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PERLGUTS(1) 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 com-
ments 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, respec-
tively. (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 rep-
resent the size of any string that perl can handle.
In the unlikely case of a SV requiring more complex ini-
tialisation, 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 argu-
ment 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 oper-
ate on values that have "magic". See "Magic Virtual
Tables" later in this document.
All SVs that contain strings should be terminated with a
NUL character. If it is not NUL-terminated there is a
risk of core dumps and corruptions from code which passes
the string to C functions or system calls which expect a
NUL-terminated string. Perl's own functions typically add
a trailing NUL for this reason. Nevertheless, you should
be very careful when you pass a string stored in an SV to
a C function or system call.
To access the actual value that an SV points to, you can
use the macros:
SvIV(SV*)
SvUV(SV*)
SvNV(SV*)
SvPV(SV*, STRLEN len)
SvPV_nolen(SV*)
which will automatically coerce the actual scalar type
into an IV, UV, double, or string.
In the "SvPV" macro, the length of the string returned is
placed into the variable "len" (this is a macro, so you do
not use &len). If you do not care what the length of the
data is, use the "SvPV_nolen" macro. Historically the
"SvPV" macro with the global variable "PL_na" has been
used in this case. But that can be quite inefficient
because "PL_na" must be accessed in thread-local storage
in threaded Perl. In any case, remember that Perl allows
arbitrary strings of data that may both contain NULs and
might not be terminated by a NUL.
Also remember that C doesn't allow you to safely say
"foo(SvPV(s, len), len);". It might work with your com-
piler, but it won't work for everyone. Break this sort of
statement up into separate assignments:
SV *s;
STRLEN len;
char * ptr;
ptr = SvPV(s, len);
foo(ptr, len);
If you want to know if the scalar value is TRUE, you can
use:
SvTRUE(SV*)
Although Perl will automatically grow strings for you, if
you need to force Perl to allocate more memory for your
SV, you can use the macro
SvGROW(SV*, STRLEN newlen)
which will determine if more memory needs to be allocated.
If so, it will call the function "sv_grow". Note that
"SvGROW" can only increase, not decrease, the allocated
memory of an SV and that it does not automatically add a
byte for the a trailing NUL (perl's own string functions
typically do "SvGROW(sv, len + 1)").
If you have an SV and want to know what kind of data Perl
thinks is stored in it, you can use the following macros
to check the type of SV you have.
SvIOK(SV*)
SvNOK(SV*)
SvPOK(SV*)
You can get and set the current length of the string
stored in an SV with the following macros:
SvCUR(SV*)
SvCUR_set(SV*, I32 val)
You can also get a pointer to the end of the string stored
in the SV with the macro:
SvEND(SV*)
But note that these last three macros are valid only if
"SvPOK()" is true.
If you want to append something to the end of string
stored in an "SV*", you can use the following functions:
void sv_catpv(SV*, const char*);
void sv_catpvn(SV*, const char*, STRLEN);
void sv_catpvf(SV*, const char*, ...);
void sv_vcatpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool);
void sv_catsv(SV*, SV*);
The first function calculates the length of the string to
be appended by using "strlen". In the second, you specify
the length of the string yourself. The third function
processes its arguments like "sprintf" and appends the
formatted output. The fourth function works like
"vsprintf". You can specify the address and length of an
array of SVs instead of the va_list argument. The fifth
function extends the string stored in the first SV with
the string stored in the second SV. It also forces the
second SV to be interpreted as a string.
The "sv_cat*()" functions are not generic enough to oper-
ate on values that have "magic". See "Magic Virtual
Tables" later in this document.
If you know the name of a scalar variable, you can get a
pointer to its SV by using the following:
SV* get_sv("package::varname", FALSE);
This returns NULL if the variable does not exist.
If you want to know if this variable (or any other SV) is
actually "defined", you can call:
SvOK(SV*)
The scalar "undef" value is stored in an SV instance
called "PL_sv_undef".
Its address can be used whenever an "SV*" is needed. Make
sure that you don't try to compare a random sv with
&PL_sv_undef. For example when interfacing Perl code,
it'll work correctly for:
foo(undef);
But won't work when called as:
$x = undef;
foo($x);
So to repeat always use SvOK() to check whether an sv is
defined.
Also you have to be careful when using &PL_sv_undef as a
value in AVs or HVs (see "AVs, HVs and undefined values").
