# Copyright (c) 2001-2011, Parrot Foundation.

=head1 PDD 10: Embedding

=head2 Abstract

Parrot, more precisely libparrot, can be embedded into applications to provide
a dynamic language runtime. A perfect example of this embedding is in the
Parrot executable, which is a thin wrapper around libparrot.



Version 1

=head2 Description

=head3 Difference Between Embedding and Extending

Embedding and Extending (PDD 11) are similar concepts. In both, we write code
that interfaces with libparrot. In an embedding situation we write an
application which loads and calls libparrot. In an extending situation,
libparrot loads and calls your module.

Extending gives libparrot more features, and allows your code to execute from
inside libparrot. From that location, the extending application has full
access to the available power and features of libparrot. This includes
knowledge about internal structure definitions, and internal-only functions
and subsystems. Because extending code is so closely tied to the internals
of libparrot, it will be more affected by changes in libparrot itself. Also,
the stability of extending code is tied to the stability of libparrot: If
either crashes, the other will likely crash with it.

Embedding, on the other hand, has much more limited access to libparrot. All
embedding applications must use the official embedding API, which is
limited and abstracted by design. Embedding applications must treat all
pointers and structures returned from the API as being opaque. This
abstraction buys stability. Changes to the internals of libparrot are unlikely
to cause changes in embedding code. If libparrot crashes or suffers an
unrecoverable error, it can return control to the embedding application more
gracefully.

=head3 The Embedding API

The Embedding API is a special set of functions found in the C<src/embed/>
directory. These functions may not be used internally by libparrot, embedding
applications may not use any other functions. Breaking either of these rules
can have serious implications for application stability.

Prior to the implementation of the new API, when libparrot had an unhandled
exception it would call the C C<exit()> library function to close the
application. This is undesirable because embedding applications want the
ability to handle errors and recover from problems in libparrot. The new API
provides error handling capabilities for cases of unhandled exceptions,
including both expected EXCEPT_exit and other types of error-related
exceptions.

The embedding API also makes sure certain details are in place, including
stack markers for the GC. Calling into libparrot without setting a valid
stack marker could cause serious (and difficult to diagnose) errors.

The embedding API provides relatively limited interaction with libparrot, at
least from the point of view of an internals developer or an extension
developer. There are many reasons for this. First and foremost, the full power
of libparrot is almost always available through the runcore. If you want to
do something with Parrot, it is almost always easier and preferred to write
your code in a language which targets Parrot, compile it down to bytecode, and
load that bytecode into Parrot to execute. Almost all applications of
libparrot will involve bytecode execution at some level, and this is where
most operations become possible.

The API also provides a powerful abstraction layer between the libparrot
internals developers and the embedding application developers. The API is
sufficiently abstracted and detached enough that even large changes to the
internals of libparrot are unlikely to require any changes in the embedding
application. For instance, libparrot could completely change its entire
object model implementation and not cause a change to the API at all.

While limited, the API is not static. If embedders need new features or
functionality, those can usually be added with relative ease.

=head2 Using the Embedding API

The embedding API follows certain guidelines that should be understood by
users, and followed by developers:

=over 4

=item * The embed API operates mostly on the 4 core Parrot data types:
Parrot_PMC, Parrot_String, Parrot_Int, and Parrot_Float. The first two of
these are pointers and should be treated as opaque.

=item * Where possible and reasonable, functions should take Parrot_Strings
instead of raw C C<char*> or C<wchar_t*> types. Functions do exist to easily
convert between C types and Parrot_String.

=item * PMCs are the primary data item. Anything more complicated than an
integer or string will be passed as a PMC. Some integers and strings will be
wrapped up in a PMC and passed.

=item * The number of API functions will stay relatively small. The purpose of
the API is not to provide the most efficient use of libparrot, but instead
the most general, stable and abstracted one.

=item * Calls into libparrot carry a performance overhead because we have to
do error handling, stack manipulation, data marshaling, etc. It is best to
do less work through the API, and more work through bytecode and the runcore.

