perlguts - Introduction to the Perl API
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.
Perl has three typedefs that handle Perl's three main data types:
- SV Scalar Value
- AV Array Value
- HV Hash Value
Each typedef has specific routines that manipulate the various data types.
Perl uses a special typedef IV which is a simple signed integer type that is guaranteed to be large enough to hold a pointer (as well as an integer). Additionally, there is the UV, which is simply an unsigned IV.
Perl also uses two special typedefs, I32 and I16, which will always be at least 32-bits and 16-bits long, respectively. (Again, there are U32 and U16, as well.) They will usually be exactly 32 and 16 bits long, but on Crays they will both be 64 bits.
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 value undef.
- SV *sv = newSV(0); /* no storage allocated */
- SV *sv = newSV(10); /* 10 (+1) bytes of uninitialised storage allocated */
To change the value of an already-existing SV, there are eight routines:
- void sv_setiv(SV*, IV);
- void sv_setuv(SV*, UV);
- void sv_setnv(SV*, double);
- void sv_setpv(SV*, const char*);
- void sv_setpvn(SV*, const char*, STRLEN)
- void sv_setpvf(SV*, const char*, ...);
- void sv_vsetpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool *);
- void sv_setsv(SV*, SV*);
Notice that you can choose to specify the length of the string to be
assigned by using sv_setpvn
, newSVpvn
, or newSVpv
, or you may
allow Perl to calculate the length by using sv_setpv
or by specifying
0 as the second argument to newSVpv
. Be warned, though, that Perl will
determine the string's length by using strlen
, which depends on the
string terminating with a NUL character.
The arguments of sv_setpvf
are processed like sprintf, and the
formatted output becomes the value.
sv_vsetpvfn
is an analogue of vsprintf
, but it allows you to specify
either a pointer to a variable argument list or the address and length of
an array of SVs. The last argument points to a boolean; on return, if that
boolean is true, then locale-specific information has been used to format
the string, and the string's contents are therefore untrustworthy (see
perlsec). This pointer may be NULL if that information is not
important. Note that this function requires you to specify the length of
the format.
The sv_set*()
functions are not generic enough to operate on values
that have "magic". See Magic Virtual Tables later in this document.
All SVs that contain strings should be terminated with a NUL character. If it is not NUL-terminated there is a risk of core dumps and corruptions from code which passes the string to C functions or system calls which expect a NUL-terminated string. Perl's own functions typically add a trailing NUL for this reason. Nevertheless, you should be very careful when you pass a string stored in an SV to a C function or system call.
To access the actual value that an SV points to, you can use the macros:
- SvIV(SV*)
- SvUV(SV*)
- SvNV(SV*)
- SvPV(SV*, STRLEN len)
- SvPV_nolen(SV*)
which will automatically coerce the actual scalar type into an IV, UV, double, or string.
In the SvPV
macro, the length of the string returned is placed into the
variable len
(this is a macro, so you do not use &len
). If you do
not care what the length of the data is, use the SvPV_nolen
macro.
Historically the SvPV
macro with the global variable PL_na
has been
used in this case. But that can be quite inefficient because PL_na
must
be accessed in thread-local storage in threaded Perl. In any case, remember
that Perl allows arbitrary strings of data that may both contain NULs and
might not be terminated by a NUL.
Also remember that C doesn't allow you to safely say foo(SvPV(s, len),
len);. It might work with your compiler, but it won't work for everyone.
Break this sort of statement up into separate assignments:
- SV *s;
- STRLEN len;
- char * ptr;
- ptr = SvPV(s, len);
- foo(ptr, len);
If you want to know if the scalar value is TRUE, you can use:
- SvTRUE(SV*)
Although Perl will automatically grow strings for you, if you need to force Perl to allocate more memory for your SV, you can use the macro
- SvGROW(SV*, STRLEN newlen)
which will determine if more memory needs to be allocated. If so, it will
call the function sv_grow
. Note that SvGROW
can only increase, not
decrease, the allocated memory of an SV and that it does not automatically
add a byte for the a trailing NUL (perl's own string functions typically do
SvGROW(sv, len + 1)
).
If you have an SV and want to know what kind of data Perl thinks is stored in it, you can use the following macros to check the type of SV you have.
- SvIOK(SV*)
- SvNOK(SV*)
- SvPOK(SV*)
You can get and set the current length of the string stored in an SV with the following macros:
- SvCUR(SV*)
- SvCUR_set(SV*, I32 val)
You can also get a pointer to the end of the string stored in the SV with the macro:
- SvEND(SV*)
But note that these last three macros are valid only if SvPOK()
is true.
If you want to append something to the end of string stored in an SV*
,
you can use the following functions:
- void sv_catpv(SV*, const char*);
- void sv_catpvn(SV*, const char*, STRLEN);
- void sv_catpvf(SV*, const char*, ...);
- void sv_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", FALSE);
This returns NULL if the variable does not exist.
If you want to know if this variable (or any other SV) is actually defined,
you can call:
- SvOK(SV*)
The scalar undef value is stored in an SV instance called PL_sv_undef
.
Its address can be used whenever an SV*
is needed. Make sure that
you don't try to compare a random sv with &PL_sv_undef
. For example
when interfacing Perl code, it'll work correctly for:
- foo(undef);
But won't work when called as:
- $x = undef;
- foo($x);
So to repeat always use SvOK() to check whether an sv is defined.
