Provided by: perl-doc_5.34.0-3ubuntu1.4_all 
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*)
You can get and set the current length of the string stored in an SV with the following macros:
SvCUR(SV*)
SvCUR_set(SV*, I32 val)
You can also get a pointer to the end of the string stored in the SV with the macro:
SvEND(SV*)
But note that these last three macros are valid only if "SvPOK()" is true.
If you want to append something to the end of string stored in an "SV*", you can use the following
functions:
void sv_catpv(SV*, const char*);
void sv_catpvn(SV*, const char*, STRLEN);
void sv_catpvf(SV*, const char*, ...);
void sv_vcatpvfn(SV*, const char*, STRLEN, va_list *, SV **,
I32, bool);
void sv_catsv(SV*, SV*);
The first function calculates the length of the string to be appended by using "strlen". In the second,
you specify the length of the string yourself. The third function processes its arguments like "sprintf"
and appends the formatted output. The fourth function works like "vsprintf". You can specify the
address and length of an array of SVs instead of the va_list argument. The fifth function extends the
string stored in the first SV with the string stored in the second SV. It also forces the second SV to
be interpreted as a string.
The "sv_cat*()" functions are not generic enough to operate on values that have "magic". See "Magic
Virtual Tables" later in this document.
If you know the name of a scalar variable, you can get a pointer to its SV by using the following:
SV* get_sv("package::varname", 0);
This returns NULL if the variable does not exist.
If you want to know if this variable (or any other SV) is actually "defined", you can call:
SvOK(SV*)
The scalar "undef" value is stored in an SV instance called "PL_sv_undef".
Its address can be used whenever an "SV*" is needed. Make sure that you don't try to compare a random sv
with &PL_sv_undef. For example when interfacing Perl code, it'll work correctly for:
foo(undef);
But won't work when called as:
$x = undef;
foo($x);
So to repeat always use SvOK() to check whether an sv is defined.
Also you have to be careful when using &PL_sv_undef as a value in AVs or HVs (see "AVs, HVs and undefined
values").
There are also the two values "PL_sv_yes" and "PL_sv_no", which contain boolean TRUE and FALSE values,
respectively. Like "PL_sv_undef", their addresses can be used whenever an "SV*" is needed.
Do not be fooled into thinking that "(SV *) 0" is the same as &PL_sv_undef. Take this code:
SV* sv = (SV*) 0;
if (I-am-to-return-a-real-value) {
sv = sv_2mortal(newSViv(42));
}
sv_setsv(ST(0), sv);
This code tries to return a new SV (which contains the value 42) if it should return a real value, or
undef otherwise. Instead it has returned a NULL pointer which, somewhere down the line, will cause a
segmentation violation, bus error, or just weird results. Change the zero to &PL_sv_undef in the first
line and all will be well.
To free an SV that you've created, call "SvREFCNT_dec(SV*)". Normally this call is not necessary (see
"Reference Counts and Mortality").
Offsets
Perl provides the function "sv_chop" to efficiently remove characters from the beginning of a string; you
give it an SV and a pointer to somewhere inside the PV, and it discards everything before the pointer.
The efficiency comes by means of a little hack: instead of actually removing the characters, "sv_chop"
sets the flag "OOK" (offset OK) to signal to other functions that the offset hack is in effect, and it
moves the PV pointer (called "SvPVX") forward by the number of bytes chopped off, and adjusts "SvCUR" and
"SvLEN" accordingly. (A portion of the space between the old and new PV pointers is used to store the
count of chopped bytes.)
Hence, at this point, the start of the buffer that we allocated lives at "SvPVX(sv) - SvIV(sv)" in memory
and the PV pointer is pointing into the middle of this allocated storage.
This is best demonstrated by example. Normally copy-on-write will prevent the substitution from operator
from using this hack, but if you can craft a string for which copy-on-write is not possible, you can see
it in play. In the current implementation, the final byte of a string buffer is used as a copy-on-write
reference count. If the buffer is not big enough, then copy-on-write is skipped. First have a look at
an empty string:
% ./perl -Ilib -MDevel::Peek -le '$a=""; $a .= ""; Dump $a'
SV = PV(0x7ffb7c008a70) at 0x7ffb7c030390
REFCNT = 1
FLAGS = (POK,pPOK)
PV = 0x7ffb7bc05b50 ""\0
CUR = 0
LEN = 10
Notice here the LEN is 10. (It may differ on your platform.) Extend the length of the string to one
less than 10, and do a substitution:
% ./perl -Ilib -MDevel::Peek -le '$a=""; $a.="123456789"; $a=~s/.//; \
Dump($a)'
SV = PV(0x7ffa04008a70) at 0x7ffa04030390
REFCNT = 1
FLAGS = (POK,OOK,pPOK)
OFFSET = 1
PV = 0x7ffa03c05b61 ( "\1" . ) "23456789"\0
CUR = 8
LEN = 9
Here the number of bytes chopped off (1) is shown next as the OFFSET. The portion of the string between
the "real" and the "fake" beginnings is shown in parentheses, and the values of "SvCUR" and "SvLEN"
reflect the fake beginning, not the real one. (The first character of the string buffer happens to have
changed to "\1" here, not "1", because the current implementation stores the offset count in the string
buffer. This is subject to change.)
Something similar to the offset hack is performed on AVs to enable efficient shifting and splicing off
the beginning of the array; while "AvARRAY" points to the first element in the array that is visible from
Perl, "AvALLOC" points to the real start of the C array. These are usually the same, but a "shift"
operation can be carried out by increasing "AvARRAY" by one and decreasing "AvFILL" and "AvMAX". Again,
the location of the real start of the C array only comes into play when freeing the array. See
"av_shift" in av.c.
What's Really Stored in an SV?
Recall that the usual method of determining the type of scalar you have is to use "Sv*OK" macros.
Because a scalar can be both a number and a string, usually these macros will always return TRUE and
calling the "Sv*V" macros will do the appropriate conversion of string to integer/double or
integer/double to string.
If you really need to know if you have an integer, double, or string pointer in an SV, you can use the
following three macros instead:
SvIOKp(SV*)
SvNOKp(SV*)
SvPOKp(SV*)
These will tell you if you truly have an integer, double, or string pointer stored in your SV. The "p"
stands for private.
There are various ways in which the private and public flags may differ. For example, in perl 5.16 and
earlier a tied SV may have a valid underlying value in the IV slot (so SvIOKp is true), but the data
should be accessed via the FETCH routine rather than directly, so SvIOK is false. (In perl 5.18 onwards,
tied scalars use the flags the same way as untied scalars.) Another is when numeric conversion has
occurred and precision has been lost: only the private flag is set on 'lossy' values. So when an NV is
converted to an IV with loss, SvIOKp, SvNOKp and SvNOK will be set, while SvIOK wont be.
