Proper fix for CvOUTSIDE weak refcounting

[p5sagit/p5-mst-13.2.git] / pod / perlintern.pod

=head1 NAME

perlintern - autogenerated documentation of purely B<internal>
                 Perl functions

=head1 DESCRIPTION

This file is the autogenerated documentation of functions in the
Perl interpreter that are documented using Perl's internal documentation
format but are not marked as part of the Perl API. In other words,
B<they are not for use in extensions>!


=head1 CV reference counts and CvOUTSIDE

=over 8

=item CvWEAKOUTSIDE

Each CV has a pointer, C<CvOUTSIDE()>, to its lexically enclosing
CV (if any). Because pointers to anonymous sub prototypes are
stored in C<&> pad slots, it is a possible to get a circular reference,
with the parent pointing to the child and vice-versa. To avoid the
ensuing memory leak, we do not increment the reference count of the CV
pointed to by C<CvOUTSIDE> in the I<one specific instance> that the parent
has a C<&> pad slot pointing back to us. In this case, we set the
C<CvWEAKOUTSIDE> flag in the child. This allows us to determine under what
circumstances we should decrement the refcount of the parent when freeing
the child.

There is a further complication with non-closure anonymous subs (ie those
that do not refer to any lexicals outside that sub). In this case, the
anonymous prototype is shared rather than being cloned. This has the
consequence that the parent may be freed while there are still active
children, eg

    BEGIN { $a = sub { eval '$x' } }

In this case, the BEGIN is freed immediately after execution since there
are no active references to it: the anon sub prototype has
C<CvWEAKOUTSIDE> set since it's not a closure, and $a points to the same
CV, so it doesn't contribute to BEGIN's refcount either.  When $a is
executed, the C<eval '$x'> causes the chain of C<CvOUTSIDE>s to be followed,
and the freed BEGIN is accessed.

To avoid this, whenever a CV and its associated pad is freed, any
C<&> entries in the pad are explicitly removed from the pad, and if the
refcount of the pointed-to anon sub is still positive, then that
child's C<CvOUTSIDE> is set to point to its grandparent. This will only
occur in the single specific case of a non-closure anon prototype
having one or more active references (such as C<$a> above).

One other thing to consider is that a CV may be merely undefined
rather than freed, eg C<undef &foo>. In this case, its refcount may
not have reached zero, but we still delete its pad and its C<CvROOT> etc.
Since various children may still have their C<CvOUTSIDE> pointing at this
undefined CV, we keep its own C<CvOUTSIDE> for the time being, so that
the chain of lexical scopes is unbroken. For example, the following
should print 123:

    my $x = 123;
    sub tmp { sub { eval '$x' } }
    my $a = tmp();
    undef &tmp;
    print  $a->();

        bool    CvWEAKOUTSIDE(CV *cv)

=for hackers
Found in file cv.h


=back

=head1 Functions in file pad.h


=over 8

=item CX_CURPAD_SAVE

Save the current pad in the given context block structure.

        void    CX_CURPAD_SAVE(struct context)

=for hackers
Found in file pad.h

=item CX_CURPAD_SV

Access the SV at offset po in the saved current pad in the given
context block structure (can be used as an lvalue).

        SV *    CX_CURPAD_SV(struct context, PADOFFSET po)

=for hackers
Found in file pad.h

=item PAD_BASE_SV       

Get the value from slot C<po> in the base (DEPTH=1) pad of a padlist

        SV *    PAD_BASE_SV     (PADLIST padlist, PADOFFSET po)

=for hackers
Found in file pad.h

=item PAD_CLONE_VARS

|CLONE_PARAMS* param
Clone the state variables associated with running and compiling pads.

        void    PAD_CLONE_VARS(PerlInterpreter *proto_perl \)

=for hackers
Found in file pad.h

=item PAD_COMPNAME_FLAGS

Return the flags for the current compiling pad name
at offset C<po>. Assumes a valid slot entry.

        U32     PAD_COMPNAME_FLAGS(PADOFFSET po)

=for hackers
Found in file pad.h

=item PAD_COMPNAME_GEN

The generation number of the name at offset C<po> in the current
compiling pad (lvalue). Note that C<SvCUR> is hijacked for this purpose.

