=head1 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
+to provide some info on the basic workings of the Perl core. It is far
from complete and probably contains many errors. Please refer any
questions or comments to the author below.
Perl also uses two special typedefs, I32 and I16, which will always be at
least 32-bits and 16-bits long, respectively. (Again, there are U32 and U16,
-as well.)
+as well.) They will usually be exactly 32 and 16 bits long, but on Crays
+they will both be 64 bits.
=head2 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).
+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).
-The six routines are:
+The seven routines are:
SV* newSViv(IV);
+ SV* newSVuv(UV);
SV* newSVnv(double);
- SV* newSVpv(const char*, int);
- SV* newSVpvn(const char*, int);
+ SV* newSVpv(const char*, STRLEN);
+ SV* newSVpvn(const char*, STRLEN);
SV* newSVpvf(const char*, ...);
SV* newSVsv(SV*);
-To change the value of an *already-existing* SV, there are seven routines:
+C<STRLEN> is an integer type (Size_t, usually defined as size_t in
+F<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 initialisation, you
+can create an empty SV with newSV(len). If C<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 value undef.
+
+ SV *sv = newSV(0); /* no storage allocated */
+ SV *sv = newSV(10); /* 10 (+1) bytes of uninitialised storage allocated */
+
+To change the value of an I<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*, int)
+ void sv_setpvn(SV*, const char*, STRLEN)
void sv_setpvf(SV*, const char*, ...);
- void sv_setpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool);
+ void sv_vsetpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool *);
void sv_setsv(SV*, SV*);
Notice that you can choose to specify the length of the string to be
The arguments of C<sv_setpvf> are processed like C<sprintf>, and the
formatted output becomes the value.
-C<sv_setpvfn> is an analogue of C<vsprintf>, but it allows you to specify
+C<sv_vsetpvfn> is an analogue of C<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
len);>. It might work with your compiler, but it won't work for everyone.
Break this sort of statement up into separate assignments:
- SV *s;
- STRLEN len;
- char * ptr;
- ptr = SvPV(s, len);
- foo(ptr, len);
+ 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:
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_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
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);
+ SV* get_sv("package::varname", 0);
This returns NULL if the variable does not exist.
SvOK(SV*)
-The scalar C<undef> value is stored in an SV instance called C<PL_sv_undef>. Its
-address can be used whenever an C<SV*> is needed.
+The scalar C<undef> value is stored in an SV instance called C<PL_sv_undef>.
+
+Its address can be used whenever an C<SV*> is needed. Make sure that
+you don't try to compare a random sv with C<&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 C<&PL_sv_undef> as a value in
+AVs or HVs (see L<AVs, HVs and undefined values>).
-There are also the two values C<PL_sv_yes> and C<PL_sv_no>, which contain Boolean
-TRUE and FALSE values, respectively. Like C<PL_sv_undef>, their addresses can
-be used whenever an C<SV*> is needed.
+There are also the two values C<PL_sv_yes> and C<PL_sv_no>, which contain
+boolean TRUE and FALSE values, respectively. Like C<PL_sv_undef>, their
+addresses can be used whenever an C<SV*> is needed.
Do not be fooled into thinking that C<(SV *) 0> is the same as C<&PL_sv_undef>.
Take this code:
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 C<&PL_sv_undef> in the first
-line and all will be well.
+bus error, or just weird results. Change the zero to C<&PL_sv_undef> in the
+first line and all will be well.
To free an SV that you've created, call C<SvREFCNT_dec(SV*)>. Normally this
call is not necessary (see L<Reference Counts and Mortality>).
Perl provides the function C<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
+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, C<sv_chop> sets the flag C<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 C<SvPVX>) forward that
-many bytes, and adjusts C<SvCUR> and C<SvLEN>.
+many bytes, and adjusts C<SvCUR> and C<SvLEN>.
Hence, at this point, the start of the buffer that we allocated lives
at C<SvPVX(sv) - SvIV(sv)> in memory and the PV pointer is pointing
shown in parentheses, and the values of C<SvCUR> and C<SvLEN> reflect
the fake beginning, not the real one.
+Something similar to the offset hack is performed on AVs to enable
+efficient shifting and splicing off the beginning of the array; while
+C<AvARRAY> points to the first element in the array that is visible from
+Perl, C<AvALLOC> points to the real start of the C array. These are
+usually the same, but a C<shift> operation can be carried out by
+increasing C<AvARRAY> by one and decreasing C<AvFILL> and C<AvMAX>.
+Again, the location of the real start of the C array only comes into
+play when freeing the array. See C<av_shift> in F<av.c>.
+
=head2 What's Really Stored in an SV?
Recall that the usual method of determining the type of scalar you have is
These will tell you if you truly have an integer, double, or string pointer
stored in your SV. The "p" stands for private.
+The are various ways in which the private and public flags may differ.
+For example, 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. 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 C<Sv*V> macros.
=head2 Working with AVs
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);
+ AV* get_av("package::varname", 0);
This returns NULL if the variable does not exist.
/* 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
+ /* 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,
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);
+ HV* get_hv("package::varname", 0);
This returns NULL if the variable does not exist.
If these functions return a NULL value, the caller will usually have to
decrement the reference count of C<val> to avoid a memory leak.
+=head2 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 C<&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. AVs use C<&PL_sv_undef> as a marker
+for indicating that an array element has not yet been initialized.
+Thus, C<exists $av[0]> would be true for the above Perl code, but
+false for the array generated by the XS code.
+
+Other problems can occur when storing C<&PL_sv_undef> in HVs:
+
+ hv_store( hv, "key", 3, &PL_sv_undef, 0 );
+
+This will indeed make the value C<undef>, but if you try to modify
+the value of C<key>, you'll get the following error:
+
+ Modification of non-creatable hash value attempted
+
+In perl 5.8.0, C<&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 C<hv_exists> function.
