PERLGUTS(1) Perl Programmers Reference Guide PERLGUTS(1)NAMEperlguts - Introduction to the Perl API
DESCRIPTION
This document attempts to describe how to use the Perl
API, as well as containing 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 com
ments to the author below.
Variables
Datatypes
Perl has three typedefs that handle Perl's three main data
types:
SV Scalar Value
AV Array Value
HV Hash Value
Each typedef has specific routines that manipulate the
various data types.
What is an "IV"?
Perl uses a special typedef IV which is a simple signed
integer type that is guaranteed to be large enough to hold
a pointer (as well as an integer). Additionally, there is
the UV, which is simply an unsigned IV.
Perl also uses two special typedefs, I32 and I16, which
will always be at least 32-bits and 16-bits long, respec
tively. (Again, there are U32 and U16, as well.)
Working with SVs
An SV can be created and loaded with one command. There
are four types of values that can be loaded: an integer
value (IV), a double (NV), a string (PV), and another
scalar (SV).
The six routines are:
SV* newSViv(IV);
SV* newSVnv(double);
SV* newSVpv(const char*, int);
SV* newSVpvn(const char*, int);
SV* newSVpvf(const char*, ...);
SV* newSVsv(SV*);
To change the value of an *already-existing* SV, there are
seven 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*, int)
void sv_setpvf(SV*, const char*, ...);
void sv_setpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool);
void sv_setsv(SV*, SV*);
Notice that you can choose to specify the length of the
string to be assigned by using "sv_setpvn", "newSVpvn", or
"newSVpv", or you may allow Perl to calculate the length
by using "sv_setpv" or by specifying 0 as the second argu
ment to "newSVpv". Be warned, though, that Perl will
determine the string's length by using "strlen", which
depends on the string terminating with a NUL character.
The arguments of "sv_setpvf" are processed like "sprintf",
and the formatted output becomes the value.
"sv_setpvfn" 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 the perlsec manpage). This
pointer may be NULL if that information is not important.
Note that this function requires you to specify the length
of the format.
STRLEN is an integer type (Size_t, usually defined as
size_t in config.h) guaranteed to be large enough to rep
resent the size of any string that perl can handle.
The "sv_set*()" functions are not generic enough to oper
ate on values that have "magic". See the Magic Virtual
Tables entry elsewhere in this document later in this doc
ument.
All SVs that contain strings should be terminated with a
NUL character. If it is not NUL-terminated there is a
risk of core dumps and corruptions from code which passes
the string to C functions or system calls which expect a
NUL-terminated string. Perl's own functions typically add
a trailing NUL for this reason. Nevertheless, you should
be very careful when you pass a string stored in an SV to
a C function or system call.
To access the actual value that an SV points to, you can
use the macros:
SvIV(SV*)
SvUV(SV*)
SvNV(SV*)
SvPV(SV*, STRLEN len)
SvPV_nolen(SV*)
which will automatically coerce the actual scalar type
into an IV, UV, double, or string.
In the "SvPV" macro, the length of the string returned is
placed into the variable "len" (this is a macro, so you do
not use "&len"). If you do not care what the length of
the data is, use the "SvPV_nolen" macro. Historically the
"SvPV" macro with the global variable "PL_na" has been
used in this case. But that can be quite inefficient
because "PL_na" must be accessed in thread-local storage
in threaded Perl. In any case, remember that Perl allows
arbitrary strings of data that may both contain NULs and
might not be terminated by a NUL.
Also remember that C doesn't allow you to safely say
"foo(SvPV(s, len), len);". It might work with your com
piler, but it won't work for everyone. Break this sort of
statement up into separate assignments:
SV *s;
STRLEN len;
char * ptr;
ptr = SvPV(s, len);
foo(ptr, len);
If you want to know if the scalar value is TRUE, you can
use:
SvTRUE(SV*)
Although Perl will automatically grow strings for you, if
you need to force Perl to allocate more memory for your
SV, you can use the macro
SvGROW(SV*, STRLEN newlen)
which will determine if more memory needs to be allocated.
If so, it will call the function "sv_grow". Note that
"SvGROW" can only increase, not decrease, the allocated
memory of an SV and that it does not automatically add a
byte for the a trailing NUL (perl's own string functions
typically do "SvGROW(sv, len + 1)").
If you have an SV and want to know what kind of data Perl
thinks is stored in it, you can use the following macros
to check the type of SV you have.
SvIOK(SV*)
SvNOK(SV*)
SvPOK(SV*)
You can get and set the current length of the string
stored in an SV with the following macros:
SvCUR(SV*)
SvCUR_set(SV*, I32 val)
You can also get a pointer to the end of the string stored
in the SV with the macro:
SvEND(SV*)
But note that these last three macros are valid only if
"SvPOK()" is true.
If you want to append something to the end of string
stored in an "SV*", you can use the following functions:
void sv_catpv(SV*, const char*);
void sv_catpvn(SV*, const char*, STRLEN);
void sv_catpvf(SV*, const char*, ...);
void sv_catpvfn(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 oper
ate on values that have "magic". See the Magic Virtual
Tables entry elsewhere in this document later in this doc
ument.
If you know the name of a scalar variable, you can get a
pointer to its SV by using the following:
SV* get_sv("package::varname", FALSE);
This returns NULL if the variable does not exist.
If you want to know if this variable (or any other SV) is
actually "defined", you can call:
SvOK(SV*)
The scalar "undef" value is stored in an SV instance
called "PL_sv_undef". Its address can be used whenever an
"SV*" is needed.
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 oth
erwise. Instead it has returned a NULL pointer which,
somewhere down the line, will cause a segmentation viola
tion, 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 "SvRE
FCNT_dec(SV*)". Normally this call is not necessary (see
the Reference Counts and Mortality entry elsewhere in this
document).
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 the PV, and it
discards everything before the pointer. The efficiency
comes by means of a little hack: instead of actually
removing the characters, "sv_chop" sets the flag "OOK"
(offset OK) to signal to other functions that the offset
hack is in effect, and it puts the number of bytes chopped
off into the IV field of the SV. It then moves the PV
pointer (called "SvPVX") forward that many bytes, and
adjusts "SvCUR" and "SvLEN".
