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).
("PV" stands for "Pointer Value". You might think that it is misnamed
because it is described as pointing only to strings. However, it is
possible to have it point to other things. For example, it could point
to an array of UVs. But,
using it for non-strings requires care, as the underlying assumption of
much of the internals is that PVs are just for strings. Often, for
example, a trailing NUL
is tacked on automatically. The non-string use
is documented only in this paragraph.)
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 initialization, you
can create an empty SV with newSV(len). If len
is 0 an empty SV of
type NULL is returned, else an SV of type PV is returned with len + 1 (for
the NUL
) bytes of storage allocated, accessible via SvPVX. In both cases
the SV has the undef value.
- SV *sv = newSV(0); /* no storage allocated */
- SV *sv = newSV(10); /* 10 (+1) bytes of uninitialised storage
- * allocated */
To change the value of an already-existing SV, there are eight routines:
- void sv_setiv(SV*, IV);
- void sv_setuv(SV*, UV);
- void sv_setnv(SV*, double);
- void sv_setpv(SV*, const char*);
- void sv_setpvn(SV*, const char*, STRLEN)
- void sv_setpvf(SV*, const char*, ...);
- void sv_vsetpvfn(SV*, const char*, STRLEN, va_list *,
- SV **, I32, bool *);
- void sv_setsv(SV*, SV*);
Notice that you can choose to specify the length of the string to be
assigned by using sv_setpvn
, newSVpvn
, or newSVpv
, or you may
allow Perl to calculate the length by using sv_setpv
or by specifying
0 as the second argument to newSVpv
. Be warned, though, that Perl will
determine the string's length by using strlen
, which depends on the
string terminating with a NUL
character, and not otherwise containing
NULs.
The arguments of sv_setpvf
are processed like sprintf, and the
formatted output becomes the value.
sv_vsetpvfn
is an analogue of vsprintf
, but it allows you to specify
either a pointer to a variable argument list or the address and length of
an array of SVs. The last argument points to a boolean; on return, if that
boolean is true, then locale-specific information has been used to format
the string, and the string's contents are therefore untrustworthy (see
perlsec). This pointer may be NULL if that information is not
important. Note that this function requires you to specify the length of
the format.
The sv_set*()
functions are not generic enough to operate on values
that have "magic". See Magic Virtual Tables later in this document.
All SVs that contain strings should be terminated with a NUL
character.
If it is not NUL
-terminated there is a risk of
core dumps and corruptions from code which passes the string to C
functions or system calls which expect a NUL
-terminated string.
Perl's own functions typically add a trailing NUL
for this reason.
Nevertheless, you should be very careful when you pass a string stored
in an SV to a C function or system call.
To access the actual value that an SV points to, you can use the macros:
- SvIV(SV*)
- SvUV(SV*)
- SvNV(SV*)
- SvPV(SV*, STRLEN len)
- SvPV_nolen(SV*)
which will automatically coerce the actual scalar type into an IV, UV, double, or string.
In the SvPV
macro, the length of the string returned is placed into the
variable len
(this is a macro, so you do not use &len
). If you do
not care what the length of the data is, use the SvPV_nolen
macro.
Historically the SvPV
macro with the global variable PL_na
has been
used in this case. But that can be quite inefficient because PL_na
must
be accessed in thread-local storage in threaded Perl. In any case, remember
that Perl allows arbitrary strings of data that may both contain NULs and
might not be terminated by a NUL
.
Also remember that C doesn't allow you to safely say foo(SvPV(s, len),
len);. It might work with your
compiler, but it won't work for everyone.
Break this sort of statement up into separate assignments:
- SV *s;
- STRLEN len;
- char *ptr;
- ptr = SvPV(s, len);
- foo(ptr, len);
If you want to know if the scalar value is TRUE, you can use:
- SvTRUE(SV*)
Although Perl will automatically grow strings for you, if you need to force Perl to allocate more memory for your SV, you can use the macro
- SvGROW(SV*, STRLEN newlen)
which will determine if more memory needs to be allocated. If so, it will
call the function sv_grow
. Note that SvGROW
can only increase, not
decrease, the allocated memory of an SV and that it does not automatically
add space for the trailing NUL
byte (perl's own string functions typically do
SvGROW(sv, len + 1)
).
If you want to write to an existing SV's buffer and set its value to a string, use SvPV_force() or one of its variants to force the SV to be a PV. This will remove any of various types of non-stringness from the SV while preserving the content of the SV in the PV. This can be used, for example, to append data from an API function to a buffer without extra copying:
- (void)SvPVbyte_force(sv, len);
- s = SvGROW(sv, len + needlen + 1);
- /* something that modifies up to needlen bytes at s+len, but
- modifies newlen bytes
- eg. newlen = read(fd, s + len, needlen);
- ignoring errors for these examples
- */
- s[len + newlen] = '\0';
- SvCUR_set(sv, len + newlen);
- SvUTF8_off(sv);
- SvSETMAGIC(sv);
If you already have the data in memory or if you want to keep your code simple, you can use one of the sv_cat*() variants, such as sv_catpvn(). If you want to insert anywhere in the string you can use sv_insert() or sv_insert_flags().
If you don't need the existing content of the SV, you can avoid some copying with:
- SvPVCLEAR(sv);
- s = SvGROW(sv, needlen + 1);
- /* something that modifies up to needlen bytes at s, but modifies
- newlen bytes
- eg. newlen = read(fd, s. needlen);
- */
- s[newlen] = '\0';
- SvCUR_set(sv, newlen);
- SvPOK_only(sv); /* also clears SVf_UTF8 */
- SvSETMAGIC(sv);
Again, if you already have the data in memory or want to avoid the complexity of the above, you can use sv_setpvn().
If you have a buffer allocated with Newx() and want to set that as the SV's value, you can use sv_usepvn_flags(). That has some requirements if you want to avoid perl re-allocating the buffer to fit the trailing NUL:
- Newx(buf, somesize+1, char);
- /* ... fill in buf ... */
- buf[somesize] = '\0';
- sv_usepvn_flags(sv, buf, somesize, SV_SMAGIC | SV_HAS_TRAILING_NUL);
- /* buf now belongs to perl, don't release it */
If you have an SV and want to know what kind of data Perl thinks is stored in it, you can use the following macros to check the type of SV you have.
- SvIOK(SV*)
- SvNOK(SV*)
- SvPOK(SV*)
You can get and set the current length of the string stored in an SV with the following macros:
- SvCUR(SV*)
- SvCUR_set(SV*, I32 val)
You can also get a pointer to the end of the string stored in the SV with the macro:
- SvEND(SV*)
But note that these last three macros are valid only if SvPOK()
is true.
If you want to append something to the end of string stored in an SV*
,
you can use the following functions:
- void sv_catpv(SV*, const char*);
- void sv_catpvn(SV*, const char*, STRLEN);
- void sv_catpvf(SV*, const char*, ...);
- void sv_vcatpvfn(SV*, const char*, STRLEN, va_list *, SV **,
- I32, bool);
- void sv_catsv(SV*, SV*);
The first function calculates the length of the string to be appended by
using strlen
. In the second, you specify the length of the string
yourself. The third function processes its arguments like sprintf and
appends the formatted output. The fourth function works like vsprintf
.
You can specify the address and length of an array of SVs instead of the
va_list argument. The fifth function
extends the string stored in the first
SV with the string stored in the second SV. It also forces the second SV
to be interpreted as a string.
The sv_cat*()
functions are not generic enough to operate on values that
have "magic". See Magic Virtual Tables later in this document.
If you know the name of a scalar variable, you can get a pointer to its SV by using the following:
- SV* get_sv("package::varname", 0);
This returns NULL if the variable does not exist.
If you want to know if this variable (or any other SV) is actually defined,
you can call:
- SvOK(SV*)
The scalar undef value is stored in an SV instance called PL_sv_undef
.
Its address can be used whenever an SV*
is needed. Make sure that
you don't try to compare a random sv with &PL_sv_undef
. For example
when interfacing Perl code, it'll work correctly for:
- foo(undef);
But won't work when called as:
- $x = undef;
- foo($x);
So to repeat always use SvOK() to check whether an sv is defined.
Also you have to be careful when using &PL_sv_undef
as a value in
AVs or HVs (see AVs, HVs and undefined values).
There are also the two values PL_sv_yes
and PL_sv_no
, which contain
boolean TRUE and FALSE values, respectively. Like PL_sv_undef
, their
addresses can be used whenever an SV*
is needed.
Do not be fooled into thinking that (SV *) 0 is the same as &PL_sv_undef
.
Take this code:
- SV* sv = (SV*) 0;
- if (I-am-to-return-a-real-value) {
- sv = sv_2mortal(newSViv(42));
- }
- sv_setsv(ST(0), sv);
This code tries to return a new SV (which contains the value 42) if it should
return a real value, or undef otherwise. Instead it has returned a NULL
pointer which, somewhere down the line, will cause a segmentation violation,
bus error, or just weird results. Change the zero to &PL_sv_undef
in the
first line and all will be well.
To free an SV that you've created, call SvREFCNT_dec(SV*)
. Normally this
call is not necessary (see Reference Counts and Mortality).
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 moves the PV pointer (called SvPVX
) forward
by the number of bytes chopped off, and adjusts SvCUR
and SvLEN
accordingly. (A portion of the space between the old and new PV
pointers is used to store the count of chopped bytes.)
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. Normally copy-on-write will prevent the substitution from operator from using this hack, but if you can craft a string for which copy-on-write is not possible, you can see it in play. In the current implementation, the final byte of a string buffer is used as a copy-on-write reference count. If the buffer is not big enough, then copy-on-write is skipped. First have a look at an empty string:
- % ./perl -Ilib -MDevel::Peek -le '$a=""; $a .= ""; Dump $a'
- SV = PV(0x7ffb7c008a70) at 0x7ffb7c030390
- REFCNT = 1
- FLAGS = (POK,pPOK)
- PV = 0x7ffb7bc05b50 ""\0
- CUR = 0
- LEN = 10
Notice here the LEN is 10. (It may differ on your platform.) Extend the length of the string to one less than 10, and do a substitution:
- % ./perl -Ilib -MDevel::Peek -le '$a=""; $a.="123456789"; $a=~s/.//; \
- Dump($a)'
- SV = PV(0x7ffa04008a70) at 0x7ffa04030390
- REFCNT = 1
- FLAGS = (POK,OOK,pPOK)
- OFFSET = 1
- PV = 0x7ffa03c05b61 ( "\1" . ) "23456789"\0
- CUR = 8
- LEN = 9
Here the number of bytes chopped off (1) is shown next as the 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. (The first character of the string
buffer happens to have changed to "\1" here, not "1", because the current
implementation stores the offset count in the string buffer. This is
subject to change.)