+
+You can run into similar problems when you store C<&PL_sv_true> or
+C<&PL_sv_false> 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
+C<&PL_sv_undef>, C<&PL_sv_true> and C<&PL_sv_false> 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 C<&PL_sv_undef>, but rather create a
+new undefined value using the C<newSV> function, for example:
+
+ av_store( av, 42, newSV(0) );
+ hv_store( hv, "foo", 3, newSV(0), 0 );
+
=head2 References
References are a special type of scalar that point to other data types
SVt_PVGV Glob (possible a file handle)
SVt_PVMG Blessed or Magical Scalar
- See the sv.h header file for more details.
+See the F<sv.h> header file for more details.
=head2 Blessed References and Class Objects
-References are also used to support object-oriented programming. In the
+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.
SV* sv_bless(SV* sv, HV* stash);
-The C<sv> argument must be a reference. The C<stash> argument specifies
-which class the reference will belong to. See
+The C<sv> argument must be a reference value. The C<stash> argument
+specifies which class the reference will belong to. See
L<Stashes and Globs> for information on converting class names into stashes.
/* Still under construction */
SV* newSVrv(SV* rv, const char* classname);
-Copies integer or double into an SV whose reference is C<rv>. SV is blessed
+Copies integer, unsigned integer or double into an SV whose reference is C<rv>. SV is blessed
if C<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);
Copies the pointer value (I<the address, not the string!>) into an SV whose
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 check if you've got an object derived from a specific class you have
to write:
if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }
To create a new Perl variable with an undef value which can be accessed from
your Perl script, use the following routines, depending on the variable type.
- SV* get_sv("package::varname", TRUE);
- AV* get_av("package::varname", TRUE);
- HV* get_hv("package::varname", TRUE);
+ 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 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
C<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.
+=over
+
+=item GV_ADDMULTI
+
+Marks the variable as multiply defined, thus preventing the:
+
+ Name <varname> used only once: possible typo
+
+warning.
+
+=item GV_ADDWARN
+
+Issues the warning:
+
+ Had to create <varname> unexpectedly
+
+if the variable did not exist before the function was called.
+
+=back
If you do not specify a package name, the variable is created in the current
package.
=head2 Reference Counts and Mortality
-Perl uses an reference count-driven garbage collection mechanism. SVs,
+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.
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 mortal within multiple contexts,
-or if you make a variable mortal multiple times.
+"Mortal" SVs are mainly used for SVs that are placed on perl's stack.
+For example an SV which is created just to pass a number to a called sub
+is made mortal to have it cleaned up automatically when it's popped off
+the stack. Similarly, results returned by XSUBs (which are pushed on the
+stack) are often made mortal.
To create a mortal variable, use the functions:
SV* sv_2mortal(SV*)
SV* sv_mortalcopy(SV*)
-The first call creates a mortal SV, the second converts an existing
+The first call creates a mortal SV (with no value), the second converts an existing
SV to a mortal SV (and thus defers a call to C<SvREFCNT_dec>), and the
third creates a mortal copy of an existing SV.
+Because C<sv_newmortal> gives the new SV no value,it must normally be given one
+via C<sv_setpv>, C<sv_setiv>, etc. :
+
+ SV *tmp = sv_newmortal();
+ sv_setiv(tmp, an_integer);
+
+As that is multiple C statements it is quite common so see this idiom instead:
+
+ SV *tmp = sv_2mortal(newSViv(an_integer));
+
+
+You should be careful about creating mortal variables. Strange things
+can happen if you make the same value mortal within multiple contexts,
+or if you make a variable mortal multiple times. Thinking of "Mortalization"
+as deferred C<SvREFCNT_dec> should help to minimize such problems.
+For example if you are passing an SV which you I<know> has high enough REFCNT
+to survive its use on the stack you need not do any mortalization.
+If you are not sure then doing an C<SvREFCNT_inc> and C<sv_2mortal>, or
+making a C<sv_mortalcopy> is safer.
The mortal routines are not just for SVs -- AVs and HVs can be
made mortal by passing their address (type-casted to C<SV*>) to the
=head2 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
+A B<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,
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.
+There is a single stash called C<PL_defstash> that holds the items that exist
+in the C<main> package. To get at the items in other packages, append the
+string "::" to the package name. The items in the C<Foo> package are in
+the stash C<Foo::> in PL_defstash. The items in the C<Bar::Baz> package are
+in the stash C<Baz::> in C<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)
+ 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
-C<HV*>. The C<create> flag will create a new package if it is set.
+C<HV*>. The C<flags> flag will create a new package if it is set to GV_ADD.
The name that C<gv_stash*v> wants is the name of the package whose symbol table
you want. The default package is called C<main>. If you have multiply nested
extern int dberror;
extern char *dberror_list;
- SV* sv = get_sv("dberror", TRUE);
+ SV* sv = get_sv("dberror", GV_ADD);
sv_setiv(sv, (IV) dberror);
sv_setpv(sv, dberror_list[dberror]);
SvIOK_on(sv);
U16 mg_private;
char mg_type;
U8 mg_flags;
+ I32 mg_len;
SV* mg_obj;
char* mg_ptr;
- I32 mg_len;
};
Note this is current as of patchlevel 0, and could change at any time.
feature.
If C<sv> is not already magical, Perl uses the C<SvUPGRADE> macro to
-set the C<SVt_PVMG> flag for the C<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
-associated with an SV.
+convert C<sv> to type C<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 C<name> and C<namlen> arguments are used to associate a string with
the magic, typically the name of a variable. C<namlen> is stored in the
-C<mg_len> field and if C<name> is non-null and C<namlen> >= 0 a malloc'd
-copy of the name is stored in C<mg_ptr> field.
+C<mg_len> field and if C<name> is non-null then either a C<savepvn> copy of
+C<name> or C<name> itself is stored in the C<mg_ptr> field, depending on
+whether C<namlen> is greater than zero or equal to zero respectively. As a
+special case, if C<(name && namlen == HEf_SVKEY)> then C<name> is assumed
+to contain an C<SV*> and is stored as-is with its REFCNT incremented.
The sv_magic function uses C<how> to determine which, if any, predefined
"Magic Virtual Table" should be assigned to the C<mg_virtual> field.
-See the "Magic Virtual Table" section below. The C<how> argument is also
-stored in the C<mg_type> field.