Hence, at this point, the start of the buffer that we
allocated lives at "SvPVX(sv) - SvIV(sv)" in memory and
the PV pointer is pointing into the middle of this
allocated storage.
This is best demonstrated by example:
% ./perl -Ilib -MDevel::Peek -le '$a="12345"; $a=~s/.//; Dump($a)'
SV = PVIV(0x8128450) at 0x81340f0
REFCNT = 1
FLAGS = (POK,OOK,pPOK)
IV = 1 (OFFSET)
PV = 0x8135781 ( "1" . ) "2345"\0
CUR = 4
LEN = 5
Here the number of bytes chopped off (1) is put into IV,
and "Devel::Peek::Dump" helpfully reminds us that this is
an offset. The portion of the string between the "real"
and the "fake" beginnings is shown in parentheses, and the
values of "SvCUR" and "SvLEN" reflect the fake beginning,
not the real one.
Something similar to the offset hack is perfomed on AVs to
enable efficient shifting and splicing off the beginning
of the array; while "AvARRAY" points to the first element
in the array that is visible from Perl, "AvALLOC" points
to the real start of the C array. These are usually the
same, but a "shift" operation can be carried out by
increasing "AvARRAY" by one and decreasing "AvFILL" and
"AvLEN". Again, the location of the real start of the C
array only comes into play when freeing the array. See
"av_shift" in av.c.
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.
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 popu
lates it with SVs:
AV* av_make(I32 num, SV **ptr);
The second argument points to an array containing "num"
"SV*"'s. Once the AV has been created, the SVs can be
destroyed, if so desired.
Once the AV has been created, the following operations are
possible on AVs:
void av_push(AV*, SV*);
SV* av_pop(AV*);
SV* av_shift(AV*);
void av_unshift(AV*, I32 num);
These should be familiar operations, with the exception of
"av_unshift". This routine adds "num" elements at the
front of the array with the "undef" value. You must then
use "av_store" (described below) to assign values to these
new elements.
Here are some other functions:
I32 av_len(AV*);
SV** av_fetch(AV*, I32 key, I32 lval);
SV** av_store(AV*, I32 key, SV* val);
The "av_len" function returns the highest index value in
array (just like $#array in Perl). If the array is empty,
-1 is returned. The "av_fetch" function returns the value
at index "key", but if "lval" is non-zero, then "av_fetch"
will store an undef value at that index. The "av_store"
function stores the value "val" at index "key", and does
not increment the reference count of "val". Thus the
caller is responsible for taking care of that, and if
"av_store" returns NULL, the caller will have to decrement
the reference count to avoid a memory leak. Note that
"av_fetch" and "av_store" both return "SV**"'s, not
"SV*"'s as their return value.
void av_clear(AV*);
void av_undef(AV*);
void av_extend(AV*, I32 key);
The "av_clear" function deletes all the elements in the
AV* array, but does not actually delete the array itself.
The "av_undef" function will delete all the elements in
the array plus the array itself. The "av_extend" function
extends the array so that it contains at least "key+1"
elements. If "key+1" is less than the currently allocated
length of the array, then nothing is done.
If you know the name of an array variable, you can get a
pointer to its AV by using the following:
AV* get_av("package::varname", FALSE);
This returns NULL if the variable does not exist.
See the Understanding the Magic of Tied Hashes and Arrays
entry elsewhere in this document 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 HVs:
SV** hv_store(HV*, const char* key, U32 klen, SV* val, U32 hash);
SV** hv_fetch(HV*, const char* key, U32 klen, I32 lval);
The "klen" parameter is the length of the key being passed
in (Note that you cannot pass 0 in as a value of "klen" to
tell Perl to measure the length of the key). The "val"
argument contains the SV pointer to the scalar being
stored, and "hash" is the precomputed hash value (zero if
you want "hv_store" to calculate it for you). The "lval"
parameter indicates whether this fetch is actually a part
of a store operation, in which case a new undefined value
will be added to the HV with the supplied key and
"hv_fetch" will return as if the value had already
existed.
Remember that "hv_store" and "hv_fetch" return "SV**"'s
and not just "SV*". To access the scalar value, you must
first dereference the return value. However, you should
check to make sure that the return value is not NULL
before dereferencing it.
These two functions check if a hash table entry exists,
and deletes it.
bool hv_exists(HV*, const char* key, U32 klen);
SV* hv_delete(HV*, const char* key, U32 klen, I32 flags);
If "flags" does not include the "G_DISCARD" flag then
"hv_delete" will create and return a mortal copy of the
deleted value.
And more miscellaneous functions:
void hv_clear(HV*);
void hv_undef(HV*);
Like their AV counterparts, "hv_clear" deletes all the
entries in the hash table but does not actually delete the
hash table. The "hv_undef" deletes both the entries and
the hash table itself.
Perl keeps the actual data in linked list of structures
with a typedef of HE. These contain the actual key and
value pointers (plus extra administrative overhead). The
key is a string pointer; the value is an "SV*". However,
once you have an "HE*", to get the actual key and value,
use the routines specified below.
I32 hv_iterinit(HV*);
/* Prepares starting point to traverse hash table */
HE* hv_iternext(HV*);
/* Get the next entry, and return a pointer to a
structure that has both the key and value */
char* hv_iterkey(HE* entry, I32* retlen);
/* Get the key from an HE structure and also return
the length of the key string */
SV* hv_iterval(HV*, HE* entry);
/* Return a SV pointer to the value of the HE
structure */
SV* hv_iternextsv(HV*, char** key, I32* retlen);
/* This convenience routine combines hv_iternext,
hv_iterkey, and hv_iterval. The key and retlen
arguments are return values for the key and its
length. The value is returned in the SV* argument */
If you know the name of a hash variable, you can get a
pointer to its HV by using the following:
HV* get_hv("package::varname", FALSE);
This returns NULL if the variable does not exist.
