README.txt
    README
    ^^^^^^
    
      o Environments
        - Installing Cygwin
        - Ubuntu Bash under Windows 10
      o Installation
        - Download and Unpack
        - Semi-Optional apps/ Package
        - Installation Directories with Spaces in the Path
        - Downloading from Repositories
        - Related Repositories
        - Notes about Header Files
      o Configuring NuttX
        - Instantiating "Canned" Configurations
        - Refreshing Configurations
        - NuttX Configuration Tool
        - Finding Selections in the Configuration Menus
        - Reveal Hidden Configuration Options
        - Make Sure that You are on the Right Platform
        - Comparing Two Configurations
        - Making defconfig Files
        - Incompatibilities with Older Configurations
        - NuttX Configuration Tool under DOS
      o Toolchains
        - Cross-Development Toolchains
        - NuttX Buildroot Toolchain
      o Shells
      o Building NuttX
        - Building
        - Re-building
        - Build Targets and Options
        - Native Windows Build
        - Installing GNUWin32
      o Cygwin Build Problems
        - Strange Path Problems
        - Window Native Toolchain Issues
      o Documentation
    
    ENVIRONMENTS
    ^^^^^^^^^^^^
    
      NuttX requires a POSIX development environment such as you would find under
      Linux or macOS.  NuttX may also be installed and built on Windows system
      if you also provide such a POSIX development environment.  Options for a
      POSIX development environment under Windows include:
    
        - An installation of Linux on a virtual machine (VM) in Windows.  I have
          not been happy using a VM myself.  I have had stability problems with
          open source VMs and commercial VMs cost more than I want to spend.
          Sharing files with Linux running in a VM is awkward;  sharing devices
          connected to the Windows box with Linux in a VM is, at the very least,
          confusing;  Using Windows tools (such as Segger J-Link) with files
          built under the Linux VM is not a possibility.
    
        - The Cygwin environment.  Instructions for installation of Cygwin on a
          Windows system are provided in the following paragraph, "Installing
          Cygwin".  Cygwin is a mature, well-tested, and very convenient
          environment.  It is especially convenient if you  need to
          integrate with Windows tools and files.  Downsides are that the
          installation time is very long and the compile times are slow.
    
        - Ubuntu/Bash shell under Windows 10.  This is a new option under
          Windows 10.  See the section "Ubuntu Bash under Windows 10" below.
          This is an improvement over Cygwin if your concern is compile time;
          its build performance is comparable to native Linux, certainly better
          than the Cygwin build time.  It also installs in a tiny fraction of
          the time as Cygwin, perhaps 20 minutes for the basic Ubuntu install
          (vs. more than a day for the complete Cygwin install).
    
          There have been even more recent ports of Linux environment to
          Windows.  I need to update this section to include some mention of
          these alternatives.
    
        - The MSYS environment.  MSYS derives from an older version of Cygwin
          simplified and adapted to work more naturally in the Windows
          environment.  See http://www.mingw.org/wiki/MSYS if you are
          interested in using MSYS.  The advantages of the MSYS environment is
          that it is better integrted with the native Windows environment and
          lighter weight; it uses only a  minimal number of add-on POSIX-land
          tools.
    
          The download link in that Wiki takes you to the SourceForge download
          site.  The SourceForge MSYS project has been stagnant for some time.
          The MSYS project has more recently moved to
          http://odsn.net/projects/sfnet_mingwbundle.  Downloads of current .zip
          files are available there but no instructions for the installation.
    
        - MSYS2 appears to be a re-write of MSYS based on a newer version of
          Cygwin.  Is it available at https://www.msys2.org.  A windows
          installer is available at that site along with very good installation
          instructions.  The download is relatively quick (at least compared to
          Cygwin) and the 'pacman' package management tool supports supports
          simple system updates.  For example, 'pacman -S git' will install the
          GIT command line utilities.
    
        - Other POSIX environments.  Check out:
    
            UnxUtils: https://sourceforge.net/projects/unxutils/,
              https://en.wikipedia.org/wiki/UnxUtils
            MobaXterm: https://mobaxterm.mobatek.net/
            Gow: https://github.com/bmatzelle/gow/wiki
    
          Disclaimer:  In principle, these should work.  However, I have never
          used any of these environments and cannot guarantee that there is
          not some less-than-obvious issues.
    
      NuttX can also be installed and built on a native Windows system, but with
      some potential tool-related issues (see the discussion "Native Windows
      Build" under "Building NuttX" below).  GNUWin32 is used to provide
      compatible native windows tools.
    
    Installing Cygwin
    -----------------
    
      Installing Cygwin on your Windows PC is simple, but time consuming.  See
      http://www.cygwin.com/ for installation instructions. Basically you just
      need to download a tiny setup.exe program and it does the real, network
      installation for you.
    
      Some Cygwin installation tips:
    
      1. Install at C:\cygwin
    
      2. Install EVERYTHING:  "Only the minimal base packages from the
         Cygwin distribution are installed by default. Clicking on categories
         and packages in the setup.exe package installation screen will
         provide you with the ability to control what is installed or updated.
         Clicking on the "Default" field next to the "All" category will
         provide you with the opportunity to install every Cygwin package.
         Be advised that this will download and install hundreds of megabytes
         to your computer."
    
         If you use the "default" installation, you will be missing many
         of the Cygwin utilities that you will need to build NuttX.  The
         build will fail in numerous places because of missing packages.
    
         NOTE: The last time I installed EVERYTHING, the download was
         about 5GiB.  The server I selected was also very slow so it took
         over a day to do the whole install!
    
         NOTE: You don't really have to install EVERYTHING but I cannot
         answer the question "Then what should I install?"  I don't know
         the answer to that and so will continue to recommend installing
         EVERYTHING.
    
         You should certainly be able to omit "Science", "Math", and
         "Publishing".  You can try omitting KDE, Gnome, GTK, and other
         graphics packages if you don't plan to use them.
    
         Perhaps a minimum set would be those packages listed below for the
         "Ubuntu Bash under Windows 10" installation?
    
         UPDATE:  Sergey Frolov had success with the following minimal
         Cygwin configuration:
    
         1. After starting the Cygwin installer, keep the recommended
            packages that are pre-selected in the default configuration.
         2. Using the installation tools, add the following packages:
    
              make (GNU make)  bison        libgmp3-dev
              gcc-core         byacc        libmpfr-dev
              gcc-g++          gperf        libmpc-dev
              flex             gdb          automake-1.15
              libncurses-dev   libgmp-dev
    
      After installing Cygwin, you will get lots of links for installed
      tools and shells.  I use the RXVT native shell.  It is fast and reliable
      and does not require you to run the Cygwin X server (which is neither
      fast nor reliable).  Unless otherwise noted, the rest of these
      instructions assume that you are at a bash command line prompt in
      either Linux or in Cygwin shell.
    
    Using MSYS
    ----------
    
      MSYS is an environment the derives from Cygwin.  Thus, most things said
      about Cygwin apply equally to MSYS.  This section will, then, focus on
      the differences when using MSYS, specifically MSYS2.
    
      Here is it assumed that you have already downloaded and installed MSYS2
      from https://www.msys2.org using the windows installer available at that
      location.  It is also assumed that you have brought in the necessary
      tools using the 'pacman' package management tool Tools needed including:
    
        pacman -S git
        pacman -S make
        pacman -S gcc
        pacman -S gdb
    
      And possibly others depending upon your usage.  Then you will need to
      build and install kconfig-frontends per the instructions of the top-level
      README.txt file in the tools repository.  This requires the following
      additional tools:
    
        pacman -S bison
        pacman -S gperf
        pacman -S ncurses-devel
        pacman -S automake-wrapper
        pacman -S autoconf
        pacman -S pkg-config
    
      Because of some versioning issues, I had to run 'aclocal' prior to
      running the kconfig-frontends configure script.  See "Configuring NuttX"
      below for further information.
    
      Unlike Cygwin, MSYS does not support symbolic links.  The 'ln -s' command
      will, in fact, copy a directory!  This means that you Make.defs file will
      have to include definitions like:
    
        ifeq ($(CONFIG_WINDOWS_MSYS),y)
          DIRLINK = $(TOPDIR)/tools/copydir.sh
          DIRUNLINK = $(TOPDIR)/tools/unlink.sh
        endif
    
      This will force the directory copies to work in a way that can be handled
      by the NuttX build system.  NOTE:  The default link.sh script has been
      updated so that is should now be MSYS2 compatible.  The above is preferred
      but no longer necessary in the Make.defs file.
    
      To build the simulator under MSYS, you also need:
    
        pacman -S zlib-devel
    
      It appears that you cannot use directory names with spaces in them like
      "/c/Program\ Files \(86\)" in the MSYS path variable.  I worked around this
      by create Windows junctions like this::
    
        1. Open the a windows command terminal,
        2. CD to c:\msys64, then
        3. mklink /j programfiles "C:/Program\ Files" and
        4. mklink /j programfiles86 "C:/Program\ Files\ \(x86\)"
    
      They then show up as /programfiles and /programfiles86 with the MSYS2
      sandbox.  Thos paths can then be used with the PATH variable.  I had
      to do something similar for the path to the GNU Tools "ARM Embedded
      Toolchain" which also has spaces in the path name.
    
    Ubuntu Bash under Windows 10
    ----------------------------
    
      A better version of a command-line only Ubuntu under Windows 10 (beta)
      has recently been made available from Microsoft.
    
      Installation
      ------------
      Installation instructions abound on the Internet complete with screen
      shots.  I will attempt to duplicate those instructions in full here.
      Here are the simplified installation steps:
    
        - Open "Settings".
        - Click on "Update & security".
        - Click on "For Developers".
        - Under "Use developer features", select the "Developer mode" option to
          setup the environment to install Bash.
        - A message box should pop up.  Click "Yes" to turn on developer mode.
        - After the necessary components install, you'll need to restart your
          computer.
    
      Once your computer reboots:
    
        - Open "Control Panel".
        - Click on "Programs".
        - Click on "Turn Windows features on or off".
        - A list of features will pop up, check the "Windows Subsystem for Linux
          (beta)" option.
        - Click OK.
        - Once the components installed on your computer, click the "Restart
          now" button to complete the task.
    
      After your computer restarts, you will notice that Bash will not appear in
      the "Recently added" list of apps, this is because Bash isn't actually
      installed yet. Now that you have setup the necessary components, use the
      following steps to complete the installation of Bash:
    
        - Open "Start", do a search for bash.exe, and press "Enter".
        - On the command prompt, type y and press Enter to download and install
          Bash from the Windows Store.  This will take awhile.
        - Then you'll need to create a default UNIX user account. This account
          doesn't have to be the same as your Windows account. Enter the
          username in the required field and press Enter (you can't use the
          username "admin").
        - Close the "bash.exe" command prompt.
    