+See the L<Magic Virtual Tables> section below. The C<how> argument is also
+stored in the C<mg_type> field. The value of C<how> should be chosen
+from the set of macros C<PERL_MAGIC_foo> found in F<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 C<PERL_MAGIC_uvar> for example.
The C<obj> argument is stored in the C<mg_obj> field of the C<MAGIC>
structure. If it is not the same as the C<sv> argument, the reference
count of the C<obj> object is incremented. If it is the same, or if
-the C<how> argument is "#", or if it is a NULL pointer, then C<obj> is
-merely stored, without the reference count being incremented.
+the C<how> argument is C<PERL_MAGIC_arylen>, or if it is a NULL pointer,
+then C<obj> is merely stored, without the reference count being incremented.
+
+See also C<sv_magicext> in L<perlapi> for a more flexible way to add magic
+to an SV.
There is also a function to add magic to an C<HV>:
=head2 Magic Virtual Tables
-The C<mg_virtual> field in the C<MAGIC> structure is a pointer to a
+The C<mg_virtual> field in the C<MAGIC> structure is a pointer to an
C<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 C<MGVTBL> has five pointers to the following routine types:
+The C<MGVTBL> has five (or sometimes eight) pointers to the following
+routine types:
int (*svt_get)(SV* sv, MAGIC* mg);
int (*svt_set)(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 C<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.
+ int (*svt_copy)(SV *sv, MAGIC* mg, SV *nsv, const char *name, I32 namlen);
+ int (*svt_dup)(MAGIC *mg, CLONE_PARAMS *param);
+ int (*svt_local)(SV *nsv, MAGIC *mg);
+
+
+This MGVTBL structure is set at compile-time in F<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 after the value of the SV is retrieved.
+ 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_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 C<vtbl_sv> (which corresponds
-to an C<mg_type> of '\0') contains:
+to an C<mg_type> of C<PERL_MAGIC_sv>) 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 C<magic_get> is called. All
-the various routines for the various magical types begin with C<magic_>.
-NOTE: the magic routines are not considered part of the Perl API, and may
-not be exported by the Perl library.
+Thus, when an SV is determined to be magical and of type C<PERL_MAGIC_sv>,
+if a get operation is being performed, the routine C<magic_get> is
+called. All the various routines for the various magical types begin
+with C<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 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
+ mg_type
+ (old-style char and macro) MGVTBL Type of magic
+ -------------------------- ------ -------------
+ \0 PERL_MAGIC_sv vtbl_sv Special scalar variable
+ A PERL_MAGIC_overload vtbl_amagic %OVERLOAD hash
+ a PERL_MAGIC_overload_elem vtbl_amagicelem %OVERLOAD hash element
+ c PERL_MAGIC_overload_table (none) Holds overload table (AMT)
+ on stash
+ B PERL_MAGIC_bm vtbl_bm Boyer-Moore (fast string search)
+ 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_fm Formline ('compiled' format)
+ g PERL_MAGIC_regex_global vtbl_mglob m//g target / study()ed string
+ 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
+ o PERL_MAGIC_collxfrm vtbl_collxfrm Locale collate 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_qr precompiled qr// regex
+ S PERL_MAGIC_sig vtbl_sig %SIG hash
+ s PERL_MAGIC_sigelem vtbl_sigelem %SIG hash element
+ t PERL_MAGIC_taint vtbl_taint Taintedness
+ U PERL_MAGIC_uvar vtbl_uvar Available for use by extensions
+ v PERL_MAGIC_vec vtbl_vec vec() lvalue
+ V PERL_MAGIC_vstring (none) v-string scalars
+ w PERL_MAGIC_utf8 vtbl_utf8 UTF-8 length+offset cache
+ x PERL_MAGIC_substr vtbl_substr substr() lvalue
+ y PERL_MAGIC_defelem vtbl_defelem Shadow "foreach" iterator
+ variable / smart parameter
+ vivification
+ # PERL_MAGIC_arylen vtbl_arylen Array length ($#ary)
+ . PERL_MAGIC_pos vtbl_pos pos() lvalue
+ < PERL_MAGIC_backref vtbl_backref back pointer to a weak ref
+ ~ PERL_MAGIC_ext (none) Available for use by extensions
+ : PERL_MAGIC_symtab (none) hash used as symbol table
+ % PERL_MAGIC_rhash (none) hash used as restricted hash
+ @ PERL_MAGIC_arylen_p vtbl_arylen_p pointer to $#a from @a
+
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 lowercase letter is used to represent an element of
-that composite 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 C<MAGIC>'s
+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 C<PERL_MAGIC_ext> and C<PERL_MAGIC_uvar> magic types are defined
+specifically for use by extensions and will not be used by perl itself.
+Extensions can use C<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, C<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 C<MAGIC>'s
C<mg_ptr> field points to a C<ufuncs> structure:
struct ufuncs {
- I32 (*uf_val)(IV, SV*);
- I32 (*uf_set)(IV, SV*);
+ 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 C<uf_val> or C<uf_set>
-function will be called with C<uf_index> as the first arg and a
-pointer to the SV as the second. A simple example of how to add 'U'
+function will be called with C<uf_index> as the first arg and a pointer to
+the SV as the second. A simple example of how to add C<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.
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.
+ sv_magic(sv, 0, PERL_MAGIC_uvar, (char*)&uf, sizeof(uf));
+
+Attaching C<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 C<PERL_MAGIC_uvar> 'get' magic
+if the "set" function in the C<ufuncs> structure is NULL. The hook
+is activated whenever the hash is accessed with a key specified as
+an C<SV> through the functions C<hv_store_ent>, C<hv_fetch_ent>,
+C<hv_delete_ent>, and C<hv_exists_ent>. Accessing the key as a string
+through the functions without the C<..._ent> suffix circumvents the
+hook. See L<Hash::Util::Fieldhash/Guts> for a detailed description.