Also you have to be careful when using &PL_sv_undef
as a value in
AVs or HVs (see AVs, HVs and undefined values).
There are also the two values PL_sv_yes
and PL_sv_no
, which contain
boolean TRUE and FALSE values, respectively. Like PL_sv_undef
, their
addresses can be used whenever an SV*
is needed.
Do not be fooled into thinking that (SV *) 0 is the same as &PL_sv_undef
.
Take this code:
- SV* sv = (SV*) 0;
- if (I-am-to-return-a-real-value) {
- sv = sv_2mortal(newSViv(42));
- }
- sv_setsv(ST(0), sv);
This code tries to return a new SV (which contains the value 42) if it should
return a real value, or undef otherwise. Instead it has returned a NULL
pointer which, somewhere down the line, will cause a segmentation violation,
bus error, or just weird results. Change the zero to &PL_sv_undef
in the
first line and all will be well.
To free an SV that you've created, call SvREFCNT_dec(SV*)
. Normally this
call is not necessary (see Reference Counts and Mortality).
Perl provides the function sv_chop
to efficiently remove characters
from the beginning of a string; you give it an SV and a pointer to
somewhere inside the 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 AvLEN
.
Again, the location of the real start of the C array only comes into
play when freeing the array. See av_shift
in av.c.
Recall that the usual method of determining the type of scalar you have is
to use Sv*OK
macros. Because a scalar can be both a number and a string,
usually these macros will always return TRUE and calling the Sv*V
macros will do the appropriate conversion of string to integer/double or
integer/double to string.
If you really need to know if you have an integer, double, or string pointer in an SV, you can use the following three macros instead:
- SvIOKp(SV*)
- SvNOKp(SV*)
- SvPOKp(SV*)
These will tell you if you truly have an integer, double, or string pointer stored in your SV. The "p" stands for private.
The are various ways in which the private and public flags may differ. For example, a tied SV may have a valid underlying value in the IV slot (so SvIOKp is true), but the data should be accessed via the FETCH routine rather than directly, so SvIOK is false. Another is when numeric conversion has occurred and precision has been lost: only the private flag is set on 'lossy' values. So when an NV is converted to an IV with loss, SvIOKp, SvNOKp and SvNOK will be set, while SvIOK wont be.
In general, though, it's best to use the Sv*V
macros.
There are two ways to create and load an AV. The first method creates an empty AV:
- AV* newAV();
The second method both creates the AV and initially populates it with SVs:
- AV* av_make(I32 num, SV **ptr);
The second argument points to an array containing num
SV*
's. Once the
AV has been created, the SVs can be destroyed, if so desired.
Once the AV has been created, the following operations are possible on AVs:
- void av_push(AV*, SV*);
- SV* av_pop(AV*);
- SV* av_shift(AV*);
- void av_unshift(AV*, I32 num);
These should be familiar operations, with the exception of av_unshift
.
This routine adds num
elements at the front of the array with the undef
value. You must then use av_store
(described below) to assign values
to these new elements.
Here are some other functions:
- I32 av_len(AV*);
- SV** av_fetch(AV*, I32 key, I32 lval);
- SV** av_store(AV*, I32 key, SV* val);
The av_len
function returns the highest index value in array (just
like $#array in Perl). If the array is empty, -1 is returned. The
av_fetch
function returns the value at index key
, but if lval
is non-zero, then av_fetch
will store an undef value at that index.
The av_store
function stores the value val
at index key
, and does
not increment the reference count of val
. Thus the caller is responsible
for taking care of that, and if av_store
returns NULL, the caller will
have to decrement the reference count to avoid a memory leak. Note that
av_fetch
and av_store
both return SV**
's, not SV*
's as their
return value.
- void av_clear(AV*);
- void av_undef(AV*);
- void av_extend(AV*, I32 key);
The av_clear
function deletes all the elements in the AV* array, but
does not actually delete the array itself. The av_undef
function will
delete all the elements in the array plus the array itself. The
av_extend
function extends the array so that it contains at least key+1
elements. If key+1
is less than the currently allocated length of the array,
then nothing is done.
If you know the name of an array variable, you can get a pointer to its AV by using the following:
- AV* get_av("package::varname", FALSE);
This returns NULL if the variable does not exist.
See Understanding the Magic of Tied Hashes and Arrays for more information on how to use the array access functions on tied arrays.
To create an HV, you use the following routine:
- HV* newHV();
Once the HV has been created, the following operations are possible on HVs:
- SV** hv_store(HV*, const char* key, U32 klen, SV* val, U32 hash);
- SV** hv_fetch(HV*, const char* key, U32 klen, I32 lval);
The klen
parameter is the length of the key being passed in (Note that
you cannot pass 0 in as a value of klen
to tell Perl to measure the
length of the key). The val
argument contains the SV pointer to the
scalar being stored, and hash
is the precomputed hash value (zero if
you want hv_store
to calculate it for you). The lval
parameter
indicates whether this fetch is actually a part of a store operation, in
which case a new undefined value will be added to the HV with the supplied
key and hv_fetch
will return as if the value had already existed.
