Question

I’m attempting to build the C++ Avro API as a DLL so I can use the Avro generated header files in a Visual Studio project. I’m able to build Avro in Windows using Cygwin, so the next step is to convert the Avro API into a DLL and use it along with the cygwin1.dll in my project (not entirely clear on how this would work either).

I’ve been looking at various online guides on how to build the DLL using cmake (by VTK) (since cmake is generally used to build Avro) and I wanted to make sure that what I’m attempting to do is actually feasible before spending a ton of time trying to do it. I’ve also noticed that the cygwin guide indicates that the gcc compiler can already build DLLs:

gcc -shared -o mydll.dll mydll.o

Generally in windows we have to specify which functions/classes needs to be exported by using the __declspec(dllexport) attribute where appropriate. Could anybody please clarify how it’s all supposed to come together:

1. In order to build my DLL with Cygwin, do I have to mark all of the functions/classes with the export attribute as shown in the VTK guide or can I just do something to tell the gcc compiler to output a DLL without using the export attributes (i.e. change the cmake options in the CMakeList.txt file)?
2. Once I generate a DLL from cygwin, then in my Visual Studio project I must reference both my.dll and the cygwin1.dll? Is it “that simple” or am I missing something?

If I have to mark all of the functions/classes in Avro with the export attribute, then I can imagine it will be a significantly more involved task since Avro has a decent amount of classes. If I don’t mark the functions/classes with the export attribute and I use GCC to output a DLL, then will the necessary export symbols be generated so they can be used in a VS project?

Answer 1

I made a sample project that attempts to generate the DLL and this is what I had in my CMakeLists.txt:

PROJECT(DLLTest)

# Allow the developer to select if Dynamic or Static libraries are built
OPTION (BUILD_SHARED_LIBS "Build Shared Libraries" ON)

# Set the LIB_TYPE variable to SHARED
SET (LIB_TYPE SHARED)

#IF (BUILD_SHARED_LIBS)
  # User wants to build Dynamic Libraries, so change the LIB_TYPE variable to CMake keyword 'SHARED'
#  SET (LIB_TYPE SHARED)
#ENDIF (BUILD_SHARED_LIBS)

# Create a target for the library
ADD_LIBRARY(DLLTest ${LIB_TYPE} hello.cpp)

SET( EXECUTABLE_OUTPUT_PATH "${CMAKE_BINARY_DIR}/bin" )
SET( LIBRARY_OUTPUT_PATH "${CMAKE_BINARY_DIR}/lib" )

The resulting output is a DLL (.dll) and a shared library (.a). However, I wasn’t able to successfully link with the DLL, although I should be able to according to an answer on the cygwin FAQ: http://cygwin.com/faq/faq.programming.html#faq.programming.msvs-mingw

The above link also provides instructions on how to generate the .def and the .lib (see the links for more info on why they’re needed).