How do I view gstreamer debug output?

The current documentation is here. Some interesting excerpts, in my opinion:

The '*' wildcard is also available. For example GST_DEBUG=2,audio*:5 will use Debug Level 5 for all categories starting with the word audio, and 2 for all the others.

Use gst-launch-1.0 --gst-debug-help to obtain the list of all registered categories.

GStreamer has the capability to output graph files. Example.

And the debug levels are:

| # | Name    | Description                                                    |
|---|---------|----------------------------------------------------------------|
| 0 | none    | No debug information is output.                                |
| 1 | ERROR   | Logs all fatal errors. These are errors that do not allow the  |
|   |         | core or elements to perform the requested action. The          |
|   |         | application can still recover if programmed to handle the      |
|   |         | conditions that triggered the error.                           |
| 2 | WARNING | Logs all warnings. Typically these are non-fatal, but          |
|   |         | user-visible problems are expected to happen.                  |
| 3 | FIXME   | Logs all "fixme" messages. Those typically that a codepath that|
|   |         | is known to be incomplete has been triggered. It may work in   |
|   |         | most cases, but may cause problems in specific instances.      |
| 4 | INFO    | Logs all informational messages. These are typically used for  |
|   |         | events in the system that only happen once, or are important   |
|   |         | and rare enough to be logged at this level.                    |
| 5 | DEBUG   | Logs all debug messages. These are general debug messages for  |
|   |         | events that happen only a limited number of times during an    |
|   |         | object's lifetime; these include setup, teardown, change of    |
|   |         | parameters, etc.                                               |
| 6 | LOG     | Logs all log messages. These are messages for events that      |
|   |         | happen repeatedly during an object's lifetime; these include   |
|   |         | streaming and steady-state conditions. This is used for log    |
|   |         | messages that happen on every buffer in an element for example.|
| 7 | TRACE   | Logs all trace messages. Those are message that happen very    |
|   |         | very often. This is for example is each time the reference     |
|   |         | count of a GstMiniObject, such as a GstBuffer or GstEvent, is  |
|   |         | modified.                                                      |
| 8 | MEMDUMP | Logs all memory dump messages. This is the heaviest logging and|
|   |         | may include dumping the content of blocks of memory.           |
+------------------------------------------------------------------------------+

To show debug information for all categories, use something like

export GST_DEBUG="*:6"

before running your command.

Debugging messages can be printed in stderr by using the GST_DEBUG environment variable (if gstreamer has been compiled with --enable-gst-debug, which is default).

For example: GST_DEBUG=audiotestsrc:5 gst-launch audiotestsrc ! fakesink will log everything (5) from the audiotestsrc element.

You can change your program debugging output at runtime using setenv("GST_DEBUG","cat:level...", 1)

Sometime reading GStreamer debugging can be tedious. You can give gst-debug-viewer a try.

You can read the Documentation for other details.