This is the documentation for the latest (main) development branch of Zephyr. If you are looking for the documentation of previous releases, use the drop-down list at the bottom of the left panel and select the desired version.


Application Debugging

This section is a quick hands-on reference to start debugging your application with QEMU. Most content in this section is already covered in QEMU and GNU_Debugger reference manuals.

In this quick reference, you’ll find shortcuts, specific environmental variables, and parameters that can help you to quickly set up your debugging environment.

The simplest way to debug an application running in QEMU is using the GNU Debugger and setting a local GDB server in your development system through QEMU.

You will need an ELF binary image for debugging purposes. The build system generates the image in the build directory. By default, the kernel binary name is zephyr.elf. The name can be changed using CONFIG_KERNEL_BIN_NAME.

GDB server

We will use the standard 1234 TCP port to open a GDB server instance. This port number can be changed for a port that best suits the development environment. There are multiple ways to do this. Each way starts a QEMU instance with the processor halted at startup and with a GDB server instance listening for a connection.

Running QEMU directly

You can run QEMU to listen for a “gdb connection” before it starts executing any code to debug it.

qemu -s -S <image>

will setup Qemu to listen on port 1234 and wait for a GDB connection to it.

The options used above have the following meaning:

  • -S Do not start CPU at startup; rather, you must type ‘c’ in the monitor.

  • -s Shorthand for -gdb tcp::1234: open a GDB server on TCP port 1234.

Running QEMU via ninja

Run the following inside the build directory of an application:

ninja debugserver

QEMU will write the console output to the path specified in ${QEMU_PIPE} via CMake, typically qemu-fifo within the build directory. You may monitor this file during the run with tail -f qemu-fifo.

Running QEMU via west

Run the following from your project root:

west build -t debugserver_qemu

QEMU will write the console output to the terminal from which you invoked west.

Configuring the gdbserver listening device

The Kconfig option CONFIG_QEMU_GDBSERVER_LISTEN_DEV controls the listening device, which can be a TCP port number or a path to a character device. GDB releases 9.0 and newer also support Unix domain sockets.

If the option is unset, then the QEMU invocation will lack a -s or a -gdb parameter. You can then use the QEMU_EXTRA_FLAGS shell environment variable to pass in your own listen device configuration.

GDB client

Connect to the server by running gdb and giving these commands:

$ path/to/gdb path/to/zephyr.elf
(gdb) target remote localhost:1234
(gdb) dir ZEPHYR_BASE


Substitute the correct ZEPHYR_BASE for your system.

You can use a local GDB configuration .gdbinit to initialize your GDB instance on every run. Your home directory is a typical location for .gdbinit, but you can configure GDB to load from other locations, including the directory from which you invoked gdb. This example file performs the same configuration as above:

target remote localhost:1234

Alternate interfaces

GDB provides a curses-based interface that runs in the terminal. Pass the --tui option when invoking gdb or give the tui enable command within gdb.


The GDB version on your development system might not support the --tui option. Please make sure you use the GDB binary from the SDK which corresponds to the toolchain that has been used to build the binary.

Finally, the command below connects to the GDB server using the DDD, a graphical frontend for GDB. The following command loads the symbol table from the ELF binary file, in this instance, zephyr.elf.

ddd --gdb --debugger "gdb zephyr.elf"

Both commands execute gdb. The command name might change depending on the toolchain you are using and your cross-development tools.

ddd may not be installed in your development system by default. Follow your system instructions to install it. For example, use sudo apt-get install ddd on an Ubuntu system.


As configured above, when you connect the GDB client, the application will be stopped at system startup. You may set breakpoints, step through code, etc. as when running the application directly within gdb.


gdb will not print the system console output as the application runs, unlike when you run a native application in GDB directly. If you just continue after connecting the client, the application will run, but nothing will appear to happen. Check the console output as described above.

Debug with Eclipse


CMake supports generating a project description file that can be imported into the Eclipse Integrated Development Environment (IDE) and used for graphical debugging.

The GNU MCU Eclipse plug-ins provide a mechanism to debug ARM projects in Eclipse with pyOCD, Segger J-Link, and OpenOCD debugging tools.

The following tutorial demonstrates how to debug a Zephyr application in Eclipse with pyOCD in Windows. It assumes you have already installed the GCC ARM Embedded toolchain and pyOCD.

Set Up the Eclipse Development Environment

  1. Download and install Eclipse IDE for C/C++ Developers.

  2. In Eclipse, install the GNU MCU Eclipse plug-ins by opening the menu Window->Eclipse Marketplace..., searching for GNU MCU Eclipse, and clicking Install on the matching result.

  3. Configure the path to the pyOCD GDB server by opening the menu Window->Preferences, navigating to MCU, and setting the Global pyOCD Path.