Something similar to the offset hack is performed on AVs to enable
efficient shifting and splicing off the beginning of the array; while
AvARRAY
points to the first element in the array that is visible from
Perl, AvALLOC
points to the real start of the C array. These are
usually the same, but a shift operation can be carried out by
increasing AvARRAY
by one and decreasing AvFILL
and AvMAX
.
Again, the location of the real start of the C array only comes into
play when freeing the array. See av_shift
in av.c.
Recall that the usual method of determining the type of scalar you have is
to use Sv*OK
macros. Because a scalar can be both a number and a string,
usually these macros will always return TRUE and calling the Sv*V
macros will do the appropriate conversion of string to integer/double or
integer/double to string.
If you really need to know if you have an integer, double, or string pointer in an SV, you can use the following three macros instead:
- SvIOKp(SV*)
- SvNOKp(SV*)
- SvPOKp(SV*)
These will tell you if you truly have an integer, double, or string pointer stored in your SV. The "p" stands for private.
There are various ways in which the private and public flags may differ. For example, in perl 5.16 and earlier 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. (In perl 5.18 onwards, tied scalars use the flags the same way as untied scalars.) 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(SSize_t num, SV **ptr);
The second argument points to an array containing num
SV*
's. Once the
AV has been created, the SVs can be destroyed, if so desired.
Once the AV has been created, the following operations are possible on it:
- void av_push(AV*, SV*);
- SV* av_pop(AV*);
- SV* av_shift(AV*);
- void av_unshift(AV*, SSize_t 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:
- SSize_t av_top_index(AV*);
- SV** av_fetch(AV*, SSize_t key, I32 lval);
- SV** av_store(AV*, SSize_t key, SV* val);
The av_top_index
function returns the highest index value in an array (just
like $#array in Perl). If the array is empty, -1 is returned. The
av_fetch
function returns the value at index key
, but if lval
is non-zero, then av_fetch
will store an undef value at that index.
The av_store
function stores the value val
at index key
, and does
not increment the reference count of val
. Thus the caller is responsible
for taking care of that, and if av_store
returns NULL, the caller will
have to decrement the reference count to avoid a memory leak. Note that
av_fetch
and av_store
both return SV**
's, not SV*
's as their
return value.
A few more:
- void av_clear(AV*);
- void av_undef(AV*);
- void av_extend(AV*, SSize_t key);
The av_clear
function deletes all the elements in the AV* array, but
does not actually delete the array itself. The av_undef
function will
delete all the elements in the array plus the array itself. The
av_extend
function extends the array so that it contains at least key+1
elements. If key+1
is less than the currently allocated length of the array,
then nothing is done.
If you know the name of an array variable, you can get a pointer to its AV by using the following:
- AV* get_av("package::varname", 0);
This returns NULL if the variable does not exist.
See Understanding the Magic of Tied Hashes and Arrays for more information on how to use the array access functions on tied arrays.
To create an HV, you use the following routine:
- HV* newHV();
Once the HV has been created, the following operations are possible on it:
- SV** hv_store(HV*, const char* key, U32 klen, SV* val, U32 hash);
- SV** hv_fetch(HV*, const char* key, U32 klen, I32 lval);
The klen
parameter is the length of the key being passed in (Note that
you cannot pass 0 in as a value of klen
to tell Perl to measure the
length of the key). The val
argument contains the SV pointer to the
scalar being stored, and hash
is the precomputed hash value (zero if
you want hv_store
to calculate it for you). The lval
parameter
indicates whether this fetch is actually a part of a store operation, in
which case a new undefined value will be added to the HV with the supplied
key and hv_fetch
will return as if the value had already existed.
Remember that hv_store
and hv_fetch
return SV**
's and not just
SV*
. To access the scalar value, you must first dereference the return
value. However, you should check to make sure that the return value is
not NULL before dereferencing it.
The first of these two functions checks if a hash table entry exists, and the second deletes it.
- bool hv_exists(HV*, const char* key, U32 klen);
- SV* hv_delete(HV*, const char* key, U32 klen, I32 flags);
If flags
does not include the G_DISCARD
flag then hv_delete
will
create and return a mortal copy of the deleted value.
And more miscellaneous functions:
- void hv_clear(HV*);
- void hv_undef(HV*);
Like their AV counterparts, hv_clear
deletes all the entries in the hash
table but does not actually delete the hash table. The hv_undef
deletes
both the entries and the hash table itself.
Perl keeps the actual data in a linked list of structures with a typedef of HE.
These contain the actual key and value pointers (plus extra administrative
overhead). The key is a string pointer; the value is an SV*
. However,
once you have an HE*
, to get the actual key and value, use the routines
specified below.
- I32 hv_iterinit(HV*);
- /* Prepares starting point to traverse hash table */
- HE* hv_iternext(HV*);
- /* Get the next entry, and return a pointer to a
- structure that has both the key and value */
- char* hv_iterkey(HE* entry, I32* retlen);
- /* Get the key from an HE structure and also return
- the length of the key string */
- SV* hv_iterval(HV*, HE* entry);
- /* Return an SV pointer to the value of the HE
- structure */
- SV* hv_iternextsv(HV*, char** key, I32* retlen);
- /* This convenience routine combines hv_iternext,
- hv_iterkey, and hv_iterval. The key and retlen
- arguments are return values for the key and its
- length. The value is returned in the SV* argument */
If you know the name of a hash variable, you can get a pointer to its HV by using the following:
- HV* get_hv("package::varname", 0);
This returns NULL if the variable does not exist.
The hash algorithm is defined in the PERL_HASH
macro:
- PERL_HASH(hash, key, klen)
The exact implementation of this macro varies by architecture and version of perl, and the return value may change per invocation, so the value is only valid for the duration of a single perl process.
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. In perl 5.18 and earlier, 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. In perl 5.20, storing
&PL_sv_undef will create a read-only element, because the scalar
&PL_sv_undef itself is stored, not a copy.
Similar problems can occur when storing &PL_sv_undef
in HVs:
- hv_store( hv, "key", 3, &PL_sv_undef, 0 );
This will indeed make the value undef, but if you try to modify
the value of key
, you'll get the following error:
- Modification of non-creatable hash value attempted
In perl 5.8.0, &PL_sv_undef
was also used to mark placeholders
in restricted hashes. This caused such hash entries not to appear
when iterating over the hash or when checking for the keys
with the hv_exists
function.
You can run into similar problems when you store &PL_sv_yes
or
&PL_sv_no
into AVs or HVs. Trying to modify such elements
will give you the following error:
- Modification of a read-only value attempted
To make a long story short, you can use the special variables
&PL_sv_undef
, &PL_sv_yes
and &PL_sv_no
with AVs and
HVs, but you have to make sure you know what you're doing.
Generally, if you want to store an undefined value in an AV
or HV, you should not use &PL_sv_undef
, but rather create a
new undefined value using the newSV
function, for example:
- av_store( av, 42, newSV(0) );
- hv_store( hv, "foo", 3, newSV(0), 0 );
References are a special type of scalar that point to other data types (including other references).
To create a reference, use either of the following functions:
- SV* newRV_inc((SV*) thing);
- SV* newRV_noinc((SV*) thing);
The thing
argument can be any of an SV*
, AV*
, or HV*
. The
functions are identical except that newRV_inc
increments the reference
count of the thing
, while newRV_noinc
does not. For historical
reasons, newRV
is a synonym for newRV_inc
.
Once you have a reference, you can use the following macro to dereference the reference:
- SvRV(SV*)
then call the appropriate routines, casting the returned SV*
to either an
AV*
or HV*
, if required.
To determine if an SV is a reference, you can use the following macro:
- SvROK(SV*)
To discover what type of value the reference refers to, use the following macro and then check the return value.
- SvTYPE(SvRV(SV*))
The most useful types that will be returned are:
- < SVt_PVAV Scalar
- SVt_PVAV Array
- SVt_PVHV Hash
- SVt_PVCV Code
- SVt_PVGV Glob (possibly a file handle)
See svtype in perlapi 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 */
The following function upgrades rv to reference if not already one.
Creates a new SV for rv to point to. If classname
is non-null, the SV
is blessed into the specified class. SV is returned.
- SV* newSVrv(SV* rv, const char* classname);
The following three functions copy integer, unsigned integer or double
into an SV whose reference is rv
. SV is blessed if classname
is
non-null.
- SV* sv_setref_iv(SV* rv, const char* classname, IV iv);
- SV* sv_setref_uv(SV* rv, const char* classname, UV uv);
- SV* sv_setref_nv(SV* rv, const char* classname, NV iv);
The following function copies the pointer value (the address, not the
string!) into an SV whose reference is rv. SV is blessed if classname
is non-null.
- SV* sv_setref_pv(SV* rv, const char* classname, void* pv);
The following function copies a string into an SV whose reference is rv
.
Set length to 0 to let Perl calculate the string length. SV is blessed if
classname
is non-null.
- SV* sv_setref_pvn(SV* rv, const char* classname, char* pv,
- STRLEN length);
The following function tests whether the SV is blessed into the specified class. It does not check inheritance relationships.
- int sv_isa(SV* sv, const char* name);
The following function tests whether the SV is a reference to a blessed object.
- int sv_isobject(SV* sv);
The following function tests whether the SV is derived from the specified
class. SV can be either a reference to a blessed object or a string
containing a class name. This is the function implementing the
UNIVERSAL::isa
functionality.
- bool sv_derived_from(SV* sv, const char* name);
To check if you've got an object derived from a specific class you have to write:
- if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }
To create a new Perl variable with an undef value which can be accessed from your Perl script, use the following routines, depending on the variable type.
- SV* get_sv("package::varname", GV_ADD);
- AV* get_av("package::varname", GV_ADD);
- HV* get_hv("package::varname", GV_ADD);
Notice the use of GV_ADD as the second parameter. The new variable can now be set, using the routines appropriate to the data type.
There are additional macros whose values may be bitwise OR'ed with the
GV_ADD
argument to enable certain extra features. Those bits are:
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 a high enough REFCNT
to survive its use on the stack you need not do any mortalization.
If you are not sure then doing an SvREFCNT_inc
and sv_2mortal
, or
making a sv_mortalcopy
is safer.
The mortal routines are not just for SVs; AVs and HVs can be
made mortal by passing their address (type-casted to SV*
) to the
sv_2mortal
or sv_mortalcopy
routines.