There are also the two values "PL_sv_yes" and "PL_sv_no",
which contain boolean TRUE and FALSE values, respectively.
Like "PL_sv_undef", their addresses can be used whenever
an "SV*" is needed.
Do not be fooled into thinking that "(SV *) 0" is the same
as &PL_sv_undef. Take this code:
SV* sv = (SV*) 0;
if (I-am-to-return-a-real-value) {
sv = sv_2mortal(newSViv(42));
}
sv_setsv(ST(0), sv);
This code tries to return a new SV (which contains the
value 42) if it should return a real value, or undef oth-
erwise. Instead it has returned a NULL pointer which,
somewhere down the line, will cause a segmentation viola-
tion, bus error, or just weird results. Change the zero
to &PL_sv_undef in the first line and all will be well.
To free an SV that you've created, call "SvRE-
FCNT_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 dis-
cards 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 allo-
cated 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 begin-
ning of the array; while "AvARRAY" points to the first
element in the array that is visible from Perl, "AvALLOC"
points to the real start of the C array. These are usually
the same, but a "shift" operation can be carried out by
increasing "AvARRAY" by one and decreasing "AvFILL" and
"AvLEN". Again, the location of the real start of the C
array only comes into play when freeing the array. See
"av_shift" in av.c.
What's Really Stored in an SV?
Recall that the usual method of determining the type of
scalar you have is to use "Sv*OK" macros. Because a
scalar can be both a number and a string, usually these
macros will always return TRUE and calling the "Sv*V"
macros will do the appropriate conversion of string to
integer/double or integer/double to string.
If you really need to know if you have an integer, double,
or string pointer in an SV, you can use the following
three macros instead:
SvIOKp(SV*)
SvNOKp(SV*)
SvPOKp(SV*)
These will tell you if you truly have an integer, double,
or string pointer stored in your SV. The "p" stands for
private.
The are various ways in which the private and public flags
may differ. For example, a tied SV may have a valid
underlying value in the IV slot (so SvIOKp is true), but
the data should be accessed via the FETCH routine rather
than directly, so SvIOK is false. Another is when numeric
conversion has 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 popu-
lates it with SVs:
AV* av_make(I32 num, SV **ptr);
The second argument points to an array containing "num"
"SV*"'s. Once the AV has been created, the SVs can be
destroyed, if so desired.
Once the AV has been created, the following operations are
possible on AVs:
void av_push(AV*, SV*);
SV* av_pop(AV*);
SV* av_shift(AV*);
void av_unshift(AV*, I32 num);
These should be familiar operations, with the exception of
"av_unshift". This routine adds "num" elements at the
front of the array with the "undef" value. You must then
use "av_store" (described below) to assign values to these
new elements.
Here are some other functions:
I32 av_len(AV*);
SV** av_fetch(AV*, I32 key, I32 lval);
SV** av_store(AV*, I32 key, SV* val);
The "av_len" function returns the highest index value in
array (just like $#array in Perl). If the array is empty,
-1 is returned. The "av_fetch" function returns the value
at index "key", but if "lval" is non-zero, then "av_fetch"
will store an undef value at that index. The "av_store"
function stores the value "val" at index "key", and does
not increment the reference count of "val". Thus the
caller is responsible for taking care of that, and if
"av_store" returns NULL, the caller will have to decrement
the reference count to avoid a memory leak. Note that
"av_fetch" and "av_store" both return "SV**"'s, not
"SV*"'s as their return value.
void av_clear(AV*);
void av_undef(AV*);
void av_extend(AV*, I32 key);
The "av_clear" function deletes all the elements in the
AV* array, but does not actually delete the array itself.
The "av_undef" function will delete all the elements in
the array plus the array itself. The "av_extend" function
extends the array so that it contains at least "key+1"
elements. If "key+1" is less than the currently allocated
length of the array, then nothing is done.
If you know the name of an array variable, you can get a
pointer to its AV by using the following:
AV* get_av("package::varname", FALSE);
This returns NULL if the variable does not exist.
See "Understanding the Magic of Tied Hashes and Arrays"
for more information on how to use the array access func-
tions on tied arrays.
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 have an "HE*", to get the actual key and value,
use the routines specified below.
