How to generate a DLL linked to a static library with CMake

Question

The main objective with this question is to write an CMakeLists.txt to generate a dynamic library, "containing/linked" a static library.

Let me create the scenario:

• My C++ code is written in mycode.cpp
• In mycode.cpp, I call some functions from libthirdparty.a (static library)
• I want to generate libmylib.so (shared library) to be dynamically linked by others
• libmylib.so must to "contain" libthirdparty.a

My attempt to write this script is at the lines bellow:

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -Wall -Werror -m64 -fPIC ")
add_executable(myapp mycode.cpp)
target_link_libraries(myapp thirdparty)
add_library(mylib SHARED myapp)

But of course this is not working and I would like some help to write it correctly.

Answer 1

For now, let's remove the myapp and focus only on the library you are trying to create.

That said, here is what you could do

cmake_minimum_required(VERSION 3.12)

project(AwesomeLib)

include(GenerateExportHeader)

set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(CMAKE_CXX_STANDARD 11)
set(CMAKE_POSITION_INDEPENDENT_CODE ON)

find_package(ThirdParty REQUIRED)

add_library(mylib SHARED mylib.cpp)
target_link_libraries(mylib PUBLIC ThirdParty)

# Note: If you library will be cross-platform, flag should be conditionally specified. Have a look at generator-expression
target_compile_options(mylib PRIVATE -Wall -Werror)

generate_export_header(mylib)

# TODO:
# * add install rules
# * generate config-file package
# * add tests

Notes:

• generate_export_header will generate "mylib_export.h" header with the MYLIB_EXPORT macro for exporting symbols. See https://cmake.org/cmake/help/latest/module/GenerateExportHeader.html
• to understand the idea behind find_package(ThirdParty REQUIRED), I recommend you read config-file package. See Correct way to use third-party libraries in cmake project
• to learn more about generator expression. See https://cmake.org/cmake/help/latest/manual/cmake-generator-expressions.7.html