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 several special typedefs to declare variables to hold
integers of (at least) a given size. Use I8, I16, I32, and I64 to
declare a signed integer variable which has at least as many bits as the
number in its name. These all evaluate to the native C type that is
closest to the given number of bits, but no smaller than that number.
For example, on many platforms, a "short" is 16 bits long, and if so,
I16 will evaluate to a "short". But on platforms where a "short" isn't
exactly 16 bits, Perl will use the smallest type that contains 16 bits
or more.
U8, U16, U32, and U64 are to declare the corresponding unsigned integer
types.
If the platform doesn't support 64-bit integers, both I64 and U64 will
be undefined. Use IV and UV to declare the largest practicable, and
""WIDEST_UTYPE" in perlapi" for the absolute maximum unsigned, but which
may not be usable in all circumstances.
A numeric constant can be specified with ""INT16_C"" in perlapi,
""UINTMAX_C"" in perlapi, and similar.
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, Perl's API exposes
several macros that coerce the actual scalar type into an IV, UV,
double, or string:
• SvIV(SV*) ("IV") and SvUV(SV*) ("UV")
• SvNV(SV*) ("double")
• Strings are a bit complicated:
• Byte string: "SvPVbyte(SV*, STRLEN len)" or SvPVbyte_nolen(SV*)
If the Perl string is "\xff\xff", then this returns a 2-byte
"char*".
This is suitable for Perl strings that represent bytes.
• UTF-8 string: "SvPVutf8(SV*, STRLEN len)" or SvPVutf8_nolen(SV*)
If the Perl string is "\xff\xff", then this returns a 4-byte
"char*".
This is suitable for Perl strings that represent characters.
CAVEAT: That "char*" will be encoded via Perl's internal UTF-8
variant, which means that if the SV contains non-Unicode code
points (e.g., 0x110000), then the result may contain extensions
over valid UTF-8. See "is_strict_utf8_string" in perlapi for
some methods Perl gives you to check the UTF-8 validity of these
macros' returns.
• You can also use "SvPV(SV*, STRLEN len)" or SvPV_nolen(SV*) to
fetch the SV's raw internal buffer. This is tricky, though; if
your Perl string is "\xff\xff", then depending on the SV's
internal encoding you might get back a 2-byte OR a 4-byte
"char*". Moreover, if it's the 4-byte string, that could come
from either Perl "\xff\xff" stored UTF-8 encoded, or Perl
"\xc3\xbf\xc3\xbf" stored as raw octets. To differentiate
between these you MUST look up the SV's UTF8 bit (cf. "SvUTF8")
to know whether the source Perl string is 2 characters ("SvUTF8"
would be on) or 4 characters ("SvUTF8" would be off).
IMPORTANT: Use of "SvPV", "SvPV_nolen", or similarly-named
macros without looking up the SV's UTF8 bit is almost certainly
a bug if non-ASCII input is allowed.
When the UTF8 bit is on, the same CAVEAT about UTF-8 validity
applies here as for "SvPVutf8".
(See "How do I pass a Perl string to a C library?" for more
details.)
In "SvPVbyte", "SvPVutf8", and "SvPV", the length of the "char*"
returned is placed into the variable "len" (these are macros, so you
do not use &len). If you do not care what the length of the data is,
use "SvPVbyte_nolen", "SvPVutf8_nolen", or "SvPV_nolen" instead.
The global variable "PL_na" can also be given to
"SvPVbyte"/"SvPVutf8"/"SvPV" 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(SvPVbyte(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 = SvPVbyte(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 SvPVbyte_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*)
Be aware that retrieving the numeric value of an SV can set IOK or NOK
on that SV, even when the SV started as a string. Prior to Perl 5.36.0
retrieving the string value of an integer could set POK, but this can no
longer occur. From 5.36.0 this can be used to distinguish the original
representation of an SV and is intended to make life simpler for
serializers:
/* references handled elsewhere */
if (SvIsBOOL(sv)) {
/* originally boolean */
...
}
else if (SvPOK(sv)) {
/* originally a string */
...
}
else if (SvNIOK(sv)) {
/* originally numeric */
...
}
else {
/* something special or undef */
}
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 main, longstanding 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.
Perl v5.36 added two new ways to create an AV and allocate a SV** array
without populating it. These are more efficient than a newAV() followed
by an av_extend().
/* Creates but does not initialize (Zero) the SV** array */
AV *av = newAV_alloc_x(1);
/* Creates and does initialize (Zero) the SV** array */
AV *av = newAV_alloc_xz(1);
The numerical argument refers to the number of array elements to
allocate, not an array index, and must be >0. The first form must only
ever be used when all elements will be initialized before any read
occurs. Reading a non-initialized SV* - i.e. treating a random memory
address as a SV* - is a serious bug.
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:
Size_t av_count(AV*);
SSize_t av_top_index(AV*);
SV** av_fetch(AV*, SSize_t key, I32 lval);
SV** av_store(AV*, SSize_t key, SV* val);
"av_count" returns the number of elements in the array (including any
empty slots (undefined ones) that are intermixed with filled-in ones).
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. It
is always equal to "av_count() - 1". 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.
More efficient working with new or vanilla AVs
Perl v5.36 and v5.38 introduced streamlined, inlined versions of some
functions:
• "av_store_simple"
• "av_fetch_simple"
• "av_push_simple"
These are drop-in replacements, but can only be used on straightforward
AVs that meet the following criteria:
• are not magical
• are not readonly
• are "real" (refcounted) AVs
• have an av_top_index value > -2
AVs created using newAV(), "av_make", "newAV_alloc_x", and
"newAV_alloc_xz" are all compatible at the time of creation. It is only
if they are declared readonly or unreal, have magic attached, or are
otherwise configured unusually that they will stop being compatible.
Note that some interpreter functions may attach magic to an AV as part
of normal operations. It is therefore safest, unless you are sure of the
lifecycle of an AV, to only use these new functions close to the point
of AV creation.
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 and "Temporaries Stack" below 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.
I/O Handles
Like AVs and HVs, IO objects are another type of non-scalar SV which may
contain input and output PerlIO objects or a "DIR *" from opendir().
You can create a new IO object:
IO* newIO();
Unlike other SVs, a new IO object is automatically blessed into the
IO::File class.
The IO object contains an input and output PerlIO handle:
PerlIO *IoIFP(IO *io);
PerlIO *IoOFP(IO *io);
Typically if the IO object has been opened on a file, the input handle
is always present, but the output handle is only present if the file is
open for output. For a file, if both are present they will be the same
PerlIO object.