+
+Note that because multiple extensions may be using C<PERL_MAGIC_ext>
+or C<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 C<PERL_MAGIC_ext> 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 C<sv_set*()> and C<sv_cat*()> functions described
earlier do B<not> invoke 'set' magic on their targets. This must
=head2 Understanding the Magic of Tied Hashes and Arrays
-Tied hashes and arrays are magical beasts of the 'P' magic type.
+Tied hashes and arrays are magical beasts of the C<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
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
+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
CODE:
hash = newHV();
tie = newRV_noinc((SV*)newHV());
- stash = gv_stashpv("MyTie", TRUE);
+ stash = gv_stashpv("MyTie", GV_ADD);
sv_bless(tie, stash);
- hv_magic(hash, tie, 'P');
+ hv_magic(hash, (GV*)tie, PERL_MAGIC_tied);
RETVAL = newRV_noinc(hash);
OUTPUT:
RETVAL
The biggest difference is that the first construction would
reinstate the initial value of $var, irrespective of how control exits
-the block: C<goto>, C<return>, C<die>/C<eval> etc. It is a little bit
+the block: C<goto>, C<return>, C<die>/C<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
Inside such a I<pseudo-block> the following service is available:
-=over
+=over 4
=item C<SAVEINT(int i)>
=item C<SAVEFREESV(SV *sv)>
The refcount of C<sv> would be decremented at the end of
-I<pseudo-block>. This is similar to C<sv_2mortal>, which should (?) be
-used instead.
+I<pseudo-block>. This is similar to C<sv_2mortal> in that it is also a
+mechanism for doing a delayed C<SvREFCNT_dec>. However, while C<sv_2mortal>
+extends the lifetime of C<sv> until the beginning of the next statement,
+C<SAVEFREESV> extends it until the end of the enclosing scope. These
+lifetimes can be wildly different.
+
+Also compare C<SAVEMORTALIZESV>.
+
+=item C<SAVEMORTALIZESV(SV *sv)>
+
+Just like C<SAVEFREESV>, but mortalizes C<sv> at the end of the current
+scope instead of decrementing its reference count. This usually has the
+effect of keeping C<sv> alive until the statement that called the currently
+live scope has finished executing.
=item C<SAVEFREEOP(OP *op)>
The following API list contains functions, thus one needs to
provide pointers to the modifiable data explicitly (either C pointers,
-or Perlish C<GV *>s). Where the above macros take C<int>, a similar
+or Perlish C<GV *>s). Where the above macros take C<int>, a similar
function takes C<int *>.
-=over
+=over 4
=item C<SV* save_scalar(GV *gv)>
Duplicates the current value of C<SV>, on the exit from the current
C<ENTER>/C<LEAVE> I<pseudo-block> will restore the value of C<SV>
-using the stored value.
+using the stored value. It doesn't handle magic. Use C<save_scalar> if
+magic is affected.
=item C<void save_list(SV **sarg, I32 maxsarg)>
=item C<SV* save_svref(SV **sptr)>
-Similar to C<save_scalar>, but will reinstate a C<SV *>.
+Similar to C<save_scalar>, but will reinstate an C<SV *>.
=item C<void save_aptr(AV **aptr)>
where C<SP> is the macro that represents the local copy of the stack pointer,
and C<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:
+Now that there is room on the stack, values can be pushed on it using C<PUSHs>
+macro. The pushed values will often need to be "mortal" (See
+L</Reference Counts and Mortality>):
- PUSHi(IV)
- PUSHn(double)
- PUSHp(char*, I32)
- PUSHs(SV*)
+ 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)))
And now the Perl program calling C<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 macros:
+to use the macro:
- XPUSHi(IV)
- XPUSHn(double)
- XPUSHp(char*, I32)
XPUSHs(SV*)
-These macros automatically adjust the stack for you, if needed. Thus, you
+This macro automatically adjust the stack for you, if needed. Thus, you
do not need to call C<EXTEND> to extend the stack.
+Despite their suggestions in earlier versions of this document the macros
+C<(X)PUSH[iunp]> are I<not> suited to XSUBs which return multiple results.
+For that, either stick to the C<(X)PUSHs> macros shown above, or use the new
+C<m(X)PUSH[iunp]> macros instead; see L</Putting a C value on Perl stack>.
+
For more information, consult L<perlxs> and L<perlxstut>.
=head2 Calling Perl Routines from within C Programs
All four routines return the number of arguments that the subroutine returned
on the Perl stack.
-These routines used to be called C<perl_call_sv> etc., before Perl v5.6.0,
+These routines used to be called C<perl_call_sv>, etc., before Perl v5.6.0,
but those names are now deprecated; macros of the same name are provided for
compatibility.
=head2 Memory Allocation
+=head3 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
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 following three macros are used to initially allocate memory :
-The first argument C<x> was a "magic cookie" that was used to keep track
-of who called the macro, to help when debugging 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.
+ Newx(pointer, number, type);
+ Newxc(pointer, number, type, cast);
+ Newxz(pointer, number, type);
-The
second argument C<pointer> should be the name of a variable that will
+The
first argument C<pointer> should be the name of a variable that will
point to the newly allocated memory.
-The third and fourth arguments C<number> and C<type> specify how many of
+The second and third arguments C<number> and C<type> specify how many of
the specified type of data structure should be allocated. The argument
-C<type> is passed to C<sizeof>. The final argument to C<Newc>, C<cast>,
+C<type> is passed to C<sizeof>. The final argument to C<Newxc>, C<cast>,
should be used if the C<pointer> argument is different from the C<type>
argument.
-Unlike the C<New> and C<Newc> macros, the C<Newz> macro calls C<memzero>
+Unlike the C<Newx> and C<Newxc> macros, the C<Newxz> macro calls C<memzero>
to zero out all the newly allocated memory.
+=head3 Reallocation
+
Renew(pointer, number, type);
Renewc(pointer, number, type, cast);
Safefree(pointer)
match those of C<New> and C<Newc> with the exception of not needing the
"magic cookie" argument.
+=head3 Moving
+
Move(source, dest, number, type);
Copy(source, dest, number, type);
Zero(dest, number, type);
instances of the size of the C<type> data structure (using the C<sizeof>
function).
