esp-idf/docs/en/api-guides/app_trace.rst

Application Level Tracing library
=================================
:link_to_translation:`zh_CN:[中文]`

Overview
--------

IDF provides a useful feature for program behavior analysis called **Application Level Tracing**. The feature can be enabled in menuconfig and allows transfer of arbitrary data between the host and {IDF_TARGET_NAME} via JTAG interface with minimal overhead on program execution.

Developers can use this library to send application specific state of execution to the host and receive commands or other type of info in the opposite direction at runtime. The main use cases of this library are:

1. Collecting application specific data, see :ref:`app_trace-application-specific-tracing`
2. Lightweight logging to the host, see :ref:`app_trace-logging-to-host`
3. System behavior analysis, see :ref:`app_trace-system-behaviour-analysis-with-segger-systemview`
4. Source code coverage, see :ref:`app_trace-gcov-source-code-coverage`

Tracing components when working over JTAG interface are shown in the figure below.

.. figure:: ../../_static/app_trace-overview.jpg
    :align: center
    :alt: Tracing Components when Working Over JTAG
    :figclass: align-center

    Tracing Components when Working Over JTAG


Modes of Operation
------------------

The library supports two modes of operation:

**Post-mortem mode**. This is the default mode. The mode does not need interaction with the host side. In this mode, the tracing module does not check whether host has read all the data from the *HW UP BUFFER* and overwrites it with new data. This mode is useful when only the latest trace data is interesting to the user, e.g. for analyzing the program's behavior just before a crash. Host can read the data later upon user request, e.g. via a special OpenOCD command in case of working via JTAG interface.

**Streaming mode.** Tracing module enters this mode when host connects to {IDF_TARGET_NAME}. In this mode, before writing new data to *HW UP BUFFER*, the tracing module checks that there is enough space in it and if necessary waits for the host to read data and free enough memory. Maximum waiting time is controlled via timeout values passed by users to corresponding API routines. When an application tries to write data to the trace buffer with a finite wait time, it is possible that the data will be dropped. This is especially true when tracing from time critical code (ISRs, OS scheduler code etc.) where infinite timeouts can lead to a system malfunction. In order to avoid loss of such critical data, developers can enable additional data buffering via menuconfig option :ref:`CONFIG_APPTRACE_PENDING_DATA_SIZE_MAX`. This macro specifies the size of data which can be buffered in such scenarios. This option can also help to overcome a situation when data transfer to the host is temporarily slowed down, e.g. due to USB bus congestions, etc. But it will not help when the average bitrate of the trace data stream exceeds the HW interface capabilities.

Configuration Options and Dependencies
--------------------------------------

Using of this feature depends on two components:

1. **Host side:** Application tracing is done over JTAG, so it needs OpenOCD to be set up and running on host machine. For instructions on how to set it up, please see :doc:`JTAG Debugging <../api-guides/jtag-debugging/index>` for details.

2. **Target side:** Application tracing functionality can be enabled in menuconfig. *Component config > Application Level Tracing* menu allows selecting destination for the trace data (HW interface for transport). Choosing any of the destinations automatically enables ``CONFIG_APPTRACE_ENABLE`` option.

.. note::

    In order to achieve higher data rates and minimize the number of dropped packets, it is recommended to optimize the setting of the JTAG clock frequency, so it is at maximum and still provides stable operation of JTAG, see :ref:`jtag-debugging-tip-optimize-jtag-speed`.

There are two additional menuconfig options that are available to users:

