PERLGUTS(1) Perl Programmers Reference Guide PERLGUTS(1)
NAME
perlguts - Introduction to the Perl API
DESCRIPTION
This document attempts to describe how to use the Perl API, as well as
to provide some info on the basic workings of the Perl core. It is far
from complete and probably contains many errors. Please refer any
questions or comments to the author below.
Variables
Datatypes
Perl has three typedefs that handle Perl's three main data types:
SV Scalar Value
AV Array Value
HV Hash Value
Each typedef has specific routines that manipulate the various data
types.
What is an "IV"?
Perl uses a special typedef IV which is a simple signed integer type
that is guaranteed to be large enough to hold a pointer (as well as an
integer). Additionally, there is the UV, which is simply an unsigned
IV.
Perl also uses two special typedefs, I32 and I16, which will always be
at least 32-bits and 16-bits long, respectively. (Again, there are U32
and U16, as well.) They will usually be exactly 32 and 16 bits long,
but on Crays they will both be 64 bits.
Working with SVs
An SV can be created and loaded with one command. There are five types
of values that can be loaded: an integer value (IV), an unsigned
integer value (UV), a double (NV), a string (PV), and another scalar
(SV). ("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 **, Size_t, 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").
Offsets
Perl provides the function "sv_chop" to efficiently remove characters
from the beginning of a string; you give it an SV and a pointer to
somewhere inside the PV, and it discards everything before the pointer.
The efficiency comes by means of a little hack: instead of actually
removing the characters, "sv_chop" sets the flag "OOK" (offset OK) to
signal to other functions that the offset hack is in effect, and it
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.
What's Really Stored in an SV?
Recall that the usual method of determining the type of scalar you have
is to use "Sv*OK" macros. Because a scalar can be both a number and a
string, usually these macros will always return TRUE and calling the
"Sv*V" macros will do the appropriate conversion of string to
integer/double or integer/double to string.
If you really need to know if you have an integer, double, or string
pointer in an SV, you can use the following three macros instead:
SvIOKp(SV*)
SvNOKp(SV*)
SvPOKp(SV*)
These will tell you if you truly have an integer, double, or string
pointer stored in your SV. The "p" stands for private.
There are various ways in which the private and public flags may
differ. For example, 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.
Working with AVs
There are two ways to create and load an AV. The first method creates
an empty AV:
AV* newAV();
The second method both creates the AV and initially populates it with
SVs:
AV* av_make(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.
Working with HVs
To create an HV, you use the following routine:
HV* newHV();
Once the HV has been created, the following operations are possible on
it:
SV** hv_store(HV*, const char* key, U32 klen, SV* val, U32 hash);
SV** hv_fetch(HV*, const char* key, U32 klen, I32 lval);
The "klen" parameter is the length of the key being passed in (Note
that you cannot pass 0 in as a value of "klen" to tell Perl to measure
the length of the key). The "val" argument contains the SV pointer to
the scalar being stored, and "hash" is the precomputed hash value (zero
if you want "hv_store" to calculate it for you). The "lval" parameter
indicates whether this fetch is actually a part of a store operation,
in which case a new undefined value will be added to the HV with the
supplied key and "hv_fetch" will return as if the value had already
existed.
Remember that "hv_store" and "hv_fetch" return "SV**"'s and not just
"SV*". To access the scalar value, you must first dereference the
return value. However, you should check to make sure that the return
value is not NULL before dereferencing it.
The first of these two functions checks if a hash table entry exists,
and the second deletes it.
bool hv_exists(HV*, const char* key, U32 klen);
SV* hv_delete(HV*, const char* key, U32 klen, I32 flags);
If "flags" does not include the "G_DISCARD" flag then "hv_delete" will
create and return a mortal copy of the deleted value.
And more miscellaneous functions:
void hv_clear(HV*);
void hv_undef(HV*);
Like their AV counterparts, "hv_clear" deletes all the entries in the
hash table but does not actually delete the hash table. The "hv_undef"
deletes both the entries and the hash table itself.
Perl keeps the actual data in a linked list of structures with a
typedef of HE. These contain the actual key and value pointers (plus
extra administrative overhead). The key is a string pointer; the value
is an "SV*". However, once you have an "HE*", to get the actual key
and value, use the routines specified below.
I32 hv_iterinit(HV*);
/* Prepares starting point to traverse hash table */
HE* hv_iternext(HV*);
/* Get the next entry, and return a pointer to a
structure that has both the key and value */
char* hv_iterkey(HE* entry, I32* retlen);
/* Get the key from an HE structure and also return
the length of the key string */
SV* hv_iterval(HV*, HE* entry);
/* Return an SV pointer to the value of the HE
structure */
SV* hv_iternextsv(HV*, char** key, I32* retlen);
/* This convenience routine combines hv_iternext,
hv_iterkey, and hv_iterval. The key and retlen
arguments are return values for the key and its
length. The value is returned in the SV* argument */
If you know the name of a hash variable, you can get a pointer to its
HV by using the following:
HV* get_hv("package::varname", 0);
This returns NULL if the variable does not exist.
The hash algorithm is defined in the "PERL_HASH" 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.
Hash API Extensions
Beginning with version 5.004, the following functions are also
supported:
HE* hv_fetch_ent (HV* tb, SV* key, I32 lval, U32 hash);
HE* hv_store_ent (HV* tb, SV* key, SV* val, U32 hash);
bool hv_exists_ent (HV* tb, SV* key, U32 hash);
SV* hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash);
SV* hv_iterkeysv (HE* entry);
Note that these functions take "SV*" keys, which simplifies writing of
extension code that deals with hash structures. These functions also
allow passing of "SV*" keys to "tie" functions without forcing you to
stringify the keys (unlike the previous set of functions).
They also return and accept whole hash entries ("HE*"), making their
use more efficient (since the hash number for a particular string
doesn't have to be recomputed every time). See perlapi for detailed
descriptions.
The following macros must always be used to access the contents of hash
entries. Note that the arguments to these macros must be simple
variables, since they may get evaluated more than once. See perlapi
for detailed descriptions of these macros.
HePV(HE* he, STRLEN len)
HeVAL(HE* he)
HeHASH(HE* he)
HeSVKEY(HE* he)
HeSVKEY_force(HE* he)
HeSVKEY_set(HE* he, SV* sv)
These two lower level macros are defined, but must only be used when
dealing with keys that are not "SV*"s:
HeKEY(HE* he)
HeKLEN(HE* he)
Note that both "hv_store" and "hv_store_ent" do not increment the
reference count of the stored "val", which is the caller's
responsibility. If these functions return a NULL value, the caller
will usually have to decrement the reference count of "val" to avoid a
memory leak.
AVs, HVs and undefined values
Sometimes you have to store undefined values in AVs or HVs. Although
this may be a rare case, it can be tricky. That's because you're used
to using &PL_sv_undef if you need an undefined SV.
For example, intuition tells you that this XS code:
AV *av = newAV();
av_store( av, 0, &PL_sv_undef );
is equivalent to this Perl code:
my @av;
$av[0] = undef;
Unfortunately, this isn't true. 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
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 Array
SVt_PVHV Hash
SVt_PVCV Code
SVt_PVGV Glob (possibly a file handle)
Any numerical value returned which is less than SVt_PVAV will be a
scalar of some form.
See "svtype" in perlapi for more details.
Blessed References and Class Objects
References are also used to support object-oriented programming. In
perl's OO lexicon, an object is simply a reference that has been
blessed into a package (or class). Once blessed, the programmer may
now use the reference to access the various methods in the class.
A reference can be blessed into a package with the following function:
SV* sv_bless(SV* sv, HV* stash);
The "sv" argument must be a reference value. The "stash" argument
specifies which class the reference will belong to. See "Stashes and
Globs" for information on converting class names into stashes.
/* Still under construction */
The following function upgrades rv to reference if not already one.
Creates a new SV for rv to point to. If "classname" is non-null, the
SV is blessed into the specified class. SV is returned.
SV* newSVrv(SV* rv, const char* classname);
The following three functions copy integer, unsigned integer or double
into an SV whose reference is "rv". SV is blessed if "classname" is
non-null.
SV* sv_setref_iv(SV* rv, const char* classname, IV iv);
SV* sv_setref_uv(SV* rv, const char* classname, UV uv);
SV* sv_setref_nv(SV* rv, const char* classname, NV iv);
The following function copies the pointer value (the address, not the
string!) into an SV whose reference is rv. SV is blessed if
"classname" is non-null.
SV* sv_setref_pv(SV* rv, const char* classname, void* pv);
The following function copies a string into an SV whose reference is
"rv". Set length to 0 to let Perl calculate the string length. SV is
blessed if "classname" is non-null.
