value (UV), a double (NV), a string (PV), and another scalar (SV).
("PV" stands for "Pointer Value". You might think that it is misnamed
because it is described as pointing only to strings. However, it is
-possible to have it point to other things For example, it could point
+possible to have it point to other things. For example, it could point
to an array of UVs. But,
using it for non-strings requires care, as the underlying assumption of
much of the internals is that PVs are just for strings. Often, for
SV* newSVpvf(const char*, ...);
SV* newSVsv(SV*);
-C<STRLEN> is an integer type (Size_t, usually defined as size_t in
+C<STRLEN> is an integer type (C<Size_t>, usually defined as C<size_t> in
F<config.h>) guaranteed to be large enough to represent the size of
any string that perl can handle.
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 *);
+ SV **, Size_t, bool *);
void sv_setsv(SV*, SV*);
Notice that you can choose to specify the length of the string to be
the format.
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.
+that have "magic". See L</Magic Virtual Tables> later in this document.
All SVs that contain strings should be terminated with a C<NUL> character.
If it is not C<NUL>-terminated there is a risk of
add space for the trailing C<NUL> byte (perl's own string functions typically do
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
+a PV. This will remove any of various types of non-stringness from
+the SV while preserving the content of the SV in the PV. This can be
+used, for example, to append data from an API function to a buffer
+without extra copying:
+
+ (void)SvPVbyte_force(sv, len);
+ s = SvGROW(sv, len + needlen + 1);
+ /* something that modifies up to needlen bytes at s+len, but
+ modifies newlen bytes
+ eg. newlen = read(fd, s + len, needlen);
+ ignoring errors for these examples
+ */
+ s[len + newlen] = '\0';
+ SvCUR_set(sv, len + newlen);
+ SvUTF8_off(sv);
+ SvSETMAGIC(sv);
+
+If you already have the data in memory or if you want to keep your
+code simple, you can use one of the sv_cat*() variants, such as
+sv_catpvn(). If you want to insert anywhere in the string you can use
+sv_insert() or sv_insert_flags().
+
+If you don't need the existing content of the SV, you can avoid some
+copying with:
+
+ SvPVCLEAR(sv);
+ s = SvGROW(sv, needlen + 1);
+ /* something that modifies up to needlen bytes at s, but modifies
+ newlen bytes
+ eg. newlen = read(fd, s. needlen);
+ */
+ s[newlen] = '\0';
+ SvCUR_set(sv, newlen);
+ SvPOK_only(sv); /* also clears SVf_UTF8 */
+ SvSETMAGIC(sv);
+
+Again, if you already have the data in memory or want to avoid the
+complexity of the above, you can use sv_setpvn().
+
+If you have a buffer allocated with Newx() and want to set that as the
+SV's value, you can use sv_usepvn_flags(). That has some requirements
+if you want to avoid perl re-allocating the buffer to fit the trailing
+NUL:
+
+ Newx(buf, somesize+1, char);
+ /* ... fill in buf ... */
+ buf[somesize] = '\0';
+ sv_usepvn_flags(sv, buf, somesize, SV_SMAGIC | SV_HAS_TRAILING_NUL);
+ /* buf now belongs to perl, don't release it */
+
If you have an SV and want to know what kind of data Perl thinks is stored
in it, you can use the following macros to check the type of SV you have.
to be interpreted as a string.
The C<sv_cat*()> functions are not generic enough to operate on values that
-have "magic". See L<Magic Virtual Tables> later in this document.
+have "magic". See L</Magic Virtual Tables> later in this document.
If you know the name of a scalar variable, you can get a pointer to its SV
by using the following:
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>).
+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
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>).
+call is not necessary (see L</Reference Counts and Mortality>).
=head2 Offsets
at C<SvPVX(sv) - SvIV(sv)> in memory and the PV pointer is pointing
into the middle of this allocated storage.
-This is best demonstrated by example:
+This is best demonstrated by example. Normally copy-on-write will prevent
+the substitution from operator from using this hack, but if you can craft a
+string for which copy-on-write is not possible, you can see it in play. In
+the current implementation, the final byte of a string buffer is used as a
+copy-on-write reference count. If the buffer is not big enough, then
+copy-on-write is skipped. First have a look at an empty string:
- % ./perl -Ilib -MDevel::Peek -le '$a="12345"; $a=~s/.//; Dump($a)'
- SV = PVIV(0x8128450) at 0x81340f0
+ % ./perl -Ilib -MDevel::Peek -le '$a=""; $a .= ""; Dump $a'
+ SV = PV(0x7ffb7c008a70) at 0x7ffb7c030390
REFCNT = 1
- FLAGS = (POK,OOK,pPOK)
- IV = 1 (OFFSET)
- PV = 0x8135781 ( "1" . ) "2345"\0
- CUR = 4
- LEN = 5
-
-Here the number of bytes chopped off (1) is put into IV, and
-C<Devel::Peek::Dump> helpfully reminds us that this is an offset. The
+ FLAGS = (POK,pPOK)
+ PV = 0x7ffb7bc05b50 ""\0
+ CUR = 0
+ LEN = 10
+
+Notice here the LEN is 10. (It may differ on your platform.) Extend the
+length of the string to one less than 10, and do a substitution:
+
+ % ./perl -Ilib -MDevel::Peek -le '$a=""; $a.="123456789"; $a=~s/.//; \
+ Dump($a)'
+ SV = PV(0x7ffa04008a70) at 0x7ffa04030390
+ REFCNT = 1
+ FLAGS = (POK,OOK,pPOK)
+ OFFSET = 1
+ PV = 0x7ffa03c05b61 ( "\1" . ) "23456789"\0
+ CUR = 8
+ LEN = 9
+
+Here the number of bytes chopped off (1) is shown next as the OFFSET. The
portion of the string between the "real" and the "fake" beginnings is
shown in parentheses, and the values of C<SvCUR> and C<SvLEN> reflect
-the fake beginning, not the real one.