In general, though, it's best to use the "Sv*V" macros.
Working with AVs
There are two ways to create and load an AV. The first method creates an empty AV:
AV* newAV();
The second method both creates the AV and initially populates it with SVs:
AV* av_make(SSize_t num, SV **ptr);
The second argument points to an array containing "num" "SV*"'s. Once the AV has been created, the SVs
can be destroyed, if so desired.
Once the AV has been created, the following operations are possible on it:
void av_push(AV*, SV*);
SV* av_pop(AV*);
SV* av_shift(AV*);
void av_unshift(AV*, SSize_t num);
These should be familiar operations, with the exception of "av_unshift". This routine adds "num"
elements at the front of the array with the "undef" value. You must then use "av_store" (described
below) to assign values to these new elements.
Here are some other functions:
SSize_t av_top_index(AV*);
SV** av_fetch(AV*, SSize_t key, I32 lval);
SV** av_store(AV*, SSize_t key, SV* val);
The "av_top_index" function returns the highest index value in an array (just like $#array in Perl). If
the array is empty, -1 is returned. The "av_fetch" function returns the value at index "key", but if
"lval" is non-zero, then "av_fetch" will store an undef value at that index. The "av_store" function
stores the value "val" at index "key", and does not increment the reference count of "val". Thus the
caller is responsible for taking care of that, and if "av_store" returns NULL, the caller will have to
decrement the reference count to avoid a memory leak. Note that "av_fetch" and "av_store" both return
"SV**"'s, not "SV*"'s as their return value.
A few more:
void av_clear(AV*);
void av_undef(AV*);
void av_extend(AV*, SSize_t key);
The "av_clear" function deletes all the elements in the AV* array, but does not actually delete the array
itself. The "av_undef" function will delete all the elements in the array plus the array itself. The
"av_extend" function extends the array so that it contains at least "key+1" elements. If "key+1" is less
than the currently allocated length of the array, then nothing is done.
If you know the name of an array variable, you can get a pointer to its AV by using the following:
AV* get_av("package::varname", 0);
This returns NULL if the variable does not exist.
See "Understanding the Magic of Tied Hashes and Arrays" for more information on how to use the array
access functions on tied arrays.
Working with HVs
To create an HV, you use the following routine:
HV* newHV();
Once the HV has been created, the following operations are possible on it:
SV** hv_store(HV*, const char* key, U32 klen, SV* val, U32 hash);
SV** hv_fetch(HV*, const char* key, U32 klen, I32 lval);
The "klen" parameter is the length of the key being passed in (Note that you cannot pass 0 in as a value
of "klen" to tell Perl to measure the length of the key). The "val" argument contains the SV pointer to
the scalar being stored, and "hash" is the precomputed hash value (zero if you want "hv_store" to
calculate it for you). The "lval" parameter indicates whether this fetch is actually a part of a store
operation, in which case a new undefined value will be added to the HV with the supplied key and
"hv_fetch" will return as if the value had already existed.
Remember that "hv_store" and "hv_fetch" return "SV**"'s and not just "SV*". To access the scalar value,
you must first dereference the return value. However, you should check to make sure that the return
value is not NULL before dereferencing it.
The first of these two functions checks if a hash table entry exists, and the second deletes it.
bool hv_exists(HV*, const char* key, U32 klen);
SV* hv_delete(HV*, const char* key, U32 klen, I32 flags);
If "flags" does not include the "G_DISCARD" flag then "hv_delete" will create and return a mortal copy of
the deleted value.
And more miscellaneous functions:
void hv_clear(HV*);
void hv_undef(HV*);
Like their AV counterparts, "hv_clear" deletes all the entries in the hash table but does not actually
delete the hash table. The "hv_undef" deletes both the entries and the hash table itself.
Perl keeps the actual data in a linked list of structures with a typedef of HE. These contain the actual
key and value pointers (plus extra administrative overhead). The key is a string pointer; the value is
an "SV*". However, once you have an "HE*", to get the actual key and value, use the routines specified
below.
I32 hv_iterinit(HV*);
/* Prepares starting point to traverse hash table */
HE* hv_iternext(HV*);
/* Get the next entry, and return a pointer to a
structure that has both the key and value */
char* hv_iterkey(HE* entry, I32* retlen);
/* Get the key from an HE structure and also return
the length of the key string */
SV* hv_iterval(HV*, HE* entry);
/* Return an SV pointer to the value of the HE
structure */
SV* hv_iternextsv(HV*, char** key, I32* retlen);
/* This convenience routine combines hv_iternext,
hv_iterkey, and hv_iterval. The key and retlen
arguments are return values for the key and its
length. The value is returned in the SV* argument */
If you know the name of a hash variable, you can get a pointer to its HV by using the following:
HV* get_hv("package::varname", 0);
This returns NULL if the variable does not exist.
The hash algorithm is defined in the "PERL_HASH" macro:
PERL_HASH(hash, key, klen)
The exact implementation of this macro varies by architecture and version of perl, and the return value
may change per invocation, so the value is only valid for the duration of a single perl process.
See "Understanding the Magic of Tied Hashes and Arrays" for more information on how to use the hash
access functions on tied hashes.
Hash API Extensions
Beginning with version 5.004, the following functions are also supported:
HE* hv_fetch_ent (HV* tb, SV* key, I32 lval, U32 hash);
HE* hv_store_ent (HV* tb, SV* key, SV* val, U32 hash);
bool hv_exists_ent (HV* tb, SV* key, U32 hash);
SV* hv_delete_ent (HV* tb, SV* key, I32 flags, U32 hash);
SV* hv_iterkeysv (HE* entry);
Note that these functions take "SV*" keys, which simplifies writing of extension code that deals with
hash structures. These functions also allow passing of "SV*" keys to "tie" functions without forcing you
to stringify the keys (unlike the previous set of functions).
They also return and accept whole hash entries ("HE*"), making their use more efficient (since the hash
number for a particular string doesn't have to be recomputed every time). See perlapi for detailed
descriptions.
The following macros must always be used to access the contents of hash entries. Note that the arguments
to these macros must be simple variables, since they may get evaluated more than once. See perlapi for
detailed descriptions of these macros.
