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Native simulator - native_sim


The native_sim board is a POSIX architecture based board. With it, a Zephyr application can be compiled together with the Zephyr kernel, and libraries, creating a normal Linux executable.

native_sim is based on the native simulator and the POSIX architecture.

This board does not intend to simulate any particular HW, but it provides a few peripherals such as an Ethernet driver, display, UART, etc., to enable developing and testing application code which would require them. See Peripherals for more information.


native_sim is an evolution of the older native_posix.
Some components, code, options names, and documentation will still use the old native_posix names. But all components which worked with native_posix will work with native_sim.

Host system dependencies

Please check the Posix Arch Dependencies

Important limitations and unsupported features

native_sim is based on the POSIX architecture, and therefore its limitations and considerations apply to it.

Similarly, it inherits the POSIX architecture unsupported features set.

Note that some drivers may have limitations, or may not support their whole driver API optional functionality.

How to use it


To build, simply specify the native_sim board as target:

west build -b native_sim samples/hello_world


The result of the compilation is an executable (zephyr.exe) placed in the zephyr/ subdirectory of the build folder. Run the zephyr.exe executable as you would any other Linux console application.

$ ./build/zephyr/zephyr.exe
# Press Ctrl+C to exit

This executable accepts several command line options depending on the compilation configuration. You can run it with the --help command line switch to get a list of available options.

$ ./build/zephyr/zephyr.exe --help

Note that the Zephyr kernel does not actually exit once the application is finished. It simply goes into the idle loop forever. Therefore you must stop the application manually (Ctrl+C in Linux).

Application tests using the ztest framework will exit after all tests have completed.

If you want your application to gracefully finish when it reaches some point, you may add a conditionally compiled (CONFIG_ARCH_POSIX) call to nsi_exit(int status) at that point.


Since the Zephyr executable is a native application, it can be debugged and instrumented as any other native program. The program is compiled with debug information, so it can be run directly in, for example, gdb or instrumented with valgrind.

Because the execution of your Zephyr application is normally deterministic (there are no asynchronous or random components), you can execute the code multiple times and get the exact same result. Instrumenting the code does not affect its execution.

To ease debugging you may want to compile your code without optimizations (e.g., -O0) by setting CONFIG_NO_OPTIMIZATIONS.

For ease of debugging consider using an IDE as GUI for your debugger.

Address Sanitizer (ASan)

You can also build Zephyr with the Address Sanitizer. To do this, set CONFIG_ASAN, for example, in the application project file, or in the west build or cmake command line invocation.

Note that you will need the ASan library installed in your system. In Debian/Ubuntu this is libasan1.

Undefined Behavior Sanitizer (UBSan)

You can also build Zephyr with the Undefined Behavior Sanitizer. To do this, set CONFIG_UBSAN, for example, in the application project file, or in the west build or cmake command line invocation.

Coverage reports

See coverage reports using the POSIX architecture.

32 and 64bit versions

native_sim comes with two targets: A 32 bit and 64 bit version. The 32 bit version, native_sim, is the default target, which will compile your code for the ILP32 ABI (i386 in a x86 or x86_64 system) where pointers and longs are 32 bits. This mimics the ABI of most embedded systems Zephyr targets, and is therefore normally best to test and debug your code, as some bugs are dependent on the size of pointers and longs. This target requires either a 64 bit system with multilib support installed or one with a 32bit userspace.

The 64 bit version, native_sim/native/64, compiles your code targeting the LP64 ABI (x86-64 in x86 systems), where pointers and longs are 64 bits. You can use this target if you cannot compile or run 32 bit binaries.

C library choice

native_sim may be compiled with a choice of C libraries. By default it will be compiled with the host C library (CONFIG_EXTERNAL_LIBC), but you can also select to build it with CONFIG_MINIMAL_LIBC or with CONFIG_PICOLIBC. If you select some feature which are not compatible with the host C library, Picolibc will be selected by default instead.

