# Step 10: Selecting Static or Shared Libraries

In this section we will show how the `BUILD_SHARED_LIBS` variable can be used to control the default behavior of `add_library()`, and allow control over how libraries without an explicit type (`STATIC`, `SHARED`, `MODULE` or `OBJECT`) are built.

To accomplish this we need to add `BUILD_SHARED_LIBS` to the top-level `CMakeLists.txt`. We use the `option()` command as it allows users to optionally select if the value should be `ON` or `OFF`.

CMakeLists.txt

    option(BUILD_SHARED_LIBS "Build using shared libraries" ON)

Next, we need to specify output directories for our static and shared libraries.

CMakeLists.txt

    set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY "${PROJECT_BINARY_DIR}")
    set(CMAKE_LIBRARY_OUTPUT_DIRECTORY "${PROJECT_BINARY_DIR}")
    set(CMAKE_RUNTIME_OUTPUT_DIRECTORY "${PROJECT_BINARY_DIR}")

    option(BUILD_SHARED_LIBS "Build using shared libraries" ON)

Finally, update `MathFunctions/MathFunctions.h` to use dll export defines:

MathFunctions/MathFunctions.h

    #if defined(_WIN32)
    #  if defined(EXPORTING_MYMATH)
    #    define DECLSPEC __declspec(dllexport)
    #  else
    #    define DECLSPEC __declspec(dllimport)
    #  endif
    #else // non windows
    #  define DECLSPEC
    #endif

    namespace mathfunctions {
    double DECLSPEC sqrt(double x);
    }

At this point, if you build everything, you may notice that linking fails as we are combining a static library without position independent code with a library that has position independent code. The solution to this is to explicitly set the `POSITION_INDEPENDENT_CODE` target property of SqrtLibrary to be `True` when building shared libraries.

MathFunctions/CMakeLists.txt

      # state that SqrtLibrary need PIC when the default is shared libraries
      set_target_properties(SqrtLibrary PROPERTIES
                            POSITION_INDEPENDENT_CODE ${BUILD_SHARED_LIBS}
                            )

Define `EXPORTING_MYMATH` stating we are using `declspec(dllexport)` when building on Windows.

MathFunctions/CMakeLists.txt

    # define the symbol stating we are using the declspec(dllexport) when
    # building on windows
    target_compile_definitions(MathFunctions PRIVATE "EXPORTING_MYMATH")

Exercise: We modified `MathFunctions.h` to use dll export defines. Using CMake documentation can you find a helper module to simplify this?