I32 hv_iterinit(HV*);
/* Prepares starting point to traverse hash table */
HE* hv_iternext(HV*);
/* Get the next entry, and return a pointer to a
structure that has both the key and value */
char* hv_iterkey(HE* entry, I32* retlen);
/* Get the key from an HE structure and also return
the length of the key string */
SV* hv_iterval(HV*, HE* entry);
/* Return an SV pointer to the value of the HE
structure */
SV* hv_iternextsv(HV*, char** key, I32* retlen);
/* This convenience routine combines hv_iternext,
hv_iterkey, and hv_iterval. The key and retlen
arguments are return values for the key and its
length. The value is returned in the SV* argument */
If you know the name of a hash variable, you can get a
pointer to its HV by using the following:
HV* get_hv("package::varname", FALSE);
This returns NULL if the variable does not exist.
The hash algorithm is defined in the "PERL_HASH(hash, key,
klen)" macro:
hash = 0;
while (klen--)
hash = (hash * 33) + *key++;
hash = hash + (hash >> 5); /* after 5.6 */
The last step was added in version 5.6 to improve distri-
bution 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 func-
tions 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);
SV* hv_iterkeysv (HE* entry);
Note that these functions take "SV*" keys, which simpli-
fies writing of extension code that deals with hash struc-
tures. 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 incre-
ment 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 ref-
erence count of "val" to avoid a memory leak.
AVs, HVs and undefined values
Sometimes you have to store undefined values in AVs or
HVs. Although this may be a rare case, it can be tricky.
That's because you're used to using &PL_sv_undef if you
need an undefined SV.
For example, intuition tells you that this XS code:
AV *av = newAV();
av_store( av, 0, &PL_sv_undef );
is equivalent to this Perl code:
my @av;
$av[0] = undef;
Unfortunately, this isn't true. AVs use &PL_sv_undef as a
marker for indicating that an array element has not yet
been initialized. Thus, "exists $av[0]" would be true for
the above Perl code, but false for the array generated by
the XS code.
Other problems can occur when storing &PL_sv_undef in HVs:
hv_store( hv, "key", 3, &PL_sv_undef, 0 );
This will indeed make the value "undef", but if you try to
modify the value of "key", you'll get the following error:
Modification of non-creatable hash value attempted
In perl 5.8.0, &PL_sv_undef was also used to mark place-
holders 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 vari-
ables &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 cre-
ate 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 func-
tions:
SV* newRV_inc((SV*) thing);
SV* newRV_noinc((SV*) thing);
The "thing" argument can be any of an "SV*", "AV*", or
"HV*". The functions are identical except that
"newRV_inc" increments the reference count of the "thing",
while "newRV_noinc" does not. For historical reasons,
"newRV" is a synonym for "newRV_inc".
Once you have a reference, you can use the following macro
to dereference the reference:
SvRV(SV*)
then call the appropriate routines, casting the returned
"SV*" to either an "AV*" or "HV*", if required.
To determine if an SV is a reference, you can use the fol-
lowing 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 pro-
gramming. 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 follow-
ing function:
SV* sv_bless(SV* sv, HV* stash);
The "sv" argument must be a reference value. The "stash"
argument specifies which class the reference will belong
to. See "Stashes and Globs" for information on converting
class names into stashes.
/* Still under construction */
Upgrades rv to reference if not already one. Creates new
SV for rv to point to. If "classname" is non-null, the SV
is blessed into the specified class. SV is returned.
SV* newSVrv(SV* rv, const char* classname);
Copies integer, unsigned integer or double into an SV
whose reference is "rv". SV is blessed if "classname" is
non-null.
SV* sv_setref_iv(SV* rv, const char* classname, IV iv);
SV* sv_setref_uv(SV* rv, const char* classname, UV uv);
SV* sv_setref_nv(SV* rv, const char* classname, NV iv);
Copies the pointer value (the address, not the string!)
into an SV whose reference is rv. SV is blessed if
"classname" is non-null.
SV* sv_setref_pv(SV* rv, const char* classname, PV iv);
Copies string into an SV whose reference is "rv". Set
length to 0 to let Perl calculate the string length. SV
is blessed if "classname" is non-null.
SV* sv_setref_pvn(SV* rv, const char* classname, PV iv, STRLEN length);
Tests whether the SV is blessed into the specified class.
It does not check inheritance relationships.
int sv_isa(SV* sv, const char* name);
Tests whether the SV is a reference to a blessed object.
int sv_isobject(SV* sv);
Tests whether the SV is derived from the specified class.
