Cross compiling Windows binaries from Linux

Recently I was helping out a friend with some C/C++ programming work for one of his classes. One issue was that his university uses Microsoft Visual Studio as their compiler which causes a bit of a problem if you don’t have that installed and want to make sure a particular bit of code actually compiles.

Now I could have installed Visual Studio Express Edition (thankfully it seems Microsoft has had a change of heart and decided that you can keep developing desktop apps with the latest 2012 release), though I have rather limited space on my Windows desktop as it’s mostly reserved for playing games 😁. Thus I decided to work out how hard it is to cross compile Windows binaries from a Linux machine.

Thankfully, due to the existence of MinGW-w64, the answer turns out to be “not too hard”. First off, on Debian based systems, you’ll require the mingw-w64 package to be installed. This will install the C compiler as i686-w64-mingw32-gcc and x86_64-w64-mingw32-gcc for 32-bit and 64-bit Windows binaries respectively.

Now since Windows isn’t a Unix operating system, it doesn’t have any of the standard headers you might expect, instead MingGW-w64 provides a series of Windows headers and libraries which can be found in either /usr/i686-w64-mingw32/ or /usr/x86_64-w64-mingw32/. These directories will automatically be included by the compiler, so you shouldn’t need to manually specify the include or library directories with -I or -L.

From this point it should just be a case of running something like i686-w64-mingw32-gcc -o prog.exe prog.c -lwsock32 to get a binary you can happily run on a Windows machine without any further problems.

Here’s some hints I found while porting an program from Unix to the Win32 API:

• You can use #ifdef _WIN32 to check if you’re compiling for the Windows API (defined for both the 32-bit and 64-bit compilers).
• Include winsock2.h for access to the socket API. The POSIX headers don’t exist, but most of the standard functions are available.
• Use WSAGetLastError instead of errno when checking why a socket function failed. While the errno.h header exists, errno will always be set to 0.
• You must call WSAStartup before you call any socket functions, otherwise they’ll return an error and WSAGetLastError will return WSANOTINITIALISED (10093). At very least you must do something like:

  // Initialize WinSock API
  WORD version = MAKEWORD(2, 2);
  WSADATA wsaData;
  int err = WSAStartup(version, &wsaData);
  if (err != 0) {
      // Fatal error
      fprintf(stderr, "ERROR: WSAStartup (%d)\n", WSAGetLastError());
      exit(1);
  }
  

• You’ll need to use the Windows Desktop API for everything (no pthreads and such). MingGW-w64 does not provide a POSIX compatibility environmental like Cygwin or Windows Subsystem for Linux (WSL).