Each typedef has specific routines that manipulate the various data types.
+=for apidoc_section $AV
+=for apidoc Ayh||AV
+=for apidoc_section $HV
+=for apidoc Ayh||HV
+=for apidoc_section $SV
+=for apidoc Ayh||SV
+
=head2 What is an "IV"?
Perl uses a special typedef IV which is a simple signed integer type that is
guaranteed to be large enough to hold a pointer (as well as an integer).
Additionally, there is the UV, which is simply an unsigned IV.
-Perl also uses two special typedefs, I32 and I16, which will always be at
-least 32-bits and 16-bits long, respectively. (Again, there are U32 and U16,
-as well.) They will usually be exactly 32 and 16 bits long, but on Crays
-they will both be 64 bits.
+Perl also uses several special typedefs to declare variables to hold
+integers of (at least) a given size.
+Use I8, I16, I32, and I64 to declare a signed integer variable which has
+at least as many bits as the number in its name. These all evaluate to
+the native C type that is closest to the given number of bits, but no
+smaller than that number. For example, on many platforms, a C<short> is
+16 bits long, and if so, I16 will evaluate to a C<short>. But on
+platforms where a C<short> isn't exactly 16 bits, Perl will use the
+smallest type that contains 16 bits or more.
+
+U8, U16, U32, and U64 are to declare the corresponding unsigned integer
+types.
+
+If the platform doesn't support 64-bit integers, both I64 and U64 will
+be undefined. Use IV and UV to declare the largest practicable, and
+C<L<perlapi/WIDEST_UTYPE>> for the absolute maximum unsigned, but which
+may not be usable in all circumstances.
+
+A numeric constant can be specified with L<perlapi/C<INT16_C>>,
+L<perlapi/C<UINTMAX_C>>, and similar.
+
+=for apidoc_section $integer
+=for apidoc Ayh||I8
+=for apidoc_item ||I16
+=for apidoc_item ||I32
+=for apidoc_item ||I64
+=for apidoc_item ||IV
+
+=for apidoc Ayh||U8
+=for apidoc_item ||U16
+=for apidoc_item ||U32
+=for apidoc_item ||U64
+=for apidoc_item ||UV
=head2 Working with SVs
example, a trailing C<NUL> is tacked on automatically. The non-string use
is documented only in this paragraph.)
+=for apidoc Ayh||NV
+
The seven routines are:
SV* newSViv(IV);
F<config.h>) guaranteed to be large enough to represent the size of
any string that perl can handle.
+=for apidoc Ayh||STRLEN
+
In the unlikely case of a SV requiring more complex initialization, 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
Nevertheless, you should be very careful when you pass a string stored
in an SV to a C function or system call.
-To access the actual value that an SV points to, you can use the macros:
+To access the actual value that an SV points to, Perl's API exposes
+several macros that coerce the actual scalar type into an IV, UV, double,
+or string:
+
+=over
+
+=item * C<SvIV(SV*)> (C<IV>) and C<SvUV(SV*)> (C<UV>)
+
+=item * C<SvNV(SV*)> (C<double>)
+
+=item * Strings are a bit complicated:
+
+=over
+
+=item * Byte string: C<SvPVbyte(SV*, STRLEN len)> or C<SvPVbyte_nolen(SV*)>
+
+If the Perl string is C<"\xff\xff">, then this returns a 2-byte C<char*>.
- SvIV(SV*)
- SvUV(SV*)
- SvNV(SV*)
- SvPV(SV*, STRLEN len)
- SvPV_nolen(SV*)
+This is suitable for Perl strings that represent bytes.
-which will automatically coerce the actual scalar type into an IV, UV, double,
-or string.
+=item * UTF-8 string: C<SvPVutf8(SV*, STRLEN len)> or C<SvPVutf8_nolen(SV*)>
-In the C<SvPV> macro, the length of the string returned is placed into the
-variable C<len> (this is a macro, so you do I<not> use C<&len>). If you do
-not care what the length of the data is, use the C<SvPV_nolen> macro.
-Historically the C<SvPV> macro with the global variable C<PL_na> has been
-used in this case. But that can be quite inefficient because C<PL_na> must
+If the Perl string is C<"\xff\xff">, then this returns a 4-byte C<char*>.
+
+This is suitable for Perl strings that represent characters.
+
+B<CAVEAT>: That C<char*> will be encoded via Perl's internal UTF-8 variant,
+which means that if the SV contains non-Unicode code points (e.g.,
+0x110000), then the result may contain extensions over valid UTF-8.
+See L<perlapi/is_strict_utf8_string> for some methods Perl gives
+you to check the UTF-8 validity of these macros' returns.
+
+=item * You can also use C<SvPV(SV*, STRLEN len)> or C<SvPV_nolen(SV*)>
+to fetch the SV's raw internal buffer. This is tricky, though; if your Perl
+string
+is C<"\xff\xff">, then depending on the SV's internal encoding you might get
+back a 2-byte B<OR> a 4-byte C<char*>.
+Moreover, if it's the 4-byte string, that could come from either Perl
+C<"\xff\xff"> stored UTF-8 encoded, or Perl C<"\xc3\xbf\xc3\xbf"> stored
+as raw octets. To differentiate between these you B<MUST> look up the
+SV's UTF8 bit (cf. C<SvUTF8>) to know whether the source Perl string
+is 2 characters (C<SvUTF8> would be on) or 4 characters (C<SvUTF8> would be
+off).
+
+B<IMPORTANT:> Use of C<SvPV>, C<SvPV_nolen>, or
+similarly-named macros I<without> looking up the SV's UTF8 bit is
+almost certainly a bug if non-ASCII input is allowed.
+
+When the UTF8 bit is on, the same B<CAVEAT> about UTF-8 validity applies
+here as for C<SvPVutf8>.
+
+=back
+
+(See L</How do I pass a Perl string to a C library?> for more details.)
+
+In C<SvPVbyte>, C<SvPVutf8>, and C<SvPV>, the length of the C<char*> returned
+is placed into the
+variable C<len> (these are macros, so you do I<not> use C<&len>). If you do
+not care what the length of the data is, use C<SvPVbyte_nolen>,
+C<SvPVutf8_nolen>, or C<SvPV_nolen> instead.