HePV(HE* he, STRLEN len)
HeVAL(HE* he)
HeHASH(HE* he)
HeSVKEY(HE* he)
HeSVKEY_force(HE* he)
HeSVKEY_set(HE* he, SV* sv)
These two lower level macros are defined, but must only be used when dealing with keys that are not
"SV*"s:
HeKEY(HE* he)
HeKLEN(HE* he)
Note that both "hv_store" and "hv_store_ent" do not increment the reference count of the stored "val",
which is the caller's responsibility. If these functions return a NULL value, the caller will usually
have to decrement the reference count of "val" to avoid a memory leak.
AVs, HVs and undefined values
Sometimes you have to store undefined values in AVs or HVs. Although this may be a rare case, it can be
tricky. That's because you're used to using &PL_sv_undef if you need an undefined SV.
For example, intuition tells you that this XS code:
AV *av = newAV();
av_store( av, 0, &PL_sv_undef );
is equivalent to this Perl code:
my @av;
$av[0] = undef;
Unfortunately, this isn't true. In perl 5.18 and earlier, AVs use &PL_sv_undef as a marker for
indicating that an array element has not yet been initialized. Thus, "exists $av[0]" would be true for
the above Perl code, but false for the array generated by the XS code. In perl 5.20, storing
&PL_sv_undef will create a read-only element, because the scalar &PL_sv_undef itself is stored, not a
copy.
Similar problems can occur when storing &PL_sv_undef in HVs:
hv_store( hv, "key", 3, &PL_sv_undef, 0 );
This will indeed make the value "undef", but if you try to modify the value of "key", you'll get the
following error:
Modification of non-creatable hash value attempted
In perl 5.8.0, &PL_sv_undef was also used to mark placeholders in restricted hashes. This caused such
hash entries not to appear when iterating over the hash or when checking for the keys with the
"hv_exists" function.
You can run into similar problems when you store &PL_sv_yes or &PL_sv_no into AVs or HVs. Trying to
modify such elements will give you the following error:
Modification of a read-only value attempted
To make a long story short, you can use the special variables &PL_sv_undef, &PL_sv_yes and &PL_sv_no with
AVs and HVs, but you have to make sure you know what you're doing.
Generally, if you want to store an undefined value in an AV or HV, you should not use &PL_sv_undef, but
rather create a new undefined value using the "newSV" function, for example:
av_store( av, 42, newSV(0) );
hv_store( hv, "foo", 3, newSV(0), 0 );
References
References are a special type of scalar that point to other data types (including other references).
To create a reference, use either of the following functions:
SV* newRV_inc((SV*) thing);
SV* newRV_noinc((SV*) thing);
The "thing" argument can be any of an "SV*", "AV*", or "HV*". The functions are identical except that
"newRV_inc" increments the reference count of the "thing", while "newRV_noinc" does not. For historical
reasons, "newRV" is a synonym for "newRV_inc".
Once you have a reference, you can use the following macro to dereference the reference:
SvRV(SV*)
then call the appropriate routines, casting the returned "SV*" to either an "AV*" or "HV*", if required.
To determine if an SV is a reference, you can use the following macro:
SvROK(SV*)
To discover what type of value the reference refers to, use the following macro and then check the return
value.
SvTYPE(SvRV(SV*))
The most useful types that will be returned are:
SVt_PVAV Array
SVt_PVHV Hash
SVt_PVCV Code
SVt_PVGV Glob (possibly a file handle)
Any numerical value returned which is less than SVt_PVAV will be a scalar of some form.
See "svtype" in perlapi for more details.
Blessed References and Class Objects
References are also used to support object-oriented programming. In perl's OO lexicon, an object is
simply a reference that has been blessed into a package (or class). Once blessed, the programmer may now
use the reference to access the various methods in the class.
A reference can be blessed into a package with the following function:
SV* sv_bless(SV* sv, HV* stash);
The "sv" argument must be a reference value. The "stash" argument specifies which class the reference
will belong to. See "Stashes and Globs" for information on converting class names into stashes.
/* Still under construction */
The following function upgrades rv to reference if not already one. Creates a new SV for rv to point to.
If "classname" is non-null, the SV is blessed into the specified class. SV is returned.
SV* newSVrv(SV* rv, const char* classname);
The following three functions copy integer, unsigned integer or double into an SV whose reference is
"rv". SV is blessed if "classname" is non-null.
SV* sv_setref_iv(SV* rv, const char* classname, IV iv);
SV* sv_setref_uv(SV* rv, const char* classname, UV uv);
SV* sv_setref_nv(SV* rv, const char* classname, NV iv);
The following function copies the pointer value (the address, not the string!) into an SV whose reference
is rv. SV is blessed if "classname" is non-null.
SV* sv_setref_pv(SV* rv, const char* classname, void* pv);
The following function copies a string into an SV whose reference is "rv". Set length to 0 to let Perl
calculate the string length. SV is blessed if "classname" is non-null.
SV* sv_setref_pvn(SV* rv, const char* classname, char* pv,
STRLEN length);
The following function tests whether the SV is blessed into the specified class. It does not check
inheritance relationships.
int sv_isa(SV* sv, const char* name);
The following function tests whether the SV is a reference to a blessed object.
int sv_isobject(SV* sv);
The following function tests whether the SV is derived from the specified class. SV can be either a
reference to a blessed object or a string containing a class name. This is the function implementing the
"UNIVERSAL::isa" functionality.
bool sv_derived_from(SV* sv, const char* name);
To check if you've got an object derived from a specific class you have to write:
if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }
Creating New Variables
To create a new Perl variable with an undef value which can be accessed from your Perl script, use the
following routines, depending on the variable type.
SV* get_sv("package::varname", GV_ADD);
AV* get_av("package::varname", GV_ADD);
HV* get_hv("package::varname", GV_ADD);
Notice the use of GV_ADD as the second parameter. The new variable can now be set, using the routines
appropriate to the data type.
There are additional macros whose values may be bitwise OR'ed with the "GV_ADD" argument to enable
certain extra features. Those bits are:
GV_ADDMULTI
Marks the variable as multiply defined, thus preventing the:
Name <varname> used only once: possible typo
warning.
GV_ADDWARN
Issues the warning:
Had to create <varname> unexpectedly
if the variable did not exist before the function was called.
If you do not specify a package name, the variable is created in the current package.
Reference Counts and Mortality
Perl uses a reference count-driven garbage collection mechanism. SVs, AVs, or HVs (xV for short in the
following) start their life with a reference count of 1. If the reference count of an xV ever drops to
0, then it will be destroyed and its memory made available for reuse. At the most basic internal level,
reference counts can be manipulated with the following macros:
int SvREFCNT(SV* sv);
SV* SvREFCNT_inc(SV* sv);
void SvREFCNT_dec(SV* sv);
(There are also suffixed versions of the increment and decrement macros, for situations where the full
generality of these basic macros can be exchanged for some performance.)