A stash is a hash that contains all variables that are defined within a package. Each key of the stash is a symbol name (shared by all the different types of objects that have the same name), and each value in the hash table is a GV (Glob Value). This GV in turn contains references to the various objects of that name, including (but not limited to) the following:
- Scalar Value
- Array Value
- Hash Value
- I/O Handle
- Format
- Subroutine
There is a single stash called PL_defstash
that holds the items that exist
in the main
package. To get at the items in other packages, append the
string "::" to the package name. The items in the Foo
package are in
the stash Foo::
in PL_defstash. The items in the Bar::Baz
package are
in the stash Baz::
in Bar::
's stash.
To get the stash pointer for a particular package, use the function:
- HV* gv_stashpv(const char* name, I32 flags)
- HV* gv_stashsv(SV*, I32 flags)
The first function takes a literal string, the second uses the string stored
in the SV. Remember that a stash is just a hash table, so you get back an
HV*
. The flags
flag will create a new package if it is set to GV_ADD.
The name that gv_stash*v
wants is the name of the package whose symbol table
you want. The default package is called main
. If you have multiply nested
packages, pass their names to gv_stash*v
, separated by ::
as in the Perl
language itself.
Alternately, if you have an SV that is a blessed reference, you can find out the stash pointer by using:
- HV* SvSTASH(SvRV(SV*));
then use the following to get the package name itself:
- char* HvNAME(HV* stash);
If you need to bless or re-bless an object you can use the following function:
- SV* sv_bless(SV*, HV* stash)
where the first argument, an SV*
, must be a reference, and the second
argument is a stash. The returned SV*
can now be used in the same way
as any other SV.
For more information on references and blessings, consult perlref.
Scalar variables normally contain only one type of value, an integer, double, pointer, or reference. Perl will automatically convert the actual scalar data from the stored type into the requested type.
Some scalar variables contain more than one type of scalar data. For
example, the variable $!
contains either the numeric value of errno
or its string equivalent from either strerror
or sys_errlist[]
.
To force multiple data values into an SV, you must do two things: use the
sv_set*v
routines to add the additional scalar type, then set a flag
so that Perl will believe it contains more than one type of data. The
four macros to set the flags are:
- SvIOK_on
- SvNOK_on
- SvPOK_on
- SvROK_on
The particular macro you must use depends on which sv_set*v
routine
you called first. This is because every sv_set*v
routine turns on
only the bit for the particular type of data being set, and turns off
all the rest.
For example, to create a new Perl variable called "dberror" that contains both the numeric and descriptive string error values, you could use the following code:
- extern int dberror;
- extern char *dberror_list;
- SV* sv = get_sv("dberror", GV_ADD);
- sv_setiv(sv, (IV) dberror);
- sv_setpv(sv, dberror_list[dberror]);
- SvIOK_on(sv);
If the order of sv_setiv
and sv_setpv
had been reversed, then the
macro SvPOK_on
would need to be called instead of SvIOK_on
.
In Perl 5.16 and earlier, copy-on-write (see the next section) shared a
flag bit with read-only scalars. So the only way to test whether
sv_setsv
, etc., will raise a "Modification of a read-only value" error
in those versions is:
- SvREADONLY(sv) && !SvIsCOW(sv)
Under Perl 5.18 and later, SvREADONLY only applies to read-only variables, and, under 5.20, copy-on-write scalars can also be read-only, so the above check is incorrect. You just want:
- SvREADONLY(sv)
If you need to do this check often, define your own macro like this:
- #if PERL_VERSION >= 18
- # define SvTRULYREADONLY(sv) SvREADONLY(sv)
- #else
- # define SvTRULYREADONLY(sv) (SvREADONLY(sv) && !SvIsCOW(sv))
- #endif
Perl implements a copy-on-write (COW) mechanism for scalars, in which string copies are not immediately made when requested, but are deferred until made necessary by one or the other scalar changing. This is mostly transparent, but one must take care not to modify string buffers that are shared by multiple SVs.
You can test whether an SV is using copy-on-write with SvIsCOW(sv)
.
You can force an SV to make its own copy of its string buffer by calling sv_force_normal(sv)
or SvPV_force_nolen(sv).
If you want to make the SV drop its string buffer, use
sv_force_normal_flags(sv, SV_COW_DROP_PV)
or simply
sv_setsv(sv, NULL)
.
All of these functions will croak on read-only scalars (see the previous section for more on those).
To test that your code is behaving correctly and not modifying COW buffers,
on systems that support mmap(2) (i.e., Unix) you can configure perl with
-Accflags=-DPERL_DEBUG_READONLY_COW
and it will turn buffer violations
into crashes. You will find it to be marvellously slow, so you may want to
skip perl's own tests.
[This section still under construction. Ignore everything here. Post no bills. Everything not permitted is forbidden.]
Any SV may be magical, that is, it has special features that a normal
SV does not have. These features are stored in the SV structure in a
linked list of struct magic
's, typedef'ed to MAGIC
.
- struct magic {
- MAGIC* mg_moremagic;
- MGVTBL* mg_virtual;
- U16 mg_private;
- char mg_type;
- U8 mg_flags;
- I32 mg_len;
- SV* mg_obj;
- char* mg_ptr;
- };
Note this is current as of patchlevel 0, and could change at any time.
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
, PERL_MAGIC_regdatum
,
PERL_MAGIC_regdata
, 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:
The type
argument should be equal to the how
value when the SV
was initially made magical.
However, note that sv_unmagic
removes all magic of a certain type
from the
SV
. If you want to remove only certain
magic of a type
based on the magic
virtual table, use sv_unmagicext
instead:
The mg_virtual
field in the MAGIC
structure is a pointer to an
MGVTBL
, which is a structure of function pointers and stands for
"Magic Virtual Table" to handle the various operations that might be
applied to that variable.
The MGVTBL
has five (or sometimes eight) pointers to the following
routine types:
- int (*svt_get) (pTHX_ SV* sv, MAGIC* mg);
- int (*svt_set) (pTHX_ SV* sv, MAGIC* mg);
- U32 (*svt_len) (pTHX_ SV* sv, MAGIC* mg);
- int (*svt_clear)(pTHX_ SV* sv, MAGIC* mg);
- int (*svt_free) (pTHX_ SV* sv, MAGIC* mg);
- int (*svt_copy) (pTHX_ SV *sv, MAGIC* mg, SV *nsv,
- const char *name, I32 namlen);
- int (*svt_dup) (pTHX_ MAGIC *mg, CLONE_PARAMS *param);
- int (*svt_local)(pTHX_ SV *nsv, MAGIC *mg);
This MGVTBL structure is set at compile-time in perl.h and there are currently 32 types. These different structures contain pointers to various routines that perform additional actions depending on which function is being called.
- Function pointer Action taken
- ---------------- ------------
- svt_get Do something before the value of the SV is
- retrieved.
- svt_set Do something after the SV is assigned a value.
- svt_len Report on the SV's length.
- svt_clear Clear something the SV represents.
- svt_free Free any extra storage associated with the SV.
- svt_copy copy tied variable magic to a tied element
- svt_dup duplicate a magic structure during thread cloning
- svt_local copy magic to local value during 'local'
For instance, the MGVTBL structure called vtbl_sv
(which corresponds
to an mg_type
of PERL_MAGIC_sv
) contains:
- { magic_get, magic_set, magic_len, 0, 0 }
Thus, when an SV is determined to be magical and of type PERL_MAGIC_sv
,
if a get operation is being performed, the routine magic_get
is
called. All the various routines for the various magical types begin
with magic_
. NOTE: the magic routines are not considered part of
the Perl API, and may not be exported by the Perl library.
The last three slots are a recent addition, and for source code compatibility they are only checked for if one of the three flags MGf_COPY, MGf_DUP or MGf_LOCAL is set in mg_flags. This means that most code can continue declaring a vtable as a 5-element value. These three are currently used exclusively by the threading code, and are highly subject to change.
The current kinds of Magic Virtual Tables are:
- mg_type
- (old-style char and macro) MGVTBL Type of magic
- -------------------------- ------ -------------
- \0 PERL_MAGIC_sv vtbl_sv Special scalar variable
- # PERL_MAGIC_arylen vtbl_arylen Array length ($#ary)
- % PERL_MAGIC_rhash (none) Extra data for restricted
- hashes
- * PERL_MAGIC_debugvar vtbl_debugvar $DB::single, signal, trace
- vars
- . PERL_MAGIC_pos vtbl_pos pos() lvalue
- : PERL_MAGIC_symtab (none) Extra data for symbol
- tables
- < PERL_MAGIC_backref vtbl_backref For weak ref data
- @ PERL_MAGIC_arylen_p (none) To move arylen out of XPVAV
- B PERL_MAGIC_bm vtbl_regexp Boyer-Moore
- (fast string search)
- c PERL_MAGIC_overload_table vtbl_ovrld Holds overload table
- (AMT) on stash
- D PERL_MAGIC_regdata vtbl_regdata Regex match position data
- (@+ and @- vars)
- d PERL_MAGIC_regdatum vtbl_regdatum Regex match position data
- element
- E PERL_MAGIC_env vtbl_env %ENV hash
- e PERL_MAGIC_envelem vtbl_envelem %ENV hash element
- f PERL_MAGIC_fm vtbl_regexp Formline
- ('compiled' format)
- g PERL_MAGIC_regex_global vtbl_mglob m//g target
- H PERL_MAGIC_hints vtbl_hints %^H hash
- h PERL_MAGIC_hintselem vtbl_hintselem %^H hash element
- I PERL_MAGIC_isa vtbl_isa @ISA array
- i PERL_MAGIC_isaelem vtbl_isaelem @ISA array element
- k PERL_MAGIC_nkeys vtbl_nkeys scalar(keys()) lvalue
- L PERL_MAGIC_dbfile (none) Debugger %_<filename
- l PERL_MAGIC_dbline vtbl_dbline Debugger %_<filename
- element
- N PERL_MAGIC_shared (none) Shared between threads
- n PERL_MAGIC_shared_scalar (none) Shared between threads
- o PERL_MAGIC_collxfrm vtbl_collxfrm Locale transformation
- P PERL_MAGIC_tied vtbl_pack Tied array or hash
- p PERL_MAGIC_tiedelem vtbl_packelem Tied array or hash element
- q PERL_MAGIC_tiedscalar vtbl_packelem Tied scalar or handle
- r PERL_MAGIC_qr vtbl_regexp Precompiled qr// regex
- S PERL_MAGIC_sig (none) %SIG hash
- s PERL_MAGIC_sigelem vtbl_sigelem %SIG hash element
- t PERL_MAGIC_taint vtbl_taint Taintedness
- U PERL_MAGIC_uvar vtbl_uvar Available for use by
- extensions
- u PERL_MAGIC_uvar_elem (none) Reserved for use by
- extensions
- V PERL_MAGIC_vstring (none) SV was vstring literal
- v PERL_MAGIC_vec vtbl_vec vec() lvalue
- w PERL_MAGIC_utf8 vtbl_utf8 Cached UTF-8 information
- x PERL_MAGIC_substr vtbl_substr substr() lvalue
- y PERL_MAGIC_defelem vtbl_defelem Shadow "foreach" iterator
- variable / smart parameter
- vivification
- \ PERL_MAGIC_lvref vtbl_lvref Lvalue reference
- constructor
- ] PERL_MAGIC_checkcall vtbl_checkcall Inlining/mutation of call
- to this CV
- ~ PERL_MAGIC_ext (none) Available for use by
- extensions
When an uppercase and lowercase letter both exist in the table, then the uppercase letter is typically used to represent some kind of composite type (a list or a hash), and the lowercase letter is used to represent an element of that composite type. Some internals code makes use of this case relationship. However, 'v' and 'V' (vec and v-string) are in no way related.