Remember that hv_store
and hv_fetch
return SV**
's and not just
SV*
. To access the scalar value, you must first dereference the return
value. However, you should check to make sure that the return value is
not NULL before dereferencing it.
These two functions check if a hash table entry exists, and deletes it.
- bool hv_exists(HV*, const char* key, U32 klen);
- SV* hv_delete(HV*, const char* key, U32 klen, I32 flags);
If flags
does not include the G_DISCARD
flag then hv_delete
will
create and return a mortal copy of the deleted value.
And more miscellaneous functions:
- void hv_clear(HV*);
- void hv_undef(HV*);
Like their AV counterparts, hv_clear
deletes all the entries in the hash
table but does not actually delete the hash table. The hv_undef
deletes
both the entries and the hash table itself.
Perl keeps the actual data in linked list of structures with a typedef of HE.
These contain the actual key and value pointers (plus extra administrative
overhead). The key is a string pointer; the value is an SV*
. However,
once you have an HE*
, to get the actual key and value, use the routines
specified below.
- I32 hv_iterinit(HV*);
- /* Prepares starting point to traverse hash table */
- HE* hv_iternext(HV*);
- /* Get the next entry, and return a pointer to a
- structure that has both the key and value */
- char* hv_iterkey(HE* entry, I32* retlen);
- /* Get the key from an HE structure and also return
- the length of the key string */
- SV* hv_iterval(HV*, HE* entry);
- /* Return an SV pointer to the value of the HE
- structure */
- SV* hv_iternextsv(HV*, char** key, I32* retlen);
- /* This convenience routine combines hv_iternext,
- hv_iterkey, and hv_iterval. The key and retlen
- arguments are return values for the key and its
- length. The value is returned in the SV* argument */
If you know the name of a hash variable, you can get a pointer to its HV by using the following:
- HV* get_hv("package::varname", FALSE);
This returns NULL if the variable does not exist.
The hash algorithm is defined in the PERL_HASH(hash, key, klen)
macro:
- hash = 0;
- while (klen--)
- hash = (hash * 33) + *key++;
- hash = hash + (hash >> 5); /* after 5.6 */
The last step was added in version 5.6 to improve distribution of lower bits in the resulting hash value.
See Understanding the Magic of Tied Hashes and Arrays for more information on how to use the hash access functions on tied hashes.
Beginning with version 5.004, the following functions are also supported:
- HE* hv_fetch_ent (HV* tb, SV* key, I32 lval, U32 hash);
- HE* hv_store_ent (HV* tb, SV* key, SV* val, U32 hash);
- bool hv_exists_ent (HV* tb, SV* key, U32 hash);
- SV* hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash);
- SV* hv_iterkeysv (HE* entry);
Note that these functions take SV*
keys, which simplifies writing
of extension code that deals with hash structures. These functions
also allow passing of SV*
keys to tie functions without forcing
you to stringify the keys (unlike the previous set of functions).
They also return and accept whole hash entries (HE*
), making their
use more efficient (since the hash number for a particular string
doesn't have to be recomputed every time). See 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.
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:
Unfortunately, this isn't true. AVs use &PL_sv_undef
as a marker
for indicating that an array element has not yet been initialized.
Thus, exists $av[0]
would be true for the above Perl code, but
false for the array generated by the XS code.
Other problems can occur when storing &PL_sv_undef
in HVs:
- hv_store( hv, "key", 3, &PL_sv_undef, 0 );
This will indeed make the value undef, but if you try to modify
the value of key
, you'll get the following error:
- Modification of non-creatable hash value attempted
In perl 5.8.0, &PL_sv_undef
was also used to mark placeholders
in restricted hashes. This caused such hash entries not to appear
when iterating over the hash or when checking for the keys
with the hv_exists
function.
You can run into similar problems when you store &PL_sv_true
or
&PL_sv_false
into AVs or HVs. Trying to modify such elements
will give you the following error:
- Modification of a read-only value attempted
To make a long story short, you can use the special variables
&PL_sv_undef
, &PL_sv_true
and &PL_sv_false
with AVs and
HVs, but you have to make sure you know what you're doing.
Generally, if you want to store an undefined value in an AV
or HV, you should not use &PL_sv_undef
, but rather create a
new undefined value using the newSV
function, for example:
- av_store( av, 42, newSV(0) );
- hv_store( hv, "foo", 3, newSV(0), 0 );
References are a special type of scalar that point to other data types (including references).
To create a reference, use either of the following functions:
- SV* newRV_inc((SV*) thing);
- SV* newRV_noinc((SV*) thing);
The thing
argument can be any of an SV*
, AV*
, or HV*
. The
functions are identical except that newRV_inc
increments the reference
count of the thing
, while newRV_noinc
does not. For historical
reasons, newRV
is a synonym for newRV_inc
.
Once you have a reference, you can use the following macro to dereference the reference:
- SvRV(SV*)
then call the appropriate routines, casting the returned SV*
to either an
AV*
or HV*
, if required.
To determine if an SV is a reference, you can use the following macro:
- SvROK(SV*)
To discover what type of value the reference refers to, use the following macro and then check the return value.
