf8_to_bytes" to go the other
way, but naturally, this will fail if the string contains
any characters above 255 that can't be represented in a
single byte.
Is there anything else I need to know?
Not really. Just remember these things:
o There's no way to tell if a string is UTF-8 or not. You
can tell if an SV is UTF-8 by looking at is "SvUTF8"
flag. Don't forget to set the flag if something should be
UTF-8. Treat the flag as part of the PV, even though it's
not - if you pass on the PV to somewhere, pass on the
flag too.
o If a string is UTF-8, always use "utf8_to_uv" to get at
the value, unless "UTF8_IS_INVARIANT(*s)" in which case
you can use *s.
o When writing a character "uv" to a UTF-8 string, always
use "uv_to_utf8", unless "UTF8_IS_INVARIANT(uv))" in
which case you can use "*s = uv".
o Mixing UTF-8 and non-UTF-8 strings is tricky. Use
"bytes_to_utf8" to get a new string which is UTF-8
encoded, and then combine them.
Custom Operators
Custom operator support is a new experimental feature that
allows you to define your own ops. This is primarily to
allow the building of interpreters for other languages in
the Perl core, but it also allows optimizations through the
creation of "macro-ops" (ops which perform the functions of
multiple ops which are usually executed together, such as
"gvsv, gvsv, add".)
This feature is implemented as a new op type, "OP_CUSTOM".
The Perl core does not "know" anything special about this op
type, and so it will not be involved in any optimizations.
This also means that you can define your custom ops to be
any op structure - unary, binary, list and so on - you like.
It's important to know what custom operators won't do for
you. They won't let you add new syntax to Perl, directly.
They won't even let you add new keywords, directly. In fact,
they won't change the way Perl compiles a program at all.
You have to do those changes yourself, after Perl has
compiled the program. You do this either by manipulating the
op tree using a "CHECK" block and the "B::Generate" module,
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or by adding a custom peephole optimizer with the "optimize"
module.
When you do this, you replace ordinary Perl ops with custom
ops by creating ops with the type "OP_CUSTOM" and the
"pp_addr" of your own PP function. This should be defined in
XS code, and should look like the PP ops in "pp_*.c". You
are responsible for ensuring that your op takes the
appropriate number of values from the stack, and you are
responsible for adding stack marks if necessary.
You should also "register" your op with the Perl interpreter
so that it can produce sensible error and warning messages.
Since it is possible to have multiple custom ops within the
one "logical" op type "OP_CUSTOM", Perl uses the value of
"o->op_ppaddr" as a key into the "PL_custom_op_descs" and
"PL_custom_op_names" hashes. This means you need to enter a
name and description for your op at the appropriate place in
the "PL_custom_op_names" and "PL_custom_op_descs" hashes.
"B::Generate" directly supports the creation of custom ops
by name.
AUTHORS
Until May 1997, this document was maintained by Jeff Okamoto
<okamoto@corp.hp.com>. It is now maintained as part of Perl
itself by the Perl 5 Porters <perl5-porters@perl.org>.
With lots of help and suggestions from Dean Roehrich,
Malcolm Beattie, Andreas Koenig, Paul Hudson, Ilya
Zakharevich, Paul Marquess, Neil Bowers, Matthew Green, Tim
Bunce, Spider Boardman, Ulrich Pfeifer, Stephen McCamant,
and Gurusamy Sarathy.
ATTRIBUTES
See attributes(5) for descriptions of the following
attributes:
+---------------+------------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+------------------+
|Availability | runtime/perl-512 |
+---------------+------------------+
|Stability | Uncommitted |
+---------------+------------------+
SEE ALSO
perlapi, perlintern, perlxs, perlembed
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NOTES
This software was built from source available at
https://java.net/projects/solaris-userland. The original
community source was downloaded from
http://www.cpan.org/src/5.0/perl-5.12.5.tar.bz2
Further information about this software can be found on the
open source community website at http://www.perl.org/.
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