+The global variable C<PL_na> can also be given to
+C<SvPVbyte>/C<SvPVutf8>/C<SvPV>
+in this case. But that can be quite inefficient because C<PL_na> must
be accessed in thread-local storage in threaded Perl. In any case, remember
that Perl allows arbitrary strings of data that may both contain NULs and
might not be terminated by a C<NUL>.
-Also remember that C doesn't allow you to safely say C<foo(SvPV(s, len),
+Also remember that C doesn't allow you to safely say C<foo(SvPVbyte(s, len),
len);>. It might work with your
compiler, but it won't work for everyone.
Break this sort of statement up into separate assignments:
SV *s;
STRLEN len;
char *ptr;
- ptr = SvPV(s, len);
+ ptr = SvPVbyte(s, len);
foo(ptr, len);
+=back
+
If you want to know if the scalar value is TRUE, you can use:
SvTRUE(SV*)
C<SvGROW(sv, len + 1)>).
If you want to write to an existing SV's buffer and set its value to a
-string, use SvPV_force() or one of its variants to force the SV to be
+string, use SvPVbyte_force() or one of its variants to force the SV to be
a PV. This will remove any of various types of non-stringness from
the SV while preserving the content of the SV in the PV. This can be
used, for example, to append data from an API function to a buffer
s = SvGROW(sv, needlen + 1);
/* something that modifies up to needlen bytes at s, but modifies
newlen bytes
- eg. newlen = read(fd, s. needlen);
+ eg. newlen = read(fd, s, needlen);
*/
s[newlen] = '\0';
SvCUR_set(sv, newlen);
once you have an C<HE*>, to get the actual key and value, use the routines
specified below.
+=for apidoc Ayh||HE
+
I32 hv_iterinit(HV*);
/* Prepares starting point to traverse hash table */
HE* hv_iternext(HV*);
See L</Understanding the Magic of Tied Hashes and Arrays> for more
information on how to use the hash access functions on tied hashes.
+=for apidoc_section $HV
+=for apidoc Amh|void|PERL_HASH|U32 hash|char *key|STRLEN klen
+
=head2 Hash API Extensions
Beginning with version 5.004, the following functions are also supported:
The most useful types that will be returned are:
- < SVt_PVAV Scalar
SVt_PVAV Array
SVt_PVHV Hash
SVt_PVCV Code
SVt_PVGV Glob (possibly a file handle)
+Any numerical value returned which is less than SVt_PVAV will be a scalar
+of some form.
+
See L<perlapi/svtype> for more details.
=head2 Blessed References and Class Objects
around at the same time, with different death dates. Internally, the
actual determinant for when mortal xV references are destroyed depends
on two macros, SAVETMPS and FREETMPS. See L<perlcall> and L<perlxs>
-for more details on these macros.
+and L</Temporaries Stack> below for more details on these macros.
Mortal references are mainly used for xVs that are placed on perl's
main stack. The stack is problematic for reference tracking, because it
For more information on references and blessings, consult L<perlref>.
+=head2 I/O Handles
+
+Like AVs and HVs, IO objects are another type of non-scalar SV which
+may contain input and output L<PerlIO|perlapio> objects or a C<DIR *>
+from opendir().
+
+You can create a new IO object:
+
+ IO* newIO();
+
+Unlike other SVs, a new IO object is automatically blessed into the
+L<IO::File> class.
+
+The IO object contains an input and output PerlIO handle:
+
+ PerlIO *IoIFP(IO *io);
+ PerlIO *IoOFP(IO *io);
+
+Typically if the IO object has been opened on a file, the input handle
+is always present, but the output handle is only present if the file
+is open for output. For a file, if both are present they will be the
+same PerlIO object.
+
+Distinct input and output PerlIO objects are created for sockets and
+character devices.
+
+The IO object also contains other data associated with Perl I/O
+handles:
+
+ IV IoLINES(io); /* $. */
+ IV IoPAGE(io); /* $% */
+ IV IoPAGE_LEN(io); /* $= */
+ IV IoLINES_LEFT(io); /* $- */
+ char *IoTOP_NAME(io); /* $^ */
+ GV *IoTOP_GV(io); /* $^ */
+ char *IoFMT_NAME(io); /* $~ */
+ GV *IoFMT_GV(io); /* $~ */
+ char *IoBOTTOM_NAME(io);
+ GV *IoBOTTOM_GV(io);
+ char IoTYPE(io);
+ U8 IoFLAGS(io);
+
+Most of these are involved with L<formats|perlform>.
+
+IoFLAGs() may contain a combination of flags, the most interesting of
+which are C<IOf_FLUSH> (C<$|>) for autoflush and C<IOf_UNTAINT>,
+settable with L<< IO::Handle's untaint() method|IO::Handle/"$io->untaint" >>.
+
+The IO object may also contains a directory handle:
+
+ DIR *IoDIRP(io);
+
+suitable for use with PerlDir_read() etc.
+
+All of these accessors macros are lvalues, there are no distinct
+C<_set()> macros to modify the members of the IO object.
+
=head2 Double-Typed SVs
Scalar variables normally contain only one type of value, an integer,
"Magic Virtual Table" to handle the various operations that might be
applied to that variable.