- SvTYPE(SvRV(SV*))
The most useful types that will be returned are:
- SVt_IV Scalar
- SVt_NV Scalar
- SVt_PV Scalar
- SVt_RV Scalar
- SVt_PVAV Array
- SVt_PVHV Hash
- SVt_PVCV Code
- SVt_PVGV Glob (possible a file handle)
- SVt_PVMG Blessed or Magical Scalar
- See the sv.h header file for more details.
References are also used to support object-oriented programming. In 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 */
Upgrades rv to reference if not already one. Creates new SV for rv to
point to. If classname
is non-null, the SV is blessed into the specified
class. SV is returned.
- SV* newSVrv(SV* rv, const char* classname);
Copies integer, unsigned integer or double into an SV whose reference is rv
. SV is blessed
if classname
is non-null.
- SV* sv_setref_iv(SV* rv, const char* classname, IV iv);
- SV* sv_setref_uv(SV* rv, const char* classname, UV uv);
- SV* sv_setref_nv(SV* rv, const char* classname, NV iv);
Copies the pointer value (the address, not the string!) into an SV whose
reference is rv. SV is blessed if classname
is non-null.
- SV* sv_setref_pv(SV* rv, const char* classname, PV iv);
Copies string into an SV whose reference is rv
. Set length to 0 to let
Perl calculate the string length. SV is blessed if classname
is non-null.
- SV* sv_setref_pvn(SV* rv, const char* classname, PV iv, STRLEN length);
Tests whether the SV is blessed into the specified class. It does not check inheritance relationships.
- int sv_isa(SV* sv, const char* name);
Tests whether the SV is a reference to a blessed object.
- int sv_isobject(SV* sv);
Tests whether the SV is derived from the specified class. SV can be either
a reference to a blessed object or a string containing a class name. This
is the function implementing the UNIVERSAL::isa
functionality.
- bool sv_derived_from(SV* sv, const char* name);
To check if you've got an object derived from a specific class you have to write:
- if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }
To create a new Perl variable with an undef value which can be accessed from your Perl script, use the following routines, depending on the variable type.
- SV* get_sv("package::varname", TRUE);
- AV* get_av("package::varname", TRUE);
- HV* get_hv("package::varname", TRUE);
Notice the use of TRUE as the second parameter. The new variable can now be set, using the routines appropriate to the data type.
There are additional macros whose values may be bitwise OR'ed with the
TRUE
argument to enable certain extra features. Those bits are:
Marks the variable as multiply defined, thus preventing the:
- Name <varname> used only once: possible typo
warning.
Issues the warning:
- Had to create <varname> unexpectedly
if the variable did not exist before the function was called.
If you do not specify a package name, the variable is created in the current package.
Perl uses 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 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.
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 create)
- HV* gv_stashsv(SV*, I32 create)
The first function takes a literal string, the second uses the string stored
in the SV. Remember that a stash is just a hash table, so you get back an
HV*
. The create
flag will create a new package if it is set.
The name that gv_stash*v
wants is the name of the package whose symbol table
you want. The default package is called main
. If you have multiply nested
packages, pass their names to gv_stash*v
, separated by ::
as in the Perl
language itself.
Alternately, if you have an SV that is a blessed reference, you can find out the stash pointer by using:
- HV* SvSTASH(SvRV(SV*));
then use the following to get the package name itself:
- char* HvNAME(HV* stash);
If you need to bless or re-bless an object you can use the following function:
- SV* sv_bless(SV*, HV* stash)
where the first argument, an SV*
, must be a reference, and the second
argument is a stash. The returned SV*
can now be used in the same way
as any other SV.
For more information on references and blessings, consult perlref.
Scalar variables normally contain only one type of value, an integer, double, pointer, or reference. Perl will automatically convert the actual scalar data from the stored type into the requested type.
Some scalar variables contain more than one type of scalar data. For
example, the variable $!
contains either the numeric value of errno
or its string equivalent from either strerror
or sys_errlist[]
.
To force multiple data values into an SV, you must do two things: use the
sv_set*v
routines to add the additional scalar type, then set a flag
so that Perl will believe it contains more than one type of data. The
four macros to set the flags are:
- SvIOK_on
- SvNOK_on
- SvPOK_on
- SvROK_on
The particular macro you must use depends on which sv_set*v
routine
you called first. This is because every sv_set*v
routine turns on
only the bit for the particular type of data being set, and turns off
all the rest.
For example, to create a new Perl variable called "dberror" that contains both the numeric and descriptive string error values, you could use the following code:
- extern int dberror;
- extern char *dberror_list;
- SV* sv = get_sv("dberror", TRUE);
- sv_setiv(sv, (IV) dberror);
- sv_setpv(sv, dberror_list[dberror]);
- SvIOK_on(sv);
If the order of sv_setiv
and sv_setpv
had been reversed, then the
macro SvPOK_on
would need to be called instead of SvIOK_on
.
[This section still under construction. Ignore everything here. Post no bills. Everything not permitted is forbidden.]
Any SV may be magical, that is, it has special features that a normal
SV does not have. These features are stored in the SV structure in a
linked list of struct magic
's, typedef'ed to MAGIC
.
- struct magic {
- MAGIC* mg_moremagic;
- MGVTBL* mg_virtual;
- U16 mg_private;
- char mg_type;
- U8 mg_flags;
- SV* mg_obj;
- char* mg_ptr;
- I32 mg_len;
- };
Note this is current as of patchlevel 0, and could change at any time.
