In this chapter you will learn how to construct the bare minimum code for a new plugin. Starting from ground zero, you will see how to get the &GStreamer; template source. Then you will learn how to use a few basic tools to copy and modify a template plugin to create a new plugin. If you follow the examples here, then by the end of this chapter you will have a functional audio filter plugin that you can compile and use in &GStreamer; applications. There are currently two ways to develop a new plugin for &GStreamer;: You can write the entire plugin by hand, or you can copy an existing plugin template and write the plugin code you need. The second method is by far the simpler of the two, so the first method will not even be described here. (Errm, that is, it is left as an exercise to the reader.) The first step is to check out a copy of the gst-template git module to get an important tool and the source code template for a basic &GStreamer; plugin. To check out the gst-template module, make sure you are connected to the internet, and type the following commands at a command console: shell $ git clone git://anongit.freedesktop.org/gstreamer/gst-template.git Initialized empty Git repository in /some/path/gst-template/.git/ remote: Counting objects: 373, done. remote: Compressing objects: 100% (114/114), done. remote: Total 373 (delta 240), reused 373 (delta 240) Receiving objects: 100% (373/373), 75.16 KiB | 78 KiB/s, done. Resolving deltas: 100% (240/240), done. This command will check out a series of files and directories into gst-template. The template you will be using is in the gst-template/gst-plugin/ directory. You should look over the files in that directory to get a general idea of the structure of a source tree for a plugin. The first thing to do when making a new element is to specify some basic details about it: what its name is, who wrote it, what version number it is, etc. We also need to define an object to represent the element and to store the data the element needs. These details are collectively known as the boilerplate. The standard way of defining the boilerplate is simply to write some code, and fill in some structures. As mentioned in the previous section, the easiest way to do this is to copy a template and add functionality according to your needs. To help you do so, there is a tool in the ./gst-plugins/tools/ directory. This tool, make_element, is a command line utility that creates the boilerplate code for you. To use make_element, first open up a terminal window. Change to the gst-template/gst-plugin/src directory, and then run the make_element command. The arguments to the make_element are: the name of the plugin, and the source file that the tool will use. By default, gstplugin is used. Note that capitalization is important for the name of the plugin. Under some operating systems, capitalization is also important when specifying directory names. For example, the following commands create the ExampleFilter plugin based on the plugin template and put the output files in the gst-template/gst-plugin/src directory: shell $ cd gst-template/gst-plugin/src shell $ ../tools/make_element ExampleFilter The last command creates two files: gstexamplefilter.c and gstexamplefilter.h. It is recommended that you create a copy of the gst-plugin directory before continuing. First we will examine the code you would be likely to place in a header file (although since the interface to the code is entirely defined by the plugin system, and doesn't depend on reading a header file, this is not crucial.) The code here can be found in examples/pwg/examplefilter/boiler/gstexamplefilter.h. #include <gst/gst.h> /* Definition of structure storing data for this element. */ typedef struct _GstMyFilter { GstElement element; GstPad *sinkpad, *srcpad; gboolean silent; } GstMyFilter; /* Standard definition defining a class for this element. */ typedef struct _GstMyFilterClass { GstElementClass parent_class; } GstMyFilterClass; /* Standard macros for defining types for this element. */ #define GST_TYPE_MY_FILTER (gst_my_filter_get_type()) #define GST_MY_FILTER(obj) \ (G_TYPE_CHECK_INSTANCE_CAST((obj),GST_TYPE_MY_FILTER,GstMyFilter)) #define GST_MY_FILTER_CLASS(klass) \ (G_TYPE_CHECK_CLASS_CAST((klass),GST_TYPE_MY_FILTER,GstMyFilterClass)) #define GST_IS_MY_FILTER(obj) \ (G_TYPE_CHECK_INSTANCE_TYPE((obj),GST_TYPE_MY_FILTER)) #define GST_IS_MY_FILTER_CLASS(klass) \ (G_TYPE_CHECK_CLASS_TYPE((klass),GST_TYPE_MY_FILTER)) /* Standard function returning type information. */ GType gst_my_filter_get_type (void); Using this header file, you can use the following macro to setup the GObject basics in your source file so that all functions will be called appropriately: #include "filter.h" GST_BOILERPLATE (GstMyFilter, gst_my_filter, GstElement, GST_TYPE_ELEMENT); The GstElementDetails structure gives a hierarchical type for the element, a human-readable description of the element, as well as author and version data. The entries are: A long, english, name for the element. The type of the element, see the docs/design/draft-klass.txt document in the GStreamer core source tree