+the fake beginning, not the real one. (The first character of the string
+buffer happens to have changed to "\1" here, not "1", because the current
+implementation stores the offset count in the string buffer. This is
+subject to change.)
Something similar to the offset hack is performed on AVs to enable
efficient shifting and splicing off the beginning of the array; while
This returns NULL if the variable does not exist.
-See L<Understanding the Magic of Tied Hashes and Arrays> for more
+See L</Understanding the Magic of Tied Hashes and Arrays> for more
information on how to use the array access functions on tied arrays.
=head2 Working with HVs
of perl, and the return value may change per invocation, so the value
is only valid for the duration of a single perl process.
-See L<Understanding the Magic of Tied Hashes and Arrays> for more
+See L</Understanding the Magic of Tied Hashes and Arrays> for more
information on how to use the hash access functions on tied hashes.
=head2 Hash API Extensions
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.
+L</Stashes and Globs> for information on converting class names into stashes.
/* Still under construction */
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.
-
-This normally doesn't happen at the Perl level unless a variable is
-undef'ed or the last variable holding a reference to it is changed or
-overwritten. At the internal level, however, reference counts can be
-manipulated with the following macros:
+At the most basic internal level, reference counts can be manipulated
+with the following macros:
int SvREFCNT(SV* sv);
SV* SvREFCNT_inc(SV* sv);
void SvREFCNT_dec(SV* sv);
-However, there is one other function which manipulates the reference
-count of its argument. The C<newRV_inc> function, you will recall,
-creates a reference to the specified argument. As a side effect,
-it increments the argument's reference count. If this is not what
-you want, use C<newRV_noinc> instead.
-
-For example, imagine you want to return a reference from an XSUB function.
-Inside the XSUB routine, you create an SV which initially has a reference
-count of one. Then you call C<newRV_inc>, passing it the just-created SV.
-This returns the reference as a new SV, but the reference count of the
-SV you passed to C<newRV_inc> has been incremented to two. Now you
-return the reference from the XSUB routine and forget about the SV.
-But Perl hasn't! Whenever the returned reference is destroyed, the
-reference count of the original SV is decreased to one and nothing happens.
-The SV will hang around without any way to access it until Perl itself
-terminates. This is a memory leak.
-
-The correct procedure, then, is to use C<newRV_noinc> instead of
-C<newRV_inc>. Then, if and when the last reference is destroyed,
-the reference count of the SV will go to zero and it will be destroyed,
-stopping any memory leak.
+(There are also suffixed versions of the increment and decrement macros,
+for situations where the full generality of these basic macros can be
+exchanged for some performance.)
+
+However, the way a programmer should think about references is not so
+much in terms of the bare reference count, but in terms of I<ownership>
+of references. A reference to an xV can be owned by any of a variety
+of entities: another xV, the Perl interpreter, an XS data structure,
+a piece of running code, or a dynamic scope. An xV generally does not
+know what entities own the references to it; it only knows how many
+references there are, which is the reference count.
+
+To correctly maintain reference counts, it is essential to keep track
+of what references the XS code is manipulating. The programmer should
+always know where a reference has come from and who owns it, and be
+aware of any creation or destruction of references, and any transfers
+of ownership. Because ownership isn't represented explicitly in the xV
+data structures, only the reference count need be actually maintained
+by the code, and that means that this understanding of ownership is not
+actually evident in the code. For example, transferring ownership of a
+reference from one owner to another doesn't change the reference count
+at all, so may be achieved with no actual code. (The transferring code
+doesn't touch the referenced object, but does need to ensure that the
+former owner knows that it no longer owns the reference, and that the
+new owner knows that it now does.)
+
+An xV that is visible at the Perl level should not become unreferenced
+and thus be destroyed. Normally, an object will only become unreferenced
+when it is no longer visible, often by the same means that makes it
+invisible. For example, a Perl reference value (RV) owns a reference to
+its referent, so if the RV is overwritten that reference gets destroyed,
+and the no-longer-reachable referent may be destroyed as a result.
+
+Many functions have some kind of reference manipulation as
+part of their purpose. Sometimes this is documented in terms
+of ownership of references, and sometimes it is (less helpfully)
+documented in terms of changes to reference counts. For example, the
+L<newRV_inc()|perlapi/newRV_inc> function is documented to create a new RV
+(with reference count 1) and increment the reference count of the referent
+that was supplied by the caller. This is best understood as creating
+a new reference to the referent, which is owned by the created RV,
+and returning to the caller ownership of the sole reference to the RV.
+The L<newRV_noinc()|perlapi/newRV_noinc> function instead does not
+increment the reference count of the referent, but the RV nevertheless
+ends up owning a reference to the referent. It is therefore implied
+that the caller of C<newRV_noinc()> is relinquishing a reference to the
+referent, making this conceptually a more complicated operation even
+though it does less to the data structures.
+
+For example, imagine you want to return a reference from an XSUB
+function. Inside the XSUB routine, you create an SV which initially
+has just a single reference, owned by the XSUB routine. This reference
+needs to be disposed of before the routine is complete, otherwise it
+will leak, preventing the SV from ever being destroyed. So to create
+an RV referencing the SV, it is most convenient to pass the SV to
+C<newRV_noinc()>, which consumes that reference. Now the XSUB routine
+no longer owns a reference to the SV, but does own a reference to the RV,
+which in turn owns a reference to the SV. The ownership of the reference
+to the RV is then transferred by the process of returning the RV from
+the XSUB.