Perl adds magic to an SV using the sv_magic function:
- void sv_magic(SV* sv, SV* obj, int how, const char* name, I32 namlen);
The sv
argument is a pointer to the SV that is to acquire a new magical
feature.
If sv
is not already magical, Perl uses the SvUPGRADE
macro to
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:
- void sv_unmagic(SV *sv, int type);
The type
argument should be equal to the how
value when the SV
was initially made magical.
The mg_virtual
field in the MAGIC
structure is a pointer to 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 pointers to the following routine types:
- int (*svt_get)(SV* sv, MAGIC* mg);
- int (*svt_set)(SV* sv, MAGIC* mg);
- U32 (*svt_len)(SV* sv, MAGIC* mg);
- int (*svt_clear)(SV* sv, MAGIC* mg);
- int (*svt_free)(SV* sv, MAGIC* mg);
This MGVTBL structure is set at compile-time in perl.h and there are currently 19 types (or 21 with overloading turned on). These different structures contain pointers to various routines that perform additional actions depending on which function is being called.
- Function pointer Action taken
- ---------------- ------------
- svt_get Do something before the value of the SV is retrieved.
- svt_set Do something after the SV is assigned a value.
- svt_len Report on the SV's length.
- svt_clear Clear something the SV represents.
- svt_free Free any extra storage associated with the SV.
For instance, the MGVTBL structure called vtbl_sv
(which corresponds
to an mg_type
of PERL_MAGIC_sv
) contains:
- { magic_get, magic_set, magic_len, 0, 0 }
Thus, when an SV is determined to be magical and of type PERL_MAGIC_sv
,
if a get operation is being performed, the routine magic_get
is
called. All the various routines for the various magical types begin
with magic_
. NOTE: the magic routines are not considered part of
the Perl API, and may not be exported by the Perl library.
The current kinds of Magic Virtual Tables are:
- mg_type
- (old-style char and macro) MGVTBL Type of magic
- -------------------------- ------ ----------------------------
- \0 PERL_MAGIC_sv vtbl_sv Special scalar variable
- A PERL_MAGIC_overload vtbl_amagic %OVERLOAD hash
- a PERL_MAGIC_overload_elem vtbl_amagicelem %OVERLOAD hash element
- c PERL_MAGIC_overload_table (none) Holds overload table (AMT)
- on stash
- B PERL_MAGIC_bm vtbl_bm Boyer-Moore (fast string search)
- D PERL_MAGIC_regdata vtbl_regdata Regex match position data
- (@+ and @- vars)
- d PERL_MAGIC_regdatum vtbl_regdatum Regex match position data
- element
- E PERL_MAGIC_env vtbl_env %ENV hash
- e PERL_MAGIC_envelem vtbl_envelem %ENV hash element
- f PERL_MAGIC_fm vtbl_fm Formline ('compiled' format)
- g PERL_MAGIC_regex_global vtbl_mglob m//g target / study()ed string
- I PERL_MAGIC_isa vtbl_isa @ISA array
- i PERL_MAGIC_isaelem vtbl_isaelem @ISA array element
- k PERL_MAGIC_nkeys vtbl_nkeys scalar(keys()) lvalue
- L PERL_MAGIC_dbfile (none) Debugger %_<filename
- l PERL_MAGIC_dbline vtbl_dbline Debugger %_<filename element
- m PERL_MAGIC_mutex vtbl_mutex ???
- o PERL_MAGIC_collxfrm vtbl_collxfrm Locale collate transformation
- P PERL_MAGIC_tied vtbl_pack Tied array or hash
- p PERL_MAGIC_tiedelem vtbl_packelem Tied array or hash element
- q PERL_MAGIC_tiedscalar vtbl_packelem Tied scalar or handle
- r PERL_MAGIC_qr vtbl_qr precompiled qr// regex
- S PERL_MAGIC_sig vtbl_sig %SIG hash
- s PERL_MAGIC_sigelem vtbl_sigelem %SIG hash element
- t PERL_MAGIC_taint vtbl_taint Taintedness
- U PERL_MAGIC_uvar vtbl_uvar Available for use by extensions
- v PERL_MAGIC_vec vtbl_vec vec() lvalue
- V PERL_MAGIC_vstring (none) v-string scalars
- w PERL_MAGIC_utf8 vtbl_utf8 UTF-8 length+offset cache
- x PERL_MAGIC_substr vtbl_substr substr() lvalue
- y PERL_MAGIC_defelem vtbl_defelem Shadow "foreach" iterator
- variable / smart parameter
- vivification
- * PERL_MAGIC_glob vtbl_glob GV (typeglob)
- # PERL_MAGIC_arylen vtbl_arylen Array length ($#ary)
- . PERL_MAGIC_pos vtbl_pos pos() lvalue
- < PERL_MAGIC_backref vtbl_backref ???
- ~ 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));
Note that because multiple extensions may be using PERL_MAGIC_ext
or PERL_MAGIC_uvar
magic, it is important for extensions to take
extra care to avoid conflict. Typically only using the magic on
objects blessed into the same class as the extension is sufficient.