        STRLEN  PAD_COMPNAME_GEN(PADOFFSET po)

=for hackers
Found in file pad.h

=item PAD_COMPNAME_OURSTASH

Return the stash associated with an C<our> variable.
Assumes the slot entry is a valid C<our> lexical.

        HV *    PAD_COMPNAME_OURSTASH(PADOFFSET po)

=for hackers
Found in file pad.h

=item PAD_COMPNAME_PV

Return the name of the current compiling pad name
at offset C<po>. Assumes a valid slot entry.

        char *  PAD_COMPNAME_PV(PADOFFSET po)

=for hackers
Found in file pad.h

=item PAD_COMPNAME_TYPE

Return the type (stash) of the current compiling pad name at offset
C<po>. Must be a valid name. Returns null if not typed.

        HV *    PAD_COMPNAME_TYPE(PADOFFSET po)

=for hackers
Found in file pad.h

=item PAD_DUP

Clone a padlist.

        void    PAD_DUP(PADLIST dstpad, PADLIST srcpad, CLONE_PARAMS* param)

=for hackers
Found in file pad.h

=item PAD_RESTORE_LOCAL

Restore the old pad saved into the local variable opad by PAD_SAVE_LOCAL()

        void    PAD_RESTORE_LOCAL(PAD *opad)

=for hackers
Found in file pad.h

=item PAD_SAVE_LOCAL

Save the current pad to the local variable opad, then make the
current pad equal to npad

        void    PAD_SAVE_LOCAL(PAD *opad, PAD *npad)

=for hackers
Found in file pad.h

=item PAD_SAVE_SETNULLPAD

Save the current pad then set it to null.

        void    PAD_SAVE_SETNULLPAD()

=for hackers
Found in file pad.h

=item PAD_SETSV 

Set the slot at offset C<po> in the current pad to C<sv>

        SV *    PAD_SETSV       (PADOFFSET po, SV* sv)

=for hackers
Found in file pad.h

=item PAD_SET_CUR       

Set the current pad to be pad C<n> in the padlist, saving
the previous current pad.

        void    PAD_SET_CUR     (PADLIST padlist, I32 n)

=for hackers
Found in file pad.h

=item PAD_SV    

Get the value at offset C<po> in the current pad

        void    PAD_SV  (PADOFFSET po)

=for hackers
Found in file pad.h

=item PAD_SVl   

Lightweight and lvalue version of C<PAD_SV>.
Get or set the value at offset C<po> in the current pad.
Unlike C<PAD_SV>, does not print diagnostics with -DX.
For internal use only.

        SV *    PAD_SVl (PADOFFSET po)

=for hackers
Found in file pad.h

=item SAVECLEARSV       

Clear the pointed to pad value on scope exit. (ie the runtime action of 'my')

        void    SAVECLEARSV     (SV **svp)

=for hackers
Found in file pad.h

=item SAVECOMPPAD

save PL_comppad and PL_curpad





        void    SAVECOMPPAD()

=for hackers
Found in file pad.h

=item SAVEPADSV 

Save a pad slot (used to restore after an iteration)

XXX DAPM it would make more sense to make the arg a PADOFFSET
        void    SAVEPADSV       (PADOFFSET po)

=for hackers
Found in file pad.h


=back

=head1 Functions in file pp_ctl.c


=over 8

=item find_runcv

Locate the CV corresponding to the currently executing sub or eval.

        CV*     find_runcv()

=for hackers
Found in file pp_ctl.c


=back

=head1 Global Variables

=over 8

=item PL_DBsingle

When Perl is run in debugging mode, with the B<-d> switch, this SV is a
boolean which indicates whether subs are being single-stepped.
Single-stepping is automatically turned on after every step.  This is the C
variable which corresponds to Perl's $DB::single variable.  See
C<PL_DBsub>.

        SV *    PL_DBsingle

=for hackers
Found in file intrpvar.h

=item PL_DBsub

When Perl is run in debugging mode, with the B<-d> switch, this GV contains
the SV which holds the name of the sub being debugged.  This is the C
variable which corresponds to Perl's $DB::sub variable.  See
C<PL_DBsingle>.