      Now that you completed the installation and setup, you can open the Bash
      tool from the Start menu like you would with any other app.
    
      Accessing Windows Files from Ubuntu
      -----------------------------------
      File systems will be mounted under "/mnt" so for example "C:\Program Files"
      appears at "/mnt/c/Program Files".  This is as opposed to Cygwin where
      the same directory would appear at "/cygdrive/c/Program Files".
    
      With these differences (perhaps a few other Windows quirks) the Ubuntu
      install works just like Ubuntu running natively on your PC.
    
      A good tip for file sharing is to use symbolic links within your Ubuntu
      home directory.  For example, suppose you have your "projects" directory
      at C:\Documents\projects.  Then you can set up a link to the projects/
      directory in your Ubuntu directory like:
    
        ln -s /mnt/c/Documents/projects projects
    
      Accessing Ubuntu Files From Windows
      -----------------------------------
      In Ubuntu Userspace for Windows, the Ubuntu file system root directory is
      at:
    
        %localappdata%\lxss\rootfs
    
      Or
    
        C:\Users\Username\AppData\Local\lxss\rootfs
    
      However, I am unable to see my files under the rootfs\home directory.
      After some looking around, I find the home directory
      %localappdata%\lxss\home.
    
      With that trick access to the /home directory, you should actually be
      able to use Windows tools outside of the Ubuntu sandbox with versions of
      NuttX built within the sandbox using that path.
    
      Executing Windows Tools from Ubuntu
      -----------------------------------
      You can also execute Windows tools from within the Ubuntu sandbox:
    
        /mnt/c/Program\ Files\ \(x86\)/Microchip/xc32/v1.43/bin/xc32-gcc.exe --version
        Unable to translate current working directory. Using C:\WINDOWS\System32
        xc32-gcc.exe (Microchip Technology) 4.8.3 MPLAB XC32 Compiler v1.43 Build date: Mar  1 2017
        ...
    
      The error message indicates that there are more issues:  You cannot mix
      Windows tools that use Windows style paths in an environment that uses
      POSIX paths.  I think you would have to use Linux tools only from within
      the Ubuntu sandbox.
    
      Install Ubuntu Software
      -----------------------
      Use "sudo apt-get install <package name>".  As examples, this is how
      you would get GIT:
    
        sudo apt-get install git
    
      This will get you a compiler for your host PC:
    
        sudo apt-get install gcc
    
      This will get you an ARM compiler for your target:
    
        sudo apt-get install gcc-arm-none-eabi
    
      NOTE: That is just an example.  I am not sure if apt-get will give you a
      current or usable compiler.  You should carefully select your toolchain
      for the needs of your project.
    
      You will also need to get the kconfig-frontends configuration as
      described below under "NuttX Configuration Tool".  In order to build the
      kconfig-frontends configuration tool you will also need:  make, gperf,
      flex, bison, and libncurses-dev.
    
      That is enough to do a basic NuttX build.
    
      Integrating with Windows Tools
      ------------------------------
      If you want to integrate with Windows native tools, then you would need
      deal with the same kind of craziness as with integrating Cygwin with
      native toolchains, see the section "Cygwin Build Problems" below.
    
      However, there is currently no build support for using Windows native
      tools with Ubuntu under Windows.  This tool combination is made to work
      with Cygwin through the use of the 'cygpath -w' tool that converts paths
      from say '/cydrive/c/Program Files' to 'C:\Program Files'.  There is,
      however, no corresponding tool to convert '/mnt/c/Program Files' in the
      Ubuntu environment.
    
      Graphics Support
      ----------------
      The Ubuntu version support by Microsoft is a command-line only version.
      There is no support for Linux graphics utilities.
    
      This limitation is not a limitation of Ubuntu, however, only in what
      Microsoft is willing to support.  If you install a X-Server, then you
      can also use basic graphics utilities.  See for example:
    
        http://www.howtogeek.com/261575/how-to-run-graphical-linux-desktop-applications-from-windows-10s-bash-shell/
    
      Many Linux graphics programs would, however, also require a graphics
      framework like GTK or Qt.  So this might be a trip down the rabbit hole.
    
    INSTALLATION
    ^^^^^^^^^^^^
    
      There are two ways to get NuttX:  You may download released, stable
      tarballs from either the Bitbucket or Sourceforge download locations.
      Or you may get NuttX by cloning the Bitbucket GIT repositories.  Let's
      consider the released tarballs first:
    
    Download and Unpack
    -------------------
    
      Download and unpack the NuttX tarball.  If you are reading this, then
      you have probably already done that.  After unpacking, you will end
      up with a directory called nuttx-version (where version is the NuttX
      version number). You might want to rename that directory nuttx to
      match the various instructions in the documentation and some scripts
      in the source tree.
    
      Download locations:
    
        https://bitbucket.org/nuttx/nuttx/downloads
        https://sourceforge.net/projects/nuttx/files/nuttx/
    
    Semi-Optional apps/ Package
    ---------------------------
    
      All NuttX libraries and example code used to be in included within
      the NuttX source tree.  As of NuttX-6.0, this application code was
      moved into a separate tarball, the apps tarball.  If you are just
      beginning with NuttX, then you will want to download the versioned
      apps tarball along with the NuttX tarball.  If you already have your
      own product application directory, then you may not need the apps
      tarball.
    
      It is called "Semi-optional" because if you don't have some apps/
      directory, NuttX will *fail* to build! You do not necessarily need
      to use the NuttX apps tarball but may, instead, provide your own
      custom application directory.  Such a custom directory would need
      to include a valid Makefile to support the build and a valid Kconfig
      file to support the configuration.  More about these files later.
    
      Download then unpack the apps tarball in the same directory where you
      unpacked the NuttX tarball.  After you unpack the apps tarball, you
      will have a new directory called apps-version (where the version
      should exactly match the version of the NuttX tarball).  Again, you
      might want to rename the directory to simply apps/ to match what
      you read in the documentation
    
      After unpacking (and renaming) the apps tarball, you will have two
      directories side by side like this:
    
                 |
            +----+----+
            |         |
          nuttx/     apps/
    
      This is important because the NuttX build will expect to find the
      apps directory in that (default) location.  That default location
      can be changed by modifying your NuttX configuration file, but that
      is another story.
    
    Installation Directories with Spaces in the Path
    ------------------------------------------------
    
      The nuttx build directory should reside in a path that contains no
      spaces in any higher level directory name.  For example, under
      Cygwin, your home directory might be formed from your first and last
      names like: "/home/First Last". That will cause strange errors when
      the make system tries to build.
    
      [Actually, that problem is probably not too difficult to fix.  Some
       Makefiles probably just need some paths within double quotes]
    
      I work around spaces in the home directory name, by creating a
      new directory that does not contain any spaces, such as /home/nuttx.
      Then I install NuttX in /home/nuttx and always build from
      /home/nuttx/nuttx-code.
    
    Downloading from Repositories
    -----------------------------
    
      Cloning the Repository
    
        BEFORE cloning repositories on any Windows platform do the following GIT
        command:
    
          git config --global core.autocrlf false
    
        That will avoid conversions of linefeeds (newlines, \n) to carriage
        return plus linefeed sequences (\r\n)
    
        The current NuttX du jour is available in from a GIT repository.  Here are
        instructions for cloning the core NuttX RTOS (corresponding to the nuttx
        tarball discussed above)::
    
          git clone https://bitbucket.org/nuttx/nuttx.git nuttx
    
        And the semi-optional apps/ application directory and be cloned like:
    
          git clone https://bitbucket.org/nuttx/apps.git apps
    
        That will give you the same directory structure like this:
    
                 |
            +----+----+
            |         |
          nuttx/     apps/
    
      Configuring the Clones
    
        The following steps need to be performed for each of the repositories.
        After changing to the clone directory:
    
        Set your identity:
    
          git config --global user.name "My Name"
          git config --global user.email my.name@example.com
    
        Colorized diffs are much easier to read:
    
          git config --global color.branch auto
          git config --global color.diff auto
          git config --global color.interactive auto
          git config --global color.status auto
    
        Checkout other settings
    
          git config --list
    
      Cloning NuttX Inside Cygwin
    
        If you are cloning the NuttX repository, it is recommended to avoid
        automatic end of lines conversions by git. These conversions may break
        some scripts like configure.sh. Before cloning, do the following:
    
          git config --global core.autocrlf false
    
    Related Repositories
    --------------------
    
      These are standalone repositories:
    
      * https://bitbucket.org/nuttx/apps
    
        This directory holds an optional package of applications and libraries
        can be used with the NuttX RTOS.  There is a README.txt file there that
        will provide more information about that package.
    
      * https://bitbucket.org/nuttx/nxwidgets
    
        This is the NuttX C++ graphics support.  This includes NxWM, the tiny
        NuttX Window Manager.
    
      * https://bitbucket.org/nuttx/uclibc
    
        This repository contains a version of the uClibc++ C++ library.  This code
        originates from http://cxx.uclibc.org/ and has been adapted for NuttX by the
        RGMP team (http://rgmp.sourceforge.net/wiki/index.php/Main_Page).
    
      * https://bitbucket.org/nuttx/buildroot
    
        A environment that you can to use to build a custom, NuttX GNU toolchain.
    
      * https://bitbucket.org/nuttx/tools
    
        There are snapshots of some tools here that you will need to work with
        NuttX:  kconfig-frontends, genromfs, and others.
    
      * https://bitbucket.org/nuttx/pascal
    
        Yes, this really is a Pascal compiler.  The Pascal p-code run-time and
        pcode debugger can be built as a part of NuttX.
    
    Notes about Header Files
    ------------------------
    
      Other C-Library Header Files.
    
        When a GCC toolchain is built, it must be built against a C library.
        The compiler together with the contents of the C library completes the
        C language definition and provides the complete C development
        environment.  NuttX provides its own, built-in C library.  So the
        complete, consistent C language definition for use with NuttX comes from
        the combination of the compiler and the header files provided by the
        NuttX C library.
    
        When a GCC toolchain is built, it incorporates the C library header
        files into the compiler internal directories and, in this way, the C
        library really becomes a part of the toolchain.  If you use the NuttX
        buildroot toolchain as described below under "NuttX Buildroot
        Toolchain", your GCC toolchain will build against the NuttX C library
        and will incorporate the NuttX C library header files as part of the
        toolchain.
    
        If you use some other, third-party tool chain, this will not be the
        case, however.  Those toolchains were probably built against some
        other, incompatible C library distribution (such as newlib).  Those
        tools will have incorporated the incompatible C library header files
        as part of the toolchain.  These incompatible header files must *not*
        be used with NuttX because they will conflict with definitions in the
        NuttX built-in C-Library.  For such toolchains that include header
        files from a foreign C-Library, NuttX must be compiled without using
        the standard header files that are distributed with your toolchain.
        This prevents including conflicting, incompatible header files such
        as stdio.h.
    