For PERL_MAGIC_ext
magic, it may also be appropriate to add an I32
'signature' at the top of the private data area and check that.
Also note that the sv_set*()
and sv_cat*()
functions described
earlier do not invoke 'set' magic on their targets. This must
be done by the user either by calling the SvSETMAGIC()
macro after
calling these functions, or by using one of the sv_set*_mg()
or
sv_cat*_mg()
functions. Similarly, generic C code must call the
SvGETMAGIC()
macro to invoke any 'get' magic if they use an SV
obtained from external sources in functions that don't handle magic.
See perlapi for a description of these functions.
For example, calls to the sv_cat*()
functions typically need to be
followed by SvSETMAGIC()
, but they don't need a prior SvGETMAGIC()
since their implementation handles 'get' magic.
- MAGIC* mg_find(SV*, int type); /* Finds the magic pointer of that type */
This routine returns a pointer to the MAGIC
structure stored in the SV.
If the SV does not have that magical feature, NULL
is returned. Also,
if the SV is not of type SVt_PVMG, Perl may core dump.
- int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen);
This routine checks to see what types of magic sv
has. If the mg_type
field is an uppercase letter, then the mg_obj is copied to nsv
, but
the mg_type field is changed to be the lowercase letter.
Tied hashes and arrays are magical beasts of the 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", TRUE);
- 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.
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.
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.
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)))
And now the Perl program calling tzname
, the two values will be assigned
as in:
- ($standard_abbrev, $summer_abbrev) = POSIX::tzname;
An alternate (and possibly simpler) method to pushing values on the stack is to use the macro:
- XPUSHs(SV*)
This macro automatically adjust the stack for you, if needed. Thus, you
do not need to call EXTEND
to extend the stack.
Despite their suggestions in earlier versions of this document the macros
(X)PUSH[iunp] are not suited to XSUBs which return multiple results.
For that, either stick to the (X)PUSHs macros shown above, or use the new
m(X)PUSH[iunp] macros instead; see Putting a C value on Perl stack.
For more information, consult perlxs and perlxstut.
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.
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.
- Renew(pointer, number, type);
- Renewc(pointer, number, type, cast);
- Safefree(pointer)
These three macros are used to change a memory buffer size or to free a
piece of memory no longer needed. The arguments to Renew
and Renewc
match those of New
and Newc
with the exception of not needing the
"magic cookie" argument.
- Move(source, dest, number, type);
- Copy(source, dest, number, type);
- Zero(dest, number, type);
These three macros are used to move, copy, or zero out previously allocated
memory. The source
and dest
arguments point to the source and
destination starting points. Perl will move, copy, or zero out number
instances of the size of the type
data structure (using the sizeof
function).
The most recent development releases of Perl has been experimenting with removing Perl's dependency on the "normal" standard I/O suite and allowing other stdio implementations to be used. This involves creating a new abstraction layer that then calls whichever implementation of stdio Perl was compiled with. All XSUBs should now use the functions in the PerlIO abstraction layer and not make any assumptions about what kind of stdio is being used.
For a complete description of the PerlIO abstraction, consult perlapio.
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
.
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.
In fact it is not 100% true that a compiled unit contains a pointer to the scratchpad AV. In fact it contains a pointer to an AV of (initially) one element, and this element is the scratchpad AV. Why do we need an extra level of indirection?
The answer is recursion, and maybe threads. Both these can create several execution pointers going into the same subroutine. For the subroutine-child not write over the temporaries for the subroutine-parent (lifespan of which covers the call to the child), the parent and the child should have different scratchpads. (And the lexicals should be separate anyway!)
So each subroutine is born with an array of scratchpads (of length 1). On each entry to the subroutine it is checked that the current depth of the recursion is not more than the length of this array, and if it is, new scratchpad is created and pushed into the array.
The targets on this scratchpad are undefs, but they are already
marked with correct flags.
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.
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.
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).
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.
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).
After the compile tree for a subroutine (or for an eval or a file)
is created, an additional pass over the code is performed. This pass
is neither top-down or bottom-up, but in the execution order (with
additional complications for conditionals). These optimizations are
done in the subroutine peep(). Optimizations performed at this stage
are subject to the same restrictions as in the pass 2.
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.
dump functionsTo 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.
The Perl interpreter can be regarded as a closed box: it has an API for feeding it code or otherwise making it do things, but it also has functions for its own use. This smells a lot like an object, and there are ways for you to build Perl so that you can have multiple interpreters, with one interpreter represented either as a C structure, or inside a thread-specific structure. These structures contain all the context, the state of that interpreter.
Two macros control the major Perl build flavors: MULTIPLICITY and USE_5005THREADS. The MULTIPLICITY build has a C structure that packages all the interpreter state, and there is a similar thread-specific data structure under USE_5005THREADS. In both cases, PERL_IMPLICIT_CONTEXT is also normally defined, and enables the support for passing in a "hidden" first argument that represents all three data structures.
All this obviously requires a way for the Perl internal functions to be either subroutines taking some kind of structure as the first argument, or subroutines taking nothing as the first argument. To enable these two very different ways of building the interpreter, the Perl source (as it does in so many other situations) makes heavy use of macros and subroutine naming conventions.