Distinct input and output PerlIO objects are created for sockets and
character devices.
The IO object also contains other data associated with Perl I/O handles:
IV IoLINES(io); /* $. */
IV IoPAGE(io); /* $% */
IV IoPAGE_LEN(io); /* $= */
IV IoLINES_LEFT(io); /* $- */
char *IoTOP_NAME(io); /* $^ */
GV *IoTOP_GV(io); /* $^ */
char *IoFMT_NAME(io); /* $~ */
GV *IoFMT_GV(io); /* $~ */
char *IoBOTTOM_NAME(io);
GV *IoBOTTOM_GV(io);
char IoTYPE(io);
U8 IoFLAGS(io);
=for apidoc_sections $io_scn, $formats_section
=for apidoc_section $reports
=for apidoc Amh|IV|IoLINES|IO *io
=for apidoc Amh|IV|IoPAGE|IO *io
=for apidoc Amh|IV|IoPAGE_LEN|IO *io
=for apidoc Amh|IV|IoLINES_LEFT|IO *io
=for apidoc Amh|char *|IoTOP_NAME|IO *io
=for apidoc Amh|GV *|IoTOP_GV|IO *io
=for apidoc Amh|char *|IoFMT_NAME|IO *io
=for apidoc Amh|GV *|IoFMT_GV|IO *io
=for apidoc Amh|char *|IoBOTTOM_NAME|IO *io
=for apidoc Amh|GV *|IoBOTTOM_GV|IO *io
=for apidoc_section $io
=for apidoc Amh|char|IoTYPE|IO *io
=for apidoc Amh|U8|IoFLAGS|IO *io
Most of these are involved with formats.
IoFLAGs() may contain a combination of flags, the most interesting of
which are "IOf_FLUSH" ($|) for autoflush and "IOf_UNTAINT", settable
with IO::Handle's untaint() method.
The IO object may also contains a directory handle:
DIR *IoDIRP(io);
suitable for use with PerlDir_read() etc.
All of these accessors macros are lvalues, there are no distinct _set()
macros to modify the members of the IO object.
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 vtbl_sig %SIG hash
s PERL_MAGIC_sigelem vtbl_sigelem %SIG hash element
t PERL_MAGIC_taint vtbl_taint Taintedness
U PERL_MAGIC_uvar vtbl_uvar Available for use by
extensions
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_destruct vtbl_destruct destruct callback
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
Z PERL_MAGIC_hook vtbl_hook %{^HOOK} hash
z PERL_MAGIC_hookelem vtbl_hookelem %{^HOOK} hash element
\ PERL_MAGIC_lvref vtbl_lvref Lvalue reference
constructor
] PERL_MAGIC_checkcall vtbl_checkcall Inlining/mutation of call
to this CV
^ PERL_MAGIC_extvalue (none) Value magic available for
use by extensions
~ PERL_MAGIC_ext (none) Variable magic 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", "PERL_MAGIC_extvalue" 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" or
"PERL_MAGIC_extvalue" 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). "PERL_MAGIC_extvalue" is value magic
(unlike "PERL_MAGIC_ext" and "PERL_MAGIC_uvar") meaning that on
localization the new value will not be magical.
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);
SvREFCNT_dec(tie); /* hv_magic() increases tie ref count */
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)"
"SAVEI8(I8 i)"
"SAVEI16(I16 i)"
"SAVEBOOL(int i)"
"SAVESTRLEN(STRLEN i)"
These macros arrange things to restore the value of integer variable
"i" at the end of the 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.
"SAVERCPV(char **ppv)"
This macro arranges to restore the value of a "char *" variable
which was allocated with a call to rcpv_new() to its previous state
when the current pseudo block is completed. The pointer stored in
*ppv at the time of the call will be refcount incremented and stored
on the save stack. Later when the current pseudo-block is completed
the value stored in *ppv will be refcount decremented, and the
previous value restored from the savestack which will also be
refcount decremented.
This is the "RCPV" equivalent of SAVEGENERICSV().
"SAVEGENERICSV(SV **psv)"
This macro arranges to restore the value of a "SV *" variable to its
previous state when the current pseudo block is completed. The
pointer stored in *psv at the time of the call will be refcount
incremented and stored on the save stack. Later when the current
pseudo-block is completed the value stored in *ppv will be refcount
decremented, and the previous value restored from the savestack
which will also be refcount decremented. This the C equivalent of
"local $sv".
"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 the current pseudo-block.
"SAVEFREERCPV(char *pv)"
Ensures that a "char *" which was created by a call to rcpv_new() is
rcpv_free()ed at the end of the current pseudo-block.
This is the RCPV equivalent of SAVEFREESV().
"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" which may be NULL.
"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" which may be NULL.
Note the end of the current pseudo-block may occur much later than
the end of the current statement. You may wish to look at the
MORTALSVFUNC_X() macro instead.
"MORTALSVFUNC_X(SVFUNC_t f, SV *sv)"
At the end of the current statement the function "f" is called with
the implicit context argument (if any), and "sv" which may be NULL.
Be aware that the parameter argument to the destructor function
differs from the related SAVEDESTRUCTOR_X() in that it MUST be
either NULL or an "SV*".
Note the end of the current statement may occur much before the the
end of the current pseudo-block. You may wish to look at the
SAVEDESTRUCTOR_X() macro instead.
"MORTALDESTRUCTOR_SV(SV *coderef, SV *args)"
At the end of the current statement the Perl function contained in
"coderef" is called with the arguments provided (if any) in "args".
See the documentation for mortal_destructor_sv() for details on the
"args" parameter is handled.
Note the end of the current statement may occur much before the the
end of the current pseudo-block. If you wish to call a perl
function at the end of the current pseudo block you should use the
SAVEDESTRUCTOR_X() API instead, which will require you create a C
wrapper to call the Perl function.
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 *".
Other macros above have functions implementing them, but its probably
best to just use the macro, and not those or the ones below.
"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 the value of "SV" will be restored
using the stored value. It doesn't handle magic. Use "save_scalar"
if magic is affected.
"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 the 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.
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 *first)
{
OP *o = first, *t = first;
for(; o = o->op_next, t = t->op_next) {
/* custom per-op optimisation goes here */
o = o->op_next;
if (!o || o == t) break;
/* custom per-op optimisation goes AND 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 MULTIPLICITY
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
is a way 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.
The macro that controls the major Perl build flavor is MULTIPLICITY.
The MULTIPLICITY build has a C structure that packages all the
interpreter state, which is being passed to various perl functions as a
"hidden" first argument. MULTIPLICITY makes multi-threaded perls
possible (with the ithreads threading model, related to the macro
USE_ITHREADS.)
