MinGW

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MinGW is a GCC-based compiler toolchain targeting Windows platforms. It is used to produce the Windows releases of Unvanquished. It can be quite complicated to use successfully, in part due to the existence of different compiler "flavors", and confusing naming schemes. This page serves as a guide to both MinGW in general, and specifically in application to building Daemon and Unvanquished. Throughout this page, "MinGW" will be used to refer to the mingw-w64 project, which is a fork of the (now long-moribund) "original MinGW" project aiming to support both 32- and 64-bit builds. So beware: something having "64" in its name does not at all imply it is for 64-bit binaries! Same goes for "32".

Flavors

Bitness: 32-bit or 64-bit

Whether to compile for 32-bit ("i686" in package names) or 64-bit ("x86-64" or "x86_64") architecture. 64-bit Windows versions are capable of running 32-bit executables in a mode called WOW (Windows-on-Windows).

Threading flavor

You can get win32 or posix thread models. Daemon must be built with the posix flavor, because code that uses std::thread does not compile with win32. In general there is no reason to use win32. However, it is unfortunately the default in the APT package manager.

Exception handling flavor

  • sjlj (setjmp/longjmp) is available for both 32- and 64-bit builds. It has the drawback of significant runtime cost.
  • dwarf is available only for 32-bit. It has good performance, but it is said to have a drawback that exceptions can't be propagated through code compiled with a different compiler (or C code?). However, propagating exceptions through C code shouldn't be needed for Daemon.
  • seh (Structured Exception Handling) is available only for 64-bit. It has good performance and always works correctly, so it should always be used for 64-bit.

One way to identify which exception model code was built with is to check the libgcc dependency. You'll find for example libgcc_s_seh-1.dll for SEH, libgcc_s_dw2-1.dll for DWARF, or libgcc_s_sjlj-1.dll for sjlj. An easy way to make a C++ test program emit a libgcc dependency is to add a try/catch block.

Mixing flavors

Mixing different thread or exception flavors can cause problems, such as missing DLL dependencies, or exception handling not working.

For the external_deps libraries, efforts have been made to avoid depending on any thread-flavor-specific or exception-flavor-specific libraries, so it should be possible to use them with any flavor of MinGW. Exception handling glitches shouldn't be an issue since the Windows depencies are C-only.

Distributions

Cross-compiling from Linux

Most Linux package managers offer some form of MinGW package. Availability of different flavor combinations varies.

Installation

APT (Debian Buster or older):

# 32-bit, sjlj. win32 threads are selected by default; switch it to posix
sudo apt install g++-mingw-w64-i686  
sudo update-alternatives --set i686-w64-mingw32-gcc /usr/bin/i686-w64-mingw32-gcc-posix
sudo update-alternatives --set i686-w64-mingw32-g++ /usr/bin/i686-w64-mingw32-g++-posix

# 64-bit, SEH. win32 threads are selected by default; switch it to posix
sudo apt install g++-mingw-w64-x86-64  
sudo update-alternatives --set x86_64-w64-mingw32-gcc /usr/bin/x86_64-w64-mingw32-gcc-posix
sudo update-alternatives --set x86_64-w64-mingw32-g++ /usr/bin/x86_64-w64-mingw32-g++-posix

APT (Debian Bullseye or newer):

  • sudo apt install gcc-mingw-w64-i686-posix (32-bit, posix, sjlj)
  • sudo apt install gcc-mingw-w64-x86-64-posix (64-bit, posix, SEH)

Usage

To build a CMake project, you need a "toolchain file". Sadly, there is no official toolchain file from MinGW or CMake. For Daemon and Unvanquished we use this (32-bit) and this (64-bit). Use it by passing -DCMAKE_TOOLCHAIN_FILE=<path> on the cmake command line.

MSYS2

MSYS2 runs on Windows hosts and provides up-to-date 32-bit/DWARF/posix and 64-bit/SEH/posix toolchains. Building is done from a Bash shell in a partial emulation of a Linux environment, similar to Cygwin.

Installation

Download from [1].

Install dependencies to build Daemon (can be done from any MSYS2 shell flavor):

# 32-bit
pacman -Sy mingw-w64-i686-gcc mingw64-i686-cmake make
# 64-bit
pacman -Sy mingw-w64-x86_64-gcc mingw64-x86_64-cmake make

Usage

You must open a different MSYS2 shell flavor according to which architecture you want to target: "MSYS2 MinGW 32-bit" or "MSYS2 MinGW 64-bit". When invoking cmake, you must pass the flag -G"MSYS Makefiles". When building Unvanquished, the flag -DDAEMON_CBSE_PYTHON_PATH=<path> may be of interest, if you want to use a Python other than the one shipped with MSYS2. <path> should be in the Unix-emulation format, e.g. /c/Python/python.exe.

"MinGW-W64-builds" (Standalone Windows version)

This distribution (see Sourceforge) provides a toolchain that can be used from the Windows command prompt, without needing a Unix emulation layer. The installer allows you to choose from every valid combination of flavors. Caveat: the builds are currently somewhat outdated. At the time of writing (Febrary 2021), the newest available gcc version is 8.1.0.

Usage

Choose the MinGW Makefiles generator in CMake. The location of the toolchain used is determined by finding the first one in your PATH environment variable. Relative to the installation root, the PATH entry should be at mingw32\bin or mingw64\bin.

After running CMake, compile by running mingw32-make in the build directory.

Work around linker error with difftime

To build Unvanquished 0.52 or higher, a small hack is needed to avoid a linker error, because Lua depends on an MSVCRT function which was not added to MinGW's import lib yet as of 8.1.0. Since you are on Windows, you can link to mscvrt.dll directly. In the CMake GUI, check "Advanced" and find the variable CMAKE_CXX_STANDARD_LIBRARIES. Add  C:/windows/system32/msvcrt.dll at the end of the list (or  C:/windows/syswow64/msvcrt.dll if you are building a 32-bit binary on a 64-bit system).

Built-in DLL dependencies

There are a few DLLs shipped as part of MinGW that most programs will depend on. These are also available as static libraries (a fully static binary can be produced with the -static flag), but dynamic is the default. These are:

  • libgcc
  • libstdc++ (for C++ programs)
  • libssp (if stack protection is enabled)
  • libwinpthread (if threads are used)

libstdc++ is both thread- and exception-flavor-dependent, but unlike libgcc, the DLL does not have the flavor written in the filename. This means that you can't easily tell if you have the right one (nor can the Windows executable loader). Dependencies is a good tool to investigate such issues.

When distributing a binary release of Daemon, you must ship these DLLs alongside the executable, else you will run into this issue. Note that unlike Daemon's other DLL dependencies (from the external_deps package), the MinGW DLLs are not automatically copied to the build directory as part of the build process.

C ABI compability with MSVC

On 32-bit, GCC has a different default stack alignment (16 bytes) than MSVC (4 bytes). This can cause problems when MSVC-compiled code calls into GCC-compiled code and the alignment is less than expected. One solution, which we use to build MSVC-compatible external_deps DLLs, is to use a GCC flag to change the presumed stack alignment. Another, which we use to handle jumps from 3rd-party binaries to our (MinGW-built) code, is to put the annotation __attribute__((force_align_arg_pointer)) on the functions which may be called with less alignment than expected, instructing the compiler to perform alignment when the function is called.

The default bitfield layout may also be different.