1.  *Threshold for flushing last trace data to host on panic* (:ref:`CONFIG_APPTRACE_POSTMORTEM_FLUSH_THRESH`). This option is useful when working over JTAG wherein the trace data is exposed to the host in 16 KB blocks. In post-mortem mode, when one block is filled it is exposed to the host and the previous one becomes unavailable. In other words, the trace data is overwritten in 16 KB granularity. On panic, the latest data from the current input block is exposed to the host and the host can read it for post-analysis. System panic may occur when a very small amount of data has been accumulated but not yet exposed to the host. In this case the previous 16 KB of collected data will be lost and the host will see the latest, very small piece of the trace. This data may be insufficient to diagnose the problem. Thus, this menuconfig option allows avoiding such situations. It controls the threshold for flushing data in case of a panic. For example, users can decide that they need no less than 512 bytes of the recent trace data for meaningful analysis. If there is less than 512 bytes of pending data at the moment of panic, it will not be flushed and will not overwrite the previous 16 KB. The option is only meaningful in post-mortem mode and when working over JTAG.
2.  *Timeout for flushing last trace data to host on panic* (:ref:`CONFIG_APPTRACE_ONPANIC_HOST_FLUSH_TMO`). The option is only meaningful in streaming mode and controls the maximum time tracing module will wait for the host to read the last data in case of panic.


How to use this library
-----------------------

This library provides APIs for transferring arbitrary data between the host and {IDF_TARGET_NAME}. When enabled in menuconfig, target application tracing module is initialized automatically at the system startup. All that the user needs to do is to call corresponding APIs to send, receive or flush the data.

.. _app_trace-application-specific-tracing:

Application Specific Tracing
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In general the user should decide what type of data should be transferred in either direction and how this data must be interpreted (processed). The following steps must be performed to transfer data between {IDF_TARGET_NAME} and the host:

1.  On target side user should implement algorithms for writing trace data to the host. The following piece of code demonstrates an example of how to do this.

    .. code-block:: c

        #include "esp_app_trace.h"
        ...
        char buf[] = "Hello World!";
        esp_err_t res = esp_apptrace_write(ESP_APPTRACE_DEST_TRAX, buf, strlen(buf), ESP_APPTRACE_TMO_INFINITE);
        if (res != ESP_OK) {
            ESP_LOGE(TAG, "Failed to write data to host!");
            return res;
        }

    ``esp_apptrace_write()`` function uses memcpy to copy user data to the internal buffer. In some cases it can be more optimal to use ``esp_apptrace_buffer_get()`` and ``esp_apptrace_buffer_put()`` functions. They allow developers to allocate buffer and fill it themselves. The following piece of code shows how to do this.

    .. code-block:: c

        #include "esp_app_trace.h"
        ...
        int number = 10;
        char *ptr = (char *)esp_apptrace_buffer_get(ESP_APPTRACE_DEST_TRAX, 32, 100/*tmo in us*/);
        if (ptr == NULL) {
            ESP_LOGE(TAG, "Failed to get buffer!");
            return ESP_FAIL;
        }
        sprintf(ptr, "Here is the number %d", number);
        esp_err_t res = esp_apptrace_buffer_put(ESP_APPTRACE_DEST_TRAX, ptr, 100/*tmo in us*/);
        if (res != ESP_OK) {
            /* in case of error host tracing tool (e.g. OpenOCD) will report incomplete user buffer */
            ESP_LOGE(TAG, "Failed to put buffer!");
            return res;
        }

    The user may also want to receive data from the host. The following piece of code shows an example of how to do this.

    .. code-block:: c

        #include "esp_app_trace.h"
        ...
        char buf[32];
        char down_buf[32];
        size_t sz = sizeof(buf);

        /* config down buffer */
        esp_apptrace_down_buffer_config(down_buf, sizeof(down_buf));
        /* check for incoming data and read them if any */
        esp_err_t res = esp_apptrace_read(ESP_APPTRACE_DEST_TRAX, buf, &sz, 0/*do not wait*/);
        if (res != ESP_OK) {
            ESP_LOGE(TAG, "Failed to read data from host!");
            return res;
        }
        if (sz > 0) {
            /* we have data, process them */
            ...
        }

    ``esp_apptrace_read()`` function uses memcpy to copy host data to user buffer. In some cases it can be more optimal to use ``esp_apptrace_down_buffer_get()`` and ``esp_apptrace_down_buffer_put()`` functions. They allow developers to occupy chunk of read buffer and process it in-place. The following piece of code shows how to do this.