        The math.h and stdarg.h are probably the two most trouble some header
        files to deal with.  These troublesome header files are discussed in
        more detail below.
    
      Header Files Provided by Your Toolchain.
    
        Certain header files, such as setjmp.h, stdarg.h, and math.h, may still
        be needed from your toolchain and your compiler may not, however, be able
        to find these if you compile NuttX without using standard header files
        (i.e., with -nostdinc).  If that is the case, one solution is to copy
        those header file from your toolchain into the NuttX include directory.
    
      Duplicated Header Files.
    
        There are also a few header files that can be found in the nuttx/include
        directory which are duplicated by the header files from your toolchain.
        stdint.h and stdbool.h are examples.  If you prefer to use the stdint.h
        and stdbool.h header files from your toolchain, those could be copied
        into the nuttx/include/ directory. Using most other header files from
        your toolchain would probably cause errors.
    
      math.h
    
        Even though you should not use a foreign C-Library, you may still need
        to use other, external libraries with NuttX.  In particular, you may
        need to use the math library, libm.a.  NuttX supports a generic, built-in
        math library that can be enabled using CONFIG_LIBM=y.  However, you may
        still want to use a higher performance external math library that has
        been tuned for your CPU.  Sometimes such tuned math libraries are
        bundled with your toolchain.
    
        The math library header file, math.h, is a then special case.  If you do
        nothing, the standard math.h header file that is provided with your
        toolchain will be used.
    
        If you have a custom, architecture specific math.h header file, then
        that header file should be placed at arch/<cpu>/include/math.h.  There
        is a stub math.h header file located at include/nuttx/lib/math.h.  This stub
        header file can be used to "redirect" the inclusion to an architecture-
        specific math.h header file.  If you add an architecture specific math.h
        header file then you should also define CONFIG_ARCH_MATH_H=y in your
        NuttX Configuration file.  If CONFIG_ARCH_MATH_H is selected, then the
        top-level Makefile will copy the stub math.h header file from
        include/nuttx/lib/math.h to include/math.h where it will become the system
        math.h header file.  The stub math.h header file does nothing other
        than to include that architecture-specific math.h header file as the
        system math.h header file.
    
      float.h
    
        If you enable the generic, built-in math library, then that math library
        will expect your toolchain to provide the standard float.h header file.
        The float.h header file defines the properties of your floating point
        implementation.  It would always be best to use your toolchain's float.h
        header file but if none is available, a default float.h header file will
        be provided if this option is selected.  However, there is no assurance
        that the settings in this float.h are actually correct for your platform!
    
      stdarg.h
    
        In most cases, the correct version of stdarg.h is the version provided
        with your toolchain.  However, sometimes there are issues with
        using your toolchains stdarg.h.  For example, it may attempt to draw in
        header files that do not exist in NuttX or perhaps the header files that
        it uses are not compatible with the NuttX header files.  In those cases,
        you can use an architecture-specific stdarg.h header file by defining
        CONFIG_ARCH_STDARG_H=y.
    
        See the discussion above for the math.h header.  This setting works
        exactly the same for the stdarg.h header file.
    
    CONFIGURING NUTTX
    ^^^^^^^^^^^^^^^^^
    
    Instantiating "Canned" Configurations
    -------------------------------------
    
      configure.sh and configure.bat:
    
        "Canned" NuttX configuration files are retained in:
    
          boards/<arch-name>/<chip-name>/<board-name>/configs/<config-dir>
    
        Where <board-name> is the name of your development board and <config-dir>
        is the name of the sub-directory containing a specific configuration for
        that board.  <arch-name> and <chip-name> refer to characteristics of the
        MCU used on the board:  <arch-name> is the CPU architecture implemented
        by the MCU; <chip-name> identifies the MCU chip family.  Only a few
        steps are required to instantiate a NuttX configuration, but to make the
        configuration even easier there are scripts available in the tools/
        sub-directory combines those simple steps into one command.
    
        There is one tool for use with any Bash-like shell that does configuration
        steps.  It is used as follows:
    
          tools/configure.sh <board-name>:<config-dir>
    
        There is an alternative Windows batch file that can be used in the windows
        native environment like:
    
          tools\configure.bat <board-name>:<config-dir>
    
        And, to make sure that other platforms are supported, there is also a
        C program at tools/configure.c that can be compiled to establish the
        board configuration.
    
        See tools/README.txt for more information about these scripts.
    
        General information about configuring NuttX can be found in:
    
         {TOPDIR}/boards/README.txt
         {TOPDIR}/boards/<arch-name>/<chip-name>/<board-name>/README.txt
    
      The Hidden Configuration Scripts:
    
        As mentioned above, there are only a few simple steps to instantiating a
        NuttX configuration.  Those steps are hidden by the configuration scripts
        but are summarized below:
    
        1. Copy Files
    
          Configuring NuttX requires only copying two files from the
          <config-dir> to the directory where you installed NuttX (TOPDIR):
    
            Copy boards/<arch-name>/<chip-name>/<board-name>/configs/<config-dir>/Make.def
            to {TOPDIR}/Make.defs
    
          OR
    
            Copy boards/<arch-name>/<chip-name>/<board-name>/scripts/Make.def
            to {TOPDIR}/Make.defs
    
          Make.defs describes the rules needed by your tool chain to compile
          and link code.  You may need to modify this file to match the
          specific needs of your toolchain.  NOTE that a configuration may
          have its own unique Make.defs file in its configuration directory or
          it may use a common Make.defs file for the board in the scripts/
          directory.  The first takes precedence.
    
            Copy boards/<arch-name>/<chip-name>/<board-name>/configs/<config-dir>/defconfig
            to{TOPDIR}/.config
    
          The defconfig file holds the actual build configuration.  This
          file is included by all other make files to determine what is
          included in the build and what is not.  This file is also used
          to generate a C configuration header at include/nuttx/config.h.
    
            Copy other, environment-specific files to{TOPDIR}
    
          This might include files like .gdbinit or IDE configuration files
          like .project or .cproject.
    
        2. Refresh the Configuration
    
          New configuration setting may be added or removed.  Existing settings
          may also change there values or options.  This must be handled by
          refreshing the configuration as described below.
    
          NOTE:  NuttX uses only compressed defconfig files.  For the NuttX
          defconfig files, this refreshing step is *NOT* optional; it is also
          necessary to uncompress and regenerate the full making file.  This is
          discussed further below.
    
    Refreshing Configurations
    -------------------------
    
      Configurations can get out of date.  As new configuration settings are
      added or removed or as dependencies between configuration settings
      change, the contents of a default configuration can become out of synch
      with the build systems.  Hence, it is a good practice to "refresh" each
      configuration after configuring and before making.  To refresh the
      configuration, use the NuttX Configuration Tool like this:
    
        make oldconfig
    
      AFTER you have instantiated the NuttX configuration as described above.
      The configuration step copied the .config file into place in the top-level
      NuttX directory; 'make oldconfig' step will then operate on that .config
      file to bring it up-to-date.
    
      If your configuration is out of date, you will be prompted by 'make oldconfig'
      to resolve the issues detected by the configuration tool, that is, to
      provide values for the new configuration options in the build system.  Doing
      this can save you a lot of problems down the road due to obsolete settings in
      the default board configuration file.  The NuttX configuration tool is
      discussed in more detail in the following paragraph.
    
      Confused about what the correct value for a new configuration item should
      be?  Enter ? in response to the 'make oldconfig' prompt and it will show
      you the help text that goes with the option.
    
      If you don't want to make any decisions are willing to just accept the
      recommended default value for each new configuration item, an even easier
      way is:
    
        make olddefconfig
    
      The olddefconfig target will simply bring your configuration up to date with
      the current Kconfig files, setting any new options to the default value.
      No questions asked.
    
    NuttX Configuration Tool
    ------------------------
    
      An automated tool has been incorporated to support re-configuration
      of NuttX.  This tool is based on the kconfig-frontends application available
      at https://bitbucket.org/nuttx/tools/src/master/kconfig-frontends/.  (This
      is a snapshot of http://ymorin.is-a-geek.org/projects/kconfig-frontends.)
      This application provides a tool called 'kconfig-mconf' that is used by the
      NuttX top-level Makefile. The following make target is provided:
    
        make menuconfig
    
      This make target will bring up NuttX configuration menus.
    
      WARNING:  Never do 'make menuconfig' on a configuration that has
      not been converted to use the kconfig-frontends tools!  This will
      damage your configuration (see
      http://www.nuttx.org/doku.php?id=wiki:howtos:convertconfig).
    
      How do we tell a new configuration from an old one? See "Incompatibilities
      with Older Configurations" below.
    
      The 'menuconfig' make target depends on two things:
    
      1. The Kconfig configuration data files that appear in almost all
         NuttX directories.  These data files are the part that is still
         under development (patches are welcome!).  The Kconfig files
         contain configuration information for the configuration settings
         relevant to the directory in which the Kconfig file resides.
    
         NOTE: For a description of the syntax of this configuration file,
         see kconfig-language.txt in the tools repository at
         https://bitbucket.org/nuttx/tools
    
      2. The 'kconfig-mconf' tool.  'kconfig-mconf' is part of the
         kconfig-frontends package.  You can download that package from the
         snapshot in the tools repository at https://bitbucket.org/nuttx/tools.
    
         Building kconfig-frontends under Linux may be as simple as
         'configure; make; make install' but there may be some build
         complexities, especially if you are building under Cygwin.  See
         the more detailed build instructions in the top-level README.txt
         file of the tools repository at https://bitbucket.org/nuttx/tools.
    
         The 'make install' step will, by default, install the 'kconfig-mconf'
         tool at /usr/local/bin/mconf.  Where ever you choose to
         install 'kconfig-mconf', make certain that your PATH variable includes
         a path to that installation directory.
    
         The kconfig-frontends tools will not build in a native Windows
         environment directly "out-of-the-box".  For the Windows native
         case, you should use the modified version of kconfig-frontends
         that can be found at
         http://uvc.de/posts/linux-kernel-configuration-tool-mconf-under-windows.html
    
      The basic configuration order is "bottom-up":
    
        - Select the build environment,
        - Select the processor,
        - Select the board,
        - Select the supported peripherals
        - Configure the device drivers,
        - Configure the application options on top of this.
    
      This is pretty straight forward for creating new configurations
      but may be less intuitive for modifying existing configurations.
    
      Another ncurses-based tool that is an option to kconfig-mconf is
      kconfig-nconf.  The differences are primary in in the aesthetics of the
      UI.  If you have kconfig-nconf built, then you can invoke that front end
      with:
    
        make nconfig
    
      If you have an environment that supports the Qt or GTK graphical systems
      (probably KDE or gnome, respectively, or Cygwin under Windows with Qt or
      GTK installed), then you can also build the graphical kconfig-frontends,
      kconfig-qconf and kconfig-gconf.  In these case, you can start the
      graphical configurator with either:
    
        make qconfig
    
      or
    
        make gconfig
    
      Some keyboard shortcuts supported by kconfig-mconf, the tool that runs
      when you do 'make menuconfig':
    
        - '?' will bring up the mconfig help display.
    