There are some convenience functions available that can help with the
destruction of xVs. These functions introduce the concept of "mortality".
-An xV that is mortal has had its reference count marked to be decremented,
-but not actually decremented, until "a short time later". Generally the
-term "short time later" means a single Perl statement, such as a call to
-an XSUB function. The actual determinant for when mortal xVs have their
-reference count decremented depends on two macros, SAVETMPS and FREETMPS.
-See L<perlcall> and L<perlxs> for more details on these macros.
-
-"Mortalization" then is at its simplest a deferred C<SvREFCNT_dec>.
-However, if you mortalize a variable twice, the reference count will
-later be decremented twice.
-
-"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:
+Much documentation speaks of an xV itself being mortal, but this is
+misleading. It is really I<a reference to> an xV that is mortal, and it
+is possible for there to be more than one mortal reference to a single xV.
+For a reference to be mortal means that it is owned by the temps stack,
+one of perl's many internal stacks, which will destroy that reference
+"a short time later". Usually the "short time later" is the end of
+the current Perl statement. However, it gets more complicated around
+dynamic scopes: there can be multiple sets of mortal references hanging
+around at the same time, with different death dates. Internally, the
+actual determinant for when mortal xV references are destroyed depends
+on two macros, SAVETMPS and FREETMPS. See L<perlcall> and L<perlxs>
+for more details on these macros.
+
+Mortal references are mainly used for xVs that are placed on perl's
+main stack. The stack is problematic for reference tracking, because it
+contains a lot of xV references, but doesn't own those references: they
+are not counted. Currently, there are many bugs resulting from xVs being
+destroyed while referenced by the stack, because the stack's uncounted
+references aren't enough to keep the xVs alive. So when putting an
+(uncounted) reference on the stack, it is vitally important to ensure that
+there will be a counted reference to the same xV that will last at least
+as long as the uncounted reference. But it's also important that that
+counted reference be cleaned up at an appropriate time, and not unduly
+prolong the xV's life. For there to be a mortal reference is often the
+best way to satisfy this requirement, especially if the xV was created
+especially to be put on the stack and would otherwise be unreferenced.
+
+To create a mortal reference, use the functions:
SV* sv_newmortal()
- SV* sv_2mortal(SV*)
SV* sv_mortalcopy(SV*)
+ SV* sv_2mortal(SV*)
-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. :
+C<sv_newmortal()> creates an SV (with the undefined value) whose sole
+reference is mortal. C<sv_mortalcopy()> creates an xV whose value is a
+copy of a supplied xV and whose sole reference is mortal. C<sv_2mortal()>
+mortalises an existing xV reference: it transfers ownership of a reference
+from the caller to the temps stack. 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);
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 a 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
C<sv_2mortal> or C<sv_mortalcopy> routines.
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 L<Magic Virtual Tables> 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 in F<perl.h>. Note that before
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 C<PERL_MAGIC_arylen>, 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>, C<PERL_MAGIC_regdatum>,
+C<PERL_MAGIC_regdata>, 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.
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);
- U32 (*svt_len)(SV* sv, MAGIC* mg);
- int (*svt_clear)(SV* sv, MAGIC* mg);
- int (*svt_free)(SV* sv, MAGIC* mg);
+ int (*svt_get) (pTHX_ SV* sv, MAGIC* mg);
+ int (*svt_set) (pTHX_ SV* sv, MAGIC* mg);
+ U32 (*svt_len) (pTHX_ SV* sv, MAGIC* mg);
+ int (*svt_clear)(pTHX_ SV* sv, MAGIC* mg);
+ int (*svt_free) (pTHX_ SV* sv, MAGIC* mg);
- int (*svt_copy)(SV *sv, MAGIC* mg, SV *nsv,
+ int (*svt_copy) (pTHX_ 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);
+ int (*svt_dup) (pTHX_ MAGIC *mg, CLONE_PARAMS *param);
+ int (*svt_local)(pTHX_ SV *nsv, MAGIC *mg);
This MGVTBL structure is set at compile-time in F<perl.h> and there are
-------------------------- ------ -------------
\0 PERL_MAGIC_sv vtbl_sv Special scalar variable
# PERL_MAGIC_arylen vtbl_arylen Array length ($#ary)
- % PERL_MAGIC_rhash (none) extra data for restricted
+ % PERL_MAGIC_rhash (none) Extra data for restricted
hashes
- & PERL_MAGIC_proto (none) my sub prototype CV
+ * PERL_MAGIC_debugvar vtbl_debugvar $DB::single, signal, trace
+ vars
. PERL_MAGIC_pos vtbl_pos pos() lvalue
- : PERL_MAGIC_symtab (none) extra data for symbol
+ : PERL_MAGIC_symtab (none) Extra data for symbol
tables
- < PERL_MAGIC_backref vtbl_backref for weak ref data
- @ PERL_MAGIC_arylen_p (none) to move arylen out of XPVAV
+ < PERL_MAGIC_backref vtbl_backref For weak ref data
+ @ PERL_MAGIC_arylen_p (none) To move arylen out of XPVAV
B PERL_MAGIC_bm vtbl_regexp Boyer-Moore
(fast string search)
c PERL_MAGIC_overload_table vtbl_ovrld Holds overload table
P PERL_MAGIC_tied vtbl_pack Tied array or hash
p PERL_MAGIC_tiedelem vtbl_packelem Tied array or hash element
q PERL_MAGIC_tiedscalar vtbl_packelem Tied scalar or handle
- r PERL_MAGIC_qr vtbl_regexp precompiled qr// regex
+ r PERL_MAGIC_qr vtbl_regexp Precompiled qr// regex
S PERL_MAGIC_sig (none) %SIG hash
s PERL_MAGIC_sigelem vtbl_sigelem %SIG hash element
t PERL_MAGIC_taint vtbl_taint Taintedness
y PERL_MAGIC_defelem vtbl_defelem Shadow "foreach" iterator
variable / smart parameter
vivification
- ] PERL_MAGIC_checkcall vtbl_checkcall inlining/mutation of call
+ \ PERL_MAGIC_lvref vtbl_lvref Lvalue reference
+ constructor
+ ] PERL_MAGIC_checkcall vtbl_checkcall Inlining/mutation of call
to this CV
~ PERL_MAGIC_ext (none) Available for use by
extensions
The perl tie function associates a variable with an object that implements
the various GET, SET, etc methods. To perform the equivalent of the perl
tie function from an XSUB, you must mimic this behaviour. The code below
-carries out the necessary steps - firstly it creates a new hash, and then
+carries out the necessary steps -- firstly it creates a new hash, and then
creates a second hash which it blesses into the class which will implement
the tie methods. Lastly it ties the two hashes together, and returns a
reference to the new tied hash. Note that the code below does NOT call the
TIEHASH method in the MyTie class -
-see L<Calling Perl Routines from within C Programs> for details on how
+see L</Calling Perl Routines from within C Programs> for details on how
to do this.