        GV *    PL_DBsub

=for hackers
Found in file intrpvar.h

=item PL_DBtrace

Trace variable used when Perl is run in debugging mode, with the B<-d>
switch.  This is the C variable which corresponds to Perl's $DB::trace
variable.  See C<PL_DBsingle>.

        SV *    PL_DBtrace

=for hackers
Found in file intrpvar.h

=item PL_dowarn

The C variable which corresponds to Perl's $^W warning variable.

        bool    PL_dowarn

=for hackers
Found in file intrpvar.h

=item PL_last_in_gv

The GV which was last used for a filehandle input operation. (C<< <FH> >>)

        GV*     PL_last_in_gv

=for hackers
Found in file thrdvar.h

=item PL_ofs_sv

The output field separator - C<$,> in Perl space.

        SV*     PL_ofs_sv

=for hackers
Found in file thrdvar.h

=item PL_rs

The input record separator - C<$/> in Perl space.

        SV*     PL_rs

=for hackers
Found in file thrdvar.h


=back

=head1 GV Functions

=over 8

=item is_gv_magical

Returns C<TRUE> if given the name of a magical GV.

Currently only useful internally when determining if a GV should be
created even in rvalue contexts.

C<flags> is not used at present but available for future extension to
allow selecting particular classes of magical variable.

        bool    is_gv_magical(char *name, STRLEN len, U32 flags)

=for hackers
Found in file gv.c


=back

=head1 IO Functions

=over 8

=item start_glob

Function called by C<do_readline> to spawn a glob (or do the glob inside
perl on VMS). This code used to be inline, but now perl uses C<File::Glob>
this glob starter is only used by miniperl during the build process.
Moving it away shrinks pp_hot.c; shrinking pp_hot.c helps speed perl up.

        PerlIO* start_glob(SV* pattern, IO *io)

=for hackers
Found in file doio.c


=back

=head1 Pad Data Structures

=over 8

=item CvPADLIST

CV's can have CvPADLIST(cv) set to point to an AV.

For these purposes "forms" are a kind-of CV, eval""s are too (except they're
not callable at will and are always thrown away after the eval"" is done
executing).

XSUBs don't have CvPADLIST set - dXSTARG fetches values from PL_curpad,
but that is really the callers pad (a slot of which is allocated by
every entersub).

The CvPADLIST AV has does not have AvREAL set, so REFCNT of component items
is managed "manual" (mostly in pad.c) rather than normal av.c rules.
The items in the AV are not SVs as for a normal AV, but other AVs:

0'th Entry of the CvPADLIST is an AV which represents the "names" or rather
the "static type information" for lexicals.

The CvDEPTH'th entry of CvPADLIST AV is an AV which is the stack frame at that
depth of recursion into the CV.
The 0'th slot of a frame AV is an AV which is @_.
other entries are storage for variables and op targets.

During compilation:
C<PL_comppad_name> is set the the the names AV.
C<PL_comppad> is set the the frame AV for the frame CvDEPTH == 1.
C<PL_curpad> is set the body of the frame AV (i.e. AvARRAY(PL_comppad)).

During execution, C<PL_comppad> and C<PL_curpad> refer to the live
frame of the currently executing sub.

Iterating over the names AV iterates over all possible pad
items. Pad slots that are SVs_PADTMP (targets/GVs/constants) end up having
&PL_sv_undef "names" (see pad_alloc()).

Only my/our variable (SVs_PADMY/SVs_PADOUR) slots get valid names.
The rest are op targets/GVs/constants which are statically allocated
or resolved at compile time.  These don't have names by which they
can be looked up from Perl code at run time through eval"" like
my/our variables can be.  Since they can't be looked up by "name"
but only by their index allocated at compile time (which is usually
in PL_op->op_targ), wasting a name SV for them doesn't make sense.