PERL_IMPLICIT_CONTEXT is a legacy synonym for MULTIPLICITY.
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".
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 MULTIPLICITY, 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
MULTIPLICITY 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 MULTIPLICITY
#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?
See also "Dealing with embedded perls and threads" in perlclib.
When Perl is built with MULTIPLICITY, 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 MULTIPLICITY
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 MULTIPLICITY 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.
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 ...
(You can always get the current context via "PERL_GET_CONTEXT".)
Future Plans and PERL_IMPLICIT_SYS
Just as MULTIPLICITY 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 first column is a set of flags, the second column the return type,
the third column the name. Columns after that are the arguments. The
flags are documented at the top of embed.fnc.
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 the spaces are required around the format in case the code is
compiled with C++, to maintain compliance with its standard.
Note that there are different "long doubles": Perl will use whatever the
compiler has.
If you are printing addresses of pointers, use %p or UVxf combined with
PTR2UV().
Formatted Printing of SVs
The contents of SVs may be printed using the "SVf" format, like so:
Perl_croak(aTHX_ "This croaked because: %" SVf "\n", SVfARG(err_msg))
where "err_msg" is an SV.
Not all scalar types are printable. Simple values certainly are: one of
IV, UV, NV, or PV. Also, if the SV is a reference to some value, either
it will be dereferenced and the value printed, or information about the
type of that value and its address are displayed. The results of
printing any other type of SV are undefined and likely to lead to an
interpreter crash. NVs are printed using a %g-ish format.
Note that the spaces are required around the "SVf" in case the code is
compiled with C++, to maintain compliance with its standard.
Note that any filehandle being printed to under UTF-8 must be expecting
UTF-8 in order to get good results and avoid Wide-character warnings.
One way to do this for typical filehandles is to invoke perl with the
"-C" parameter. (See "-C [number/list]" in perlrun.
You can use this to concatenate two scalars:
SV *var1 = get_sv("var1", GV_ADD);
SV *var2 = get_sv("var2", GV_ADD);
SV *var3 = newSVpvf("var1=%" SVf " and var2=%" SVf,
SVfARG(var1), SVfARG(var2));
"SVf_QUOTEDPREFIX" is similar to "SVf" except that it restricts the
number of the characters printed, showing at most the first
"PERL_QUOTEDPREFIX_LEN" characters of the argument, and rendering it
with double quotes and with the contents escaped using double quoted
string escaping rules. If the string is longer than this then ellipses
"..." will be appended after the trailing quote. This is intended for
error messages where the string is assumed to be a class name.
"HvNAMEf" and "HvNAMEf_QUOTEDPREFIX" are similar to "SVf" except they
extract the string, length and utf8 flags from the argument using the
HvNAME(), HvNAMELEN(), HvNAMEUTF8() macros. This is intended for
stringifying a class name directly from an stash HV.
Formatted Printing of Strings
If you just want the bytes printed in a 7bit NUL-terminated string, you
can just use %s (assuming they are all really only 7bit). But if there
is a possibility the value will be encoded as UTF-8 or contains bytes
above 0x7F (and therefore 8bit), you should instead use the "UTF8f"
format. And as its parameter, use the UTF8fARG() macro:
chr * msg;
/* U+2018: \xE2\x80\x98 LEFT SINGLE QUOTATION MARK
U+2019: \xE2\x80\x99 RIGHT SINGLE QUOTATION MARK */
if (can_utf8)
msg = "\xE2\x80\x98Uses fancy quotes\xE2\x80\x99";
else
msg = "'Uses simple quotes'";
Perl_croak(aTHX_ "The message is: %" UTF8f "\n",
UTF8fARG(can_utf8, strlen(msg), msg));
The first parameter to "UTF8fARG" is a boolean: 1 if the string is in
UTF-8; 0 if string is in native byte encoding (Latin1). The second
parameter is the number of bytes in the string to print. And the third
and final parameter is a pointer to the first byte in the string.
Note that any filehandle being printed to under UTF-8 must be expecting
UTF-8 in order to get good results and avoid Wide-character warnings.
One way to do this for typical filehandles is to invoke perl with the
"-C" parameter. (See "-C [number/list]" in perlrun.
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().
Formatted Printing of "Ptrdiff_t", "intmax_t", "short" and other special
sizes
There are modifiers for these special situations if you are using
PerlIO_printf(). See "size" in perlfunc.
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);
There are also
PTR2nat(pointer) /* pointer to integer of PTRSIZE */
PTR2ul(pointer) /* pointer to unsigned long */
And "PTRV" which gives the native type for an integer the same size as
pointers, such as "unsigned" or "unsigned long".
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 "use bytes". If
so, the underlying bytes that comprise the UTF-8 string are to be
exposed, rather than the character they represent. But this pragma
should only really be used for debugging and perhaps low-level testing
at the byte level. Hence most XS code need not concern itself with
this, but various areas of the perl core do need to support it.
And this isn't the whole story. Starting in Perl v5.12, strings that
aren't encoded in UTF-8 may also be treated as Unicode under various
conditions (see "ASCII Rules versus Unicode Rules" in perlunicode).
This is only really a problem for characters whose ordinals are between
128 and 255, and their behavior varies under ASCII versus Unicode rules
in ways that your code cares about (see "The "Unicode Bug"" in
perlunicode). There is no published API for dealing with this, as it is
subject to change, but you can look at the code for "pp_lc" in pp.c for
an example as to how it's currently done.
How do I pass a Perl string to a C library?
A Perl string, conceptually, is an opaque sequence of code points. Many
C libraries expect their inputs to be "classical" C strings, which are
arrays of octets 1-255, terminated with a NUL byte. Your job when
writing an interface between Perl and a C library is to define the
mapping between Perl and that library.
Generally speaking, "SvPVbyte" and related macros suit this task well.
These assume that your Perl string is a "byte string", i.e., is either
raw, undecoded input into Perl or is pre-encoded to, e.g., UTF-8.
Alternatively, if your C library expects UTF-8 text, you can use
"SvPVutf8" and related macros. This has the same effect as encoding to
UTF-8 then calling the corresponding "SvPVbyte"-related macro.
Some C libraries may expect other encodings (e.g., UTF-16LE). To give
Perl strings to such libraries you must either do that encoding in Perl
then use "SvPVbyte", or use an intermediary C library to convert from
however Perl stores the string to the desired encoding.
