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131 lines
6 KiB
Markdown
131 lines
6 KiB
Markdown
---
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title: Things to check when writing an application
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...
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# Things to check when writing an application
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This chapter contains a fairly random selection of things that can be
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useful to keep in mind when writing GStreamer-based applications. It's
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up to you how much you're going to use the information provided here. We
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will shortly discuss how to debug pipeline problems using GStreamer
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applications. Also, we will touch upon how to acquire knowledge about
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plugins and elements and how to test simple pipelines before building
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applications around them.
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## Good programming habits
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- Always add a `GstBus` handler to your pipeline. Always report errors
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in your application, and try to do something with warnings and
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information messages, too.
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- Always check return values of GStreamer functions. Especially, check
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return values of `gst_element_link ()` and `gst_element_set_state
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()`.
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- Dereference return values of all functions returning a non-base
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type, such as `gst_element_get_pad ()`. Also, always free non-const
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string returns, such as `gst_object_get_name ()`.
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- Always use your pipeline object to keep track of the current state
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of your pipeline. Don't keep private variables in your application.
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Also, don't update your user interface if a user presses the “play”
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button. Instead, listen for the “state-changed” message on the
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`GstBus` and only update the user interface whenever this message is
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received.
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- Report all bugs that you find in GStreamer bugzilla at
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[http://bugzilla.gnome.org/](http://bugzilla.gnome.org).
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## Debugging
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Applications can make use of the extensive GStreamer debugging system to
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debug pipeline problems. Elements will write output to this system to
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log what they're doing. It's not used for error reporting, but it is
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very useful for tracking what an element is doing exactly, which can
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come in handy when debugging application issues (such as failing seeks,
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out-of-sync media, etc.).
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Most GStreamer-based applications accept the commandline option
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`--gst-debug=LIST` and related family members. The list consists of a
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comma-separated list of category/level pairs, which can set the
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debugging level for a specific debugging category. For example,
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`--gst-debug=oggdemux:5` would turn on debugging for the Ogg demuxer
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element. You can use wildcards as well. A debugging level of 0 will turn
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off all debugging, and a level of 9 will turn on all debugging.
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Intermediate values only turn on some debugging (based on message
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severity; 2, for example, will only display errors and warnings). Here's
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a list of all available options:
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- `--gst-debug-help` will print available debug categories and exit.
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- `--gst-debug-level=LEVEL` will set the default debug level (which
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can range from 0 (no output) to 9 (everything)).
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- `--gst-debug=LIST` takes a comma-separated list of
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category\_name:level pairs to set specific levels for the individual
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categories. Example: `GST_AUTOPLUG:5,avidemux:3`. Alternatively, you
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can also set the `GST_DEBUG` environment variable, which has the
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same effect.
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- `--gst-debug-no-color` will disable color debugging. You can also
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set the GST\_DEBUG\_NO\_COLOR environment variable to 1 if you want
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to disable colored debug output permanently. Note that if you are
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disabling color purely to avoid messing up your pager output, try
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using `less -R`.
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- `--gst-debug-color-mode=MODE` will change debug log coloring mode.
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MODE can be one of the following: `on`, `off`, `auto`, `disable`,
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`unix`. You can also set the GST\_DEBUG\_COLOR\_MODE environment
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variable if you want to change colored debug output permanently.
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Note that if you are disabling color purely to avoid messing up your
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pager output, try using `less -R`.
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- `--gst-debug-disable` disables debugging altogether.
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- `--gst-plugin-spew` enables printout of errors while loading
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GStreamer plugins.
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## Conversion plugins
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GStreamer contains a bunch of conversion plugins that most applications
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will find useful. Specifically, those are videoscalers (videoscale),
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colorspace convertors (videoconvert), audio format convertors and
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channel resamplers (audioconvert) and audio samplerate convertors
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(audioresample). Those convertors don't do anything when not required,
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they will act in passthrough mode. They will activate when the hardware
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doesn't support a specific request, though. All applications are
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recommended to use those elements.
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## Utility applications provided with GStreamer
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GStreamer comes with a default set of command-line utilities that can
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help in application development. We will discuss only `gst-launch` and
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`gst-inspect` here.
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### `gst-launch`
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`gst-launch` is a simple script-like commandline application that can be
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used to test pipelines. For example, the command `gst-launch
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audiotestsrc ! audioconvert !
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audio/x-raw,channels=2 ! alsasink` will run a pipeline which generates a
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sine-wave audio stream and plays it to your ALSA audio card.
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`gst-launch` also allows the use of threads (will be used automatically
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as required or as queue elements are inserted in the pipeline) and bins
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(using brackets, so “(” and “)”). You can use dots to imply padnames on
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elements, or even omit the padname to automatically select a pad. Using
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all this, the pipeline `gst-launch filesrc location=file.ogg ! oggdemux
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name=d
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d. ! queue ! theoradec ! videoconvert ! xvimagesink
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d. ! queue ! vorbisdec ! audioconvert ! audioresample ! alsasink
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` will play an Ogg file containing a Theora video-stream and a Vorbis
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audio-stream. You can also use autopluggers such as decodebin on the
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commandline. See the manual page of `gst-launch` for more information.
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### `gst-inspect`
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`gst-inspect` can be used to inspect all properties, signals, dynamic
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parameters and the object hierarchy of an element. This can be very
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useful to see which `GObject` properties or which signals (and using
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what arguments) an element supports. Run `gst-inspect fakesrc` to get an
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idea of what it does. See the manual page of `gst-inspect` for more
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information.
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