-Here is a handy table of equivalents between ordinary C and Perl's
-memory abstraction layer:
-
- Instead Of: Use:
-
- malloc New
- calloc Newz
- realloc Renew
- memcopy Copy
- memmove Move
- free Safefree
- strdup savepv
- strndup savepvn (Hey, strndup doesn't exist!)
- memcpy/*(struct foo *) StructCopy
-
=head2 PerlIO
The most recent development releases of Perl has been experimenting with
The macro to put this target on stack is C<PUSHTARG>, and it is
directly used in some opcodes, as well as indirectly in zillions of
-others, which use it via C<(X)PUSH[pni]>.
+others, which use it via C<(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 C<TARG> to 10, push a pointer to C<TARG> onto
+the stack; set C<TARG> to 20, push a pointer to C<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 C<TARG>, which we have set
+to 20.
+
+If you need to push multiple different values then you should either use
+the C<(X)PUSHs> macros, or else use the new C<m(X)PUSH[iunp]> macros,
+none of which make use of C<TARG>. The C<(X)PUSHs> macros simply push an
+SV* on the stack, which, as noted under L</XSUBs and the Argument Stack>,
+will often need to be "mortal". The new C<m(X)PUSH[iunp]> macros make
+this a little easier to achieve by creating a new mortal for you (via
+C<(X)PUSHmortal>), pushing that onto the stack (extending it if necessary
+in the case of the C<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 C<(X)PUSH[iunp]>, then you're going to
+need a C<dTARG> in your variable declarations so that the C<*PUSH*>
+macros can make use of the local variable C<TARG>. See also C<dTARGET>
+and C<dXSTARG>.
=head2 Scratchpads
(initially) one element, and this element is the scratchpad AV. Why do
we need an extra level of indirection?
-The answer is B<recursion>, and maybe (sometime soon) B<threads>. Both
+The answer is B<recursion>, and maybe B<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
=head2 Examining the tree
-If you have your perl compiled for debugging (usually done with C<-D
-optimize=-g> on C<Configure> command line), you may examine the
+If you have your perl compiled for debugging (usually done with
+C<-DDEBUGGING> on the C<Configure> command line), you may examine the
compiled tree by specifying C<-Dx> on the Perl command line. The
output takes several lines per node, and for C<$b+$c> it looks like
this:
4 5 6> (node C<6> is not included into above listing), i.e.,
C<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
+F<pp*.c> files; the function which implements the op with type C<gvsv>
+is C<pp_gvsv>, and so on. As the tree above shows, different ops have
+different numbers of children: C<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 C<OP>: this has no children. Unary
+operators, C<UNOP>s, have one child, and this is pointed to by the
+C<op_first> field. Binary operators (C<BINOP>s) have not only an
+C<op_first> field but also an C<op_last> field. The most complex type of
+op is a C<LISTOP>, which has any number of children. In this case, the
+first child is pointed to by C<op_first> and the last child by
+C<op_last>. The children in between can be found by iteratively
+following the C<op_sibling> pointer from the first child to the last.
+
+There are also two other op types: a C<PMOP> holds a regular expression,
+and has no children, and a C<LOOP> may or may not have children. If the
+C<op_children> field is non-zero, it behaves like a C<LISTOP>. To
+complicate matters, if a C<UNOP> is actually a C<null> op after
+optimization (see L</Compile pass 2: context propagation>) it will still
+have children in accordance with its former type.
+
+Another way to examine the tree is to use a compiler back-end module, such
+as L<B::Concise>.
+
=head2 Compile pass 1: check routines
The tree is created by the compiler while I<yacc> code feeds it
done in the subroutine peep(). Optimizations performed at this stage
are subject to the same restrictions as in the pass 2.
+=head2 Pluggable runops
+
+The compile tree is executed in a runops function. There are two runops
+functions, in F<run.c> and in F<dump.c>. C<Perl_runops_debug> is used
+with DEBUGGING and C<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.
+
+=head1 Examining internal data structures with the C<dump> functions
+
+To aid debugging, the source file F<dump.c> contains a number of
+functions which produce formatted output of internal data structures.
+
+The most commonly used of these functions is C<Perl_sv_dump>; it's used
+for dumping SVs, AVs, HVs, and CVs. The C<Devel::Peek> module calls
+C<sv_dump> to produce debugging output from Perl-space, so users of that
+module should already be familiar with its format.
+
+C<Perl_op_dump> can be used to dump an C<OP> structure or any of its
+derivatives, and produces output similar to C<perl -Dx>; in fact,
+C<Perl_dump_eval> will dump the main root of the code being evaluated,
+exactly like C<-Dx>.
+
+Other useful functions are C<Perl_dump_sub>, which turns a C<GV> into an
+op tree, C<Perl_dump_packsubs> which calls C<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 C<Perl_dump_all>, which dumps all the subroutines in the stash and
+the op tree of the main root.
+
=head1 How multiple interpreters and concurrency are supported
=head2 Background and PERL_IMPLICIT_CONTEXT
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 structure, or
-the C++ object will contain all the context, the state of that
-interpreter.
-
-Three macros control the major Perl build flavors: MULTIPLICITY,
-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 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.
+interpreters, with one interpreter represented either as a C structure,
+or inside a thread-specific structure. These structures contain all
+the context, the state of that interpreter.
+
+One macro controls the major Perl build flavor: MULTIPLICITY. The
+MULTIPLICITY build has a C structure that packages all the interpreter
+state. With multiplicity-enabled perls, PERL_IMPLICIT_CONTEXT is also
+normally defined, and enables the support for passing in a "hidden" first
+argument that represents all three data structures. MULTIPLICITY makes
+mutli-threaded perls possible (with the ithreads threading model, related
+to the macro USE_ITHREADS.)