The hash algorithm is defined in the "PERL_HASH(hash, key,
klen)" macro:
hash = 0;
while (klen--)
hash = (hash * 33) + *key++;
hash = hash + (hash >> 5); /* after 5.6 */
The last step was added in version 5.6 to improve distri
bution of lower bits in the resulting hash value.
See the Understanding the Magic of Tied Hashes and Arrays
entry elsewhere in this document 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 simpli
fies writing of extension code that deals with hash struc
tures. 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 the perlapi manpage 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 the perlapi manpage 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 incre
ment 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 ref
erence count of "val" to avoid a memory leak.
References
References are a special type of scalar that point to
other data types (including references).
To create a reference, use either of the following func
tions:
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 fol
lowing macro:
SvROK(SV*)
To discover what type of value the reference refers to,
use the following macro and then check the return value.
SvTYPE(SvRV(SV*))
The most useful types that will be returned are:
SVt_IV Scalar
SVt_NV Scalar
SVt_PV Scalar
SVt_RV Scalar
SVt_PVAV Array
SVt_PVHV Hash
SVt_PVCV Code
SVt_PVGV Glob (possible a file handle)
SVt_PVMG Blessed or Magical Scalar
See the sv.h header file for more details.
Blessed References and Class Objects
References are also used to support object-oriented pro
gramming. In the OO lexicon, an object is simply a refer
ence 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 follow
ing function:
SV* sv_bless(SV* sv, HV* stash);
The "sv" argument must be a reference. The "stash" argu
ment specifies which class the reference will belong to.
See the Stashes and Globs entry elsewhere in this document
for information on converting class names into stashes.
/* Still under construction */
Upgrades rv to reference if not already one. Creates new
SV for rv to point to. If "classname" is non-null, the SV
is blessed into the specified class. SV is returned.
SV* newSVrv(SV* rv, const char* classname);
Copies integer 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_nv(SV* rv, const char* classname, NV iv);
Copies the pointer value (the address, not the string!)
into an SV whose reference is rv. SV is blessed if
"classname" is non-null.
SV* sv_setref_pv(SV* rv, const char* classname, PV iv);
Copies string into an SV whose reference is "rv". Set
length to 0 to let Perl calculate the string length. SV
is blessed if "classname" is non-null.
SV* sv_setref_pvn(SV* rv, const char* classname, PV iv, STRLEN length);
Tests whether the SV is blessed into the specified class.
It does not check inheritance relationships.
int sv_isa(SV* sv, const char* name);
Tests whether the SV is a reference to a blessed object.
int sv_isobject(SV* sv);
Tests whether the SV is derived from the specified class.
SV can be either a reference to a blessed object or a
string containing a class name. This is the function
implementing the "UNIVERSAL::isa" functionality.
bool sv_derived_from(SV* sv, const char* name);
To check if you've got an object derived from a specific
class you have to write:
if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }
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", TRUE);
AV* get_av("package::varname", TRUE);
HV* get_hv("package::varname", TRUE);
Notice the use of TRUE as the second parameter. The new
variable can now be set, using the routines appropriate to
the data type.
There are additional macros whose values may be bitwise
OR'ed with the "TRUE" argument to enable certain extra
features. Those bits are:
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 cre
ated in the current package.
Reference Counts and Mortality
Perl uses an reference count-driven garbage collection
mechanism. SVs, AVs, or HVs (xV for short in the follow
ing) start their life with a reference count of 1. If the
reference count of an xV ever drops to 0, then it will be
destroyed and its memory made available for reuse.
This normally doesn't happen at the Perl level unless a
variable is undef'ed or the last variable holding a refer
ence to it is changed or overwritten. At the internal
level, however, reference counts can be manipulated with
the following macros:
int SvREFCNT(SV* sv);
SV* SvREFCNT_inc(SV* sv);
void SvREFCNT_dec(SV* sv);
However, there is one other function which manipulates the
reference count of its argument. The "newRV_inc" func
tion, you will recall, creates a reference to the speci
fied argument. As a side effect, it increments the argu
ment's reference count. If this is not what you want, use
"newRV_noinc" instead.
For example, imagine you want to return a reference from
an XSUB function. Inside the XSUB routine, you create an
SV which initially has a reference count of one. Then you
call "newRV_inc", passing it the just-created SV. This
returns the reference as a new SV, but the reference count
of the SV you passed to "newRV_inc" has been incremented
to two. Now you return the reference from the XSUB rou
tine and forget about the SV. But Perl hasn't! Whenever
the returned reference is destroyed, the reference count
of the original SV is decreased to one and nothing hap
pens. The SV will hang around without any way to access
it until Perl itself terminates. This is a memory leak.
The correct procedure, then, is to use "newRV_noinc"
instead of "newRV_inc". Then, if and when the last refer
ence is destroyed, the reference count of the SV will go
to zero and it will be destroyed, stopping any memory
leak.
There are some convenience functions available that can
help with the destruction of xVs. These functions intro
duce the concept of "mortality". An xV that is mortal has
had its reference count marked to be decremented, but not
actually decremented, until "a short time later". Gener
ally the term "short time later" means a single Perl
statement, such as a call to an XSUB function. The actual
determinant for when mortal xVs have their reference count
decremented depends on two macros, SAVETMPS and FREETMPS.
See the perlcall manpage and the perlxs manpage for more
details on these macros.
"Mortalization" then is at its simplest a deferred "SvRE
FCNT_dec". However, if you mortalize a variable twice,
the reference count will later be decremented twice.
You should be careful about creating mortal variables.
Strange things can happen if you make the same value mor
tal within multiple contexts, or if you make a variable
mortal multiple times.
To create a mortal variable, use the functions:
SV* sv_newmortal()
SV* sv_2mortal(SV*)
SV* sv_mortalcopy(SV*)
The first call creates a mortal SV, the second converts an
existing SV to a mortal SV (and thus defers a call to
"SvREFCNT_dec"), and the third creates a mortal copy of an
existing SV.