SV* sv_setref_pvn(SV* rv, const char* classname, char* pv,
STRLEN length);
The following function tests whether the SV is blessed into the
specified class. It does not check inheritance relationships.
int sv_isa(SV* sv, const char* name);
The following function tests whether the SV is a reference to a blessed
object.
int sv_isobject(SV* sv);
The following function tests whether the SV is derived from the
specified class. SV can be either a reference to a blessed object or a
string containing a class name. This is the function implementing the
"UNIVERSAL::isa" functionality.
bool sv_derived_from(SV* sv, const char* name);
To check if you've got an object derived from a specific class you have
to write:
if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }
Creating New Variables
To create a new Perl variable with an undef value which can be accessed
from your Perl script, use the following routines, depending on the
variable type.
SV* get_sv("package::varname", GV_ADD);
AV* get_av("package::varname", GV_ADD);
HV* get_hv("package::varname", GV_ADD);
Notice the use of GV_ADD as the second parameter. The new variable can
now be set, using the routines appropriate to the data type.
There are additional macros whose values may be bitwise OR'ed with the
"GV_ADD" argument to enable certain extra features. Those bits are:
GV_ADDMULTI
Marks the variable as multiply defined, thus preventing the:
Name <varname> used only once: possible typo
warning.
GV_ADDWARN
Issues the warning:
Had to create <varname> unexpectedly
if the variable did not exist before the function was called.
If you do not specify a package name, the variable is created in the
current package.
Reference Counts and Mortality
Perl uses a reference count-driven garbage collection mechanism. SVs,
AVs, or HVs (xV for short in the following) start their life with a
reference count of 1. If the reference count of an xV ever drops to 0,
then it will be destroyed and its memory made available for reuse. At
the most basic internal level, reference counts can be manipulated with
the following macros:
int SvREFCNT(SV* sv);
SV* SvREFCNT_inc(SV* sv);
void SvREFCNT_dec(SV* sv);
(There are also suffixed versions of the increment and decrement
macros, for situations where the full generality of these basic macros
can be exchanged for some performance.)
However, the way a programmer should think about references is not so
much in terms of the bare reference count, but in terms of ownership of
references. A reference to an xV can be owned by any of a variety of
entities: another xV, the Perl interpreter, an XS data structure, a
piece of running code, or a dynamic scope. An xV generally does not
know what entities own the references to it; it only knows how many
references there are, which is the reference count.
To correctly maintain reference counts, it is essential to keep track
of what references the XS code is manipulating. The programmer should
always know where a reference has come from and who owns it, and be
aware of any creation or destruction of references, and any transfers
of ownership. Because ownership isn't represented explicitly in the xV
data structures, only the reference count need be actually maintained
by the code, and that means that this understanding of ownership is not
actually evident in the code. For example, transferring ownership of a
reference from one owner to another doesn't change the reference count
at all, so may be achieved with no actual code. (The transferring code
doesn't touch the referenced object, but does need to ensure that the
former owner knows that it no longer owns the reference, and that the
new owner knows that it now does.)
An xV that is visible at the Perl level should not become unreferenced
and thus be destroyed. Normally, an object will only become
unreferenced when it is no longer visible, often by the same means that
makes it invisible. For example, a Perl reference value (RV) owns a
reference to its referent, so if the RV is overwritten that reference
gets destroyed, and the no-longer-reachable referent may be destroyed
as a result.
Many functions have some kind of reference manipulation as part of
their purpose. Sometimes this is documented in terms of ownership of
references, and sometimes it is (less helpfully) documented in terms of
changes to reference counts. For example, the newRV_inc() function is
documented to create a new RV (with reference count 1) and increment
the reference count of the referent that was supplied by the caller.
This is best understood as creating a new reference to the referent,
which is owned by the created RV, and returning to the caller ownership
of the sole reference to the RV. The newRV_noinc() function instead
does not increment the reference count of the referent, but the RV
nevertheless ends up owning a reference to the referent. It is
therefore implied that the caller of "newRV_noinc()" is relinquishing a
reference to the referent, making this conceptually a more complicated
operation even though it does less to the data structures.
For example, imagine you want to return a reference from an XSUB
function. Inside the XSUB routine, you create an SV which initially
has just a single reference, owned by the XSUB routine. This reference
needs to be disposed of before the routine is complete, otherwise it
will leak, preventing the SV from ever being destroyed. So to create
an RV referencing the SV, it is most convenient to pass the SV to
"newRV_noinc()", which consumes that reference. Now the XSUB routine
no longer owns a reference to the SV, but does own a reference to the
RV, which in turn owns a reference to the SV. The ownership of the
reference to the RV is then transferred by the process of returning the
RV from the XSUB.
There are some convenience functions available that can help with the
destruction of xVs. These functions introduce the concept of
"mortality". Much documentation speaks of an xV itself being mortal,
but this is misleading. It is really a reference to an xV that is
mortal, and it is possible for there to be more than one mortal
reference to a single xV. For a reference to be mortal means that it
is owned by the temps stack, one of perl's many internal stacks, which
will destroy that reference "a short time later". Usually the "short
time later" is the end of the current Perl statement. However, it gets
more complicated around dynamic scopes: there can be multiple sets of
mortal references hanging around at the same time, with different death
dates. Internally, the actual determinant for when mortal xV
references are destroyed depends on two macros, SAVETMPS and FREETMPS.
See perlcall and perlxs for more details on these macros.
Mortal references are mainly used for xVs that are placed on perl's
main stack. The stack is problematic for reference tracking, because
it contains a lot of xV references, but doesn't own those references:
they are not counted. Currently, there are many bugs resulting from
xVs being destroyed while referenced by the stack, because the stack's
uncounted references aren't enough to keep the xVs alive. So when
putting an (uncounted) reference on the stack, it is vitally important
to ensure that there will be a counted reference to the same xV that
will last at least as long as the uncounted reference. But it's also
important that that counted reference be cleaned up at an appropriate
time, and not unduly prolong the xV's life. For there to be a mortal
reference is often the best way to satisfy this requirement, especially
if the xV was created especially to be put on the stack and would
otherwise be unreferenced.
To create a mortal reference, use the functions:
SV* sv_newmortal()
SV* sv_mortalcopy(SV*)
SV* sv_2mortal(SV*)
"sv_newmortal()" creates an SV (with the undefined value) whose sole
reference is mortal. "sv_mortalcopy()" creates an xV whose value is a
copy of a supplied xV and whose sole reference is mortal.
"sv_2mortal()" mortalises an existing xV reference: it transfers
ownership of a reference from the caller to the temps stack. 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));
The mortal routines are not just for SVs; AVs and HVs can be made
mortal by passing their address (type-casted to "SV*") to the
"sv_2mortal" or "sv_mortalcopy" routines.
Stashes and Globs
A stash is a hash that contains all variables that are defined within a
package. Each key of the stash is a symbol name (shared by all the
different types of objects that have the same name), and each value in
the hash table is a GV (Glob Value). This GV in turn contains
references to the various objects of that name, including (but not
limited to) the following:
Scalar Value
Array Value
Hash Value
I/O Handle
Format
Subroutine
There is a single stash called "PL_defstash" that holds the items that
exist in the "main" package. To get at the items in other packages,
append the string "::" to the package name. The items in the "Foo"
package are in the stash "Foo::" in PL_defstash. The items in the
"Bar::Baz" package are in the stash "Baz::" in "Bar::"'s stash.
To get the stash pointer for a particular package, use the function:
HV* gv_stashpv(const char* name, I32 flags)
HV* gv_stashsv(SV*, I32 flags)
The first function takes a literal string, the second uses the string
stored in the SV. Remember that a stash is just a hash table, so you
get back an "HV*". The "flags" flag will create a new package if it is
set to GV_ADD.
The name that "gv_stash*v" wants is the name of the package whose
symbol table you want. The default package is called "main". If you
have multiply nested packages, pass their names to "gv_stash*v",
separated by "::" as in the Perl language itself.
Alternately, if you have an SV that is a blessed reference, you can
find out the stash pointer by using:
HV* SvSTASH(SvRV(SV*));
then use the following to get the package name itself:
char* HvNAME(HV* stash);
If you need to bless or re-bless an object you can use the following
function:
SV* sv_bless(SV*, HV* stash)
where the first argument, an "SV*", must be a reference, and the second
argument is a stash. The returned "SV*" can now be used in the same
way as any other SV.
For more information on references and blessings, consult perlref.
Double-Typed SVs
Scalar variables normally contain only one type of value, an integer,
double, pointer, or reference. Perl will automatically convert the
actual scalar data from the stored type into the requested type.
Some scalar variables contain more than one type of scalar data. For
example, the variable $! contains either the numeric value of "errno"
or its string equivalent from either "strerror" or "sys_errlist[]".
To force multiple data values into an SV, you must do two things: use
the "sv_set*v" routines to add the additional scalar type, then set a
flag so that Perl will believe it contains more than one type of data.