+
+Two other "encapsulation" macros are the PERL_GLOBAL_STRUCT and
+PERL_GLOBAL_STRUCT_PRIVATE (the latter turns on the former, and the
+former turns on MULTIPLICITY.) The PERL_GLOBAL_STRUCT causes all the
+internal variables of Perl to be wrapped inside a single global struct,
+struct perl_vars, accessible as (globals) &PL_Vars or PL_VarsPtr or
+the function Perl_GetVars(). The PERL_GLOBAL_STRUCT_PRIVATE goes
+one step further, there is still a single struct (allocated in main()
+either from heap or from stack) but there are no global data symbols
+pointing to it. In either case the global struct should be initialised
+as the very first thing in main() using Perl_init_global_struct() and
+correspondingly tear it down after perl_free() using Perl_free_global_struct(),
+please see F<miniperlmain.c> for usage details. You may also need
+to use C<dVAR> in your coding to "declare the global variables"
+when you are using them. dTHX does this for you automatically.
+
+To see whether you have non-const data you can use a BSD-compatible C<nm>:
+
+ nm libperl.a | grep -v ' [TURtr] '
+
+If this displays any C<D> or C<d> symbols, you have non-const data.
+
+For backward compatibility reasons defining just PERL_GLOBAL_STRUCT
+doesn't actually hide all symbols inside a big global struct: some
+PerlIO_xxx vtables are left visible. The PERL_GLOBAL_STRUCT_PRIVATE
+then hides everything (see how the PERLIO_FUNCS_DECL is used).
All this obviously requires a way for the Perl internal functions to be
-C++ methods, subroutines taking some kind of structure as the first
+either 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,
+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 C<S_> are private
+which will be private. All functions whose names begin C<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 L</Internal Functions>.) The easiest way to be B<sure> a
-function is part of the API is to find its entry in L<perlapi>.
-If it exists in L<perlapi>, it's part of the API. If it doesn't, and you
-think it should be (i.e., you need it for your extension), send mail via
+part of the API. (See L</Internal Functions>.) The easiest way to be B<sure> a
+function is part of the API is to find its entry in L<perlapi>.
+If it exists in L<perlapi>, it's part of the API. If it doesn't, and you
+think it should be (i.e., you need it for your extension), send mail via
L<perlbug> explaining why you think it should be.
Second problem: there must be a syntax so that the same subroutine
STATIC void
S_incline(pTHX_ char *s)
-STATIC becomes "static" in C, and is #define'd to nothing in C++.
+STATIC becomes "static" in C, and may be #define'd to nothing in some
+configurations in 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_setsv(pTHX_ SV* dsv, SV* ssv)
+ Perl_sv_setiv(pTHX_ SV* dsv, IV num)
-C<pTHX_> is one of a number of macros (in perl.h) that hide the
+C<pTHX_> is one of a number of macros (in F<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 B<p>rototype, 'a' for B<a>rgument,
-or 'd' for B<d>eclaration.
+or 'd' for B<d>eclaration, so we have C<pTHX>, C<aTHX> and C<dTHX>, and
+their variants.
-When Perl is built without PERL_IMPLICIT_CONTEXT, there is no first
-argument containing the interpreter's context. The trailing underscore
+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
explicit arguments.
When a core function calls another, it must pass the context. This
-is normally hidden via macros. Consider C<sv_setsv>. It expands
+is normally hidden via macros. Consider C<sv_setiv>. It expands into
something like this:
- ifdef PERL_IMPLICIT_CONTEXT
- define sv_setsv(a,b) Perl_sv_setsv(aTHX_ a, b)
+ #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_setsv Perl_sv_setsv
- endif
+ #else
+ #define sv_setiv Perl_sv_setiv
+ #endif
This works well, and means that XS authors can gleefully write:
- sv_setsv(foo, bar);
+ sv_setiv(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 w/ PERL_IMPLICIT_CONTEXT in both core
-and extensions, it will be:
-
- 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 C<aTHX_> as the first
compatibility 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 headers/sources.
+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.
+=head2 So what happened to dTHR?
+
+C<dTHR> was introduced in perl 5.005 to support the older thread model.
+The older thread model now uses the C<THX> mechanism to pass context
+pointers around, so C<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.
+
=head2 How do I use all this in extensions?
When Perl is built with PERL_IMPLICIT_CONTEXT, extensions that call
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
+with extensions: whenever F<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);
+ sv_setiv(sv, num);
in your extension will translate to this when PERL_IMPLICIT_CONTEXT is
in effect:
- Perl_sv_setsv(Perl_get_context(), asv, bsv);
+ Perl_sv_setiv(Perl_get_context(), sv, num);
or to this otherwise:
- Perl_sv_setsv(asv, bsv);
+ Perl_sv_setiv(sv, num);
You have to do nothing new in your extension to get this; since
the Perl library provides Perl_get_context(), it will all just
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"
+ #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 void my_private_function(int arg1, int arg2);
- static SV *
- my_private_function(int arg1, int arg2)
- {
- dTHX; /* fetch context */
- ... call many Perl API functions ...
- }
+ STATIC void
+ my_private_function(int arg1, int arg2)
+ {
+ dTHX; /* fetch context */
+ ... call many Perl API functions ...
+ }
[... etc ...]
- MODULE = Foo PACKAGE = Foo
+ MODULE = Foo PACKAGE = Foo
- /* typical XSUB */
+ /* typical XSUB */
- void
- my_xsub(arg)
- int arg
- CODE:
- my_private_function(arg, 10);
+ 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 C<#define PERL_NO_GET_CONTEXT> before
the Perl guts:
- #define PERL_NO_GET_CONTEXT /* we want efficiency */
- #include "EXTERN.h"
- #include "perl.h"
- #include "XSUB.h"
+ #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 void 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 ...
- }
+ 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
+ MODULE = Foo PACKAGE = Foo
- /* typical XSUB */
+ /* typical XSUB */
- void
- my_xsub(arg)
- int arg
- CODE:
- my_private_function(aTHX_ arg, 10);
+ 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
macro with the underscore for functions that take explicit arguments,
or the form without the argument for functions with no explicit arguments.
+If one is compiling Perl with the C<-DPERL_GLOBAL_STRUCT> the C<dVAR>
+definition is needed if the Perl global variables (see F<perlvars.h>
+or F<globvar.sym>) are accessed in the function and C<dTHX> is not
+used (the C<dTHX> includes the C<dVAR> if necessary). One notices
+the need for C<dVAR> only with the said compile-time define, because
+otherwise the Perl global variables are visible as-is.
