Constructing the Boilerplate

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.

Getting the GStreamer Plugin Templates

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 https://gitlab.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.

If for some reason you can't access the git repository, you can also download a snapshot of the latest revision via the gitlab web interface.

Using the Project Stamp

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-plugin/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.

For example, the following commands create the MyFilter 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 MyFilter

Note

Capitalization is important for the name of the plugin. Keep in mind that under some operating systems, capitalization is also important when specifying directory and file names in general.

The last command creates two files: gstmyfilter.c and gstmyfilter.h.

Note

It is recommended that you create a copy of the gst-plugin directory before continuing.

Now one needs to run meson setup build from the parent directory to bootstrap the build environment. After that, the project can be built and installed using the well known ninja -C build commands.

Note

Be aware that by default meson will choose /usr/local as a default location. One would need to add /usr/local/lib/gstreamer-1.0 to GST_PLUGIN_PATH in order to make the new plugin show up in a gstreamer that's been installed from packages.

Note

FIXME: this section is slightly outdated. gst-template is still useful as an example for a minimal plugin build system skeleton. However, for creating elements the tool gst-element-maker from gst-plugins-bad is recommended these days.

Examining the Basic Code

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.)

#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);

GST_ELEMENT_REGISTER_DECLARE(my_filter)

Using this header file, you can use the following macros to setup the Element basics in your source file so that all functions will be called appropriately:

#include "filter.h"

G_DEFINE_TYPE (GstMyFilter, gst_my_filter, GST_TYPE_ELEMENT);
GST_ELEMENT_REGISTER_DEFINE(my_filter, "my-filter", GST_RANK_NONE, GST_TYPE_MY_FILTER);

The macro GST_ELEMENT_REGISTER_DEFINE in combination with GST_ELEMENT_REGISTER_DECLARE allows to register the element from within the plugin or from any other plugin/application by calling GST_ELEMENT_REGISTER (my_filter).

Element metadata

The Element metadata provides extra element information. It is configured with gst_element_class_set_metadata or gst_element_class_set_static_metadata which takes the following parameters:

A long, English, name for the element.
The type of the element, see the docs/additional/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:

gst_element_class_set_static_metadata (klass,
  "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 _class_init () function, which is part of the GObject system. The _class_init () function should be set for this GObject in the function where you register the type with GLib.

static void
gst_my_filter_class_init (GstMyFilterClass * klass)
{
  GstElementClass *element_class = GST_ELEMENT_CLASS (klass);

[..]
  gst_element_class_set_static_metadata (element_class,
    "An example plugin",
    "Example/FirstExample",
    "Shows the basic structure of a plugin",
    "your name <your.name@your.isp>");

}

GstStaticPadTemplate

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 _class_init () function with the gst_element_class_add_pad_template (). For this function you need a handle to the GstPadTemplate which you can create from the static pad template with gst_static_pad_template_get (). See below for more details on this.

Pads are created from these static templates in the element's _init () function using gst_pad_new_from_static_template (). In order to create a new pad from this template using gst_pad_new_from_static_template (), you will need to declare the pad template as a global variable. More on this subject in Specifying the pads.

static GstStaticPadTemplate sink_factory = [..],
    src_factory = [..];

static void
gst_my_filter_class_init (GstMyFilterClass * 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 media type 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 media type, 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, "
      "format = (string) " GST_AUDIO_NE (S16) ", "
      "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: Specifying the pads.

Constructor Functions

Each element has two functions which are used for construction of an element. 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.

The plugin_init function

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 (my_filter, plugin);
}

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.

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