Take care also that NULs in your Perl string don't confuse the C
library. If possible, give the string's length to the C library; if
that's not possible, consider rejecting strings that contain NUL bytes.
What about "SvPV", "SvPV_nolen", etc.?
Consider a 3-character Perl string "$foo = "\x64\x78\x8c"". Perl can
store these 3 characters either of two ways:
• bytes: 0x64 0x78 0x8c
• UTF-8: 0x64 0x78 0xc2 0x8c
Now let's say you convert $foo to a C string thus:
STRLEN strlen;
char *str = SvPV(foo_sv, strlen);
At this point "str" could point to a 3-byte C string or a 4-byte one.
Generally speaking, we want "str" to be the same regardless of how Perl
stores $foo, so the ambiguity here is undesirable. "SvPVbyte" and
"SvPVutf8" solve that by giving predictable output: use "SvPVbyte" if
your C library expects byte strings, or "SvPVutf8" if it expects UTF-8.
If your C library happens to support both encodings, then "SvPV"--always
in tandem with lookups to "SvUTF8"!--may be safe and (slightly) more
efficient.
TESTING TIP: Use utf8's "upgrade" and "downgrade" functions in your
tests to ensure consistent handling regardless of Perl's internal
encoding.
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:
• There's no way to tell if a "char *" or "U8 *" string is UTF-8 or
not. But you can tell if an SV is to be treated as UTF-8 by calling
"DO_UTF8" on it, after stringifying it with "SvPV" or a similar
macro. And, you can tell if SV is actually UTF-8 (even if it is not
to be treated as such) by looking at its "SvUTF8" flag (again after
stringifying it). Don't forget to set the flag if something should
be UTF-8. Treat the flag as part of the PV, even though it's not --
if you pass on the PV to somewhere, pass on the flag too.
• If a string is UTF-8, always use "utf8_to_uvchr_buf" to get at the
value, unless UTF8_IS_INVARIANT(*s) in which case you can use *s.
• When writing a character UV to a UTF-8 string, always use
"uvchr_to_utf8", unless "UVCHR_IS_INVARIANT(uv))" in which case you
can use "*s = uv".
• Mixing UTF-8 and non-UTF-8 strings is tricky. Use "bytes_to_utf8" to
get a new string which is UTF-8 encoded, and then combine them.
Custom 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.
Stacks
Descriptions above occasionally refer to "the stack", but there are in
fact many stack-like data structures within the perl interpreter. When
otherwise unqualified, "the stack" usually refers to the value stack.
The various stacks have different purposes, and operate in slightly
different ways. Their differences are noted below.
Value Stack
This stack stores the values that regular perl code is operating on,
usually intermediate values of expressions within a statement. The stack
itself is formed of an array of SV pointers.
The base of this stack is pointed to by the interpreter variable
"PL_stack_base", of type "SV **".
The head of the stack is "PL_stack_sp", and points to the most recently-
pushed item.
Items are pushed to the stack by using the PUSHs() macro or its variants
described above; XPUSHs(), mPUSHs(), mXPUSHs() and the typed versions.
Note carefully that the non-"X" versions of these macros do not check
the size of the stack and assume it to be big enough. These must be
paired with a suitable check of the stack's size, such as the "EXTEND"
macro to ensure it is large enough. For example
EXTEND(SP, 4);
mPUSHi(10);
mPUSHi(20);
mPUSHi(30);
mPUSHi(40);
This is slightly more performant than making four separate checks in
four separate mXPUSHi() calls.
As a further performance optimisation, the various "PUSH" macros all
operate using a local variable "SP", rather than the interpreter-global
variable "PL_stack_sp". This variable is declared by the "dSP" macro -
though it is normally implied by XSUBs and similar so it is rare you
have to consider it directly. Once declared, the "PUSH" macros will
operate only on this local variable, so before invoking any other perl
core functions you must use the "PUTBACK" macro to return the value from
the local "SP" variable back to the interpreter variable. Similarly,
after calling a perl core function which may have had reason to move the
stack or push/pop values to it, you must use the "SPAGAIN" macro which
refreshes the local "SP" value back from the interpreter one.
Items are popped from the stack by using the "POPs" macro or its typed
versions, There is also a macro "TOPs" that inspects the topmost item
without removing it.
Note specifically that SV pointers on the value stack do not contribute
to the overall reference count of the xVs being referred to. If newly-
created xVs are being pushed to the stack you must arrange for them to
be destroyed at a suitable time; usually by using one of the "mPUSH*"
macros or sv_2mortal() to mortalise the xV.
Mark Stack
The value stack stores individual perl scalar values as temporaries
between expressions. Some perl expressions operate on entire lists; for
that purpose we need to know where on the stack each list begins. This
is the purpose of the mark stack.
The mark stack stores integers as I32 values, which are the height of
the value stack at the time before the list began; thus the mark itself
actually points to the value stack entry one before the list. The list
itself starts at "mark + 1".
The base of this stack is pointed to by the interpreter variable
"PL_markstack", of type "I32 *".
The head of the stack is "PL_markstack_ptr", and points to the most
recently-pushed item.
Items are pushed to the stack by using the PUSHMARK() macro. Even though
the stack itself stores (value) stack indices as integers, the
"PUSHMARK" macro should be given a stack pointer directly; it will
calculate the index offset by comparing to the "PL_stack_sp" variable.
Thus almost always the code to perform this is
PUSHMARK(SP);
Items are popped from the stack by the "POPMARK" macro. There is also a
macro "TOPMARK" that inspects the topmost item without removing it.
These macros return I32 index values directly. There is also the "dMARK"
macro which declares a new SV double-pointer variable, called "mark",
which points at the marked stack slot; this is the usual macro that C
code will use when operating on lists given on the stack.
As noted above, the "mark" variable itself will point at the most
recently pushed value on the value stack before the list begins, and so
the list itself starts at "mark + 1". The values of the list may be
iterated by code such as
for(SV **svp = mark + 1; svp <= PL_stack_sp; svp++) {
SV *item = *svp;
...
}
Note specifically in the case that the list is already empty, "mark"
will equal "PL_stack_sp".
Because the "mark" variable is converted to a pointer on the value
stack, extra care must be taken if "EXTEND" or any of the "XPUSH" macros
are invoked within the function, because the stack may need to be moved
to extend it and so the existing pointer will now be invalid. If this
may be a problem, a possible solution is to track the mark offset as an
integer and track the mark itself later on after the stack had been
moved.
I32 markoff = POPMARK;
...
SP **mark = PL_stack_base + markoff;
Temporaries Stack
As noted above, xV references on the main value stack do not contribute
to the reference count of an xV, and so another mechanism is used to
track when temporary values which live on the stack must be released.