        - '/' can be used find configuration selections.
    
        - 'Z' can be used to reveal hidden configuration options
    
      These last two shortcuts are described further in the following
      paragraphs.
    
    Finding Selections in the Configuration Menus
    ---------------------------------------------
    
      The NuttX configuration options have gotten complex and it can be very
      difficult to find options in the menu trees if you are not sure where
      to look.  The "basic configuration order" describe above can help to
      narrow things down.
    
      But if you know exactly what configuration setting you want to select,
      say CONFIG_XYZ, but not where to find it, then the 'make menuconfig'
      version of the tool offers some help:  By pressing the '/' key, the
      tool will bring up a menu that will allow you to search for a
      configuration item.  Just enter the string CONFIG_XYZ and press 'ENTER'.
      It will show you not only where to find the configuration item, but
      also all of the dependencies related to the configuration item.
    
    Reveal Hidden Configuration Options
    -----------------------------------
    
      If you type 'Z', then kconfig-mconf will change what is displayed.
      Normally, only enabled features that have all of their dependencies met
      are displayed.  That is, of course, not very useful if you would like to
      discover new options or if you are looking for an option and do not
      realize that the dependencies have not yet been selected and, hence, it
      is not displayed.
    
      But if you enter 'Z', then every option will be shown, whether or not its
      dependencies have been met.  You can then see everything that could be
      selected with the right dependency selections.  These additional options
      will be shown the '-' for the selection and for the value (since it
      cannot be selected and has no value).  About all you do is to select
      the <Help> option to see what the dependencies are.
    
    Make Sure that You are on the Right Platform
    --------------------------------------------
    
      Saved configurations may run on Linux, Cygwin (32- or 64-bit), or other
      platforms.  The platform characteristics can be changed use 'make
      menuconfig'.  Sometimes this can be confusing due to the differences
      between the platforms.  Enter sethost.sh
    
      sethost.sh is a simple script that changes a configuration to your
      host platform.  This can greatly simplify life if you use many different
      configurations.  For example, if you are running on Linux and you
      configure like this:
    
        tools/configure.sh board:configuration
    
      The you can use the following command to both (1) make sure that the
      configuration is up to date, AND (2) the configuration is set up
      correctly for Linux:
    
        tools/sethost.sh -l
    
      Or, if you are on a Windows/Cygwin 64-bit platform:
    
        tools/sethost.sh -c
    
      Or, for MSYS/MSYS2:
    
        tools/sethost.sh -g
    
      Other options are available from the help option built into the
      script.  You can see all options with:
    
        tools/sethost.sh -h
    
      Recently, the options to the configure.sh (and configure.bat) scripts have
      been extended so that you both setup the configuration, select for the host
      platform that you use, and uncompress and refresh the defconfig file all in
      one command like:
    
        tools/configure.sh -l board:configuration
    
      For a Linux host or for a Windows/Cygwin host:
    
        tools/configure.sh -h board:configuration
    
      Other options are available from the help option built into the
      script.  You can see all options with:
    
        tools/configure.sh -h
    
    Comparing Two Configurations
    ----------------------------
    
      If you try to compare two configurations using 'diff', you will probably
      not be happy with the result.  There are superfluous things added to
      the configuration files that make comparisons with the human eye
      difficult.
    
      There is a tool at nuttx/tools/cmpconfig.c that can be built to simplify
      these comparisons.  The output from this difference tool will show only
      the meaningful differences between two configuration files.  This tool is
      built as follows:
    
        cd nuttx/tools
        make -f Makefile.host
    
      This will create a program called 'cmpconfig' or 'comconfig.exe' on Windows.
    
      Why would you want to compare two configuration files?  Here are a few
      of the reasons why I do this
    
      1. When I create a new configuration I usually base it on an older
         configuration and I want to know, "What are the options that I need to
         change to add the new feature to the older configurations?"  For example,
         suppose that I have a boardA/nsh configuration and I want to create a
         boardA/nxwm configuration.  Suppose I already have boardB/nsh and
         boardB/nxwm configurations.  Then by comparing the boardB/nsh with the
         boardB/nxwm I can see the modifications that I would need to make to my
         boardA/nsh to create a new  boardA/nxwm.
    
      2. But the most common reason that I use the 'cmpconfig' program is to
         check the results of "refreshing" a configuration with 'make oldconfig'
         (see the paragraph "Refreshing Configurations" above).  The 'make
         oldconfig' command will make changes to my configuration and using
         'cmpconfig', I can see precisely what those changes were and if any
         should be of concern to me.
    
      3. The 'cmpconfig' tool can also be useful when converting older, legacy
         manual configurations to the current configurations based on the
         kconfig-frontends tools.  See the following paragraph.
    
    Making defconfig Files
    ----------------------
    
      .config Files as defconfig Files:
    
        The minimum defconfig file is simply the generated .config file with
        CONFIG_APPS_DIR setting removed or commented out.  That setting provides
        the name and location of the apps/ directory relative to the nuttx build
        directory.  The default is ../apps/, however, the apps directory may be
        any other location and may have a different name.  For example, the name
        of versioned NuttX releases are always in the form apps-xx.yy where xx.yy
        is the version number.
    
      Finding the apps/ Directory Path:
    
        When the default configuration is installed using one of the scripts or
        programs in the NuttX tools directory, there will be an option to provide
        the path to the apps/ directory.  If not provided, then the configure tool
        will look around and try to make a reasonable decision about where the
        apps/ directory is located.
    
      Compressed defconfig Files:
    
        The Makefile also supports an option to generate very small defconfig
        files.  The .config files are quite large and complex.  But most of the
        settings in the .config file simply have the default settings from the
        Kconfig files.  These .config files can be converted into small defconfig
        file:
    
          make savedefconfig
    
        That make target will generate a defconfig file in the top-level
        directory.  The size reduction is really quite remarkable:
    
          wc -l .config defconfig
           1085 .config
             82 defconfig
           1167 total
    
         In order to be usable, the .config file installed from the compressed
         defconfig file must be reconstituted using:
    
           make olddefconfig
    
         NOTE 1:  Only compressed defconfig files are retained in the NuttX repository.
         All patches and PRs that attempt to add or modify a defconfig file MUST
         use the compressed defconfig format as created by 'make savdefconfig.'
    
         NOTE 2:  When 'make savedefconfig' runs it will try several things some of
         which are expected to fail.  In these cases you will see an error message
         from make followed by "(ignored)."  You should also ignore these messages
    
         CAUTION:  This size reduction was accomplished by removing all setting
         from the .config file that were at the default value.  'make olddefconfig'
         can regenerate the original .config file by simply restoring those default
         settings.  The underlying assumption here is, of course, that the default
         settings do not change.  If the default settings change, and they often
         do, then the original .config may not be reproducible.
    
         So if your project requires 100% reproducibility over a long period of
         time, you make want to save the complete .config files vs. the standard,
         compressed defconfig file.
    
      Configuring with "Compressed" defconfig Files:
    
        As described above defconfig, all NuttX defconfig files are compressed
        using 'make savedeconfig'.  These compressed defconfig files are
        generally not fully usable as they are and may not build the target
        binaries that you want because the compression process removed all of
        the default settings from the defconfig file.  To restore the default
        settings, you should run the following after configuring:
    
          make olddefconfig
    
        That will restore the the missing defaulted values.
    
        Using this command after configuring is generally a good practice anyway:
        Even if the defconfig files are not "compressed" in this fashion, the
        defconfig file may be old and the only way to assure that the installed
        .config is is up to date is via 'make oldconfig' or 'make olddefconfig'.
        See the paragraph above entitled ""Refreshing Configurations" for
        additional information.
    
    Incompatibilities with Older Configurations
    -------------------------------------------
    
      ***** WARNING *****
    
      The current NuttX build system supports *only* the new compressed,
      defconfig configuration files generated using the kconfig-frontends tools
      as described in the preceding section.  Support for the older, legacy,
      manual configurations was eliminated in NuttX 7.0; support for
      uncompressed .config-files-as-defconfig files was eliminated after
      NuttX-7.21.  All configurations must now be done using the
      kconfig-frontends tool.  The older manual configurations and the new
      kconfig-frontends configurations are not compatible.  Old legacy
      configurations can *not* be used with the kconfig-frontends tool and,
      hence, cannot be used with releases of NuttX 7.0 and beyond:
    
      If you run 'make menuconfig' with a legacy configuration the resulting
      configuration will probably not be functional.
    
      Q: How can I tell if a configuration is a new kconfig-frontends
         configuration or an older, manual configuration?
    
      A: Only old, manual configurations will have an appconfig file
    
    
      Q: How can I convert a older, manual configuration into a new,
         kconfig-frontends toolchain.
    
      A: Refer to http://www.nuttx.org/doku.php?id=wiki:howtos:convertconfig
    
      ***** WARNING *****
    
      As described above, whenever you use a configuration, you really should
      always refresh the configuration with the following command *before* you
      make NuttX:
    
        make oldconfig
    
      OR
    
        make olddefconfig
    
      This will make sure that the configuration is up-to-date in the event that
      it has lapsed behind the current NuttX development (see the paragraph
      "Refreshing Configurations" above).  But this only works with *new*
      configuration files created with the kconfig-frontends tools.
    
      Further, this step is *NOT* optional with the new, compressed defconfig
      files.  It is a necessary step that will also uncompress the defconfig
      file, regenerating the .config and making it usable for NuttX builds.
    
      Never do 'make oldconfig' (OR 'make menuconfig') on a  configuration that
      has not been converted to use the kconfig-frontends tools!  This will
      damage your configuration (see
      http://www.nuttx.org/doku.php?id=wiki:howtos:convertconfig).
    
    NuttX Configuration Tool under DOS
    ----------------------------------
    
      Recent versions of NuttX support building NuttX from a native Windows
      console window (see "Native Windows Build" below).  But kconfig-frontends
      is a Linux tool.  At one time this was a problem for Windows users, but
      now there is a specially modified version of the kconfig-frontends tools
      that can be used:
      http://uvc.de/posts/linux-kernel-configuration-tool-mconf-under-windows.html
    
      The configuration steps most recent versions of NuttX require the
      kconfig-tweak tool that is not not available in the the above.  However,
      there has been an update to this Kconfig Windows tools that does include
      kconfig-tweak:  http://reclonelabs.com/more-kconfig-awesomeness-for-windows/
    
      Source code is available here: https://github.com/reclone/kconfig-frontends-win32
      and https://github.com/reclone/kconfig-frontends-win32/releases
    
      It is also possible to use the version of kconfig-frontends built
      under Cygwin outside of the Cygwin "sandbox" in a native Windows
      environment:
    
      1. You can run the configuration tool using Cygwin.  However, the
         Cygwin Makefile.win will complain so to do this will, you have
         to manually edit the .config file:
    
          a. Delete the line: CONFIG_WINDOWS_NATIVE=y
          b. Change the apps/ directory path, CONFIG_APPS_DIR to use Unix
             style delimiters.  For example, change "..\apps" to "../apps"
    
         And of course, after you use the configuration tool you need to
         restore CONFIG_WINDOWS_NATIVE=y and the correct CONFIG_APPS_DIR.
    