When building with either minimal or Picolibc you will build your code in a more similar way as when building for the embedded target, you will be able to test your code interacting with that C library, and there will be no conflicts with the POSIX OS abstraction shim, but, accessing the host for test purposes from your embedded code will be more difficult, and you will have a limited choice of drivers and backends to chose from.

Rationale for this port and comparison with other options

The native_sim board shares the overall intent of the POSIX architecture, while being a HW agnostic test platform which in some cases utilizes the host OS peripherals. It does not intend to model any particular HW, and as such can only be used to develop and test application code which is far decoupled from the HW.

For developing and testing SW which requires specific HW, while retaining the benefits of the POSIX architecture other solutions like the bsim boards should be considered.

Check the POSIX architecture comparison with other development and test options for more insights.


This board is based on the POSIX architecture port of Zephyr and shares its basic architecture regarding threading and CPU/HW scheduling.

If you are interested on the inner workings of the native simulator itself, you can check its documentation.

This board does not try to emulate any particular embedded CPU or SOC. The code is compiled natively for the host system (typically x86).

About time in native_sim

Normally simulated time runs fully decoupled from the real host time and as fast as the host compute power would allow. This is desirable when running in a debugger or testing in batch, but not if interacting with external interfaces based on the real host time.

The Zephyr kernel is only aware of the simulated time as provided by the HW models. Therefore any normal Zephyr thread will also know only about simulated time.

The only link between the simulated time and the real/host time, if any, is created by the clock and timer model.

This model can be configured to slow down the execution of native_sim to real time. You can do this with the --rt and --no-rt options from the command line. The default behavior is set with CONFIG_NATIVE_SIM_SLOWDOWN_TO_REAL_TIME.

Note that all this model does is wait before raising the next system tick interrupt until the corresponding real/host time. If, for some reason, native_sim runs slower than real time, all this model can do is “catch up” as soon as possible by not delaying the following ticks. So if the host load is too high, or you are running in a debugger, you will see simulated time lagging behind the real host time. This solution ensures that normal runs are still deterministic while providing an illusion of real timeness to the observer.

When locked to real time, simulated time can also be set to run faster or slower than real time. This can be controlled with the --rt-ratio=<ratio> and -rt-drift=<drift> command line options. Note that both of these options control the same underlying mechanism, and that drift is by definition equal to ratio - 1. It is also possible to adjust this clock speed on the fly with native_rtc_adjust_clock().

In this way if, for example, --rt-ratio=2 is given, the simulated time will advance at twice the real time speed. Similarly if --rt-drift=-100e-6 is given, the simulated time will progress 100ppm slower than real time. Note that these 2 options have no meaning when running in non real-time mode.

How simulated time and real time relate to each other

Simulated time (st) can be calculated from real time (rt) as

\[st = (rt - last\_rt) \times ratio + last\_st\]

And vice-versa:

\[rt = (st - last\_st) / ratio + last\_rt\]

Where last_rt and last_st are respectively the real time and the simulated time when the last clock ratio adjustment took place.

All times are kept in microseconds.


The following peripherals are currently provided with this board:

Interrupt controller

A simple yet generic interrupt controller is provided. It can nest interrupts and provides interrupt priorities. Interrupts can be individually masked or unmasked. SW interrupts are also supported.

Clock, timer and system tick model

This model provides the system tick timer. By default CONFIG_SYS_CLOCK_TICKS_PER_SEC configures it to tick every 10ms.

Please refer to the section About time in native_sim for more information.


Two optional native UART drivers are available:


With this driver, one or two Zephyr UART devices can be created. These can be connected to the Linux process stdin/stdout or a newly created pseudo-tty. For more information refer to the section PTTY UART.


An UART driver for interacting with host-attached serial port devices (eg. USB to UART dongles). For more information refer to the section TTY UART.

Real time clock

The real time clock model provides a model of a constantly powered clock. By default this is initialized to the host time at boot.

This RTC can also be set to start from time 0 with the --rtc-reset command line option.