    .. code-block:: c

        #include "esp_app_trace.h"
        ...
        char down_buf[32];
        uint32_t *number;
        size_t sz = 32;

        /* config down buffer */
        esp_apptrace_down_buffer_config(down_buf, sizeof(down_buf));
        char *ptr = (char *)esp_apptrace_down_buffer_get(ESP_APPTRACE_DEST_TRAX, &sz, 100/*tmo in us*/);
        if (ptr == NULL) {
            ESP_LOGE(TAG, "Failed to get buffer!");
            return ESP_FAIL;
        }
        if (sz > 4) {
            number = (uint32_t *)ptr;
            printf("Here is the number %d", *number);
        } else {
            printf("No data");
        }
        esp_err_t res = esp_apptrace_down_buffer_put(ESP_APPTRACE_DEST_TRAX, ptr, 100/*tmo in us*/);
        if (res != ESP_OK) {
            /* in case of error host tracing tool (e.g. OpenOCD) will report incomplete user buffer */
            ESP_LOGE(TAG, "Failed to put buffer!");
            return res;
        }

2.  The next step is to build the program image and download it to the target as described in the :ref:`Getting Started Guide <get-started-build>`.
3.  Run OpenOCD (see :doc:`JTAG Debugging <../api-guides/jtag-debugging/index>`).
4.  Connect to OpenOCD telnet server. It can be done using the following command in terminal ``telnet <oocd_host> 4444``. If telnet session is opened on the same machine which runs OpenOCD you can use ``localhost`` as ``<oocd_host>`` in the command above.
5.  Start trace data collection using special OpenOCD command. This command will transfer tracing data and redirect them to specified file or socket (currently only files are supported as trace data destination). For description of the corresponding commands see `OpenOCD Application Level Tracing Commands`_.
6.  The final step is to process received data. Since format of data is defined by user the processing stage is out of the scope of this document. Good starting points for data processor are python scripts in ``$IDF_PATH/tools/esp_app_trace``: ``apptrace_proc.py`` (used for feature tests) and ``logtrace_proc.py`` (see more details in section `Logging to Host`_).


OpenOCD Application Level Tracing Commands
""""""""""""""""""""""""""""""""""""""""""

*HW UP BUFFER* is shared between user data blocks and filling of the allocated memory is performed on behalf of the API caller (in task or ISR context). In a multithreaded environment, it can happen that the task/ISR which fills the buffer is preempted by another high priority task/ISR. It is possible that the user data preparation process is not complete when that chunk is read by the host. To handle such scenarios, the tracing module prepends all user data chunks with a header that contains allocated user buffer size (2 bytes) and the length of the actual written data (2 bytes). So total length of the header is 4 bytes. OpenOCD command which reads trace data reports an error when it reads an incomplete user data chunk. In any case, it puts the contents of the whole user chunk (including unfilled area) to the output file.

Below is the description of available OpenOCD application tracing commands.

.. note::

    Currently, OpenOCD does not provide commands to send arbitrary user data to the target.


Command usage:

``esp apptrace [start <options>] | [stop] | [status] | [dump <cores_num> <outfile>]``

Sub-commands:

``start``
    Start tracing (continuous streaming).
``stop``
    Stop tracing.
``status``
    Get tracing status.
``dump``
    Dump all data from  (post-mortem dump).


Start command syntax:

  ``start <outfile> [poll_period [trace_size [stop_tmo [wait4halt [skip_size]]]]``

``outfile``
    Path to file to save data from both CPUs. This argument should have the following format: ``file://path/to/file``.
``poll_period``
    Data polling period (in ms) for available trace data. If greater than 0 then command runs in non-blocking mode. By default, 1 ms.
``trace_size``
    Maximum size of data to collect (in bytes). Tracing is stopped after specified amount of data is received. By default -1 (trace size stop trigger is disabled).
``stop_tmo``
    Idle timeout (in sec). Tracing is stopped if there is no data for a specified period of time. By default -1 (disable this stop trigger). Optionally set it to a value longer than the longest pause between tracing commands from the target.
``wait4halt``
    If 0 start tracing immediately, otherwise command waits for the target to be halted (after reset, by breakpoint etc.) and then automatically resumes it and starts tracing. By default, 0.
``skip_size``
    Number of bytes to skip at the start. By default, 0.

