Directory Structure

This is included for reference, and it’s not necessary to know all the details at first.

The general directory layout for NuttX is very similar to the directory structure of the Linux kernel – at least at the most superficial layers. At the top level is the main makefile and a series of sub-directories identified below and discussed in the following paragraphs:

Configuration Files. The NuttX configuration consists of logic in processor architecture directories, chip/SoC directories, and board configuration directories. The complete configuration is specified by several settings in the NuttX configuration file.

  • Processor architecture specific files. These are the files contained in the arch/<arch-name>/ directory and are discussed in a paragraph below. As an example, all ARM processor architectures are provided under the arch/arm/ directory which is selected with the CONFIG_ARCH="arm" configuration option.

    Variants of the processor architecture may be provided in sub-directories of the Extending this example, the ARMv7-M ARM family is supported by logic in arch/arm/include/armv7-m and arch/arm/src/armv7-m directories which are selected by the CONFIG_ARCH_CORTEXM3=y, CONFIG_ARCH_CORTEXM4=y, or CONFIG_ARCH_CORTEXM7=y configuration options

  • Chip/SoC specific files. Each processor architecture is embedded in a System-on-a-Chip (SoC) architecture. The full SoC architecture includes the processor architecture plus chip-specific interrupt logic, clocking logic, general purpose I/O (GPIO) logic, and specialized, internal peripherals (such as UARTs, USB, etc.).

    These chip-specific files are contained within chip-specific sub-directories also under the arch/<arch-name>/ directory and are selected via the CONFIG_ARCH_CHIP selection.

    As an example, the STMicro STM32 SoC architecture is based on the ARMv7-M processor and is supported by logic in the arch/arm/include/stm32 and arch/arm/src/stm32 directories which are selected with the CONFIG_ARCH_CHIP="stm32" configuration setting.

  • Board specific configurations. In order to be usable, the chip must be contained in a board environment. The board configuration defines additional properties of the board including such things as peripheral LEDs, external peripherals (such as networks, USB, etc.).

    These board-specific configuration files can be found in the boards/<arch-name>/<chip-name>/<board-name>/ sub-directories and are discussed in a paragraph below.

    The directory boards/arm/stm32/stm32f4disovery/, as an example, holds board-specific logic for the STM32F4 Discovery board and is selected via the CONFIG_ARCH_BOARD="stm32f4discovery" configuration setting.


This directory holds the NuttX documentation. It’s made with the Sphinx documentation system. See the file for information on how to build it.


Subdirectory Structure

This directory contains several sub-directories, each containing architecture-specific logic. The task of porting NuttX to a new processor consists of add a new subdirectory under arch/ containing logic specific to the new architecture. The complete board port in is defined by the architecture-specific code in this directory (plus the board-specific configurations in the config/ subdirectory). Each architecture must provide a subdirectory, <arch-name> under arch/ with the following characteristics:

Summary of Files

  • include/<chip-name>/ This sub-directory contains chip-specific header files.

  • include/arch.h: This is a hook for any architecture specific definitions that may be needed by the system. It is included by include/nuttx/arch.h.

  • include/types.h: This provides architecture/toolchain-specific definitions for standard types. This file should typedef:

    and if the architecture supports 24- or 64-bit integers

    NOTE that these type names have a leading underscore character. This file will be included(indirectly) by include/stdint.h and typedef’ed to the final name without the underscore character. This roundabout way of doings things allows the stdint.h to be removed from the include/ directory in the event that the user prefers to use the definitions provided by their toolchain header files

    And finally

    Must be defined to the be the size required to hold the interrupt enable/disable state.

    This file will be included by include/sys/types.h and be made available to all files.

  • include/irq.h: This file needs to define some architecture specific functions (usually inline if the compiler supports inlining) and some structures. These include:

    • struct xcptcontext: This structures represents the saved context of a thread.

    • irqstate_t up_irq_save(void): Used to disable all interrupts. In the case of multi-CPU platforms, this function disables interrupts on CPUs.

    • void up_irq_restore(irqstate_t flags): Used to restore interrupt enables to the same state as before up_irq_save() was called.

    This file must also define NR_IRQS, the total number of IRQs supported by the board.

  • include/syscall.h: This file needs to define some architecture specific functions (usually inline if the compiler supports inlining) to support software interrupts or syscalls that can be used all from user-mode applications into kernel-mode NuttX functions. This directory must always be provided to prevent compilation errors. However, it need only contain valid function declarations if the architecture supports the CONFIG_BUILD_PROTECTED or CONFIG_BUILD_KERNELconfigurations.

    • uintptr_t sys_call0(unsigned int nbr): nbr is one of the system call numbers that can be found in include/sys/syscall.h. This function will perform a system call with no (additional) parameters.

    • uintptr_t sys_call1(unsigned int nbr, uintptr_t parm1): nbr is one of the system call numbers that can be found in include/sys/syscall.h. This function will perform a system call with one (additional) parameter.