This is the job of the temporaries stack.
The temporaries stack stores pointers to xVs whose reference counts will
be decremented soon.
The base of this stack is pointed to by the interpreter variable
"PL_tmps_stack", of type "SV **".
The head of the stack is indexed by "PL_tmps_ix", an integer which
stores the index in the array of the most recently-pushed item.
There is no public API to directly push items to the temporaries stack.
Instead, the API function sv_2mortal() is used to mortalize an xV,
adding its address to the temporaries stack.
Likewise, there is no public API to read values from the temporaries
stack. Instead, the macros "SAVETMPS" and "FREETMPS" are used. The
"SAVETMPS" macro establishes the base levels of the temporaries stack,
by capturing the current value of "PL_tmps_ix" into "PL_tmps_floor" and
saving the previous value to the save stack. Thereafter, whenever
"FREETMPS" is invoked all of the temporaries that have been pushed since
that level are reclaimed.
While it is common to see these two macros in pairs within an "ENTER"/
"LEAVE" pair, it is not necessary to match them. It is permitted to
invoke "FREETMPS" multiple times since the most recent "SAVETMPS"; for
example in a loop iterating over elements of a list. While you can
invoke "SAVETMPS" multiple times within a scope pair, it is unlikely to
be useful. Subsequent invocations will move the temporaries floor
further up, thus effectively trapping the existing temporaries to only
be released at the end of the scope.
Save Stack
The save stack is used by perl to implement the "local" keyword and
other similar behaviours; any cleanup operations that need to be
performed when leaving the current scope. Items pushed to this stack
generally capture the current value of some internal variable or state,
which will be restored when the scope is unwound due to leaving,
"return", "die", "goto" or other reasons.
Whereas other perl internal stacks store individual items all of the
same type (usually SV pointers or integers), the items pushed to the
save stack are formed of many different types, having multiple fields to
them. For example, the "SAVEt_INT" type needs to store both the address
of the "int" variable to restore, and the value to restore it to. This
information could have been stored using fields of a "struct", but would
have to be large enough to store three pointers in the largest case,
which would waste a lot of space in most of the smaller cases.
Instead, the stack stores information in a variable-length encoding of
"ANY" structures. The final value pushed is stored in the "UV" field
which encodes the kind of item held by the preceding items; the count
and types of which will depend on what kind of item is being stored. The
kind field is pushed last because that will be the first field to be
popped when unwinding items from the stack.
The base of this stack is pointed to by the interpreter variable
"PL_savestack", of type "ANY *".
The head of the stack is indexed by "PL_savestack_ix", an integer which
stores the index in the array at which the next item should be pushed.
(Note that this is different to most other stacks, which reference the
most recently-pushed item).
Items are pushed to the save stack by using the various "SAVE...()"
macros. Many of these macros take a variable and store both its address
and current value on the save stack, ensuring that value gets restored
on scope exit.
SAVEI8(i8)
SAVEI16(i16)
SAVEI32(i32)
SAVEINT(i)
...
There are also a variety of other special-purpose macros which save
particular types or values of interest. "SAVETMPS" has already been
mentioned above. Others include "SAVEFREEPV" which arranges for a PV
(i.e. a string buffer) to be freed, or "SAVEDESTRUCTOR" which arranges
for a given function pointer to be invoked on scope exit. A full list of
such macros can be found in scope.h.
There is no public API for popping individual values or items from the
save stack. Instead, via the scope stack, the "ENTER" and "LEAVE" pair
form a way to start and stop nested scopes. Leaving a nested scope via
"LEAVE" will restore all of the saved values that had been pushed since
the most recent "ENTER".
Scope Stack
As with the mark stack to the value stack, the scope stack forms a pair
with the save stack. The scope stack stores the height of the save stack
at which nested scopes begin, and allows the save stack to be unwound
back to that point when the scope is left.
When perl is built with debugging enabled, there is a second part to
this stack storing human-readable string names describing the type of
stack context. Each push operation saves the name as well as the height
of the save stack, and each pop operation checks the topmost name with
what is expected, causing an assertion failure if the name does not
match.
The base of this stack is pointed to by the interpreter variable
"PL_scopestack", of type "I32 *". If enabled, the scope stack names are
stored in a separate array pointed to by "PL_scopestack_name", of type
"const char **".
The head of the stack is indexed by "PL_scopestack_ix", an integer which
stores the index of the array or arrays at which the next item should be
pushed. (Note that this is different to most other stacks, which
reference the most recently-pushed item).
Values are pushed to the scope stack using the "ENTER" macro, which
begins a new nested scope. Any items pushed to the save stack are then
restored at the next nested invocation of the "LEAVE" macro.
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 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".
Reference-counted argument stack
Introduction
As of perl 5.40, there is a build option, "PERL_RC_STACK", not enabled
by default, which requires that items pushed onto, or popped off the
argument stack have their reference counts adjusted. It is intended that
eventually this will be the default way (and finally the only way) to
configure perl.
The macros which manipulate the stack such as PUSHs() and POPs() don't
adjust the reference count of the SV. Most of the time this is fine,
since something else is keeping the SV alive while on the argument
stack, such a pointer from the TEMPs stack, or from the pad (e.g. a
lexical variable or a "PADTMP"). Occasionally this can go horribly
wrong. For example, this code:
my @a = (1,2,3);
sub f { @a = (); print "(@_)\n" };
f(@a, 4);
may print undefined or random freed values, since some of the elements
of @_, which have been aliased to the elements of @a, have been freed.
"PERL_RC_STACK" is intended to fix this by making each SV pointer on the
argument stack increment the reference count (RC) of the SV by one.
In this new environment, unmodified existing PP and XS functions, which
have been written assuming a non reference-counted stack (non-RC for
short), are called via special wrapper functions which adjust the stack
before and after. At the moment there is no API to write an RC XS
function, so all XS code will continue to be called via a wrapper (which
makes them slightly slower), but means that in general, CPAN
distributions containing XS code code continue to work without
modification.
However, PP functions, either in perl core, or those in XS functions
used to implement custom ops or to override the PP functions for built-
in ops, need dealing with specially. For the latter, they can just be
wrapped; this involves the least work, but has a performance impact. In
the longer term, and for core PP functions, they need unwrapping and
rewriting using a new API. With this, the old macros such as PUSHs()
have been replaced with a new set of (mostly inline) functions with a
common prefix, such as rpp_push_1(). "RPP" stands for "reference-counted
push and pop functions". The new functions modify the reference count
on "PERL_RC_STACK" builds, while leaving them unadjusted otherwise. Thus
in core they generally work in both cases, while in XS code they are
portable to older perl versions via "PPPort" (XXX assuming that they get
been added to "PPPort").
