=head1 TITLE

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

=head1 PMC Structure Items Access

Don't use PMC structure items directly, always use the macros, which are
defined in F<include/parrot/pobj.h>.

=head1 PMC Storage

The PMC structure contains two places of data storage: the B<union UnionVal>
and the B<PMC_data> pointer.

=head2 Storage memory washing

During PMC recycling the B<UnionVal> data members are B<not> cleared.
B<PMC_data> or B<pmc_ext> is set to NULL. The flags are set to their default
value, especially the private flags are 0.

=head2 UnionVal usage

There are no special rules, where to store what: use whatever fits best. If
your PMC points to a B<STRING *>, hang it off the B<PMC_str_val()>, if it's an
B<INTVAL>, place it in B<PMC_int_val()>. If you need to store two items, try
to use union members that have distinct storage like B<PObj_bustart()> /
B<PObj_buf_len()> or B<PMC_struct_val()> / B<PMC_pmc_val()> in parallel with
B<PMC_num_val()>.

=head2 PMC_data()

If your PMC contains other PMCs that possibly would allow the creation of
self-referential or arbitrary deeply nested containers, you have to allocate
the B<PMC_EXT> structure by specifying the B<need_ext> flag on the B<pmclass>
definition line. The B<PMC_data()> pointer is currently located in the
B<PMC_EXT> structure too. Using B<PMC_data()> therefore adds one more
indirection to access these data.

=head1 PMC flags

Each PMC has 8 private flags B<PObj_private0_FLAG> -  B<PObj_private7_FLAG>,
which can be used for storing 8 bits.

=head1 PMCs and DOD

=head2 Overview

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

=head2 DOD related flags

=over 4

=item PObj_is_buffer_ptr_FLAG

=item PObj_is_PMC_ptr_FLAG

B<PMC_data> points to a B<PObj> object. This PMC or Buffer gets marked
automatically. (Since the unification of PMCs and Buffers, these two flags
are the same).

=item PObj_is_buffer_of_PMCs_ptr_FLAG

B<PMC_data> points to a buffer holding an array of B<PObj*>s.

=item PObj_custom_mark_FLAG

If your PMC refers to any Parrot objects and above standard flags don't cover
this usage, a custom B<mark> vtable has to be implemented, which has to call
B<pobject_lives()> for all contained B<PObj>s.

=back

=head1 PMCs and System Resources

Whenever a PMC B<malloc()>s system memory or opens a file or a database
connection, it has to take care of freeing or closing these system resources.

=head2 Flags for PMC destruction

=over 4

=item PObj_active_destroy_FLAG

The PMC's B<destroy> vtable is called, when this PMC is found to be dead
during DOD.

=item PObj_needs_early_DOD_FLAG

Set this flag too, if the 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/dod.c>, F<docs/pdds/pdd02_vtables.pod>

=head1 AUTHOR

Leopold Toetsch E<lt>lt@toetsch.atE<gt>

=head1 VERSION

=over 4

=item 0.1

2004.06.14 - Initial

=back