+
+=head2 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 C<perl_alloc> and C<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 C<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 ...
+
=head2 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 PERL_OBJECT,
-but is mostly there for MULTIPLICITY and USE_THREADS (see inside
-iperlsys.h).
+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
+calls (see F<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.
=head1 Internal Functions
All of Perl's internal functions which will be exposed to the outside
-world are
be prefixed by C<Perl_> so that they will not conflict with XS
+world are prefixed by C<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 C<PL_>. (By convention,
-static functions start with C<S_>)
+static functions start with C<S_>.)
Inside the Perl core, you can get at the functions either with or
without the C<Perl_> prefix, thanks to a bunch of defines that live in
F<embed.h>. This header file is generated automatically from
-F<embed.pl>. F<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 F<embed.pl> as well. Here's a sample entry from that table:
+F<embed.pl> and F<embed.fnc>. F<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 F<embed.fnc> as well. Here's a sample entry from
+that table:
Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval
=item A
-This function is a part of the public API.
+This function is a part of the public API. All such functions should also
+have 'd', very few do not.
=item p
-This function has a C<Perl_> prefix; ie, it is defined as C<Perl_av_fetch>
+This function has a C<Perl_> prefix; i.e. it is defined as
+C<Perl_av_fetch>.
=item d
This function has documentation using the C<apidoc> feature which we'll
-look at in a second.
+look at in a second. Some functions have 'd' but not 'A'; docs are good.
=back
=item s
-This is a static function and is defined as C<S_whatever>.
+This is a static function and is defined as C<STATIC S_whatever>, and
+usually called within the sources as C<whatever(...)>.
=item n
-This does not use C<aTHX_> and C<pTHX> to pass interpreter context. (See
+This does not need a interpreter context, so the definition has no
+C<pTHX>, and it follows that callers don't use C<aTHX>. (See
L<perlguts/Background and PERL_IMPLICIT_CONTEXT>.)
=item r
Afprd |void |croak |const char* pat|...
-=item m
+=item M
-This function is part of the experimental development API, and may change
+This function is part of the experimental development API, and may change
or disappear without notice.
=item o
This function should not have a compatibility macro to define, say,
C<Perl_parse> to C<parse>. It must be called as C<Perl_parse>.
-=item j
-
-This function is not a member of C<CPerlObj>. If you don't know
-what this means, don't use it.
-
=item x
This function isn't exported out of the Perl core.
+=item m
+
+This is implemented as a macro.
+
+=item X
+
+This function is explicitly exported.
+
+=item E
+
+This function is visible to extensions included in the Perl core.
+
+=item b
+
+Binary backward compatibility; this function is a macro but also has
+a C<Perl_> implementation (which is exported).
+
+=item others
+
+See the comments at the top of C<embed.fnc> for others.
+
=back
-If you edit F<embed.pl>, you will need to run C<make regen_headers> to
-force a rebuild of F<embed.h> and other auto-generated files.
+If you edit F<embed.pl> or F<embed.fnc>, you will need to run
+C<make regen_headers> to force a rebuild of F<embed.h> and other
+auto-generated files.
=head2 Formatted Printing of IVs, UVs, and NVs
formatting codes like C<%d>, C<%ld>, C<%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
+ 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);
+ printf("IV is %"IVdf"\n", iv);
The IVdf will expand to whatever is the correct format for the IVs.
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)
+ PTR2UV(pointer)
+ PTR2IV(pointer)
+ PTR2NV(pointer)
+ INT2PTR(pointertotype, integer)
For example:
- IV iv = ...;
- SV *sv = INT2PTR(SV*, iv);
+ IV iv = ...;
+ SV *sv = INT2PTR(SV*, iv);
and
- AV *av = ...;
- UV uv = PTR2UV(av);
+ AV *av = ...;
+ UV uv = PTR2UV(av);
+
+=head2 Exception Handling
+
+There are a couple of macros to do very basic exception handling in XS
+modules. You have to define C<NO_XSLOCKS> before including F<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 C<call_*> function.
+
+The advantage of using the above macros is that you don't have
+to setup an extra function for C<call_*>, and that using these
+macros is faster than using C<call_*>.
=head2 Source Documentation
Please try and supply some documentation if you add functions to the
Perl core.
+=head2 Backwards compatibility
+
+The Perl API changes over time. New functions are added or the interfaces
+of existing functions are changed. The C<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.
+
+C<Devel::PPPort> generates a C header file F<ppport.h> that can also
+be run as a Perl script. To generate F<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 C<--api-info>
+command line switch. For example:
+
+ % perl ppport.h --api-info=sv_magicext
+
+For details, see C<perldoc ppport.h>.
+
=head1 Unicode Support
Perl 5.6.0 introduced Unicode support. It's important for porters and XS
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 UTF8. UTF8 uses
-a variable number of bytes to represent a character, instead of just
-one. You can learn more about Unicode at http://www.unicode.org/
+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 L<perlunicode>.
-=head2 How can I recognise a UTF8 string?
+=head2 How can I recognise a UTF-8 string?
-You can't. This is because UTF8 data is stored in bytes just like
-non-UTF8 data. The Unicode character 200, (C<0xC8> for you hex types)
+You can't. This is because UTF-8 data is stored in bytes just like
+non-UTF-8 data. The Unicode character 200, (C<0xC8> for you hex types)
capital E with a grave accent, is represented by the two bytes
C<v196.172>. Unfortunately, the non-Unicode string C<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 C<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, C<is_utf8_char> will tell you
-whether the current character in a string is valid UTF8.
+In general, you either have to know what you're dealing with, or you
+have to guess. The API function C<is_utf8_string> can help; it'll tell
+you if a string contains only valid UTF-8 characters. However, it can't
+do the work for you. On a character-by-character basis, C<is_utf8_char>
+will tell you whether the current character in a string is valid UTF-8.
-=head2 How does UTF8 represent Unicode characters?
