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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 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 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 the undef value.
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, and
not otherwise containing NULs.
The arguments of "sv_setpvf" are processed like "sprintf", and the formatted output becomes the
value.
"sv_vsetpvfn" is an analogue of "vsprintf", but it allows you to specify either a pointer to a
variable argument list or the address and length of an array of SVs. The last argument points to a
boolean; on return, if that boolean is true, then locale-specific information has been used to format
the string, and the string's contents are therefore untrustworthy (see perlsec). This pointer may be
NULL if that information is not important. Note that this function requires you to specify the
length of the format.
The "sv_set*()" functions are not generic enough to operate on values that have "magic". See "Magic
Virtual Tables" later in this document.
All SVs that contain strings should be terminated with a NUL character. If it is not NUL-terminated
there is a risk of core dumps and corruptions from code which passes the string to C functions or
system calls which expect a NUL-terminated string. Perl's own functions typically add a trailing NUL
for this reason. Nevertheless, you should be very careful when you pass a string stored in an SV to
a C function or system call.
To access the actual value that an SV points to, you can use the macros:
SvIV(SV*)
SvUV(SV*)
SvNV(SV*)
SvPV(SV*, STRLEN len)
SvPV_nolen(SV*)
which will automatically coerce the actual scalar type into an IV, UV, double, or string.
In the "SvPV" macro, the length of the string returned is placed into the variable "len" (this is a
macro, so you do not use &len). If you do not care what the length of the data is, use the
"SvPV_nolen" macro. Historically the "SvPV" macro with the global variable "PL_na" has been used in
this case. But that can be quite inefficient because "PL_na" must be accessed in thread-local
storage in threaded Perl. In any case, remember that Perl allows arbitrary strings of data that may
both contain NULs and might not be terminated by a NUL.
Also remember that C doesn't allow you to safely say "foo(SvPV(s, len), len);". It might work with
your compiler, but it won't work for everyone. Break this sort of statement up into separate
assignments:
SV *s;
STRLEN len;
char *ptr;
ptr = SvPV(s, len);
foo(ptr, len);
If you want to know if the scalar value is TRUE, you can use:
SvTRUE(SV*)
Although Perl will automatically grow strings for you, if you need to force Perl to allocate more
memory for your SV, you can use the macro
SvGROW(SV*, STRLEN newlen)
which will determine if more memory needs to be allocated. If so, it will call the function
"sv_grow". Note that "SvGROW" can only increase, not decrease, the allocated memory of an SV and
that it does not automatically add space for the trailing NUL byte (perl's own string functions
typically do "SvGROW(sv, len + 1)").
If you have an SV and want to know what kind of data Perl thinks is stored in it, you can use the
following macros to check the type of SV you have.
SvIOK(SV*)
SvNOK(SV*)
SvPOK(SV*)
You can get and set the current length of the string stored in an SV with the following macros:
SvCUR(SV*)
SvCUR_set(SV*, I32 val)
You can also get a pointer to the end of the string stored in the SV with the macro:
SvEND(SV*)
But note that these last three macros are valid only if "SvPOK()" is true.
If you want to append something to the end of string stored in an "SV*", you can use the following
functions:
void sv_catpv(SV*, const char*);
void sv_catpvn(SV*, const char*, STRLEN);
void sv_catpvf(SV*, const char*, ...);
void sv_vcatpvfn(SV*, const char*, STRLEN, va_list *, SV **,
I32, bool);
void sv_catsv(SV*, SV*);
The first function calculates the length of the string to be appended by using "strlen". In the
second, you specify the length of the string yourself. The third function processes its arguments
like "sprintf" and appends the formatted output. The fourth function works like "vsprintf". You can
specify the address and length of an array of SVs instead of the va_list argument. The fifth function
extends the string stored in the first SV with the string stored in the second SV. It also forces
the second SV to be interpreted as a string.