    • uintptr_t sys_call2(unsigned int nbr, uintptr_t parm1, uintptr_t parm2): nbr is one of the system call numbers that can be found in include/sys/syscall.h. This function will perform a system call with two (additional) parameters.

    • uintptr_t sys_call3(unsigned int nbr, uintptr_t parm1, uintptr_t parm2, uintptr_t parm3): nbr is one of the system call numbers that can be found in include/sys/syscall.h. This function will perform a system call with three (additional) parameters.

    • uintptr_t sys_call4(unsigned int nbr, uintptr_t parm1, uintptr_t parm2, uintptr_t parm3, uintptr_t parm4): nbr is one of the system call numbers that can be found in include/sys/syscall.h. This function will perform a system call with four (additional) parameters.

    • uintptr_t sys_call5(unsigned int nbr, uintptr_t parm1, uintptr_t parm2, uintptr_t parm3, uintptr_t parm4, uintptr_t parm5): nbr is one of the system call numbers that can be found in include/sys/syscall.h. This function will perform a system call with five (additional) parameters.

    • uintptr_t sys_call6(unsigned int nbr, uintptr_t parm1, uintptr_t parm2, uintptr_t parm3, uintptr_t parm4, uintptr_t parm5, uintptr_t parm6): nbr is one of the system call numbers that can be found in include/sys/syscall.h. This function will perform a system call with six (additional) parameters.

    This file must also define NR_IRQS, the total number of IRQs supported by the board.

  • src/<chip-name>/ This sub-directory contains chip-specific source files.

  • src/Makefile: This makefile will be executed to build the targets src/libup.a and src/up_head.o. The up_head.o file holds the entry point into the system (power-on reset entry point, for example). It will be used in the final link with libup.a and other system archives to generate the final executable.

  • (architecture-specific source files). The file include/nuttx/arch.h identifies all of the APIs that must be provided by the architecture specific logic. (It also includes arch/<arch-name>/arch.h as described above).

Supported Architectures

Architecture- and Chip-Specific Directories. All processor architecture-specific directories are maintained in sub-directories of the arch/ directory. Different chips or SoC’s may implement the same processor core. Chip-specific logic can be found in sub-directories under the architecture directory. Current architecture/chip directories are summarized below:

  • arch/sim: A user-mode port of NuttX to the x86 Linux or Cygwin platform is available. The purpose of this port is primarily to support OS feature development. This port does not support interrupts or a real timer (and hence no round robin scheduler) Otherwise, it is complete.

  • arch/arm: This directory holds common ARM architectures.

  • arch/avr: This directory holds common AVR and AVR32 architectures.

  • arch/mips: This directory holds common MIPS architectures. This include PIC32MX and PIC32MZ.

  • arch/misoc: This directory supports the Misoc LM3 architecture.

  • arch/or1K: This directory supports the OpenRISC mor1kx architecture.

  • arch/renesas: This directory is the home for various Renesas architectures, currently only the M16C and vererable SuperH-1 architectures.

  • arch/risc-v: This directory supports the RISC-V NR5 architecture.

  • arch/xtensa: This directory supports the Xtensa LX6 architecture as used by the ESP32.

  • arch/z16f: Zilog z16f Microcontroller.

  • arch/z80: This directory holds 8-bit ZiLOG architectures. At present, this includes the Zilog z80, ez80Acclaim! and z8Encore! Microcontrollers.


The binfmt/ subdirectory contains logic for loading binaries in the file system into memory in a form that can be used to execute them.


The audio/ subdirectory contains the NuttX audio sub-system.


The boards/ subdirectory contains custom logic and board configuration data for each board. These board-specific configurations plus the architecture-specific configurations in the arch/ subdirectory complete define a customized port of NuttX.

Subdirectory Structure

The boards/ directory contains board specific configuration files. Each board must provide a sub-directory <board-name> under boards/<arch-name>/><chip-name>/ with the following characteristics:

Summary of Files

Board Specific Logic

  • include/: This directory contains board specific header files. This directory will be linked as include/arch/board at configuration time and can be included via #include <arch/board/header.h>. These header file can only be included by files in arch/<arch-name>/include/ and arch/<arch-name>/src/.

  • src/: This directory contains board specific drivers. This directory will be linked as <config>/arch/<arch-name>/src/board at configuration time and will be integrated into the build system.

  • src/Makefile: This makefile will be invoked to build the board specific drivers. It must support the following targets: libext$(LIBEXT), clean, and distclean.

Board Specific Configuration Sub-Directories

The boards/<arch-name>/<chip-name>/<board-name>/configs sub-directory holds all of the files that are necessary to configure NuttX for the particular board. A board may have various different configurations using the common source files. Each board configuration is described by two files: Make.defs and defconfig. Typically, each set of configuration files is retained in a separate configuration sub-directory (<config1-dir>, <config2-dir>, .. in the above diagram).