SV can be either a reference to a blessed object or a
string containing a class name. This is the function
implementing the "UNIVERSAL::isa" functionality.
bool sv_derived_from(SV* sv, const char* name);
To check if you've got an object derived from a specific
class you have to write:
if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }
Creating New Variables
To create a new Perl variable with an undef value which
can be accessed from your Perl script, use the following
routines, depending on the variable type.
SV* get_sv("package::varname", TRUE);
AV* get_av("package::varname", TRUE);
HV* get_hv("package::varname", TRUE);
Notice the use of TRUE as the second parameter. The new
variable can now be set, using the routines appropriate to
the data type.
There are additional macros whose values may be bitwise
OR'ed with the "TRUE" argument to enable certain extra
features. Those bits are:
GV_ADDMULTI
Marks the variable as multiply defined, thus prevent-
ing the:
Name used only once: possible typo
warning.
GV_ADDWARN
Issues the warning:
Had to create unexpectedly
if the variable did not exist before the function was
called.
If you do not specify a package name, the variable is cre-
ated 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 follow-
ing) 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 refer-
ence to it is changed or overwritten. At the internal
level, however, reference counts can be manipulated with
the following macros:
int SvREFCNT(SV* sv);
SV* SvREFCNT_inc(SV* sv);
void SvREFCNT_dec(SV* sv);
However, there is one other function which manipulates the
reference count of its argument. The "newRV_inc" func-
tion, you will recall, creates a reference to the speci-
fied argument. As a side effect, it increments the argu-
ment'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 rou-
tine 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 hap-
pens. 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 refer-
ence 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 intro-
duce 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". Gener-
ally 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 "SvRE-
FCNT_dec". However, if you mortalize a variable twice,
the reference count will later be decremented twice.
"Mortal" SVs are mainly used for SVs that are placed on
perl's stack. For example an SV which is created just to
pass a number to a called sub is made mortal to have it
cleaned up automatically when it's popped off the stack.
Similarly, results returned by XSUBs (which are pushed on
the stack) are often made mortal.
To create a mortal variable, use the functions:
SV* sv_newmortal()
SV* sv_2mortal(SV*)
SV* sv_mortalcopy(SV*)
The first call creates a mortal SV (with no value), the
second converts an existing SV to a mortal SV (and thus
defers a call to "SvREFCNT_dec"), and the third creates a
mortal copy of an existing SV. Because "sv_newmortal"
gives the new SV no value,it must normally be given one
via "sv_setpv", "sv_setiv", etc. :
SV *tmp = sv_newmortal();
sv_setiv(tmp, an_integer);
As that is multiple C statements it is quite common so see
this idiom instead:
SV *tmp = sv_2mortal(newSViv(an_integer));
You should be careful about creating mortal variables.
Strange things can happen if you make the same value mor-
tal within multiple contexts, or if you make a variable
mortal multiple times. Thinking of "Mortalization" as
deferred "SvREFCNT_dec" should help to minimize such prob-
lems. 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 sym-
bol name (shared by all the different types of objects
that have the same name), and each value in the hash table
is a GV (Glob Value). This GV in turn contains references
to the various objects of that name, including (but not
limited to) the following:
Scalar Value
Array Value
Hash Value
I/O Handle
Format
Subroutine
There is a single stash called "PL_defstash" that holds
the items that exist in the "main" package. To get at the
items in other packages, append the string "::" to the
package name. The items in the "Foo" package are in the
stash "Foo::" in PL_defstash. The items in the "Bar::Baz"
package are in the stash "Baz::" in "Bar::"'s stash.
To get the stash pointer for a particular package, use the
function:
HV* gv_stashpv(const char* name, I32 create)
HV* gv_stashsv(SV*, I32 create)
The first function takes a literal string, the second uses
the string stored in the SV. Remember that a stash is
just a hash table, so you get back an "HV*". The "create"
flag will create a new package if it is set.
The name that "gv_stash*v" wants is the name of the pack-
age 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 refer-
ence, 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
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 "dber-
ror" that contains both the numeric and descriptive string
error values, you could use the following code:
extern int dberror;
extern char *dberror_list;
SV* sv = get_sv("dberror", TRUE);
sv_setiv(sv, (IV) dberror);
sv_setpv(sv, dberror_list[dberror]);
SvIOK_on(sv);
If the order of "sv_setiv" and "sv_setpv" had been
reversed, then the macro "SvPOK_on" would need to be
called instead of "SvIOK_on".
Magic Variables
[This section still under construction. Ignore everything
here. Post no bills. Everything not permitted is forbid-
den.]