The "sv_cat*()" functions are not generic enough to operate on values that have "magic". See "Magic
Virtual Tables" later in this document.
If you know the name of a scalar variable, you can get a pointer to its SV by using the following:
SV* get_sv("package::varname", 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
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.
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.
There 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 it:
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 an 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.
A few more:
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.
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 it:
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.
The first of these two functions checks if a hash table entry exists, and the second 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 a 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", 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);
SV* hv_iterkeysv (HE* entry);
Note that these functions take "SV*" keys, which simplifies writing of extension code that deals with
hash structures. These functions also allow passing of "SV*" keys to "tie" functions without forcing
you to stringify the keys (unlike the previous set of functions).
They also return and accept whole hash entries ("HE*"), making their use more efficient (since the
hash number for a particular string doesn't have to be recomputed every time). See perlapi for
detailed descriptions.
The following macros must always be used to access the contents of hash entries. Note that the
arguments to these macros must be simple variables, since they may get evaluated more than once. See
perlapi for detailed descriptions of these macros.
HePV(HE* he, STRLEN len)
HeVAL(HE* he)
HeHASH(HE* he)
HeSVKEY(HE* he)
HeSVKEY_force(HE* he)
HeSVKEY_set(HE* he, SV* sv)
These two lower level macros are defined, but must only be used when dealing with keys that are not
"SV*"s:
HeKEY(HE* he)
HeKLEN(HE* he)
Note that both "hv_store" and "hv_store_ent" do not increment the reference count of the stored
"val", which is the caller's responsibility. If these functions return a NULL value, the caller will
usually have to decrement the reference count of "val" to avoid a memory leak.
AVs, HVs and undefined values
Sometimes you have to store undefined values in AVs or HVs. Although this may be a rare case, it can
be tricky. That's because you're used to using &PL_sv_undef if you need an undefined SV.
For example, intuition tells you that this XS code:
AV *av = newAV();
av_store( av, 0, &PL_sv_undef );
is equivalent to this Perl code:
my @av;
$av[0] = undef;
Unfortunately, this isn't true. AVs use &PL_sv_undef as a marker for indicating that an array element
has not yet been initialized. Thus, "exists $av[0]" would be true for the above Perl code, but false
for the array generated by the XS code.
Other problems can occur when storing &PL_sv_undef in HVs:
hv_store( hv, "key", 3, &PL_sv_undef, 0 );
This will indeed make the value "undef", but if you try to modify the value of "key", you'll get the
following error:
Modification of non-creatable hash value attempted
In perl 5.8.0, &PL_sv_undef was also used to mark placeholders in restricted hashes. This caused such
hash entries not to appear when iterating over the hash or when checking for the keys with the
"hv_exists" function.
You can run into similar problems when you store &PL_sv_yes or &PL_sv_no 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_yes and &PL_sv_no
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 other references).
To create a reference, use either of the following functions:
SV* newRV_inc((SV*) thing);
SV* newRV_noinc((SV*) thing);
The "thing" argument can be any of an "SV*", "AV*", or "HV*". The functions are identical except
that "newRV_inc" increments the reference count of the "thing", while "newRV_noinc" does not. For
historical reasons, "newRV" is a synonym for "newRV_inc".
Once you have a reference, you can use the following macro to dereference the reference:
SvRV(SV*)
then call the appropriate routines, casting the returned "SV*" to either an "AV*" or "HV*", if
required.
To determine if an SV is a reference, you can use the following macro:
SvROK(SV*)
To discover what type of value the reference refers to, use the following macro and then check the
return value.
SvTYPE(SvRV(SV*))
The most useful types that will be returned are:
SVt_IV Scalar
SVt_NV Scalar
SVt_PV Scalar
SVt_RV Scalar
SVt_PVAV Array
SVt_PVHV Hash
SVt_PVCV Code
SVt_PVGV Glob (possibly 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. See "Stashes and Globs" for information on converting class names into
stashes.