for details and examples. A brief description of the purpose of the element. The name of the author of the element, optionally followed by a contact email address in angle brackets. For example: static const GstElementDetails my_filter_details = { "An example plugin", "Example/FirstExample", "Shows the basic structure of a plugin", "your name <your.name@your.isp>" }; The element details are registered with the plugin during the _base_init () function, which is part of the GObject system. The _base_init () function should be set for this GObject in the function where you register the type with GLib. static void gst_my_filter_base_init (gpointer klass) { GstElementClass *element_class = GST_ELEMENT_CLASS (klass); static const GstElementDetails my_filter_details = { [..] }; [..] gst_element_class_set_details (element_class, &my_filter_details); } A GstStaticPadTemplate is a description of a pad that the element will (or might) create and use. It contains: A short name for the pad. Pad direction. Existence property. This indicates whether the pad exists always (an always pad), only in some cases (a sometimes pad) or only if the application requested such a pad (a request pad). Supported types by this element (capabilities). For example: static GstStaticPadTemplate sink_factory = GST_STATIC_PAD_TEMPLATE ( "sink", GST_PAD_SINK, GST_PAD_ALWAYS, GST_STATIC_CAPS ("ANY") ); Those pad templates are registered during the _base_init () function. Pads are created from these templates in the element's _init () function using gst_pad_new_from_template (). The template can be retrieved from the element class using gst_element_class_get_pad_template (). See below for more details on this. In order to create a new pad from this template using gst_pad_new_from_template (), you will need to declare the pad template as a global variable. More on this subject in . static GstStaticPadTemplate sink_factory = [..], src_factory = [..]; static void gst_my_filter_base_init (gpointer klass) { GstElementClass *element_class = GST_ELEMENT_CLASS (klass); [..] gst_element_class_add_pad_template (element_class, gst_static_pad_template_get (&src_factory)); gst_element_class_add_pad_template (element_class, gst_static_pad_template_get (&sink_factory)); } The last argument in a template is its type or list of supported types. In this example, we use 'ANY', which means that this element will accept all input. In real-life situations, you would set a mimetype and optionally a set of properties to make sure that only supported input will come in. This representation should be a string that starts with a mimetype, then a set of comma-separates properties with their supported values. In case of an audio filter that supports raw integer 16-bit audio, mono or stereo at any samplerate, the correct template would look like this: static GstStaticPadTemplate sink_factory = GST_STATIC_PAD_TEMPLATE ( "sink", GST_PAD_SINK, GST_PAD_ALWAYS, GST_STATIC_CAPS ( "audio/x-raw-int, " "width = (int) 16, " "depth = (int) 16, " "endianness = (int) BYTE_ORDER, " "channels = (int) { 1, 2 }, " "rate = (int) [ 8000, 96000 ]" ) ); Values surrounded by curly brackets ({ and }) are lists, values surrounded by square brackets ([ and ]) are ranges. Multiple sets of types are supported too, and should be separated by a semicolon (;). Later, in the chapter on pads, we will see how to use types to know the exact format of a stream: . Each element has three functions which are used for construction of an element. These are the _base_init() function which is meant to initialize class and child class properties during each new child class creation; the _class_init() function, which is used to initialise the class only once (specifying what signals, arguments and virtual functions the class has and setting up global state); and the _init() function, which is used to initialise a specific instance of this type. Once we have written code defining all the parts of the plugin, we need to write the plugin_init() function. This is a special function, which is called as soon as the plugin is loaded, and should return TRUE or FALSE depending on whether it loaded initialized any dependencies correctly. Also, in this function, any supported element type in the plugin should be registered. static gboolean plugin_init (GstPlugin *plugin) { return gst_element_register (plugin, "my_filter", GST_RANK_NONE, GST_TYPE_MY_FILTER); } GST_PLUGIN_DEFINE ( GST_VERSION_MAJOR, GST_VERSION_MINOR, "my_filter", "My filter plugin", plugin_init, VERSION, "LGPL", "GStreamer", "http://gstreamer.net/" ) Note that the information returned by the plugin_init() function will be cached in a central registry. For this reason, it is important that the same information is always returned by the function: for example, it must not make element factories available based on runtime conditions. If an element can only work in certain conditions (for example, if the soundcard is not being used by some other process) this must be reflected by the element being unable to enter the READY state if unavailable, rather than the plugin attempting to deny existence of the plugin.