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 of the different
objects that are contained within a package. Each key of
the stash is a symbol name (shared by all the different
types of objects that have the same name), and each value
in the hash table is a GV (Glob Value). This GV in turn
contains references to the various objects of that name,
including (but not limited to) the following:
Scalar Value
Array Value
Hash Value
I/O Handle
Format
Subroutine
There is a single stash called "PL_defstash" that holds
the items that exist in the "main" package. To get at the
items in other packages, append the string "::" to the
package name. The items in the "Foo" package are in the
stash "Foo::" in PL_defstash. The items in the "Bar::Baz"
package are in the stash "Baz::" in "Bar::"'s stash.
To get the stash pointer for a particular package, use the
function:
HV* gv_stashpv(const char* name, I32 create)
HV* gv_stashsv(SV*, I32 create)
The first function takes a literal string, the second uses
the string stored in the SV. Remember that a stash is
just a hash table, so you get back an "HV*". The "create"
flag will create a new package if it is set.
The name that "gv_stash*v" wants is the name of the pack
age 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 refer
ence, 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
the perlref manpage.
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 "dber
ror" that contains both the numeric and descriptive string
error values, you could use the following code:
extern int dberror;
extern char *dberror_list;
SV* sv = get_sv("dberror", TRUE);
sv_setiv(sv, (IV) dberror);
sv_setpv(sv, dberror_list[dberror]);
SvIOK_on(sv);
If the order of "sv_setiv" and "sv_setpv" had been
reversed, then the macro "SvPOK_on" would need to be
called instead of "SvIOK_on".
Magic Variables
[This section still under construction. Ignore everything
here. Post no bills. Everything not permitted is forbid
den.]
Any SV may be magical, that is, it has special features
that a normal SV does not have. These features are stored
in the SV structure in a linked list of "struct magic"'s,
typedef'ed to "MAGIC".
struct magic {
MAGIC* mg_moremagic;
MGVTBL* mg_virtual;
U16 mg_private;
char mg_type;
U8 mg_flags;
SV* mg_obj;
char* mg_ptr;
I32 mg_len;
};
Note this is current as of patchlevel 0, and could change
at any time.
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 set the "SVt_PVMG" flag for the "sv". Perl then
continues by adding it 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 asso
ciated 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 and "namlen" >= 0 a malloc'd copy of the name is
stored in "mg_ptr" field.
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 Table"
section below. The "how" argument is also stored in the
"mg_type" field.
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 "#", or if it is a NULL pointer, then "obj" is merely
stored, without the reference count being incremented.
There is also a function to add magic to an "HV":
void hv_magic(HV *hv, GV *gv, int how);
This simply calls "sv_magic" and coerces the "gv" argument
into an "SV".
To remove the magic from an SV, call the function
sv_unmagic:
void sv_unmagic(SV *sv, int type);
The "type" argument should be equal to the "how" value
when the "SV" was initially made magical.
Magic Virtual Tables
The "mg_virtual" field in the "MAGIC" structure is a
pointer to a "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 vari
able.
The "MGVTBL" has five pointers to the following routine
types:
int (*svt_get)(SV* sv, MAGIC* mg);
int (*svt_set)(SV* sv, MAGIC* mg);
U32 (*svt_len)(SV* sv, MAGIC* mg);
int (*svt_clear)(SV* sv, MAGIC* mg);
int (*svt_free)(SV* sv, MAGIC* mg);
This MGVTBL structure is set at compile-time in "perl.h"
and there are currently 19 types (or 21 with overloading
turned on). These different structures contain pointers
to various routines that perform additional actions
depending on which function is being called.
Function pointer Action taken
----------------------------
svt_get Do something after the value of the SV is retrieved.
svt_set Do something after the SV is assigned a value.
svt_len Report on the SV's length.
svt_clear Clear something the SV represents.
svt_free Free any extra storage associated with the SV.
For instance, the MGVTBL structure called "vtbl_sv" (which
corresponds to an "mg_type" of '\0') contains:
{ magic_get, magic_set, magic_len, 0, 0 }
Thus, when an SV is determined to be magical and of type
'\0', if a get operation is being performed, the routine
"magic_get" is called. All the various routines for the
various magical types begin with "magic_". NOTE: the
magic routines are not considered part of the Perl API,
and may not be exported by the Perl library.
The current kinds of Magic Virtual Tables are:
mg_type MGVTBL Type of magic
-----------------------------------------
\0 vtbl_sv Special scalar variable
A vtbl_amagic %OVERLOAD hash
a vtbl_amagicelem %OVERLOAD hash element
c (none) Holds overload table (AMT) on stash
B vtbl_bm Boyer-Moore (fast string search)
D vtbl_regdata Regex match position data (@+ and @- vars)
d vtbl_regdatum Regex match position data element
E vtbl_env %ENV hash
e vtbl_envelem %ENV hash element
f vtbl_fm Formline ('compiled' format)
g vtbl_mglob m//g target / study()ed string
I vtbl_isa @ISA array
i vtbl_isaelem @ISA array element
k vtbl_nkeys scalar(keys()) lvalue
L (none) Debugger %_<filename
l vtbl_dbline Debugger %_<filename element
o vtbl_collxfrm Locale transformation
P vtbl_pack Tied array or hash
p vtbl_packelem Tied array or hash element
q vtbl_packelem Tied scalar or handle
S vtbl_sig %SIG hash
s vtbl_sigelem %SIG hash element
t vtbl_taint Taintedness
U vtbl_uvar Available for use by extensions
v vtbl_vec vec() lvalue
x vtbl_substr substr() lvalue
y vtbl_defelem Shadow "foreach" iterator variable /
smart parameter vivification
* vtbl_glob GV (typeglob)
# vtbl_arylen Array length ($#ary)
. vtbl_pos pos() lvalue
~ (none) Available for use by extensions
When an uppercase and lowercase letter both exist in the
table, then the uppercase letter is used to represent some
kind of composite type (a list or a hash), and the lower
case letter is used to represent an element of that com
posite type.
The '~' and 'U' magic types are defined specifically for
use by extensions and will not be used by perl itself.