/* Still under construction */
The following function upgrades rv to reference if not already one. Creates a 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);
The following three functions copy 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);
The following function 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, void* pv);
The following function copies a 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, char* pv,
STRLEN length);
The following function tests whether the SV is blessed into the specified class. It does not check
inheritance relationships.
int sv_isa(SV* sv, const char* name);
The following function tests whether the SV is a reference to a blessed object.
int sv_isobject(SV* sv);
The following function 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", GV_ADD);
AV* get_av("package::varname", GV_ADD);
HV* get_hv("package::varname", GV_ADD);
Notice the use of GV_ADD 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 "GV_ADD" 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);
However, there is one other function which manipulates the reference count of its argument. The
"newRV_inc" function, you will recall, creates a reference to the specified argument. As a side
effect, it increments the argument's reference count. If this is not what you want, use
"newRV_noinc" instead.
For example, imagine you want to return a reference from an XSUB function. Inside the XSUB routine,
you create an SV which initially has a reference count of one. Then you call "newRV_inc", passing it
the just-created SV. This returns the reference as a new SV, but the reference count of the SV you
passed to "newRV_inc" has been incremented to two. Now you return the reference from the XSUB
routine and forget about the SV. But Perl hasn't! Whenever the returned reference is destroyed, the
reference count of the original SV is decreased to one and nothing happens. The SV will hang around
without any way to access it until Perl itself terminates. This is a memory leak.
The correct procedure, then, is to use "newRV_noinc" instead of "newRV_inc". Then, if and when the
last reference is destroyed, the reference count of the SV will go to zero and it will be destroyed,
stopping any memory leak.
There are some convenience functions available that can help with the destruction of xVs. These
functions introduce the concept of "mortality". An xV that is mortal has had its reference count
marked to be decremented, but not actually decremented, until "a short time later". Generally the
term "short time later" means a single Perl statement, such as a call to an XSUB function. The
actual determinant for when mortal xVs have their reference count decremented depends on two macros,
SAVETMPS and FREETMPS. See perlcall and perlxs for more details on these macros.
"Mortalization" then is at its simplest a deferred "SvREFCNT_dec". However, if you mortalize a
variable twice, the reference count will later be decremented twice.
"Mortal" SVs are mainly used for SVs that are placed on perl's stack. For example an SV which is
created just to pass a number to a called sub is made mortal to have it cleaned up automatically when
it's popped off the stack. Similarly, results returned by XSUBs (which are pushed on the stack) are
often made mortal.
To create a mortal variable, use the functions:
SV* sv_newmortal()
SV* sv_2mortal(SV*)
SV* sv_mortalcopy(SV*)
The first call creates a mortal SV (with no value), the second converts an existing SV to a mortal SV
(and thus defers a call to "SvREFCNT_dec"), and the third creates a mortal copy of an existing SV.
Because "sv_newmortal" gives the new SV no value, it must normally be given one via "sv_setpv",
"sv_setiv", etc. :
SV *tmp = sv_newmortal();
sv_setiv(tmp, an_integer);
As that is multiple C statements it is quite common so see this idiom instead:
SV *tmp = sv_2mortal(newSViv(an_integer));
You should be careful about creating mortal variables. Strange things can happen if you make the
same value mortal within multiple contexts, or if you make a variable mortal multiple times. Thinking
of "Mortalization" as deferred "SvREFCNT_dec" should help to minimize such problems. For example if
you are passing an SV which you know has a 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.
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 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.
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);
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:
int sv_unmagic(SV *sv, int type);
The "type" argument should be equal to the "how" value when the "SV" was initially made magical.