+=for apidoc Ayh||MGVTBL
+
The C<MGVTBL> has five (or sometimes eight) pointers to the following
routine types:
~ PERL_MAGIC_ext (none) Available for use by
extensions
+
+=for apidoc AmnhU||PERL_MAGIC_arylen
+=for apidoc_item ||PERL_MAGIC_arylen_p
+=for apidoc_item ||PERL_MAGIC_backref
+=for apidoc_item ||PERL_MAGIC_bm
+=for apidoc_item ||PERL_MAGIC_checkcall
+=for apidoc_item ||PERL_MAGIC_collxfrm
+=for apidoc_item ||PERL_MAGIC_dbfile
+=for apidoc_item ||PERL_MAGIC_dbline
+=for apidoc_item ||PERL_MAGIC_debugvar
+=for apidoc_item ||PERL_MAGIC_defelem
+=for apidoc_item ||PERL_MAGIC_env
+=for apidoc_item ||PERL_MAGIC_envelem
+=for apidoc_item ||PERL_MAGIC_ext
+=for apidoc_item ||PERL_MAGIC_fm
+=for apidoc_item ||PERL_MAGIC_hints
+=for apidoc_item ||PERL_MAGIC_hintselem
+=for apidoc_item ||PERL_MAGIC_isa
+=for apidoc_item ||PERL_MAGIC_isaelem
+=for apidoc_item ||PERL_MAGIC_lvref
+=for apidoc_item ||PERL_MAGIC_nkeys
+=for apidoc_item ||PERL_MAGIC_nonelem
+=for apidoc_item ||PERL_MAGIC_overload_table
+=for apidoc_item ||PERL_MAGIC_pos
+=for apidoc_item ||PERL_MAGIC_qr
+=for apidoc_item ||PERL_MAGIC_regdata
+=for apidoc_item ||PERL_MAGIC_regdatum
+=for apidoc_item ||PERL_MAGIC_regex_global
+=for apidoc_item ||PERL_MAGIC_rhash
+=for apidoc_item ||PERL_MAGIC_shared
+=for apidoc_item ||PERL_MAGIC_shared_scalar
+=for apidoc_item ||PERL_MAGIC_sig
+=for apidoc_item ||PERL_MAGIC_sigelem
+=for apidoc_item ||PERL_MAGIC_substr
+=for apidoc_item ||PERL_MAGIC_sv
+=for apidoc_item ||PERL_MAGIC_symtab
+=for apidoc_item ||PERL_MAGIC_taint
+=for apidoc_item ||PERL_MAGIC_tied
+=for apidoc_item ||PERL_MAGIC_tiedelem
+=for apidoc_item ||PERL_MAGIC_tiedscalar
+=for apidoc_item ||PERL_MAGIC_utf8
+=for apidoc_item ||PERL_MAGIC_uvar
+=for apidoc_item ||PERL_MAGIC_uvar_elem
+=for apidoc_item ||PERL_MAGIC_vec
+=for apidoc_item ||PERL_MAGIC_vstring
+
=for mg_vtable.pl end
When an uppercase and lowercase letter both exist in the table, then the
=item C<SAVELONG(long i)>
+=item C<SAVEI8(I8 i)>
+
+=item C<SAVEI16(I16 i)>
+
+=item C<SAVEBOOL(int i)>
+
These macros arrange things to restore the value of integer variable
-C<i> at the end of enclosing I<pseudo-block>.
+C<i> at the end of the enclosing I<pseudo-block>.
+
+=for apidoc_section $stack
+=for apidoc Amh||SAVEINT|int i
+=for apidoc Amh||SAVEIV|IV i
+=for apidoc Amh||SAVEI32|I32 i
+=for apidoc Amh||SAVELONG|long i
+=for apidoc Amh||SAVEI8|I8 i
+=for apidoc Amh||SAVEI16|I16 i
+=for apidoc Amh||SAVEBOOL|bool i
=item C<SAVESPTR(s)>
C<SV*> and back, C<p> should be able to survive conversion to C<char*>
and back.
+=for apidoc Amh||SAVESPTR|SV * s
+=for apidoc Amh||SAVEPPTR|char * p
+
=item C<SAVEFREESV(SV *sv)>
The refcount of C<sv> will be decremented at the end of
Also compare C<SAVEMORTALIZESV>.
+=for apidoc Amh||SAVEFREESV|SV* sv
+
=item C<SAVEMORTALIZESV(SV *sv)>
Just like C<SAVEFREESV>, but mortalizes C<sv> at the end of the current
effect of keeping C<sv> alive until the statement that called the currently
live scope has finished executing.
+=for apidoc Amh||SAVEMORTALIZESV|SV* sv
+
=item C<SAVEFREEOP(OP *op)>
The C<OP *> is op_free()ed at the end of I<pseudo-block>.
+=for apidoc Amh||SAVEFREEOP|OP *op
+
=item C<SAVEFREEPV(p)>
The chunk of memory which is pointed to by C<p> is Safefree()ed at the
end of I<pseudo-block>.
+=for apidoc Amh||SAVEFREEPV|void * p
+
=item C<SAVECLEARSV(SV *sv)>
Clears a slot in the current scratchpad which corresponds to C<sv> at
SAVEDELETE(PL_defstash, savepv(tmpbuf), strlen(tmpbuf));
+=for apidoc Amh||SAVEDELETE|HV * hv|char * key|I32 length
+
=item C<SAVEDESTRUCTOR(DESTRUCTORFUNC_NOCONTEXT_t f, void *p)>
At the end of I<pseudo-block> the function C<f> is called with the
only argument C<p>.
+=for apidoc Ayh||DESTRUCTORFUNC_NOCONTEXT_t
+=for apidoc Amh||SAVEDESTRUCTOR|DESTRUCTORFUNC_NOCONTEXT_t f|void *p
+
=item C<SAVEDESTRUCTOR_X(DESTRUCTORFUNC_t f, void *p)>
At the end of I<pseudo-block> the function C<f> is called with the
implicit context argument (if any), and C<p>.
+=for apidoc Ayh||DESTRUCTORFUNC_t
+=for apidoc Amh||SAVEDESTRUCTOR_X|DESTRUCTORFUNC_t f|void *p
+
=item C<SAVESTACK_POS()>
The current offset on the Perl internal stack (cf. C<SP>) is restored
at the end of I<pseudo-block>.
+=for apidoc Amh||SAVESTACK_POS
+
=back
The following API list contains functions, thus one needs to
or Perlish C<GV *>s). Where the above macros take C<int>, a similar
function takes C<int *>.
+Other macros above have functions implementing them, but its probably
+best to just use the macro, and not those or the ones below.
+
=over 4
=item C<SV* save_scalar(GV *gv)>
+=for apidoc save_scalar
+
Equivalent to Perl code C<local $gv>.
=item C<AV* save_ary(GV *gv)>
+=for apidoc save_ary
+
=item C<HV* save_hash(GV *gv)>
+=for apidoc save_hash
+
Similar to C<save_scalar>, but localize C<@gv> and C<%gv>.
=item C<void save_item(SV *item)>
-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>
+=for apidoc save_item
+
+Duplicates the current value of C<SV>. On the exit from the current
+C<ENTER>/C<LEAVE> I<pseudo-block> the value of C<SV> will be restored
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)>
+=for apidoc save_list
+
A variant of C<save_item> which takes multiple arguments via an array
C<sarg> of C<SV*> of length C<maxsarg>.
=item C<SV* save_svref(SV **sptr)>
+=for apidoc save_svref
+
Similar to C<save_scalar>, but will reinstate an C<SV *>.