However, the way a programmer should think about references is not so much in terms of the bare reference
count, but in terms of ownership of references. A reference to an xV can be owned by any of a variety of
entities: another xV, the Perl interpreter, an XS data structure, a piece of running code, or a dynamic
scope. An xV generally does not know what entities own the references to it; it only knows how many
references there are, which is the reference count.
To correctly maintain reference counts, it is essential to keep track of what references the XS code is
manipulating. The programmer should always know where a reference has come from and who owns it, and be
aware of any creation or destruction of references, and any transfers of ownership. Because ownership
isn't represented explicitly in the xV data structures, only the reference count need be actually
maintained by the code, and that means that this understanding of ownership is not actually evident in
the code. For example, transferring ownership of a reference from one owner to another doesn't change
the reference count at all, so may be achieved with no actual code. (The transferring code doesn't touch
the referenced object, but does need to ensure that the former owner knows that it no longer owns the
reference, and that the new owner knows that it now does.)
An xV that is visible at the Perl level should not become unreferenced and thus be destroyed. Normally,
an object will only become unreferenced when it is no longer visible, often by the same means that makes
it invisible. For example, a Perl reference value (RV) owns a reference to its referent, so if the RV is
overwritten that reference gets destroyed, and the no-longer-reachable referent may be destroyed as a
result.
Many functions have some kind of reference manipulation as part of their purpose. Sometimes this is
documented in terms of ownership of references, and sometimes it is (less helpfully) documented in terms
of changes to reference counts. For example, the newRV_inc() function is documented to create a new RV
(with reference count 1) and increment the reference count of the referent that was supplied by the
caller. This is best understood as creating a new reference to the referent, which is owned by the
created RV, and returning to the caller ownership of the sole reference to the RV. The newRV_noinc()
function instead does not increment the reference count of the referent, but the RV nevertheless ends up
owning a reference to the referent. It is therefore implied that the caller of "newRV_noinc()" is
relinquishing a reference to the referent, making this conceptually a more complicated operation even
though it does less to the data structures.
For example, imagine you want to return a reference from an XSUB function. Inside the XSUB routine, you
create an SV which initially has just a single reference, owned by the XSUB routine. This reference
needs to be disposed of before the routine is complete, otherwise it will leak, preventing the SV from
ever being destroyed. So to create an RV referencing the SV, it is most convenient to pass the SV to
"newRV_noinc()", which consumes that reference. Now the XSUB routine no longer owns a reference to the
SV, but does own a reference to the RV, which in turn owns a reference to the SV. The ownership of the
reference to the RV is then transferred by the process of returning the RV from the XSUB.
There are some convenience functions available that can help with the destruction of xVs. These
functions introduce the concept of "mortality". Much documentation speaks of an xV itself being mortal,
but this is misleading. It is really a reference to an xV that is mortal, and it is possible for there
to be more than one mortal reference to a single xV. For a reference to be mortal means that it is owned
by the temps stack, one of perl's many internal stacks, which will destroy that reference "a short time
later". Usually the "short time later" is the end of the current Perl statement. However, it gets more
complicated around dynamic scopes: there can be multiple sets of mortal references hanging around at the
same time, with different death dates. Internally, the actual determinant for when mortal xV references
are destroyed depends on two macros, SAVETMPS and FREETMPS. See perlcall and perlxs 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); /* $~ */
char *IoBOTTOM_NAME(io);
GV *IoBOTTOM_GV(io);
char IoTYPE(io);
U8 IoFLAGS(io);
Most of these are involved with formats.
IoFLAGs() may contain a combination of flags, the most interesting of which are "IOf_FLUSH" ($|) 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 (none) %SIG hash
s PERL_MAGIC_sigelem vtbl_sigelem %SIG hash element
t PERL_MAGIC_taint vtbl_taint Taintedness
U PERL_MAGIC_uvar vtbl_uvar Available for use by
extensions
u PERL_MAGIC_uvar_elem (none) Reserved for use by
extensions
V PERL_MAGIC_vstring (none) SV was vstring literal
v PERL_MAGIC_vec vtbl_vec vec() lvalue
w PERL_MAGIC_utf8 vtbl_utf8 Cached UTF-8 information
x PERL_MAGIC_substr vtbl_substr substr() lvalue
Y PERL_MAGIC_nonelem vtbl_nonelem Array element that does not
exist
y PERL_MAGIC_defelem vtbl_defelem Shadow "foreach" iterator
variable / smart parameter
vivification
\ PERL_MAGIC_lvref vtbl_lvref Lvalue reference
constructor
] PERL_MAGIC_checkcall vtbl_checkcall Inlining/mutation of call
to this CV
~ PERL_MAGIC_ext (none) Available for use by
extensions
When an uppercase and lowercase letter both exist in the table, then the uppercase letter is typically
used to represent some kind of composite type (a list or a hash), and the lowercase letter is used to
represent an element of that composite type. Some internals code makes use of this case relationship.
However, 'v' and 'V' (vec and v-string) are in no way related.
The "PERL_MAGIC_ext" and "PERL_MAGIC_uvar" magic types are defined specifically for use by extensions and
will not be used by perl itself. Extensions can use "PERL_MAGIC_ext" magic to 'attach' private
information to variables (typically objects). This is especially useful because there is no way for
normal perl code to corrupt this private information (unlike using extra elements of a hash object).
Similarly, "PERL_MAGIC_uvar" magic can be used much like tie() to call a C function any time a scalar's
value is used or changed. The "MAGIC"'s "mg_ptr" field points to a "ufuncs" structure:
struct ufuncs {
I32 (*uf_val)(pTHX_ IV, SV*);
I32 (*uf_set)(pTHX_ IV, SV*);
IV uf_index;
};
When the SV is read from or written to, the "uf_val" or "uf_set" function will be called with "uf_index"
as the first arg and a pointer to the SV as the second. A simple example of how to add "PERL_MAGIC_uvar"
magic is shown below. Note that the ufuncs structure is copied by sv_magic, so you can safely allocate
it on the stack.
void
Umagic(sv)
SV *sv;
PREINIT:
struct ufuncs uf;
CODE:
uf.uf_val = &my_get_fn;
uf.uf_set = &my_set_fn;
uf.uf_index = 0;
sv_magic(sv, 0, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf));
Attaching "PERL_MAGIC_uvar" to arrays is permissible but has no effect.
