C-Pluff C API 0.2.0
Plug-in

Overview

Plug-in is the core element of an extensible application. The extensions and also the main application logic are implemented as plug-ins. Plug-ins can be developed, distributed and deployed independently, subject to inter-plugin dependencies. Deploying a new plug-in does not require recompilation or relinking if the operating system platform supports required dynamic linking facilities.

Structure

A plug-in includes the following structural elements.

Plug-in descriptor

A plug-in descriptor is an XML document describing a plug-in. It includes information about the contents of the plug-in, the features provided by the plug-in, plug-in version information and static dependencies of the plug-in. Most of the elements are optional. Most of the descriptor information described here is available to software via cp_plugin_info_t structure. The plug-in descriptor must be located in the plug-in directory as plugin.xml.

The formal declaration of plug-in descriptor is available as XML Schema Definition in plugin.xsd located in the top level source directory. Currently there is no namespace associated with the plug-in descriptor. Here is an example of a plug-in descriptor. Click element name to jump into documentation for that element.

<plugin id="org.c-pluff.example" name="Example Plug-in" version="0.3.2" provider-name="Johannes Lehtinen">
    <backwards-compatibility abi="0.3" api="0.2.8"/>
    <requires>
        <c-pluff version="0.1"/>
        <import plugin="org.c-pluff.util" version="0.2"/>
        <import plugin="org.c-pluff.extra" optional="true"/> 
    </requires>
    <runtime library="libruntime" funcs="org_cpluff_example_funcs"/>
    <extension-point id="editors" name="Text Editors" schema="editors_schema.xsd"/>
    <extension-point id="url-families"/>
    <extension point="org.c-pluff.util.archivers" id="tar" name="Tar Archiver Support">
        <type random-access="false"/>
        <exec bin="tar"/>
    </extension>
    <extension point="org.c-pluff.example.editors</span>&gt;
        &lt;editor name=<span class="charliteral">"Text Editor"</span> runtime=<span class="charliteral">"org_cpluff_example_txteditor_runtime"</span>&gt;
            &lt;file-types&gt;
                &lt;file-type mime-type=<span class="charliteral">"text/plain"</span>/&gt;
            &lt;/file-types&gt;
        &lt;/editor&gt;
    &lt;/<a class="code" href="#pluginDescPluginE">extension</a>&gt;
&lt;/<a class="code" href="#pluginDescPlugin">plugin</a>&gt;</pre>
</div>

A descriptor can also be much simpler, depending on the plug-in.
Here is an example of a minimal descriptor (of a useless plug-in).

<div class="fragment">
<pre class="fragment">
&lt;<a class="code" href="#pluginDescPlugin">plugin</a> id=<span class="charliteral">"org.c-pluff.useless"</span>/&gt;</pre>
</div>

@subsubsection pluginDescPlugin plugin

This is the top level element of the plug-in descriptor. It can have
following attributes.

- @a id: A mandatory unique identifier for the plug-in. Plug-in identifiers
  should preferably be generated using a reversed DNS domain name as
  prefix to prevent identifier conflicts.
- @a name: An optional human-readable name for the plug-in.
- @a version: An optional version number for the plug-in. Version numbers
  are used for checking compatibility when resolving versioned plug-in
  dependencies. See also information about
  @ref pluginVersions "plug-in versions".
- @a provider-name: The name of the plug-in provider or author. Optional.

This element can contain following elements.

- @ref pluginDescPluginBWC "backwards-compatibility": Optional information about backwards
  compatibility of this plug-in version.
- @ref pluginDescPluginRequires "requires": Information about static plug-in dependencies. Can be omitted
  if the plug-in does not have static dependencies.
- @ref pluginDescPluginRuntime "runtime": Information about the plug-in runtime library. Can be omitted
  if the plug-in does not have a runtime library but only data.
- @ref pluginDescPluginEP "extension-point": Information about extension points provided by the
  plug-in. This element is repeated if there are multiple extension points
  and omitted if there are none.
- @ref pluginDescPluginE "extension": Information about extensions provided by the plug-in.
  This element is repeated if there are multiple extensions and omitted
  if there are none.

@subsubsection pluginDescPluginBWC backwards-compatibility

This element includes optional information about the backwards compatibility
of this plug-in version. It can have following attributes.

- @a abi: Backwards compatibility of the application binary interface (ABI)
  of the plug-in. ABI includes any public symbols exported by the plug-in,
  data structures associated with exported symbols and any extension points
  provided by the plug-in. The ABI of the current plug-in version is
  backwards compatible with any plug-in version from the version specified
  here to the current version. This information is used when resolving
  versioned plug-in dependencies. See also information about
  @ref pluginVersions "plug-in versions".
- @a api: Backwards compatibility of the application programming interface
  (API) of the plug-in. API compatibility means that source code developed
  against one version of the plug-in also compiles against another version
  of the plug-in. This information is not used by framework but it can be
  used by a developer developing dependent plug-ins.

These apply to plug-ins that provide header files and runtime libraries.
For example, a plug-in might export global functions to other plug-ins or it
might provide an extension point where an extension installed by other
plug-in must conform to data structures defined by the plug-in.
Both attributes are optional.

@subsubsection pluginDescPluginRequires requires

This element includes information about static plug-in dependencies.
It can be omitted if there are no dependencies. It can contain following
elements.