It is possible to offset the RTC clock value at boot with the --rtc-offset=<offset> option, or to adjust it dynamically with the function native_rtc_offset().

After start, this RTC advances with the simulated time, and is therefore affected by the simulated time speed ratio. See About time in native_sim for more information.

The time can be queried with the functions native_rtc_gettime_us() and native_rtc_gettime(). Both accept as parameter the clock source:

  • RTC_CLOCK_BOOT: It counts the simulated time passed since boot. It is not subject to offset adjustments

  • RTC_CLOCK_REALTIME: RTC persistent time. It is affected by offset adjustments.

  • RTC_CLOCK_PSEUDOHOSTREALTIME: A version of the real host time, as if the host was also affected by the clock speed ratio and offset adjustments performed to the simulated clock and this RTC. Normally this value will be a couple of hundredths of microseconds ahead of the simulated time, depending on the host execution speed. This clock source should be used with care, as depending on the actual execution speed of native_sim and the host load, it may return a value considerably ahead of the simulated time.

Note this device does not yet have an RTC API compatible driver.

Entropy device

An entropy device based on the host random() API. This device will generate the same sequence of random numbers if initialized with the same random seed. You can change this random seed value by using the command line option: --seed=<random_seed> where the value specified is a 32-bit integer such as 97229 (decimal), 0x17BCD (hex), or 0275715 (octal).

Ethernet driver

A simple TAP based ethernet driver is provided. The driver expects that the zeth network interface already exists in the host system. The zeth network interface can be created by the script found in the net-tools zephyr project repository. User can communicate with the Zephyr instance via the zeth network interface. Multiple TAP based network interfaces can be created if needed. The IP address configuration can be specified for each network interface instance.

Note that this device can only be used with Linux hosts.

Offloaded sockets driver

This driver is an alternative to the TAP based ethernet driver. Instead of using a virtual network in the Linux side, this driver utilizes Linux’s standard BSD socket API. With this, multiple Zephyr applications can communicate over the Linux loopback interface. The benefit of this approach is that root privileges are not required and that the process is connected to the same interface as other Linux processes instead of a virtual network, facilitating testing without the need for extra setup in the host. The drawback is that the L2 layer of Zephyr’s networking stack is not exercised.

Bluetooth controller

It’s possible to use the host’s Bluetooth adapter as a Bluetooth controller for Zephyr. To do this the HCI device needs to be passed as a command line option to zephyr.exe. For example, to use hci0, use sudo zephyr.exe --bt-dev=hci0. Using the device requires root privileges (or the CAP_NET_ADMIN POSIX capability, to be exact) so zephyr.exe needs to be run through sudo. The chosen HCI device must be powered down and support Bluetooth Low Energy (i.e. support the Bluetooth specification version 4.0 or greater).

Another possibility is to use a HCI TCP server which acts as a virtual Bluetooth controller over TCP. To connect to a HCI TCP server its IP address and port number must be specified. For example, to connect to a HCI TCP server with IP address and port number 1020 use zephyr.exe --bt-dev= This alternative option is mainly aimed for testing Bluetooth connectivity over a virtual Bluetooth controller that does not depend on the Linux Bluetooth stack and its HCI interface.

USB controller

It’s possible to use the Virtual USB controller working over USB/IP protocol. More information can be found in Testing USB over USP/IP in native_sim.

Display driver

A display driver is provided that creates a window on the host machine to render display content.

This driver requires a 32-bit version of the SDL2 library on the host machine and pkg-config settings to correctly pickup the SDL2 install path and compiler flags.

On a Ubuntu 22.04 host system, for example, install the pkg-config and libsdl2-dev:i386 packages, and configure the pkg-config search path with these commands:

$ sudo dpkg --add-architecture i386
$ sudo apt update
$ sudo apt-get install pkg-config libsdl2-dev:i386
$ export PKG_CONFIG_PATH=/usr/lib/i386-linux-gnu/pkgconfig
EEPROM simulator

The EEPROM simulator can also be used in the native targets. In these, you have the added feature of keeping the EEPROM content on a file on the host filesystem. By default this is kept in the file eeprom.bin in the current working directory, but you can select the location of this file and its name with the command line parameter --eeprom. Some more information can be found in the emulators page.