NOTE: That the Make.defs file may reside in one of two locations: There may be a unique Make.defs file for each configuration in the configuration directory OR if that file is absent, there may be a common board Make.defs file in the /scripts directory. The Make.defs file in the configuration takes precedence if it is present.

The procedure for configuring NuttX is described below, This paragraph will describe the contents of these configuration files.

  • Make.defs: This makefile fragment provides architecture and tool-specific build options. It will be included by all other makefiles in the build (once it is installed). This make fragment should define:


    • Tool options: CFLAGS, LDFLAGS

    When this makefile fragment runs, it will be passed TOPDIR which is the path to the root directory of the build. This makefile fragment should include:

    • $(TOPDIR)/.config : Nuttx configuration

    • $(TOPDIR)/tools/ : Common definitions

    Definitions in the Make.defs file probably depend on some of the settings in the .config file. For example, the CFLAGS will most likely be different if CONFIG_DEBUG_FEATURES=y.

    The included tools/ file contains additional definitions that may be overridden in the architecture-specific Make.defs file as necessary:


  • defconfig: This is a configuration file similar to the Linux configuration file. In contains variable/value pairs like:


    This configuration file will be used at build time:

    1. As a makefile fragment included in other makefiles, and

    2. to generate include/nuttx/config.h which is included by most C files in the system.

Supported Boards

All of the specific boards supported by NuttX are identified in the README.txt file.

Adding a New Board Configuration

Okay, so you have created a new board configuration directory. Now, how do you hook this board into the configuration system so that you can select with make menuconfig?

You will need modify the file boards/Kconfig. Let’s look at the STM32F4-Discovery configuration in the Kconfig file and see how we would add a new board directory to the configuration. For this configuration let’s say that you new board resides in the directory boards/myarch/mychip/myboard; It uses an MCU selected with CONFIG_ARCH_CHIP_MYMCU; and you want the board to be selected with CONFIG_ARCH_BOARD_MYBOARD. Then here is how you can clone the STM32F4-Discovery configuration in boards/Kconfig to support your new board configuration.

In boards/Kconfig for the stm32f4-discovery, you will see a configuration definition like this:

The above selects the STM32F4-Discovery board. The select lines say that the board has both LEDs and buttons and that the board can generate interrupts from the button presses. You can just copy the above configuration definition to a new location (notice that they the configurations are in alphabetical order). Then you should edit the configuration to support your board. The final configuration definition might look something like:

Later in the boards/Kconfig file, you will see a long, long string configuration with lots of defaults like this:

This logic will assign string value to a configuration variable called CONFIG_ARCH_BOARD that will name the directory where the board-specific files reside. In our case, these files reside in boards/myarch/mychip/myboard and we add the following to the long list of defaults (again in alphabetical order):

Now the build system knows where to find your board configuration!

And finally, add something like this near the bottom of boards/myarch/mychip/myboard:

This includes additional, board-specific configuration variable definitions in boards/myarch/mychip/myboard/Kconfig.


This sub-directory holds the NuttX cryptographic sub-system.


This directory holds architecture-independent device drivers.


This directory contains the NuttX file system. This file system is described below.


This directory contains files for graphics/video support under NuttX.


This directory holds NuttX header files. Standard header files file retained in can be included in the normal fashion:


This is a (almost) empty directory that has a holding place for generated static libraries. The NuttX build system generates a collection of such static libraries in this directory during the compile phase. These libraries are then in a known place for the final link phase where they are accessed to generated the final binaries.


This directory holds a collection of standard libc-like functions with custom interfaces into NuttX.

Normally the logic in this file builds to a single library (libc.a). However, if NuttX is built as a separately compiled kernel (with CONFIG_BUILD_PROTECTED=y or CONFIG_BUILD_KERNEL=y), then the contents of this directory are built as two libraries: One for use by user programs (libc.a) and one for use only within the <kernel> space (libkc.a).

These user/kernel space libraries (along with the sycalls of `nuttx/syscall <#DirStructSyscall>`__) are needed to support the two differing protection domains.

Directory structure:


This directory holds a tiny, minimal standard std C++ that can be used to build some, simple C++ applications in NuttX.


This is the NuttX memory manager.


This directory contains the implementation of the NuttX networking layer including internal socket APIs.


The files forming core of the NuttX RTOS reside here.


If NuttX is built as a separately compiled kernel (with CONFIG_BUILD_PROTECTED=y or CONFIG_BUILD_KERNEL=y), then the contents of this directory are built. This directory holds a syscall interface that can be used for communication between user-mode applications and the kernel-mode RTOS.


This directory holds a collection of tools and scripts to simplify configuring, building and maintaining NuttX.

Refer to the README file in the tools directory for more information about the individual files. Some of these tools are discussed below as well in the discussion of configuring and building NuttX.


This directory holds support for hardware-independent wireless support.


The top-level Makefile in the $(TOPDIR) directory contains all of the top-level control logic to build NuttX. Use of this Makefile to build NuttX is described below.

Next up is Build and Make Details.