SV*
=item C<SAVEFREESV(SV *sv)>
-The refcount of C<sv> would be decremented at the end of
+The refcount of C<sv> will be decremented at the end of
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,
not constantly freed/created.
Each of the targets is created only once (but see
-L<Scratchpads and recursion> below), and when an opcode needs to put
+L</Scratchpads and recursion> below), and when an opcode needs to put
an integer, a double, or a string on stack, it just sets the
corresponding parts of its I<target> and puts the I<target> on stack.
targets for opcodes. A previous version of this document
stated that one can deduce that an SV lives on a scratchpad
by looking on its flags: lexicals have C<SVs_PADMY> set, and
-I<target>s have C<SVs_PADTMP> set. But this have never been fully true.
+I<target>s have C<SVs_PADTMP> set. But this has never been fully true.
C<SVs_PADMY> could be set on a variable that no longer resides in any pad.
While I<target>s do have C<SVs_PADTMP> set, it can also be set on variables
-that have never resided in a pad, but nonetheless act like I<target>s.
+that have never resided in a pad, but nonetheless act like I<target>s. As
+of perl 5.21.5, the C<SVs_PADMY> flag is no longer used and is defined as
+0. C<SvPADMY()> now returns true for anything without C<SVs_PADTMP>.
The correspondence between OPs and I<target>s is not 1-to-1. Different
OPs in the compile tree of the unit can use the same target, if this
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.
+following the C<OpSIBLING> pointer from the first child to the last (but
+see below).
-There are also
two other op types: a C<PMOP> holds a regular expression,
+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
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.
+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
+last child. Instead it has an C<op_other> field, which is comparable to
+the C<op_next> field described below, and represents an alternate
+execution path. Operators like C<and>, C<or> and C<?> are C<LOGOP>s. Note
+that in general, C<op_other> may not point to any of the direct children
+of the C<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
+C<OpSIBLING> chain. This frees up the C<op_sibling> field on the last
+sibling to point back to the parent op. Under this build, that field is
+also renamed C<op_sibparent> to reflect its joint role. The macro
+C<OpSIBLING(o)> wraps this special behaviour, and always returns NULL on
+the last sibling. With this build the C<op_parent(o)> function can be
+used to find the parent of any op. Thus for forward compatibility, you
+should always use the C<OpSIBLING(o)> macro rather than accessing
+C<op_sibling> directly.
+
Another way to examine the tree is to use a compiler back-end module, such
as L<B::Concise>.
creates two scopes: the first starts at the C<(> and has C<full == 1>,
the second starts at the C<{> and has C<full == 0>. Both end at the
-C<}>, so calls to C<start> and C<pre/post_end> will match. Anything
+C<}>, so calls to C<start> and C<pre>/C<post_end> will match. Anything
pushed onto the save stack by this hook will be popped just before the
scope ends (between the C<pre_> and C<post_end> hooks, in fact).
is also possible to use the C<BhkDISABLE> and C<BhkENABLE> macros to
temporarily switch entries on and off. You should also be aware that
generally speaking at least one scope will have opened before your
-extension is loaded, so you will see some C<pre/post_end> pairs that
+extension is loaded, so you will see some C<pre>/C<post_end> pairs that
didn't have a matching C<start>.
=head1 Examining internal data structures with the C<dump> functions
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>:
+To see whether you have non-const data you can use a BSD (or GNU)
+compatible C<nm>:
nm libperl.a | grep -v ' [TURtr] '
-If this displays any C<D> or C<d> symbols, you have non-const data.
+If this displays any C<D> or C<d> symbols (or possibly C<C> or C<c>),
+you have non-const data. The symbols the C<grep> removed are as follows:
+C<Tt> are I<text>, or code, the C<Rr> are I<read-only> (const) data,
+and the C<U> is <undefined>, external symbols referred to.
+
+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
The IVdf will expand to whatever is the correct format for the IVs.
+Note that there are different "long doubles": Perl will use
+whatever the compiler has.
+
If you are printing addresses of pointers, use UVxf combined
with PTR2UV(), do not use %lx or %p.
+=head2 Formatted Printing of C<Size_t> and C<SSize_t>
+
+The most general way to do this is to cast them to a UV or IV, and
+print as in the
+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>):
+
+ PerlIO_printf("STRLEN is %zu\n", len);
+
+This modifier is not portable, so its use should be restricted to
+C<PerlIO_printf()>.