=item C<void save_aptr(AV **aptr)>
=item C<void save_hptr(HV **hptr)>
+=for apidoc save_aptr
+=for apidoc save_hptr
+
Similar to C<save_svref>, but localize C<AV *> and C<HV *>.
=back
transparency between differences in the actual malloc implementation that is
used within perl.
-It is suggested that you enable the version of malloc that is distributed
-with Perl. It keeps pools of various sizes of unallocated memory in
-order to satisfy allocation requests more quickly. However, on some
-platforms, it may cause spurious malloc or free errors.
-
The following three macros are used to initially allocate memory :
Newx(pointer, number, type);
following the C<OpSIBLING> pointer from the first child to the last (but
see below).
+=for apidoc Ayh||OP
+=for apidoc Ayh||BINOP
+=for apidoc Ayh||LISTOP
+=for apidoc Ayh||UNOP
+
There are also some 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
optimization (see L</Compile pass 2: context propagation>) it will still
have children in accordance with its former type.
+=for apidoc Ayh||LOOP
+=for apidoc Ayh||PMOP
+
Finally, there is a C<LOGOP>, or logic op. Like a C<LISTOP>, this has one
or more children, but it doesn't have an C<op_last> field: so you have to
follow C<op_first> and then the C<OpSIBLING> chain itself to find the
that in general, C<op_other> may not point to any of the direct children
of the C<LOGOP>.
+=for apidoc Ayh||LOGOP
+
Starting in version 5.21.2, perls built with the experimental
define C<-DPERL_OP_PARENT> add an extra boolean flag for each op,
C<op_moresib>. When not set, this indicates that this is the last op in an
PL_peepp = my_peep;
static peep_t prev_rpeepp;
- static void my_rpeep(pTHX_ OP *o)
+ static void my_rpeep(pTHX_ OP *first)
{
- OP *orig_o = o;
- for(; o; o = o->op_next) {
+ OP *o = first, *t = first;
+ for(; o = o->op_next, t = t->op_next) {
/* custom per-op optimisation goes here */
+ o = o->op_next;
+ if (!o || o == t) break;
+ /* custom per-op optimisation goes AND here */
}
prev_rpeepp(aTHX_ orig_o);
}
prev_rpeepp = PL_rpeepp;
PL_rpeepp = my_rpeep;
+=for apidoc Ayh||peep_t
+
=head2 Pluggable runops
The compile tree is executed in a runops function. There are two runops
PL_runops = my_runops;
+=for apidoc Amnh|runops_proc_t|PL_runops
+
This function should be as efficient as possible to keep your programs
running as fast as possible.
This will arrange to have C<my_start_hook> called at the start of
compiling every lexical scope. The available hooks are:
+=for apidoc Ayh||BHK
+
=over 4
=item C<void bhk_start(pTHX_ int full)>
The Perl interpreter can be regarded as a closed box: it has an API
for feeding it code or otherwise making it do things, but it also has
functions for its own use. This smells a lot like an object, and
-there are ways for you to build Perl so that you can have multiple
+there is a way for you to build Perl so that you can have multiple
interpreters, with one interpreter represented either as a C structure,
or inside a thread-specific structure. These structures contain all
the context, the state of that interpreter.
-One macro controls the major Perl build flavor: MULTIPLICITY. The
+The macro that controls the major Perl build flavor is MULTIPLICITY. The
MULTIPLICITY build has a C structure that packages all the interpreter
state. With multiplicity-enabled perls, PERL_IMPLICIT_CONTEXT is also
normally defined, and enables the support for passing in a "hidden" first
multi-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 initialized
-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 (or GNU)
compatible C<nm>:
The test F<t/porting/libperl.t> does this kind of symbol sanity
checking on C<libperl.a>.
-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
either subroutines taking some kind of structure as the first
argument, or subroutines taking nothing as the first argument. To
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.
+think it should be (i.e., you need it for your extension), submit an issue at
+L<https://github.com/Perl/perl5/issues> explaining why you think it should be.
Second problem: there must be a syntax so that the same subroutine
declarations and calls can pass a structure as their first argument,
STATIC becomes "static" in C, and may be #define'd to nothing in some
configurations in the future.
+=for apidoc_section $directives
+=for apidoc Ayh||STATIC
+
A public function (i.e. part of the internal API, but not necessarily
sanctioned for use in extensions) begins like this:
or 'd' for B<d>eclaration, so we have C<pTHX>, C<aTHX> and C<dTHX>, and
their variants.
+=for apidoc_section $concurrency
+=for apidoc Amnh||aTHX
+=for apidoc Amnh||aTHX_
+=for apidoc Amnh||dTHX
+=for apidoc Amnh||pTHX
+=for apidoc Amnh||pTHX_
+
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
The context-free version of Perl_warner is called
Perl_warner_nocontext, and does not take the extra argument. Instead
-it does dTHX; to get the context from thread-local storage. We
+it does C<dTHX;> to get the context from thread-local storage. We
C<#define warner Perl_warner_nocontext> so that extensions get source
compatibility at the expense of performance. (Passing an arg is
cheaper than grabbing it from thread-local storage.)
=head2 So what happened to dTHR?
+=for apidoc Amnh||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
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
Apd |SV** |av_fetch |AV* ar|I32 key|I32 lval
-The second column is the return type, the third column the name. Columns
-after that are the arguments. The first column is a set of flags:
+The first column is a set of flags, the second column the return type,
+the third column the name. Columns after that are the arguments.
+The flags are documented at the top of F<embed.fnc>.
-=over 3
-
-=item A
-
-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; 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. Some functions have 'd' but not 'A'; docs are good.
-
-=back
-
-Other available flags are:
-
-=over 3
-
-=item s
-
-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 need an interpreter context, so the definition has no
-C<pTHX>, and it follows that callers don't use C<aTHX>. (See
-L</Background and PERL_IMPLICIT_CONTEXT>.)
-
-=item r
-
-This function never returns; C<croak>, C<exit> and friends.
-
-=item f
-
-This function takes a variable number of arguments, C<printf> style.
-The argument list should end with C<...>, like this:
+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.
- Afprd |void |croak |const char* pat|...
+=head2 Formatted Printing of IVs, UVs, and NVs
-=item M
+If you are printing IVs, UVs, or NVS instead of the stdio(3) style
+formatting codes like C<%d>, C<%ld>, C<%f>, you should use the
+following macros for portability
-This function is part of the experimental development API, and may change
-or disappear without notice.
