# Copyright (C) 2001-2004, The Perl Foundation.
# $Id: pdd11_extending.pod 12839 2006-05-30 14:34:31Z coke $

=head1 NAME

docs/pdds/pdd11_extending.pod - The Parrot Extension System

=head1 ABSTRACT

The extension API for Parrot is a simple, somewhat abstract, interface to
Parrot for code written in C or other compiled languages. It provides about the
same level of access to Parrot  that bytecode programs have.

=head1 DESCRIPTION

The API presents to C programs roughly the same interface presented to bytecode
programs--that is, a C extension can see everything that a bytecode program can
see, including Parrot's base architecture, registers, stacks, and whatnot. This
view isn't required, however, and often extension code won't need or want to
know what Parrot's internal structures look like. For this reason the functions
in the extension API are divided into two broad groups, one that has no
particular knowledge of the internals and one that does.

The stability of the two API groups is guaranteed separately. Group 1, the
internals unaware group, should be good basically forever. Group 2, the
internals aware group, is only guaranteed for the lifetime of the current
architecture. (It's likely that both groups will last equally long; however,
the Group 1 interface could reasonably be emulated on a different engine, while
the Group 2 interface is more closely tied to Parrot).

B<Note:> The extension API has not yet been completely specified. New functions
may be added, and those described below may change or be removed. You have been
warned...

=head1 API - Group 1: Internals-unaware functions

These functions are the ones that are largely unaware of the structure and
architecture of Parrot. They deal mainly in data as abstract entities, and
Parrot itself as a black box that, at best, can make subroutine or method
calls. This is sufficient for many extensions which act as black box processing
units and in turn treat Parrot itself as a black box.

=head2 PMC access functions

The following functions are for storing and retrieving data inside PMCs. Note
that use of the _keyed functions with non-aggregate PMCs will generally just
result in Parrot throwing an exception.

=over 4

=item C<Parrot_PMC_get_string(interp, pmc)>

Returns a Parrot_STRING that represents the string value of the PMC.

=item C<Parrot_PMC_get_string_intkey(interp, pmc, Parrot_Int key)>

Keyed version of C<Parrot_PMC_get_string>. Returns a Parrot_STRING 
representing the string value of whatever is stored at the element of the  PMC
indexed by C<key>.

=item C<Parrot_PMC_get_pointer(interp, pmc)>

Returns a pointer to some item of data. The details of what the pointer points
to depend on the particular PMC. This function is useful for dealing with PMCs
that hold pointers to arbitrary data.

=item C<Parrot_PMC_get_pointer_intkey(interp, pmc, Parrot_Int key)>

A keyed version of C<Parrot_PMC_get_pointer>. Returns the pointer value of
whatever is stored at the element of the PMC indexed by C<key>.

=item C<Parrot_PMC_get_intval(interp, pmc)>

Returns the integer value of the PMC.

=item C<Parrot_PMC_get_intval_intkey(interp, pmc, Parrot_Int key)>

A keyed version of C<Parrot_PMC_get_intval>. Returns the integer value of
whatever is stored at the element of the PMC indexed by C<key>.

=item C<Parrot_PMC_get_numval(interp, pmc)>

Returns the numeric value of the PMC.

=item C<Parrot_PMC_get_numval_intkey(interp, pmc, Parrot_Int key)>

A keyed version of C<Parrot_PMC_get_numval>. Returns the numeric value of
whatever is stored at the element of the PMC indexed by C<key>.

=item C<Parrot_PMC_get_cstring(interp, pmc)>

Returns a C-string (char *) that represents the string value of the PMC. The
memory the char * points to is a copy of the original value, and changing it
will not change the original in any way.

This memory will I<not> be reclaimed by garbage collection, nor may it be
returned to the system with C<free>. It must be returned with
C<Parrot_free_cstring>.

=item C<Parrot_PMC_get_cstring_intkey(interp, pmc, Parrot_Int key)>

A keyed version of C<Parrot_PMC_get_cstring>. Returns a C-string representing
the string value of whatever is stored at the element of  the PMC indexed by
C<key>.