Any SV may be magical, that is, it has special features
that a normal SV does not have. These features are stored
in the SV structure in a linked list of "struct magic"'s,
typedef'ed to "MAGIC".
struct magic {
MAGIC* mg_moremagic;
MGVTBL* mg_virtual;
U16 mg_private;
char mg_type;
U8 mg_flags;
SV* mg_obj;
char* mg_ptr;
I32 mg_len;
};
Note this is current as of patchlevel 0, and could change
at any time.
Assigning Magic
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 con-
tinues 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 overrid-
den, 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 occasion-
ally 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);
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 vari-
able.
The "MGVTBL" has five pointers to the following routine
types:
int (*svt_get)(SV* sv, MAGIC* mg);
int (*svt_set)(SV* sv, MAGIC* mg);
U32 (*svt_len)(SV* sv, MAGIC* mg);
int (*svt_clear)(SV* sv, MAGIC* mg);
int (*svt_free)(SV* sv, MAGIC* mg);
This MGVTBL structure is set at compile-time in perl.h and
there are currently 19 types (or 21 with overloading
turned on). These different structures contain pointers
to various routines that perform additional actions
depending on which function is being called.
Function pointer Action taken
---------------- ------------
svt_get Do something before the value of the SV is retrieved.
svt_set Do something after the SV is assigned a value.
svt_len Report on the SV's length.
svt_clear Clear something the SV represents.
svt_free Free any extra storage associated with the SV.
For instance, the MGVTBL structure called "vtbl_sv" (which
corresponds to an "mg_type" of "PERL_MAGIC_sv") contains:
{ magic_get, magic_set, magic_len, 0, 0 }
Thus, when an SV is determined to be magical and of type
"PERL_MAGIC_sv", if a get operation is being performed,
the routine "magic_get" is called. All the various rou-
tines for the various magical types begin with "magic_".
NOTE: the magic routines are not considered part of the
Perl API, and may not be exported by the Perl library.
The current kinds of Magic Virtual Tables are:
mg_type
(old-style char and macro) MGVTBL Type of magic
-------------------------- ------ ----------------------------
\0 PERL_MAGIC_sv vtbl_sv Special scalar variable
A PERL_MAGIC_overload vtbl_amagic %OVERLOAD hash
a PERL_MAGIC_overload_elem vtbl_amagicelem %OVERLOAD hash element
c PERL_MAGIC_overload_table (none) Holds overload table (AMT)
on stash
B PERL_MAGIC_bm vtbl_bm Boyer-Moore (fast string search)
D PERL_MAGIC_regdata vtbl_regdata Regex match position data
(@+ and @- vars)
d PERL_MAGIC_regdatum vtbl_regdatum Regex match position data
element
E PERL_MAGIC_env vtbl_env %ENV hash
e PERL_MAGIC_envelem vtbl_envelem %ENV hash element
f PERL_MAGIC_fm vtbl_fm Formline ('compiled' format)
g PERL_MAGIC_regex_global vtbl_mglob m//g target / study()ed string
I PERL_MAGIC_isa vtbl_isa @ISA array
i PERL_MAGIC_isaelem vtbl_isaelem @ISA array element
k PERL_MAGIC_nkeys vtbl_nkeys scalar(keys()) lvalue
L PERL_MAGIC_dbfile (none) Debugger %_ $c ---> + ---> $a ---> assign-to
But with the actual compile tree for "$a = $b + $c" it is
different: some nodes optimized away. As a corollary,
though the actual tree contains more nodes than our sim-
plified example, the execution order is the same as in our
example.
Examining the tree
If you have your perl compiled for debugging (usually done
with "-DDEBUGGING" on the "Configure" command line), you
may examine the compiled tree by specifying "-Dx" on the
Perl command line. The output takes several lines per
node, and for "$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)
{
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 corre-
spond 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 opera-
tion 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 differ-
ent 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 vari-
ous 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 opera-
tors ("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 regu-
lar 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 opti-
mization (see "Compile pass 2: context propagation") it
will still have children in accordance with its former
type.
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 bot-
tom-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 con-
structed except for the execution-order thread. Since at
this time there are no back-links to the currently con-
structed 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 mod-
ified, 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 con-
text 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 con-
tains back-references (via "thread" pointers), nodes can-
not be free()d now. To allow optimized-away nodes at this
stage, such nodes are null()ified instead of free()ing
(i.e. their type is changed to OP_NULL).