.. note::

    If ``poll_period`` is 0, OpenOCD telnet command line will not be available until tracing is stopped. You must stop it manually by resetting the board or pressing Ctrl+C in OpenOCD window (not one with the telnet session). Another option is to set ``trace_size`` and wait until this size of data is collected. At this point tracing stops automatically.

Command usage examples:

.. highlight:: none

1.  Collect 2048 bytes of tracing data to a file "trace.log". The file will be saved in "openocd-esp32" directory.

    ::

        esp apptrace start file://trace.log 1 2048 5 0 0

    The tracing data will be retrieved and saved in non-blocking mode. This process will stop automatically after 2048 bytes are collected, or if no data are available for more than 5 seconds.

    .. note::

        Tracing data is buffered before it is made available to OpenOCD. If you see a "Data timeout!" message, it is likely that the target is not sending enough data to empty the buffer to OpenOCD before the timeout. Either increase the timeout or use a function ``esp_apptrace_flush()`` to flush the data on specific intervals.

2.  Retrieve tracing data indefinitely in non-blocking mode.

    ::

        esp apptrace start file://trace.log 1 -1 -1 0 0

    There is no limitation on the size of collected data and there is no data timeout set. This process may be stopped by issuing ``esp apptrace stop`` command on OpenOCD telnet prompt, or by pressing Ctrl+C in OpenOCD window.

3.  Retrieve tracing data and save them indefinitely.

    ::

        esp apptrace start file://trace.log 0 -1 -1 0 0

    OpenOCD telnet command line prompt will not be available until tracing is stopped. To stop tracing press Ctrl+C in OpenOCD window.

4.  Wait for target to be halted. Then resume target's operation and start data retrieval. Stop after collecting 2048 bytes of data:

    ::

        esp apptrace start file://trace.log 0 2048 -1 1 0

    To configure tracing immediately after reset use the openocd ``reset halt`` command.


.. _app_trace-logging-to-host:

Logging to Host
^^^^^^^^^^^^^^^

IDF implements a useful feature: logging to host via application level tracing library. This is a kind of semihosting when all `ESP_LOGx` calls are redirected to the host instead of UART. This can be useful because "printing to host" eliminates some steps performed when logging to UART. Most part of the work is done on the host.

By default, IDF's logging library uses a vprintf-like function to write formatted output to dedicated UART. In general, it involves the following steps:

1. Format string is parsed to obtain type of each argument.
2. According to its type every argument is converted to string representation.
3. Format string combined with converted arguments is sent to UART.

Though the implementation of a vprintf-like function can be optimized to a certain level, all steps above have to be performed in any case and every step takes some time (especially item 3). Hence, it is quite common to observe that with additional logging added to a program for debugging, the program behavior changes and the problem is not reproduced. In the worst case, the program may not work normally at all and ends up with an error or even hangs.

Possible ways to overcome this problem are to use higher UART bitrates (or another faster interface) and/or move string formatting procedure to the host.

Application level tracing feature can be used to transfer log information to the host using ``esp_apptrace_vprintf`` function. This function does not perform full parsing of the format string and arguments, and instead just calculates the number of arguments passed and sends them along with the format string address to the host. On the host, log data are processed and printed out by a special Python script.

Limitations
"""""""""""

Current implementation of logging over JTAG has some limitations:

1. Tracing from ``ESP_EARLY_LOGx`` macros is not supported.
2. No support for printf arguments which size exceeds 4 bytes (e.g. ``double`` and ``uint64_t``).
3. Only strings from .rodata section are supported as format strings and arguments.
4. Maximum number of printf arguments is 256.


How To Use It
"""""""""""""

In order to use logging via trace module user needs to perform the following steps:

1. On target side, the user must use the :cpp:func:`esp_apptrace_vprintf` function to send log data to the host. Example code is provided in :example:`system/app_trace_to_host`.
2. Follow instructions in items 2-5 in `Application Specific Tracing`_.
3. To print out collected log records, run the following command in terminal: ``$IDF_PATH/tools/esp_app_trace/logtrace_proc.py /path/to/trace/file /path/to/program/elf/file``.


