8.7 KiB
Basic tutorial 11: Debugging tools
Goal
Sometimes things won’t go as expected and the error messages retrieved from the bus (if any) just don’t provide enough information. Luckily, GStreamer ships with massive amounts of debug information, which usually hint what the problem might be. This tutorial shows:
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How to get more debug information from GStreamer.
-
How to print your own debug information into the GStreamer log.
-
How to get pipeline graphs
Printing debug information
The debug log
GStreamer and its plugins are full of debug traces, this is, places in the code where a particularly interesting piece of information is printed to the console, along with time stamping, process, category, source code file, function and element information.
The debug output is controlled with the GST_DEBUG environment
variable. Here’s an example with
GST_DEBUG=2:
0:00:00.868050000 1592 09F62420 WARN filesrc gstfilesrc.c:1044:gst_file_src_start:<filesrc0> error: No such file "non-existing-file.webm"
As you can see, this is quite a bit of information. In fact, the GStreamer debug log is so verbose, that when fully enabled it can render applications unresponsive (due to the console scrolling) or fill up megabytes of text files (when redirected to a file). For this reason, the logs are categorized, and you seldom need to enable all categories at once.
The first category is the Debug Level, which is a number specifying the amount of desired output:
| # | 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 mauy 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, ... |
| 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 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 enable debug output, set the GST_DEBUG environment variable to the
desired debug level. All levels below that will also be shown (i.e., if
you set GST_DEBUG=2, you will get both ERROR and
WARNING messages).
Furthermore, each plugin or part of the GStreamer defines its own
category, so you can specify a debug level for each individual category.
For example, GST_DEBUG=2,audiotestsrc:6, will use Debug Level 6 for
the audiotestsrc element, and 2 for all the others.
The GST_DEBUG environment variable, then, is a comma-separated list of
category:level pairs, with an optional level at the beginning,
representing the default debug level for all categories.
The '*' wildcard is also available. For example
GST_DEBUG=2,audio*:6 will use Debug Level 5 for all categories
starting with the word audio. GST_DEBUG=*:2 is equivalent to
GST_DEBUG=2.
Use gst-launch-1.0 --gst-debug-help to obtain the list of all
registered categories. Bear in mind that each plugin registers its own
categories, so, when installing or removing plugins, this list can
change.
Use GST_DEBUG when the error information posted on the GStreamer bus
does not help you nail down a problem. It is common practice to redirect
the output log to a file, and then examine it later, searching for
specific messages.
The content of each line in the debug output is:
0:00:00.868050000 1592 09F62420 WARN filesrc gstfilesrc.c:1044:gst_file_src_start:<filesrc0> error: No such file "non-existing-file.webm"
| Example | Explained |
|---|---|
0:00:00.868050000 |
Time stamp in HH:MM:SS.sssssssss format since the start of the program |
1592 |
Process ID from which the message was issued. Useful when your problem involves multiple processes |
09F62420 |
Thread ID from which the message was issued. Useful when your problem involves multiple threads |
WARN |
Debug level of the message |
filesrc |
Debug Category of the message |
gstfilesrc.c:1044 |
Source file and line in the GStreamer source code where this message is printed |
gst_file_src_start |
Function from which the message was issued |
<filesrc0> |
Name of the object that issued the message. It can be an element, a Pad, or something else. Useful when you have multiple elements of the same kind and need to distinguish among them. Naming your elements with the name property will make this debug output more readable (otherwise, GStreamer assigns each new element a unique name). |
error: No such file "non-existing-file.webm" |
The actual message. |
Adding your own debug information
In the parts of your code that interact with GStreamer, it is interesting to use GStreamer’s debugging facilities. In this way, you have all debug output in the same file and the temporal relationship between different messages is preserved.
To do so, use the GST_ERROR(), GST_WARNING(), GST_INFO(),
GST_LOG() and GST_DEBUG() macros. They accept the same parameters as
printf, and they use the default category (default will be shown
as the Debug category in the output log).
To change the category to something more meaningful, add these two lines at the top of your code:
GST_DEBUG_CATEGORY_STATIC (my_category);
#define GST_CAT_DEFAULT my_category
And then this one after you have initialized GStreamer with
gst_init():
GST_DEBUG_CATEGORY_INIT (my_category, "my category", 0, "This is my very own");
This registers a new category (this is, for the duration of your
application: it is not stored in any file), and sets it as the default
category for your code. See the documentation
for GST_DEBUG_CATEGORY_INIT().
Getting pipeline graphs
For those cases where your pipeline starts to grow too large and you
lose track of what is connected with what, GStreamer has the capability
to output graph files. These are .dot files, readable with free
programs like GraphViz, that describe the
topology of your pipeline, along with the caps negotiated in each link.
This is also very handy when using all-in-one elements like playbin
or uridecodebin, which instantiate several elements inside them. Use
the .dot files to learn what pipeline they have created inside (and
learn a bit of GStreamer along the way).
To obtain .dot files, simply set
the GST_DEBUG_DUMP_DOT_DIR environment variable to point to the
folder where you want the files to be placed. gst-launch-1.0 will create
a .dot file at each state change, so you can see the evolution of the
caps negotiation. Unset the variable to disable this facility. From
within your application, you can use the
GST_DEBUG_BIN_TO_DOT_FILE() and
GST_DEBUG_BIN_TO_DOT_FILE_WITH_TS() macros to generate .dot files
at your convenience.
Here you have an example of the kind of pipelines that playbin
generates. It is very complex because playbin can handle many
different cases: Your manual pipelines normally do not need to be this
long. If your manual pipeline is starting to get very big, consider
using playbin.
To download the full-size picture, use the attachments link at the top of this page (It's the paperclip icon).
Conclusion
This tutorial has shown:
- How to get more debug information from GStreamer using the
GST_DEBUGenvironment variable. - How to print your own debug information into the GStreamer log with
the
GST_ERROR()macro and relatives. - How to get pipeline graphs with the
GST_DEBUG_DUMP_DOT_DIRenvironment variable.
It has been a pleasure having you here, and see you soon!