+ 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
-=item o
+These will take care of 64-bit integers and long doubles.
+For example:
-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>.
+ printf("IV is %" IVdf "\n", iv);
-=item x
+The C<IVdf> will expand to whatever is the correct format for the IVs.
+Note that the spaces are required around the format in case the code is
+compiled with C++, to maintain compliance with its standard.
-This function isn't exported out of the Perl core.
+Note that there are different "long doubles": Perl will use
+whatever the compiler has.
-=item m
+If you are printing addresses of pointers, use %p or UVxf combined
+with PTR2UV().
-This is implemented as a macro.
+=head2 Formatted Printing of SVs
-=item X
+The contents of SVs may be printed using the C<SVf> format, like so:
-This function is explicitly exported.
+ Perl_croak(aTHX_ "This croaked because: %" SVf "\n", SVfARG(err_msg))
-=item E
+where C<err_msg> is an SV.
-This function is visible to extensions included in the Perl core.
+=for apidoc Amnh||SVf
+=for apidoc Amh||SVfARG|SV *sv
-=item b
+Not all scalar types are printable. Simple values certainly are: one of
+IV, UV, NV, or PV. Also, if the SV is a reference to some value,
+either it will be dereferenced and the value printed, or information
+about the type of that value and its address are displayed. The results
+of printing any other type of SV are undefined and likely to lead to an
+interpreter crash. NVs are printed using a C<%g>-ish format.
-Binary backward compatibility; this function is a macro but also has
-a C<Perl_> implementation (which is exported).
+Note that the spaces are required around the C<SVf> in case the code is
+compiled with C++, to maintain compliance with its standard.
-=item others
+Note that any filehandle being printed to under UTF-8 must be expecting
+UTF-8 in order to get good results and avoid Wide-character warnings.
+One way to do this for typical filehandles is to invoke perl with the
+C<-C>> parameter. (See L<perlrun/-C [numberE<sol>list]>.
-See the comments at the top of C<embed.fnc> for others.
+You can use this to concatenate two scalars:
-=back
+ SV *var1 = get_sv("var1", GV_ADD);
+ SV *var2 = get_sv("var2", GV_ADD);
+ SV *var3 = newSVpvf("var1=%" SVf " and var2=%" SVf,
+ SVfARG(var1), SVfARG(var2));
-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 Strings
-=head2 Formatted Printing of IVs, UVs, and NVs
+If you just want the bytes printed in a 7bit NUL-terminated string, you can
+just use C<%s> (assuming they are all really only 7bit). But if there is a
+possibility the value will be encoded as UTF-8 or contains bytes above
+C<0x7F> (and therefore 8bit), you should instead use the C<UTF8f> format.
+And as its parameter, use the C<UTF8fARG()> macro:
-If you are printing IVs, UVs, or NVS instead of the stdio(3) style
-formatting codes like C<%d>, C<%ld>, C<%f>, you should use the
-following macros for portability
+ chr * msg;
- 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
+ /* U+2018: \xE2\x80\x98 LEFT SINGLE QUOTATION MARK
+ U+2019: \xE2\x80\x99 RIGHT SINGLE QUOTATION MARK */
+ if (can_utf8)
+ msg = "\xE2\x80\x98Uses fancy quotes\xE2\x80\x99";
+ else
+ msg = "'Uses simple quotes'";
-These will take care of 64-bit integers and long doubles.
-For example:
+ Perl_croak(aTHX_ "The message is: %" UTF8f "\n",
+ UTF8fARG(can_utf8, strlen(msg), msg));
- printf("IV is %"IVdf"\n", iv);
+The first parameter to C<UTF8fARG> is a boolean: 1 if the string is in
+UTF-8; 0 if string is in native byte encoding (Latin1).
+The second parameter is the number of bytes in the string to print.
+And the third and final parameter is a pointer to the first byte in the
+string.
-The IVdf will expand to whatever is the correct format for the IVs.
+Note that any filehandle being printed to under UTF-8 must be expecting
+UTF-8 in order to get good results and avoid Wide-character warnings.
+One way to do this for typical filehandles is to invoke perl with the
+C<-C>> parameter. (See L<perlrun/-C [numberE<sol>list]>.
-Note that there are different "long doubles": Perl will use
-whatever the compiler has.
+=for apidoc_section $formats
+=for apidoc Amnh||UTF8f
+=for apidoc Amh||UTF8fARG|bool is_utf8|Size_t byte_len|char *str
-If you are printing addresses of pointers, use UVxf combined
-with PTR2UV(), do not use %lx or %p.
+=cut
=head2 Formatted Printing of C<Size_t> and C<SSize_t>
L<previous section|/Formatted Printing of IVs, UVs, and NVs>.
But if you're using C<PerlIO_printf()>, it's less typing and visual
-clutter to use the C<"%z"> length modifier (for I<siZe>):
+clutter to use the C<%z> length modifier (for I<siZe>):
PerlIO_printf("STRLEN is %zu\n", len);
This modifier is not portable, so its use should be restricted to
C<PerlIO_printf()>.
+=head2 Formatted Printing of C<Ptrdiff_t>, C<intmax_t>, C<short> and other special sizes
+
+There are modifiers for these special situations if you are using
+C<PerlIO_printf()>. See L<perlfunc/size>.
+
=head2 Pointer-To-Integer and Integer-To-Pointer
Because pointer size does not necessarily equal integer size,
PTR2NV(pointer)
INT2PTR(pointertotype, integer)
+=for apidoc_section $casting
+=for apidoc Amh|type|INT2PTR|type|int value
+=for apidoc Amh|UV|PTR2UV|void * ptr
+=for apidoc Amh|IV|PTR2IV|void * ptr
+=for apidoc Amh|NV|PTR2NV|void * ptr
+
For example:
IV iv = ...;
AV *av = ...;
UV uv = PTR2UV(av);
+There are also
+
+ PTR2nat(pointer) /* pointer to integer of PTRSIZE */
+ PTR2ul(pointer) /* pointer to unsigned long */
+
+=for apidoc Amh|IV|PTR2nat|void *
+=for apidoc Amh|unsigned long|PTR2ul|void *
+
+And C<PTRV> which gives the native type for an integer the same size as
+pointers, such as C<unsigned> or C<unsigned long>.