The four macros to set the flags are:
SvIOK_on
SvNOK_on
SvPOK_on
SvROK_on
The particular macro you must use depends on which "sv_set*v" routine
you called first. This is because every "sv_set*v" routine turns on
only the bit for the particular type of data being set, and turns off
all the rest.
For example, to create a new Perl variable called "dberror" that
contains both the numeric and descriptive string error values, you
could use the following code:
extern int dberror;
extern char *dberror_list;
SV* sv = get_sv("dberror", GV_ADD);
sv_setiv(sv, (IV) dberror);
sv_setpv(sv, dberror_list[dberror]);
SvIOK_on(sv);
If the order of "sv_setiv" and "sv_setpv" had been reversed, then the
macro "SvPOK_on" would need to be called instead of "SvIOK_on".
Read-Only Values
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
Copy on Write
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.
Magic Variables
[This section still under construction. Ignore everything here. Post
no bills. Everything not permitted is forbidden.]
Any SV may be magical, that is, it has special features that a normal
SV does not have. These features are stored in the SV structure in a
linked list of "struct magic"'s, typedef'ed to "MAGIC".
struct magic {
MAGIC* mg_moremagic;
MGVTBL* mg_virtual;
U16 mg_private;
char mg_type;
U8 mg_flags;
I32 mg_len;
SV* mg_obj;
char* mg_ptr;
};
Note this is current as of patchlevel 0, and could change at any time.
Assigning Magic
Perl adds magic to an SV using the sv_magic function:
void sv_magic(SV* sv, SV* obj, int how, const char* name, I32 namlen);
The "sv" argument is a pointer to the SV that is to acquire a new
magical feature.
If "sv" is not already magical, Perl uses the "SvUPGRADE" macro to
convert "sv" to type "SVt_PVMG". Perl then continues by adding new
magic to the beginning of the linked list of magical features. Any
prior entry of the same type of magic is deleted. Note that this can
be overridden, and multiple instances of the same type of magic can be
associated with an SV.
The "name" and "namlen" arguments are used to associate a string with
the magic, typically the name of a variable. "namlen" is stored in the
"mg_len" field and if "name" is non-null then either a "savepvn" copy
of "name" or "name" itself is stored in the "mg_ptr" field, depending
on whether "namlen" is greater than zero or equal to zero respectively.
As a special case, if "(name && namlen == HEf_SVKEY)" then "name" is
assumed to contain an "SV*" and is stored as-is with its REFCNT
incremented.
The sv_magic function uses "how" to determine which, if any, predefined
"Magic Virtual Table" should be assigned to the "mg_virtual" field.
See the "Magic Virtual Tables" section below. The "how" argument is
also stored in the "mg_type" field. The value of "how" should be
chosen from the set of macros "PERL_MAGIC_foo" found in perl.h. Note
that before these macros were added, Perl internals used to directly
use character literals, so you may occasionally come across old code or
documentation referring to 'U' magic rather than "PERL_MAGIC_uvar" for
example.
The "obj" argument is stored in the "mg_obj" field of the "MAGIC"
structure. If it is not the same as the "sv" argument, the reference
count of the "obj" object is incremented. If it is the same, or if the
"how" argument is "PERL_MAGIC_arylen", "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:
int sv_unmagic(SV *sv, int type);
The "type" argument should be equal to the "how" value when the "SV"
was initially made magical.
However, note that "sv_unmagic" removes all magic of a certain "type"
from the "SV". If you want to remove only certain magic of a "type"
based on the magic virtual table, use "sv_unmagicext" instead:
int sv_unmagicext(SV *sv, int type, MGVTBL *vtbl);
Magic Virtual Tables
The "mg_virtual" field in the "MAGIC" structure is a pointer to an
"MGVTBL", which is a structure of function pointers and stands for
"Magic Virtual Table" to handle the various operations that might be
applied to that variable.
The "MGVTBL" has five (or sometimes eight) pointers to the following
routine types:
int (*svt_get) (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_nonelem vtbl_nonelem Array element that does not
exist
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.
Finding Magic
MAGIC *mg_find(SV *sv, int type); /* Finds the magic pointer of that
* type */
This routine returns a pointer to a "MAGIC" structure stored in the SV.
If the SV does not have that magical feature, "NULL" is returned. If
the SV has multiple instances of that magical feature, the first one
will be returned. "mg_findext" can be used to find a "MAGIC" structure
of an SV based on both its magic type and its magic virtual table:
MAGIC *mg_findext(SV *sv, int type, MGVTBL *vtbl);
Also, if the SV passed to "mg_find" or "mg_findext" is not of type
SVt_PVMG, Perl may core dump.
int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen);
This routine checks to see what types of magic "sv" has. If the
mg_type field is an uppercase letter, then the mg_obj is copied to
"nsv", but the mg_type field is changed to be the lowercase letter.
Understanding the Magic of Tied Hashes and Arrays
Tied hashes and arrays are magical beasts of the "PERL_MAGIC_tied"
magic type.
WARNING: As of the 5.004 release, proper usage of the array and hash
access functions requires understanding a few caveats. Some of these
caveats are actually considered bugs in the API, to be fixed in later
releases, and are bracketed with [MAYCHANGE] below. If you find
yourself actually applying such information in this section, be aware
that the behavior may change in the future, umm, without warning.
The perl tie function associates a variable with an object that
implements the various GET, SET, etc methods. To perform the
equivalent of the perl tie function from an XSUB, you must mimic this
behaviour. The code below carries out the necessary steps -- firstly
it creates a new hash, and then creates a second hash which it blesses
into the class which will implement the tie methods. Lastly it ties
the two hashes together, and returns a reference to the new tied hash.
Note that the code below does NOT call the TIEHASH method in the MyTie
class - see "Calling Perl Routines from within C Programs" for details
on how to do this.
SV*
mytie()
PREINIT:
HV *hash;
HV *stash;
SV *tie;
CODE:
hash = newHV();
tie = newRV_noinc((SV*)newHV());
stash = gv_stashpv("MyTie", GV_ADD);
sv_bless(tie, stash);
hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
RETVAL = newRV_noinc(hash);
OUTPUT:
RETVAL
The "av_store" function, when given a tied array argument, merely
copies the magic of the array onto the value to be "stored", using
"mg_copy". It may also return NULL, indicating that the value did not
actually need to be stored in the array. [MAYCHANGE] After a call to
"av_store" on a tied array, the caller will usually need to call
"mg_set(val)" to actually invoke the perl level "STORE" method on the
TIEARRAY object. If "av_store" did return NULL, a call to
"SvREFCNT_dec(val)" will also be usually necessary to avoid a memory
leak. [/MAYCHANGE]
The previous paragraph is applicable verbatim to tied hash access using
the "hv_store" and "hv_store_ent" functions as well.
"av_fetch" and the corresponding hash functions "hv_fetch" and
"hv_fetch_ent" actually return an undefined mortal value whose magic
has been initialized using "mg_copy". Note the value so returned does
not need to be deallocated, as it is already mortal. [MAYCHANGE] But
you will need to call "mg_get()" on the returned value in order to
actually invoke the perl level "FETCH" method on the underlying TIE
object. Similarly, you may also call "mg_set()" on the return value
after possibly assigning a suitable value to it using "sv_setsv",
which will invoke the "STORE" method on the TIE object. [/MAYCHANGE]
[MAYCHANGE] In other words, the array or hash fetch/store functions
don't really fetch and store actual values in the case of tied arrays
and hashes. They merely call "mg_copy" to attach magic to the values
that were meant to be "stored" or "fetched". Later calls to "mg_get"
and "mg_set" actually do the job of invoking the TIE methods on the
underlying objects. Thus the magic mechanism currently implements a
kind of lazy access to arrays and hashes.
Currently (as of perl version 5.004), use of the hash and array access
functions requires the user to be aware of whether they are operating
on "normal" hashes and arrays, or on their tied variants. The API may
be changed to provide more transparent access to both tied and normal
data types in future versions. [/MAYCHANGE]
You would do well to understand that the TIEARRAY and TIEHASH
interfaces are mere sugar to invoke some perl method calls while using
the uniform hash and array syntax. The use of this sugar imposes some
overhead (typically about two to four extra opcodes per FETCH/STORE
operation, in addition to the creation of all the mortal variables
required to invoke the methods). This overhead will be comparatively
small if the TIE methods are themselves substantial, but if they are
only a few statements long, the overhead will not be insignificant.
Localizing changes
Perl has a very handy construction
{
local $var = 2;
...
}
This construction is approximately equivalent to
{
my $oldvar = $var;
$var = 2;
...
$var = $oldvar;
}
The biggest difference is that the first construction would reinstate
the initial value of $var, irrespective of how control exits the block:
"goto", "return", "die"/"eval", etc. It is a little bit more efficient
as well.