The rest of this section is mainly concerned with how to convert
existing PP functions, and how to write new PP functions to use the new
"rpp_" API.
A reference-counted perl can be built using the PERL_RC_STACK define.
For development and debugging purposes, it is best to enable leaking
scalar debugging too, as that displays extra information about scalars
that have leaked or been prematurely freed.
Configure -DDEBUGGING \
-Accflags='-DPERL_RC_STACK -DDEBUG_LEAKING_SCALARS'
Reference counted stack states
In the new regime, the current argument stack can be in one of three
states, which can be determined by the shown expression.
• not reference-counted
!AvREAL(PL_curstack)
In this case, perl will assume when emptying the stack (such as
during a croak()) that the items on it don't need freeing. This is
the traditional perl behaviour. On "PERL_RC_STACK" builds, such
stacks will be rarely encountered.
• fully reference-counted
AvREAL(PL_curstack) && !PL_curstackinfo->si_stack_nonrc_base
All the items on the stack are reference counted, and will be freed
by functions like rpp_popfree_1() or if perl croak()s. This is the
normal state of the stack in "PERL_RC_STACK" builds.
• partially reference-counted (split)
AvREAL(PL_curstack) && PL_curstackinfo->si_stack_nonrc_base > 0
In this case, items on the stack from the index
"si_stack_nonrc_base" upwards are non-RC; those below are RC. This
state occurs when a PP or XS function has been wrapped. In this
case, the wrapper function pushes a non-RC copy of the arg pointers
above the cut then calls the real function. When that returns, the
wrapper function bumps up the RC of any returned args. See below for
more details.
Note that perl uses a stack-of-stacks, and the AvREAL() and
"si_stack_nonrc_base" states are per stack. When perl starts up, the
main stack is RC, but by default, new stacks pushed in XS code via
PUSHSTACKi() are non-RC, so it is quite possible to get a mixture. The
perl core itself uses the new push_stackinfo() function which replaces
PUSHSTACKi() and allows you to specify that the new stack should be RC
by default. (XXX core mostly hasn't actually been updated yet to use
push_stackinfo())
Most places in the core assume a particular RC environment. In
particular, it is assumed that within a runops loop, all the PP
functions are RC-aware, either because they have been (re)written to be
aware, or because they have been wrapped. Whenever a runops loop is
entered via CALLRUNOPS(), it will check the current state of the stack,
and if it's not fully RC, will temporarily update its contents to be
fully RC before entering the main runops loop. Then if necessary it will
restore the stack to its old state on return. This means that functions
like call_sv(), which can be called from any environment (e.g. RC core
or wrapped and temporarily non-RC XS code) will always do the Right
Thing when invoking the runops loop, no matter what the current stack
state is.
Similarly, croaks and the like (which can occur anywhere) have to be
able to handle both stack types. So there are a few places in core -
call_sv(), eval_sv() etc, Perl_die_unwind() and S_my_exit_jump() - which
have been specially crafted to handle both cases; everything else can
assume a fixed environment.
Wrapping
Normally a core PP function is declared like this:
PP(pp_foo)
{
...
}
This expands to something like:
OP* Perl_pp_foo(pTHX)
{
...
}
When such a function needs to be wrapped, it is instead declared as:
PP_wrapped(pp_foo, nargs, nlists)
{
...
}
which on non-RC builds, expands to the same as PP() (the extra args are
ignored). On RC builds it expands to something like
OP* Perl_pp_foo(pTHX)
{
return Perl_pp_wrap(aTHX_ S_Perl_pp_foo_norc, nargs, nlists);
}
STATIC OP* S_Perl_pp_foo_norc(pTHX)
{
...
}
Here the externally visible PP function calls pp_wrap(), which adjusts
the stack contents, then calls the hidden real body of the PP function,
then on return, adjusts the stack back.
There is an API macro, XSPP_wrapped(), intended for use on PP functions
declared in XS code, It is identical to PP_wrapped(), except that it
doesn't prepend a "Perl_" prefix to the function name.
The "nargs" and "nlists" parameters to the macro are numeric constants
or simple expressions which specify how many arguments the PP function
expects, or how many lists it expects. For example,
PP_wrapped(pp_add, 2, 0); /* consumes two args off the stack */
PP_wrapped(pp_readline, /* consumes one or two args */
((PL_op->op_flags & OPf_STACKED) ? 2 : 1), 0);
PP_wrapped(pp_push, 0, 1); /* consumes one list */
PP_wrapped(pp_aassign, 0, 2); /* consumes two lists */
To understand what pp_wrap() does, consider calling Perl_pp_foo() which
expects three arguments. On entry the stack may look like:
... A+ B+ C+
(where the "+" indicates that the pointers to A, B and C are each
reference counted). The wrapper function pp_wrap() marks a cut at the
current stack position using "si_stack_nonrc_base", then, based on the
value of "nargs", pushes a copy of those three pointers above the cut:
... A+ B+ C+ | A0 B0 C0
(where the 0 indicates that the pointers aren't RC), then calls the real
PP function, S_Perl_pp_foo_norc(). That function processes A, B and C,
pops them off the stack, and pushes some result SVs. None of this
manipulation adjusts any RCs. On return to pp_wrap(), the stack may look
something like:
... A+ B+ C+ | X0 Y0
The wrapper function bumps up the RCs of X and Y, decrements A B C,
shifts the results down and sets "si_stack_nonrc_base" to zero, leaving
the stack as:
... X+ Y+
In places like pp_entersub(), a similar wrapping (via the functions
rpp_invoke_xs() and then xs_wrap()) is done when calling XS subs.
When "nlists" is positive, a similar action takes place, except that the
mark stack is examined (and adjusted) in order to determine the number
of args that need copying.
A complex calling environment might have multiple nested stacks with
different RC states. Perl starts off with an RC stack. Then for example,
pp_entersub() is called, which (via xs_wrap()) splits the stack and
executes the XS function in a non-RC environment. That function may call
PUSHSTACKi(), which creates a new non-RC stack, then calls call_sv(),
which does CALLRUNOPS(), which causes the new stack to temporarily
become RC. Then a tied method is called, which pushes a new RC stack,
and so on. (XXX currently tied methods actually push a non-RC stack. To
be fixed soon).