Generate and Import an Eclipse Project

  1. Set up a GNU Arm Embedded toolchain as described in GNU Arm Embedded.

  2. Navigate to a folder outside of the Zephyr tree to build your application.

    # On Windows
    cd %userprofile%


    If the build directory is a subdirectory of the source directory, as is usually done in Zephyr, CMake will warn:

    “The build directory is a subdirectory of the source directory.

    This is not supported well by Eclipse. It is strongly recommended to use a build directory which is a sibling of the source directory.”

  3. Configure your application with CMake and build it with ninja. Note the different CMake generator specified by the -G"Eclipse CDT4 - Ninja" argument. This will generate an Eclipse project description file, .project, in addition to the usual ninja build files.

    Using west:

    west build -b frdm_k64f %ZEPHYR_BASE%\samples\synchronization -- -G"Eclipse CDT4 - Ninja"

    Using CMake and ninja:

    cmake -Bbuild -GNinja -DBOARD=frdm_k64f -G"Eclipse CDT4 - Ninja" %ZEPHYR_BASE%\samples\synchronization
    ninja -Cbuild
  4. In Eclipse, import your generated project by opening the menu File->Import... and selecting the option Existing Projects into Workspace. Browse to your application build directory in the choice, Select root directory:. Check the box for your project in the list of projects found and click the Finish button.

Create a Debugger Configuration

  1. Open the menu Run->Debug Configurations....

  2. Select GDB PyOCD Debugging, click the New button, and configure the following options:

    • In the Main tab:

      • Project: my_zephyr_app@build

      • C/C++ Application: zephyr/zephyr.elf

    • In the Debugger tab:

      • pyOCD Setup

        • Executable path: $pyocd_path\$pyocd_executable

        • Uncheck “Allocate console for semihosting”

      • Board Setup

        • Bus speed: 8000000 Hz

        • Uncheck “Enable semihosting”

      • GDB Client Setup

        • Executable path example (use your GNUARMEMB_TOOLCHAIN_PATH): C:\gcc-arm-none-eabi-6_2017-q2-update\bin\arm-none-eabi-gdb.exe

    • In the SVD Path tab:

      • File path: <workspace top>\modules\hal\nxp\mcux\devices\MK64F12\MK64F12.xml


      This is optional. It provides the SoC’s memory-mapped register addresses and bitfields to the debugger.

  3. Click the Debug button to start debugging.

RTOS Awareness

Support for Zephyr RTOS awareness is implemented in pyOCD v0.11.0 and later. It is compatible with GDB PyOCD Debugging in Eclipse, but you must enable CONFIG_DEBUG_THREAD_INFO=y in your application.

Debugging I2C communication

There is a possibility to log all or some of the I2C transactions done by the application. This feature is enabled by the Kconfig option CONFIG_I2C_DUMP_MESSAGES, but it uses the LOG_DBG function to print the contents so the CONFIG_I2C_LOG_LEVEL_DBG option must also be enabled.

The sample output of the dump looks like this:

D: I2C msg: io_i2c_ctrl7_port0, addr=50
D:    W      len=01: 00
D:    R Sr P len=08:
D: contents:
D: 43 42 41 00 00 00 00 00 |CBA.....

The first line indicates the I2C controller and the target address of the transaction. In above example, the I2C controller is named io_i2c_ctrl7_port0 and the target device address is 0x50


the address, length and contents values are in hexadecimal, but lack the 0x prefix

Next lines contain messages, both sent and received. The contents of write messages is always shown, while the content of read messages is controlled by a parameter to the function i2c_dump_msgs_rw. This function is available for use by user, but is also called internally by i2c_transfer API function with read content dump enabled. Before the length parameter, the header of the message is printed using abbreviations:

  • W - write message

  • R - read message

  • Sr - restart bit

  • P - stop bit

The above example shows one write message with byte 0x00 representing the address of register to read from the I2C target. After that the log shows the length of received message and following that, the bytes read from the target 43 42 41 00 00 00 00 00. The content dump consist of both the hex and ASCII representation.

Filtering the I2C communication dump

By default, all I2C communication is logged between all I2C controllers and I2C targets. It may litter the log with unrelated devices and make it difficult to effectively debug the communication with a device of interest.

Enable the Kconfig option CONFIG_I2C_DUMP_MESSAGES_ALLOWLIST to create an allowlist of I2C targets to log. The allowlist of devices is configured using the devicetree, for example:

/ {
    i2c {
        display0: some-display@a {
        sensor3: some-sensor@b {

    i2c-dump-allowlist {
        compatible = "zephyr,i2c-dump-allowlist";
        devices = < &display0 >, < &sensor3 >;

The filters nodes are identified by the compatible string with zephyr,i2c-dump-allowlist value. The devices are selected using the devices property with phandles to the devices on the I2C bus.

In the above example, the communication with device display0 and sensor3 will be displayed in the log.