There is a way to achieve a similar task from C via Perl API: create a
pseudo-block, and arrange for some changes to be automatically undone
at the end of it, either explicit, or via a non-local exit (via die()).
A block-like construct is created by a pair of "ENTER"/"LEAVE" macros
(see "Returning a Scalar" in perlcall). Such a construct may be
created specially for some important localized task, or an existing one
(like boundaries of enclosing Perl subroutine/block, or an existing
pair for freeing TMPs) may be used. (In the second case the overhead
of additional localization must be almost negligible.) Note that any
XSUB is automatically enclosed in an "ENTER"/"LEAVE" pair.
Inside such a pseudo-block the following service is available:
"SAVEINT(int i)"
"SAVEIV(IV i)"
"SAVEI32(I32 i)"
"SAVELONG(long i)"
These macros arrange things to restore the value of integer
variable "i" at the end of enclosing pseudo-block.
SAVESPTR(s)
SAVEPPTR(p)
These macros arrange things to restore the value of pointers "s"
and "p". "s" must be a pointer of a type which survives conversion
to "SV*" and back, "p" should be able to survive conversion to
"char*" and back.
"SAVEFREESV(SV *sv)"
The refcount of "sv" 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.
Subroutines
XSUBs and the Argument Stack
The XSUB mechanism is a simple way for Perl programs to access C
subroutines. An XSUB routine will have a stack that contains the
arguments from the Perl program, and a way to map from the Perl data
structures to a C equivalent.
The stack arguments are accessible through the ST(n) macro, which
returns the "n"'th stack argument. Argument 0 is the first argument
passed in the Perl subroutine call. These arguments are "SV*", and can
be used anywhere an "SV*" is used.
Most of the time, output from the C routine can be handled through use
of the RETVAL and OUTPUT directives. However, there are some cases
where the argument stack is not already long enough to handle all the
return values. An example is the POSIX tzname() call, which takes no
arguments, but returns two, the local time zone's standard and summer
time abbreviations.
To handle this situation, the PPCODE directive is used and the stack is
extended using the macro:
EXTEND(SP, num);
where "SP" is the macro that represents the local copy of the stack
pointer, and "num" is the number of elements the stack should be
extended by.
Now that there is room on the stack, values can be pushed on it using
"PUSHs" macro. The pushed values will often need to be "mortal" (See
"Reference Counts and Mortality"):
PUSHs(sv_2mortal(newSViv(an_integer)))
PUSHs(sv_2mortal(newSVuv(an_unsigned_integer)))
PUSHs(sv_2mortal(newSVnv(a_double)))
PUSHs(sv_2mortal(newSVpv("Some String",0)))
/* Although the last example is better written as the more
* efficient: */
PUSHs(newSVpvs_flags("Some String", SVs_TEMP))
And now the Perl program calling "tzname", the two values will be
assigned as in:
($standard_abbrev, $summer_abbrev) = POSIX::tzname;
An alternate (and possibly simpler) method to pushing values on the
stack is to use the macro:
XPUSHs(SV*)
This macro automatically adjusts the stack for you, if needed. Thus,
you do not need to call "EXTEND" to extend the stack.
Despite their suggestions in earlier versions of this document the
macros "(X)PUSH[iunp]" are not suited to XSUBs which return multiple
results. For that, either stick to the "(X)PUSHs" macros shown above,
or use the new "m(X)PUSH[iunp]" macros instead; see "Putting a C value
on Perl stack".
For more information, consult perlxs and perlxstut.
Autoloading with XSUBs
If an AUTOLOAD routine is an XSUB, as with Perl subroutines, Perl puts
the fully-qualified name of the autoloaded subroutine in the $AUTOLOAD
variable of the XSUB's package.
But it also puts the same information in certain fields of the XSUB
itself:
HV *stash = CvSTASH(cv);
const char *subname = SvPVX(cv);
STRLEN name_length = SvCUR(cv); /* in bytes */
U32 is_utf8 = SvUTF8(cv);
"SvPVX(cv)" contains just the sub name itself, not including the
package. For an AUTOLOAD routine in UNIVERSAL or one of its
superclasses, "CvSTASH(cv)" returns NULL during a method call on a
nonexistent package.
Note: Setting $AUTOLOAD stopped working in 5.6.1, which did not support
XS AUTOLOAD subs at all. Perl 5.8.0 introduced the use of fields in
the XSUB itself. Perl 5.16.0 restored the setting of $AUTOLOAD. If
you need to support 5.8-5.14, use the XSUB's fields.
Calling Perl Routines from within C Programs
There are four routines that can be used to call a Perl subroutine from
within a C program. These four are:
I32 call_sv(SV*, I32);
I32 call_pv(const char*, I32);
I32 call_method(const char*, I32);
I32 call_argv(const char*, I32, 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.
Putting a C value on Perl stack
A lot of opcodes (this is an elementary operation in the internal perl
stack machine) put an SV* on the stack. However, as an optimization
the corresponding SV is (usually) not recreated each time. The opcodes
reuse specially assigned SVs (targets) which are (as a corollary) not
constantly freed/created.
Each of the targets is created only once (but see "Scratchpads and
recursion" below), and when an opcode needs to put an integer, a
double, or a string on stack, it just sets the corresponding parts of
its target and puts the target on stack.
The macro to put this target on stack is "PUSHTARG", and it is directly
used in some opcodes, as well as indirectly in zillions of others,
which use it via "(X)PUSH[iunp]".
Because the target is reused, you must be careful when pushing multiple
values on the stack. The following code will not do what you think:
XPUSHi(10);
XPUSHi(20);
This translates as "set "TARG" to 10, push a pointer to "TARG" onto the
stack; set "TARG" to 20, push a pointer to "TARG" onto the stack". At
the end of the operation, the stack does not contain the values 10 and
20, but actually contains two pointers to "TARG", which we have set to
20.
If you need to push multiple different values then you should either
use the "(X)PUSHs" macros, or else use the new "m(X)PUSH[iunp]" macros,
none of which make use of "TARG". The "(X)PUSHs" macros simply push an
SV* on the stack, which, as noted under "XSUBs and the Argument Stack",
will often need to be "mortal". The new "m(X)PUSH[iunp]" macros make
this a little easier to achieve by creating a new mortal for you (via
"(X)PUSHmortal"), pushing that onto the stack (extending it if
necessary in the case of the "mXPUSH[iunp]" macros), and then setting
its value. Thus, instead of writing this to "fix" the example above:
XPUSHs(sv_2mortal(newSViv(10)))
XPUSHs(sv_2mortal(newSViv(20)))
you can simply write:
mXPUSHi(10)mXPUSHi(20)
On a related note, if you do use "(X)PUSH[iunp]", then you're going to
need a "dTARG" in your variable declarations so that the "*PUSH*"
macros can make use of the local variable "TARG". See also "dTARGET"
and "dXSTARG".
Scratchpads
The question remains on when the SVs which are targets for opcodes are
created. The answer is that they are created when the current unit--a
subroutine or a file (for opcodes for statements outside of
subroutines)--is compiled. During this time a special anonymous Perl
array is created, which is called a scratchpad for the current unit.
A scratchpad keeps SVs which are lexicals for the current unit and are
targets for opcodes. 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.
Scratchpads and recursion
In fact it is not 100% true that a compiled unit contains a pointer to
the scratchpad AV. In fact it contains a pointer to an AV of
(initially) one element, and this element is the scratchpad AV. Why do
we need an extra level of indirection?
The answer is recursion, and maybe threads. Both these can create
several execution pointers going into the same subroutine. For the
subroutine-child not write over the temporaries for the subroutine-
parent (lifespan of which covers the call to the child), the parent and
the child should have different scratchpads. (And the lexicals should
be separate anyway!)
So each subroutine is born with an array of scratchpads (of length 1).
On each entry to the subroutine it is checked that the current depth of
the recursion is not more than the length of this array, and if it is,
new scratchpad is created and pushed into the array.
The targets on this scratchpad are "undef"s, but they are already
marked with correct flags.
Memory Allocation
Allocation
All memory meant to be used with the Perl API functions should be
manipulated using the macros described in this section. The macros
provide the necessary transparency between differences in the actual
malloc implementation that is used within perl.
It is suggested that you enable the version of malloc that is
distributed with Perl. It keeps pools of various sizes of unallocated
memory in order to satisfy allocation requests more quickly. However,
on some platforms, it may cause spurious malloc or free errors.
The following three macros are used to initially allocate memory :
Newx(pointer, number, type);
Newxc(pointer, number, type, cast);
Newxz(pointer, number, type);
The first argument "pointer" should be the name of a variable that will
point to the newly allocated memory.