- @ref pluginDescPluginReqCP "c-pluff": An optional version dependency
  on the C-Pluff implementation.
- @ref pluginDescPluginReqImport "import": Declares a static dependency
  on other plug-in. This element is repeated if there are multiple
  dependencies and omitted if there are none.

@subsubsection pluginDescPluginReqCP c-pluff

This element declares a version dependency on the C-Pluff
implementation. It can be used to ensure that the plug-in is not loaded by
incompatible C-Pluff version. It has the following attribute.

- @a version: The required version of the C-Pluff implementation.
  This is used when resolving the plug-in. It is checked that the used
  C-Pluff implementation is backwards compatible with the version specified
  here when it comes to the application binary interface (ABI) of C-Pluff.

@subsubsection pluginDescPluginReqImport import

This element declares a static dependency on other plug-in. It must be
used when a plug-in uses global symbols or data from other plug-in or when
a plug-in uses an extension point defined by other plug-in or whenever some
other plug-in needs to be there for the plug-in to work. The framework takes
care of resolving and starting the dependencies whenever the plug-in is
resolved or started.

This element can have following attributes.

- @a plugin: The identifier of the imported plug-in.
- @a version: An optional version dependency on the imported plug-in.
  The plug-in can be resolved only if the version of the imported plug-in
  is backwards compatible with the version specified here when it comes
  to the application binary interface (ABI) of the imported plug-in.
- @a optional: Is the import optional or not ("true" or "false"). Default is
  false, a mandatory import.
  An optional import behaves just like the mandatory import as long as the
  imported plug-in is present. However, if it is not present then the
  import is ignored. Optional import can be used if the plug-in works
  in limited capacity even without the specified plug-in.

@subsubsection pluginDescPluginRuntime runtime

This element contains information about the plug-in runtime library. It is
omitted if the plug-in does not have a runtime library but contains only
data. It can have following attributes.

- @a library: The name of the plug-in runtime library in the plug-in
  directory. A platform specific extension (for example, ".so" or ".dll")
  is added to the value specified here when loading the library.
- funcs: The functions to be used to create an instance of the plug-in
  runtime. This attribute is optional. It is needed if the plug-in has
  a start or stop function. The value specified here is a name of an
  exported symbol which contains a pointer to cp_plugin_runtime_t
  structure.

extension-point

This element defines an extension point provided by the plug-in.
It can have following attributes.
  • id: The local identifier of the extension point. The value specified here is prefixed with the identifier of the plug-in and dot to construct the global identifier of the extension point.
  • name: An optional human-readable name describing the use of the extension point.
  • schema: An optional path to the extension point XML schema in the plug-in directory. This information is not currently used by the framework. But it can be used by a developer to determine what information should be provided by extensions attached to this extension point.

extension

This element defines an extension installed into a specified extension
point provided by the defining plug-in or some other plug-in.
It can have following attributes.
  • point: The global identifier of the associated extension point.
  • id: An optional local identifier for the extension. The value specified here is prefixed with the identifier of the plug-in and dot to construct the global identifier for the extension.
  • name: An optional human-readable name describing the extension.
The extension element can contain XML elements specific to the associated
extension point (conforming to the schema defined by the extension point).

Plug-in runtime library

A plug-in runtime library is an optional plug-in element. Plug-ins only
supplying static data in form of XML data and files do not need a runtime
library. However, a typical plug-in does provide program logic as well.
The plug-in runtime library includes all program logic and program
data provided by the plug-in. It is simply a shared library, or a
dynamically linked library, which is linked in to the application when
the plug-in is started. When plug-in is unloaded, the runtime library is
unloaded as well. The framework has been designed to manage dependencies
so that unloading of the runtime library does not cause problems, provided
that plug-ins behave properly.
A plug-in can expose functionality to other plug-ins either as exported
global symbols that are directly resolved by other plug-ins or by supplying
extensions. When other plug-ins are directly using exported symbols the
plug-in acts just like any standard shared library. Nothing special there.
The more interesting case is exposing functionality as extensions. Because
the extension is registered at a specific extension point, the logic in
other plug-ins can use the extension and the associated program logic even
if they are not aware of the existence of the extension supplying plug-in.
The extension points accepting program logic as extensions define a way
to specify the name of the symbol pointing to the supplied logic. This is
typically an attribute of an XML element contained in the extension
definition. The plug-in supplying the extension can then export the program
logic as a global symbol with arbitrary name and then place the name of the
symbol in extension data. Alternatively, the plug-in can define a virtual
symbol at runtime using cp_define_symbol. Other plug-ins that are using
extensions registered at the extension point can then resolve the named
symbol using cp_resolve_symbol at runtime. The framework automatically
creates a dependency from the symbol using plug-in to the symbol supplying
plug-in to prevent failures in case the symbol supplying plug-in is stopped
or unloaded.

Static plug-in data

Plug-in can supply static data to other plug-ins using at least two
different mechanisms. A plug-in can easily provide static XML data as part
of extension elements. Additionally, a plug-in directory can contain
files that may be accessed by other plug-ins. Currently the platform does
not provide assistance in accessing data files provided by other plug-ins.
However, a plug-in can locate the plug-in directory and thus any included
data files by using plug-in path available in cp_plugin_info_t
structure of the data providing plug-in.

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