+
=head2 Pointer-To-Integer and Integer-To-Pointer
Because pointer size does not necessarily equal integer size,
=head2 Source Documentation
There's an effort going on to document the internal functions and
-automatically produce reference manuals from them - L<perlapi> is one
+automatically produce reference manuals from them -
- L<perlapi> is one
such manual which details all the functions which are available to XS
writers. L<perlintern> is the autogenerated manual for the functions
which are not part of the API and are supposedly for internal use only.
=for apidoc sv_setiv
Copies an integer into the given SV. Does not handle 'set' magic. See
- C<sv_setiv_mg>.
+ L<perlapi/sv_setiv_mg>.
=cut
*/
a variable number of bytes to represent a character. You can learn more
about Unicode and Perl's Unicode model in L<perlunicode>.
+(On EBCDIC platforms, Perl uses instead UTF-EBCDIC, which is a form of
+UTF-8 adapted for EBCDIC platforms. Below, we just talk about UTF-8.
+UTF-EBCDIC is like UTF-8, but the details are different. The macros
+hide the differences from you, just remember that the particular numbers
+and bit patterns presented below will differ in UTF-EBCDIC.)
+
=head2 How can I recognise a UTF-8 string?
You can't. This is because UTF-8 data is stored in bytes just like
non-UTF-8 data. The Unicode character 200, (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
+has that byte sequence as well. So you can't tell just by looking -- this
is what makes Unicode input an interesting problem.
In general, you either have to know what you're dealing with, or you
have to guess. The API function 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<isUTF8_CHAR>
+you if a string contains only valid UTF-8 characters, and the chances
+of a non-UTF-8 string looking like valid UTF-8 become very small very
+quickly with increasing string length. On a character-by-character
+basis, C<isUTF8_CHAR>
will tell you whether the current character in a string is valid UTF-8.
=head2 How does UTF-8 represent Unicode characters?
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
+C<10111111>) so we move on; character 192 is C<v195.128>. And
so it goes on, moving to three bytes at character 2048.
+L<perlunicode/Unicode Encodings> has pictures of how this works.
Assuming you know you're dealing with a UTF-8 string, you can find out
how long the first character in it is with the C<UTF8SKIP> macro:
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
+character like this (the C<UTF8_IS_INVARIANT()> is a macro that tests
whether the byte is encoded as a single byte even in UTF-8):
U8 *utf;
value of the character; the inverse function C<uvchr_to_utf8> is available
for putting a UV into UTF-8:
- if (!UTF8_IS_INVARIANT(uv))
+ if (!UVCHR_IS_INVARIANT(uv))
/* Must treat this as UTF8 */
utf8 = uvchr_to_utf8(utf8, uv);
else
that character, you can never match a C<chr(200)> in a non-UTF-8 string.
So don't do that!
+(Note that we don't have to test for invariant characters in the
+examples above. The functions work on any well-formed UTF-8 input.
+It's just that its faster to avoid the function overhead when it's not
+needed.)
+
=head2 How does Perl store UTF-8 strings?
-Currently, Perl deals with Unicode strings and non-Unicode strings
+Currently, Perl deals with UTF-8 strings and non-UTF-8 strings
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, but C<use feature 'unicode_strings'> is needed to get iso-8859-1
-semantics). This flag is only meaningful if the SV is C<SvPOK>
-or immediately after stringification via C<SvPV> or a similar
-macro. You can check and manipulate this flag with the
+codepoint number and vice versa. This flag is only meaningful if the SV
+is C<SvPOK> or immediately after stringification via C<SvPV> or a
+similar macro. You can check and manipulate this flag with the
following macros:
SvUTF8(sv)
SvUTF8_off(sv)
This flag has an important effect on Perl's treatment of the string: if
-Unicode data is not properly distinguished, regular expressions,
+UTF-8 data is not properly distinguished, regular expressions,
C<length>, C<substr> and other string handling operations will have
-undesirable results.
+undesirable (wrong) results.
The problem comes when you have, for instance, a string that isn't
-flagged as UTF-8, and contains a byte sequence that could be UTF-8 -
+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
+Never forget that the C<SVf_UTF8> flag is separate from the PV value; you
+need to be sure you don't accidentally knock it off while you're
manipulating SVs. More specifically, you cannot expect to do this:
SV *sv;
accordingly:
p = SvPV(sv, len);
- frobnicate(p);
+ is_utf8 = SvUTF8(sv);
+ frobnicate(p, is_utf8);
nsv = newSVpvn(p, len);
- if (SvUTF8(sv))
+ if (is_utf8)
SvUTF8_on(nsv);
-In fact, your C<frobnicate> function should be made aware of whether or
-not it's dealing with UTF-8 data, so that it can handle the string
-appropriately.
+In the above, your C<frobnicate> function has been changed to be made
+aware of whether or not it's dealing with UTF-8 data, so that it can
+handle the string appropriately.
Since just passing an SV to an XS function and copying the data of
the SV is not enough to copy the UTF8 flags, even less right is just
-passing a C<char *> to an XS function.
+passing a S<C<char *>> to an XS function.
+
+For full generality, use the L<C<DO_UTF8>|perlapi/DO_UTF8> macro to see if the
+string in an SV is to be I<treated> as UTF-8. This takes into account
+if the call to the XS function is being made from within the scope of
+L<S<C<use bytes>>|bytes>. If so, the underlying bytes that comprise the
+UTF-8 string are to be exposed, rather than the character they
+represent. But this pragma should only really be used for debugging and
+perhaps low-level testing at the byte level. Hence most XS code need
+not concern itself with this, but various areas of the perl core do need
+to support it.