+
+=for apidoc Ayh|type|PTRV
+
=head2 Exception Handling
There are a couple of macros to do very basic exception handling in XS
change, but you can look at the code for C<pp_lc> in F<pp.c> for an
example as to how it's currently done.
+=head2 How do I pass a Perl string to a C library?
+
+A Perl string, conceptually, is an opaque sequence of code points.
+Many C libraries expect their inputs to be "classical" C strings, which are
+arrays of octets 1-255, terminated with a NUL byte. Your job when writing
+an interface between Perl and a C library is to define the mapping between
+Perl and that library.
+
+Generally speaking, C<SvPVbyte> and related macros suit this task well.
+These assume that your Perl string is a "byte string", i.e., is either
+raw, undecoded input into Perl or is pre-encoded to, e.g., UTF-8.
+
+Alternatively, if your C library expects UTF-8 text, you can use
+C<SvPVutf8> and related macros. This has the same effect as encoding
+to UTF-8 then calling the corresponding C<SvPVbyte>-related macro.
+
+Some C libraries may expect other encodings (e.g., UTF-16LE). To give
+Perl strings to such libraries
+you must either do that encoding in Perl then use C<SvPVbyte>, or
+use an intermediary C library to convert from however Perl stores the
+string to the desired encoding.
+
+Take care also that NULs in your Perl string don't confuse the C
+library. If possible, give the string's length to the C library; if that's
+not possible, consider rejecting strings that contain NUL bytes.
+
+=head3 What about C<SvPV>, C<SvPV_nolen>, etc.?
+
+Consider a 3-character Perl string C<$foo = "\x64\x78\x8c">.
+Perl can store these 3 characters either of two ways:
+
+=over
+
+=item * bytes: 0x64 0x78 0x8c
+
+=item * UTF-8: 0x64 0x78 0xc2 0x8c
+
+=back
+
+Now let's say you convert C<$foo> to a C string thus:
+
+ STRLEN strlen;
+ char *str = SvPV(foo_sv, strlen);
+
+At this point C<str> could point to a 3-byte C string or a 4-byte one.
+
+Generally speaking, we want C<str> to be the same regardless of how
+Perl stores C<$foo>, so the ambiguity here is undesirable. C<SvPVbyte>
+and C<SvPVutf8> solve that by giving predictable output: use
+C<SvPVbyte> if your C library expects byte strings, or C<SvPVutf8>
+if it expects UTF-8.
+
+If your C library happens to support both encodings, then C<SvPV>--always
+in tandem with lookups to C<SvUTF8>!--may be safe and (slightly) more
+efficient.
+
+B<TESTING> B<TIP:> Use L<utf8>'s C<upgrade> and C<downgrade> functions
+in your tests to ensure consistent handling regardless of Perl's
+internal encoding.
+
=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
C<XopENTRY_set>, and register the structure against the ppaddr using
C<Perl_custom_op_register>. A trivial example might look like:
+=for apidoc Ayh||XOP
+
static XOP my_xop;
static OP *my_pp(pTHX);
by the peephole optimizer. I<o> is the OP that needs optimizing;
I<oldop> is the previous OP optimized, whose C<op_next> points to I<o>.
+=for apidoc Ayh||Perl_cpeep_t
+
=back
C<B::Generate> directly supports the creation of custom ops by name.
+=head1 Stacks
+
+Descriptions above occasionally refer to "the stack", but there are in fact
+many stack-like data structures within the perl interpreter. When otherwise
+unqualified, "the stack" usually refers to the value stack.
+
+The various stacks have different purposes, and operate in slightly different
+ways. Their differences are noted below.
+
+=head2 Value Stack
+
+This stack stores the values that regular perl code is operating on, usually
+intermediate values of expressions within a statement. The stack itself is
+formed of an array of SV pointers.
+
+The base of this stack is pointed to by the interpreter variable
+C<PL_stack_base>, of type C<SV **>.
+
+The head of the stack is C<PL_stack_sp>, and points to the most
+recently-pushed item.
+
+Items are pushed to the stack by using the C<PUSHs()> macro or its variants
+described above; C<XPUSHs()>, C<mPUSHs()>, C<mXPUSHs()> and the typed
+versions. Note carefully that the non-C<X> versions of these macros do not
+check the size of the stack and assume it to be big enough. These must be
+paired with a suitable check of the stack's size, such as the C<EXTEND> macro
+to ensure it is large enough. For example
+
+ EXTEND(SP, 4);
+ mPUSHi(10);
+ mPUSHi(20);
+ mPUSHi(30);
+ mPUSHi(40);
+
+This is slightly more performant than making four separate checks in four
+separate C<mXPUSHi()> calls.
+
+As a further performance optimisation, the various C<PUSH> macros all operate
+using a local variable C<SP>, rather than the interpreter-global variable
+C<PL_stack_sp>. This variable is declared by the C<dSP> macro - though it is
+normally implied by XSUBs and similar so it is rare you have to consider it
+directly. Once declared, the C<PUSH> macros will operate only on this local
+variable, so before invoking any other perl core functions you must use the
+C<PUTBACK> macro to return the value from the local C<SP> variable back to
+the interpreter variable. Similarly, after calling a perl core function which
+may have had reason to move the stack or push/pop values to it, you must use
+the C<SPAGAIN> macro which refreshes the local C<SP> value back from the
+interpreter one.
+
+Items are popped from the stack by using the C<POPs> macro or its typed
+versions, There is also a macro C<TOPs> that inspects the topmost item without
+removing it.
+
+Note specifically that SV pointers on the value stack do not contribute to the
+overall reference count of the xVs being referred to. If newly-created xVs are
+being pushed to the stack you must arrange for them to be destroyed at a
+suitable time; usually by using one of the C<mPUSH*> macros or C<sv_2mortal()>
+to mortalise the xV.
+
+=head2 Mark Stack
+
+The value stack stores individual perl scalar values as temporaries between
+expressions. Some perl expressions operate on entire lists; for that purpose
+we need to know where on the stack each list begins. This is the purpose of the
+mark stack.
+
+The mark stack stores integers as I32 values, which are the height of the
+value stack at the time before the list began; thus the mark itself actually
+points to the value stack entry one before the list. The list itself starts at
+C<mark + 1>.
+
+The base of this stack is pointed to by the interpreter variable
+C<PL_markstack>, of type C<I32 *>.
+
+The head of the stack is C<PL_markstack_ptr>, and points to the most
+recently-pushed item.