Log Trace Processor Command Options
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Command usage:

``logtrace_proc.py [-h] [--no-errors] <trace_file> <elf_file>``

Positional arguments:

``trace_file``
    Path to log trace file
``elf_file``
    Path to program ELF file

Optional arguments:

``-h``, ``--help``
    show this help message and exit
``--no-errors``, ``-n``
    Do not print errors

.. _app_trace-system-behaviour-analysis-with-segger-systemview:

System Behavior Analysis with SEGGER SystemView
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Another useful IDF feature built on top of application tracing library is the system level tracing which produces traces compatible with SEGGER SystemView tool (see `SystemView <https://www.segger.com/products/development-tools/systemview/>`_). SEGGER SystemView is a real-time recording and visualization tool that allows to analyze runtime behavior of an application.

.. note::

    Currently, IDF based applications are able to generate SystemView compatible traces in form of files to be opened in the SystemView application. The tracing process cannot yet be controlled using that tool.


How To Use It
"""""""""""""

Support for this feature is enabled by *Component config > Application Level Tracing > FreeRTOS SystemView Tracing* (:ref:`CONFIG_APPTRACE_SV_ENABLE`) menuconfig option. There are several other options enabled under the same menu:

1. {IDF_TARGET_NAME} timer to use as SystemView timestamp source: (:ref:`CONFIG_APPTRACE_SV_TS_SOURCE`) selects the source of timestamps for SystemView events. In single core mode timestamps are generated using {IDF_TARGET_NAME} internal cycle counter running at maximum 240 Mhz (~4 ns granularity). In dual-core mode external timer working at 40 Mhz is used, so timestamp granularity is 25 ns.
2. Individually enabled or disabled collection of SystemView events (``CONFIG_APPTRACE_SV_EVT_XXX``):

    - Trace Buffer Overflow Event
    - ISR Enter Event
    - ISR Exit Event
    - ISR Exit to Scheduler Event
    - Task Start Execution Event
    - Task Stop Execution Event
    - Task Start Ready State Event
    - Task Stop Ready State Event
    - Task Create Event
    - Task Terminate Event
    - System Idle Event
    - Timer Enter Event
    - Timer Exit Event

IDF has all the code required to produce SystemView compatible traces, so user can just configure necessary project options (see above), build, download the image to target and use OpenOCD to collect data as described in the previous sections.


OpenOCD SystemView Tracing Command Options
""""""""""""""""""""""""""""""""""""""""""

Command usage:

``esp sysview [start <options>] | [stop] | [status]``

Sub-commands:

``start``
    Start tracing (continuous streaming).
``stop``
    Stop tracing.
``status``
    Get tracing status.

Start command syntax:

  ``start <outfile1> [outfile2] [poll_period [trace_size [stop_tmo]]]``

``outfile1``
    Path to file to save data from PRO CPU. This argument should have the following format: ``file://path/to/file``.
``outfile2``
    Path to file to save data from APP CPU. This argument should have the following format: ``file://path/to/file``.
``poll_period``
    Data polling period (in ms) for available trace data. If greater than 0 then command runs in non-blocking mode. By default, 1 ms.
``trace_size``
    Maximum size of data to collect (in bytes). Tracing is stopped after specified amount of data is received. By default -1 (trace size stop trigger is disabled).
``stop_tmo``
    Idle timeout (in sec). Tracing is stopped if there is no data for specified period of time. By default -1 (disable this stop trigger).

.. note::

    If ``poll_period`` is 0 OpenOCD telnet command line will not be available until tracing is stopped. You must stop it manually by resetting the board or pressing Ctrl+C in OpenOCD window (not one with the telnet session). Another option is to set ``trace_size`` and wait until this size of data is collected. At this point tracing stops automatically.