For hashes there is a specialized hook that gives control over hash keys (but not values). This hook
calls "PERL_MAGIC_uvar" 'get' magic if the "set" function in the "ufuncs" structure is NULL. The hook is
activated whenever the hash is accessed with a key specified as an "SV" through the functions
"hv_store_ent", "hv_fetch_ent", "hv_delete_ent", and "hv_exists_ent". Accessing the key as a string
through the functions without the "..._ent" suffix circumvents the hook. See "GUTS" in
Hash::Util::FieldHash for a detailed description.
Note that because multiple extensions may be using "PERL_MAGIC_ext" or "PERL_MAGIC_uvar" magic, it is
important for extensions to take extra care to avoid conflict. Typically only using the magic on objects
blessed into the same class as the extension is sufficient. For "PERL_MAGIC_ext" magic, it is usually a
good idea to define an "MGVTBL", even if all its fields will be 0, so that individual "MAGIC" pointers
can be identified as a particular kind of magic using their magic virtual table. "mg_findext" provides
an easy way to do that:
STATIC MGVTBL my_vtbl = { 0, 0, 0, 0, 0, 0, 0, 0 };
MAGIC *mg;
if ((mg = mg_findext(sv, PERL_MAGIC_ext, &my_vtbl))) {
/* this is really ours, not another module's PERL_MAGIC_ext */
my_priv_data_t *priv = (my_priv_data_t *)mg->mg_ptr;
...
}
Also note that the "sv_set*()" and "sv_cat*()" functions described earlier do not invoke 'set' magic on
their targets. This must be done by the user either by calling the "SvSETMAGIC()" macro after calling
these functions, or by using one of the "sv_set*_mg()" or "sv_cat*_mg()" functions. Similarly, generic C
code must call the "SvGETMAGIC()" macro to invoke any 'get' magic if they use an SV obtained from
external sources in functions that don't handle magic. See perlapi for a description of these functions.
For example, calls to the "sv_cat*()" functions typically need to be followed by "SvSETMAGIC()", but they
don't need a prior "SvGETMAGIC()" since their implementation handles 'get' magic.
Finding Magic
MAGIC *mg_find(SV *sv, int type); /* Finds the magic pointer of that
* type */
This routine returns a pointer to a "MAGIC" structure stored in the SV. If the SV does not have that
magical feature, "NULL" is returned. If the SV has multiple instances of that magical feature, the first
one will be returned. "mg_findext" can be used to find a "MAGIC" structure of an SV based on both its
magic type and its magic virtual table:
MAGIC *mg_findext(SV *sv, int type, MGVTBL *vtbl);
Also, if the SV passed to "mg_find" or "mg_findext" is not of type SVt_PVMG, Perl may core dump.
int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen);
This routine checks to see what types of magic "sv" has. If the mg_type field is an uppercase letter,
then the mg_obj is copied to "nsv", but the mg_type field is changed to be the lowercase letter.
Understanding the Magic of Tied Hashes and Arrays
Tied hashes and arrays are magical beasts of the "PERL_MAGIC_tied" magic type.
WARNING: As of the 5.004 release, proper usage of the array and hash access functions requires
understanding a few caveats. Some of these caveats are actually considered bugs in the API, to be fixed
in later releases, and are bracketed with [MAYCHANGE] below. If you find yourself actually applying such
information in this section, be aware that the behavior may change in the future, umm, without warning.
The perl tie function associates a variable with an object that implements the various GET, SET, etc
methods. To perform the equivalent of the perl tie function from an XSUB, you must mimic this behaviour.
The code below carries out the necessary steps -- firstly it creates a new hash, and then creates a
second hash which it blesses into the class which will implement the tie methods. Lastly it ties the two
hashes together, and returns a reference to the new tied hash. Note that the code below does NOT call
the TIEHASH method in the MyTie class - see "Calling Perl Routines from within C Programs" for details on
how to do this.
SV*
mytie()
PREINIT:
HV *hash;
HV *stash;
SV *tie;
CODE:
hash = newHV();
tie = newRV_noinc((SV*)newHV());
stash = gv_stashpv("MyTie", GV_ADD);
sv_bless(tie, stash);
hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
RETVAL = newRV_noinc(hash);
OUTPUT:
RETVAL
The "av_store" function, when given a tied array argument, merely copies the magic of the array onto the
value to be "stored", using "mg_copy". It may also return NULL, indicating that the value did not
actually need to be stored in the array. [MAYCHANGE] After a call to "av_store" on a tied array, the
caller will usually need to call "mg_set(val)" to actually invoke the perl level "STORE" method on the
TIEARRAY object. If "av_store" did return NULL, a call to "SvREFCNT_dec(val)" will also be usually
necessary to avoid a memory leak. [/MAYCHANGE]
The previous paragraph is applicable verbatim to tied hash access using the "hv_store" and "hv_store_ent"
functions as well.
"av_fetch" and the corresponding hash functions "hv_fetch" and "hv_fetch_ent" actually return an
undefined mortal value whose magic has been initialized using "mg_copy". Note the value so returned does
not need to be deallocated, as it is already mortal. [MAYCHANGE] But you will need to call "mg_get()" on
the returned value in order to actually invoke the perl level "FETCH" method on the underlying TIE
object. Similarly, you may also call "mg_set()" on the return value after possibly assigning a suitable
value to it using "sv_setsv", which will invoke the "STORE" method on the TIE object. [/MAYCHANGE]
[MAYCHANGE] In other words, the array or hash fetch/store functions don't really fetch and store actual
values in the case of tied arrays and hashes. They merely call "mg_copy" to attach magic to the values
that were meant to be "stored" or "fetched". Later calls to "mg_get" and "mg_set" actually do the job of
invoking the TIE methods on the underlying objects. Thus the magic mechanism currently implements a kind
of lazy access to arrays and hashes.
Currently (as of perl version 5.004), use of the hash and array access functions requires the user to be
aware of whether they are operating on "normal" hashes and arrays, or on their tied variants. The API
may be changed to provide more transparent access to both tied and normal data types in future versions.
[/MAYCHANGE]
You would do well to understand that the TIEARRAY and TIEHASH interfaces are mere sugar to invoke some
perl method calls while using the uniform hash and array syntax. The use of this sugar imposes some
overhead (typically about two to four extra opcodes per FETCH/STORE operation, in addition to the
creation of all the mortal variables required to invoke the methods). This overhead will be
comparatively small if the TIE methods are themselves substantial, but if they are only a few statements
long, the overhead will not be insignificant.
Localizing changes
Perl has a very handy construction
{
local $var = 2;
...
}
This construction is approximately equivalent to
{
my $oldvar = $var;
$var = 2;
...