Extensions can use '~' 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, 'U' 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" struc
ture:
struct ufuncs {
I32 (*uf_val)(IV, SV*);
I32 (*uf_set)(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 'U' 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, 'U', (char*)&uf, sizeof(uf));
Note that because multiple extensions may be using '~' or
'U' 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 '~' magic, it may also be appropriate to
add an I32 'signature' at the top of the private data area
and check that.
Also note that the "sv_set*()" and "sv_cat*()" functions
described earlier do not invoke 'set' magic on their tar
gets. 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()" func
tions. Similarly, generic C code must call the "SvGET
MAGIC()" macro to invoke any 'get' magic if they use an SV
obtained from external sources in functions that don't
handle magic. See the perlapi manpage 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*, int type); /* Finds the magic pointer of that type */
This routine returns a pointer to the "MAGIC" structure
stored in the SV. If the SV does not have that magical
feature, "NULL" is returned. Also, if the SV is not of
type SVt_PVMG, Perl may core dump.
int mg_copy(SV* sv, SV* nsv, const char* key, STRLEN klen);
This routine checks to see what types of magic "sv" has.
If the mg_type field is an uppercase letter, then the
mg_obj is copied to "nsv", but the mg_type field is
changed to be the lowercase letter.
Understanding the Magic of Tied Hashes and Arrays
Tied hashes and arrays are magical beasts of the 'P' 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 consid
ered 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 per
form 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 the Calling Perl
Routines from within C Programs entry elsewhere in this
document for details on how to do this.
SV*
mytie()
PREINIT:
HV *hash;
HV *stash;
SV *tie;
CODE:
hash = newHV();
tie = newRV_noinc((SV*)newHV());
stash = gv_stashpv("MyTie", TRUE);
sv_bless(tie, stash);
hv_magic(hash, tie, 'P');
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, indi
cating 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 deallo
cated, 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 cur
rently 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 pro
vide 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 syn
tax. The use of this sugar imposes some overhead (typi
cally 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 them
selves 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 the Returning a Scalar entry
in the perlcall manpage). Such a construct may be created
specially for some important localized task, or an exist
ing one (like boundaries of enclosing Perl subrou
tine/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 avail
able:
""SAVEINT(int i)""
""SAVEIV(IV i)""
""SAVEI32(I32 i)""
""SAVELONG(long i)""
These macros arrange things to restore the value of
integer variable "i" at the end of enclosing pseudo-
block.
""SAVESPTR(s)""
""SAVEPPTR(p)""
These macros arrange things to restore the value of
pointers "s" and "p". "s" must be a pointer of a type
which survives conversion to "SV*" and back, "p"
should be able to survive conversion to "char*" and
back.
""SAVEFREESV(SV *sv)""
The refcount of "sv" would be decremented at the end
of pseudo-block. This is similar to "sv_2mortal" in
that it is also a mechanism for doing a delayed "SvRE
FCNT_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 ref
erence count. This usually has the effect of keeping
"sv" alive until the statement that called the cur
rently 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 corre
sponds 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 corre
sponding string may be reallocated like this:
SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));
""SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)""
At the end of pseudo-block the function "f" is called
with the only argument "p".
""SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)""
At the end of pseudo-block the function "f" is called
with the implicit context argument (if any), and "p".
""SAVESTACK_POS()""
The current offset on the Perl internal stack (cf.
"SP") is restored at the end of pseudo-block.
The following API list contains functions, thus one needs
to provide pointers to the modifiable data explicitly
(either C pointers, or Perlish "GV *"s). Where the above
macros take "int", a similar function takes "int *".
""SV* save_scalar(GV *gv)""
Equivalent to Perl code "local $gv".
""AV* save_ary(GV *gv)""
""HV* save_hash(GV *gv)""
Similar to "save_scalar", but localize "@gv" and
"%gv".
""void save_item(SV *item)""
Duplicates the current value of "SV", on the exit from
the current "ENTER"/"LEAVE" pseudo-block will restore
the value of "SV" using the stored value.
""void save_list(SV **sarg, I32 maxsarg)""
A variant of "save_item" which takes multiple argu
ments via an array "sarg" of "SV*" of length
"maxsarg".
""SV* save_svref(SV **sptr)""
Similar to "save_scalar", but will reinstate a "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 inter
ested 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 equiva
lent.
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 argu
ments, 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 the macros to push IVs, doubles, strings, and
SV pointers respectively:
PUSHi(IV)PUSHn(double)
PUSHp(char*, I32)
PUSHs(SV*)
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 val
ues on the stack is to use the macros:
XPUSHi(IV)XPUSHn(double)
XPUSHp(char*, I32)
XPUSHs(SV*)
These macros automatically adjust the stack for you, if
needed. Thus, you do not need to call "EXTEND" to extend
the stack. However, see the Putting a C value on Perl
stack entry elsewhere in this document
For more information, consult the perlxs manpage and the
perlxstut manpage.
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, register char**);
The routine most often used is "call_sv". The "SV*" argu
ment 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 subrou
tine 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 the perlcall manpage.
Memory Allocation
All memory meant to be used with the Perl API functions
should be manipulated using the macros described in this
section. The macros provide the necessary transparency
between differences in the actual malloc implementation
that is used within perl.
It is suggested that you enable the version of malloc that
is distributed with Perl. It keeps pools of various sizes
of unallocated memory in order to satisfy allocation
requests more quickly. However, on some platforms, it may
cause spurious malloc or free errors.
New(x, pointer, number, type);
Newc(x, pointer, number, type, cast);
Newz(x, pointer, number, type);
These three macros are used to initially allocate memory.
The first argument "x" was a "magic cookie" that was used
to keep track of who called the macro, to help when debug
ging memory problems. However, the current code makes no
use of this feature (most Perl developers now use run-time
memory checkers), so this argument can be any number.
The second argument "pointer" should be the name of a
variable that will point to the newly allocated memory.
The third and fourth 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 "Newc", "cast", should be used if
the "pointer" argument is different from the "type" argu
ment.
Unlike the "New" and "Newc" macros, the "Newz" macro calls
"memzero" to zero out all the newly allocated memory.
