WINDOWS BUILD NOTES
====================

Below are some notes on how to build Bitcoin Core for Windows.

Most developers use cross-compilation from Ubuntu to build executables for
Windows. This is also used to build the release binaries.

Building on Ubuntu Trusty 14.04 is recommended.
At the time of writing the Windows Subsystem for Linux installs Ubuntu Xenial 16.04. The default cross
compiler package for Ubuntu Xenial does not produce working executables for some of the bitcoin applications.
It is possible to build on Ubuntu Xenial by installing the cross compiler packages from
Ubuntu Zesty, see the steps below.
Building on Ubuntu Zesty 17.04 up to 17.10 has been verified to work.

While there are potentially a number of ways to build on Windows (for example using msys / mingw-w64),
using the Windows Subsystem For Linux is the most straightforward. If you are building with
another method, please contribute the instructions here for others who are running versions
of Windows that are not compatible with the Windows Subsystem for Linux.

Compiling with Windows Subsystem For Linux
-------------------------------------------

With Windows 10, Microsoft has released a new feature named the [Windows
Subsystem for Linux (WSL)](https://msdn.microsoft.com/commandline/wsl/about). This
feature allows you to run a bash shell directly on Windows in an Ubuntu-based
environment. Within this environment you can cross compile for Windows without
the need for a separate Linux VM or server.

This feature is not supported in versions of Windows prior to Windows 10 or on
Windows Server SKUs. In addition, it is available [only for 64-bit versions of
Windows](https://msdn.microsoft.com/en-us/commandline/wsl/install_guide).

Full instructions to install WSL are available on the above link.
To install WSL on Windows 10 with Fall Creators Update installed (version >= 16215.0) do the following:

1. Enable the Windows Subsystem for Linux feature
  * From Start, search for "Turn Windows features on or off" (type 'turn')
  * Select Windows Subsystem for Linux
  * Click OK
  * Restart if necessary
2. Install Ubuntu
  * Open Microsoft Store and search for Ubuntu or use [this link](https://www.microsoft.com/store/productId/9NBLGGH4MSV6)
  * Click Install
3. Complete Installation
  * Open a cmd prompt and type "Ubuntu"
  * Create a new UNIX user account (this is a separate account from your Windows account)

After the bash shell is active, you can follow the instructions below, starting
with the "Cross-compilation" section. Compiling the 64-bit version is
recommended but it is possible to compile the 32-bit version.

Cross-compilation
-------------------

These steps can be performed on, for example, an Ubuntu VM. The depends system
will also work on other Linux distributions, however the commands for
installing the toolchain will be different.

First, install the general dependencies:

    sudo apt install build-essential libtool autotools-dev automake pkg-config bsdmainutils curl git

A host toolchain (`build-essential`) is necessary because some dependency
packages (such as `protobuf`) need to build host utilities that are used in the
build process.

See also: [dependencies.md](dependencies.md).

## Building for 64-bit Windows

The first step is to install the mingw-w64 cross-compilation tool chain. Due to different Ubuntu
packages for each distribution and problems with the Xenial packages the steps for each are different.

Common steps to install mingw32 cross compiler tool chain:

    sudo apt install g++-mingw-w64-x86-64

Ubuntu Trusty 14.04:

    No further steps required

Ubuntu Xenial 16.04 and Windows Subsystem for Linux <sup>[1](#footnote1),[2](#footnote2)</sup>:

    sudo apt install software-properties-common
    sudo add-apt-repository "deb http://archive.ubuntu.com/ubuntu zesty universe"
    sudo apt update
    sudo apt upgrade
    sudo update-alternatives --config x86_64-w64-mingw32-g++ # Set the default mingw32 g++ compiler option to posix.

Ubuntu Zesty 17.04 <sup>[2](#footnote2)</sup>:

    sudo update-alternatives --config x86_64-w64-mingw32-g++ # Set the default mingw32 g++ compiler option to posix.

Once the tool chain is installed the build steps are common:

    PATH=$(echo "$PATH" | sed -e 's/:\/mnt.*//g') # strip out problematic Windows %PATH% imported var
    cd depends
    make HOST=x86_64-w64-mingw32
    cd ..
    ./autogen.sh # not required when building from tarball
    CONFIG_SITE=$PWD/depends/x86_64-w64-mingw32/share/config.site ./configure --prefix=/
    make

## Building for 32-bit Windows

To build executables for Windows 32-bit, install the following dependencies:

    sudo apt install g++-mingw-w64-i686 mingw-w64-i686-dev

For Ubuntu Xenial 16.04, Ubuntu Zesty 17.04 and Windows Subsystem for Linux <sup>[2](#footnote2)</sup>:

    sudo update-alternatives --config i686-w64-mingw32-g++  # Set the default mingw32 g++ compiler option to posix.

Then build using:

    PATH=$(echo "$PATH" | sed -e 's/:\/mnt.*//g') # strip out problematic Windows %PATH% imported var
    cd depends
    make HOST=i686-w64-mingw32
    cd ..
    ./autogen.sh # not required when building from tarball
    CONFIG_SITE=$PWD/depends/i686-w64-mingw32/share/config.site ./configure --prefix=/
    make

## Depends system

For further documentation on the depends system see [README.md](../depends/README.md) in the depends directory.

Installation
-------------

After building using the Windows subsystem it can be useful to copy the compiled
executables to a directory on the windows drive in the same directory structure
as they appear in the release `.zip` archive. This can be done in the following
way. This will install to `c:\workspace\bitcoin`, for example:

    make install DESTDIR=/mnt/c/workspace/bitcoin

Footnotes
---------

<a name="footnote1">1</a>: There is currently a bug in the 64 bit mingw-w64 cross compiler packaged for WSL/Ubuntu Xenial 16.04 that
causes two of the bitcoin executables to crash shortly after start up. The bug is related to the
-fstack-protector-all g++ compiler flag which is used to mitigate buffer overflows.
Installing the mingw-w64 packages from the Ubuntu 17 distribution solves the issue, however, this is not
an officially supported approach and it's only recommended if you are prepared to reinstall WSL/Ubutntu should
something break.

<a name="footnote2">2</a>: Starting from Ubuntu Xenial 16.04 both the 32 and 64 bit mingw-w64 packages install two different
compiler options to allow a choice between either posix or win32 threads. The default option is win32 threads which is the more
efficient since it will result in binary code that links directly with the Windows kernel32.lib. Unfortunately, the headers
required to support win32 threads conflict with some of the classes in the C++11 standard library in particular std::mutex.
It's not possible to build the bitcoin code using the win32 version of the mingw-w64 cross compilers (at least not without
modifying headers in the bitcoin source code).