=item C<Parrot_PMC_get_cstringn(interp, pmc, &len)>

Returns a C-string (char *) that represents the string value of the PMC. The
memory the char * points to is a copy of the original value, and changing it
will not change the original in any way. The C<len> parameter is the address of
an integer that will get the length of the string as Parrot knows it.

This memory will I<not> be reclaimed by garbage collection, nor may it be
returned to the system with C<free>. It must be returned with
C<Parrot_free_cstring>.

=item C<Parrot_PMC_get_cstringn_intkey(interp, pmc, &len, Parrot_Int key)>

A keyed version of C<Parrot_PMC_get_cstringn>. Returns a C-string representing
the string value of whatever is stored at the element of  the PMC indexed by
C<key>. Stores the length of the string in C<len>.

=item C<Parrot_PMC_get_pmc_intkey(interp, pmc, Parrot_Int key)>

Returns the PMC stored at the element of the passed-in PMC that is  indexed by
C<key>.

=item C<Parrot_PMC_set_string(interp, pmc, Parrot_STRING value)>

Assign the passed-in Parrot_STRING to the passed-in PMC.

=item C<Parrot_PMC_set_string_intkey(interp, 
pmc, Parrot_STRING value, Parrot_Int key)>

Keyed version of C<Parrot_PMC_set_string>. Assigns C<value> to the PMC stored 
at element <key> of the passed-in PMC.

=item C<Parrot_PMC_set_pointer(interp, pmc, void *value)>

Assign the passed-in pointer to the passed-in PMC.

=item C<Parrot_PMC_set_pointer_intkey(interp, pmc, void *value, Parrot_Int key)>

Keyed version of C<Parrot_PMC_set_pointer>. Assigns C<value> to the PMC stored 
at element <key> of the passed-in PMC.

=item C<Parrot_PMC_set_pmc_intkey(interp, pmc, Parrot_PMC value, Parrot_Int key)>

Assigns C<value> to the PMC stored at element <key> of the passed-in PMC.

=item C<Parrot_PMC_set_intval(interp, pmc, Parrot_Int value)>

Assign the passed-in Parrot integer to the passed-in PMC.

=item C<Parrot_PMC_set_intval_intkey(interp, 
pmc, Parrot_Int value, Parrot_Int key)>

Keyed version of C<Parrot_PMC_set_intval>. Assigns C<value> to the PMC stored 
at element <key> of the passed-in PMC.

=item C<Parrot_PMC_set_numval(interp, pmc, Parrot_Float value)>

Assign the passed-in Parrot number to the passed-in PMC.

=item C<Parrot_PMC_set_numval_intkey(interp, 
pmc, Parrot_Float value, Parrot_Int key)>

Keyed version of C<Parrot_PMC_set_numval>. Assigns C<value> to the PMC stored 
at element <key> of the passed-in PMC.

=item C<Parrot_PMC_set_cstring(interp, pmc, const char *value)>

Convert the passed-in null-terminated C string to a Parrot_STRING and assign it
to the passed-in PMC.

=item C<Parrot_PMC_set_cstring_intkey(interp, 
pmc, const char *value, Parrot_Int key)>

Keyed version of C<Parrot_PMC_set_cstring>. Converts C<value> to a 
Parrot_STRING and assigns it to the PMC stored at element <key> of the 
passed-in PMC.

=item C<Parrot_PMC_set_cstringn(interp, 
pmc, const char *value, Parrot_Int length)>

Convert the passed-in null-terminated C string to a Parrot_STRING of length
C<length> and assign it to the passed-in PMC. If C<value> is longer than
C<length>, then only the first C<length> characters will be converted. If
C<value> is shorter than C<length>, then the extra characters in the 
Parrot_STRING should be assumed to contain garbage.

=item C<Parrot_PMC_set_cstringn_intkey(interp, 
pmc, const char *value, Parrot_int length, Parrot_Int key)>

Keyed version of C<Parrot_PMC_set_cstringn>. Converts the first C<length>
characters of C<value> to a Parrot_STRING and assigns the resulting string to
the PMC stored at element <key> of the passed-in PMC.

=back

=head2 Creation and destruction

Functions used to create and destroy PMCs, Parrot_STRINGs, etc.