      2) You can, with some effort, run the Cygwin kconfig-mconf tool
         directly in the Windows console window.  In this case, you do not
         have to modify the .config file, but there are other complexities:
    
          a. You need to temporarily set the Cygwin directories in the PATH
             variable then run kconfig-mconf manually like:
    
              kconfig-mconf Kconfig
    
             There is a Windows batch file at tools/kconfig.bat that automates
             these steps:
    
             tools/kconfig menuconfig
    
           b. There is an issue with accessing DOS environment variables from
              the Cygwin kconfig-mconf running in the Windows console.  The
              following change to the top-level Kconfig file seems to work
              around these problems:
    
              config APPSDIR
                  string
              -   option env="APPSDIR"
              +   default "../apps"
    
    TOOLCHAINS
    ^^^^^^^^^^
    
    Cross-Development Toolchains
    ----------------------------
    
      In order to build NuttX for your board, you will have to obtain a cross-
      compiler to generate code for your target CPU.  For each board,
      configuration, there is a README.txt file (at
      boards/<arch-name>/<chip-name>/<board-name>/README.txt).
      That README file contains suggestions and information about appropriate
      tools and development environments for use with your board.
    
      In any case, the PATH environment variable will need to be updated to
      include the location where the build can find the toolchain binaries.
    
    NuttX Buildroot Toolchain
    -------------------------
    
      For many configurations, a DIY set of tools is available for NuttX.  These
      tools can be downloaded from the NuttX Bitbucket.org file repository.  After
      unpacking the buildroot tarball, you can find instructions for building
      the tools in the buildroot/boards/README.txt file.
    
      Check the README.txt file in the configuration directory for your board
      to see if you can use the buildroot toolchain with your board (this
      README.txt file is located in
      boards/<arch-name>/<chip-name>/<board-name>/README.txt).
    
      This toolchain is available for both the Linux and Cygwin development
      environments.
    
      Advantages:  (1) NuttX header files are built into the tool chain,
      and (2) related support tools like NXFLAT tools, the ROMFS
      genromfs tools, and the kconfig-frontends tools can be built into your
      toolchain.
    
      Disadvantages:  This tool chain is not was well supported as some other
      toolchains.  GNU tools are not my priority and so the buildroot tools
      often get behind.  For example, until recently there was no EABI support
      in the NuttX buildroot toolchain for ARM.
    
      NOTE: For Cortex-M3/4, there are OABI and EABI versions of the buildroot
      toolchains.  If you are using the older OABI toolchain the prefix for
      the tools will be arm-nuttx-elf-; for the EABI toolchain the prefix will
      be arm-nuttx-eabi-.  If you are using the older OABI toolchain with
      an ARM Cortex-M3/4, you will need to set CONFIG_ARMV7M_OABI_TOOLCHAIN
      in the .config file in order to pick the right tool prefix.
    
      If the make system ever picks the wrong prefix for your toolchain, you
      can always specify the prefix on the command to override the default
      like:
    
        make CROSSDEV=arm-nuttx-elf
    
    SHELLS
    ^^^^^^
    
      The NuttX build relies on some shell scripts.  Some are inline in the
      Makefiles and many are executable scripts in the tools/. directory.  The
      scripts were all developed using bash and many contain bash shell
      dependencies.
    
      Most of the scripts begin with #!/bin/bash to specifically select the
      bash shell.  Some still have #!/bin/sh but I haven't heard any complaints
      so these must not have bash dependencies.
    
      There are two shell issues that I have heard of:
    
      1. Linux where /bin/sh refers to an incompatible shell (like ksh or csh).
    
         In this case, bash is probably available and the #!/bin/bash at the
         beginning of the file should do the job.  If any scripts with #!/bin/sh
         fail, try changing that to #!/bin/bash and let me know about the change.
    
      2. FreeBSD with the Bourne Shell and no bash shell.
    
         The other, reverse case has also been reported on FreeBSD setups that
         have the Bourne shell, but not bash.  In this base, #!/bin/bash fails
         but #!/bin/sh works okay.  My recommendation in this case is to create
         a symbolic link at /bin/bash that refers to the Bourne shell.
    
         There may still be issues, however, with certain the bash-centric scripts
         that will require modifications.
    
    BUILDING NUTTX
    ^^^^^^^^^^^^^^
    
    Building
    --------
    
      NuttX builds in-place in the source tree.  You do not need to create
      any special build directories.  Assuming that your Make.defs is setup
      properly for your tool chain and that PATH environment variable contains
      the path to where your cross-development tools are installed, the
      following steps are all that are required to build NuttX:
    
        cd{TOPDIR}
        make
    
      At least one configuration (eagle100) requires additional command line
      arguments on the make command.  Read
      {TOPDIR}/boards/<arch-name>/<chip-name>/<board-name>/README.txt to see
      if that applies to your target.
    
    Re-building
    -----------
    
      Re-building is normally simple -- just type make again.
    
      But there are some things that can "get you" when you use the Cygwin
      development environment with Windows native tools.  The native Windows
      tools do not understand Cygwin's symbolic links, so the NuttX make system
      does something weird:  It copies the configuration directories instead of
      linking to them (it could, perhaps, use the NTFS 'mklink' command, but it
      doesn't).
    
      A consequence of this is that you can easily get confused when you edit
      a file in one of the linked (i.e., copied) directories, re-build NuttX,
      and then not see your changes when you run the program.  That is because
      build is still using the version of the file in the copied directory, not
      your modified file!
    
      Older versions of NuttX did not support dependencies in this
      configuration.  So a simple work around this annoying behavior in this
      case was the following when you re-build:
    
         make clean_context all
    
      This 'make' command will remove of the copied directories, re-copy them,
      then make NuttX.
    
      However, more recent versions of NuttX do support dependencies for the
      Cygwin build.  As a result, the above command will cause everything to be
      rebuilt (because it removes and will cause recreating the
      include/nuttx/config.h header file).  A much less gracefully but still
      effective command in this case is the following for the ARM configuration:
    
        rm -rf arch/arm/src/chip arch/arm/src/board
    
      This "kludge" simple removes the copied directories.  These directories
      will be re-created when you do a normal 'make' and your edits will then be
      effective.
    
    Build Targets and Options
    -------------------------
    
      Build Targets:
      Below is a summary of the build targets available in the top-level
      NuttX Makefile:
    
      all
    
        The default target builds the NuttX executable in the selected output
        formats.
    
      clean
    
        Removes derived object files, archives, executables, and temporary
        files, but retains the configuration and context files and directories.
    
      distclean
    
        Does 'clean' then also removes all configuration and context files.
        This essentially restores the directory structure to its original,
        unconfigured stated.
    
      Application housekeeping targets.  The APPDIR variable refers to the user
      application directory.  A sample apps/ directory is included with NuttX,
      however, this is not treated as part of NuttX and may be replaced with a
      different application directory.  For the most part, the application
      directory is treated like any other build directory in the Makefile script.
      However, as a convenience, the following targets are included to support
      housekeeping functions in the user application directory from the NuttX
      build directory.
    
      apps_clean
    
        Perform the clean operation only in the user application directory
    
      apps_distclean
    
        Perform the distclean operation only in the user application directory.
        The apps/.config file is preserved so that this is not a "full" distclean
        but more of a configuration "reset" for the application directory.
    
      export
    
        The export target will package the NuttX libraries and header files into
        an exportable package.  Caveats: (1) These needs some extension for the KERNEL
        build. (2) The logic in tools/mkexport.sh only supports GCC and, for example,
        explicitly assumes that the archiver is 'ar'
    
      download
    
        This is a helper target that will rebuild NuttX and download it to the target
        system in one step.  The operation of this target depends completely upon
        implementation of the DOWNLOAD command in the user Make.defs file.  It will
        generate an error an error if the DOWNLOAD command is not defined.
    
      The following targets are used internally by the make logic but can be invoked
      from the command under certain conditions if necessary.
    
      depend
    
        Create build dependencies. (NOTE:  There is currently no support for build
        dependencies under Cygwin using Windows-native toolchains.)
    
      context
    
        The context target is invoked on each target build to assure that NuttX is
        properly configured.  The basic configuration steps include creation of the
        the config.h and version.h header files in the include/nuttx directory and
        the establishment of symbolic links to configured directories.
    
      clean_context
    
        This is part of the distclean target.  It removes all of the header files
        and symbolic links created by the context target.
    
      Build Options:
      Of course, the value any make variable an be overridden from the make command
      line.  However, there is one particular variable assignment option that may
      be useful to you:
    
      V=1
    
        This is the build "verbosity flag."  If you specify V=1 on the make command
        line, you will see the exact commands used in the build. This can be very
        useful when adding new boards or tracking down compile time errors and
        warnings (Contributed by Richard Cochran).
    
    Native Windows Build
    --------------------
    
      The beginnings of a Windows native build are in place but still not often
      used as of this writing.  The build was functional but because of lack of
      use may find some issues to be resolved with this build configuration.
    
      The windows native build logic initiated if CONFIG_WINDOWS_NATIVE=y is
      defined in the NuttX configuration file:
    
      This build:
    
        - Uses all Windows style paths
        - Uses primarily Windows batch commands from cmd.exe, with
        - A few extensions from GNUWin32
    
      In this build, you cannot use a Cygwin or MSYS shell. Rather the build must
      be performed in a Windows console window. Here is a better terminal than the
      standard issue, CMD.exe terminal:  ConEmu which can be downloaded from:
      https://sourceforge.net/projects/conemu/ or https://conemu.github.io/ .
    
      Build Tools.  The build still relies on some Unix-like commands.  I use
      the GNUWin32 tools that can be downloaded from http://gnuwin32.sourceforge.net/
      using the 'Download all' selection.  Individual packages can be download
      instead if you know what you are doing and want a faster download (No, I
      can't tell you which packages you should or should not download).
    
      Host Compiler:  I use the MingGW GCC compiler which can be downloaded from
      http://www.mingw.org/.  If you are using GNUWin32, then it is recommended
      the you not install the optional MSYS components as there may be conflicts.
    
      This capability should still be considered a work in progress because:
    
        (1) It has not been verified on all targets and tools, and
        (2) it still lacks some of the creature-comforts of the more mature
            environments.
    
