Building libpqxx on MSYS2 MinGW 64 bit

This walkthrough explains how to build the shared library (dll) for libpqxx on MSYS2.

1) Install PostgreSQL

Download the latest PostgreSQL installer. Remember to choose the 64-bit version. This walkthrough uses postgresql-10.3-3-windows-x64.exe. The installation is trivial.

2) Get the libpqxx source code

According to the wiki, the recommended method is to get the source from GitHub: git clone https://github.com/jtv/libpqxx. The revision used here is 120ba161b928fcb5c0a6c6a78370cb21f2e3c6d6.

3) Configure

1. In the MSYS2 shell, set the path to pg_config: export PATH=$PATH:"/c/Progra~1/PostgreSQL/10/bin". Take note of the use of Progra~1, this is because spaces don't play well with the provided configure script.
2. Run ./configure --enable-shared=yes --enable-static=no. It should complete without errors.

4) Tweak the <thread> C++ header

I didn't bother to find out why, but if you run make now, compilation will fail for util.cxx with something like the following:
In file included from util.cxx:17:0:
C:/msys64/mingw64/include/c++/7.3.0/thread: In function 'bool std::operator==(std::thread::id, std::thread::id)':
C:/msys64/mingw64/include/c++/7.3.0/thread:276:26: error: no match for 'operator==' (operand types are 'std::thread::native_handle_type {aka ptw32_handle_t}' and 'std::thread::native_handle_type {aka ptw32_handle_t}')
     return __x._M_thread == __y._M_thread;
            ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~
<---snip--->

In file included from util.cxx:17:0:
C:/msys64/mingw64/include/c++/7.3.0/thread: In function 'bool std::operator<(std::thread::id, std::thread::id)':
C:/msys64/mingw64/include/c++/7.3.0/thread:288:26: error: no match for 'operator<' (operand types are 'std::thread::native_handle_type {aka ptw32_handle_t}' and 'std::thread::native_handle_type {aka ptw32_handle_t}')
     return __x._M_thread < __y._M_thread;
            ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~
<---snip--->


The workaround is to edit the header file C:/msys64/mingw64/include/c++/7.3.0/thread. Comment out the offending functions:
//   inline bool
//   operator==(thread::id __x, thread::id __y) noexcept
//   {
//     // pthread_equal is undefined if either thread ID is not valid, so we
//     // can't safely use __gthread_equal on default-constructed values (nor
//     // the non-zero value returned by this_thread::get_id() for
//     // single-threaded programs using GNU libc). Assume EqualityComparable.
//     return __x._M_thread == __y._M_thread;
//   }

  inline bool
  operator!=(thread::id __x, thread::id __y) noexcept
  { return !(__x == __y); }

//   inline bool
//   operator<(thread::id __x, thread::id __y) noexcept
//   {
//     // Pthreads doesn't define any way to do this, so we just have to
//     // assume native_handle_type is LessThanComparable.
//     return __x._M_thread < __y._M_thread;
//   }


Save the changes. Remember to revert them later!

5) Add the -no-undefined flag for libtool

If you run make now, another error will occur:
libtool: error: can't build x86_64-w64-mingw32 shared library unless -no-undefined is specified
Manually edit the makefile in src/ (not the top level one!):
LDFLAGS = -LC:/Progra~1/POSTGR~1/10/lib -no-undefined

6) make

Now you can run make. The dll should build without further errors and can be found at src/.libs/libpqxx-6-2.dll. Congratulations! Note that in order to use the library, you still need to link against libpq.dll, which can be found in C:\Program Files\PostgreSQL\10\lib.

UPDATE: Having trouble with the DLL? Segmentation fault? Read this post regarding the use of libpq from the Windows installer.