=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_vsetpvfn(SV*, const char*, STRLEN, va_list *, SV **, I32, bool *);
void sv_setsv(SV*, SV*);
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 represent the size of
-any string that perl can handle.
-
The C<sv_set*()> functions are not generic enough to operate on values
that have "magic". See L<Magic Virtual Tables> later in this document.
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.
+However, 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>).
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
=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 */
warning.
-=over
-
=item GV_ADDWARN
Issues the warning:
"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 stack is popped.
-Similarly results returned by XSUBs (which go in the stack) are often
-made mortal.
+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:
=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:
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
+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 perl.h. Note that before
+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.
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
+This MGVTBL structure is set at compile-time in F<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.
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
~ PERL_MAGIC_ext (none) Available for use by extensions
When an uppercase and lowercase letter both exist in the table, then the
-uppercase letter is 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.
+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.
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 C<PUSHs>
-macro. The values pushed will often need to be "mortal" (See L</Reference Counts and Mortality>).
+macro. The pushed values will often need to be "mortal" (See
+L</Reference Counts and Mortality>).
PUSHs(sv_2mortal(newSViv(an_integer)))
PUSHs(sv_2mortal(newSVpv("Some String",0)))
=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.
+The following three macros are used to initially allocate memory :
+
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 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
Unlike the C<New> and C<Newc> macros, the C<Newz> 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);
=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:
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
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
details of the interpreter's context. THX stands for "thread", "this",
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 into
+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.
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
=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
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 UTF8 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)
+ if (!UTF8_IS_INVARIANT(*utf))
/* Must treat this as UTF8 */
uv = utf8_to_uv(utf);
else
value of the character; the inverse function C<uv_to_utf8> is available
for putting a UV into UTF8:
- if (uv > 0x80)
+ if (!UTF8_IS_INVARIANT(uv))
/* Must treat this as UTF8 */
utf8 = uv_to_utf8(utf8, uv);
else
not it's dealing with UTF8 data, so that it can handle the string
appropriately.
+Since just passing an SV to an XS function and copying the data of
+the SV is not enough to copy the UTF8 flags, even less right is just
+passing a C<char *> to an XS function.
+
=head2 How do I convert a string to UTF8?
If you're mixing UTF8 and non-UTF8 strings, you might find it necessary
=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>.
+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 UTF8 string, B<always> use
+C<uv_to_utf8>, unless C<UTF8_IS_INVARIANT(uv))> in which case
+you can use C<*s = uv>.
=item *
Bowers, Matthew Green, Tim Bunce, Spider Boardman, Ulrich Pfeifer,
Stephen McCamant, and Gurusamy Sarathy.
-API Listing originally by Dean Roehrich E<lt>roehrich@cray.comE<gt>.
-
-Modifications to autogenerate the API listing (L<perlapi>) by Benjamin
-Stuhl.
-
=head1 SEE ALSO
perlapi(1), perlintern(1), perlxs(1), perlembed(1)
projects / p5sagit/p5-mst-13.2.git / blobdiff