First problem: deciding which functions will be public API functions and
which will be private. All functions whose names begin S_
are private
(think "S" for "secret" or "static"). All other functions begin with
"Perl_", but just because a function begins with "Perl_" does not mean it is
part of the API. (See Internal Functions.) The easiest way to be sure a
function is part of the API is to find its entry in perlapi.
If it exists in perlapi, it's part of the API. If it doesn't, and you
think it should be (i.e., you need it for your extension), send mail via
perlbug explaining why you think it should be.
Second problem: there must be a syntax so that the same subroutine declarations and calls can pass a structure as their first argument, or pass nothing. To solve this, the subroutines are named and declared in a particular way. Here's a typical start of a static function used within the Perl guts:
- STATIC void
- S_incline(pTHX_ char *s)
STATIC becomes "static" in C, and may be #define'd to nothing in some configurations in future.
A public function (i.e. part of the internal API, but not necessarily sanctioned for use in extensions) begins like this:
- void
- Perl_sv_setiv(pTHX_ SV* dsv, IV num)
pTHX_
is one of a number of macros (in perl.h) that hide the
details of the interpreter's context. THX stands for "thread", "this",
or "thingy", as the case may be. (And no, George Lucas is not involved. :-)
The first character could be 'p' for a prototype, 'a' for argument,
or 'd' for declaration, so we have pTHX
, aTHX
and dTHX
, and
their variants.
When Perl is built without options that set PERL_IMPLICIT_CONTEXT, there is no first argument 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.
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.
When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call any functions in the Perl API will need to pass the initial context argument somehow. The kicker is that you will need to write it in such a way that the extension still compiles when Perl hasn't been built with PERL_IMPLICIT_CONTEXT enabled.
There are three ways to do this. First, the easy but inefficient way, which is also the default, in order to maintain source compatibility with extensions: whenever XSUB.h is #included, it redefines the aTHX and aTHX_ macros to call a function that will return the context. Thus, something like:
- sv_setiv(sv, num);
in your extension will translate to this when PERL_IMPLICIT_CONTEXT is in effect:
- Perl_sv_setiv(Perl_get_context(), sv, num);
or to this otherwise:
- Perl_sv_setiv(sv, num);
You have to do nothing new in your extension to get this; since the Perl library provides Perl_get_context(), it will all just work.
The second, more efficient way is to use the following template for your Foo.xs:
- #define PERL_NO_GET_CONTEXT /* we want efficiency */
- #include "EXTERN.h"
- #include "perl.h"
- #include "XSUB.h"
- static my_private_function(int arg1, int arg2);
- static SV *
- my_private_function(int arg1, int arg2)
- {
- dTHX; /* fetch context */
- ... call many Perl API functions ...
- }
- [... etc ...]
- MODULE = Foo PACKAGE = Foo
- /* typical XSUB */
- void
- my_xsub(arg)
- int arg
- CODE:
- my_private_function(arg, 10);
Note that the only two changes from the normal way of writing an
extension is the addition of a #define PERL_NO_GET_CONTEXT
before
including the Perl headers, followed by a dTHX;
declaration at
the start of every function that will call the Perl API. (You'll
know which functions need this, because the C compiler will complain
that there's an undeclared identifier in those functions.) No changes
are needed for the XSUBs themselves, because the XS() macro is
correctly defined to pass in the implicit context if needed.
The third, even more efficient way is to ape how it is done within the Perl guts:
- #define PERL_NO_GET_CONTEXT /* we want efficiency */
- #include "EXTERN.h"
- #include "perl.h"
- #include "XSUB.h"
- /* pTHX_ only needed for functions that call Perl API */
- static my_private_function(pTHX_ int arg1, int arg2);
- static SV *
- my_private_function(pTHX_ int arg1, int arg2)
- {
- /* dTHX; not needed here, because THX is an argument */
- ... call Perl API functions ...
- }
- [... etc ...]
- MODULE = Foo PACKAGE = Foo
- /* typical XSUB */
- void
- my_xsub(arg)
- int arg
- CODE:
- my_private_function(aTHX_ arg, 10);
This implementation never has to fetch the context using a function call, since it is always passed as an extra 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 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 ...
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 and USE_5005THREADS on Windows (see inside iperlsys.h).
This allows the ability to provide an extra pointer (called the "host" environment) for all the system calls. This makes it possible for all the system stuff to 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.
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, you can get at the functions either with or
without the Perl_
prefix, thanks to a bunch of defines that live in
embed.h. This header file is 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:
This function is a part of the public API. All such functions should also have 'd', very few do not.
This function has a Perl_
prefix; i.e. it is defined as
Perl_av_fetch
.
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:
This is a static function and is defined as STATIC S_whatever
, and
usually called within the sources as whatever(...)
.
This does not need a interpreter context, so the definition has no
pTHX
, and it follows that callers don't use aTHX
. (See
Background and PERL_IMPLICIT_CONTEXT in perlguts.)
This function never returns; croak
, exit and friends.
This function takes a variable number of arguments, printf style.
The argument list should end with ...
, like this:
- Afprd |void |croak |const char* pat|...
This function is part of the experimental development API, and may change or disappear without notice.
This function should not have a compatibility macro to define, say,
Perl_parse
to parse
. It must be called as Perl_parse
.