+
+And this isn't the whole story. Starting in Perl v5.12, strings that
+aren't encoded in UTF-8 may also be treated as Unicode under various
+conditions (see L<perlunicode/ASCII Rules versus Unicode Rules>).
+This is only really a problem for characters whose ordinals are between
+128 and 255, and their behavior varies under ASCII versus Unicode rules
+in ways that your code cares about (see L<perlunicode/The "Unicode Bug">).
+There is no published API for dealing with this, as it is subject to
+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 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
+the non-UTF-8 strings to UTF-8. If you've got an SV, the easiest way to do
this is:
sv_utf8_upgrade(sv);
the string contains any characters above 255 that can't be represented
in a single byte.
+=head2 How do I compare strings?
+
+L<perlapi/sv_cmp> and L<perlapi/sv_cmp_flags> do a lexigraphic
+comparison of two SV's, and handle UTF-8ness properly. Note, however,
+that Unicode specifies a much fancier mechanism for collation, available
+via the L<Unicode::Collate> module.
+
+To just compare two strings for equality/non-equality, you can just use
+L<C<memEQ()>|perlapi/memEQ> and L<C<memNE()>|perlapi/memEQ> as usual,
+except the strings must be both UTF-8 or not UTF-8 encoded.
+
+To compare two strings case-insensitively, use
+L<C<foldEQ_utf8()>|perlapi/foldEQ_utf8> (the strings don't have to have
+the same UTF-8ness).
+
=head2 Is there anything else I need to know?
Not really. Just remember these things:
=item *
-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 its C<SvUTF8> flag after stringifying it
-with C<SvPV> or a similar macro. 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.
+There's no way to tell if a S<C<char *>> or S<C<U8 *>> string is UTF-8
+or not. But you can tell if an SV is to be treated as UTF-8 by calling
+C<DO_UTF8> on it, after stringifying it with C<SvPV> or a similar
+macro. And, you can tell if SV is actually UTF-8 (even if it is not to
+be treated as such) by looking at its C<SvUTF8> flag (again after
+stringifying it). Don't forget to set the flag if something should be
+UTF-8.
+Treat the flag as part of the PV, even though it's not -- if you pass on
+the PV to somewhere, pass on the flag too.
=item *
=item *
-When writing a character
C<uv> to a UTF-8 string, B<always> use
-C<uvchr_to_utf8>, unless C<UTF8_IS_INVARIANT(uv))> in which case
+When writing a character
UV to a UTF-8 string, B<always> use
+C<uvchr_to_utf8>, unless C<UVCHR_IS_INVARIANT(uv))> in which case
you can use C<*s = uv>.
=item *
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.
+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
C<B::Generate> directly supports the creation of custom ops by name.
+
+=head1 Dynamic Scope and the Context Stack
+
+B<Note:> this section describes a non-public internal API that is subject
+to change without notice.
+
+=head2 Introduction to the context stack
+
+In Perl, dynamic scoping refers to the runtime nesting of things like
+subroutine calls, evals etc, as well as the entering and exiting of block
+scopes. For example, the restoring of a C<local>ised variable is
+determined by the dynamic scope.
+
+Perl tracks the dynamic scope by a data structure called the context
+stack, which is an array of C<PERL_CONTEXT> structures, and which is
+itself a big union for all the types of context. Whenever a new scope is
+entered (such as a block, a C<for> loop, or a subroutine call), a new
+context entry is pushed onto the stack. Similarly when leaving a block or
+returning from a subroutine call etc. a context is popped. Since the
+context stack represents the current dynamic scope, it can be searched.
+For example, C<next LABEL> searches back through the stack looking for a
+loop context that matches the label; C<return> pops contexts until it
+finds a sub or eval context or similar; C<caller> examines sub contexts on
+the stack.
+
+Each context entry is labelled with a context type, C<cx_type>. Typical
+context types are C<CXt_SUB>, C<CXt_EVAL> etc., as well as C<CXt_BLOCK>
+and C<CXt_NULL> which represent a basic scope (as pushed by C<pp_enter>)
+and a sort block. The type determines which part of the context union are
+valid.
+
+The main division in the context struct is between a substitution scope
+(C<CXt_SUBST>) and block scopes, which are everything else. The former is
+just used while executing C<s///e>, and won't be discussed further
+here.
+
+All the block scope types share a common base, which corresponds to
+C<CXt_BLOCK>. This stores the old values of various scope-related
+variables like C<PL_curpm>, as well as information about the current
+scope, such as C<gimme>. On scope exit, the old variables are restored.
+
+Particular block scope types store extra per-type information. For
+example, C<CXt_SUB> stores the currently executing CV, while the various
+for loop types might hold the original loop variable SV. On scope exit,
+the per-type data is processed; for example the CV has its reference count
+decremented, and the original loop variable is restored.
+
+The macro C<cxstack> returns the base of the current context stack, while
+C<cxstack_ix> is the index of the current frame within that stack.
+
+In fact, the context stack is actually part of a stack-of-stacks system;
+whenever something unusual is done such as calling a C<DESTROY> or tie
+handler, a new stack is pushed, then popped at the end.
+
+Note that the API described here changed considerably in perl 5.24; prior
+to that, big macros like C<PUSHBLOCK> and C<POPSUB> were used; in 5.24
+they were replaced by the inline static functions described below. In
+addition, the ordering and detail of how these macros/function work
+changed in many ways, often subtly. In particular they didn't handle
+saving the savestack and temps stack positions, and required additional
+C<ENTER>, C<SAVETMPS> and C<LEAVE> compared to the new functions. The
+old-style macros will not be described further.