The second and third arguments "number" and "type" specify how many of
the specified type of data structure should be allocated. The argument
"type" is passed to "sizeof". The final argument to "Newxc", "cast",
should be used if the "pointer" argument is different from the "type"
argument.
Unlike the "Newx" and "Newxc" macros, the "Newxz" macro calls "memzero"
to zero out all the newly allocated memory.
Reallocation
Renew(pointer, number, type);
Renewc(pointer, number, type, cast);
Safefree(pointer)
These three macros are used to change a memory buffer size or to free a
piece of memory no longer needed. The arguments to "Renew" and
"Renewc" match those of "New" and "Newc" with the exception of not
needing the "magic cookie" argument.
Moving
Move(source, dest, number, type);
Copy(source, dest, number, type);
Zero(dest, number, type);
These three macros are used to move, copy, or zero out previously
allocated memory. The "source" and "dest" arguments point to the
source and destination starting points. Perl will move, copy, or zero
out "number" instances of the size of the "type" data structure (using
the "sizeof" function).
PerlIO
The most recent development releases of Perl have been experimenting
with removing Perl's dependency on the "normal" standard I/O suite and
allowing other stdio implementations to be used. This involves
creating a new abstraction layer that then calls whichever
implementation of stdio Perl was compiled with. All XSUBs should now
use the functions in the PerlIO abstraction layer and not make any
assumptions about what kind of stdio is being used.
For a complete description of the PerlIO abstraction, consult perlapio.
Compiled code
Code tree
Here we describe the internal form your code is converted to by Perl.
Start with a simple example:
$a = $b + $c;
This is converted to a tree similar to this one:
assign-to
/ \
+ $a
/ \
$b $c
(but slightly more complicated). This tree reflects the way Perl
parsed your code, but has nothing to do with the execution order.
There is an additional "thread" going through the nodes of the tree
which shows the order of execution of the nodes. In our simplified
example above it looks like:
$b ---> $c ---> + ---> $a ---> assign-to
But with the actual compile tree for "$a = $b + $c" it is different:
some nodes optimized away. As a corollary, though the actual tree
contains more nodes than our simplified example, the execution order is
the same as in our example.
Examining the tree
If you have your perl compiled for debugging (usually done with
"-DDEBUGGING" on the "Configure" command line), you may examine the
compiled tree by specifying "-Dx" on the Perl command line. The output
takes several lines per node, and for "$b+$c" it looks like this:
5 TYPE = add ===> 6
TARG = 1
FLAGS = (SCALAR,KIDS)
{
TYPE = null ===> (4)
(was rv2sv)
FLAGS = (SCALAR,KIDS)
{
3 TYPE = gvsv ===> 4
FLAGS = (SCALAR)
GV = main::b
}
}
{
TYPE = null ===> (5)
(was rv2sv)
FLAGS = (SCALAR,KIDS)
{
4 TYPE = gvsv ===> 5
FLAGS = (SCALAR)
GV = main::c
}
}
This tree has 5 nodes (one per "TYPE" specifier), only 3 of them are
not optimized away (one per number in the left column). The immediate
children of the given node correspond to "{}" pairs on the same level
of indentation, thus this listing corresponds to the tree:
add
/ \
null null
| |
gvsv gvsv
The execution order is indicated by "===>" marks, thus it is "3 4 5 6"
(node 6 is not included into above listing), i.e., "gvsv gvsv add
whatever".
Each of these nodes represents an op, a fundamental operation inside
the Perl core. The code which implements each operation can be found
in the pp*.c files; the function which implements the op with type
"gvsv" is "pp_gvsv", and so on. As the tree above shows, different ops
have different numbers of children: "add" is a binary operator, as one
would expect, and so has two children. To accommodate the various
different numbers of children, there are various types of op data
structure, and they link together in different ways.
The simplest type of op structure is "OP": this has no children. Unary
operators, "UNOP"s, have one child, and this is pointed to by the
"op_first" field. Binary operators ("BINOP"s) have not only an
"op_first" field but also an "op_last" field. The most complex type of
op is a "LISTOP", which has any number of children. In this case, the
first child is pointed to by "op_first" and the last child by
"op_last". The children in between can be found by iteratively
following the "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.
Compile pass 1: check routines
The tree is created by the compiler while yacc code feeds it the
constructions it recognizes. Since yacc works bottom-up, so does the
first pass of perl compilation.
What makes this pass interesting for perl developers is that some
optimization may be performed on this pass. This is optimization by
so-called "check routines". The correspondence between node names and
corresponding check routines is described in opcode.pl (do not forget
to run "make regen_headers" if you modify this file).
A check routine is called when the node is fully constructed except for
the execution-order thread. Since at this time there are no back-links
to the currently constructed node, one can do most any operation to the
top-level node, including freeing it and/or creating new nodes
above/below it.
The check routine returns the node which should be inserted into the
tree (if the top-level node was not modified, check routine returns its
argument).
By convention, check routines have names "ck_*". They are usually
called from "new*OP" subroutines (or "convert") (which in turn are
called from perly.y).
Compile pass 1a: constant folding
Immediately after the check routine is called the returned node is
checked for being compile-time executable. If it is (the value is
judged to be constant) it is immediately executed, and a constant node
with the "return value" of the corresponding subtree is substituted
instead. The subtree is deleted.
If constant folding was not performed, the execution-order thread is
created.
Compile pass 2: context propagation
When a context for a part of compile tree is known, it is propagated
down through the tree. At this time the context can have 5 values
(instead of 2 for runtime context): void, boolean, scalar, list, and
lvalue. In contrast with the pass 1 this pass is processed from top to
bottom: a node's context determines the context for its children.
Additional context-dependent optimizations are performed at this time.
Since at this moment the compile tree contains back-references (via
"thread" pointers), nodes cannot be free()d now. To allow optimized-
away nodes at this stage, such nodes are null()ified instead of
free()ing (i.e. their type is changed to OP_NULL).
Compile pass 3: peephole optimization
After the compile tree for a subroutine (or for an "eval" or a file) is
created, an additional pass over the code is performed. This pass is
neither top-down or bottom-up, but in the execution order (with
additional complications for conditionals). Optimizations performed at
this stage are subject to the same restrictions as in the pass 2.
Peephole optimizations are done by calling the function pointed to by
the global variable "PL_peepp". By default, "PL_peepp" just calls the
function pointed to by the global variable "PL_rpeepp". By default,
that performs some basic op fixups and optimisations along the
execution-order op chain, and recursively calls "PL_rpeepp" for each
side chain of ops (resulting from conditionals). Extensions may
provide additional optimisations or fixups, hooking into either the
per-subroutine or recursive stage, like this:
static peep_t prev_peepp;
static void my_peep(pTHX_ OP *o)
{
/* custom per-subroutine optimisation goes here */
prev_peepp(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;
Pluggable runops
The compile tree is executed in a runops function. There are two
runops functions, in run.c and in dump.c. "Perl_runops_debug" is used
with DEBUGGING and "Perl_runops_standard" is used otherwise. For fine
control over the execution of the compile tree it is possible to
provide your own runops function.
It's probably best to copy one of the existing runops functions and
change it to suit your needs. Then, in the BOOT section of your XS
file, add the line:
PL_runops = my_runops;
This function should be as efficient as possible to keep your programs
running as fast as possible.
Compile-time scope hooks
As of perl 5.14 it is possible to hook into the compile-time lexical
scope mechanism using "Perl_blockhook_register". This is used like
this:
STATIC void my_start_hook(pTHX_ int full);
STATIC BHK my_hooks;
BOOT:
BhkENTRY_set(&my_hooks, bhk_start, my_start_hook);
Perl_blockhook_register(aTHX_ &my_hooks);
This will arrange to have "my_start_hook" called at the start of
compiling every lexical scope. The available hooks are:
"void bhk_start(pTHX_ int full)"
This is called just after starting a new lexical scope. Note that
Perl code like
if ($x) { ... }
creates two scopes: the first starts at the "(" and has "full ==
1", the second starts at the "{" and has "full == 0". Both end at
the "}", so calls to "start" and "pre"/"post_end" will match.
Anything pushed onto the save stack by this hook will be popped
just before the scope ends (between the "pre_" and "post_end"
hooks, in fact).
"void bhk_pre_end(pTHX_ OP **o)"
This is called at the end of a lexical scope, just before unwinding
the stack. o is the root of the optree representing the scope; it
is a double pointer so you can replace the OP if you need to.
"void bhk_post_end(pTHX_ OP **o)"
This is called at the end of a lexical scope, just after unwinding
the stack. o is as above. Note that it is possible for calls to
"pre_" and "post_end" to nest, if there is something on the save
stack that calls string eval.
"void bhk_eval(pTHX_ OP *const o)"
This is called just before starting to compile an "eval STRING",
"do FILE", "require" or "use", after the eval has been set up. o
is the OP that requested the eval, and will normally be an
"OP_ENTEREVAL", "OP_DOFILE" or "OP_REQUIRE".
