How to generate import library for MinGW

Microsoft Windows has this thing called "import libraries". You don't need it in MinGW because the ld linker from GNU Binutils is smart, but you may still want it for whatever reason.

Make the .def

Import libraries are compiled from a list of what symbols to use, .def. This should be already in your exports directory: cd OPENBLAS_TOP_DIR/exports.

Making a MinGW import library

MinGW import libraries have the suffix .a, same as static libraries. (It's actually more common to do .dll.a...)

You need to first prepend libopenblas.def with a line LIBRARY libopenblas.dll:

cat <(echo "LIBRARY libopenblas.dll") libopenblas.def > libopenblas.def.1
mv libopenblas.def.1 libopenblas.def

Now it probably looks like:

LIBRARY libopenblas.dll
EXPORTS
   caxpy=caxpy_  @1
   caxpy_=caxpy_  @2
       ...

Then, generate the import library: dlltool -d libopenblas.def -l libopenblas.a

Again, there is basically no point in making an import library for use in MinGW. It actually slows down linking.

Making a MSVC import library

Unlike MinGW, MSVC absolutely requires an import library. Now the C ABI of MSVC and MinGW are actually identical, so linking is actually okay. (Any incompatibility in the C ABI would be a bug.)

The import libraries of MSVC have the suffix .lib. They are generated from a .def file using MSVC's lib.exe. See the MSVC instructions.

Notes

• Always remember that MinGW is not the same as MSYS2 or Cygwin. MSYS2 and Cygwin are full POSIX environments with a lot of magic such as fork() and its own malloc(). MinGW, which builds on the normal Microsoft C Runtime, has none of that. Be clear about which one you are building for.