+
+Items are pushed to the stack by using the C<PUSHMARK()> macro. Even though
+the stack itself stores (value) stack indices as integers, the C<PUSHMARK>
+macro should be given a stack pointer directly; it will calculate the index
+offset by comparing to the C<PL_stack_sp> variable. Thus almost always the
+code to perform this is
+
+ PUSHMARK(SP);
+
+Items are popped from the stack by the C<POPMARK> macro. There is also a macro
+C<TOPMARK> that inspects the topmost item without removing it. These macros
+return I32 index values directly. There is also the C<dMARK> macro which
+declares a new SV double-pointer variable, called C<mark>, which points at the
+marked stack slot; this is the usual macro that C code will use when operating
+on lists given on the stack.
+
+As noted above, the C<mark> variable itself will point at the most recently
+pushed value on the value stack before the list begins, and so the list itself
+starts at C<mark + 1>. The values of the list may be iterated by code such as
+
+ for(SV **svp = mark + 1; svp <= PL_stack_sp; svp++) {
+ SV *item = *svp;
+ ...
+ }
+
+Note specifically in the case that the list is already empty, C<mark> will
+equal C<PL_stack_sp>.
+
+Because the C<mark> variable is converted to a pointer on the value stack,
+extra care must be taken if C<EXTEND> or any of the C<XPUSH> macros are
+invoked within the function, because the stack may need to be moved to
+extend it and so the existing pointer will now be invalid. If this may be a
+problem, a possible solution is to track the mark offset as an integer and
+track the mark itself later on after the stack had been moved.
+
+ I32 markoff = POPMARK;
+
+ ...
+
+ SP **mark = PL_stack_base + markoff;
+
+=head2 Temporaries Stack
+
+As noted above, xV references on the main value stack do not contribute to the
+reference count of an xV, and so another mechanism is used to track when
+temporary values which live on the stack must be released. This is the job of
+the temporaries stack.
+
+The temporaries stack stores pointers to xVs whose reference counts will be
+decremented soon.
+
+The base of this stack is pointed to by the interpreter variable
+C<PL_tmps_stack>, of type C<SV **>.
+
+The head of the stack is indexed by C<PL_tmps_ix>, an integer which stores the
+index in the array of the most recently-pushed item.
+
+There is no public API to directly push items to the temporaries stack. Instead,
+the API function C<sv_2mortal()> is used to mortalize an xV, adding its
+address to the temporaries stack.
+
+Likewise, there is no public API to read values from the temporaries stack.
+Instead, the macros C<SAVETMPS> and C<FREETMPS> are used. The C<SAVETMPS>
+macro establishes the base levels of the temporaries stack, by capturing the
+current value of C<PL_tmps_ix> into C<PL_tmps_floor> and saving the previous
+value to the save stack. Thereafter, whenever C<FREETMPS> is invoked all of
+the temporaries that have been pushed since that level are reclaimed.
+
+While it is common to see these two macros in pairs within an C<ENTER>/
+C<LEAVE> pair, it is not necessary to match them. It is permitted to invoke
+C<FREETMPS> multiple times since the most recent C<SAVETMPS>; for example in a
+loop iterating over elements of a list. While you can invoke C<SAVETMPS>
+multiple times within a scope pair, it is unlikely to be useful. Subsequent
+invocations will move the temporaries floor further up, thus effectively
+trapping the existing temporaries to only be released at the end of the scope.
+
+=head2 Save Stack
+
+The save stack is used by perl to implement the C<local> keyword and other
+similar behaviours; any cleanup operations that need to be performed when
+leaving the current scope. Items pushed to this stack generally capture the
+current value of some internal variable or state, which will be restored when
+the scope is unwound due to leaving, C<return>, C<die>, C<goto> or other
+reasons.
+
+Whereas other perl internal stacks store individual items all of the same type
+(usually SV pointers or integers), the items pushed to the save stack are
+formed of many different types, having multiple fields to them. For example,
+the C<SAVEt_INT> type needs to store both the address of the C<int> variable
+to restore, and the value to restore it to. This information could have been
+stored using fields of a C<struct>, but would have to be large enough to store
+three pointers in the largest case, which would waste a lot of space in most
+of the smaller cases.
+
+Instead, the stack stores information in a variable-length encoding of C<ANY>
+structures. The final value pushed is stored in the C<UV> field which encodes
+the kind of item held by the preceeding items; the count and types of which
+will depend on what kind of item is being stored. The kind field is pushed
+last because that will be the first field to be popped when unwinding items
+from the stack.
+
+The base of this stack is pointed to by the interpreter variable
+C<PL_savestack>, of type C<ANY *>.
+
+The head of the stack is indexed by C<PL_savestack_ix>, an integer which
+stores the index in the array at which the next item should be pushed. (Note
+that this is different to most other stacks, which reference the most
+recently-pushed item).
+
+Items are pushed to the save stack by using the various C<SAVE...()> macros.
+Many of these macros take a variable and store both its address and current
+value on the save stack, ensuring that value gets restored on scope exit.
+
+ SAVEI8(i8)
+ SAVEI16(i16)
+ SAVEI32(i32)
+ SAVEINT(i)
+ ...
+
+There are also a variety of other special-purpose macros which save particular
+types or values of interest. C<SAVETMPS> has already been mentioned above.
+Others include C<SAVEFREEPV> which arranges for a PV (i.e. a string buffer) to
+be freed, or C<SAVEDESTRUCTOR> which arranges for a given function pointer to
+be invoked on scope exit. A full list of such macros can be found in
+F<scope.h>.
+
+There is no public API for popping individual values or items from the save
+stack. Instead, via the scope stack, the C<ENTER> and C<LEAVE> pair form a way
+to start and stop nested scopes. Leaving a nested scope via C<LEAVE> will
+restore all of the saved values that had been pushed since the most recent
+C<ENTER>.
+
+=head2 Scope Stack
+
+As with the mark stack to the value stack, the scope stack forms a pair with
+the save stack. The scope stack stores the height of the save stack at which
+nested scopes begin, and allows the save stack to be unwound back to that
+point when the scope is left.
+
+When perl is built with debugging enabled, there is a second part to this
+stack storing human-readable string names describing the type of stack
+context. Each push operation saves the name as well as the height of the save
+stack, and each pop operation checks the topmost name with what is expected,
+causing an assertion failure if the name does not match.