Command usage examples:

.. highlight:: none

1.  Collect SystemView tracing data to files "pro-cpu.SVDat" and "app-cpu.SVDat". The files will be saved in "openocd-esp32" directory.

    ::

        esp sysview start file://pro-cpu.SVDat file://app-cpu.SVDat

    The tracing data will be retrieved and saved in non-blocking mode. To stop data this process enter ``esp sysview stop`` command on OpenOCD telnet prompt, optionally pressing Ctrl+C in OpenOCD window.

2.  Retrieve tracing data and save them indefinitely.

    ::

        esp sysview start file://pro-cpu.SVDat file://app-cpu.SVDat 0 -1 -1

    OpenOCD telnet command line prompt will not be available until tracing is stopped. To stop tracing, press Ctrl+C in OpenOCD window.


Data Visualization
""""""""""""""""""

After trace data is collected, users can use a special tool to visualize the results and inspect behavior of the program.

.. only:: not CONFIG_FREERTOS_UNICORE

    Unfortunately SystemView does not support tracing from multiple cores. So when tracing from {IDF_TARGET_NAME} working in dual-core mode two files are generated: one for PRO CPU and another one for APP CPU. Users can load both files into separate instances of the tool.

It is uneasy and awkward to analyze data for every core in separate instance of the tool. Fortunately there is an Eclipse plugin called *Impulse* which can load several trace files and makes it possible to inspect events from both cores in one view. Also, this plugin has no limitation of 1,000,000 events as compared to free version of SystemView.

Good instruction on how to install, configure and visualize data in Impulse from one core can be found `here <https://mcuoneclipse.com/2016/07/31/impulse-segger-systemview-in-eclipse/>`_.

.. note::

    IDF uses its own mapping for SystemView FreeRTOS events IDs, so user needs to replace original file with mapping ``$SYSVIEW_INSTALL_DIR/Description/SYSVIEW_FreeRTOS.txt`` with ``$IDF_PATH/docs/api-guides/SYSVIEW_FreeRTOS.txt``.
    Also contents of that IDF specific file should be used when configuring SystemView serializer using above link.

.. only:: not CONFIG_FREERTOS_UNICORE

    Configure Impulse for Dual Core Traces
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    After installing Impulse and ensuring that it can successfully load trace files for each core in separate tabs， users can add special Multi Adapter port and load both files into one view. To do this, users need to do the following in Eclipse:

    1. Open 'Signal Ports' view. Go to Windows->Show View->Other menu. Find 'Signal Ports' view in Impulse folder and double-click on it.
    2. In 'Signal Ports' view right-click on 'Ports' and select 'Add ...'->New Multi Adapter Port
    3. In open dialog Press 'Add' button and select 'New Pipe/File'.
    4. In open dialog select 'SystemView Serializer' as Serializer and set path to PRO CPU trace file. Press OK.
    5. Repeat steps 3-4 for APP CPU trace file.
    6. Double-click on created port. View for this port should open.
    7. Click Start/Stop Streaming button. Data should be loaded.
    8. Use 'Zoom Out', 'Zoom In' and 'Zoom Fit' button to inspect data.
    9. For settings measurement cursors and other features please see `Impulse documentation <https://toem.de/index.php/projects/impulse>`_).

    .. note::

        If you have problems with visualization (no data are shown or strange behavior of zoom action is observed) you can try to delete current signal hierarchy and double-click on the necessary file or port. Eclipse will ask you to create new signal hierarchy.


.. _app_trace-gcov-source-code-coverage:

Gcov (Source Code Coverage)
^^^^^^^^^^^^^^^^^^^^^^^^^^^

Basics of Gcov and Gcovr
""""""""""""""""""""""""

Source code coverage is data indicating the count and frequency of every program execution path that has been taken within a program's runtime. `Gcov <https://en.wikipedia.org/wiki/Gcov>`_ is a GCC tool that, when used in concert with the compiler, can generate log files indicating the execution count of each line of a source file. The `Gcovr <https://gcovr.com>`_ tool is utility for managing Gcov and generating summarized code coverage results.