The PERL_MAGIC_ext
and PERL_MAGIC_uvar
magic types are defined
specifically for use by extensions and will not be used by perl itself.
Extensions can use PERL_MAGIC_ext
magic to 'attach' private information
to variables (typically objects). This is especially useful because
there is no way for normal perl code to corrupt this private information
(unlike using extra elements of a hash object).
Similarly, PERL_MAGIC_uvar
magic can be used much like tie() to call a
C function any time a scalar's value is used or changed. The MAGIC
's
mg_ptr
field points to a ufuncs
structure:
- struct ufuncs {
- I32 (*uf_val)(pTHX_ IV, SV*);
- I32 (*uf_set)(pTHX_ IV, SV*);
- IV uf_index;
- };
When the SV is read from or written to, the uf_val
or uf_set
function will be called with uf_index
as the first arg and a pointer to
the SV as the second. A simple example of how to add PERL_MAGIC_uvar
magic is shown below. Note that the ufuncs structure is copied by
sv_magic, so you can safely allocate it on the stack.
- void
- Umagic(sv)
- SV *sv;
- PREINIT:
- struct ufuncs uf;
- CODE:
- uf.uf_val = &my_get_fn;
- uf.uf_set = &my_set_fn;
- uf.uf_index = 0;
- sv_magic(sv, 0, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf));
Attaching PERL_MAGIC_uvar
to arrays is permissible but has no effect.
For hashes there is a specialized hook that gives control over hash
keys (but not values). This hook calls PERL_MAGIC_uvar
'get' magic
if the "set" function in the ufuncs
structure is NULL. The hook
is activated whenever the hash is accessed with a key specified as
an SV
through the functions hv_store_ent
, hv_fetch_ent
,
hv_delete_ent
, and hv_exists_ent
. Accessing the key as a string
through the functions without the ..._ent
suffix circumvents the
hook. See GUTS in Hash::Util::FieldHash for a detailed description.
Note that because multiple extensions may be using PERL_MAGIC_ext
or PERL_MAGIC_uvar
magic, it is important for extensions to take
extra care to avoid conflict. Typically only using the magic on
objects blessed into the same class as the extension is sufficient.
For PERL_MAGIC_ext
magic, it is usually a good idea to define an
MGVTBL
, even if all its fields will be 0
, so that individual
MAGIC
pointers can be identified as a particular kind of magic
using their magic virtual table. mg_findext
provides an easy way
to do that:
- STATIC MGVTBL my_vtbl = { 0, 0, 0, 0, 0, 0, 0, 0 };
- MAGIC *mg;
- if ((mg = mg_findext(sv, PERL_MAGIC_ext, &my_vtbl))) {
- /* this is really ours, not another module's PERL_MAGIC_ext */
- my_priv_data_t *priv = (my_priv_data_t *)mg->mg_ptr;
- ...
- }
Also note that the sv_set*()
and sv_cat*()
functions described
earlier do not invoke 'set' magic on their targets. This must
be done by the user either by calling the SvSETMAGIC()
macro after
calling these functions, or by using one of the sv_set*_mg()
or
sv_cat*_mg()
functions. Similarly, generic C code must call the
SvGETMAGIC()
macro to invoke any 'get' magic if they use an SV
obtained from external sources in functions that don't handle magic.
See perlapi for a description of these functions.
For example, calls to the sv_cat*()
functions typically need to be
followed by SvSETMAGIC()
, but they don't need a prior SvGETMAGIC()
since their implementation handles 'get' magic.
- MAGIC *mg_find(SV *sv, int type); /* Finds the magic pointer of that
- * type */
This routine returns a pointer to a MAGIC
structure stored in the SV.
If the SV does not have that magical
feature, NULL
is returned. If the
SV has multiple instances of that magical feature, the first one will be
returned. mg_findext
can be used
to find a MAGIC
structure of an SV
based on both its magic type and its magic virtual table:
- MAGIC *mg_findext(SV *sv, int type, MGVTBL *vtbl);
Also, if the SV passed to mg_find
or mg_findext
is not of type
SVt_PVMG, Perl may core dump.
- int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen);
This routine checks to see what types of magic sv
has. If the mg_type
field is an uppercase letter, then the mg_obj is copied to nsv
, but
the mg_type field is changed to be the lowercase letter.
Tied hashes and arrays are magical beasts of the PERL_MAGIC_tied
magic type.
WARNING: As of the 5.004 release, proper usage of the array and hash access functions requires understanding a few caveats. Some of these caveats are actually considered bugs in the API, to be fixed in later releases, and are bracketed with [MAYCHANGE] below. If you find yourself actually applying such information in this section, be aware that the behavior may change in the future, umm, without warning.
The perl tie function associates a variable with an object that implements the various GET, SET, etc methods. To perform the equivalent of the perl tie function from an XSUB, you must mimic this behaviour. The code below carries out the necessary steps -- firstly it creates a new hash, and then creates a second hash which it blesses into the class which will implement the tie methods. Lastly it ties the two hashes together, and returns a reference to the new tied hash. Note that the code below does NOT call the TIEHASH method in the MyTie class - see Calling Perl Routines from within C Programs for details on how to do this.
- SV*
- mytie()
- PREINIT:
- HV *hash;
- HV *stash;
- SV *tie;
- CODE:
- hash = newHV();
- tie = newRV_noinc((SV*)newHV());
- stash = gv_stashpv("MyTie", GV_ADD);
- sv_bless(tie, stash);
- hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
- RETVAL = newRV_noinc(hash);
- OUTPUT:
- RETVAL
The av_store
function, when given a tied array argument, merely
copies the magic of the array onto the value to be "stored", using
mg_copy
. It may also return NULL, indicating that the value did not
actually need to be stored in the array. [MAYCHANGE] After a call to
av_store
on a tied array, the caller will usually need to call
mg_set(val)
to actually invoke the perl level "STORE" method on the
TIEARRAY object. If av_store
did return NULL, a call to
SvREFCNT_dec(val)
will also be usually necessary to avoid a memory
leak. [/MAYCHANGE]
The previous paragraph is applicable verbatim to tied hash access using the
hv_store
and hv_store_ent
functions as well.
av_fetch
and the corresponding hash functions hv_fetch
and
hv_fetch_ent
actually return an undefined mortal value whose magic
has been initialized using mg_copy
. Note the value so returned does not
need to be deallocated, as it is already mortal. [MAYCHANGE] But you will
need to call mg_get()
on the returned value in order to actually invoke
the perl level "FETCH" method on the underlying TIE object. Similarly,
you may also call mg_set()
on the return value after possibly assigning
a suitable value to it using sv_setsv
, which will invoke the "STORE"
method on the TIE object. [/MAYCHANGE]
[MAYCHANGE]
In other words, the array or hash fetch/store functions don't really
fetch and store actual values in the case of tied arrays and hashes. They
merely call mg_copy
to attach magic to the values that were meant to be
"stored" or "fetched". Later calls to mg_get
and mg_set
actually
do the job of invoking the TIE methods on the underlying objects. Thus
the magic mechanism currently implements a kind of lazy access to arrays
and hashes.
Currently (as of perl version 5.004), use of the hash and array access functions requires the user to be aware of whether they are operating on "normal" hashes and arrays, or on their tied variants. The API may be changed to provide more transparent access to both tied and normal data types in future versions. [/MAYCHANGE]
You would do well to understand that the TIEARRAY and TIEHASH interfaces are mere sugar to invoke some perl method calls while using the uniform hash and array syntax. The use of this sugar imposes some overhead (typically about two to four extra opcodes per FETCH/STORE operation, in addition to the creation of all the mortal variables required to invoke the methods). This overhead will be comparatively small if the TIE methods are themselves substantial, but if they are only a few statements long, the overhead will not be insignificant.
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
will be decremented at the end of
pseudo-block. This is similar to sv_2mortal
in that it is also a
mechanism for doing a delayed SvREFCNT_dec
. However, while sv_2mortal
extends the lifetime of sv
until the beginning of the next statement,
SAVEFREESV
extends it until the end of the enclosing scope. These
lifetimes can be wildly different.
Also compare SAVEMORTALIZESV
.
SAVEMORTALIZESV(SV *sv)
Just like SAVEFREESV
, but mortalizes sv
at the end of the current
scope instead of decrementing its reference count. This usually has the
effect of keeping sv
alive until the statement that called the currently
live scope has finished executing.
SAVEFREEOP(OP *op)
The OP *
is op_free()ed at the end of pseudo-block.
SAVEFREEPV(p)
The chunk of memory which is pointed to by p
is Safefree()ed at the
end of pseudo-block.
SAVECLEARSV(SV *sv)
Clears a slot in the current scratchpad which corresponds to sv
at
the end of pseudo-block.
SAVEDELETE(HV *hv, char *key, I32 length)
The key key
of hv
is deleted at the end of pseudo-block. The
string pointed to by key
is Safefree()ed. If one has a key in
short-lived storage, the corresponding string may be reallocated like
this:
- SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));
SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)
At the end of pseudo-block the function f
is called with the
only argument p
.
SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)
At the end of pseudo-block the function f
is called with the
implicit context argument (if any), and p
.
SAVESTACK_POS()
The current offset on the Perl internal stack (cf. SP
) is restored
at the end of pseudo-block.
The following API list contains functions, thus one needs to
provide pointers to the modifiable data explicitly (either C pointers,
or Perlish GV *
s). Where the above macros take int, a similar
function takes int *
.
SV* save_scalar(GV *gv)
Equivalent to Perl code local $gv
.