+
+
+=head2 Pushing contexts
+
+For pushing a new context, the two basic functions are
+C<cx = cx_pushblock()>, which pushes a new basic context block and returns
+its address, and a family of similar functions with names like
+C<cx_pushsub(cx)> which populate the additional type-dependent fields in
+the C<cx> struct. Note that C<CXt_NULL> and C<CXt_BLOCK> don't have their
+own push functions, as they don't store any data beyond that pushed by
+C<cx_pushblock>.
+
+The fields of the context struct and the arguments to the C<cx_*>
+functions are subject to change between perl releases, representing
+whatever is convenient or efficient for that release.
+
+A typical context stack pushing can be found in C<pp_entersub>; the
+following shows a simplified and stripped-down example of a non-XS call,
+along with comments showing roughly what each function does.
+
+ dMARK;
+ U8 gimme = GIMME_V;
+ bool hasargs = cBOOL(PL_op->op_flags & OPf_STACKED);
+ OP *retop = PL_op->op_next;
+ I32 old_ss_ix = PL_savestack_ix;
+ CV *cv = ....;
+
+ /* ... make mortal copies of stack args which are PADTMPs here ... */
+
+ /* ... do any additional savestack pushes here ... */
+
+ /* Now push a new context entry of type 'CXt_SUB'; initially just
+ * doing the actions common to all block types: */
+
+ cx = cx_pushblock(CXt_SUB, gimme, MARK, old_ss_ix);
+
+ /* this does (approximately):
+ CXINC; /* cxstack_ix++ (grow if necessary) */
+ cx = CX_CUR(); /* and get the address of new frame */
+ cx->cx_type = CXt_SUB;
+ cx->blk_gimme = gimme;
+ cx->blk_oldsp = MARK - PL_stack_base;
+ cx->blk_oldsaveix = old_ss_ix;
+ cx->blk_oldcop = PL_curcop;
+ cx->blk_oldmarksp = PL_markstack_ptr - PL_markstack;
+ cx->blk_oldscopesp = PL_scopestack_ix;
+ cx->blk_oldpm = PL_curpm;
+ cx->blk_old_tmpsfloor = PL_tmps_floor;
+
+ PL_tmps_floor = PL_tmps_ix;
+ */
+
+
+ /* then update the new context frame with subroutine-specific info,
+ * such as the CV about to be executed: */
+
+ cx_pushsub(cx, cv, retop, hasargs);
+
+ /* this does (approximately):
+ cx->blk_sub.cv = cv;
+ cx->blk_sub.olddepth = CvDEPTH(cv);
+ cx->blk_sub.prevcomppad = PL_comppad;
+ cx->cx_type |= (hasargs) ? CXp_HASARGS : 0;
+ cx->blk_sub.retop = retop;
+ SvREFCNT_inc_simple_void_NN(cv);
+ */
+
+Note that C<cx_pushblock()> sets two new floors: for the args stack (to
+C<MARK>) and the temps stack (to C<PL_tmps_ix>). While executing at this
+scope level, every C<nextstate> (amongst others) will reset the args and
+tmps stack levels to these floors. Note that since C<cx_pushblock> uses
+the current value of C<PL_tmps_ix> rather than it being passed as an arg,
+this dictates at what point C<cx_pushblock> should be called. In
+particular, any new mortals which should be freed only on scope exit
+(rather than at the next C<nextstate>) should be created first.
+
+Most callers of C<cx_pushblock> simply set the new args stack floor to the
+top of the previous stack frame, but for C<CXt_LOOP_LIST> it stores the
+items being iterated over on the stack, and so sets C<blk_oldsp> to the
+top of these items instead. Note that, contrary to its name, C<blk_oldsp>
+doesn't always represent the value to restore C<PL_stack_sp> to on scope
+exit.
+
+Note the early capture of C<PL_savestack_ix> to C<old_ss_ix>, which is
+later passed as an arg to C<cx_pushblock>. In the case of C<pp_entersub>,
+this is because, although most values needing saving are stored in fields
+of the context struct, an extra value needs saving only when the debugger
+is running, and it doesn't make sense to bloat the struct for this rare
+case. So instead it is saved on the savestack. Since this value gets
+calculated and saved before the context is pushed, it is necessary to pass
+the old value of C<PL_savestack_ix> to C<cx_pushblock>, to ensure that the
+saved value gets freed during scope exit. For most users of
+C<cx_pushblock>, where nothing needs pushing on the save stack,
+C<PL_savestack_ix> is just passed directly as an arg to C<cx_pushblock>.
+
+Note that where possible, values should be saved in the context struct
+rather than on the save stack; it's much faster that way.
+
+Normally C<cx_pushblock> should be immediately followed by the appropriate
+C<cx_pushfoo>, with nothing between them; this is because if code
+in-between could die (e.g. a warning upgraded to fatal), then the context
+stack unwinding code in C<dounwind> would see (in the example above) a
+C<CXt_SUB> context frame, but without all the subroutine-specific fields
+set, and crashes would soon ensue.
+
+Where the two must be separate, initially set the type to C<CXt_NULL> or
+C<CXt_BLOCK>, and later change it to C<CXt_foo> when doing the
+C<cx_pushfoo>. This is exactly what C<pp_enteriter> does, once it's
+determined which type of loop it's pushing.
+
+=head2 Popping contexts
+
+Contexts are popped using C<cx_popsub()> etc. and C<cx_popblock()>. Note
+however, that unlike C<cx_pushblock>, neither of these functions actually
+decrement the current context stack index; this is done separately using
+C<CX_POP()>.
+
+There are two main ways that contexts are popped. During normal execution
+as scopes are exited, functions like C<pp_leave>, C<pp_leaveloop> and
+C<pp_leavesub> process and pop just one context using C<cx_popfoo> and
+C<cx_popblock>. On the other hand, things like C<pp_return> and C<next>
+may have to pop back several scopes until a sub or loop context is found,
+and exceptions (such as C<die>) need to pop back contexts until an eval
+context is found. Both of these are accomplished by C<dounwind()>, which
+is capable of processing and popping all contexts above the target one.