However, note that "sv_unmagic" removes all magic of a certain "type" from the "SV". If you want to
remove only certain magic of a "type" based on the magic virtual table, use "sv_unmagicext" instead:
int sv_unmagicext(SV *sv, int type, MGVTBL *vtbl);
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 -----------svt_get
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 }
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 ------------\0
\0 PERL_MAGIC_sv vtbl_sv Special scalar variable
# PERL_MAGIC_arylen vtbl_arylen Array length ($#ary)
% PERL_MAGIC_rhash (none) extra data for restricted
hashes
. PERL_MAGIC_pos vtbl_pos pos() lvalue
: PERL_MAGIC_symtab (none) extra data for symbol
tables
< PERL_MAGIC_backref vtbl_backref for weak ref data
@ PERL_MAGIC_arylen_p (none) to move arylen out of
XPVAV
A PERL_MAGIC_overload vtbl_amagic %OVERLOAD hash
a PERL_MAGIC_overload_elem vtbl_amagicelem %OVERLOAD hash element
B PERL_MAGIC_bm vtbl_regexp Boyer-Moore
(fast string search)
c PERL_MAGIC_overload_table vtbl_ovrld Holds overload table
(AMT) on stash
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_regdata Formline
('compiled' format)
G PERL_MAGIC_study vtbl_regexp study()ed string
g PERL_MAGIC_regex_global vtbl_mglob m//g target
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
N PERL_MAGIC_shared (none) Shared between threads
n PERL_MAGIC_shared_scalar (none) Shared between threads
o PERL_MAGIC_collxfrm vtbl_collxfrm Locale 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_regexp precompiled qr// regex
S PERL_MAGIC_sig (none) %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
u PERL_MAGIC_uvar_elem (none) Reserved for use by
extensions
V PERL_MAGIC_vstring vtbl_vstring SV was vstring literal
v PERL_MAGIC_vec vtbl_vec vec() lvalue
w PERL_MAGIC_utf8 vtbl_utf8 Cached UTF-8 information
x PERL_MAGIC_substr vtbl_substr substr() lvalue
y PERL_MAGIC_defelem vtbl_defelem Shadow "foreach" iterator
variable / smart parameter
vivification
] PERL_MAGIC_checkcall (none) inlining/mutation of call
to this CV
~ PERL_MAGIC_ext (none) Available for use by
extensions
When an uppercase and lowercase letter both exist in the table, then the uppercase letter is
typically used to represent some kind of composite type (a list or a hash), and the lowercase letter
is used to represent an element of that composite type. Some internals code makes use of this case
relationship. However, 'v' and 'V' (vec and v-string) are in no way related.
The "PERL_MAGIC_ext" and "PERL_MAGIC_uvar" magic types are defined specifically for use by extensions
and will not be used by perl itself. Extensions can use "PERL_MAGIC_ext" magic to 'attach' private
information to variables (typically objects). This is especially useful because there is no way for
normal perl code to corrupt this private information (unlike using extra elements of a hash object).
Similarly, "PERL_MAGIC_uvar" magic can be used much like tie() to call a C function any time a
scalar's value is used or changed. The "MAGIC"'s "mg_ptr" field points to a "ufuncs" structure:
struct ufuncs {
I32 (*uf_val)(pTHX_ IV, SV*);
I32 (*uf_set)(pTHX_ IV, SV*);
IV uf_index;
};
When the SV is read from or written to, the "uf_val" or "uf_set" function will be called with
"uf_index" as the first arg and a pointer to the SV as the second. A simple example of how to add
"PERL_MAGIC_uvar" magic is shown below. Note that the ufuncs structure is copied by sv_magic, so you
can safely allocate it on the stack.
void
Umagic(sv)
SV *sv;
PREINIT:
struct ufuncs uf;
CODE:
uf.uf_val = &my_get_fn;
uf.uf_set = &my_set_fn;
uf.uf_index = 0;
sv_magic(sv, 0, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf));
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
is usually a good idea to define an "MGVTBL", even if all its fields will be 0, so that individual
"MAGIC" pointers can be identified as a particular kind of magic using their magic virtual table.