Flash simulator

The flash simulator can also be used in the native targets. In this you have the option to keep the flash content in a binary file on the host file system or in RAM. The behavior of the flash device can be configured through the native_sim board devicetree or Kconfig settings under CONFIG_FLASH_SIMULATOR.

By default the binary data is located in the file flash.bin in the current working directory. The location of this file can be changed through the command line parameter --flash. The flash data will be stored in raw format and the file will be truncated to match the size specified in the devicetree configuration. In case the file does not exists the driver will take care of creating the file, else the existing file is used.

Some more information can be found in the emulators page.

The flash content can be accessed from the host system, as explained in the Host based flash access section.

Input events

Two optional native input drivers are available:

evdev driver

A driver is provided to read input events from a Linux evdev input device and inject them back into the Zephyr input subsystem.

The driver is automatically enabled when CONFIG_INPUT is enabled and the devicetree contains a node such as:

evdev {
  compatible = "zephyr,native-linux-evdev";

The application then has to be run with a command line option to specify which evdev device node has to be used, for example zephyr.exe --evdev=/dev/input/event0.

Input SDL touch

This driver emulates a touch panel input using the SDL library. It can be enabled with CONFIG_INPUT_SDL_TOUCH and configured with the device tree binding zephyr,input-sdl-touch.

More information on using SDL and the Display driver can be found in its section.

CAN controller

It is possible to use a host CAN controller with the native SockerCAN Linux driver. It can be enabled with CONFIG_CAN_NATIVE_LINUX and configured with the device tree binding zephyr,native-linux-can.


This driver can be configured with CONFIG_UART_NATIVE_POSIX to instantiate up to two UARTs. By default only one UART is enabled. With CONFIG_UART_NATIVE_POSIX_PORT_1_ENABLE you can enable the second one.

For the first UART, it can link it to a new pseudoterminal (i.e. /dev/pts<nbr>), or map the UART input and output to the executable’s stdin and stdout. This is chosen by selecting either CONFIG_NATIVE_UART_0_ON_OWN_PTY or CONFIG_NATIVE_UART_0_ON_STDINOUT For interactive use with the Shell, choose the first (OWN_PTY) option. The second (STDINOUT) option can be used with the shell for automated testing, such as when piping other processes’ output to control it. This is because the shell subsystem expects access to a raw terminal, which (by default) a normal Linux terminal is not.

When CONFIG_NATIVE_UART_0_ON_OWN_PTY is chosen, the name of the newly created UART pseudo-terminal will be displayed in the console. If you want to interact with it manually, you should attach a terminal emulator to it. This can be done, for example with the command:

$ xterm -e screen /dev/<ttyn> &

where /dev/tty<n> should be replaced with the actual TTY device.

You may also chose to automatically attach a terminal emulator to the first UART by passing the command line option -attach_uart to the executable. The command used for attaching to the new shell can be set with the command line option -attach_uart_cmd=<"cmd">. Where the default command is given by CONFIG_NATIVE_UART_AUTOATTACH_DEFAULT_CMD. Note that the default command assumes both xterm and screen are installed in the system.

This driver only supports poll mode. Interrupt and async mode are not supported. Neither runtime configuration or line control are supported.


With this driver an application can use the polling UART API (uart_poll_out, uart_poll_in) to write and read characters to and from a connected serial port device.

This driver is automatically enabled when a devicetree contains a node with "zephyr,native-tty-uart" compatible property and okay status, such as one below.

uart {
  status = "okay";
  compatible = "zephyr,native-tty-uart";
  serial-port = "/dev/ttyUSB0";
  current-speed = <115200>;

Interaction with serial ports can be configured in several different ways:

  • The default serial port and baud rate can be set via the device tree properties serial-port and current-speed respectively. The serial-port property is optional.