AV* save_ary(GV *gv)
HV* save_hash(GV *gv)
Similar to save_scalar
, but localize @gv
and %gv
.
void save_item(SV *item)
Duplicates the current value of SV
, on the exit from the current
ENTER
/LEAVE
pseudo-block will restore the value of SV
using the stored value. It doesn't handle magic. Use save_scalar
if
magic is affected.
void save_list(SV **sarg, I32 maxsarg)
A variant of save_item
which takes multiple arguments via an array
sarg
of SV*
of length maxsarg
.
SV* save_svref(SV **sptr)
Similar to save_scalar
, but will reinstate an SV *
.
void save_aptr(AV **aptr)
void save_hptr(HV **hptr)
Similar to save_svref
, but localize AV *
and HV *
.
The Alias
module implements localization of the basic types within the
caller's scope. People who are interested in how to localize things in
the containing scope should take a look there too.
The XSUB mechanism is a simple way for Perl programs to access C subroutines. An XSUB routine will have a stack that contains the arguments from the Perl program, and a way to map from the Perl data structures to a C equivalent.
The stack arguments are accessible through the ST(n)
macro, which returns
the n
'th stack argument. Argument 0 is the first argument passed in the
Perl subroutine call. These arguments are SV*
, and can be used anywhere
an SV*
is used.
Most of the time, output from the C routine can be handled through use of the RETVAL and OUTPUT directives. However, there are some cases where the argument stack is not already long enough to handle all the return values. An example is the POSIX tzname() call, which takes no arguments, but returns two, the local time zone's standard and summer time abbreviations.
To handle this situation, the PPCODE directive is used and the stack is extended using the macro:
- EXTEND(SP, num);
where SP
is the macro that represents the local copy of the stack pointer,
and num
is the number of elements the stack should be extended by.
Now that there is room on the stack, values can be pushed on it using PUSHs
macro. The pushed values will often need to be "mortal" (See
Reference Counts and Mortality):
- PUSHs(sv_2mortal(newSViv(an_integer)))
- PUSHs(sv_2mortal(newSVuv(an_unsigned_integer)))
- PUSHs(sv_2mortal(newSVnv(a_double)))
- PUSHs(sv_2mortal(newSVpv("Some String",0)))
- /* Although the last example is better written as the more
- * efficient: */
- PUSHs(newSVpvs_flags("Some String", SVs_TEMP))
And now the Perl program calling tzname
, the two values will be assigned
as in:
- ($standard_abbrev, $summer_abbrev) = POSIX::tzname;
An alternate (and possibly simpler) method to pushing values on the stack is to use the macro:
- XPUSHs(SV*)
This macro automatically adjusts the stack for you, if needed. Thus, you
do not need to call EXTEND
to extend the stack.
Despite their suggestions in earlier versions of this document the macros
(X)PUSH[iunp] are not suited to XSUBs which return multiple results.
For that, either stick to the (X)PUSHs macros shown above, or use the new
m(X)PUSH[iunp] macros instead; see Putting a C value on Perl stack.
For more information, consult perlxs and perlxstut.
If an AUTOLOAD routine is an XSUB, as with Perl subroutines, Perl puts the fully-qualified name of the autoloaded subroutine in the $AUTOLOAD variable of the XSUB's package.
But it also puts the same information in certain fields of the XSUB itself:
- HV *stash = CvSTASH(cv);
- const char *subname = SvPVX(cv);
- STRLEN name_length = SvCUR(cv); /* in bytes */
- U32 is_utf8 = SvUTF8(cv);
SvPVX(cv)
contains just the sub name itself, not including the package.
For an AUTOLOAD routine in UNIVERSAL or one of its superclasses,
CvSTASH(cv)
returns NULL during a method call on a nonexistent package.
Note: Setting $AUTOLOAD stopped working in 5.6.1, which did not support XS AUTOLOAD subs at all. Perl 5.8.0 introduced the use of fields in the XSUB itself. Perl 5.16.0 restored the setting of $AUTOLOAD. If you need to support 5.8-5.14, use the XSUB's fields.
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, 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.
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. A previous version of this document
stated that 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. But this has never been fully true.
SVs_PADMY
could be set on a variable that no longer resides in any pad.
While targets do have SVs_PADTMP
set, it can also be set on variables
that have never resided in a pad, but nonetheless act like targets. As
of perl 5.21.5, the SVs_PADMY
flag is no longer used and is defined as
0. SvPADMY()
now returns true for anything without SVs_PADTMP
.
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.
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 have been experimenting with removing Perl's dependency on the "normal" standard I/O suite and allowing other stdio implementations to be used. This involves creating a new abstraction layer that then calls whichever implementation of stdio Perl was compiled with. All XSUBs should now use the functions in the PerlIO abstraction layer and not make any assumptions about what kind of stdio is being used.
For a complete description of the PerlIO abstraction, consult perlapio.
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 OpSIBLING
pointer from the first child to the last (but
see below).
There are also some 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.
Finally, there is a LOGOP
, or logic op. Like a LISTOP
, this has one
or more children, but it doesn't have an op_last
field: so you have to
follow op_first
and then the OpSIBLING
chain itself to find the
last child. Instead it has an op_other
field, which is comparable to
the op_next
field described below, and represents an alternate
execution path. Operators like and
, or
and ? are LOGOP
s. Note
that in general, op_other
may not point to any of the direct children
of the LOGOP
.
Starting in version 5.21.2, perls built with the experimental
define -DPERL_OP_PARENT
add an extra boolean flag for each op,
op_moresib
. When not set, this indicates that this is the last op in an
OpSIBLING
chain. This frees up the op_sibling
field on the last
sibling to point back to the parent op. Under this build, that field is
also renamed op_sibparent
to reflect its joint role. The macro
OpSIBLING(o)
wraps this special behaviour, and always returns NULL on
the last sibling. With this build the op_parent(o)
function can be
used to find the parent of any op. Thus for forward compatibility, you
should always use the OpSIBLING(o)
macro rather than accessing
op_sibling
directly.
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). Optimizations performed
at this stage are subject to the same restrictions as in the pass 2.
Peephole optimizations are done by calling the function pointed to
by the global variable PL_peepp
. By default, PL_peepp
just
calls the function pointed to by the global variable PL_rpeepp
.
By default, that performs some basic op fixups and optimisations along
the execution-order op chain, and recursively calls PL_rpeepp
for
each side chain of ops (resulting from conditionals). Extensions may
provide additional optimisations or fixups, hooking into either the
per-subroutine or recursive stage, like this:
- static peep_t prev_peepp;
- static void my_peep(pTHX_ OP *o)
- {
- /* custom per-subroutine optimisation goes here */
- prev_peepp(aTHX_ o);
- /* custom per-subroutine optimisation may also go here */
- }
- BOOT:
- prev_peepp = PL_peepp;
- PL_peepp = my_peep;
- static peep_t prev_rpeepp;
- static void my_rpeep(pTHX_ OP *o)
- {
- OP *orig_o = o;
- for(; o; o = o->op_next) {
- /* custom per-op optimisation goes here */
- }
- prev_rpeepp(aTHX_ orig_o);
- }
- BOOT:
- prev_rpeepp = PL_rpeepp;
- PL_rpeepp = my_rpeep;
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.
As of perl 5.14 it is possible to hook into the compile-time lexical
scope mechanism using Perl_blockhook_register
. This is used like
this:
- STATIC void my_start_hook(pTHX_ int full);
- STATIC BHK my_hooks;
- BOOT:
- BhkENTRY_set(&my_hooks, bhk_start, my_start_hook);
- Perl_blockhook_register(aTHX_ &my_hooks);
This will arrange to have my_start_hook
called at the start of
compiling every lexical scope. The available hooks are:
void bhk_start(pTHX_ int full)
This is called just after starting a new lexical scope. Note that Perl code like
- if ($x) { ... }
creates two scopes: the first starts at the ( and has full == 1
,
the second starts at the { and has full == 0
. Both end at the
}, so calls to start
and pre
/post_end
will match. Anything
pushed onto the save stack by this hook will be popped just before the
scope ends (between the pre_
and post_end
hooks, in fact).
void bhk_pre_end(pTHX_ OP **o)
This is called at the end of a lexical scope, just before unwinding the stack. o is the root of the optree representing the scope; it is a double pointer so you can replace the OP if you need to.
void bhk_post_end(pTHX_ OP **o)
This is called at the end of a lexical scope, just after unwinding the
stack. o is as above. Note that it is possible for calls to pre_
and post_end
to nest, if there is something on the save stack that
calls string eval.
void bhk_eval(pTHX_ OP *const o)
This is called just before starting to compile an eval STRING
, do
FILE
, require or use, after the eval has been set up. o is the
OP that requested the eval, and will normally be an OP_ENTEREVAL
,
OP_DOFILE
or OP_REQUIRE
.
Once you have your hook functions, you need a BHK
structure to put
them in. It's best to allocate it statically, since there is no way to
free it once it's registered. The function pointers should be inserted
into this structure using the BhkENTRY_set
macro, which will also set
flags indicating which entries are valid. If you do need to allocate
your BHK
dynamically for some reason, be sure to zero it before you
start.
Once registered, there is no mechanism to switch these hooks off, so if
that is necessary you will need to do this yourself. An entry in %^H
is probably the best way, so the effect is lexically scoped; however it
is also possible to use the BhkDISABLE
and BhkENABLE
macros to
temporarily switch entries on and off. You should also be aware that
generally speaking at least one scope will have opened before your
extension is loaded, so you will see some pre
/post_end
pairs that
didn't have a matching start
.
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.
One macro controls the major Perl build flavor: MULTIPLICITY. The MULTIPLICITY build has a C structure that packages all the interpreter state. With multiplicity-enabled perls, PERL_IMPLICIT_CONTEXT is also normally defined, and enables the support for passing in a "hidden" first argument that represents all three data structures. MULTIPLICITY makes multi-threaded perls possible (with the ithreads threading model, related to the macro USE_ITHREADS.)
Two other "encapsulation" macros are the PERL_GLOBAL_STRUCT and
PERL_GLOBAL_STRUCT_PRIVATE (the latter turns on the former, and the
former turns on MULTIPLICITY.) The PERL_GLOBAL_STRUCT causes all the
internal variables of Perl to be wrapped inside a single global struct,
struct perl_vars, accessible as (globals) &PL_Vars or PL_VarsPtr or
the function Perl_GetVars(). The PERL_GLOBAL_STRUCT_PRIVATE goes
one step further, there is still a single struct (allocated in main()
either from heap or from stack) but there are no global data symbols
pointing to it. In either case the global struct should be initialized
as the very first thing in main() using Perl_init_global_struct() and
correspondingly tear it down after perl_free() using Perl_free_global_struct(),
please see miniperlmain.c for usage details. You may also need
to use dVAR
in your coding to "declare the global variables"
when you are using them. dTHX does this for you automatically.