"mg_findext" provides an easy way to do that:
STATIC MGVTBL my_vtbl = { 0, 0, 0, 0, 0, 0, 0, 0 };
MAGIC *mg;
if ((mg = mg_findext(sv, PERL_MAGIC_ext, &my_vtbl))) {
/* this is really ours, not another module's PERL_MAGIC_ext */
my_priv_data_t *priv = (my_priv_data_t *)mg->mg_ptr;
...
}
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 *sv, int type); /* Finds the magic pointer of that
* type */
This routine returns a pointer to a "MAGIC" structure stored in the SV. If the SV does not have that
magical feature, "NULL" is returned. If the SV has multiple instances of that magical feature, the
first one will be returned. "mg_findext" can be used to find a "MAGIC" structure of an SV based on
both its magic type and its magic virtual table:
MAGIC *mg_findext(SV *sv, int type, MGVTBL *vtbl);
Also, if the SV passed to "mg_find" or "mg_findext" 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
The "av_store" function, when given a tied array argument, merely copies the magic of the array onto
the value to be "stored", using "mg_copy". It may also return NULL, indicating that the value did
not actually need to be stored in the array. [MAYCHANGE] After a call to "av_store" on a tied array,
the caller will usually need to call "mg_set(val)" to actually invoke the perl level "STORE" method
on the TIEARRAY object. If "av_store" did return NULL, a call to "SvREFCNT_dec(val)" will also be
usually necessary to avoid a memory leak. [/MAYCHANGE]
The previous paragraph is applicable verbatim to tied hash access using the "hv_store" and
"hv_store_ent" functions as well.
"av_fetch" and the corresponding hash functions "hv_fetch" and "hv_fetch_ent" actually return an
undefined mortal value whose magic has been initialized using "mg_copy". Note the value so returned
does not need to be deallocated, as it is already mortal. [MAYCHANGE] But you will need to call
"mg_get()" on the returned value in order to actually invoke the perl level "FETCH" method on the
underlying TIE object. Similarly, you may also call "mg_set()" on the return value after possibly
assigning a suitable value to it using "sv_setsv", which will invoke the "STORE" method on the TIE
object. [/MAYCHANGE]
[MAYCHANGE] In other words, the array or hash fetch/store functions don't really fetch and store
actual values in the case of tied arrays and hashes. They merely call "mg_copy" to attach magic to
the values that were meant to be "stored" or "fetched". Later calls to "mg_get" and "mg_set"
actually do the job of invoking the TIE methods on the underlying objects. Thus the magic mechanism
currently implements a kind of lazy access to arrays and hashes.
Currently (as of perl version 5.004), use of the hash and array access functions requires the user to
be aware of whether they are operating on "normal" hashes and arrays, or on their tied variants. The
API may be changed to provide more transparent access to both tied and normal data types in future
versions. [/MAYCHANGE]
You would do well to understand that the TIEARRAY and TIEHASH interfaces are mere sugar to invoke
some perl method calls while using the uniform hash and array syntax. The use of this sugar imposes
some overhead (typically about two to four extra opcodes per FETCH/STORE operation, in addition to
the creation of all the mortal variables required to invoke the methods). This overhead will be
comparatively small if the TIE methods are themselves substantial, but if they are only a few
statements long, the overhead will not be insignificant.
Localizing changes
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 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.
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
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)))
/* Although the last example is better written as the more
* efficient: */
PUSHs(newSVpvs_flags("Some String", SVs_TEMP))
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 adjusts 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.
Autoloading with XSUBs
If an AUTOLOAD routine is an XSUB, as with Perl subroutines, Perl puts the fully-qualified name of
the autoloaded subroutine in the $AUTOLOAD variable of the XSUB's package.
But it also puts the same information in certain fields of the XSUB itself:
HV *stash = CvSTASH(cv);
const char *subname = SvPVX(cv);
STRLEN name_length = SvCUR(cv); /* in bytes */
U32 is_utf8 = SvUTF8(cv);
"SvPVX(cv)" contains just the sub name itself, not including the package. For an AUTOLOAD routine in
UNIVERSAL or one of its superclasses, "CvSTASH(cv)" returns NULL during a method call on a
nonexistent package.