    Installing GNUWin32
    -------------------
    
      The Windows native build will depend upon a few Unix-like tools that can be
      provided either by MSYS or GNUWin32.  The GNUWin32 are available from
      http://gnuwin32.sourceforge.net/.  GNUWin32 provides ports of tools with a
      GPL or similar open source license to modern MS-Windows (Microsoft Windows
      2000 / XP / 2003 / Vista / 2008 / 7).  See
      http://gnuwin32.sourceforge.net/packages.html for a list of all of the tools
      available in the GNUWin32 package.
    
      The SourceForge project is located here:
      http://sourceforge.net/projects/gnuwin32/.  The project is still being
      actively supported (although some of the Windows ports have gotten very old).
    
      Some commercial toolchains include a subset of the GNUWin32 tools in the
      installation.  My recommendation is that you download the GNUWin32 tools
      directly from the sourceforge.net website so that you will know what you are
      using and can reproduce your build environment.
    
      GNUWin32 Installation Steps:
    
      The following steps will download and execute the GNUWin32 installer.
    
      1. Download GetGNUWin32-x.x.x.exe from
         http://sourceforge.net/projects/getgnuwin32/files/.  This is the
         installer.  The current version as of this writing is 0.6.3.
    
      2. Run the installer.
    
      3. Accept the license.
    
      4. Select the installation directory.  My recommendation is the
         directory that contains this README file (<this-directory>).
    
      5. After running GetGNUWin32-0.x.x.exe, you will have a new directory
         <this-directory>/GetGNUWin32
    
      Note that the GNUWin32 installer didn't install GNUWin32.  Instead, it
      installed another, smarter downloader.  That downloader is the GNUWin32
      package management tool developed by the Open SSL project.
    
      The following steps probably should be performed from inside a DOS shell.
    
      6. Change to the directory created by GetGNUWin32-x.x.x.exe
    
        cd GetGNUWin32
    
      7. Execute the download.bat script.  The download.bat script will download
         about 446 packages!  Enough to have a very complete Linux-like environment
         under the DOS shell.  This will take awhile.  This step only downloads
         the packages and the next step will install the packages.
    
         download
    
      8. This step will install the downloaded packages.  The argument of the
         install.bat script is the installation location.  C:\gnuwin32 is the
         standard install location:
    
         install C:\gnuwin32
    
      NOTE:  This installation step will install *all* GNUWin32 packages... far
      more than you will ever need.  If disc space is a problem for you, you might
      need to perform a manual installation of the individual ZIP files that you
      will find in the <this directory>/GetGNUWin32/packages directory.
    
    CYGWIN BUILD PROBLEMS
    ^^^^^^^^^^^^^^^^^^^^^
    
    Performance
    -----------
    
      Build performance under Cygwin is really not so bad, certainly not as good
      as a Linux build.  However, often you will find that the performance is
      not just bad but terrible.  If you are seeing awful performance.. like two
      or three compilations per second.. the culprit is usually your Windows
      Anti-Virus protection interfering with the build tool program execution.
    
      I use Cygwin quite often and I use Windows Defender.  In order to get good
      build performance, I routinely keep the Windows Defender "Virus & Threat
      Protections Settings" screen up:  I disable "Real-Time Protection" just
      before entering 'make' then turn "Real-Time Protection" back on when the
      build completes.  With this additional nuisance step, I find that build
      performance under Cygwin is completely acceptable.
    
    Strange Path Problems
    ---------------------
    
      If you see strange behavior when building under Cygwin then you may have
      a problem with your PATH variable.  For example, if you see failures to
      locate files that are clearly present, that may mean that you are using
      the wrong version of a tool.  For example, you may not be using Cygwin's
      'make' program at /usr/bin/make.  Try:
    
        which make
        /usr/bin/make
    
      When you install some toolchains (such as Yargarto or CodeSourcery tools),
      they may modify your PATH variable to include a path to their binaries.
      At that location, they may have GNUWin32 versions of the tools.  So you
      might actually be using a version of make that does not understand Cygwin
      paths.
    
      The solution is either:
    
      1. Edit your PATH to remove the path to the GNUWin32 tools, or
      2. Put /usr/local/bin, /usr/bin, and /bin at the front of your path:
    
         export PATH=/usr/local/bin:/usr/bin:/bin:$PATH
    
    Window Native Toolchain Issues
    ------------------------------
    
      There are many popular Windows native toolchains that may be used with NuttX.
      Examples include CodeSourcery (for Windows), devkitARM, and several vendor-
      provided toolchains.  There are several limitations with using a and Windows
      based toolchain in a Cygwin environment.  The three biggest are:
    
      1. The Windows toolchain cannot follow Cygwin paths.  Path conversions are
         performed automatically in the Cygwin makefiles using the 'cygpath' utility
         but you might easily find some new path problems.  If so, check out 'cygpath -w'
    
      2. Windows toolchains cannot follow Cygwin symbolic links.  Many symbolic links
         are used in Nuttx (e.g., include/arch).  The make system works around these
         problems for the Windows tools by copying directories instead of linking them.
         But this can also cause some confusion for you:  For example, you may edit
         a file in a "linked" directory and find that your changes had no effect.
         That is because you are building the copy of the file in the "fake" symbolic
         directory.  If you use a Windows toolchain, you should get in the habit of
         making like this:
    
           make clean_context all
    
         An alias in your .bashrc file might make that less painful.  The rebuild
         is not a long as you might think because there is no dependency checking
         if you are using a native Windows toolchain.  That bring us to #3:
    
    General Pre-built Toolchain Issues
    ----------------------------------
    
      To continue with the list of "Window Native Toolchain Issues" we can add
      the following.  These, however, are really just issues that you will have
      if you use any pre-built toolchain (vs. building the NuttX toolchain from
      the NuttX buildroot package):
    
      There may be incompatibilities with header files, libraries, and compiler
      built-in functions detailed below.  For the most part, these issues
      are handled in the existing make logic.  But if you are breaking new ground,
      then you may encounter these:
    
      4. Header Files.  Most pre-built toolchains will build with a foreign C
         library (usually newlib, but maybe uClibc or glibc if you are using a
         Linux toolchain).  This means that the header files from the foreign
         C library will be built into the toolchain.  So if you "include <stdio.h>",
         you will get the stdio.h from the incompatible, foreign C library and
         not the nuttx stdio.h (at nuttx/include/stdio.h) that you wanted.
    
         This can cause confusion in the builds and you must always be
         sure the -nostdinc is included in the CFLAGS.  That will assure that
         you take the include files only from
    
      5. Libraries.  What was said above header files applies to libraries.
         You do not want to include code from the libraries of any foreign
         C libraries built into your toolchain.  If this happens you will get
         perplexing errors about undefined symbols.  To avoid these errors,
         you will need to add -nostdlib to your CFLAGS flags to assure that
         you only take code from the NuttX libraries.
    
         This, however, may causes other issues for libraries in the toolchain
         that you do want (like libgcc.a or libm.a).  These are special-cased
         in most Makefiles, but you could still run into issues of missing
         libraries.
    
      6. Built-Ins.  Some compilers target a particular operating system.
         Many people would, for example, like to use the same toolchain to
         develop Linux and NuttX software.  Compilers built for other
         operating systems may generate incompatible built-in logic and,
         for this reason, -fno-builtin should also be included in your
         C flags
    
      And finally you may not be able to use NXFLAT.
    
      7. NXFLAT. If you use a pre-built toolchain, you will lose all support
         for NXFLAT.  NXFLAT is a binary format described in
         Documentation/NuttXNxFlat.html.  It may be possible to build
         standalone versions of the NXFLAT tools; there are a few examples
         of this in the buildroot repository at https://bitbucket.org/nuttx/buildroot
         However, it is possible that there could be interoperability issues
         with your toolchain since they will be using different versions of
         binutils and possibly different ABIs.
    
    Building Original Linux Boards in Cygwin
    ----------------------------------------
    
      Some default board configurations are set to build under Linux and others
      to build under Windows with Cygwin.  Various default toolchains may also
      be used in each configuration.  It is possible to change the default
      setup.  Here, for example, is what you must do in order to compile a
      default Linux configuration in the Cygwin environment using the
      CodeSourcery for Windows toolchain.  After instantiating a "canned"
      NuttX configuration, run the target 'menuconfig' and set the following
      items:
    
        Build Setup->Build Host Platform->Windows
        Build Setup->Windows Build Environment->Cygwin
        System Type->Toolchain Selection->CodeSourcery GNU Toolchain under Windows
    
      In Windows 7 it may be required to open the Cygwin shell as Administrator
      ("Run As" option, right button) you find errors like "Permission denied".
    
    Recovering from Bad Configurations
    ----------------------------------
    
      Many people make the mistake of configuring NuttX with the "canned"
      configuration and then just typing 'make' with disastrous consequences;
      the build may fail with mysterious, uninterpretable, and irrecoverable
      build errors.  If, for example, you do this with an unmodified Linux
      configuration in a Windows/Cgwin environment, you will corrupt the
      build environment.  The environment will be corrupted because of POSIX vs
      Windows path issues and with issues related to symbolic links.  If you
      make the mistake of doing this, the easiest way to recover is to just
      start over: Do 'make distclean' to remove every trace of the corrupted
      configuration, reconfigure from scratch, and make certain that the set
      the configuration correctly for your platform before attempting to make
      again.
    
      Just fixing the configuration file after you have instantiated the bad
      configuration with 'make' is not enough.
    
    DOCUMENTATION
    ^^^^^^^^^^^^^
    
    Additional information can be found in the Documentation/ directory and
    also in README files that are scattered throughout the source tree.  The
    documentation is in HTML and can be access by loading the following file
    into your Web browser:
    
      Documentation/index.html
    
    NuttX documentation is also available online at http://www.nuttx.org.
    
