Selecting Static or Shared Libraries — CMake 4.0.2 Documentation (original) (raw)

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.

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?