APACHE NUTTX (INCUBATING)
^^^^^^^^^^^^^^^^^^^^^^^^^

  o Introduction
    - Incubation Status
  o Community
    - Getting Help
    - Mailing Lists
    - Issue Tracker
    - Source Code
    - Website Source Code
  o Environments
    - Installing Cygwin
    - Ubuntu Bash under Windows 10
    - Using macOS
  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

INTRODUCTION
^^^^^^^^^^^^

  Apache NuttX (Incubating) is a real-time operating system (RTOS) with an
  emphasis on standards compliance and small footprint.  Scalable from 8-bit
  to 32-bit microcontroller environments, the primary governing standards in
  NuttX are POSIX and ANSI standards. Additional standard APIs from Unix and
  other common RTOSs (such as VxWorks) are adopted for functionality not
  available under these standards, or for functionality that is not
  appropriate for deeply-embedded environments (such as fork()).

  Extensive documentation can be found on the project wiki:

    https://cwiki.apache.org/NUTTX/Nuttx

Incubation Status
-----------------

  Apache NuttX (Incubating) is an effort undergoing Incubation at The Apache
  Software Foundation (ASF), sponsored by the Incubator.  For more on our
  incubation effort, please see the file DISCLAIMER-WIP, in the same
  directory as this README.

  For brevity, the rest of this file will refer to it as Apache NuttX or
  simply NuttX.

COMMUNITY
^^^^^^^^^

  Every volunteer project obtains its strength from the people involved in
  it.  We invite you to participate as much or as little as you choose.

  We encourage you to:

  - Use our project and provide feedback.
  - Provide us with use-cases.
  - Report bugs and submit patches.
  - Contribute code or documentation.

Getting Help
------------

  The best place to get help is the developer's mailing list.  Please see
  the following section:

Mailing Lists
-------------

  Get help using NuttX or contribute to the project on our mailing lists:

  [email protected] is for people who want to contribute code to NuttX.

  - To subscribe, send an email to [email protected].
  - To unsubscribe, send an email to [email protected].
  - View the archives at:

      https://www.mail-archive.com/[email protected]/

  [email protected] is a read-only list that notifies subscribers
  about commit messages and patches to NuttX.

  - To subscribe, send an email to [email protected].
  - To unsubscribe, send an email to [email protected].
  - View the archives at:

      https://www.mail-archive.com/[email protected]/

Issue Tracker
-------------

  Bug Reports:

  Found bug? Send an email to the dev list [email protected].

  Before submitting an issue, please:

  - Verify that the bug does in fact exist.
  - Search the mailing list archives to verify there is no existing issue
    reporting the bug you've found.
  - Consider tracking down the bug yourself in the NuttX source code and
    submitting a patch along with your bug report.  This is a great time
    saver for the NuttX developers and helps ensure the bug will be fixed
    quickly.

  Feature Requests:

  Enhancement requests for new features are also welcome. The more concrete
  and rational the request is, the greater the chance it will incorporated
  into future releases.

Source Code
-----------

  The project sources are in two Git repositories.  The core OS is in
  incubator-nuttx and the apps repository is in incubator-nuttx-apps.  These
  are housed in GitBox on ASF servers and also mirrored at GitHub.  These
  are kept in sync, so you can use whichever option you prefer.

  - NuttX core OS repository:

      Primary:
      https://gitbox.apache.org/repos/asf?p=incubator-nuttx.git

      GitHub Mirror:
      https://github.com/apache/incubator-nuttx

  - Apps repository:

      Primary:
      https://gitbox.apache.org/repos/asf?p=incubator-nuttx-apps.git

      GitHub Mirror:
      https://github.com/apache/incubator-nuttx-apps

Website Source Code
-------------------

  The project website sources are accessible via the website source code
  repository which is also mirrored in GitHub:

      Primary:
      https://gitbox.apache.org/repos/asf?p=incubator-nuttx-website.git

      GitHub Mirror:
      https://github.com/apache/incubator-nuttx-website

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.  Those 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.

Using macOS
-----------

  You need to install at least the following tools specific to macOS.

  * flock (used by APPDIR build logic)

    A macOS port is available at: https://github.com/discoteq/flock

      brew tap discoteq/discoteq
      brew install flock

  If you want to build the sim:

    * Xcode (the native compiler and the rest of the toolchain)

    * ELF toolchain (if you want to build modules for CONFIG_LIBC_MODLIB)

        brew install x86_64-elf-gc

INSTALLATION
^^^^^^^^^^^^

  There are two ways to get NuttX:  You may download released, stable
  tarballs from either the project website.  Or you may get NuttX by
  cloning the 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 location:

    https://nuttx.apache.org/download/

  Legacy 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://gitbox.apache.org/repos/asf/incubator-nuttx.git nuttx
      -or-
      git clone https://github.com/apache/incubator-nuttx.git nuttx

    And the semi-optional apps/ application directory and be cloned like:

      git clone https://gitbox.apache.org/repos/asf/incubator-nuttx-apps.git apps
      -or-
      git clone https://github.com/apache/incubator-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 [email protected]

    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://gitbox.apache.org/repos/asf/incubator-nuttx-apps
    or
    https://github.com/apache/incubator-nuttx-apps.git

    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.

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 the old http://ymorin.is-a-geek.org/projects/kconfig-frontends
  which is no longer available.)  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 can use the modified version of kconfig-frontends
     that can be found at
     http://uvc.de/posts/linux-kernel-configuration-tool-kconfig-under-windows.html
     or a more recent port that can be found at
     http://reclonelabs.com/more-kconfig-awesomeness-for-windows/.

  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 -c 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 are two specially modified versions of the kconfig-frontends
  tools that can be used.  One can be found here:
  http://uvc.de/posts/linux-kernel-configuration-tool-kconfig-under-windows.html

  The configuration steps of the 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).

  NOTE:  It should be possible to use Cygwin or MSYS2 in place of the GNUWin32
  tools.  There are, however, complexities in doing that because those tools
  depend on the shell environment and use DLLs that are not found (at least
  not without the correct setup).

  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.

  Kconfig-frontends:  See the section entitled "NuttX Configuration Tool
  under DOS" for information about installing the kconfig-frontend tools to
  run natively under Windows.

  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.

  9. Make sure that you add the GNUWin32 tools to your path variable:

    set PATH=C:\gnuwin32\bin;%PATH%

  WARNING:  Make sure you have C:\MinGW\bin in your path before any other
  directory that contains libiconv-2.dll. Apparently the as.exe in some
  MinGW distributions are dependent on that DLL, and having an old
  version of it in the path somewhere (for example GnuWin32 tools) will
  cause as.exe to pick up the older version that doesn't have the entry
  point it's looking for.

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
 |- 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/
 |   |   `- m9s12/
 |   |       |- 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/
 |   |   |- m16c/
 |   |   |   `- skp16c26/
 |   |   |       `- README.txt
 |   |   `-sh1/
 |   |       `- 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/
 |   |   `- z16f/
 |   |       `- 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
 |   |   |   `- z80x/
 |   |   |       |- 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