    Below is a guide to the available README files in the NuttX source tree:
    
    nuttx/
     |
     |- arch/
     |   |
     |   |- arm/
     |   |   `- src
     |   |       |- common
     |   |       |   `- README_lwl_console.txt
     |   |       |- lpc214x
     |   |       |    `-README.txt
     |   |       `- stm32l4
     |   |           `- README.txt
     |   |- renesas/
     |   |   |- include/
     |   |   |   `-README.txt
     |   |   |- src/
     |   |   |   `-README.txt
     |   |- x86/
     |   |   |- include/
     |   |   |   `-README.txt
     |   |   `- src/
     |   |       `-README.txt
     |   `- z80/
     |   |   `- src/
     |   |       |- z80/README.txt
     |   |       `- z180/README.txt, z180_mmu.txt
     |   `- README.txt
     |- audio/
     |   `-README.txt
     |- binfmt/
     |   `-libpcode/
     |       `-README.txt
     |- boards/
     |   |- arm/
     |   |   |- a1x/
     |   |   |   `- pcduino-a10/
     |   |   |       `- README.txt
     |   |   |- am335x/
     |   |   |   `- beaglebone-black/
     |   |   |       `- README.txt
     |   |   |- c5471/
     |   |   |   `- c5471evm/
     |   |   |       `- README.txt
     |   |   |- cxd56xx/
     |   |   |   `- spresense/
     |   |   |       `- README.txt
     |   |   |- dm320/
     |   |   |   `- ntosd-dm320/
     |   |   |       |- doc/README.txt
     |   |   |       `- README.txt
     |   |   |- efm32/
     |   |   |   |- efm32-g8xx-stk/
     |   |   |   |   `- README.txt
     |   |   |   |- efm32gg-stk3700/
     |   |   |   |   `- README.txt
     |   |   |   `- olimex-efm32g880f128-stk/
     |   |   |       `- README.txt
     |   |   |- imx6/
     |   |   |   `- sabre-6quad/
     |   |   |       `- README.txt
     |   |   |- imxrt/
     |   |   |   |- imxrt1050-evk/
     |   |   |   |   `- README.txt
     |   |   |   `- imxrt1060-evk/
     |   |   |       `- README.txt
     |   |   |- kinetis/
     |   |   |   |- freedom-k28f/
     |   |   |   |   `- README.txt
     |   |   |   |- freedom-k64f/
     |   |   |   |   `- README.txt
     |   |   |   |- freedom-k66f/
     |   |   |   |   `- README.txt
     |   |   |   |- kwikstik-k40/
     |   |   |   |   `- README.txt
     |   |   |   |- teensy-3.x/
     |   |   |   |   `- README.txt
     |   |   |   |- twr-k60n512/
     |   |   |   |   `- README.txt
     |   |   |   `- twr-k64f120m/
     |   |   |       `- README.txt
     |   |   |- kl/
     |   |   |   |- freedom-kl25z/
     |   |   |   |   `- README.txt
     |   |   |   |- freedom-kl26z/
     |   |   |   |   `- README.txt
     |   |   |   `- teensy-lc/
     |   |   |       `- README.txt
     |   |   |- lc823450/
     |   |   |   `- lc823450-xgevk/
     |   |   |       `- README.txt
     |   |   |- lpc17xx_40xx/
     |   |   |   |- lincoln60/
     |   |   |   |   `- README.txt
     |   |   |   |- lpc4088-devkit/
     |   |   |   |   `- README.txt
     |   |   |   |- lpc4088-quickstart/
     |   |   |   |   `- README.txt
     |   |   |   |- lpcxpresso-lpc1768/
     |   |   |   |   `- README.txt
     |   |   |   |- lx_cpu/
     |   |   |   |   `- README.txt
     |   |   |   |- mbed/
     |   |   |   |   `- README.txt
     |   |   |   |- mcb1700/
     |   |   |   |   `- README.txt
     |   |   |   |- olimex-lpc1766stk/
     |   |   |   |   `- README.txt
     |   |   |   |- open1788/
     |   |   |   |   `- README.txt
     |   |   |   |- pnev5180b/
     |   |   |   |   `- README.txt
     |   |   |   |- u-blox-c027/
     |   |   |   |   `- README.txt
     |   |   |   `- zkit-arm-1769/
     |   |   |       `- README.txt
     |   |   |- lpc214x/
     |   |   |   |- mcu123-lpc214x/
     |   |   |   |   `- README.txt
     |   |   |   `- zp214xpa/
     |   |   |       `- README.txt
     |   |   |- lpc2378/
     |   |   |   `- olimex-lpc2378/
     |   |   |       `- README.txt
     |   |   |- lpc31xx/
     |   |   |   |- ea3131/
     |   |   |   |   `- README.txt
     |   |   |   |- ea3152/
     |   |   |   |   `- README.txt
     |   |   |   `- olimex-lpc-h3131/
     |   |   |       `- README.txt
     |   |   |- lpc43xx/
     |   |   |   |- bambino-200e/
     |   |   |   |   `- README.txt
     |   |   |   |- lpc4330-xplorer/
     |   |   |   |   `- README.txt
     |   |   |   |- lpc4337-ws/
     |   |   |   |   `- README.txt
     |   |   |   |- lpc4357-evb/
     |   |   |   |   `- README.txt
     |   |   |   `- lpc4370-link2/
     |   |   |       `- README.txt
     |   |   |- lpc54xx/
     |   |   |   `- lpcxpresso-lpc54628/
     |   |   |       `- README.txt
     |   |   |- max326xx/
     |   |   |   `- max32660-evsys/
     |   |   |       `- README.txt
     |   |   |- moxart/
     |   |   |   `- moxa/
     |   |   |- nrf52/
     |   |   |   `- nrf52-generic/
     |   |   |       `- README.txt
     |   |   |- nuc1xx/
     |   |   |   `- nutiny-nuc120/
     |   |   |       `- README.txt
     |   |   |- s32k1xx/
     |   |   |   |- s32k118evb/
     |   |   |   |   `- README.txt
     |   |   |   |- s32k146evb/
     |   |   |   |   `- README.txt
     |   |   |   `- s32k148evb/
     |   |   |       `- README.txt
     |   |   |- sam34/
     |   |   |   |- arduino-due/
     |   |   |   |   `- README.txt
     |   |   |   |- flipnclick-sam3x/
     |   |   |   |   `- README.txt
     |   |   |   |- sam3u-ek/
     |   |   |   |   `- README.txt
     |   |   |   |- sam4cmp-db/
     |   |   |   |   `- README.txt
     |   |   |   |- sam4e-ek/
     |   |   |   |   `- README.txt
     |   |   |   |- sam4l-xplained/
     |   |   |   |   `- README.txt
     |   |   |   |- sam4s-xplained/
     |   |   |   |   `- README.txt
     |   |   |   `- sam4s-xplained-pro/
     |   |   |       `- README.txt
     |   |   |- sama5/
     |   |   |   |- sama5d2-xult/
     |   |   |   |   `- README.txt
     |   |   |   |- sama5d3x-ek/
     |   |   |   |   `- README.txt
     |   |   |   |- sama5d3-xplained/
     |   |   |   |   `- README.txt
     |   |   |   `- sama5d4-ek/
     |   |   |       `- README.txt
     |   |   |- samd2l2/
     |   |   |   |- arduino-m0/
     |   |   |   |   `- README.txt
     |   |   |   |- samd20-xplained/
     |   |   |   |   `- README.txt
     |   |   |   |- samd21-xplained/
     |   |   |   |   `- README.txt
     |   |   |   `- saml21-xplained/
     |   |   |       `- README.txt
     |   |   |- samd5e5/
     |   |   |   `- metro-m4/
     |   |   |      `- README.txt
     |   |   |- samv7/
     |   |   |   |- same70-xplained/
     |   |   |   |   `- README.txt
     |   |   |   `- samv71-xult/
     |   |   |      `- README.txt
     |   |   |- stm32/
     |   |   |   |- axoloti/
     |   |   |   |   `- README.txt
     |   |   |   |- clicker2-stm32/
     |   |   |   |   `- README.txt
     |   |   |   |- cloudctrl/
     |   |   |   |   `- README.txt
     |   |   |   |- fire-stm32v2/
     |   |   |   |   `- README.txt
     |   |   |   |- hymini-stm32v/
     |   |   |   |   `- README.txt
     |   |   |   |- maple/
     |   |   |   |   `- README.txt
     |   |   |   |- mikroe-stm32f4/
     |   |   |   |   `- README.txt
     |   |   |   |- nucleo-f103rb/
     |   |   |   |   `- README.txt
     |   |   |   |- nucleo-f207zg/
     |   |   |   |   `- README.txt
     |   |   |   |- nucleo-f302r8/
     |   |   |   |   `- README.txt
     |   |   |   |- nucleo-f303re/
     |   |   |   |   `- README.txt
     |   |   |   |- nucleo-f303ze/
     |   |   |   |   `- README.txt
     |   |   |   |- nucleo-f334r8/
     |   |   |   |   `- README.txt
     |   |   |   |- nucleo-f410rb/
     |   |   |   |   `- README.txt
     |   |   |   |- nucleo-f446re/
     |   |   |   |   `- README.txt
     |   |   |   |- nucleo-f4x1re/
     |   |   |   |   `- README.txt
     |   |   |   |- nucleo-l152re/
     |   |   |   |   `- README.txt
     |   |   |   |- olimexino-stm32/
     |   |   |   |- olimex-stm32-e407/
     |   |   |   |   `- README.txt
     |   |   |   |- olimex-stm32-h405/
     |   |   |   |   `- README.txt
     |   |   |   |- olimex-stm32-h407/
     |   |   |   |   `- README.txt
     |   |   |   |- olimex-stm32-p107/
     |   |   |   |- olimex-stm32-p207/
     |   |   |   |   `- README.txt
     |   |   |   |- olimex-stm32-p407/
     |   |   |   |   `- README.txt
     |   |   |   |- omnibusf4/
     |   |   |   |   `- README.txt
     |   |   |   |- photon/
     |   |   |   |   `- README.txt
     |   |   |   |- shenzhou/
     |   |   |   |   `- README.txt
     |   |   |   |- stm32_tiny/
     |   |   |   |   `- README.txt
     |   |   |   |- stm3210e-eval/
     |   |   |   |   `- README.txt
     |   |   |   |- stm3220g-eval/
     |   |   |   |   `- README.txt
     |   |   |   |- stm3240g-eval/
     |   |   |   |   `- README.txt
     |   |   |   |- stm32butterfly2/
     |   |   |   |- stm32f103-minimum/
     |   |   |   |   `- README.txt
     |   |   |   |- stm32f334-disco/
     |   |   |   |   `- README.txt
     |   |   |   |- stm32f3discovery/
     |   |   |   |   `- README.txt
     |   |   |   |- stm32f411e-disco/
     |   |   |   |   `- README.txt
     |   |   |   |- stm32f429i-disco/
     |   |   |   |   `- README.txt
     |   |   |   |- stm32f4discovery/
     |   |   |   |   `- README.txt
     |   |   |   |- stm32ldiscovery/
     |   |   |   |   `- README.txt
     |   |   |   |- stm32vldiscovery/
     |   |   |   |   `- README.txt
     |   |   |   `- viewtool-stm32f107/
     |   |   |       `- README.txt
     |   |   |- stm32f0l0g0/
     |   |   |   |- b-l072z-lrwan1/
     |   |   |   |   `- README.txt
     |   |   |   |- nucleo-f072rb/
     |   |   |   |   `- README.txt
     |   |   |   |- nucleo-f091rc/
     |   |   |   |   `- README.txt
     |   |   |   |- nucleo-g070rb/
     |   |   |   |   `- README.txt
     |   |   |   |- nucleo-g071rb/
     |   |   |   |   `- README.txt
     |   |   |   |- nucleo-l073rz/
     |   |   |   |   `- README.txt
     |   |   |   |- stm32f051-discovery/
     |   |   |   |   `- README.txt
     |   |   |   `- stm32f072-discovery/
     |   |   |       `- README.txt
     |   |   |- stm32f7/
     |   |   |   |- nucleo-144/
     |   |   |   |   `- README.txt
     |   |   |   |- stm32f746g-disco/
     |   |   |   |   |- configs/fb/README.txt
     |   |   |   |   |- configs/nxdemo/README.txt
     |   |   |   |   |- configs/nxterm/README.txt
     |   |   |   |   `- README.txt
     |   |   |   |- stm32f746-ws/
     |   |   |   `- stm32f769i-disco/
     |   |   |       `- README.txt
     |   |   |- stm32h7/
     |   |   |   `- nucleo-h743zi/
     |   |   |       `- README.txt
     |   |   |- stm32l4/
     |   |   |   |- b-l475e-iot01a/
     |   |   |   |   `- README.txt
     |   |   |   |- nucleo-l432kc/
     |   |   |   |   `- README.txt
     |   |   |   |- nucleo-l452re/
     |   |   |   |   `- README.txt
     |   |   |   |- nucleo-l476rg/
     |   |   |   |   `- README.txt
     |   |   |   |- nucleo-l496zg/
     |   |   |   |   `- README.txt
     |   |   |   |- stm32l476-mdk/
     |   |   |   |   `- README.txt
     |   |   |   |- stm32l476vg-disco/
     |   |   |   |   `- README.txt
     |   |   |   `- stm32l4r9ai-disco/
     |   |   |       `- README.txt
     |   |   |- str71x/
     |   |   |   `- olimex-strp711/
     |   |   |       `- README.txt
     |   |   |- tiva/
     |   |   |   |- dk-tm4c129x/
     |   |   |   |   `- README.txt
     |   |   |   |- eagle100/
     |   |   |   |   `- README.txt
     |   |   |   |- ekk-lm3s9b96/
     |   |   |   |   `- README.txt
     |   |   |   |- launchxl-cc1310/
     |   |   |   |   `- README.txt
     |   |   |   |- launchxl-cc1312r1/
     |   |   |   |   `- README.txt
     |   |   |   |- lm3s6432-s2e/
     |   |   |   |   `- README.txt
     |   |   |   |- lm3s6965-ek/
     |   |   |   |   `- README.txt
     |   |   |   |- lm3s8962-ek/
     |   |   |   |   `- README.txt
     |   |   |   |- lm4f120-launchpad/
     |   |   |   |   `- README.txt
     |   |   |   |- tm4c123g-launchpad/
     |   |   |   |   `- README.txt
     |   |   |   `- tm4c1294-launchpad/
     |   |   |       `- README.txt
     |   |   |- tms570/
     |   |   |   |- launchxl-tms57004/
     |   |   |   |   `- README.txt
     |   |   |   `- tms570ls31x-usb-kit/
     |   |   |       `- README.txt
     |   |   `- xmc4/
     |   |       `- xmc4500-relax/
     |   |           `- README.txt
     |   |- avr/
     |   |   |- at32uc3/
     |   |   |   `- avr32dev1/
     |   |   |       `- README.txt
     |   |   |- at90usb/
     |   |   |   |- micropendous3/
     |   |   |   |   `- README.txt
     |   |   |   `- teensy-2.0/
     |   |   |       `- README.txt
     |   |   `- atmega/
     |   |       |- amber/
     |   |       |   `- README.txt
     |   |       |- arduino-mega2560/
     |   |       |   `- README.txt
     |   |       `- moteino-mega/
     |   |           `- README.txt
     |   |- hc/
     |   |   `- mcs92s12ne64/
     |   |       |- demo9s12ne64/
     |   |       |   `- README.txt
     |   |       `- ne64badge/
     |   |           `- README.txt
     |   |- mips/
     |   |   |- pic32mx/
     |   |   |   |- mirtoo/
     |   |   |   |   `- README.txt
     |   |   |   |- pic32mx7mmb/
     |   |   |   |   `- README.txt
     |   |   |   |- pic32mx-starterkit/
     |   |   |   |   `- README.txt
     |   |   |   |- sure-pic32mx/
     |   |   |   |   `- README.txt
     |   |   |   `- ubw32/
     |   |   |       `- README.txt
     |   |   `-pic32mz/
     |   |       |- flipnclick-pic32mz/
     |   |       |   `- README.txt
     |   |       `- pic32mz-starterkit/
     |   |           `- README.txt
     |   |- misoc/
     |   |   `- lm32/
     |   |       `- misoc/
     |   |           `- README.txt
     |   |- or1k/
     |   |   `- mor1kx/
     |   |       `- or1k/
     |   |           `- README.txt
     |   |- renesas/
     |   |   |- m32262f8/
     |   |   |   `- skp16c26/
     |   |   |       `- README.txt
     |   |   `-sh7032/
     |   |       `- us7032evb1/
     |   |           `- README.txt
     |   |- risc-v/
     |   |   |- gap8/
     |   |   |   `- gapuino/
     |   |   |       `- README.txt
     |   |   `-nr5m100/
     |   |       `- nr5m100-nexys4/
     |   |           `- README.txt
     |   |- sim/
     |   |   `- sim/
     |   |       `- sim/
     |   |           |- include/README.txt
     |   |           `- README.txt
     |   |- x86/
     |   |   `- qemu/
     |   |       `- qemu-i486/
     |   |           `- README.txt
     |   |- xtensa/
     |   |   `- esp32/
     |   |       `- esp32-core/
     |   |           `- README.txt
     |   |- z16/
     |   |   `- z16f2811/
     |   |       `- z16f2800100zcog/
     |   |           |- configs/nsh/README.txt
     |   |           |- configs/ostest/README.txt
     |   |           |- configs/pashello/README.txt
     |   |           `- README.txt
     |   |- z80/
     |   |   |- ez80/
     |   |   |   |- ez80f910200kitg/
     |   |   |   |   |- configs/ostest/README.txt
     |   |   |   |   `- README.txt
     |   |   |   |- ez80f910200zco/
     |   |   |   |   |- configs/dhcpd/README.txt
     |   |   |   |   |- configs/httpd/README.txt
     |   |   |   |   |- configs/nettest/README.txt
     |   |   |   |   |- configs/nsh/README.txt
     |   |   |   |   |- configs/poll/README.txt
     |   |   |   |   `- README.txt
     |   |   |   `- makerlisp/
     |   |   |       |- configs/nsh_flash/README.txt
     |   |   |       |- configs/nsh_ram/README.txt
     |   |   |       |- configs/sdboot/README.txt
     |   |   |       `- README.txt
     |   |   |- z180/
     |   |   |   `- p112/
     |   |   |       `- README.txt
     |   |   |- z8/
     |   |   |   |- z8encore000zco/
     |   |   |   |   |- configs/ostest/README.txt
     |   |   |   |   `- README.txt
     |   |   |   `- z8f64200100kit/
     |   |   |       |- configs/ostest/README.txt
     |   |   |       `- README.txt
     |   |   `- z80/
     |   |       `- z80sim/
     |   |           `- README.txt
     |   `-README.txt
     |- drivers/
     |   |- eeprom/
     |   |   `- README.txt
     |   |- lcd/
     |   |   | README.txt
     |   |   `- pcf8574_lcd_backpack_readme.txt
     |   |- mtd/
     |   |   `- README.txt
     |   |- sensors/
     |   |   `- README.txt
     |   |- syslog/
     |   |   `- README.txt
     |   `- README.txt
     |- fs/
     |   |- binfs/
     |   |   `- README.txt
     |   |- cromfs/
     |   |   `- README.txt
     |   |- mmap/
     |   |   `- README.txt
     |   |- nxffs/
     |   |   `- README.txt
     |   |- smartfs/
     |   |   `- README.txt
     |   |- procfs/
     |   |   `- README.txt
     |   |- spiffs/
     |   |   `- README.md
     |   `- unionfs/
     |       `- README.txt
     |- graphics/
     |   `- README.txt
     |- libs/
     |   |- README.txt
     |   |- libc/
     |   |   |- zoneinfo
     |   |   |   `- README.txt
     |   |   `- README.txt
     |   |- libdsp/
     |   |   `- README.txt
     |   |- libnx/
     |   |   |- nxfongs
     |   |   |   `- README.txt
     |   |   `- README.txt
     |   |- libxx/
     |   `- README.txt
     |- mm/
     |   |- shm/
     |   |   `- README.txt
     |   `- README.txt
     |- net/
     |   |- sixlowpan
     |   |   `- README.txt
     |   `- README.txt
     |- pass1/
     |   `- README.txt
     |- syscall/
     |   `- README.txt
     `- tools/
         `- README.txt
    