(Re)writing a PP function using the rpp_() API
Wrapping a PP function has a performance overhead, and is there mainly
as a temporary crutch. Eventually, PP functions should be updated to use
rpp_() functions, and any new PP functions should be written this way
from scratch and thus not ever need wrapping.
A couple examples of core PP functions being converted can be seen in
the commits "v5.39.1-304-g205fcd8410" and "v5.39.1-303-g2fe263a83a",
which demonstrate a unary and a binary op being converted (pp_not() and
pp_and()).
The traditional PP stack API consisted of a "dSP" declaration, plus a
number of macros to push, pop and extend the stack. A very simplified
pp_add() function might look something like:
PP(pp_add)
{
dSP;
dTARGET;
IV right = SvIV(POPs);
IV left = SvIV(POPs);
TARGi(left + right, 1);
PUSHs(TARG);
PUTBACK;
return NORMAL;
}
which expands to something like:
{
SV **sp = PL_stack_sp;
SV *targ = PAD_SV(PL_op->op_targ);
IV right = SvIV(*sp--);
IV left = SvIV(*sp--);
sv_setiv(targ, left + right);
*++sp = targ;
PL_stack_sp = sp;
return PL_op->op_next;
}
The whole "dSP" thing harks back to the days before decent optimising
compilers. It was always error-prone, e.g. if you forgot a "PUTBACK" or
"SPAGAIN". The new API always just accesses "PL_stack_sp" directly. In
fact the first step of upgrading a PP function is always to remove the
"dSP" declaration. This has the happy side effect that any old-style
macros left in the pp function which implicitly use "sp" will become
compile errors. The existence of a "dSP" somewhere in core is a good
sign that that function still needs updating.
An obvious question is: why not just modify the definitions of the
PUSHs() etc macros to modify reference counts on RC builds? The basic
problem is that an SV may now be kept alive only by a single reference
count from the stack (formerly, they tended to be on the TEMPs stack
too). So in code like:
SV *sv = POPs;
IV i = SvIV(sv);
including an SvREFCNT_dec() in the "POPs" macro definition would cause
"sv" to be freed immediately, before its integer value can be read.
A potential issue with the new regime is that perl can croak at
basically any point in execution (e.g. the SvIV() above might call
FETCH() on a tied variable which then croaks). Thus at all times, the RC
of each SV must be properly accounted for. In the example above, a naive
approach to avoiding a premature free of "sv" might be:
SV *sv = *PL_stack_sp--;
IV i = SvIV(sv);
SvREFCNT_dec(sv); // got i, so ok to free sv now
but that means that "sv" leaks if SvIV() triggers a croak.
To avoid that, the new regime has the general outline that arguments are
left on the stack until they are finished with, then removed and their
reference count adjusted at that point. With the new API, the pp_add()
function looks something like:
{
dTARGET;
IV right = SvIV(PL_stack_sp[ 0]); // NB: arguments left on stack
IV left = SvIV(PL_stack_sp[-1]);
TARGi(left + right, 1);
rpp_replace_2_1(targ);
return NORMAL;
}
The rpp_replace_2_1() function pops two values off the stack and pushes
one new value on, while adjusting reference counts as appropriate
(depending on whether built with "PERL_RC_STACK" or not).
The rpp_() functions in the new API will be described in detail below,
but in summary:
new function approximate old equivant
------------ -----------------------
rpp_extend(n) EXTEND(SP, n)
rpp_push_1(sv) PUSHs(sv)
rpp_push_2(sv1, sv2)) PUSHs(sv1); PUSHs(sv2)
rpp_xpush_1(sv) XPUSHs(sv)
rpp_xpush_2(sv1, sv2)) EXTEND(SP,2); PUSHs(sv1); PUSHs(sv2);
rpp_push_1_norc(sv) mPUSHs(sv) // on RC bulds, skips RC++;
// on non-RC builds, mortalises
rpp_popfree_1() (void)POPs;
rpp_popfree_2() (void)POPs; (void)POPs;
rpp_popfree_to(svp) PL_stack_sp = svp;
rpp_obliterate_stack_to(ix) // see description below
sv = rpp_pop_1_norc() sv = SvREFCNT_inc(POPs)
rpp_replace_1_1(sv) (void)POPs; PUSHs(sv);
rpp_replace_2_1(sv) (void)POPs; (void)POPs; PUSHs(sv);
rpp_replace_at(sp, sv) *sp = sv;
rpp_replace_at_norc(sp, sv) *sp = sv_2mortal(sv);
rpp_context(mark, gimme,
extra) SP -= extra;
// impose void/scalar/list context on return args
SP = (gimme == G_VOID) ? mark : ....
rpp_try_AMAGIC_1() tryAMAGICun_MG()
rpp_try_AMAGIC_2() tryAMAGICbin_MG()
rpp_is_lone(sv) SvTEMP(sv) && SvREFCNT(sv) == 1
rpp_stack_is_rc() no equivalent
rpp_invoke_xs(cv) CvXSUB(cv)(aTHX_ cv);
(no replacement) dATARGET // just write the macro body in full
There are also some "_NN" variants which assume that any items being
removed from the stack are non-NULL, and so are slightly more efficient:
rpp_popfree_1_NN()
rpp_popfree_2_NN()
rpp_popfree_to_NN(svp)
rpp_replace_1_1_NN(sv)
rpp_replace_2_1_NN(sv)
rpp_replace_at_NN(sp, sv)
rpp_replace_at_norc_NN(sp, sv)
There are also a few "_IMM" variants, which expect the single pushed or
replacement value to be an immortal, such as &PL_sv_undef - this skips
incrementing the ref count of the immortal SV. It doesn't matter if the
ref count of the SV prematurely reaches zero, as sv_free2() will just
resurrect it. Not every variant is provided; if a suitable one doesn't
exist, just using a standard "_1" version is fine, albeit slightly
slower.
rpp_push_IMM(&PL_sv_undef)
rpp_xpush_IMM(&PL_sv_zero)
rpp_replace_1_IMM_NN(&PL_sv_yes)
rpp_replace_2_IMM_NN(&PL_sv_no)
Other new C and perl functions related to reference-counted stacks are:
push_stackinfo(type,rc) PUSHSTACKi(type)
pop_stackinfo() POPSTACK()
switch_argstack(to) SWITCHSTACK(from,to)
(Internals::stack_refcounted() & 1) # perl built with PERL_RC_STACK
Some of these new functions are trivial, but should be used in
preference to writing direct code because they will work on both RC and
non-RC builds, and may do extra checks and assertions on "DEBUGGING"
builds.
