Things to check when writing an application

This chapter contains a fairly random selection of things that can be useful to keep in mind when writing GStreamer-based applications. It's up to you how much you're going to use the information provided here. We will shortly discuss how to debug pipeline problems using GStreamer applications. Also, we will touch upon how to acquire knowledge about plugins and elements and how to test simple pipelines before building applications around them.

Good programming habits

  • Always add a GstBus handler to your pipeline. Always report errors in your application, and try to do something with warnings and information messages, too.

  • Always check return values of GStreamer functions. Especially, check return values of gst_element_link () and gst_element_set_state ().

  • Dereference return values of all functions returning a non-base type, such as gst_element_get_pad (). Also, always free non-const string returns, such as gst_object_get_name ().

  • Always use your pipeline object to keep track of the current state of your pipeline. Don't keep private variables in your application. Also, don't update your user interface if a user presses the “play” button. Instead, listen for the “state-changed” message on the GstBus and only update the user interface whenever this message is received.

  • Report all bugs that you find in GStreamer bugzilla at http://bugzilla.gnome.org/.

Debugging

Applications can make use of the extensive GStreamer debugging system to debug pipeline problems. Elements will write output to this system to log what they're doing. It's not used for error reporting, but it is very useful for tracking what an element is doing exactly, which can come in handy when debugging application issues (such as failing seeks, out-of-sync media, etc.).

Most GStreamer-based applications accept the commandline option --gst-debug=LIST and related family members. The list consists of a comma-separated list of category/level pairs, which can set the debugging level for a specific debugging category. For example, --gst-debug=oggdemux:5 would turn on debugging for the Ogg demuxer element. You can use wildcards as well. A debugging level of 0 will turn off all debugging, and a level of 9 will turn on all debugging. Intermediate values only turn on some debugging (based on message severity; 2, for example, will only display errors and warnings). Here's a list of all available options:

  • --gst-debug-help will print available debug categories and exit.

  • --gst-debug-level=LEVEL will set the default debug level (which can range from 0 (no output) to 9 (everything)).

  • --gst-debug=LIST takes a comma-separated list of category_name:level pairs to set specific levels for the individual categories. Example: GST_AUTOPLUG:5,avidemux:3. Alternatively, you can also set the GST_DEBUG environment variable, which has the same effect.

  • --gst-debug-no-color will disable color debugging. You can also set the GST_DEBUG_NO_COLOR environment variable to 1 if you want to disable colored debug output permanently. Note that if you are disabling color purely to avoid messing up your pager output, try using less -R.

  • --gst-debug-color-mode=MODE will change debug log coloring mode. MODE can be one of the following: on, off, auto, disable, unix. You can also set the GST_DEBUG_COLOR_MODE environment variable if you want to change colored debug output permanently. Note that if you are disabling color purely to avoid messing up your pager output, try using less -R.

  • --gst-debug-disable disables debugging altogether.

  • --gst-plugin-spew enables printout of errors while loading GStreamer plugins.

Conversion plugins

GStreamer contains a bunch of conversion plugins that most applications will find useful. Specifically, those are videoscalers (videoscale), colorspace convertors (videoconvert), audio format convertors and channel resamplers (audioconvert) and audio samplerate convertors (audioresample). Those convertors don't do anything when not required, they will act in passthrough mode. They will activate when the hardware doesn't support a specific request, though. All applications are recommended to use those elements.

Utility applications provided with GStreamer

GStreamer comes with a default set of command-line utilities that can help in application development. We will discuss only gst-launch and gst-inspect here.

gst-launch

gst-launch is a simple script-like commandline application that can be used to test pipelines. For example, the command gst-launch audiotestsrc ! audioconvert ! audio/x-raw,channels=2 ! alsasink will run a pipeline which generates a sine-wave audio stream and plays it to your ALSA audio card. gst-launch also allows the use of threads (will be used automatically as required or as queue elements are inserted in the pipeline) and bins (using brackets, so “(” and “)”). You can use dots to imply padnames on elements, or even omit the padname to automatically select a pad. Using all this, the pipeline gst-launch filesrc location=file.ogg ! oggdemux name=d d. ! queue ! theoradec ! videoconvert ! xvimagesink d. ! queue ! vorbisdec ! audioconvert ! audioresample ! alsasink will play an Ogg file containing a Theora video-stream and a Vorbis audio-stream. You can also use autopluggers such as decodebin on the commandline. See the manual page of gst-launch for more information.

gst-inspect

gst-inspect can be used to inspect all properties, signals, dynamic parameters and the object hierarchy of an element. This can be very useful to see which GObject properties or which signals (and using what arguments) an element supports. Run gst-inspect fakesrc to get an idea of what it does. See the manual page of gst-inspect for more information.

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