Generally, using Gcov to compile and run programs on the Host will undergo these steps:

1. Compile the source code using GCC with the ``--coverage`` option enabled. This will cause the compiler to generate a ``.gcno`` notes files during compilation. The notes files contain information to reconstruct execution path block graphs and map each block to source code line numbers. Each source file compiled with the ``--coverage`` option should have their own ``.gcno`` file of the same name (e.g., a ``main.c`` will generate a ``main.gcno`` when compiled).

2. Execute the program. During execution, the program should generate ``.gcda`` data files. These data files contain the counts of the number of times an execution path was taken. The program will generate a ``.gcda`` file for each source file compiled with the ``--coverage`` option (e.g., ``main.c`` will generate a ``main.gcda``.

3. Gcov or Gcovr can be used generate a code coverage based on the ``.gcno``, ``.gcda``, and source files. Gcov will generate a text based coverage report for each source file in the form of a ``.gcov`` file, whilst Gcovr will generate a coverage report in HTML format.

Gcov and Gcovr in ESP-IDF
"""""""""""""""""""""""""""

Using Gcov in ESP-IDF is complicated due to the fact that the program is running remotely and not on the host (i.e., on the target). The code coverage data (i.e., the ``.gcda`` files) is initially stored on the target itself. OpenOCD is then used to dump the code coverage data from the target to the host via JTAG during runtime. Using Gcov in ESP-IDF can be split into the following steps.

1. :ref:`app_trace-gcov-setup-project`
2. :ref:`app_trace-gcov-dumping-data`
3. :ref:`app_trace-gcov-generate-report`

.. _app_trace-gcov-setup-project:

Setting Up a Project for Gcov
"""""""""""""""""""""""""""""

Compiler Option
~~~~~~~~~~~~~~~

In order to obtain code coverage data in a project, one or more source files within the project must be compiled with the ``--coverage`` option. In ESP-IDF, this can be achieved at the component level or the individual source file level:

- To cause all source files in a component to be compiled with the ``--coverage`` option, you can add ``target_compile_options(${COMPONENT_LIB} PRIVATE --coverage)`` to the ``CMakeLists.txt`` file of the component.
- To cause a select number of source files (e.g. ``sourec1.c`` and ``source2.c``) in the same component to be compiled with the ``--coverage`` option, you can add ``set_source_files_properties(source1.c source2.c PROPERTIES COMPILE_FLAGS --coverage)`` to the ``CMakeLists.txt`` file of the component.

When a source file is compiled with the ``--coverage`` option (e.g. ``gcov_example.c``), the compiler will generate the ``gcov_example.gcno`` file in the project's build directory.

Project Configuration
~~~~~~~~~~~~~~~~~~~~~

Before building a project with source code coverage, ensure that the following project configuration options are enabled by running ``idf.py menuconfig``.

- Enable the application tracing module by choosing *Trace Memory* for the  :ref:`CONFIG_APPTRACE_DESTINATION` option.
- Enable Gcov to host via the :ref:`CONFIG_APPTRACE_GCOV_ENABLE`

.. _app_trace-gcov-dumping-data:

Dumping Code Coverage Data
""""""""""""""""""""""""""

Once a project has been complied with the ``--coverage`` option and flashed onto the target, code coverage data will be stored internally on the target (i.e., in trace memory) whilst the application runs. The process of transferring code coverage data from the target to the Host is known as dumping.

The dumping of coverage data is done via OpenOCD (see :doc:`JTAG Debugging <../api-guides/jtag-debugging/index>` on how to setup and run OpenOCD). A dump is triggered by issuing commands to OpenOCD, therefore a telnet session to OpenOCD must be opened to issue such commands (run ``telnet localhost 4444``). Note that GDB could be used instead of telnet to issue commands to OpenOCD, however all commands issued from GDB will need to be prefixed as ``mon <oocd_command>``.

When the target dumps code coverage data, the ``.gcda`` files are stored in the project's build directory. For example, if ``gcov_example_main.c`` of the ``main`` component was compiled with the ``--coverage`` option, then dumping the code coverage data would generate a ``gcov_example_main.gcda`` in ``build/esp-idf/main/CMakeFiles/__idf_main.dir/gcov_example_main.c.gcda``. Note that the ``.gcno`` files produced during compilation are also placed in the same directory.

