=head1 Title

docs/pmc.pod - PMC (PMC Makers Compendium)

=head1 PMC Structure Items Access

Ideally, there should be minimal direct access to a PMC's internals.  In order
to enforce encapsulation, most interaction with a PMC should be performed
through VTABLE function calls, which allow code to remain robust as PMC
internals are changed.  

When it is not possible or practical to use VTABLE functions (for instance when
implementing PMC internals), ATTRs should be used.  ATTRs are declared after
the C<pmclass> line in a .pmc file.  For a given pmc ("Foo"), an ATTR ("bar")
can be accessed either directly: C<< PARROT_FOO(pmc)->bar >> or via a
SETATTR/GETATTR accessor macro: C<GETATTR_Foo_bar(INTERP, x)>.  Note that
inside a PMC's source file, these can be shortened to C<GET_ATTR_bar(INTERP, x)>.

=head1 PMC Storage

PMCs can store data in two places.  8 bits can be stored in the PMC's flags.
These are accessed via PObj_private0_FLAG, PObj_private1_FLAG, etc, although
these flags should be #define'd on a per-PMC basis to have more meaningful
names.  If a PMC needs more than 8 bits of storage, it should declare ATTRs of
the appropriate type.  Storage for ATTRs hangs off of C<PMC_data()>.  See
src/pmc/exporter.pmc for example code that does this.

The PMC UnionVal is an obsolete storage area which was previously used to
provide a limited amount of storage.  The use of this storage encouraged poor
encapsulation and hidden dependencies between PMCs.  Any new code should not
use the UnionVal, which will eventually be removed from Parrot.

=head2 ATTRs and C<PMC_data()>

If your PMC needs to store more data than will fit into the 8 bits of the PMC
flags, it should declare ATTRs of the appropriate type.  The pmc2c code will
generate a struct containing all ATTRs, including those inherited via
C<extends> declarations.  This struct will be named in the form
C<Parrot_x_attributes>, where C<x> is the name of your PMC, e.g.
C<Parrot_FixedIntegerArray_attributes>. 

When creating a PMC that has one or more ATTRs, the C<Parrot_x_attributes>
struct must be allocated and assigned to C<PMC_data>, and freed on PMC
destruction. This can be done automatically by using the auto_attrs flag in
the PMC declaration, or manually by using the manual_attrs flag. You must set
one of those flags, a warning is emitted otherwise. In future releases
auto_attrs will be the default.

If manual_attrs is specified or assumed the struct must be manually allocated
in the PMC's C<init()> and C<init_pmc()> VTABLE functions (if used)
and it must be destroyed in the C<destroy()> VTABLE function, the PMC must
also indicate that they need active destruction by calling
C<PObj_custom_destroy_SET()> or C<PObj_custom_mark_destroy_SETALL()>.

If your PMC only needs to store a single pointer, it can use C<PMC_data> directly.
Note that this may make maintaining your PMC difficult, should more data ever
need to be stored.

=head1 PMC flags

Each PMC has 8 private flags named B<PObj_private0_FLAG> through
B<PObj_private7_FLAG>.  These may be used for storing 8 bits of PMC-specific
information.  See C<include/parrot/key.h> and C<src/pmc/key.pmc> for examples.

=head1 PMCs and GC

The GC system doesn't make any assumptions about your PMC's layout. Whenever a
PMC is found in the root set, B<Parrot_gc_mark_PObj_alive()> is called with that PMC.  The
PMC is responsible to mark all contained or referenced active Parrot objects
(Buffers, STRINGs or other PMCs) when its C<mark()> VTABLE function is called.

=head2 PMCs and System Resources

Whenever a PMC B<malloc()>s memory or opens a file or a database connection, it
has to take care of freeing or closing these resources.  This is done by
implementing the appropriate VTABLE functions (C<mark()> and C<destroy()>) and
setting the appropriate PObj flags.  The commonly used flags are described
below.

=head2 GC related flags

=over 4

=item PObj_custom_mark_FLAG

If your PMC contains any other B<PObj>s (STRINGs, PMCs, etc), your PMC must
implement the B<mark()> VTABLE function and set this flag.  The B<mark()>
VTABLE function must call B<Parrot_gc_mark_PObj_alive()> on all B<PObj>s which your PMC
contains.

=item PObj_custom_destroy_FLAG

If your PMC allocates any memory or opens any resources during its lifetime, it
must set B<PObj_custom_destroy> and implement the B<destroy()> VTABLE function to
free those resources.

=item PObj_needs_early_gc_FLAG

Set this flag if your PMC needs timely destruction, e.g. to close a file handle
at the end of a block scope if the PMC isn't alive any more.

=back

=head1 See also

F<include/parrot/pobj.h>, F<src/gc/api.c>, F<docs/pdds/pdd02_vtables.pod>