This function isn't exported out of the Perl core.
This is implemented as a macro.
This function is explicitly exported.
This function is visible to extensions included in the Perl core.
Binary backward compatibility; this function is a macro but also has
a Perl_
implementation (which is exported).
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.
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.
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);
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.
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
.
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.
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, instead of just one. You can learn more about Unicode at http://www.unicode.org/
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.
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.
As mentioned above, UTF-8 uses a variable number of bytes to store a
character. Characters with values 1...128 are stored in one byte, just
like good ol' ASCII. Character 129 is stored as v194.129
; this
continues up to 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;
- UV uv; /* Note: a UV, not a U8, not a char */
- if (!UTF8_IS_INVARIANT(*utf))
- /* Must treat this as UTF-8 */
- uv = utf8_to_uv(utf);
- else
- /* OK to treat this character as a byte */
- uv = *utf;
You can also see in that example that we use utf8_to_uv
to get the
value of the character; the inverse function uv_to_utf8
is available
for putting a UV into UTF-8:
- if (!UTF8_IS_INVARIANT(uv))
- /* Must treat this as UTF8 */
- utf8 = uv_to_utf8(utf8, uv);
- else
- /* OK to treat this character as a byte */
- *utf8++ = uv;
You must convert characters to UVs using the above functions if
you're ever in a situation where you have to match UTF-8 and non-UTF-8
characters. You may not skip over UTF-8 characters in this case. If you
do this, you'll lose the ability to match hi-bit non-UTF-8 characters;
for instance, if your UTF-8 string contains v196.172
, and you skip
that character, you can never match a chr(200) in a non-UTF-8 string.
So don't do that!
Currently, Perl deals with Unicode strings and non-Unicode strings
slightly differently. If a string has been identified as being UTF-8
encoded, Perl will set a flag in the SV, SVf_UTF8
. You can check and
manipulate this flag with the following macros:
- SvUTF8(sv)
- SvUTF8_on(sv)
- SvUTF8_off(sv)
This flag has an important effect on Perl's treatment of the string: if
Unicode data is not properly distinguished, regular expressions,
length, substr and other string handling operations will have
undesirable results.
The problem comes when you have, for instance, a string that isn't flagged is UTF-8, and contains a byte sequence that could be UTF-8 - especially when combining non-UTF-8 and UTF-8 strings.
Never forget that the SVf_UTF8
flag is separate to the PV value; you
need be sure you don't accidentally knock it off while you're
manipulating SVs. More specifically, you cannot expect to do this:
- SV *sv;
- SV *nsv;
- STRLEN len;
- char *p;
- p = SvPV(sv, len);
- frobnicate(p);
- nsv = newSVpvn(p, len);
The char*
string does not tell you the whole story, and you can't
copy or reconstruct an SV just by copying the string value. Check if the
old SV has the UTF-8 flag set, and act accordingly:
- p = SvPV(sv, len);
- frobnicate(p);
- nsv = newSVpvn(p, len);
- if (SvUTF8(sv))
- SvUTF8_on(nsv);
In fact, your frobnicate
function should be made aware of whether or
not it's dealing with UTF-8 data, so that it can handle the string
appropriately.
Since just passing an SV to an XS function and copying the data of
the SV is not enough to copy the UTF-8 flags, even less right is just
passing a char *
to an XS function.
If you're mixing UTF-8 and non-UTF-8 strings, you might find it necessary to upgrade one of the strings to UTF-8. If you've got an SV, the easiest way to do this is:
- sv_utf8_upgrade(sv);
However, you must not do this, for example:
- if (!SvUTF8(left))
- sv_utf8_upgrade(left);
If you do this in a binary operator, you will actually change one of the strings that came into the operator, and, while it shouldn't be noticeable by the end user, it can cause problems.
Instead, bytes_to_utf8
will give you a UTF-8-encoded copy of its
string argument. This is useful for having the data available for
comparisons and so on, without harming the original SV. There's also
utf8_to_bytes
to go the other way, but naturally, this will fail if
the string contains any characters above 255 that can't be represented
in a single byte.
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 is SvUTF8
flag. Don't forget to set the flag if
something should be UTF-8. Treat the flag as part of the PV, even though
it's not - if you pass on the PV to somewhere, pass on the flag too.
If a string is UTF-8, always use utf8_to_uv
to get at the value,
unless UTF8_IS_INVARIANT(*s)
in which case you can use *s
.
When writing a character uv
to a UTF-8 string, always use
uv_to_utf8
, unless UTF8_IS_INVARIANT(uv)) in which case
you can use *s = uv
.
Mixing UTF-8 and non-UTF-8 strings is tricky. Use bytes_to_utf8
to get
a new string which is UTF-8 encoded. There are tricks you can use to
delay deciding whether you need to use a UTF-8 string until you get to a
high character - HALF_UPGRADE
is one of those.
Custom 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
as a key into the
PL_custom_op_descs
and PL_custom_op_names
hashes. This means you
need to enter a name and description for your op at the appropriate
place in the PL_custom_op_names
and PL_custom_op_descs
hashes.
Forthcoming versions of B::Generate
(version 1.0 and above) should
directly support the creation of custom ops by name.
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.
perlapi(1), perlintern(1), perlxs(1), perlembed(1)