Renew(pointer, number, type);
Renewc(pointer, number, type, cast);
Safefree(pointer)
These three macros are used to change a memory buffer size
or to free a piece of memory no longer needed. The argu
ments to "Renew" and "Renewc" match those of "New" and
"Newc" with the exception of not needing the "magic
cookie" argument.
Move(source, dest, number, type);
Copy(source, dest, number, type);
Zero(dest, number, type);
These three macros are used to move, copy, or zero out
previously allocated memory. The "source" and "dest"
arguments point to the source and destination starting
points. Perl will move, copy, or zero out "number"
instances of the size of the "type" data structure (using
the "sizeof" function).
PerlIO
The most recent development releases of Perl has been
experimenting with removing Perl's dependency on the "nor
mal" standard I/O suite and allowing other stdio implemen
tations to be used. This involves creating a new abstrac
tion 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, con
sult the perlapio manpage.
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. How
ever, 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 con
stantly freed/created.
Each of the targets is created only once (but see the
Scratchpads and recursion entry elsewhere in this document
below), and when an opcode needs to put an integer, a dou
ble, 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[pni]".
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, use
"XPUSHs", which bypasses "TARG".
On a related note, if you do use "(X)PUSH[npi]", then
you're going to need a "dTARG" in your variable declara
tions so that the "*PUSH*" macros can make use of the
local variable "TARG".
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 com
piled. During this time a special anonymous Perl array is
created, which is called a scratchpad for the current
unit.
A scratchpad keeps SVs which are lexicals for the current
unit and are targets for opcodes. One can deduce that an
SV lives on a scratchpad by looking on its flags: lexicals
have "SVs_PADMY" set, and targets have "SVs_PADTMP" set.
The correspondence between OPs and targets is not 1-to-1.
Different OPs in the compile tree of the unit can use the
same target, if this would not conflict with the expected
life of the temporary.
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 ele
ment is the scratchpad AV. Why do we need an extra level
of indirection?
The answer is recursion, and maybe (sometime soon)
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 par
ent 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.
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 + $c" it is
different: some nodes optimized away. As a corollary,
though the actual tree contains more nodes than our sim
plified example, the execution order is the same as in our
example.
Examining the tree
If you have your perl compiled for debugging (usually done
with "-D optimize=-g" on "Configure" command line), you
may examine the compiled tree by specifying "-Dx" on the
Perl command line. The output takes several lines per
node, and for "$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 corre
spond 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 list
ing), i.e., "gvsv gvsv add whatever".
Each of these nodes represents an op, a fundamental opera
tion 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 differ
ent 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 vari
ous 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 opera
tors ("BINOP"s) have not only an "op_first" field but also
an "op_last" field. The most complex type of op is a
"LISTOP", which has any number of children. In this case,
the first child is pointed to by "op_first" and the last
child by "op_last". The children in between can be found
by iteratively following the "op_sibling" pointer from the
first child to the last.
There are also two other op types: a "PMOP" holds a regu
lar 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 the Compile pass 2: context propagation entry else
where in this document) it will still have children in
accordance with its former type.
Compile pass 1: check routines
The tree is created by the compiler while yacc code feeds
it the constructions it recognizes. Since yacc works bot
tom-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 con
structed except for the execution-order thread. Since at
this time there are no back-links to the currently con
structed 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 mod
ified, 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 con
text 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 con
tains back-references (via "thread" pointers), nodes can
not 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). These optimizations are done in the sub
routine peep(). Optimizations performed at this stage are
subject to the same restrictions as in the pass 2.
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 similiar to
"perl -Dx"; in fact, "Perl_dump_eval" will dump the main
root of the code being evaluated, exactly like "-Dx".
Other useful functions are "Perl_dump_sub", which turns a
"GV" into an op tree, "Perl_dump_packsubs" which calls
"Perl_dump_sub" on all the subroutines in a package like
so: (Thankfully, these are all xsubs, so there is no op
tree)
(gdb) print Perl_dump_packsubs(PL_defstash)
SUB attributes::bootstrap = (xsub 0x811fedc 0)
SUB UNIVERSAL::can = (xsub 0x811f50c 0)
SUB UNIVERSAL::isa = (xsub 0x811f304 0)
SUB UNIVERSAL::VERSION = (xsub 0x811f7ac 0)
SUB DynaLoader::boot_DynaLoader = (xsub 0x805b188 0)
and "Perl_dump_all", which dumps all the subroutines in
the stash and the op tree of the main root.
How multiple interpreters and concurrency are supported
Background and PERL_IMPLICIT_CONTEXT
The Perl interpreter can be regarded as a closed box: it
has an API for feeding it code or otherwise making it do
things, but it also has functions for its own use. This
smells a lot like an object, and there are ways for you to
build Perl so that you can have multiple interpreters,
with one interpreter represented either as a C++ object, a
C structure, or inside a thread. The thread, the C struc
ture, or the C++ object will contain all the context, the
state of that interpreter.
Three macros control the major Perl build flavors: MULTI
PLICITY, USE_THREADS and PERL_OBJECT. The MULTIPLICITY
build has a C structure that packages all the interpreter
state, there is a similar thread-specific data structure
under USE_THREADS, and the (now deprecated) PERL_OBJECT
build has a C++ class to maintain interpreter state. In
all three cases, PERL_IMPLICIT_CONTEXT is also normally
defined, and enables the support for passing in a "hidden"
first argument that represents all three data structures.
All this obviously requires a way for the Perl internal
functions to be C++ methods, subroutines taking some kind
of structure as the first argument, or subroutines taking
nothing as the first argument. To enable these three 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 the Internal Functions entry
elsewhere in this document.) The easiest way to be sure a
function is part of the API is to find its entry in the
perlapi manpage. If it exists in the perlapi manpage,
it's part of the API. If it doesn't, and you think it
should be (i.e., you need it for your extension), send
mail via the perlbug manpage 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 is #define'd to nothing
in C++.
A public function (i.e. part of the internal API, but not
necessarily sanctioned for use in extensions) begins like
this:
void
Perl_sv_setsv(pTHX_ SV* dsv, SV* ssv)
"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 contain
ing 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 addi
tional explicit arguments.