+
+The base of this stack is pointed to by the interpreter variable
+C<PL_scopestack>, of type C<I32 *>. If enabled, the scope stack names are
+stored in a separate array pointed to by C<PL_scopestack_name>, of type
+C<const char **>.
+
+The head of the stack is indexed by C<PL_scopestack_ix>, an integer which
+stores the index of the array or arrays at which the next item should be
+pushed. (Note that this is different to most other stacks, which reference the
+most recently-pushed item).
+
+Values are pushed to the scope stack using the C<ENTER> macro, which begins a
+new nested scope. Any items pushed to the save stack are then restored at the
+next nested invocation of the C<LEAVE> macro.
=head1 Dynamic Scope and the Context Stack
The steps above are in a very specific order, designed to be the reverse
order of when the context was pushed. The first thing to do is to copy
-and/or protect any any return arguments and free any temps in the current
+and/or protect any return arguments and free any temps in the current
scope. Scope exits like an rvalue sub normally return a mortal copy of
their return args (as opposed to lvalue subs). It is important to make
this copy before the save stack is popped or variables are restored, or
C<FREETMPS>.
+=head1 Slab-based operator allocation
+
+B<Note:> this section describes a non-public internal API that is subject
+to change without notice.
+
+Perl's internal error-handling mechanisms implement C<die> (and its internal
+equivalents) using longjmp. If this occurs during lexing, parsing or
+compilation, we must ensure that any ops allocated as part of the compilation
+process are freed. (Older Perl versions did not adequately handle this
+situation: when failing a parse, they would leak ops that were stored in
+C C<auto> variables and not linked anywhere else.)
+
+To handle this situation, Perl uses I<op slabs> that are attached to the
+currently-compiling CV. A slab is a chunk of allocated memory. New ops are
+allocated as regions of the slab. If the slab fills up, a new one is created
+(and linked from the previous one). When an error occurs and the CV is freed,
+any ops remaining are freed.
+
+Each op is preceded by two pointers: one points to the next op in the slab, and
+the other points to the slab that owns it. The next-op pointer is needed so
+that Perl can iterate over a slab and free all its ops. (Op structures are of
+different sizes, so the slab's ops can't merely be treated as a dense array.)
+The slab pointer is needed for accessing a reference count on the slab: when
+the last op on a slab is freed, the slab itself is freed.
+
+The slab allocator puts the ops at the end of the slab first. This will tend to
+allocate the leaves of the op tree first, and the layout will therefore
+hopefully be cache-friendly. In addition, this means that there's no need to
+store the size of the slab (see below on why slabs vary in size), because Perl
+can follow pointers to find the last op.
+
+It might seem possible to eliminate slab reference counts altogether, by having
+all ops implicitly attached to C<PL_compcv> when allocated and freed when the
+CV is freed. That would also allow C<op_free> to skip C<FreeOp> altogether, and
+thus free ops faster. But that doesn't work in those cases where ops need to
+survive beyond their CVs, such as re-evals.
+
+The CV also has to have a reference count on the slab. Sometimes the first op
+created is immediately freed. If the reference count of the slab reaches 0,
+then it will be freed with the CV still pointing to it.
+
+CVs use the C<CVf_SLABBED> flag to indicate that the CV has a reference count
+on the slab. When this flag is set, the slab is accessible via C<CvSTART> when
+C<CvROOT> is not set, or by subtracting two pointers C<(2*sizeof(I32 *))> from
+C<CvROOT> when it is set. The alternative to this approach of sneaking the slab
+into C<CvSTART> during compilation would be to enlarge the C<xpvcv> struct by
+another pointer. But that would make all CVs larger, even though slab-based op
+freeing is typically of benefit only for programs that make significant use of
+string eval.
+
+When the C<CVf_SLABBED> flag is set, the CV takes responsibility for freeing
+the slab. If C<CvROOT> is not set when the CV is freed or undeffed, it is
+assumed that a compilation error has occurred, so the op slab is traversed and
+all the ops are freed.
+
+Under normal circumstances, the CV forgets about its slab (decrementing the
+reference count) when the root is attached. So the slab reference counting that
+happens when ops are freed takes care of freeing the slab. In some cases, the
+CV is told to forget about the slab (C<cv_forget_slab>) precisely so that the
+ops can survive after the CV is done away with.
+
+Forgetting the slab when the root is attached is not strictly necessary, but
+avoids potential problems with C<CvROOT> being written over. There is code all
+over the place, both in core and on CPAN, that does things with C<CvROOT>, so
+forgetting the slab makes things more robust and avoids potential problems.
+
+Since the CV takes ownership of its slab when flagged, that flag is never
+copied when a CV is cloned, as one CV could free a slab that another CV still
+points to, since forced freeing of ops ignores the reference count (but asserts
+that it looks right).
+
+To avoid slab fragmentation, freed ops are marked as freed and attached to the
+slab's freed chain (an idea stolen from DBM::Deep). Those freed ops are reused
+when possible. Not reusing freed ops would be simpler, but it would result in
+significantly higher memory usage for programs with large C<if (DEBUG) {...}>
+blocks.
+
+C<SAVEFREEOP> is slightly problematic under this scheme. Sometimes it can cause
+an op to be freed after its CV. If the CV has forcibly freed the ops on its
+slab and the slab itself, then we will be fiddling with a freed slab. Making
+C<SAVEFREEOP> a no-op doesn't help, as sometimes an op can be savefreed when
+there is no compilation error, so the op would never be freed. It holds
+a reference count on the slab, so the whole slab would leak. So C<SAVEFREEOP>
+now sets a special flag on the op (C<< ->op_savefree >>). The forced freeing of
+ops after a compilation error won't free any ops thus marked.
+
+Since many pieces of code create tiny subroutines consisting of only a few ops,
+and since a huge slab would be quite a bit of baggage for those to carry
+around, the first slab is always very small. To avoid allocating too many
+slabs for a single CV, each subsequent slab is twice the size of the previous.
+
+Smartmatch expects to be able to allocate an op at run time, run it, and then
+throw it away. For that to work the op is simply malloced when PL_compcv hasn't
+been set up. So all slab-allocated ops are marked as such (C<< ->op_slabbed >>),
+to distinguish them from malloced ops.
+
+
=head1 AUTHORS
Until May 1997, this document was maintained by Jeff Okamoto
https://perl5.git.perl.org / perl5.git / blobdiff