Once you have your hook functions, you need a "BHK" structure to put
them in. It's best to allocate it statically, since there is no way to
free it once it's registered. The function pointers should be inserted
into this structure using the "BhkENTRY_set" macro, which will also set
flags indicating which entries are valid. If you do need to allocate
your "BHK" dynamically for some reason, be sure to zero it before you
start.
Once registered, there is no mechanism to switch these hooks off, so if
that is necessary you will need to do this yourself. An entry in "%^H"
is probably the best way, so the effect is lexically scoped; however it
is also possible to use the "BhkDISABLE" and "BhkENABLE" macros to
temporarily switch entries on and off. You should also be aware that
generally speaking at least one scope will have opened before your
extension is loaded, so you will see some "pre"/"post_end" pairs that
didn't have a matching "start".
Examining internal data structures with the "dump" functions
To aid debugging, the source file dump.c contains a number of functions
which produce formatted output of internal data structures.
The most commonly used of these functions is "Perl_sv_dump"; it's used
for dumping SVs, AVs, HVs, and CVs. The "Devel::Peek" module calls
"sv_dump" to produce debugging output from Perl-space, so users of that
module should already be familiar with its format.
"Perl_op_dump" can be used to dump an "OP" structure or any of its
derivatives, and produces output similar to "perl -Dx"; in fact,
"Perl_dump_eval" will dump the main root of the code being evaluated,
exactly like "-Dx".
Other useful functions are "Perl_dump_sub", which turns a "GV" into an
op tree, "Perl_dump_packsubs" which calls "Perl_dump_sub" on all the
subroutines in a package like so: (Thankfully, these are all xsubs, so
there is no op tree)
(gdb) print Perl_dump_packsubs(PL_defstash)
SUB attributes::bootstrap = (xsub 0x811fedc 0)
SUB UNIVERSAL::can = (xsub 0x811f50c 0)
SUB UNIVERSAL::isa = (xsub 0x811f304 0)
SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0)
SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0)
and "Perl_dump_all", which dumps all the subroutines in the stash and
the op tree of the main root.
How multiple interpreters and concurrency are supported
Background and PERL_IMPLICIT_CONTEXT
The Perl interpreter can be regarded as a closed box: it has an API for
feeding it code or otherwise making it do things, but it also has
functions for its own use. This smells a lot like an object, and there
are ways for you to build Perl so that you can have multiple
interpreters, with one interpreter represented either as a C structure,
or inside a thread-specific structure. These structures contain all
the context, the state of that interpreter.
One macro controls the major Perl build flavor: MULTIPLICITY. The
MULTIPLICITY build has a C structure that packages all the interpreter
state. With multiplicity-enabled perls, PERL_IMPLICIT_CONTEXT is also
normally defined, and enables the support for passing in a "hidden"
first argument that represents all three data structures. MULTIPLICITY
makes multi-threaded perls possible (with the ithreads threading model,
related to the macro USE_ITHREADS.)
Two other "encapsulation" macros are the PERL_GLOBAL_STRUCT and
PERL_GLOBAL_STRUCT_PRIVATE (the latter turns on the former, and the
former turns on MULTIPLICITY.) The PERL_GLOBAL_STRUCT causes all the
internal variables of Perl to be wrapped inside a single global struct,
struct perl_vars, accessible as (globals) &PL_Vars or PL_VarsPtr or the
function Perl_GetVars(). The PERL_GLOBAL_STRUCT_PRIVATE goes one step
further, there is still a single struct (allocated in main() either
from heap or from stack) but there are no global data symbols pointing
to it. In either case the global struct should be 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), submit an issue at <https://github.com/Perl/perl5/issues>
explaining why you think it should be.
Second problem: there must be a syntax so that the same subroutine
declarations and calls can pass a structure as their first argument, or
pass nothing. To solve this, the subroutines are named and declared in
a particular way. Here's a typical start of a static function used
within the Perl guts:
STATIC void
S_incline(pTHX_ char *s)
STATIC becomes "static" in C, and may be #define'd to nothing in some
configurations in the future.
A public function (i.e. part of the internal API, but not necessarily
sanctioned for use in extensions) begins like this:
void
Perl_sv_setiv(pTHX_ SV* dsv, IV num)
"pTHX_" is one of a number of macros (in perl.h) that hide the details
of the interpreter's context. THX stands for "thread", "this", or
"thingy", as the case may be. (And no, George Lucas is not involved.
:-) The first character could be 'p' for a prototype, 'a' for argument,
or 'd' for declaration, so we have "pTHX", "aTHX" and "dTHX", and their
variants.
When Perl is built without options that set PERL_IMPLICIT_CONTEXT,
there is no first argument containing the interpreter's context. The
trailing underscore in the pTHX_ macro indicates that the macro
expansion needs a comma after the context argument because other
arguments follow it. If PERL_IMPLICIT_CONTEXT is not defined, pTHX_
will be ignored, and the subroutine is not prototyped to take the extra
argument. The form of the macro without the trailing underscore is
used when there are no additional explicit arguments.
When a core function calls another, it must pass the context. This is
normally hidden via macros. Consider "sv_setiv". It expands into
something like this:
#ifdef PERL_IMPLICIT_CONTEXT
#define sv_setiv(a,b) Perl_sv_setiv(aTHX_ a, b)
/* can't do this for vararg functions, see below */
#else
#define sv_setiv Perl_sv_setiv
#endif
This works well, and means that XS authors can gleefully write:
sv_setiv(foo, bar);
and still have it work under all the modes Perl could have been
compiled with.
This doesn't work so cleanly for varargs functions, though, as macros
imply that the number of arguments is known in advance. Instead we
either need to spell them out fully, passing "aTHX_" as the first
argument (the Perl core tends to do this with functions like
Perl_warner), or use a context-free version.
The context-free version of Perl_warner is called
Perl_warner_nocontext, and does not take the extra argument. Instead
it does dTHX; to get the context from thread-local storage. We
"#define warner Perl_warner_nocontext" so that extensions get source
compatibility at the expense of performance. (Passing an arg is
cheaper than grabbing it from thread-local storage.)
You can ignore [pad]THXx when browsing the Perl headers/sources. Those
are strictly for use within the core. Extensions and embedders need
only be aware of [pad]THX.
So what happened to dTHR?
"dTHR" was introduced in perl 5.005 to support the older thread model.
The older thread model now uses the "THX" mechanism to pass context
pointers around, so "dTHR" is not useful any more. Perl 5.6.0 and
later still have it for backward source compatibility, but it is
defined to be a no-op.
How do I use all this in extensions?
When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call any
functions in the Perl API will need to pass the initial context
argument somehow. The kicker is that you will need to write it in such
a way that the extension still compiles when Perl hasn't been built
with PERL_IMPLICIT_CONTEXT enabled.
There are three ways to do this. First, the easy but inefficient way,
which is also the default, in order to maintain source compatibility
with extensions: whenever XSUB.h is #included, it redefines the aTHX
and aTHX_ macros to call a function that will return the context.
Thus, something like:
sv_setiv(sv, num);
in your extension will translate to this when PERL_IMPLICIT_CONTEXT is
in effect:
Perl_sv_setiv(Perl_get_context(), sv, num);
or to this otherwise:
Perl_sv_setiv(sv, num);
You don't have to do anything new in your extension to get this; since
the Perl library provides Perl_get_context(), it will all just work.
The second, more efficient way is to use the following template for
your Foo.xs:
#define PERL_NO_GET_CONTEXT /* we want efficiency */
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
STATIC void my_private_function(int arg1, int arg2);
STATIC void
my_private_function(int arg1, int arg2)
{
dTHX; /* fetch context */
... call many Perl API functions ...
}
[... etc ...]
MODULE = Foo PACKAGE = Foo
/* typical XSUB */
void
my_xsub(arg)
int arg
CODE:
my_private_function(arg, 10);
Note that the only two changes from the normal way of writing an
extension is the addition of a "#define PERL_NO_GET_CONTEXT" before
including the Perl headers, followed by a "dTHX;" declaration at the
start of every function that will call the Perl API. (You'll know
which functions need this, because the C compiler will complain that
there's an undeclared identifier in those functions.) No changes are
needed for the XSUBs themselves, because the XS() macro is correctly
defined to pass in the implicit context if needed.
The third, even more efficient way is to ape how it is done within the
Perl guts:
#define PERL_NO_GET_CONTEXT /* we want efficiency */
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
/* pTHX_ only needed for functions that call Perl API */
STATIC void my_private_function(pTHX_ int arg1, int arg2);
STATIC void
my_private_function(pTHX_ int arg1, int arg2)
{
/* dTHX; not needed here, because THX is an argument */
... call Perl API functions ...