When a core function calls another, it must pass the con
text. This is normally hidden via macros. Consider
"sv_setsv". It expands into something like this:
ifdef PERL_IMPLICIT_CONTEXT
define sv_setsv(a,b) Perl_sv_setsv(aTHX_ a, b)
/* can't do this for vararg functions, see below */
else
define sv_setsv Perl_sv_setsv
endif
This works well, and means that XS authors can gleefully
write:
sv_setsv(foo, bar);
and still have it work under all the modes Perl could have
been compiled with.
Under PERL_OBJECT in the core, that will translate to
either:
CPerlObj::Perl_sv_setsv(foo,bar); # in CPerlObj functions,
# C++ takes care of 'this'
or
pPerl->Perl_sv_setsv(foo,bar); # in truly static functions,
# see objXSUB.h
Under PERL_OBJECT in extensions (aka PERL_CAPI), or under
MULTIPLICITY/USE_THREADS with PERL_IMPLICIT_CONTEXT in
both core and extensions, it will become:
Perl_sv_setsv(aTHX_ foo, bar); # the canonical Perl "API"
# for all build flavors
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 argu
ment. Instead it does dTHX; to get the context from
thread-local storage. We "#define warner
Perl_warner_nocontext" so that extensions get source com
patibility at the expense of performance. (Passing an arg
is cheaper than grabbing it from thread-local storage.)
You can ignore [pad]THX[xo] when browsing the Perl head
ers/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_setsv(asv, bsv);
in your extension will translate to this when
PERL_IMPLICIT_CONTEXT is in effect:
Perl_sv_setsv(Perl_get_context(), asv, bsv);
or to this otherwise:
Perl_sv_setsv(asv, bsv);
You have to do nothing new in your extension to get this;
since the Perl library provides Perl_get_context(), it
will all just work.
The second, more efficient way is to use the following
template for your Foo.xs:
#define PERL_NO_GET_CONTEXT /* we want efficiency */
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
static my_private_function(int arg1, int arg2);
static SV *
my_private_function(int arg1, int arg2)
{
dTHX; /* fetch context */
... call many Perl API functions ...
}
[... etc ...]
MODULE = Foo PACKAGE = Foo
/* typical XSUB */
void
my_xsub(arg)
int arg
CODE:
my_private_function(arg, 10);
Note that the only two changes from the normal way of
writing an extension is the addition of a "#define
PERL_NO_GET_CONTEXT" before including the Perl headers,
followed by a "dTHX;" declaration at the start of every
function that will call the Perl API. (You'll know which
functions need this, because the C compiler will complain
that there's an undeclared identifier in those functions.)
No changes are needed for the XSUBs themselves, because
the XS() macro is correctly defined to pass in the
implicit context if needed.
The third, even more efficient way is to ape how it is
done within the Perl guts:
#define PERL_NO_GET_CONTEXT /* we want efficiency */
#include "EXTERN.h"
#include "perl.h"
#include "XSUB.h"
/* pTHX_ only needed for functions that call Perl API */
static my_private_function(pTHX_ int arg1, int arg2);
static SV *
my_private_function(pTHX_ int arg1, int arg2)
{
/* dTHX; not needed here, because THX is an argument */
... call Perl API functions ...
}
[... etc ...]
MODULE = Foo PACKAGE = Foo
/* typical XSUB */
void
my_xsub(arg)
int arg
CODE:
my_private_function(aTHX_ arg, 10);
This implementation never has to fetch the context using a
function call, since it is always passed as an extra argu
ment. Depending on your needs for simplicity or effi
ciency, 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 auto
matically set the TLS slot to the interpreter they cre
ated, 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 inter
preter. 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 inter
preter to bundle up everything it knows about the environ
ment it's running on. This is enabled with the
PERL_IMPLICIT_SYS macro. Currently it only works with
PERL_OBJECT and USE_THREADS on Windows (see inside iperl
sys.h).
This allows the ability to provide an extra pointer
(called the "host" environment) for all the system calls.
This makes it possible for all the system stuff to main
tain 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 dif
ferent 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 be 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 func
tions start with "S_")
Inside the Perl core, you can get at the functions either
with or without the "Perl_" prefix, thanks to a bunch of
defines that live in embed.h. This header file is gener
ated automatically from embed.pl. 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 at the end of embed.pl as well. Here's a sample
entry from that table:
Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval
The second column is the return type, the third column the
name. Columns after that are the arguments. The first col
umn is a set of flags:
A This function is a part of the public API.
p This function has a "Perl_" prefix; ie, it is defined
as "Perl_av_fetch"
d This function has documentation using the "apidoc" fea
ture which we'll look at in a second.
Other available flags are:
s This is a static function and is defined as "S_what
ever", and usually called within the sources as "what
ever(...)".
n This does not use "aTHX_" and "pTHX" to pass inter
preter context. (See the Background and
PERL_IMPLICIT_CONTEXT entry in the perlguts manpage.)
r This function never returns; "croak", "exit" and
friends.
f This function takes a variable number of arguments,
"printf" style. The argument list should end with
"...", like this:
Afprd |void |croak |const char* pat|...
M This function is part of the experimental development
API, and may change or disappear without notice.
o This function should not have a compatibility macro to
define, say, "Perl_parse" to "parse". It must be called
as "Perl_parse".
j This function is not a member of "CPerlObj". If you
don't know what this means, don't use it.
x This function isn't exported out of the Perl core.
If you edit embed.pl, 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.
If you are printing addresses of pointers, use UVxf com
bined with PTR2UV(), do not use %lx or %p.
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);
Source Documentation
There's an effort going on to document the internal func
tions and automatically produce reference manuals from
them - the perlapi manpage is one such manual which
details all the functions which are available to XS writ
ers. the perlintern manpage is the autogenerated manual
for the functions which are not part of the API and are
supposedly for internal use only.
Source documentation is created by putting POD comments
into the C source, like this:
/*
=for apidoc sv_setiv
Copies an integer into the given SV. Does not handle 'set' magic. See
C<sv_setiv_mg>.