    Below is a guide to the available README files in the semi-optional apps/
    source tree:
    
    apps/
     |- examples/
     |   |- bastest/README.txt
     |   |- json/README.txt
     |   |- pashello/README.txt
     |   `- README.txt
     |- gpsutils/
     |   `- minmea/README.txt
     |- graphics/
     |   |- tiff/README.txt
     |   `- traveler/tools/tcledit/README.txt
     |- interpreters/
     |   |- bas/
     |   |  `- README.txt
     |   |- ficl/
     |   |  `- README.txt
     |   `- README.txt
     |- modbus/
     |   `- README.txt
     |- netutils/
     |   |- discover/
     |   |  `- README.txt
     |   |- ftpc/
     |   |  `- README.txt
     |   |- json/
     |   |  `- README.txt
     |   |- telnetd/
     |   |  `- README.txt
     |   `- README.txt
     |- nshlib/
     |   `- README.txt
     |- NxWidgets/
     |   `- README.txt
     |- system/
     |   |- cdcacm/
     |   |  `- README.txt
     |   |- i2c/
     |   |  `- README.txt
     |   |- inifile/
     |   |  `- README.txt
     |   |- install/
     |   |  `- README.txt
     |   |- nsh/
     |   |  `- README.txt
     |   |- nxplayer/
     |   |  `- README.txt
     |   |- psmq/
     |   |  `- README.txt
     |   |- symtab/
     |   |   `- README.txt
     |   |- termcurses/
     |   |   `- README.txt
     |   |- usbmsc/
     |   |  `- README.txt
     |   `- zmodem/
     |      `- README.txt
     `- wireless
         |- bluetooth/
         |  `- btsak/
         |     `- README.txt
         `- ieee802154
            `- i8sak/
               `- README.txt
    
    Additional README.txt files in the other, related repositories:
    
    NxWidgets/
     |- Doxygen
     |   `- README.txt
     |- tools
     |   `- README.txt
     |- UnitTests
     |   `- README.txt
     `- README.txt
    
    buildroot/
     `- README.txt
    
    tools/
     `- README.txt
    
    uClibc++/
     `- README.txt
    
    pascal/
     `- README.txt
    

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    三方开源软件NuttX

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