+
+Here is a typical example of context popping, as found in C<pp_leavesub>
+(simplified slightly):
+
+ U8 gimme;
+ PERL_CONTEXT *cx;
+ SV **oldsp;
+ OP *retop;
+
+ cx = CX_CUR();
+
+ gimme = cx->blk_gimme;
+ oldsp = PL_stack_base + cx->blk_oldsp; /* last arg of previous frame */
+
+ if (gimme == G_VOID)
+ PL_stack_sp = oldsp;
+ else
+ leave_adjust_stacks(oldsp, oldsp, gimme, 0);
+
+ CX_LEAVE_SCOPE(cx);
+ cx_popsub(cx);
+ cx_popblock(cx);
+ retop = cx->blk_sub.retop;
+ CX_POP(cx);
+
+ return retop;
+
+The steps above are in a very specific order, designed to be the reverse
+order of when the context was pushed. The first thing to do is to copy
+and/or protect any any return arguments and free any temps in the current
+scope. Scope exits like an rvalue sub normally return a mortal copy of
+their return args (as opposed to lvalue subs). It is important to make
+this copy before the save stack is popped or variables are restored, or
+bad things like the following can happen:
+
+ sub f { my $x =...; $x } # $x freed before we get to copy it
+ sub f { /(...)/; $1 } # PL_curpm restored before $1 copied
+
+Although we wish to free any temps at the same time, we have to be careful
+not to free any temps which are keeping return args alive; nor to free the
+temps we have just created while mortal copying return args. Fortunately,
+C<leave_adjust_stacks()> is capable of making mortal copies of return args,
+shifting args down the stack, and only processing those entries on the
+temps stack that are safe to do so.
+
+In void context no args are returned, so it's more efficient to skip
+calling C<leave_adjust_stacks()>. Also in void context, a C<nextstate> op
+is likely to be imminently called which will do a C<FREETMPS>, so there's
+no need to do that either.
+
+The next step is to pop savestack entries: C<CX_LEAVE_SCOPE(cx)> is just
+defined as C<<LEAVE_SCOPE(cx->blk_oldsaveix)>>. Note that during the
+popping, it's possible for perl to call destructors, call C<STORE> to undo
+localisations of tied vars, and so on. Any of these can die or call
+C<exit()>. In this case, C<dounwind()> will be called, and the current
+context stack frame will be re-processed. Thus it is vital that all steps
+in popping a context are done in such a way to support reentrancy. The
+other alternative, of decrementing C<cxstack_ix> I<before> processing the
+frame, would lead to leaks and the like if something died halfway through,
+or overwriting of the current frame.
+
+C<CX_LEAVE_SCOPE> itself is safely re-entrant: if only half the savestack
+items have been popped before dying and getting trapped by eval, then the
+C<CX_LEAVE_SCOPE>s in C<dounwind> or C<pp_leaveeval> will continue where
+the first one left off.
+
+The next step is the type-specific context processing; in this case
+C<cx_popsub>. In part, this looks like:
+
+ cv = cx->blk_sub.cv;
+ CvDEPTH(cv) = cx->blk_sub.olddepth;
+ cx->blk_sub.cv = NULL;
+ SvREFCNT_dec(cv);
+
+where its processing the just-executed CV. Note that before it decrements
+the CV's reference count, it nulls the C<blk_sub.cv>. This means that if
+it re-enters, the CV won't be freed twice. It also means that you can't
+rely on such type-specific fields having useful values after the return
+from C<cx_popfoo>.
+
+Next, C<cx_popblock> restores all the various interpreter vars to their
+previous values or previous high water marks; it expands to:
+
+ PL_markstack_ptr = PL_markstack + cx->blk_oldmarksp;
+ PL_scopestack_ix = cx->blk_oldscopesp;
+ PL_curpm = cx->blk_oldpm;
+ PL_curcop = cx->blk_oldcop;
+ PL_tmps_floor = cx->blk_old_tmpsfloor;
+
+Note that it I<doesn't> restore C<PL_stack_sp>; as mentioned earlier,
+which value to restore it to depends on the context type (specifically
+C<for (list) {}>), and what args (if any) it returns; and that will
+already have been sorted out earlier by C<leave_adjust_stacks()>.
+
+Finally, the context stack pointer is actually decremented by C<CX_POP(cx)>.
+After this point, it's possible that that the current context frame could
+be overwritten by other contexts being pushed. Although things like ties
+and C<DESTROY> are supposed to work within a new context stack, it's best
+not to assume this. Indeed on debugging builds, C<CX_POP(cx)> deliberately
+sets C<cx> to null to detect code that is still relying on the field
+values in that context frame. Note in the C<pp_leavesub()> example above,
+we grab C<blk_sub.retop> I<before> calling C<CX_POP>.
+
+=head2 Redoing contexts
+
+Finally, there is C<cx_topblock(cx)>, which acts like a super-C<nextstate>
+as regards to resetting various vars to their base values. It is used in
+places like C<pp_next>, C<pp_redo> and C<pp_goto> where rather than
+exiting a scope, we want to re-initialise the scope. As well as resetting
+C<PL_stack_sp> like C<nextstate>, it also resets C<PL_markstack_ptr>,
+C<PL_scopestack_ix> and C<PL_curpm>. Note that it doesn't do a
+C<FREETMPS>.
+
+
=head1 AUTHORS
Until May 1997, this document was maintained by Jeff Okamoto
https://perl5.git.perl.org / perl5.git / blobdiff