=cut
*/
Please try and supply some documentation if you add func
tions to the Perl core.
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 vari
ants 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 rep
resenting these characters, and the one Perl uses is
called UTF8. UTF8 uses a variable number of bytes to rep
resent a character, instead of just one. You can learn
more about Unicode at http://www.unicode.org/
How can I recognise a UTF8 string?
You can't. This is because UTF8 data is stored in bytes
just like non-UTF8 data. The Unicode character 200,
("0xC8" for you hex types) capital E with a grave accent,
is represented by the two bytes "v196.172". Unfortunately,
the non-Unicode string "chr(196).chr(172)" has that byte
sequence as well. So you can't tell just by looking - this
is what makes Unicode input an interesting problem.
The API function "is_utf8_string" can help; it'll tell you
if a string contains only valid UTF8 characters. However,
it can't do the work for you. On a character-by-character
basis, "is_utf8_char" will tell you whether the current
character in a string is valid UTF8.
How does UTF8 represent Unicode characters?
As mentioned above, UTF8 uses a variable number of bytes
to store a character. Characters with values 1...128 are
stored in one byte, just like good ol' ASCII. Character
129 is stored as "v194.129"; this continues up to charac
ter 191, which is "v194.191". Now we've run out of bits
(191 is binary "10111111") so we move on; 192 is
"v195.128". And so it goes on, moving to three bytes at
character 2048.
Assuming you know you're dealing with a UTF8 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 UTF8 string is to
use "utf8_hop", which takes a string and a number of char
acters to skip over. You're on your own about bounds
checking, though, so don't use it lightly.
All bytes in a multi-byte UTF8 character will have the
high bit set, so you can test if you need to do something
special with this character like this:
UV uv;
if (utf & 0x80)
/* Must treat this as UTF8 */
uv = utf8_to_uv(utf);
else
/* OK to treat this character as a byte */
uv = *utf;
You can also see in that example that we use "utf8_to_uv"
to get the value of the character; the inverse function
"uv_to_utf8" is available for putting a UV into UTF8:
if (uv > 0x80)
/* Must treat this as UTF8 */
utf8 = uv_to_utf8(utf8, uv);
else
/* OK to treat this character as a byte */
*utf8++ = uv;
You must convert characters to UVs using the above func
tions if you're ever in a situation where you have to
match UTF8 and non-UTF8 characters. You may not skip over
UTF8 characters in this case. If you do this, you'll lose
the ability to match hi-bit non-UTF8 characters; for
instance, if your UTF8 string contains "v196.172", and you
skip that character, you can never match a "chr(200)" in a
non-UTF8 string. So don't do that!
How does Perl store UTF8 strings?
Currently, Perl deals with Unicode strings and non-Unicode
strings slightly differently. If a string has been identi
fied as being UTF-8 encoded, Perl will set a flag in the
SV, "SVf_UTF8". You can check and manipulate this flag
with the following macros:
SvUTF8(sv)SvUTF8_on(sv)SvUTF8_off(sv)
This flag has an important effect on Perl's treatment of
the string: if Unicode data is not properly distinguished,
regular expressions, "length", "substr" and other string
handling operations will have undesirable results.
The problem comes when you have, for instance, a string
that isn't flagged is UTF8, and contains a byte sequence
that could be UTF8 - especially when combining non-UTF8
and UTF8 strings.
Never forget that the "SVf_UTF8" flag is separate to the
PV value; you need be sure you don't accidentally knock it
off while you're manipulating SVs. More specifically, you
cannot expect to do this:
SV *sv;
SV *nsv;
STRLEN len;
char *p;
p = SvPV(sv, len);
frobnicate(p);
nsv = newSVpvn(p, len);
The "char*" string does not tell you the whole story, and
you can't copy or reconstruct an SV just by copying the
string value. Check if the old SV has the UTF8 flag set,
and act accordingly:
p = SvPV(sv, len);
frobnicate(p);
nsv = newSVpvn(p, len);
if (SvUTF8(sv))
SvUTF8_on(nsv);
In fact, your "frobnicate" function should be made aware
of whether or not it's dealing with UTF8 data, so that it
can handle the string appropriately.
How do I convert a string to UTF8?
If you're mixing UTF8 and non-UTF8 strings, you might find
it necessary to upgrade one of the strings to UTF8. If
you've got an SV, the easiest way to do this is:
sv_utf8_upgrade(sv);
However, you must not do this, for example:
if (!SvUTF8(left))
sv_utf8_upgrade(left);
If you do this in a binary operator, you will actually
change one of the strings that came into the operator,
and, while it shouldn't be noticeable by the end user, it
can cause problems.
Instead, "bytes_to_utf8" will give you a UTF8-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.
Is there anything else I need to know?
Not really. Just remember these things:
There's no way to tell if a string is UTF8 or not. You
can tell if an SV is UTF8 by looking at is "SvUTF8"
flag. Don't forget to set the flag if something should
be UTF8. 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 UTF8, always use "utf8_to_uv" to get at
the value, unless "!(*s & 0x80)" in which case you can
use "*s".
When writing to a UTF8 string, always use "uv_to_utf8",
unless "uv < 0x80" in which case you can use "*s = uv".
Mixing UTF8 and non-UTF8 strings is tricky. Use
"bytes_to_utf8" to get a new string which is UTF8
encoded. There are tricks you can use to delay deciding
whether you need to use a UTF8 string until you get to
a high character - "HALF_UPGRADE" is one of those.
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, Mal
colm Beattie, Andreas Koenig, Paul Hudson, Ilya Zakhare
vich, Paul Marquess, Neil Bowers, Matthew Green, Tim
Bunce, Spider Boardman, Ulrich Pfeifer, Stephen McCamant,
and Gurusamy Sarathy.
API Listing originally by Dean Roehrich
<roehrich@cray.com>.
Modifications to autogenerate the API listing (the perlapi
manpage) by Benjamin Stuhl.
SEE ALSOperlapi(1), perlintern(1), perlxs(1), perlembed(1)2001-04-07 perl v5.6.1 PERLGUTS(1)