=over 4

=item C<Parrot_PMC_new(interp, Parrot_Int typenum)>

Creates and returns a new PMC. C<typenum> is an integer identifier that 
specifies the type of PMC required. The C<typenum> corresponding to a 
particular PMC class name can be found using C<Parrot_PMC_typenum>.

=item C<Parrot_PMC_typenum(interp, const char* class)>

Returns the internal integer identifier corresponding to a PMC with class name
C<class>.

=item C<Parrot_PMC_null()>

Returns the special C<NULL> PMC.

=item C<Parrot_new_string(interp, 
char *buffer, int length, Parrot_Encoding encoding, 
Parrot_CharType charset, Parrot_Language language, Parrot_Int flags)>

Create a new Parrot string from a passed-in buffer. If the C<encoding>,
C<charset>, or C<language> are unspecified (i.e. if you pass in 0), then  the
defaults are used. Otherwise, the functions C<Parrot_find_encoding>, C<
Parrot_find_chartype> and C<Parrot_find_language> (all described below) can be
used to find the appropriate values for a particular choice of  encoding,
chartype or language.

Flag values are currently undocumented.

Note that a copy of the buffer is made.

=item C<Parrot_find_encoding(interp, char *encoding_name)>

Find the magic token for an encoding, by name.

=item C<Parrot_find_chartype(interp, char *chartype)>

Find the magic token for a chartype, by name.

=item C<Parrot_find_language(interp, char *language)>

Find the magic token for a language, by language name.

=item C<Parrot_free_cstring(char* string)>

Deallocates a C string that the interpreter has handed to you. This function
must be used to free strings produced by  C<Parrot_PMC_get_cstring> and
C<Parrot_PMC_get_cstringn>, as these will not be reclaimed by the garbage
collector. It should not be used to deallocate strings that do not come from
Parrot.

=item C<Parrot_register_pmc(interp, pmc)>

Add a reference to the PMC to the interpreter's DOD registry. This prevents
PMCs known only to extensions from getting destroyed during  DOD runs.

=item C<Parrot_unregister_pmc(interp, pmc)>

Remove a reference to the PMC from the interpreter's DOD registry.  If the
reference count reaches zero, the PMC will be destroyed during the  next DOD
run.

=back

=head2 Subroutine and method calls

Functions to call Parrot subroutines and methods

=over 4

=item C<Parrot_call_sub(interp, Parrot_PMC sub, Parrot_Int argcount, ...)>

Calls a Parrot subroutine, with C<argcount> PMC parameters. This function sets
up Parrot's registers in line with the Parrot calling conventions; see
L<pdd03_calling_conventions.pod> for more details.

=item C<Parrot_call_method(interp, 
Parrot_PMC sub, Parrot_STRING method, Parrot_Int argcount, ...)>

Calls a Parrot method named C<method> with C<argcount> PMC parameters. NB. This
is not yet implemented and may change.

=back

=head1 API - Group 2: Internals aware

The internals-aware functions are for those extensions that need to query or
alter the state of Parrot's internals in some way.

Register access functions

The following functions allow the values stored in Parrot's registers to be
accessed. An attempt to access a non-existent register (e.g. string register
-123) will cause the function to throw an exception (well, it will once we
actually implement some bounds-checking...).  The value stored in an
uninitialized register is undefined; it may well be zero (or NULL), but do not
rely on this being the case.

=over 4

=item C<Parrot_get_intreg(interp, Parrot_Int regnum)>

Return the value of an integer register.

=item C<Parrot_get_numreg(interp, Parrot_Int regnum)>

Return the value of a numeric register.

=item C<Parrot_get_strreg(interp, Parrot_Int regnum)>

Return the value of a string register.

=item C<Parrot_get_pmcreg(interp, Parrot_Int regnum)>

Return the value of a PMC register.

=back

=head1 ATTACHMENTS

None.

=head1 FOOTNOTES

None.

=head1 REFERENCES

F<docs/glossary.pod>

=head1 VERSION

=head2 CURRENT

    Maintainer: 
    Class: Internals
    PDD Number: 11
    Version: 1.0
    Status: Developing
    Last Modified: February 20, 2004 
    PDD Format: 1
    Language: English

=head2 HISTORY

=over 4

=item Version 1

None. First version

=back

=head1 CHANGES

=over 4

=item Version 1.0

None. First version

=back

=cut