  • Serial port and baud rate can also be set via command line options X_port and X_baud respectively, where X is a name of a node. Command line options override values from the devicetree.

  • The rest of the configuration options such as number of data and stop bits, parity, as well as baud rate can be set at runtime with uart_configure.

  • This driver can emulate an interrupt-driven UART by enabling CONFIG_UART_INTERRUPT_DRIVEN.

Multiple instances of such uart drivers are supported.

The Native TTY UART sample app provides a working example of the driver.

This driver only supports poll mode and interrupt mode. Async mode is not supported. It has runtime configuration support, but no line control support.

Subsystems backends

Apart from its own peripherals, the native_sim board also has some dedicated backends for some of Zephyr’s subsystems. These backends are designed to ease development by integrating more seamlessly with the host operating system:

Console backend:

A console backend which by default is configured to redirect any printk() write to the native host application’s stdout.

This driver is selected by default if the PTTY UART is not compiled in. Otherwise CONFIG_UART_CONSOLE will be set to select the UART as console backend.

Logger backend:

A backend which prints all logger output to the process stdout. It supports timestamping, which can be enabled with CONFIG_LOG_BACKEND_FORMAT_TIMESTAMP; and colored output which can be enabled with CONFIG_LOG_BACKEND_SHOW_COLOR and controlled with the command line options --color, --no-color and --force-color.

In native_sim, by default, the logger is configured with CONFIG_LOG_MODE_IMMEDIATE.

This backend can be selected with CONFIG_LOG_BACKEND_NATIVE_POSIX and is enabled by default.


A backend/”bottom” for Zephyr’s CTF tracing subsystem which writes the tracing data to a file in the host filesystem. More information can be found in Common Tracing Format


All available HW emulators can be used with native_sim.

Host based flash access

If a flash device is present, the file system partitions on the flash device can be exposed through the host file system by enabling CONFIG_FUSE_FS_ACCESS. This option enables a FUSE (File system in User space) layer that maps the Zephyr file system calls to the required UNIX file system calls, and provides access to the flash file system partitions with normal operating system commands such as cd, ls and mkdir.

By default the partitions are exposed through the directory flash/ in the current working directory. This directory can be changed via the command line option --flash-mount. As this directory operates as a mount point for FUSE you have to ensure that it exists before starting the native_sim board.

On exit, the native_sim board application will take care of unmounting the directory. In the unfortunate case that the native_sim board application crashes, you can cleanup the stale mount point by using the program fusermount:

$ fusermount -u flash

Note that this feature requires a 32-bit version of the FUSE library, with a minimal version of 2.6, on the host system and pkg-config settings to correctly pickup the FUSE install path and compiler flags.

On a Ubuntu 22.04 host system, for example, install the pkg-config and libfuse-dev:i386 packages, and configure the pkg-config search path with these commands:

$ sudo dpkg --add-architecture i386
$ sudo apt update
$ sudo apt-get install pkg-config libfuse-dev:i386
$ export PKG_CONFIG_PATH=/usr/lib/i386-linux-gnu/pkgconfig

Peripherals and backends C library compatibility

Today, some native_sim peripherals and backends are, so far, only available when compiling with the host libC (CONFIG_EXTERNAL_LIBC):

Drivers/backends vs libC choice

Driver class

driver name

driver kconfig

libC choices


ADC emul






Host libC


CAN native Linux



Console backend

POSIX arch console




Display SDL




Native posix entropy




EEPROM simulator


Host libC


EEPROM emulator




Eth native_posix




Flash simulator




Host based flash access


Host libC


GPIO emulator




SDL GPIO emulator




I2C emulator




Input SDL touch




Linux evdev



Logger backend

Native backend



Offloaded sockets

Offloaded sockets driver




RTC emul




UART native posix/PTTY




UART native TTY




SPI emul



System tick

Native_posix timer




Posix tracing backend




USB native posix


Host libC