$var = $oldvar;
}
The biggest difference is that the first construction would reinstate the initial value of $var,
irrespective of how control exits the block: "goto", "return", "die"/"eval", etc. It is a little bit
more efficient as well.
There is a way to achieve a similar task from C via Perl API: create a pseudo-block, and arrange for some
changes to be automatically undone at the end of it, either explicit, or via a non-local exit (via
die()). A block-like construct is created by a pair of "ENTER"/"LEAVE" macros (see "Returning a Scalar"
in perlcall). Such a construct may be created specially for some important localized task, or an
existing one (like boundaries of enclosing Perl subroutine/block, or an existing pair for freeing TMPs)
may be used. (In the second case the overhead of additional localization must be almost negligible.)
Note that any XSUB is automatically enclosed in an "ENTER"/"LEAVE" pair.
Inside such a pseudo-block the following service is available:
"SAVEINT(int i)"
"SAVEIV(IV i)"
"SAVEI32(I32 i)"
"SAVELONG(long i)"
"SAVEI8(I8 i)"
"SAVEI16(I16 i)"
"SAVEBOOL(int 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.
"SAVEFREESV(SV *sv)"
The refcount of "sv" will be decremented at the end of pseudo-block. This is similar to "sv_2mortal"
in that it is also a mechanism for doing a delayed "SvREFCNT_dec". However, while "sv_2mortal"
extends the lifetime of "sv" until the beginning of the next statement, "SAVEFREESV" extends it until
the end of the enclosing scope. These lifetimes can be wildly different.
Also compare "SAVEMORTALIZESV".
"SAVEMORTALIZESV(SV *sv)"
Just like "SAVEFREESV", but mortalizes "sv" at the end of the current scope instead of decrementing
its reference count. This usually has the effect of keeping "sv" alive until the statement that
called the currently live scope has finished executing.
"SAVEFREEOP(OP *op)"
The "OP *" is op_free()ed at the end of pseudo-block.
SAVEFREEPV(p)
The chunk of memory which is pointed to by "p" is Safefree()ed at the end of pseudo-block.
"SAVECLEARSV(SV *sv)"
Clears a slot in the current scratchpad which corresponds to "sv" at the end of pseudo-block.
"SAVEDELETE(HV *hv, char *key, I32 length)"
The key "key" of "hv" is deleted at the end of pseudo-block. The string pointed to by "key" is
Safefree()ed. If one has a key in short-lived storage, the corresponding string may be reallocated
like this:
SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));
"SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)"
At the end of pseudo-block the function "f" is called with the only argument "p".
"SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)"
At the end of pseudo-block the function "f" is called with the implicit context argument (if any),
and "p".
"SAVESTACK_POS()"
The current offset on the Perl internal stack (cf. "SP") is restored at the end of pseudo-block.
The following API list contains functions, thus one needs to provide pointers to the modifiable data
explicitly (either C pointers, or Perlish "GV *"s). Where the above macros take "int", a similar
function takes "int *".
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.
"void save_list(SV **sarg, I32 maxsarg)"
A variant of "save_item" which takes multiple arguments via an array "sarg" of "SV*" of length
"maxsarg".
"SV* save_svref(SV **sptr)"
Similar to "save_scalar", but will reinstate an "SV *".
"void save_aptr(AV **aptr)"
"void save_hptr(HV **hptr)"
Similar to "save_svref", but localize "AV *" and "HV *".
The "Alias" module implements localization of the basic types within the caller's scope. People who are
interested in how to localize things in the containing scope should take a look there too.
Subroutines
XSUBs and the Argument Stack
The XSUB mechanism is a simple way for Perl programs to access C subroutines. An XSUB routine will have
a stack that contains the arguments from the Perl program, and a way to map from the Perl data structures
to a C equivalent.
The stack arguments are accessible through the ST(n) macro, which returns the "n"'th stack argument.
Argument 0 is the first argument passed in the Perl subroutine call. These arguments are "SV*", and can
be used anywhere an "SV*" is used.
Most of the time, output from the C routine can be handled through use of the RETVAL and OUTPUT
directives. However, there are some cases where the argument stack is not already long enough to handle
all the return values. An example is the POSIX tzname() call, which takes no arguments, but returns two,
the local time zone's standard and summer time abbreviations.
To handle this situation, the PPCODE directive is used and the stack is extended using the macro:
EXTEND(SP, num);
where "SP" is the macro that represents the local copy of the stack pointer, and "num" is the number of
elements the stack should be extended by.
Now that there is room on the stack, values can be pushed on it using "PUSHs" macro. The pushed values
will often need to be "mortal" (See "Reference Counts and Mortality"):
PUSHs(sv_2mortal(newSViv(an_integer)))
PUSHs(sv_2mortal(newSVuv(an_unsigned_integer)))
PUSHs(sv_2mortal(newSVnv(a_double)))
PUSHs(sv_2mortal(newSVpv("Some String",0)))
/* Although the last example is better written as the more
* efficient: */
PUSHs(newSVpvs_flags("Some String", SVs_TEMP))
And now the Perl program calling "tzname", the two values will be assigned as in:
($standard_abbrev, $summer_abbrev) = POSIX::tzname;
An alternate (and possibly simpler) method to pushing values on the stack is to use the macro:
XPUSHs(SV*)
This macro automatically adjusts the stack for you, if needed. Thus, you do not need to call "EXTEND" to
extend the stack.
Despite their suggestions in earlier versions of this document the macros "(X)PUSH[iunp]" are not suited
to XSUBs which return multiple results. For that, either stick to the "(X)PUSHs" macros shown above, or
use the new "m(X)PUSH[iunp]" macros instead; see "Putting a C value on Perl stack".
For more information, consult perlxs and perlxstut.
Autoloading with XSUBs
If an AUTOLOAD routine is an XSUB, as with Perl subroutines, Perl puts the fully-qualified name of the
autoloaded subroutine in the $AUTOLOAD variable of the XSUB's package.
But it also puts the same information in certain fields of the XSUB itself:
HV *stash = CvSTASH(cv);
const char *subname = SvPVX(cv);
STRLEN name_length = SvCUR(cv); /* in bytes */
U32 is_utf8 = SvUTF8(cv);
"SvPVX(cv)" contains just the sub name itself, not including the package. For an AUTOLOAD routine in
UNIVERSAL or one of its superclasses, "CvSTASH(cv)" returns NULL during a method call on a nonexistent
package.
Note: Setting $AUTOLOAD stopped working in 5.6.1, which did not support XS AUTOLOAD subs at all. Perl
5.8.0 introduced the use of fields in the XSUB itself. Perl 5.16.0 restored the setting of $AUTOLOAD.