To see whether you have non-const data you can use a BSD (or GNU)
compatible nm
:
- nm libperl.a | grep -v ' [TURtr] '
If this displays any D
or d
symbols (or possibly C
or c
),
you have non-const data. The symbols the grep removed are as follows:
Tt
are text, or code, the Rr
are read-only (const) data,
and the U
is <undefined>, external symbols referred to.
The test t/porting/libperl.t does this kind of symbol sanity
checking on libperl.a
.
For backward compatibility reasons defining just PERL_GLOBAL_STRUCT doesn't actually hide all symbols inside a big global struct: some PerlIO_xxx vtables are left visible. The PERL_GLOBAL_STRUCT_PRIVATE then hides everything (see how the PERLIO_FUNCS_DECL is used).
All this obviously requires a way for the Perl internal functions to be either subroutines taking some kind of structure as the first argument, or subroutines taking nothing as the first argument. To enable these two very different ways of building the interpreter, the Perl source (as it does in so many other situations) makes heavy use of macros and subroutine naming conventions.
First problem: deciding which functions will be public API functions and
which will be private. All functions whose names begin S_
are private
(think "S" for "secret" or "static"). All other functions begin with
"Perl_", but just because a function begins with "Perl_" does not mean it is
part of the API. (See Internal Functions.) The easiest way to be sure a
function is part of the API is to find its entry in perlapi.
If it exists in perlapi, it's part of the API. If it doesn't, and you
think it should be (i.e., you need it for your extension), send mail via
perlbug explaining why you think it should be.
Second problem: there must be a syntax so that the same subroutine declarations and calls can pass a structure as their first argument, or pass nothing. To solve this, the subroutines are named and declared in a particular way. Here's a typical start of a static function used within the Perl guts:
- STATIC void
- S_incline(pTHX_ char *s)
STATIC becomes "static" in C, and may be #define'd to nothing in some configurations in the future.
A public function (i.e. part of the internal API, but not necessarily sanctioned for use in extensions) begins like this:
- void
- Perl_sv_setiv(pTHX_ SV* dsv, IV num)
pTHX_
is one of a number of macros (in perl.h) that hide the
details of the interpreter's context. THX stands for "thread", "this",
or "thingy", as the case may be. (And no, George Lucas is not involved. :-)
The first character could be 'p' for a prototype, 'a' for argument,
or 'd' for declaration, so we have pTHX
, aTHX
and dTHX
, and
their variants.
When Perl is built without options that set PERL_IMPLICIT_CONTEXT, there is no first argument containing the interpreter's context. The trailing underscore in the pTHX_ macro indicates that the macro expansion needs a comma after the context argument because other arguments follow it. If PERL_IMPLICIT_CONTEXT is not defined, pTHX_ will be ignored, and the subroutine is not prototyped to take the extra argument. The form of the macro without the trailing underscore is used when there are no additional explicit arguments.
When a core function calls another, it must pass the context. This
is normally hidden via macros. Consider sv_setiv
. It expands into
something like this:
- #ifdef PERL_IMPLICIT_CONTEXT
- #define sv_setiv(a,b) Perl_sv_setiv(aTHX_ a, b)
- /* can't do this for vararg functions, see below */
- #else
- #define sv_setiv Perl_sv_setiv
- #endif
This works well, and means that XS authors can gleefully write:
- sv_setiv(foo, bar);
and still have it work under all the modes Perl could have been compiled with.
This doesn't work so cleanly for varargs functions, though, as macros
imply that the number of arguments is known in advance. Instead we
either need to spell them out fully, passing aTHX_
as the first
argument (the Perl core tends to do this with functions like
Perl_warner), or use a context-free version.
The context-free version of Perl_warner is called
Perl_warner_nocontext, and does not take the extra argument. Instead
it does dTHX; to get the context from thread-local storage. We
#define warner Perl_warner_nocontext
so that extensions get source
compatibility at the expense of performance. (Passing an arg is
cheaper than grabbing it from thread-local storage.)
You can ignore [pad]THXx when browsing the Perl headers/sources. Those are strictly for use within the core. Extensions and embedders need only be aware of [pad]THX.
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 don't have to do anything new in your extension to get this; since the Perl library provides Perl_get_context(), it will all just work.
The second, more efficient way is to use the following template for your Foo.xs:
- #define PERL_NO_GET_CONTEXT /* we want efficiency */
- #include "EXTERN.h"
- #include "perl.h"
- #include "XSUB.h"
- STATIC void my_private_function(int arg1, int arg2);
- STATIC void
- my_private_function(int arg1, int arg2)
- {
- dTHX; /* fetch context */
- ... call many Perl API functions ...
- }
- [... etc ...]
- MODULE = Foo PACKAGE = Foo
- /* typical XSUB */
- void
- my_xsub(arg)
- int arg
- CODE:
- my_private_function(arg, 10);
Note that the only two changes from the normal way of writing an
extension is the addition of a #define PERL_NO_GET_CONTEXT
before
including the Perl headers, followed by a dTHX;
declaration at
the start of every function that will call the Perl API. (You'll
know which functions need this, because the C compiler will complain
that there's an undeclared identifier in those functions.) No changes
are needed for the XSUBs themselves, because the XS() macro is
correctly defined to pass in the implicit context if needed.
The third, even more efficient way is to ape how it is done within the Perl guts:
- #define PERL_NO_GET_CONTEXT /* we want efficiency */
- #include "EXTERN.h"
- #include "perl.h"
- #include "XSUB.h"
- /* pTHX_ only needed for functions that call Perl API */
- STATIC void my_private_function(pTHX_ int arg1, int arg2);
- STATIC void
- my_private_function(pTHX_ int arg1, int arg2)
- {
- /* dTHX; not needed here, because THX is an argument */
- ... call Perl API functions ...
- }
- [... etc ...]
- MODULE = Foo PACKAGE = Foo
- /* typical XSUB */
- void
- my_xsub(arg)
- int arg
- CODE:
- my_private_function(aTHX_ arg, 10);
This implementation never has to fetch the context using a function call, since it is always passed as an extra argument. Depending on your needs for simplicity or efficiency, you may mix the previous two approaches freely.
Never add a comma after pTHX
yourself--always use the form of the
macro with the underscore for functions that take explicit arguments,
or the form without the argument for functions with no explicit arguments.
If one is compiling Perl with the -DPERL_GLOBAL_STRUCT
the dVAR
definition is needed if the Perl global variables (see perlvars.h
or globvar.sym) are accessed in the function and dTHX
is not
used (the dTHX
includes the dVAR
if necessary). One notices
the need for dVAR
only with the said compile-time define, because
otherwise the Perl global variables are visible as-is.
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 on Windows.
This allows the ability to provide an extra pointer (called the "host" environment) for all the system calls. This makes it possible for all the system stuff to maintain their own state, broken down into seven C structures. These are thin wrappers around the usual system calls (see win32/perllib.c) for the default perl executable, but for a more ambitious host (like the one that would do fork() emulation) all the extra work needed to pretend that different interpreters are actually different "processes", would be done here.
The Perl engine/interpreter and the host are orthogonal entities. There could be one or more interpreters in a process, and one or more "hosts", with free association between them.
All of Perl's internal functions which will be exposed to the outside
world are prefixed by Perl_
so that they will not conflict with XS
functions or functions used in a program in which Perl is embedded.
Similarly, all global variables begin with PL_
. (By convention,
static functions start with S_
.)
Inside the Perl core (PERL_CORE
defined), you can get at the functions
either with or without the Perl_
prefix, thanks to a bunch of defines
that live in embed.h. Note that extension code should not set
PERL_CORE
; this exposes the full perl internals, and is likely to cause
breakage of the XS in each new perl release.
The file embed.h is generated automatically from embed.pl and embed.fnc. embed.pl also creates the prototyping header files for the internal functions, generates the documentation and a lot of other bits and pieces. It's important that when you add a new function to the core or change an existing one, you change the data in the table in embed.fnc as well. Here's a sample entry from that table:
- Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval
The second column is the return type, the third column the name. Columns after that are the arguments. The first column is a set of flags:
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 an interpreter context, so the definition has no
pTHX
, and it follows that callers don't use aTHX
. (See
Background and PERL_IMPLICIT_CONTEXT.)
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.
Note that there are different "long doubles": Perl will use whatever the compiler has.
If you are printing addresses of pointers, use UVxf combined with PTR2UV(), do not use %lx or %p.
The most general way to do this is to cast them to a UV or IV, and print as in the previous section.
But if you're using PerlIO_printf()
, it's less typing and visual
clutter to use the "%z"
length modifier (for siZe):
- PerlIO_printf("STRLEN is %zu\n", len);
This modifier is not portable, so its use should be restricted to
PerlIO_printf()
.
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 are a couple of macros to do very basic exception handling in XS
modules. You have to define NO_XSLOCKS
before including XSUB.h to
be able to use these macros:
- #define NO_XSLOCKS
- #include "XSUB.h"
You can use these macros if you call code that may croak, but you need to do some cleanup before giving control back to Perl. For example:
- dXCPT; /* set up necessary variables */
- XCPT_TRY_START {
- code_that_may_croak();
- } XCPT_TRY_END
- XCPT_CATCH
- {
- /* do cleanup here */
- XCPT_RETHROW;
- }
Note that you always have to rethrow an exception that has been
caught. Using these macros, it is not possible to just catch the
exception and ignore it. If you have to ignore the exception, you
have to use the call_*
function.
The advantage of using the above macros is that you don't have
to setup an extra function for call_*
, and that using these
macros is faster than using call_*
.
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
- L<perlapi/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. You can learn more about Unicode and Perl's Unicode model in perlunicode.
(On EBCDIC platforms, Perl uses instead UTF-EBCDIC, which is a form of UTF-8 adapted for EBCDIC platforms. Below, we just talk about UTF-8. UTF-EBCDIC is like UTF-8, but the details are different. The macros hide the differences from you, just remember that the particular numbers and bit patterns presented below will differ in UTF-EBCDIC.)
You can't. This is because UTF-8 data is stored in bytes just like
non-UTF-8 data. The Unicode character 200, (0xC8
for you hex types)
capital E with a grave accent, is represented by the two bytes
v196.172
. Unfortunately, the non-Unicode string chr(196).chr(172)
has that byte sequence as well. So you can't tell just by looking -- this
is what makes Unicode input an interesting problem.