Note that rpp_popfree_1() etc aren't direct replacements for "POPs". The
rpp_() variants don't return a value and are intended to be called when
the SV is finished with. So
SV *sv = POPs;
... do stuff with sv ...
becomes
SV *sv = *PL_stack_sp;
... do stuff with sv ...
rpp_popfree_1(); /* does SvREFCNT_dec(*PL_stack_sp--) */
The rpp_replace_M_N() functions are shortcuts for popping and freeing
"M" items then pushing and bumping up the RCs of "N" items. Note that
they handle edge cases such as an old and new SV being the same.
rpp_replace_at(sp, sv) is similar to rpp_replace_1_1(), except that it
replaces an SV at an address in the stack rather than at the top.
rpp_replace_at_norc(sp, sv) is similar to rpp_replace_at(), except that
it assumes that "sv" already has a bumped reference count. So, a bit
like rpp_push_1_norc() (see below), it doesn't bother increasing "sv"'s
reference count, or on non-RC builds it mortalises it instead.
rpp_popfree_to(svp) is designed to replace code like
PL_stack_sp = PL_stack_base + cx->blk_oldsp;
which typically appears in list ops or scope exits when the arguments
are finished with. Left unaltered, all the SVs above "oldsp" would leak.
The new approach is
rpp_popfree_to(PL_stack_base + cx->blk_oldsp);
There is a rarely-used variant of this, rpp_obliterate_stack_to(), which
pops the stack back to the specified index regardless of the current RC
state of the stack. So for example if the stack is split, it will only
adjust the RCs of any SVs which are below the split point, while
rpp_popfree_to() would mindlessly free all SVs (on RC builds anyway).
For normal PP functions you should only ever use rpp_popfree_to(), which
is faster.
There are no new equivalents for all the convenience macros like POPi()
and (shudder) dPOPPOPiirl(). These should be replaced with the rpp_()
functions above and with the conversions and variable declarations being
made explicit, e.g. dPOPPOPiirl() becomes:
IV right = SvIV(PL_stack_sp[ 0]);
IV left = SvIV(PL_stack_sp[-1]);
rpp_popfree_2();
A couple of the rpp_() functions with "norc" in their names don't adjust
the reference count on RC builds (but, conversely, do on non-RC builds).
rpp_push_1_norc(sv) does a simple "*++PL_stack_sp = sv" on RC builds. It
is typically used to "root" a newly-created SV, which already has an RC
of 1. On non-RC builds it mortalises the SV instead. So for example,
code which used to look like
mPUSHs(newSViv(i));
and which expanded to the equivalent of:
PUSHs(sv_2mortal(newSViv(i));
should be rewritten as:
rpp_push_1_norc(newSViv(i));
This is because newSViv() and similar create a new SV with a reference
count one too high (1 rather than 0). This count is then "donated" to
the stack by pushing it. Conversely on non-RC builds, the count is
donated to the TEMPs stack.
Similarly, on RC builds, "sv = rpp_pop_1_norc()" does a simple "sv =
*PL_stack_sv--" without adjusting the reference count, while on non-RC
builds it actually increments the SV's reference count. It is intended
for cases where you immediately want to increment the reference count
again after popping, e.g. where the SV is to be immediately embedded
somewhere. For example this code:
SV *sv = PL_stack_sp[0];
SvREFCNT_inc(sv);
av_store(av, i, sv); /* in real life should check return value */
rpp_popfree_1();
can be more efficiently written as
av_store(av, i, rpp_pop_1_norc());
By using this function, the code works correctly on both RC and non-RC
builds.
A common operation on list ops is to impose void, scalar or list context
on the return arguments, possibly discarding all, or all except one, of
them. rpp_context(mark, gimme, extra) does this. As a first step (for
convenience and efficiency) it notionally pops "extra" args off the
stack. Then for list context, leaves things as is. For void context, the
stack pointer is reset to mark, and everything above is popped. For
scalar, the top argument (or &PL_sv_undef) is moved from the top to
mark+1 and everything above is discarded.
The macros which appear at the start of many PP functions to check for
unary or binary op overloading (among other things) have been replaced
with rpp_try_AMAGIC_1() and _2() inline functions, which now rely on the
calling PP function to choose whether to return immediately rather than
the return being hidden away in the macro.
The rpp_invoke_xs() function calls the XS function associated with the
CV, but may do so via a wrapper function to adjust the stack as
necessary.
In the spirit of hiding away less in macros, "dATARGET" hasn't been
given a replacement; where its effect is needed, it is now written out
in full; see pp_add() for an example.
Finally, a couple of rpp() functions provide information rather than
manipulate the stack.
rpp_is_lone(sv) indicates whether "sv", assumed to be still on the
stack, it kept alive only by a single reference-counted pointer from the
argument and/or temps stacks, and thus is a candidate for some
optimisations (like skipping the copying of return arguments from a
subroutine call).
rpp_stack_is_rc() indicates whether the current stack is currently
reference-counted. It's used mainly in a few places like call_sv() which
can be called from anywhere, and thus have to deal with both cases.
So for example, rather than using rpp_xpush_1(), call_sv() has lines
like:
rpp_extend(1);
*++PL_stack_sp = sv;
#ifdef PERL_RC_STACK
if (rpp_stack_is_rc())
SvREFCNT_inc_simple_void_NN(sv);
#endif
which works on both standard builds and RC builds, and works whether
call_sv() is called from a standard PP function (rpp_stack_is_rc() is
true) or from a wrapped PP or XS function (rpp_stack_is_rc() is false).
Note that you're unlikely to need to use this function, as in most
places, such as PP or XS functions, it is always RC or non-RC
respectively. In fact on debugging builds under "PERL_RC_STACK", PUSHs()
and similar macros include an "assert(!rpp_stack_is_rc())", while
rpp_push_1() and similar functions have "assert(rpp_stack_is_rc())".
The macros for pushing new stackinfos have been replaced with inline
functions which don't rely on "dSP" being in scope, and which have less
ambiguous names: they make it clear that a new stackinfo is being
pushed, rather than just some sort of stack. push_stackinfo() also has a
boolean argument indicating whether the new argument stack should be
reference-counted or not. For backwards compatibility, PUSHSTACKi(type)
is defined to be push_stackinfo(type, 0).
Some test scripts check for things like leaks by testing that the
reference count of a particular variable has an expected value. If this
is different on a perl built with "PERL_RC_STACK", then the perl
function Internals::stack_refcounted() can be used. This returns an
integer, the lowest bit of which indicates that perl was built with
"PERL_RC_STACK". Other bits are reserved for future use and should be
masked out.
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 to 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
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