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216 lines
10 KiB
Markdown
216 lines
10 KiB
Markdown
# Basic tutorial 11: Debugging tools
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## Goal
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Sometimes things won’t go as expected and the error messages retrieved
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from the bus (if any) just don’t provide enough information. Luckily,
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GStreamer ships with massive amounts of debug information, which usually
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hint what the problem might be. This tutorial shows:
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- How to get more debug information from GStreamer.
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- How to print your own debug information into the GStreamer log.
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- How to get pipeline graphs
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## Printing debug information
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### The debug log
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GStreamer and its plugins are full of debug traces, this is, places in
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the code where a particularly interesting piece of information is
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printed to the console, along with time stamping, process, category,
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source code file, function and element information.
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The debug output is controlled with the `GST_DEBUG` environment
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variable. Here’s an example with `GST_DEBUG=2`:
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```
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0:00:00.868050000 1592 09F62420 WARN filesrc gstfilesrc.c:1044:gst_file_src_start:<filesrc0> error: No such file "non-existing-file.webm"
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```
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As you can see, this is quite a bit of information. In fact, the
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GStreamer debug log is so verbose, that when fully enabled it can render
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applications unresponsive (due to the console scrolling) or fill up
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megabytes of text files (when redirected to a file). For this reason,
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the logs are categorized, and you seldom need to enable all categories
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at once.
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The first category is the Debug Level, which is a number specifying the
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amount of desired output:
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```
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| # | Name | Description |
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|---|---------|----------------------------------------------------------------|
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| 0 | none | No debug information is output. |
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| 1 | ERROR | Logs all fatal errors. These are errors that do not allow the |
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| | | core or elements to perform the requested action. The |
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| | | application can still recover if programmed to handle the |
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| | | conditions that triggered the error. |
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| 2 | WARNING | Logs all warnings. Typically these are non-fatal, but |
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| | | user-visible problems are expected to happen. |
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| 3 | FIXME | Logs all "fixme" messages. Those typically that a codepath that|
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| | | is known to be incomplete has been triggered. It may work in |
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| | | most cases, but may cause problems in specific instances. |
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| 4 | INFO | Logs all informational messages. These are typically used for |
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| | | events in the system that only happen once, or are important |
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| | | and rare enough to be logged at this level. |
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| 5 | DEBUG | Logs all debug messages. These are general debug messages for |
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| | | events that happen only a limited number of times during an |
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| | | object's lifetime; these include setup, teardown, change of |
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| | | parameters, etc. |
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| 6 | LOG | Logs all log messages. These are messages for events that |
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| | | happen repeatedly during an object's lifetime; these include |
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| | | streaming and steady-state conditions. This is used for log |
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| | | messages that happen on every buffer in an element for example.|
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| 7 | TRACE | Logs all trace messages. Those are message that happen very |
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| | | very often. This is for example is each time the reference |
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| | | count of a GstMiniObject, such as a GstBuffer or GstEvent, is |
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| | | modified. |
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| 8 | MEMDUMP | Logs all memory dump messages. This is the heaviest logging and|
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| | | may include dumping the content of blocks of memory. |
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+------------------------------------------------------------------------------+
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```
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To enable debug output, set the `GST_DEBUG` environment variable to the
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desired debug level. All levels below that will also be shown (i.e., if
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you set `GST_DEBUG=2`, you will get both `ERROR` and
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`WARNING` messages).
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Furthermore, each plugin or part of the GStreamer defines its own
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category, so you can specify a debug level for each individual category.
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For example, `GST_DEBUG=2,audiotestsrc:6`, will use Debug Level 6 for
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the `audiotestsrc` element, and 2 for all the others.
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The `GST_DEBUG` environment variable, then, is a comma-separated list of
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*category*:*level* pairs, with an optional *level* at the beginning,
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representing the default debug level for all categories.
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The `'*'` wildcard is also available. For example
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`GST_DEBUG=2,audio*:6` will use Debug Level 5 for all categories
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starting with the word `audio`. `GST_DEBUG=*:2` is equivalent to
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`GST_DEBUG=2`.
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Use `gst-launch-1.0 --gst-debug-help` to obtain the list of all
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registered categories. Bear in mind that each plugin registers its own
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categories, so, when installing or removing plugins, this list can
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change.
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Use `GST_DEBUG` when the error information posted on the GStreamer bus
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does not help you nail down a problem. It is common practice to redirect
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the output log to a file, and then examine it later, searching for
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specific messages.