In general, you either have to know what you're dealing with, or you
have to guess. The API function is_utf8_string
can help; it'll tell
you if a string contains only valid UTF-8 characters, and the chances
of a non-UTF-8 string looking like valid UTF-8 become very small very
quickly with increasing string length. On a character-by-character
basis, isUTF8_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 0...127 are stored in one
byte, just like good ol' ASCII. Character 128 is stored as
v194.128
; this continues up to character 191, which is
v194.191
. Now we've run out of bits (191 is binary
10111111
) so we move on; character 192 is v195.128
. And
so it goes on, moving to three bytes at character 2048.
Unicode Encodings in perlunicode has pictures of how this works.
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 is encoded as a single byte even in UTF-8):
- U8 *utf;
- U8 *utf_end; /* 1 beyond buffer pointed to by utf */
- UV uv; /* Note: a UV, not a U8, not a char */
- STRLEN len; /* length of character in bytes */
- if (!UTF8_IS_INVARIANT(*utf))
- /* Must treat this as UTF-8 */
- uv = utf8_to_uvchr_buf(utf, utf_end, &len);
- else
- /* OK to treat this character as a byte */
- uv = *utf;
You can also see in that example that we use utf8_to_uvchr_buf
to get the
value of the character; the inverse function uvchr_to_utf8
is available
for putting a UV into UTF-8:
- if (!UVCHR_IS_INVARIANT(uv))
- /* Must treat this as UTF8 */
- utf8 = uvchr_to_utf8(utf8, uv);
- else
- /* OK to treat this character as a byte */
- *utf8++ = uv;
You must convert characters to UVs using the above functions if
you're ever in a situation where you have to match UTF-8 and non-UTF-8
characters. You may not skip over UTF-8 characters in this case. If you
do this, you'll lose the ability to match hi-bit non-UTF-8 characters;
for instance, if your UTF-8 string contains v196.172
, and you skip
that character, you can never match a chr(200) in a non-UTF-8 string.
So don't do that!
(Note that we don't have to test for invariant characters in the examples above. The functions work on any well-formed UTF-8 input. It's just that its faster to avoid the function overhead when it's not needed.)
Currently, Perl deals with UTF-8 strings and non-UTF-8 strings
slightly differently. A flag in the SV, SVf_UTF8
, indicates that the
string is internally encoded as UTF-8. Without it, the byte value is the
codepoint number and vice versa. This flag is only meaningful if the SV
is SvPOK
or immediately after stringification via SvPV
or a
similar macro. 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
UTF-8 data is not properly distinguished, regular expressions,
length, substr and other string handling operations will have
undesirable (wrong) results.
The problem comes when you have, for instance, a string that isn't flagged as UTF-8, and contains a byte sequence that could be UTF-8 -- especially when combining non-UTF-8 and UTF-8 strings.
Never forget that the SVf_UTF8
flag is separate from the PV value; you
need to be sure you don't accidentally knock it off while you're
manipulating SVs. More specifically, you cannot expect to do this:
- SV *sv;
- SV *nsv;
- STRLEN len;
- char *p;
- p = SvPV(sv, len);
- frobnicate(p);
- nsv = newSVpvn(p, len);
The char*
string does not tell you the whole story, and you can't
copy or reconstruct an SV just by copying the string value. Check if the
old SV has the UTF8 flag set (after the SvPV
call), and act
accordingly:
- p = SvPV(sv, len);
- is_utf8 = SvUTF8(sv);
- frobnicate(p, is_utf8);
- nsv = newSVpvn(p, len);
- if (is_utf8)
- SvUTF8_on(nsv);
In the above, your frobnicate
function has been changed to be made
aware of whether or not it's dealing with UTF-8 data, so that it can
handle the string appropriately.
Since just passing an SV to an XS function and copying the data of
the SV is not enough to copy the UTF8 flags, even less right is just
passing a char *
to an XS function.
For full generality, use the DO_UTF8 macro to see if the string in an SV is to be treated as UTF-8. This takes into account if the call to the XS function is being made from within the scope of use bytes . If so, the underlying bytes that comprise the UTF-8 string are to be exposed, rather than the character they represent. But this pragma should only really be used for debugging and perhaps low-level testing at the byte level. Hence most XS code need not concern itself with this, but various areas of the perl core do need to support it.
And this isn't the whole story. Starting in Perl v5.12, strings that
aren't encoded in UTF-8 may also be treated as Unicode under various
conditions (see ASCII Rules versus Unicode Rules in perlunicode).
This is only really a problem for characters whose ordinals are between
128 and 255, and their behavior varies under ASCII versus Unicode rules
in ways that your code cares about (see The Unicode Bug in perlunicode).
There is no published API for dealing with this, as it is subject to
change, but you can look at the code for pp_lc
in pp.c for an
example as to how it's currently done.
If you're mixing UTF-8 and non-UTF-8 strings, it is necessary to upgrade the non-UTF-8 strings to UTF-8. If you've got an SV, the easiest way to do this is:
- sv_utf8_upgrade(sv);
However, you must not do this, for example:
- if (!SvUTF8(left))
- sv_utf8_upgrade(left);
If you do this in a binary operator, you will actually change one of the strings that came into the operator, and, while it shouldn't be noticeable by the end user, it can cause problems in deficient code.
Instead, bytes_to_utf8
will give you a UTF-8-encoded copy of its
string argument. This is useful for having the data available for
comparisons and so on, without harming the original SV. There's also
utf8_to_bytes
to go the other way, but naturally, this will fail if
the string contains any characters above 255 that can't be represented
in a single byte.
sv_cmp in perlapi and sv_cmp_flags in perlapi do a lexigraphic comparison of two SV's, and handle UTF-8ness properly. Note, however, that Unicode specifies a much fancier mechanism for collation, available via the Unicode::Collate module.
To just compare two strings for equality/non-equality, you can just use memEQ() and memNE() as usual, except the strings must be both UTF-8 or not UTF-8 encoded.
To compare two strings case-insensitively, use foldEQ_utf8() (the strings don't have to have the same UTF-8ness).
Not really. Just remember these things:
There's no way to tell if a char *
or U8 *
string is UTF-8
or not. But you can tell if an SV is to be treated as UTF-8 by calling
DO_UTF8
on it, after stringifying it with SvPV
or a similar
macro. And, you can tell if SV is actually UTF-8 (even if it is not to
be treated as such) by looking at its SvUTF8
flag (again after
stringifying it). Don't forget to set the flag if something should be
UTF-8.
Treat the flag as part of the PV, even though it's not -- if you pass on
the PV to somewhere, pass on the flag too.
If a string is UTF-8, always use utf8_to_uvchr_buf
to get at the value,
unless UTF8_IS_INVARIANT(*s)
in which case you can use *s
.
When writing a character UV to a UTF-8 string, always use
uvchr_to_utf8
, unless UVCHR_IS_INVARIANT(uv))
in which case
you can use *s = uv
.
Mixing UTF-8 and non-UTF-8 strings is
tricky. Use bytes_to_utf8
to get
a new string which is UTF-8 encoded, and then combine them.
Custom operator support is an 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 op_ppaddr
of your own
PP function. This should be defined in XS code, and should look like
the PP ops in pp_*.c. You are responsible for ensuring that your op
takes the appropriate number of values from the stack, and you are
responsible for adding stack marks if necessary.
You should also "register" your op with the Perl interpreter so that it
can produce sensible error and warning messages. Since it is possible to
have multiple custom ops within the one "logical" op type OP_CUSTOM
,
Perl uses the value of o->op_ppaddr
to determine which custom op
it is dealing with. You should create an XOP
structure for each
ppaddr you use, set the properties of the custom op with
XopENTRY_set
, and register the structure against the ppaddr using
Perl_custom_op_register
. A trivial example might look like:
- static XOP my_xop;
- static OP *my_pp(pTHX);
- BOOT:
- XopENTRY_set(&my_xop, xop_name, "myxop");
- XopENTRY_set(&my_xop, xop_desc, "Useless custom op");
- Perl_custom_op_register(aTHX_ my_pp, &my_xop);
The available fields in the structure are:
A short name for your op. This will be included in some error messages,
and will also be returned as $op->name
by the B module, so
it will appear in the output of module like B::Concise.
A short description of the function of the op.
Which of the various *OP
structures this op uses. This should be one of
the OA_*
constants from op.h, namely
This should be interpreted as 'PVOP
' only. The _OR_SVOP
is because
the only core PVOP
, OP_TRANS
, can sometimes be a SVOP
instead.
The other OA_*
constants should not be used.
This member is of type Perl_cpeep_t
, which expands to void
(*Perl_cpeep_t)(aTHX_ OP *o, OP *oldop)
. If it is set, this function
will be called from Perl_rpeep
when ops of this type are encountered
by the peephole optimizer. o is the OP that needs optimizing;
oldop is the previous OP optimized, whose op_next
points to o.
B::Generate
directly supports the creation of custom ops by name.
Note: this section describes a non-public internal API that is subject to change without notice.
In Perl, dynamic scoping refers to the runtime nesting of things like
subroutine calls, evals etc, as well as the entering and exiting of block
scopes. For example, the restoring of a localised variable is
determined by the dynamic scope.
Perl tracks the dynamic scope by a data structure called the context
stack, which is an array of PERL_CONTEXT
structures, and which is
itself a big union for all the types of context. Whenever a new scope is
entered (such as a block, a for
loop, or a subroutine call), a new
context entry is pushed onto the stack. Similarly when leaving a block or
returning from a subroutine call etc. a context is popped. Since the
context stack represents the current dynamic scope, it can be searched.
For example, next LABEL
searches back through the stack looking for a
loop context that matches the label; return pops contexts until it
finds a sub or eval context or similar; caller examines sub contexts on
the stack.
Each context entry is labelled with a context type, cx_type
. Typical
context types are CXt_SUB
, CXt_EVAL
etc., as well as CXt_BLOCK
and CXt_NULL
which represent a basic scope (as pushed by pp_enter
)
and a sort block. The type determines which part of the context union are
valid.
The main division in the context struct is between a substitution scope
(CXt_SUBST
) and block scopes, which are everything else. The former is
just used while executing s///e, and won't be discussed further
here.
All the block scope types share a common base, which corresponds to
CXt_BLOCK
. This stores the old values of various scope-related
variables like PL_curpm
, as well as information about the current
scope, such as gimme
. On scope exit, the old variables are restored.
Particular block scope types store extra per-type information. For
example, CXt_SUB
stores the currently executing CV, while the various
for loop types might hold the original loop variable SV. On scope exit,
the per-type data is processed; for example the CV has its reference count
decremented, and the original loop variable is restored.
The macro cxstack
returns the base of the current context stack, while
cxstack_ix
is the index of the current frame within that stack.
