Goal

有事我們會出現一些錯誤，GSteam會給我們提供一些除錯資訊幫助我們來定位問題。主要包括：如何獲取更多除錯資訊，如何將除錯資訊列印到日誌裡，如何獲取pipeline圖。

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附帶 massive amounts of大量的 debug information, which usually hint暗示 what the problem might be. This tutorial shows:

• 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的除錯資訊包括：時間，檔名，行號，函式名等資訊。下邊程式碼是一條日誌資訊。

GStreamer and its plugins外掛 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"

GStream的日誌資訊非常全面，如果全部開啟，可能會使控制欄卡死或者檔案卡死。可以通過日誌等級來定義顯示哪些資訊。

日誌分類通過設定環境變數GST_DEBUG來實現：0：無錯誤。1：致命錯誤。2，警告。3：需要修復的。4，資訊(流程性的)。5：除錯資訊。6：所有message。7：跟蹤資訊。8：堆疊資訊。其中等級是包含關係，比如設定level為4的話，1,2,3,4型別的資訊都會列印。

可以針對指定的element進行日誌等級設定，可以通過萬用字元*設定包含指定內容的element的日誌等級。GStream日誌可以寫到檔案裡邊，通過對日誌檔案的檢索，定位問題原因。

As you can see, this is quite a bit of information. In fact, the GStreamer debug log is so verbose冗餘, that when fully enabled it can render致使 applications unresponsive (due to the console scrolling) or fill up megabytes百萬位元組 of text files (when redirected to a file). For this reason, the logs are categorized分類, and you seldom很少 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, 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 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 audiotestsrcelement, and 2 for all the others.

The GST_DEBUG environment variable, then, is a comma-separated list of category:level pairs, with an optional levelat the beginning, representing the default debug level for all categories.

The '*' wildcard萬用字元 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確定 a problem. It is common practice to redirect the output log to a file, and then examine it later, searching for specific messages.

GStreamer allows for custom debugging information handlers but when using the default one, the content of each line in the debug output looks like:

0:00:00.868050000  1592   09F62420 WARN                 filesrc gstfilesrc.c:1044:gst_file_src_start:<filesrc0> error: No such file "non-existing-file.webm"

And this is how the information formatted:

| 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 was issued.                                  |
|gst_file_src_start| Function that issued the message.                         |
|<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   |
|                  | makes this debug output more readable but GStreamer       |
|                  | assigns each new element a unique name by default.        |
| error: No such   |                                                           |
| file ....        | The actual message.                                       |
+------------------------------------------------------------------------------+

Adding your own debug information

通過巨集GST_DEBUG_CATEGORY_INIT以及GST_ERROR(), GST_WARNING(), GST_INFO(), GST_LOG() and GST_DEBUG()可以利用GStream的日誌系統新增自己的除錯資訊。GST_DEBUG_CATEGORY_INIT需要在gst_init()之後呼叫。

In the parts of your code that interact with相互作用 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

獲取通道構件圖。.dot圖片檔案可以自動生成。(.dot是一種類似於visio的圖片檔案，使用開源工具GraphViz可以開啟，會顯示空間之間的關聯)

通過巨集GST_DEBUG_DUMP_DOT_DIR設定圖片.dot的儲存目錄後，gst-launch-1.0在每次狀態變化的時候，自動建立.dot檔案。

開發人員也可以通過巨集GST_DEBUG_BIN_TO_DOT_FILE()、GST_DEBUG_BIN_TO_DOT_FILE_WITH_TS()實時生成.dot檔案。

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獲取 .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_DEBUG environment 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_DIR environment variable.

It has been a pleasure having you here, and see you soon!