The dumping of code coverage data can be done multiple times throughout an application's lifetime. Each dump will simply update the ``.gcda`` file with the newest code coverage information. Code coverage data is accumulative, thus the newest data will contain the total execution count of each code path over the application's entire lifetime.

ESP-IDF supports two methods of dumping code coverage data form the target to the host:

* Instant Run-Time Dump
* Hard-coded Dump

Instant Run-Time Dump
~~~~~~~~~~~~~~~~~~~~~

An Instant Run-Time Dump is triggered by calling the ``{IDF_TARGET_NAME} gcov`` OpenOCD command (via a telnet session). Once called, OpenOCD will immediately preempt the {IDF_TARGET_NAME}'s current state and execute a builtin IDF Gcov debug stub function. The debug stub function will handle the dumping of data to the Host. Upon completion, the {IDF_TARGET_NAME} will resume its current state.

Hard-coded Dump
~~~~~~~~~~~~~~~

A Hard-coded Dump is triggered by the application itself by calling :cpp:func:`esp_gcov_dump` from somewhere within the application. When called, the application will halt and wait for OpenOCD to connect and retrieve the code coverage data. Once :cpp:func:`esp_gcov_dump` is called, the Host must execute the ``esp gcov dump`` OpenOCD command (via a telnet session). The ``esp gcov dump`` command will cause OpenOCD to connect to the {IDF_TARGET_NAME}, retrieve the code coverage data, then disconnect from the {IDF_TARGET_NAME} thus allowing the application to resume. Hard-coded Dumps can also be triggered multiple times throughout an application's lifetime.

Hard-coded dumps are useful if code coverage data is required at certain points of an application's lifetime by placing :cpp:func:`esp_gcov_dump` where necessary (e.g., after application initialization, during each iteration of an application's main loop).

GDB can be used to set a breakpoint on :cpp:func:`esp_gcov_dump`, then call ``mon esp gcov dump`` automatically via the use a ``gdbinit`` script (see  Using GDB from :ref:`jtag-debugging-using-debugger-command-line`).

The following GDB script is will add a breakpoint at :cpp:func:`esp_gcov_dump`, then call the ``mon esp gcov dump`` OpenOCD command.

.. code-block:: none

    b esp_gcov_dump
    commands
    mon esp gcov dump
    end


.. note::
    Note that all OpenOCD commands should be invoked in GDB as: ``mon <oocd_command>``.

.. _app_trace-gcov-generate-report:

Generating Coverage Report
""""""""""""""""""""""""""

Once the code coverage data has been dumped, the ``.gcno``, ``.gcda`` and the source files can be used to generate a code coverage report. A code coverage report is simply a report indicating the number of times each line in a source file has been executed.

Both Gcov and Gcovr can be used to generate code coverage reports. Gcov is provided along with the Xtensa toolchain, whilst Gcovr may need to be installed separately. For details on how to use Gcov or Gcovr, refer to `Gcov documentation <https://gcc.gnu.org/onlinedocs/gcc/Gcov.html>`_ and `Gcovr documentation <http://gcovr.com/>`_.

Adding Gcov Build Target to Project
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

To make report generation more convenient, users can define additional build targets in their projects such that the report generation can be done with a single build command.

Add the following lines to the ``CMakeLists.txt`` file of your project.

.. code-block:: none

    include($ENV{IDF_PATH}/tools/cmake/gcov.cmake)
    idf_create_coverage_report(${CMAKE_CURRENT_BINARY_DIR}/coverage_report)
    idf_clean_coverage_report(${CMAKE_CURRENT_BINARY_DIR}/coverage_report)

The following commands can now be used:

    * ``cmake --build build/ --target gcovr-report`` will generate an HTML coverage report in ``$(BUILD_DIR_BASE)/coverage_report/html`` directory.
    * ``cmake --build build/ --target cov-data-clean`` will remove all coverage data files.