In fact, the context stack is actually part of a stack-of-stacks system;
whenever something unusual is done such as calling a DESTROY
or tie
handler, a new stack is pushed, then popped at the end.
Note that the API described here changed considerably in perl 5.24; prior
to that, big macros like PUSHBLOCK
and POPSUB
were used; in 5.24
they were replaced by the inline static functions described below. In
addition, the ordering and detail of how these macros/function work
changed in many ways, often subtly. In particular they didn't handle
saving the savestack and temps stack positions, and required additional
ENTER
, SAVETMPS
and LEAVE
compared to the new functions. The
old-style macros will not be described further.
For pushing a new context, the two basic functions are
cx = cx_pushblock()
, which pushes a new basic context block and returns
its address, and a family of similar functions with names like
cx_pushsub(cx)
which populate the additional type-dependent fields in
the cx
struct. Note that CXt_NULL
and CXt_BLOCK
don't have their
own push functions, as they don't store any data beyond that pushed by
cx_pushblock
.
The fields of the context struct and the arguments to the cx_*
functions are subject to change between perl releases, representing
whatever is convenient or efficient for that release.
A typical context stack pushing can be found in pp_entersub
; the
following shows a simplified and stripped-down example of a non-XS call,
along with comments showing roughly what each function does.
- dMARK;
- U8 gimme = GIMME_V;
- bool hasargs = cBOOL(PL_op->op_flags & OPf_STACKED);
- OP *retop = PL_op->op_next;
- I32 old_ss_ix = PL_savestack_ix;
- CV *cv = ....;
- /* ... make mortal copies of stack args which are PADTMPs here ... */
- /* ... do any additional savestack pushes here ... */
- /* Now push a new context entry of type 'CXt_SUB'; initially just
- * doing the actions common to all block types: */
- cx = cx_pushblock(CXt_SUB, gimme, MARK, old_ss_ix);
- /* this does (approximately):
- CXINC; /* cxstack_ix++ (grow if necessary) */
- cx = CX_CUR(); /* and get the address of new frame */
- cx->cx_type = CXt_SUB;
- cx->blk_gimme = gimme;
- cx->blk_oldsp = MARK - PL_stack_base;
- cx->blk_oldsaveix = old_ss_ix;
- cx->blk_oldcop = PL_curcop;
- cx->blk_oldmarksp = PL_markstack_ptr - PL_markstack;
- cx->blk_oldscopesp = PL_scopestack_ix;
- cx->blk_oldpm = PL_curpm;
- cx->blk_old_tmpsfloor = PL_tmps_floor;
- PL_tmps_floor = PL_tmps_ix;
- */
- /* then update the new context frame with subroutine-specific info,
- * such as the CV about to be executed: */
- cx_pushsub(cx, cv, retop, hasargs);
- /* this does (approximately):
- cx->blk_sub.cv = cv;
- cx->blk_sub.olddepth = CvDEPTH(cv);
- cx->blk_sub.prevcomppad = PL_comppad;
- cx->cx_type |= (hasargs) ? CXp_HASARGS : 0;
- cx->blk_sub.retop = retop;
- SvREFCNT_inc_simple_void_NN(cv);
- */
Note that cx_pushblock()
sets two new floors: for the args stack (to
MARK
) and the temps stack (to PL_tmps_ix
). While executing at this
scope level, every nextstate
(amongst others) will reset the args and
tmps stack levels to these floors. Note that since cx_pushblock
uses
the current value of PL_tmps_ix
rather than it being passed as an arg,
this dictates at what point cx_pushblock
should be called. In
particular, any new mortals which should be freed only on scope exit
(rather than at the next nextstate
) should be created first.
Most callers of cx_pushblock
simply set the new args stack floor to the
top of the previous stack frame, but for CXt_LOOP_LIST
it stores the
items being iterated over on the stack, and so sets blk_oldsp
to the
top of these items instead. Note that, contrary to its name, blk_oldsp
doesn't always represent the value to restore PL_stack_sp
to on scope
exit.
Note the early capture of PL_savestack_ix
to old_ss_ix
, which is
later passed as an arg to cx_pushblock
. In the case of pp_entersub
,
this is because, although most values needing saving are stored in fields
of the context struct, an extra value needs saving only when the debugger
is running, and it doesn't make sense to bloat the struct for this rare
case. So instead it is saved on the savestack. Since this value gets
calculated and saved before the context is pushed, it is necessary to pass
the old value of PL_savestack_ix
to cx_pushblock
, to ensure that the
saved value gets freed during scope exit. For most users of
cx_pushblock
, where nothing needs pushing on the save stack,
PL_savestack_ix
is just passed directly as an arg to cx_pushblock
.
Note that where possible, values should be saved in the context struct rather than on the save stack; it's much faster that way.
Normally cx_pushblock
should be immediately followed by the appropriate
cx_pushfoo
, with nothing between them; this is because if code
in-between could die (e.g. a warning upgraded to fatal), then the context
stack unwinding code in dounwind
would see (in the example above) a
CXt_SUB
context frame, but without all the subroutine-specific fields
set, and crashes would soon ensue.
Where the two must be separate, initially set the type to CXt_NULL
or
CXt_BLOCK
, and later change it to CXt_foo
when doing the
cx_pushfoo
. This is exactly what pp_enteriter
does, once it's
determined which type of loop it's pushing.
Contexts are popped using cx_popsub()
etc. and cx_popblock()
. Note
however, that unlike cx_pushblock
, neither of these functions actually
decrement the current context stack index; this is done separately using
CX_POP()
.
There are two main ways that contexts are popped. During normal execution
as scopes are exited, functions like pp_leave
, pp_leaveloop
and
pp_leavesub
process and pop just one context using cx_popfoo
and
cx_popblock
. On the other hand, things like pp_return
and next
may have to pop back several scopes until a sub or loop context is found,
and exceptions (such as die) need to pop back contexts until an eval
context is found. Both of these are accomplished by dounwind()
, which
is capable of processing and popping all contexts above the target one.
Here is a typical example of context popping, as found in pp_leavesub
(simplified slightly):
- U8 gimme;
- PERL_CONTEXT *cx;
- SV **oldsp;
- OP *retop;
- cx = CX_CUR();
- gimme = cx->blk_gimme;
- oldsp = PL_stack_base + cx->blk_oldsp; /* last arg of previous frame */
- if (gimme == G_VOID)
- PL_stack_sp = oldsp;
- else
- leave_adjust_stacks(oldsp, oldsp, gimme, 0);
- CX_LEAVE_SCOPE(cx);
- cx_popsub(cx);
- cx_popblock(cx);
- retop = cx->blk_sub.retop;
- CX_POP(cx);
- return retop;
The steps above are in a very specific order, designed to be the reverse order of when the context was pushed. The first thing to do is to copy and/or protect any any return arguments and free any temps in the current scope. Scope exits like an rvalue sub normally return a mortal copy of their return args (as opposed to lvalue subs). It is important to make this copy before the save stack is popped or variables are restored, or bad things like the following can happen:
- sub f { my $x =...; $x } # $x freed before we get to copy it
- sub f { /(...)/; $1 } # PL_curpm restored before $1 copied
Although we wish to free any temps at the same time, we have to be careful
not to free any temps which are keeping return args alive; nor to free the
temps we have just created while mortal copying return args. Fortunately,
leave_adjust_stacks()
is capable of making mortal copies of return args,
shifting args down the stack, and only processing those entries on the
temps stack that are safe to do so.
In void context no args are returned, so it's more efficient to skip
calling leave_adjust_stacks()
. Also in void context, a nextstate
op
is likely to be imminently called which will do a FREETMPS
, so there's
no need to do that either.
The next step is to pop savestack entries: CX_LEAVE_SCOPE(cx)
is just
defined as <LEAVE_SCOPE(cx-
blk_oldsaveix)>>. Note that during the
popping, it's possible for perl to call destructors, call STORE
to undo
localisations of tied vars, and so on. Any of these can die or call
exit(). In this case, dounwind()
will be called, and the current
context stack frame will be re-processed. Thus it is vital that all steps
in popping a context are done in such a way to support reentrancy. The
other alternative, of decrementing cxstack_ix
before processing the
frame, would lead to leaks and the like if something died halfway through,
or overwriting of the current frame.
CX_LEAVE_SCOPE
itself is safely re-entrant: if only half the savestack
items have been popped before dying and getting trapped by eval, then the
CX_LEAVE_SCOPE
s in dounwind
or pp_leaveeval
will continue where
the first one left off.
The next step is the type-specific context processing; in this case
cx_popsub
. In part, this looks like:
- cv = cx->blk_sub.cv;
- CvDEPTH(cv) = cx->blk_sub.olddepth;
- cx->blk_sub.cv = NULL;
- SvREFCNT_dec(cv);
where its processing the just-executed CV. Note that before it decrements
the CV's reference count, it nulls the blk_sub.cv
. This means that if
it re-enters, the CV won't be freed twice. It also means that you can't
rely on such type-specific fields having useful values after the return
from cx_popfoo
.
Next, cx_popblock
restores all the various interpreter vars to their
previous values or previous high water marks; it expands to:
- PL_markstack_ptr = PL_markstack + cx->blk_oldmarksp;
- PL_scopestack_ix = cx->blk_oldscopesp;
- PL_curpm = cx->blk_oldpm;
- PL_curcop = cx->blk_oldcop;
- PL_tmps_floor = cx->blk_old_tmpsfloor;
Note that it doesn't restore PL_stack_sp
; as mentioned earlier,
which value to restore it to depends on the context type (specifically
for (list) {}
), and what args (if any) it returns; and that will
already have been sorted out earlier by leave_adjust_stacks()
.
Finally, the context stack pointer is actually decremented by CX_POP(cx)
.
After this point, it's possible that that the current context frame could
be overwritten by other contexts being pushed. Although things like ties
and DESTROY
are supposed to work within a new context stack, it's best
not to assume this. Indeed on debugging builds, CX_POP(cx)
deliberately
sets cx
to null to detect code that is still relying on the field
values in that context frame. Note in the pp_leavesub()
example above,
we grab blk_sub.retop
before calling CX_POP
.
Finally, there is cx_topblock(cx)
, which acts like a super-nextstate
as regards to resetting various vars to their base values. It is used in
places like pp_next
, pp_redo
and pp_goto
where rather than
exiting a scope, we want to re-initialise the scope. As well as resetting
PL_stack_sp
like nextstate
, it also resets PL_markstack_ptr
,
PL_scopestack_ix
and PL_curpm
. Note that it doesn't do a
FREETMPS
.
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, perlintern, perlxs, perlembed