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GStreamer allows for custom debugging information handlers but when
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using the default one, the content of each line in the debug output
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looks like:
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```
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0:00:00.868050000 1592 09F62420 WARN filesrc gstfilesrc.c:1044:gst_file_src_start:<filesrc0> error: No such file "non-existing-file.webm"
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```
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And this is how the information formatted:
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```
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| Example | Explained |
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|------------------|-----------------------------------------------------------|
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|0:00:00.868050000 | Time stamp in HH:MM:SS.sssssssss format since the start of|
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| | the program. |
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|1592 | Process ID from which the message was issued. Useful when |
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| | your problem involves multiple processes. |
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|09F62420 | Thread ID from which the message was issued. Useful when |
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| | your problem involves multiple threads. |
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|WARN | Debug level of the message. |
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|filesrc | Debug Category of the message. |
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|gstfilesrc.c:1044 | Source file and line in the GStreamer source code where |
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| | this message was issued. |
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|gst_file_src_start| Function that issued the message. |
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|<filesrc0> | Name of the object that issued the message. It can be an |
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| | element, a pad, or something else. Useful when you have |
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| | multiple elements of the same kind and need to distinguish|
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| | among them. Naming your elements with the name property |
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| | makes this debug output more readable but GStreamer |
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| | assigns each new element a unique name by default. |
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| error: No such | |
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| file .... | The actual message. |
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+------------------------------------------------------------------------------+
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```
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### Adding your own debug information
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In the parts of your code that interact with GStreamer, it is
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interesting to use GStreamer’s debugging facilities. In this way, you
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have all debug output in the same file and the temporal relationship
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between different messages is preserved.
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To do so, use the `GST_ERROR()`, `GST_WARNING()`, `GST_INFO()`,
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`GST_LOG()` and `GST_DEBUG()` macros. They accept the same parameters as
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`printf`, and they use the `default` category (`default` will be shown
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as the Debug category in the output log).
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To change the category to something more meaningful, add these two lines
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at the top of your code:
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``` c
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GST_DEBUG_CATEGORY_STATIC (my_category);
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#define GST_CAT_DEFAULT my_category
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```
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And then this one after you have initialized GStreamer with
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`gst_init()`:
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``` c
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GST_DEBUG_CATEGORY_INIT (my_category, "my category", 0, "This is my very own");
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```
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This registers a new category (this is, for the duration of your
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application: it is not stored in any file), and sets it as the default
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category for your code. See the documentation
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for `GST_DEBUG_CATEGORY_INIT()`.
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### Getting pipeline graphs
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For those cases where your pipeline starts to grow too large and you
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lose track of what is connected with what, GStreamer has the capability
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to output graph files. These are `.dot` files, readable with free
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programs like [GraphViz](http://www.graphviz.org), that describe the
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topology of your pipeline, along with the caps negotiated in each link.
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This is also very handy when using all-in-one elements like `playbin`
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or `uridecodebin`, which instantiate several elements inside them. Use
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the `.dot` files to learn what pipeline they have created inside (and
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learn a bit of GStreamer along the way).
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To obtain `.dot` files, simply set
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the `GST_DEBUG_DUMP_DOT_DIR` environment variable to point to the
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folder where you want the files to be placed. `gst-launch-1.0` will create
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a `.dot` file at each state change, so you can see the evolution of the
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caps negotiation. Unset the variable to disable this facility. From
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within your application, you can use the
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`GST_DEBUG_BIN_TO_DOT_FILE()` and
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`GST_DEBUG_BIN_TO_DOT_FILE_WITH_TS()` macros to generate `.dot` files
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at your convenience.
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Here you have an example of the kind of pipelines that playbin
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generates. It is very complex because `playbin` can handle many
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different cases: Your manual pipelines normally do not need to be this
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long. If your manual pipeline is starting to get very big, consider
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using `playbin`.
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To download the full-size picture, use the attachments link at the top
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of this page (It's the paperclip icon).
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## Conclusion
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This tutorial has shown:
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- How to get more debug information from GStreamer using the
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`GST_DEBUG` environment variable.
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- How to print your own debug information into the GStreamer log with
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the `GST_ERROR()` macro and relatives.
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- How to get pipeline graphs with the
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`GST_DEBUG_DUMP_DOT_DIR` environment variable.
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It has been a pleasure having you here, and see you soon!
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