The SVs in the names AV have their PV being the name of the variable.
NV+1..IV inclusive is a range of cop_seq numbers for which the name is
valid.  For typed lexicals name SV is SVt_PVMG and SvSTASH points at the
type.  For C<our> lexicals, the type is SVt_PVGV, and GvSTASH points at the
stash of the associated global (so that duplicate C<our> delarations in the
same package can be detected).  SvCUR is sometimes hijacked to
store the generation number during compilation.

If SvFAKE is set on the name SV then slot in the frame AVs are
a REFCNT'ed references to a lexical from "outside". In this case,
the name SV does not have a cop_seq range, since it is in scope
throughout.

If the 'name' is '&' the the corresponding entry in frame AV
is a CV representing a possible closure.
(SvFAKE and name of '&' is not a meaningful combination currently but could
become so if C<my sub foo {}> is implemented.)

        AV *    CvPADLIST(CV *cv)

=for hackers
Found in file pad.c

=item cv_clone

Clone a CV: make a new CV which points to the same code etc, but which
has a newly-created pad built by copying the prototype pad and capturing
any outer lexicals.

        CV*     cv_clone(CV* proto)

=for hackers
Found in file pad.c

=item cv_dump

dump the contents of a CV

        void    cv_dump(CV *cv, char *title)

=for hackers
Found in file pad.c

=item do_dump_pad

Dump the contents of a padlist

        void    do_dump_pad(I32 level, PerlIO *file, PADLIST *padlist, int full)

=for hackers
Found in file pad.c

=item intro_my

"Introduce" my variables to visible status.

        U32     intro_my()

=for hackers
Found in file pad.c

=item pad_add_anon

Add an anon code entry to the current compiling pad

        PADOFFSET       pad_add_anon(SV* sv, OPCODE op_type)

=for hackers
Found in file pad.c

=item pad_add_name

Create a new name in the current pad at the specified offset.
If C<typestash> is valid, the name is for a typed lexical; set the
name's stash to that value.
If C<ourstash> is valid, it's an our lexical, set the name's
GvSTASH to that value

Also, if the name is @.. or %.., create a new array or hash for that slot

If fake, it means we're cloning an existing entry

        PADOFFSET       pad_add_name(char *name, HV* typestash, HV* ourstash, bool clone)

=for hackers
Found in file pad.c

=item pad_alloc

Allocate a new my or tmp pad entry. For a my, simply push a null SV onto
the end of PL_comppad, but for a tmp, scan the pad from PL_padix upwards
for a slot which has no name and and no active value.

        PADOFFSET       pad_alloc(I32 optype, U32 tmptype)

=for hackers
Found in file pad.c

=item pad_block_start

Update the pad compilation state variables on entry to a new block

        void    pad_block_start(int full)

=for hackers
Found in file pad.c

=item pad_check_dup

Check for duplicate declarations: report any of:
     * a my in the current scope with the same name;
     * an our (anywhere in the pad) with the same name and the same stash
       as C<ourstash>
C<is_our> indicates that the name to check is an 'our' declaration

        void    pad_check_dup(char* name, bool is_our, HV* ourstash)

=for hackers
Found in file pad.c

=item pad_findlex

Find a named lexical anywhere in a chain of nested pads. Add fake entries
in the inner pads if it's found in an outer one. innercv is the CV *inside*
the chain of outer CVs to be searched. If newoff is non-null, this is a
run-time cloning: don't add fake entries, just find the lexical and add a
ref to it at newoff in the current pad.

        PADOFFSET       pad_findlex(char* name, PADOFFSET newoff, CV* innercv)

=for hackers
Found in file pad.c

=item pad_findmy

Given a lexical name, try to find its offset, first in the current pad,
or failing that, in the pads of any lexically enclosing subs (including
the complications introduced by eval). If the name is found in an outer pad,
then a fake entry is added to the current pad.
Returns the offset in the current pad, or NOT_IN_PAD on failure.