}
[... etc ...]
MODULE = Foo PACKAGE = Foo
/* typical XSUB */
void
my_xsub(arg)
int arg
CODE:
my_private_function(aTHX_ arg, 10);
This implementation never has to fetch the context using a function
call, since it is always passed as an extra argument. Depending on
your needs for simplicity or efficiency, you may mix the previous two
approaches freely.
Never add a comma after "pTHX" yourself--always use the form of the
macro with the underscore for functions that take explicit arguments,
or the form without the argument for functions with no explicit
arguments.
If one is compiling Perl with the "-DPERL_GLOBAL_STRUCT" the "dVAR"
definition is needed if the Perl global variables (see perlvars.h or
globvar.sym) are accessed in the function and "dTHX" is not used (the
"dTHX" includes the "dVAR" if necessary). One notices the need for
"dVAR" only with the said compile-time define, because otherwise the
Perl global variables are visible as-is.
Should I do anything special if I call perl from multiple threads?
If you create interpreters in one thread and then proceed to call them
in another, you need to make sure perl's own Thread Local Storage (TLS)
slot is initialized correctly in each of those threads.
The "perl_alloc" and "perl_clone" API functions will automatically set
the TLS slot to the interpreter they created, so that there is no need
to do anything special if the interpreter is always accessed in the
same thread that created it, and that thread did not create or call any
other interpreters afterwards. If that is not the case, you have to
set the TLS slot of the thread before calling any functions in the Perl
API on that particular interpreter. This is done by calling the
"PERL_SET_CONTEXT" macro in that thread as the first thing you do:
/* do this before doing anything else with some_perl */
PERL_SET_CONTEXT(some_perl);
... other Perl API calls on some_perl go here ...
Future Plans and PERL_IMPLICIT_SYS
Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything
that the interpreter knows about itself and pass it around, so too are
there plans to allow the interpreter to bundle up everything it knows
about the environment it's running on. This is enabled with the
PERL_IMPLICIT_SYS macro. Currently it only works with USE_ITHREADS on
Windows.
This allows the ability to provide an extra pointer (called the "host"
environment) for all the system calls. This makes it possible for all
the system stuff to maintain their own state, broken down into seven C
structures. These are thin wrappers around the usual system calls (see
win32/perllib.c) for the default perl executable, but for a more
ambitious host (like the one that would do fork() emulation) all the
extra work needed to pretend that different interpreters are actually
different "processes", would be done here.
The Perl engine/interpreter and the host are orthogonal entities.
There could be one or more interpreters in a process, and one or more
"hosts", with free association between them.
Internal Functions
All of Perl's internal functions which will be exposed to the outside
world are prefixed by "Perl_" so that they will not conflict with XS
functions or functions used in a program in which Perl is embedded.
Similarly, all global variables begin with "PL_". (By convention,
static functions start with "S_".)
Inside the Perl core ("PERL_CORE" defined), you can get at the
functions either with or without the "Perl_" prefix, thanks to a bunch
of defines that live in embed.h. Note that extension code should not
set "PERL_CORE"; this exposes the full perl internals, and is likely to
cause breakage of the XS in each new perl release.
The file embed.h is generated automatically from embed.pl and
embed.fnc. embed.pl also creates the prototyping header files for the
internal functions, generates the documentation and a lot of other bits
and pieces. It's important that when you add a new function to the
core or change an existing one, you change the data in the table in
embed.fnc as well. Here's a sample entry from that table:
Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval
The second column is the return type, the third column the name.
Columns after that are the arguments. The first column is a set of
flags:
A This function is a part of the public API. All such functions
should also have 'd', very few do not.
p This function has a "Perl_" prefix; i.e. it is defined as
"Perl_av_fetch".
d This function has documentation using the "apidoc" feature which
we'll look at in a second. Some functions have 'd' but not 'A';
docs are good.
Other available flags are:
s This is a static function and is defined as "STATIC S_whatever", and
usually called within the sources as "whatever(...)".
n This does not need an interpreter context, so the definition has no
"pTHX", and it follows that callers don't use "aTHX". (See
"Background and PERL_IMPLICIT_CONTEXT".)
r This function never returns; "croak", "exit" and friends.
f This function takes a variable number of arguments, "printf" style.
The argument list should end with "...", like this:
Afprd |void |croak |const char* pat|...
M This function is part of the experimental development API, and may
change or disappear without notice.
o This function should not have a compatibility macro to define, say,
"Perl_parse" to "parse". It must be called as "Perl_parse".
x This function isn't exported out of the Perl core.
m This is implemented as a macro.
X This function is explicitly exported.
E This function is visible to extensions included in the Perl core.
b Binary backward compatibility; this function is a macro but also has
a "Perl_" implementation (which is exported).
others
See the comments at the top of "embed.fnc" for others.
If you edit embed.pl or embed.fnc, you will need to run "make
regen_headers" to force a rebuild of embed.h and other auto-generated
files.
Formatted Printing of IVs, UVs, and NVs
If you are printing IVs, UVs, or NVS instead of the stdio(3) style
formatting codes like %d, %ld, %f, you should use the following macros
for portability
IVdf IV in decimal
UVuf UV in decimal
UVof UV in octal
UVxf UV in hexadecimal
NVef NV %e-like
NVff NV %f-like
NVgf NV %g-like
These will take care of 64-bit integers and long doubles. For example:
printf("IV is %"IVdf"\n", iv);
The IVdf will expand to whatever is the correct format for the IVs.
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.
Formatted Printing of "Size_t" and "SSize_t"
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()".
Pointer-To-Integer and Integer-To-Pointer
Because pointer size does not necessarily equal integer size, use the
follow macros to do it right.
PTR2UV(pointer)
PTR2IV(pointer)
PTR2NV(pointer)
INT2PTR(pointertotype, integer)
For example:
IV iv = ...;
SV *sv = INT2PTR(SV*, iv);
and
AV *av = ...;
UV uv = PTR2UV(av);
Exception Handling
There are a couple of macros to do very basic exception handling in XS
modules. You have to define "NO_XSLOCKS" before including XSUB.h to be
able to use these macros:
#define NO_XSLOCKS
#include "XSUB.h"
You can use these macros if you call code that may croak, but you need
to do some cleanup before giving control back to Perl. For example:
dXCPT; /* set up necessary variables */
XCPT_TRY_START {
code_that_may_croak();
} XCPT_TRY_END
XCPT_CATCH
{
/* do cleanup here */
XCPT_RETHROW;
}
Note that you always have to rethrow an exception that has been caught.
Using these macros, it is not possible to just catch the exception and
ignore it. If you have to ignore the exception, you have to use the
"call_*" function.
The advantage of using the above macros is that you don't have to setup
an extra function for "call_*", and that using these macros is faster
than using "call_*".
Source Documentation
There's an effort going on to document the internal functions and
automatically produce reference manuals from them -- perlapi is one
such manual which details all the functions which are available to XS
writers. perlintern is the autogenerated manual for the functions
which are not part of the API and are supposedly for internal use only.
Source documentation is created by putting POD comments into the C
source, like this:
/*
=for apidoc sv_setiv
Copies an integer into the given SV. Does not handle 'set' magic. See
L<perlapi/sv_setiv_mg>.
=cut
*/
Please try and supply some documentation if you add functions to the
Perl core.
Backwards compatibility
The Perl API changes over time. New functions are added or the
interfaces of existing functions are changed. The "Devel::PPPort"
module tries to provide compatibility code for some of these changes,
so XS writers don't have to code it themselves when supporting multiple
versions of Perl.
"Devel::PPPort" generates a C header file ppport.h that can also be run
as a Perl script. To generate ppport.h, run:
perl -MDevel::PPPort -eDevel::PPPort::WriteFile
Besides checking existing XS code, the script can also be used to
retrieve compatibility information for various API calls using the
"--api-info" command line switch. For example:
% perl ppport.h --api-info=sv_magicext
For details, see "perldoc ppport.h".
Unicode Support
Perl 5.6.0 introduced Unicode support. It's important for porters and
XS writers to understand this support and make sure that the code they
write does not corrupt Unicode data.
What is Unicode, anyway?
In the olden, less enlightened times, we all used to use ASCII. Most
of us did, anyway. The big problem with ASCII is that it's American.
Well, no, that's not actually the problem; the problem is that it's not
particularly useful for people who don't use the Roman alphabet. What
used to happen was that particular languages would stick their own
alphabet in the upper range of the sequence, between 128 and 255. Of
course, we then ended up with plenty of variants that weren't quite
ASCII, and the whole point of it being a standard was lost.
Worse still, if you've got a language like Chinese or Japanese that has
hundreds or thousands of characters, then you really can't fit them
into a mere 256, so they had to forget about ASCII altogether, and
build their own systems using pairs of numbers to refer to one
character.