Note: Setting $AUTOLOAD stopped working in 5.6.1, which did not support XS AUTOLOAD subs at all.
Perl 5.8.0 introduced the use of fields in the XSUB itself. Perl 5.16.0 restored the setting of
$AUTOLOAD. If you need to support 5.8-5.14, use the XSUB's fields.
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**);
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 :
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 have been experimenting with removing Perl's dependency
on the "normal" standard I/O suite and allowing other stdio implementations to be used. This
involves creating a new abstraction layer that then calls whichever implementation of stdio Perl was
compiled with. All XSUBs should now use the functions in the PerlIO abstraction layer and not make
any assumptions about what kind of stdio is being used.
For a complete description of the PerlIO abstraction, consult 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:
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 subroutineparent
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 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)
{
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.
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 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). Optimizations performed at this stage are subject
to the same restrictions as in the pass 2.
Peephole optimizations are done by calling the function pointed to by the global variable "PL_peepp".
By default, "PL_peepp" just calls the function pointed to by the global variable "PL_rpeepp". By
default, that performs some basic op fixups and optimisations along the execution-order op chain, and
recursively calls "PL_rpeepp" for each side chain of ops (resulting from conditionals). Extensions
may provide additional optimisations or fixups, hooking into either the per-subroutine or recursive
stage, like this:
static peep_t prev_peepp;
static void my_peep(pTHX_ OP *o)
{
/* custom per-subroutine optimisation goes here */
prev_peepp(o);
/* custom per-subroutine optimisation may also go here */
}
BOOT:
prev_peepp = PL_peepp;
PL_peepp = my_peep;
static peep_t prev_rpeepp;
static void my_rpeep(pTHX_ OP *o)
{
OP *orig_o = o;
for(; o; o = o->op_next) {
/* custom per-op optimisation goes here */
}
prev_rpeepp(orig_o);
}
BOOT:
prev_rpeepp = PL_rpeepp;
PL_rpeepp = my_rpeep;
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.
Compile-time scope hooks
As of perl 5.14 it is possible to hook into the compile-time lexical scope mechanism using
"Perl_blockhook_register". This is used like this:
STATIC void my_start_hook(pTHX_ int full);
STATIC BHK my_hooks;
BOOT:
BhkENTRY_set(&my_hooks, bhk_start, my_start_hook);
Perl_blockhook_register(aTHX_ &my_hooks);
This will arrange to have "my_start_hook" called at the start of compiling every lexical scope. The
available hooks are:
"void bhk_start(pTHX_ int full)"
This is called just after starting a new lexical scope. Note that Perl code like
if ($x) { ... }
creates two scopes: the first starts at the "(" and has "full == 1", the second starts at the "{"
and has "full == 0". Both end at the "}", so calls to "start" and "pre/post_end" will match.
Anything pushed onto the save stack by this hook will be popped just before the scope ends
(between the "pre_" and "post_end" hooks, in fact).
"void bhk_pre_end(pTHX_ OP **o)"
This is called at the end of a lexical scope, just before unwinding the stack. o is the root of
the optree representing the scope; it is a double pointer so you can replace the OP if you need
to.
"void bhk_post_end(pTHX_ OP **o)"
This is called at the end of a lexical scope, just after unwinding the stack. o is as above. Note
that it is possible for calls to "pre_" and "post_end" to nest, if there is something on the save
stack that calls string eval.
"void bhk_eval(pTHX_ OP *const o)"
This is called just before starting to compile an "eval STRING", "do FILE", "require" or "use",
after the eval has been set up. o is the OP that requested the eval, and will normally be an
"OP_ENTEREVAL", "OP_DOFILE" or "OP_REQUIRE".