=item * The embed API uses a single header file: L<include/parrot/api.h>.
Embedding applications should use only this header file and no other header
files from Parrot. Embedding applications should NOT use
L<include/parrot/parrot.h> or L<include/parrot/embed.h>, or any other files.

=item * libparrot does little to no signal handling. Those are typically the
responsibility of the embedder.

=item * File descriptors and resource handles are typically owned by whoever
opens them first. If the embedding application tells libparrot to open a file
with a FileHandle PMC, libparrot will keep and manage that file descriptor.
Functionality may be provided to import and export sharable resources like
these.

=item * Resources such as allocated memory are managed by whoever creates
them. If the embedding application allocates a structure and passes it in to
libparrot, the embedding application is in charge of managing and freeing that
structure. If libparrot allocates data, it will be in charge of managing and
freeing it. In many cases, data passed to or from libparrot through the API
will be copied to a new memory buffer.

=item * libparrot will not output error information to C<stderr> unless
specifically requested to. Instead, libparrot will gather all error
information and make it available to the user through function calls.

=item * API calls should be nestable. Unhandled exceptions or other error
conditions should cause a jump back to the inner-most API call. The
interpreter should be able to recover from errors in inner-most frames and
continue executing.

I<Note: Currently, the API does not support nested calls. This is planned>

=back

=head2 Implementation

The embedding API has two goals: To allow access to libparrot as a dynamic
language runtime and bytecode interpreter, and to encapsulate implementation
details internal to libparrot from the embedding application.

There are several guidelines for the embedding API implementation that
developers of it should follow:

=over 4

=item * It should never be possible to crash or corrupt the interpreter when
following the interface as documented. The interpreter should be able to be
used and reused until it is explicitly destroyed. The interpreter should
be reusable after it deals with an unhandled exception, including EXCEPT_exit
exceptions.

=item * It should never be possible for libparrot to crash, corrupt, or
forcibly exit the embedding application. Also, libparrot should never use
resources which haven't been assigned to it, such as standard IO handles
C<stdout>, C<stdin>, and C<stderr>.

=item * Each API function should have a single purpose, and should avoid
duplication of functionality as much as possible. A course-grained API is
preferable to a fine-grained one, even if some performance must be
sacrificed.

=item * names must be consistent in the API documentation and the examples.
All API functions are named C<Parrot_api_*>.

=item * The return value of every API function should be an integer value. The
return value should be 1 on success, and 0 on failure. No other results should
be returned.

The marker C<PARROT_API> is special and is used to denote API functions. This
macro sets the function up to be exported from the shared library and may
also provide some hints to the compiler.

All API functions return a Parrot_Int to signal status.

=back

=head3 Working with Interpreters

It is the external code's duty to create, manage, and destroy interpreters.

C<Parrot_api_make_interpreter> returns an opaque pointer to a new interpreter,
with some options set in it. The definition of C<Parrot_api_make_interpreter>
is as follows:

    Parrot_Int
    Parrot_api_make_interpreter(Parrot_PMC parent, Parrot_Int flags,
            Parrot_Init_Args *args, Parrot_PMC * interp);

A common usage pattern for making an interpreter is:

    Parrot_PMC interp = NULL;
    Parrot_Init_Args *args = NULL;
    GET_INIT_ARGS(args);
    if (!Parrot_api_make_interpreter(NULL, 0, args, &interp)) {
        fprintf(stderr, "Could not create interpreter");
        exit(EXIT_FAILURE);
    }

C<parent> can be NULL for the I<first> interpreter created, or where the
interpreter does not have a logical parent. If a parent is provided, the new
interpreter will have a child/parent relationship with the parent interp.

The C<flags> parameter contains a bit-wise combination of certain startup
flags that govern interpreter creation. It is safe to set this to 0 unless
special needs require it to be otherwise.

The C<args> parameter is a structure containing a series of options that must
be set on the interpreter during initialization. These options, many of which
deal with the memory subsystem and other deep internals can typically be
ignored. C<args> can be C<NULL> if no special options need to be set.

The new interpreter PMC is returned in the last parameter.

C<Parrot_api_destroy_interpreter ( interp )> destroys an interpreter and frees
its resources.