Compile pass 3: peephole optimization
After the compile tree for a subroutine (or for an "eval"
or a file) is created, an additional pass over the code is
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 sub-
routine 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 con-
trol 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)
(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.
Two macros control the major Perl build flavors: MULTI-
PLICITY and USE_5005THREADS. The MULTIPLICITY build has a
C structure that packages all the interpreter state, and
there is a similar thread-specific data structure under
USE_5005THREADS. In both cases, PERL_IMPLICIT_CONTEXT is
also normally defined, and enables the support for passing
in a "hidden" first argument that represents all three
data structures.
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
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 contain-
ing 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 addi-
tional explicit arguments.
When a core function calls another, it must pass the con-
text. 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 argu-
ment. Instead it does dTHX; to get the context from
thread-local storage. We "#define warner
Perl_warner_nocontext" so that extensions get source com-
patibility 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 head-
ers/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:
#define PERL_NO_GET_CONTEXT /* we want efficiency */
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
static my_private_function(int arg1, int arg2);
static SV *
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 my_private_function(pTHX_ int arg1, int arg2);
static SV *
my_private_function(pTHX_ int arg1, int arg2)
{
/* dTHX; not needed here, because THX is an argument */
... 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 argu-
ment. Depending on your needs for simplicity or effi-
ciency, 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.
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 auto-
matically set the TLS slot to the interpreter they cre-
ated, 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 inter-
preter. This is done by calling the "PERL_SET_CONTEXT"
macro in that thread as the first thing you do:
/* 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 inter-
preter to bundle up everything it knows about the environ-
ment it's running on. This is enabled with the
PERL_IMPLICIT_SYS macro. Currently it only works with
USE_ITHREADS and USE_5005THREADS on Windows (see inside
iperlsys.h).
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 main-
tain 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 dif-
ferent 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 func-
tions start with "S_".)
Inside the Perl core, you can get at the functions either
with or without the "Perl_" prefix, thanks to a bunch of
defines that live in embed.h. This header file is gener-
ated 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 sam-
ple entry from that table:
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 col-
umn 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" fea-
ture 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 defi-
nition has no "pTHX", and it follows that callers don't
use "aTHX". (See "Background and PERL_IMPLICIT_CON-
TEXT" 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
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 com-
bined 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);
Source Documentation
There's an effort going on to document the internal func-
tions 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.
=cut
*/
Please try and supply some documentation if you add func-
tions 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 vari-
ants 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 rep-
resenting these characters, and the one Perl uses is
called UTF-8. UTF-8 uses a variable number of bytes to
represent a character, instead of just one. You can learn
more about Unicode at http://www.unicode.org/
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 rep-
resented 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.
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 1...128 are
stored in one byte, just like good ol' ASCII. Character
129 is stored as "v194.129"; this continues up to charac-
ter 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_INVARI-
ANT() is a macro that tests whether the byte can be
encoded as a single byte even in UTF-8):
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 func-
tions 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. If a string has been identi-
fied as being UTF-8 encoded, Perl will set a flag in the
SV, "SVf_UTF8". 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 is UTF-8, and contains a byte sequence
that could be UTF-8 - especially when combining non-UTF-8
and 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 UTF-8 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 UTF-8 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, you might
find it 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.
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 repre-
sented 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. There are tricks you can use to delay deciding
whether you need to use a UTF-8 string until you get to
a high character - "HALF_UPGRADE" is one of those.
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 func-
tions 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 optimiza-
tions. 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, or by adding a custom peephole optimizer with the
"optimize" module.
When you do this, you replace ordinary Perl ops with cus-
tom 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 inter-
preter 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_cus-
tom_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.
Forthcoming versions of "B::Generate" (version 1.0 and
above) should directly support the creation of custom ops
by name.
AUTHORS
Until May 1997, this document was maintained by Jeff
Okamoto . It is now maintained as
part of Perl itself by the Perl 5 Porters
.
With lots of help and suggestions from Dean Roehrich, Mal-
colm Beattie, Andreas Koenig, Paul Hudson, Ilya Zakhare-
vich, Paul Marquess, Neil Bowers, Matthew Green, Tim
Bunce, Spider Boardman, Ulrich Pfeifer, Stephen McCamant,
and Gurusamy Sarathy.
SEE ALSO
perlapi(1), perlintern(1), perlxs(1), perlembed(1)
perl v5.8.8 2006-01-07 PERLGUTS(1)