Once you have your hook functions, you need a "BHK" structure to put them in. It's best to allocate
it statically, since there is no way to free it once it's registered. The function pointers should be
inserted into this structure using the "BhkENTRY_set" macro, which will also set flags indicating
which entries are valid. If you do need to allocate your "BHK" dynamically for some reason, be sure
to zero it before you start.
Once registered, there is no mechanism to switch these hooks off, so if that is necessary you will
need to do this yourself. An entry in "%^H" is probably the best way, so the effect is lexically
scoped; however it is also possible to use the "BhkDISABLE" and "BhkENABLE" macros to temporarily
switch entries on and off. You should also be aware that generally speaking at least one scope will
have opened before your extension is loaded, so you will see some "pre/post_end" pairs that didn't
have a matching "start".
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.
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" 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 the 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
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. 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 don't have to do anything 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 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 */
... 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:
/* 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:
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 an interpreter context, so the definition has no "pTHX", and it follows that
callers don't use "aTHX". (See "Background and PERL_IMPLICIT_CONTEXT".)
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 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);
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
*/
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 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):
U8 *utf;
U8 *utf_end; /* 1 beyond buffer pointed to by utf */
UV uv; /* Note: a UV, not a U8, not a char */
STRLEN len; /* length of character in bytes */
if (!UTF8_IS_INVARIANT(*utf))
/* Must treat this as UTF-8 */
uv = utf8_to_uvchr_buf(utf, utf_end, &len);
else
/* OK to treat this character as a byte */
uv = *utf;
You can also see in that example that we use "utf8_to_uvchr_buf" to get the value of the character;
the inverse function "uvchr_to_utf8" is available for putting a UV into UTF-8:
if (!UTF8_IS_INVARIANT(uv))
/* Must treat this as UTF8 */
utf8 = uvchr_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 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.
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:
• 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
its "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.
• If a string is UTF-8, always use "utf8_to_uvchr_buf" to get at the value, unless
"UTF8_IS_INVARIANT(*s)" in which case you can use *s.
• When writing a character "uv" to a UTF-8 string, always use "uvchr_to_utf8", unless
"UTF8_IS_INVARIANT(uv))" in which case you can use "*s = uv".
• 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, 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" to determine which custom op it is dealing
with. You should create an "XOP" structure for each ppaddr you use, set the properties of the custom
op with "XopENTRY_set", and register the structure against the ppaddr using
"Perl_custom_op_register". A trivial example might look like:
static XOP my_xop;
static OP *my_pp(pTHX);
BOOT:
XopENTRY_set(&my_xop, xop_name, "myxop");
XopENTRY_set(&my_xop, xop_desc, "Useless custom op");
Perl_custom_op_register(aTHX_ my_pp, &my_xop);
The available fields in the structure are:
xop_name
A short name for your op. This will be included in some error messages, and will also be returned
as "$op->name" by the B module, so it will appear in the output of module like B::Concise.
xop_desc
A short description of the function of the op.
xop_class
Which of the various *OP structures this op uses. This should be one of the "OA_*" constants from
op.h, namely
OA_BASEOP
OA_UNOP
OA_BINOP
OA_LOGOP
OA_LISTOP
OA_PMOP
OA_SVOP
OA_PADOP
OA_PVOP_OR_SVOP
This should be interpreted as '"PVOP"' only. The "_OR_SVOP" is because the only core "PVOP",
"OP_TRANS", can sometimes be a "SVOP" instead.
OA_LOOP
OA_COP
The other "OA_*" constants should not be used.
xop_peep
This member is of type "Perl_cpeep_t", which expands to "void (*Perl_cpeep_t)(aTHX_ OP *o, OP
*oldop)". If it is set, this function will be called from "Perl_rpeep" when ops of this type are
encountered by the peephole optimizer. o is the OP that needs optimizing; oldop is the previous
OP optimized, whose "op_next" points to o.
"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.
SEE ALSO
perlapi, perlintern, perlxs, perlembed
perl v5.16.2 2012-10-25 PERLGUTS(1)
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