    Parrot_Int Parrot_api_destroy_interpreter(Parrot_Interp);

It is a good idea to destroy child interpreters before destroying their
parents.

=head3 Working with Source Code and PBC Files

libparrot natively executes .pbc bytecode files. These are manipulated in
Parrot through a PMC interface. PBC PMCs can be obtained in a number of ways:
they can be returned from a compiler, they can be loaded from PBC, or they can
be constructed on the fly.

I<Note: There are PackFile PMCs which represent bytecode, but the PBC object
returned or consumed by any individual compiler or API function may have a
different type. Treat these objects as opaque until a good interface for them
is worked out.>

Once a PBC PMC is obtained, several things can be done with it: It can be
loaded into libparrot as a library and individual calls can be made into it.
It can also be executed directly as an application, which will trigger the
C<:main> function, if any. The PMC can also be written out to a .pbc file for
later use.

Currently there are two functions to get a bytecode PMC.

    Parrot_api_load_bytecode(interp, filename, *pbc)
    Parrot_api_load_bytecode_bytes(interp, bytecode, size, *pbc)

The first function loads bytecode from a file. The second loads bytecode in
from an in-memory byte array. Both return a bytecode PMC. That PMC can be
passed as an argument to any of the following functions:

    Parrot_api_ready_bytecode(interp, pbc, *main_sub)
    Parrot_api_run_bytecode(interp, pbc, args)
    Parrot_api_disassemble_bytecode(interp, pbc, outfilename, opts)

C<Parrot_api_ready_bytecode> loads the bytecode into memory and returns a
reference to the C<:main> Sub PMC, if any. It does not automatically execute
the bytecode. C<Parrot_api_run_bytecode> loads bytecode and automatically
executes C<:main> with the given arguments, and sets these arguments in the
IGLOBALS array for later access. C<Parrot_api_disassemble_bytecode> is used
primarily by the pbc_disassemble frontend.

=head3 Settings and Configuration

The interpreter is configured in many ways. When the interpreter is created
there is a structure C<Parrot_Init_Args> that we can use to optionally change
some of the low-level internal options of the interpreter. Thereafter, we can
set configurations using the Config Hash or a series of API calls.

The Configuration hash is a hash of named settings that can be set in the
interpreter. The primary purpose of this is to help set information such as
standard search paths. The configuration hash will also be available from the
interpreter's IGLOBALS array, and may be used by various tools and utilities
to inform certain decisions. To set a configuration hash, call

    Parrot_api_set_configuration_hash

This function is only really intended to be called once per interpreter. It
is possible to set a new configuration hash at any time, but settings from
the old hash will not be removed first. Currently, the configuration hash is
set as a global, and any interpreter created after a config hash is set will
automatically inherit the last set configuration hash. Setting a new config
hash on a child interpreter does not affect any existing interpreters.

In addition to the configuration hash, library search paths can be appended
to through the following functions:

    Parrot_api_add_library_search_path
    Parrot_api_add_include_search_path
    Parrot_api_add_dynext_search_path

There is currently no way to remove search paths once set, or to examine the
complete list of search paths. This may be added later.

=head3 Strings and PMCs

Embedding API functions which perform general operations on any PMC are
named C<Parrot_api_pmc_*> and are located in the file C<src/embed/pmc.c>.
Functions which perform general operations on a Parrot String are named
C<Parrot_api_string_*> and are located in the file C<src/embed/strings.c>.

PMC functions are general and explicitly limited. The API does not, and does
not want to, provide complete access to the entire suite of internal
operations and VTABLEs. The API does not provide convenience methods to do
all operations in a single call. Some common operations may take multiple
API calls to perform.

String operations are broken down into two types: The first are the set of
functions used to marshal between C-level strings and Parrot Strings. Import
functions take a C string (char* or wchar_t*), and copy their contents into a
new String buffer. Export functions perform the opposite operation: The
internal buffer is copied to a freshly allocated memory block and returned.
When you are done with the string, there is an associated free function to
deallocate that memory.

The second type of string API functions are functions to perform operations on
strings. These could be operations such as string analysis or string
manipulation.

=head2 References

=cut

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