If you need to support 5.8-5.14, use the XSUB's fields.
Calling Perl Routines from within C Programs
There are four routines that can be used to call a Perl subroutine from within a C program. These four
are:
I32 call_sv(SV*, I32);
I32 call_pv(const char*, I32);
I32 call_method(const char*, I32);
I32 call_argv(const char*, I32, char**);
The routine most often used is "call_sv". The "SV*" argument contains either the name of the Perl
subroutine to be called, or a reference to the subroutine. The second argument consists of flags that
control the context in which the subroutine is called, whether or not the subroutine is being passed
arguments, how errors should be trapped, and how to treat return values.
All four routines return the number of arguments that the subroutine returned on the Perl stack.
These routines used to be called "perl_call_sv", etc., before Perl v5.6.0, but those names are now
deprecated; macros of the same name are provided for compatibility.
When using any of these routines (except "call_argv"), the programmer must manipulate the Perl stack.
These include the following macros and functions:
dSP
SP
PUSHMARK()
PUTBACK
SPAGAIN
ENTER
SAVETMPS
FREETMPS
LEAVE
XPUSH*()
POP*()
For a detailed description of calling conventions from C to Perl, consult perlcall.
Putting a C value on Perl stack
A lot of opcodes (this is an elementary operation in the internal perl stack machine) put an SV* on the
stack. However, as an optimization the corresponding SV is (usually) not recreated each time. The
opcodes reuse specially assigned SVs (targets) which are (as a corollary) not constantly freed/created.
Each of the targets is created only once (but see "Scratchpads and recursion" below), and when an opcode
needs to put an integer, a double, or a string on stack, it just sets the corresponding parts of its
target and puts the target on stack.
The macro to put this target on stack is "PUSHTARG", and it is directly used in some opcodes, as well as
indirectly in zillions of others, which use it via "(X)PUSH[iunp]".
Because the target is reused, you must be careful when pushing multiple values on the stack. The
following code will not do what you think:
XPUSHi(10);
XPUSHi(20);
This translates as "set "TARG" to 10, push a pointer to "TARG" onto the stack; set "TARG" to 20, push a
pointer to "TARG" onto the stack". At the end of the operation, the stack does not contain the values 10
and 20, but actually contains two pointers to "TARG", which we have set to 20.
If you need to push multiple different values then you should either use the "(X)PUSHs" macros, or else
use the new "m(X)PUSH[iunp]" macros, none of which make use of "TARG". The "(X)PUSHs" macros simply push
an SV* on the stack, which, as noted under "XSUBs and the Argument Stack", will often need to be
"mortal". The new "m(X)PUSH[iunp]" macros make this a little easier to achieve by creating a new mortal
for you (via "(X)PUSHmortal"), pushing that onto the stack (extending it if necessary in the case of the
"mXPUSH[iunp]" macros), and then setting its value. Thus, instead of writing this to "fix" the example
above:
XPUSHs(sv_2mortal(newSViv(10)))
XPUSHs(sv_2mortal(newSViv(20)))
you can simply write:
mXPUSHi(10)
mXPUSHi(20)
On a related note, if you do use "(X)PUSH[iunp]", then you're going to need a "dTARG" in your variable
declarations so that the "*PUSH*" macros can make use of the local variable "TARG". See also "dTARGET"
and "dXSTARG".
Scratchpads
The question remains on when the SVs which are targets for opcodes are created. The answer is that they
are created when the current unit--a subroutine or a file (for opcodes for statements outside of
subroutines)--is compiled. During this time a special anonymous Perl array is created, which is called a
scratchpad for the current unit.
A scratchpad keeps SVs which are lexicals for the current unit and are targets for opcodes. A previous
version of this document stated that one can deduce that an SV lives on a scratchpad by looking on its
flags: lexicals have "SVs_PADMY" set, and targets have "SVs_PADTMP" set. But this has never been fully
true. "SVs_PADMY" could be set on a variable that no longer resides in any pad. While targets do have
"SVs_PADTMP" set, it can also be set on variables that have never resided in a pad, but nonetheless act
like targets. As of perl 5.21.5, the "SVs_PADMY" flag is no longer used and is defined as 0.
"SvPADMY()" now returns true for anything without "SVs_PADTMP".
The correspondence between OPs and targets is not 1-to-1. Different OPs in the compile tree of the unit
can use the same target, if this would not conflict with the expected life of the temporary.
Scratchpads and recursion
In fact it is not 100% true that a compiled unit contains a pointer to the scratchpad AV. In fact it
contains a pointer to an AV of (initially) one element, and this element is the scratchpad AV. Why do we
need an extra level of indirection?
The answer is recursion, and maybe threads. Both these can create several execution pointers going into
the same subroutine. For the subroutine-child not write over the temporaries for the subroutine-parent
(lifespan of which covers the call to the child), the parent and the child should have different
scratchpads. (And the lexicals should be separate anyway!)
So each subroutine is born with an array of scratchpads (of length 1). On each entry to the subroutine
it is checked that the current depth of the recursion is not more than the length of this array, and if
it is, new scratchpad is created and pushed into the array.
The targets on this scratchpad are "undef"s, but they are already marked with correct flags.
Memory Allocation
Allocation
All memory meant to be used with the Perl API functions should be manipulated using the macros described
in this section. The macros provide the necessary transparency between differences in the actual malloc
implementation that is used within perl.
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 PERL_IMPLICIT_CONTEXT
The Perl interpreter can be regarded as a closed box: it has an API for feeding it code or otherwise
making it do things, but it also has functions for its own use. This smells a lot like an object, and
there 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. With multiplicity-enabled perls,
PERL_IMPLICIT_CONTEXT is also normally defined, and enables the support for passing in a "hidden" first
argument that represents all three data structures. MULTIPLICITY makes multi-threaded perls possible
(with the ithreads threading model, related to the macro USE_ITHREADS.)
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 PERL_IMPLICIT_CONTEXT, there is no first argument containing
the interpreter's context. The trailing underscore in the pTHX_ macro indicates that the macro expansion
needs a comma after the context argument because other arguments follow it. If PERL_IMPLICIT_CONTEXT is
not defined, pTHX_ will be ignored, and the subroutine is not prototyped to take the extra argument. The
form of the macro without the trailing underscore is used when there are no additional explicit
arguments.
When a core function calls another, it must pass the context. This is normally hidden via macros.