+=head2 How does UTF-8 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 C<v194.129>; this
+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 C<v194.128>; this
continues up to character 191, which is C<v194.191>. Now we've run out of
bits (191 is binary C<10111111>) so we move on; 192 is C<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
+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 C<UTF8SKIP> macro:
char *utf = "\305\233\340\240\201";
utf += len;
len = UTF8SKIP(utf); /* len is 3 here */
-Another way to skip over characters in a UTF8 string is to use
+Another way to skip over characters in a UTF-8 string is to use
C<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 UTF8 character will have the high bit set, so
-you can test if you need to do something special with this character
-like this:
+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 can be encoded as a single byte even in UTF-8):
- UV uv;
+ U8 *utf;
+ UV uv; /* Note: a UV, not a U8, not a char */
- if (utf & 0x80)
- /* Must treat this as UTF8 */
+ if (!UTF8_IS_INVARIANT(*utf))
+ /* Must treat this as UTF-8 */
uv = utf8_to_uv(utf);
else
/* OK to treat this character as a byte */
You can also see in that example that we use C<utf8_to_uv> to get the
value of the character; the inverse function C<uv_to_utf8> is available
-for putting a UV into UTF8:
+for putting a UV into UTF-8:
- if (uv > 0x80)
+ if (!UTF8_IS_INVARIANT(uv))
/* Must treat this as UTF8 */
utf8 = uv_to_utf8(utf8, uv);
else
*utf8++ = uv;
You B<must> convert characters to UVs using the above functions 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 C<v196.172>, and you skip
-that character, you can never match a C<chr(200)> in a non-UTF8 string.
+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 C<v196.172>, and you skip
+that character, you can never match a C<chr(200)> in a non-UTF-8 string.
So don't do that!
-=head2 How does Perl store UTF8 strings?
+=head2 How does Perl store UTF-8 strings?
Currently, Perl deals with Unicode strings and non-Unicode strings
-slightly differently. If a string has been identified as being UTF-8
-encoded, Perl will set a flag in the SV, C<SVf_UTF8>. You can check and
-manipulate this flag with the following macros:
+slightly differently. A flag in the SV, C<SVf_UTF8>, indicates that the
+string is internally encoded as UTF-8. Without it, the byte value is the
+codepoint number and vice versa (in other words, the string is encoded
+as iso-8859-1). You can check and manipulate this flag with the
+following macros:
SvUTF8(sv)
SvUTF8_on(sv)
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.
+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 C<SVf_UTF8> flag is separate to the PV value; you
need be sure you don't accidentally knock it off while you're
SvUTF8_on(nsv);
In fact, your C<frobnicate> function should be made aware of whether or
-not it's dealing with UTF8 data, so that it can handle the string
+not it's dealing with UTF-8 data, so that it can handle the string
appropriately.
-=head2 How do I convert a string to UTF8?
+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 C<char *> to an XS function.
-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:
+=head2 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
+one of the strings to UTF-8. If you've got an SV, the easiest way to do
+this is:
sv_utf8_upgrade(sv);
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.
+by the end user, it can cause problems in deficient code.
-Instead, C<bytes_to_utf8> will give you a UTF8-encoded B<copy> of its
+Instead, C<bytes_to_utf8> will give you a UTF-8-encoded B<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
C<utf8_to_bytes> to go the other way, but naturally, this will fail if
=item *
-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 C<SvUTF8> flag. Don't forget to set the flag if
-something should be UTF8. Treat the flag as part of the PV, even though
+There's no way to tell if a string is UTF-8 or not. You can tell if an SV
+is UTF-8 by looking at is C<SvUTF8> flag. 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.
=item *
-If a string is UTF8, B<always> use C<utf8_to_uv> to get at the value,
-unless C<!(*s & 0x80)> in which case you can use C<*s>.
+If a string is UTF
-8, B<always> use C<utf8_to_uv> to get at the value,
+unless C<UTF8_IS_INVARIANT(*s)> in which case you can use C<*s>.
=item *
-When writing to a UTF8 string, B<always> use C<uv_to_utf8>, unless
-C<uv < 0x80> in which case you can use C<*s = uv>.
+When writing a character C<uv> to a UTF-8 string, B<always> use
+C<uv_to_utf8>, unless C<UTF8_IS_INVARIANT(uv))> in which case
+you can use C<*s = uv>.
=item *
-Mixing UTF8 and non-UTF8 strings is tricky. Use C<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 - C<HALF_UPGRADE> is one of those.
+Mixing UTF-8 and non-UTF-8 strings is tricky. Use C<bytes_to_utf8> to get
+a new string which is UTF-8 encoded, and then combine them.
=back
+=head1 Custom Operators
+
+Custom operator support is a new 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
+C<gvsv, gvsv, add>.)
+
+This feature is implemented as a new op type, C<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 C<CHECK> block and the C<B::Generate> module, or by adding
+a custom peephole optimizer with the C<optimize> module.
+
+When you do this, you replace ordinary Perl ops with custom ops by
+creating ops with the type C<OP_CUSTOM> and the C<pp_addr> of your own
+PP function. This should be defined in XS code, and should look like
+the PP ops in C<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 C<OP_CUSTOM>,
+Perl uses the value of C<< o->op_ppaddr >> as a key into the
+C<PL_custom_op_descs> and C<PL_custom_op_names> hashes. This means you
+need to enter a name and description for your op at the appropriate
+place in the C<PL_custom_op_names> and C<PL_custom_op_descs> hashes.
+
+Forthcoming versions of C<B::Generate> (version 1.0 and above) should
+directly support the creation of custom ops by name.
+
=head1 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>.
+E<lt>okamoto@corp.hp.comE<gt>. It is now maintained as part of Perl
+itself by the Perl 5 Porters E<lt>perl5-porters@perl.orgE<gt>.
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.
-API Listing originally by Dean Roehrich <roehrich@cray.com>.
-
-Modifications to autogenerate the API listing (L<perlapi>) by Benjamin
-Stuhl.
-
=head1 SEE ALSO
perlapi(1), perlintern(1), perlxs(1), perlembed(1)
https://perl5.git.perl.org / perl5.git / blobdiff