To fix this, some people formed Unicode, Inc. and produced a new
character set containing all the characters you can possibly think of
and more. There are several ways of representing these characters, and
the one Perl uses is called UTF-8. UTF-8 uses a variable number of
bytes to represent a character. You can learn more about Unicode and
Perl's Unicode model in perlunicode.
(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.)
How can I recognise a UTF-8 string?
You can't. This is because UTF-8 data is stored in bytes just like
non-UTF-8 data. The Unicode character 200, (0xC8 for you hex types)
capital E with a grave accent, is represented by the two bytes
"v196.172". Unfortunately, the non-Unicode string "chr(196).chr(172)"
has that byte sequence as well. So you can't tell just by looking --
this is what makes Unicode input an interesting problem.
In general, you either have to know what you're dealing with, or you
have to guess. The API function "is_utf8_string" can help; it'll tell
you if a string contains only valid UTF-8 characters, 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.
How does UTF-8 represent Unicode characters?
As mentioned above, UTF-8 uses a variable number of bytes to store a
character. Characters with values 0...127 are stored in one byte, just
like good ol' ASCII. Character 128 is stored as "v194.128"; this
continues up to character 191, which is "v194.191". Now we've run out
of bits (191 is binary 10111111) so we move on; 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; /* Initialize this to point to the beginning of the
sequence to convert */
U8 *utf_end; /* Initialize this to 1 beyond the end of the sequence
pointed to by 'utf' */
UV uv; /* Returned code point; note: a UV, not a U8, not a
char */
STRLEN len; /* Returned 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.)
How does Perl store UTF-8 strings?
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 "usebytes". 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.
How do I convert a string to UTF-8?
If you're mixing UTF-8 and non-UTF-8 strings, it is necessary to
upgrade 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.
How do I compare strings?
"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).
Is there anything else I need to know?
Not really. Just remember these things:
o 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.
o 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.
o 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".
o Mixing UTF-8 and non-UTF-8 strings is tricky. Use "bytes_to_utf8"
to get a new string which is UTF-8 encoded, and then combine them.
Custom Operators
Custom operator support is 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:
xop_name
A short name for your op. This will be included in some error
messages, and will also be returned as "$op->name" by the B module,
so it will appear in the output of module like B::Concise.
xop_desc
A short description of the function of the op.
xop_class
Which of the various *OP structures this op uses. This should be
one of the "OA_*" constants from op.h, namely
OA_BASEOP
OA_UNOP
OA_BINOP
OA_LOGOP
OA_LISTOP
OA_PMOP
OA_SVOP
OA_PADOP
OA_PVOP_OR_SVOP
This should be interpreted as '"PVOP"' only. The "_OR_SVOP" is
because the only core "PVOP", "OP_TRANS", can sometimes be a
"SVOP" instead.
OA_LOOP
OA_COP
The other "OA_*" constants should not be used.
xop_peep
This member is of type "Perl_cpeep_t", which expands to "void
(*Perl_cpeep_t)(aTHX_ OP *o, OP *oldop)". If it is set, this
function will be called from "Perl_rpeep" when ops of this type are
encountered by the peephole optimizer. o is the OP that needs
optimizing; oldop is the previous OP optimized, whose "op_next"
points to o.
"B::Generate" directly supports the creation of custom ops by name.
Dynamic Scope and the Context Stack
Note: this section describes a non-public internal API that is subject
to change without notice.
Introduction to the context stack
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 "local"ised 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.
Pushing contexts
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.
Popping contexts
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".
Redoing contexts
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".
Slab-based operator allocation
Note: this section describes a non-public internal API that is subject
to change without notice.
Perl's internal error-handling mechanisms implement "die" (and its
internal equivalents) using longjmp. If this occurs during lexing,
parsing or compilation, we must ensure that any ops allocated as part
of the compilation process are freed. (Older Perl versions did not
adequately handle this situation: when failing a parse, they would leak
ops that were stored in C "auto" variables and not linked anywhere
else.)
To handle this situation, Perl uses op slabs that are attached to the
currently-compiling CV. A slab is a chunk of allocated memory. New ops
are allocated as regions of the slab. If the slab fills up, a new one
is created (and linked from the previous one). When an error occurs and
the CV is freed, any ops remaining are freed.
Each op is preceded by two pointers: one points to the next op in the
slab, and the other points to the slab that owns it. The next-op
pointer is needed so that Perl can iterate over a slab and free all its
ops. (Op structures are of different sizes, so the slab's ops can't
merely be treated as a dense array.) The slab pointer is needed for
accessing a reference count on the slab: when the last op on a slab is
freed, the slab itself is freed.
The slab allocator puts the ops at the end of the slab first. This will
tend to allocate the leaves of the op tree first, and the layout will
therefore hopefully be cache-friendly. In addition, this means that
there's no need to store the size of the slab (see below on why slabs
vary in size), because Perl can follow pointers to find the last op.
It might seem possible eliminate slab reference counts altogether, by
having all ops implicitly attached to "PL_compcv" when allocated and
freed when the CV is freed. That would also allow "op_free" to skip
"FreeOp" altogether, and thus free ops faster. But that doesn't work in
those cases where ops need to survive beyond their CVs, such as re-
evals.
The CV also has to have a reference count on the slab. Sometimes the
first op created is immediately freed. If the reference count of the
slab reaches 0, then it will be freed with the CV still pointing to it.
CVs use the "CVf_SLABBED" flag to indicate that the CV has a reference
count on the slab. When this flag is set, the slab is accessible via
"CvSTART" when "CvROOT" is not set, or by subtracting two pointers
"(2*sizeof(I32 *))" from "CvROOT" when it is set. The alternative to
this approach of sneaking the slab into "CvSTART" during compilation
would be to enlarge the "xpvcv" struct by another pointer. But that
would make all CVs larger, even though slab-based op freeing is
typically of benefit only for programs that make significant use of
string eval.
When the "CVf_SLABBED" flag is set, the CV takes responsibility for
freeing the slab. If "CvROOT" is not set when the CV is freed or
undeffed, it is assumed that a compilation error has occurred, so the
op slab is traversed and all the ops are freed.
Under normal circumstances, the CV forgets about its slab (decrementing
the reference count) when the root is attached. So the slab reference
counting that happens when ops are freed takes care of freeing the
slab. In some cases, the CV is told to forget about the slab
("cv_forget_slab") precisely so that the ops can survive after the CV
is done away with.
Forgetting the slab when the root is attached is not strictly
necessary, but avoids potential problems with "CvROOT" being written
over. There is code all over the place, both in core and on CPAN, that
does things with "CvROOT", so forgetting the slab makes things more
robust and avoids potential problems.
Since the CV takes ownership of its slab when flagged, that flag is
never copied when a CV is cloned, as one CV could free a slab that
another CV still points to, since forced freeing of ops ignores the
reference count (but asserts that it looks right).
To avoid slab fragmentation, freed ops are marked as freed and attached
to the slab's freed chain (an idea stolen from DBM::Deep). Those freed
ops are reused when possible. Not reusing freed ops would be simpler,
but it would result in significantly higher memory usage for programs
with large "if (DEBUG) {...}" blocks.
"SAVEFREEOP" is slightly problematic under this scheme. Sometimes it
can cause an op to be freed after its CV. If the CV has forcibly freed
the ops on its slab and the slab itself, then we will be fiddling with
a freed slab. Making "SAVEFREEOP" a no-op doesn't help, as sometimes an
op can be savefreed when there is no compilation error, so the op would
never be freed. It holds a reference count on the slab, so the whole
slab would leak. So "SAVEFREEOP" now sets a special flag on the op
("->op_savefree"). The forced freeing of ops after a compilation error
won't free any ops thus marked.
Since many pieces of code create tiny subroutines consisting of only a
few ops, and since a huge slab would be quite a bit of baggage for
those to carry around, the first slab is always very small. To avoid
allocating too many slabs for a single CV, each subsequent slab is
twice the size of the previous.
Smartmatch expects to be able to allocate an op at run time, run it,
and then throw it away. For that to work the op is simply malloced when
PL_compcv hasn't been set up. So all slab-allocated ops are marked as
such ("->op_slabbed"), to distinguish them from malloced ops.
AUTHORS
Until May 1997, this document was maintained by Jeff Okamoto
<okamoto@corp.hp.com>. It is now maintained as part of Perl itself by
the Perl 5 Porters <perl5-porters@perl.org>.
With lots of help and suggestions from Dean Roehrich, Malcolm Beattie,
Andreas Koenig, Paul Hudson, Ilya Zakharevich, Paul Marquess, Neil
Bowers, Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer,
Stephen McCamant, and Gurusamy Sarathy.
SEE ALSO
perlapi, perlintern, perlxs, perlembed
perl v5.30.3 2020-06-07 PERLGUTS(1)