Consider "sv_setiv". It expands into something like this:
#ifdef PERL_IMPLICIT_CONTEXT
#define sv_setiv(a,b) Perl_sv_setiv(aTHX_ a, b)
/* can't do this for vararg functions, see below */
#else
#define sv_setiv Perl_sv_setiv
#endif
This works well, and means that XS authors can gleefully write:
sv_setiv(foo, bar);
and still have it work under all the modes Perl could have been compiled with.
This doesn't work so cleanly for varargs functions, though, as macros imply that the number of arguments
is known in advance. Instead we either need to spell them out fully, passing "aTHX_" as the first
argument (the Perl core tends to do this with functions like Perl_warner), or use a context-free version.
The context-free version of Perl_warner is called Perl_warner_nocontext, and does not take the extra
argument. Instead it does "dTHX;" to get the context from thread-local storage. We "#define warner
Perl_warner_nocontext" so that extensions get source compatibility at the expense of performance.
(Passing an arg is cheaper than grabbing it from thread-local storage.)
You can ignore [pad]THXx when browsing the Perl headers/sources. Those are strictly for use within the
core. Extensions and embedders need only be aware of [pad]THX.
So what happened to dTHR?
"dTHR" was introduced in perl 5.005 to support the older thread model. The older thread model now uses
the "THX" mechanism to pass context pointers around, so "dTHR" is not useful any more. Perl 5.6.0 and
later still have it for backward source compatibility, but it is defined to be a no-op.
How do I use all this in extensions?
When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call any functions in the Perl API will
need to pass the initial context argument somehow. The kicker is that you will need to write it in such
a way that the extension still compiles when Perl hasn't been built with PERL_IMPLICIT_CONTEXT enabled.
There are three ways to do this. First, the easy but inefficient way, which is also the default, in
order to maintain source compatibility with extensions: whenever XSUB.h is #included, it redefines the
aTHX and aTHX_ macros to call a function that will return the context. Thus, something like:
sv_setiv(sv, num);
in your extension will translate to this when PERL_IMPLICIT_CONTEXT is in effect:
Perl_sv_setiv(Perl_get_context(), sv, num);
or to this otherwise:
Perl_sv_setiv(sv, num);
You don't have to do anything new in your extension to get this; since the Perl library provides
Perl_get_context(), it will all just work.
The second, more efficient way is to use the following template for your Foo.xs:
#define PERL_NO_GET_CONTEXT /* we want efficiency */
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
STATIC void my_private_function(int arg1, int arg2);
STATIC void
my_private_function(int arg1, int arg2)
{
dTHX; /* fetch context */
... call many Perl API functions ...
}
[... etc ...]
MODULE = Foo PACKAGE = Foo
/* typical XSUB */
void
my_xsub(arg)
int arg
CODE:
my_private_function(arg, 10);
Note that the only two changes from the normal way of writing an extension is the addition of a "#define
PERL_NO_GET_CONTEXT" before including the Perl headers, followed by a "dTHX;" declaration at the start of
every function that will call the Perl API. (You'll know which functions need this, because the C
compiler will complain that there's an undeclared identifier in those functions.) No changes are needed
for the XSUBs themselves, because the XS() macro is correctly defined to pass in the implicit context if
needed.
The third, even more efficient way is to ape how it is done within the Perl guts:
#define PERL_NO_GET_CONTEXT /* we want efficiency */
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
/* pTHX_ only needed for functions that call Perl API */
STATIC void my_private_function(pTHX_ int arg1, int arg2);
STATIC void
my_private_function(pTHX_ int arg1, int arg2)
{
/* dTHX; not needed here, because THX is an argument */
... call Perl API functions ...
}
[... etc ...]
MODULE = Foo PACKAGE = Foo
/* typical XSUB */
void
my_xsub(arg)
int arg
CODE:
my_private_function(aTHX_ arg, 10);
This implementation never has to fetch the context using a function call, since it is always passed as an
extra argument. Depending on your needs for simplicity or efficiency, you may mix the previous two
approaches freely.
Never add a comma after "pTHX" yourself--always use the form of the macro with the underscore for
functions that take explicit arguments, or the form without the argument for functions with no explicit
arguments.
Should I do anything special if I call perl from multiple threads?
If you create interpreters in one thread and then proceed to call them in another, you need to make sure
perl's own Thread Local Storage (TLS) slot is initialized correctly in each of those threads.
The "perl_alloc" and "perl_clone" API functions will automatically set the TLS slot to the interpreter
they created, so that there is no need to do anything special if the interpreter is always accessed in
the same thread that created it, and that thread did not create or call any other interpreters
afterwards. If that is not the case, you have to set the TLS slot of the thread before calling any
functions in the Perl API on that particular interpreter. This is done by calling the "PERL_SET_CONTEXT"
macro in that thread as the first thing you do:
/* do this before doing anything else with some_perl */
PERL_SET_CONTEXT(some_perl);
... other Perl API calls on some_perl go here ...
Future Plans and PERL_IMPLICIT_SYS
Just as PERL_IMPLICIT_CONTEXT provides a way to bundle up everything that the interpreter knows about
itself and pass it around, so too are there plans to allow the interpreter to bundle up everything it
knows about the environment it's running on. This is enabled with the PERL_IMPLICIT_SYS macro.
Currently it only works with USE_ITHREADS on Windows.
This allows the ability to provide an extra pointer (called the "host" environment) for all the system
calls. This makes it possible for all the system stuff to maintain their own state, broken down into
seven C structures. These are thin wrappers around the usual system calls (see win32/perllib.c) for the
default perl executable, but for a more ambitious host (like the one that would do fork() emulation) all
the extra work needed to pretend that different interpreters are actually different "processes", would be
done here.
The Perl engine/interpreter and the host are orthogonal entities. There could be one or more
interpreters in a process, and one or more "hosts", with free association between them.
Internal Functions
All of Perl's internal functions which will be exposed to the outside world are prefixed by "Perl_" so
that they will not conflict with XS functions or functions used in a program in which Perl is embedded.
Similarly, all global variables begin with "PL_". (By convention, static functions start with "S_".)
Inside the Perl core ("PERL_CORE" defined), you can get at the functions either with or without the
"Perl_" prefix, thanks to a bunch of defines that live in embed.h. Note that extension code should not
set "PERL_CORE"; this exposes the full perl internals, and is likely to cause breakage of the XS in each
new perl release.
The file embed.h is generated automatically from embed.pl and embed.fnc. embed.pl also creates the
prototyping header files for the internal functions, generates the documentation and a lot of other bits
and pieces. It's important that when you add a new function to the core or change an existing one, you
change the data in the table in embed.fnc as well. Here's a sample entry from that table:
Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval
The 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));
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 preceeding 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".
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
perl v5.34.0 2025-04-08 PERLGUTS(1)