        PADOFFSET       pad_findmy(char* name)

=for hackers
Found in file pad.c

=item pad_fixup_inner_anons

For any anon CVs in the pad, change CvOUTSIDE of that CV from
old_cv to new_cv if necessary. Needed when a newly-compiled CV has to be
moved to a pre-existing CV struct.

        void    pad_fixup_inner_anons(PADLIST *padlist, CV *old_cv, CV *new_cv)

=for hackers
Found in file pad.c

=item pad_free

Free the SV at offet po in the current pad.

        void    pad_free(PADOFFSET po)

=for hackers
Found in file pad.c

=item pad_leavemy

Cleanup at end of scope during compilation: set the max seq number for
lexicals in this scope and warn of any lexicals that never got introduced.

        void    pad_leavemy()

=for hackers
Found in file pad.c

=item pad_new

Create a new compiling padlist, saving and updating the various global
vars at the same time as creating the pad itself. The following flags
can be OR'ed together:

    padnew_CLONE        this pad is for a cloned CV
    padnew_SAVE         save old globals
    padnew_SAVESUB      also save extra stuff for start of sub

        PADLIST*        pad_new(padnew_flags flags)

=for hackers
Found in file pad.c

=item pad_push

Push a new pad frame onto the padlist, unless there's already a pad at
this depth, in which case don't bother creating a new one.
If has_args is true, give the new pad an @_ in slot zero.

        void    pad_push(PADLIST *padlist, int depth, int has_args)

=for hackers
Found in file pad.c

=item pad_reset

Mark all the current temporaries for reuse

        void    pad_reset()

=for hackers
Found in file pad.c

=item pad_setsv

Set the entry at offset po in the current pad to sv.
Use the macro PAD_SETSV() rather than calling this function directly.

        void    pad_setsv(PADOFFSET po, SV* sv)

=for hackers
Found in file pad.c

=item pad_swipe

Abandon the tmp in the current pad at offset po and replace with a
new one.

        void    pad_swipe(PADOFFSET po, bool refadjust)

=for hackers
Found in file pad.c

=item pad_tidy

Tidy up a pad after we've finished compiling it:
    * remove most stuff from the pads of anonsub prototypes;
    * give it a @_;
    * mark tmps as such.

        void    pad_tidy(padtidy_type type)

=for hackers
Found in file pad.c

=item pad_undef

Free the padlist associated with a CV.
If parts of it happen to be current, we null the relevant
PL_*pad* global vars so that we don't have any dangling references left.
We also repoint the CvOUTSIDE of any about-to-be-orphaned
inner subs to the outer of this cv.

(This function should really be called pad_free, but the name was already
taken)

        void    pad_undef(CV* cv)

=for hackers
Found in file pad.c


=back

=head1 Stack Manipulation Macros

=over 8

=item djSP

Declare Just C<SP>. This is actually identical to C<dSP>, and declares
a local copy of perl's stack pointer, available via the C<SP> macro.
See C<SP>.  (Available for backward source code compatibility with the
old (Perl 5.005) thread model.)

                djSP;

=for hackers
Found in file pp.h

=item LVRET

True if this op will be the return value of an lvalue subroutine

=for hackers
Found in file pp.h


=back

=head1 SV Manipulation Functions

=over 8

=item report_uninit

Print appropriate "Use of uninitialized variable" warning

        void    report_uninit()

=for hackers
Found in file sv.c

=item sv_add_arena

Given a chunk of memory, link it to the head of the list of arenas,
and split it into a list of free SVs.

        void    sv_add_arena(char* ptr, U32 size, U32 flags)

=for hackers
Found in file sv.c

=item sv_clean_all

Decrement the refcnt of each remaining SV, possibly triggering a
cleanup. This function may have to be called multiple times to free
SVs which are in complex self-referential hierarchies.

        I32     sv_clean_all()

=for hackers
Found in file sv.c

=item sv_clean_objs

Attempt to destroy all objects not yet freed

        void    sv_clean_objs()

=for hackers
Found in file sv.c

=item sv_free_arenas

Deallocate the memory used by all arenas. Note that all the individual SV
heads and bodies within the arenas must already have been freed.

        void    sv_free_arenas()

=for hackers
Found in file sv.c


=back

=head1 AUTHORS

The autodocumentation system was originally added to the Perl core by
Benjamin Stuhl. Documentation is by whoever was kind enough to
document their functions.

=head1 SEE ALSO

perlguts(1), perlapi(1)