Writing C++ Shared Libraries for Linux

I've recently been getting an old Windows-only module cross compiling for Linux. One of the challenges has been figuring out how to make the consuming project find & properly open the .so file. Most posts suggest that you must put your libraries in a specific folder, but a developer friend with stronger Linux kung-fu than mine confirmed that you can simply drop the .so files in the same directory as your application, & consume them using dlopen.

Here's a couple examples: first, a trivial hello world to show the basic consumption, and second, an example of what a reusable interface might look like. The latter shows how to use custom parameter & return types. I am new to designing C interfaces and deploying on Linux, so any comments are welcome.

Hello World from a Linux shared library:

	cmake_minimum_required (VERSION 3.13)
	project (ConsumingProject)
	add_executable (ConsumingProject "ConsumingProject.cpp" "ConsumingProject.h")
	if (UNIX)
	target_link_libraries(ConsumingProject "${CMAKE_DL_LIBS}") #Allows us to include dlfcn.h to link shared (dynamic) libraries
	endif()
	find_library(SHARED_LIB_LOCATION SharedLib PATHS ${CMAKE_BINARY_DIR}) #Assumes library's .so files have been copied into the same directory as this file
	target_link_libraries(ConsumingProject "${SHARED_LIB_LOCATION}")

view raw CMakeLists.txt (for consuming project) hosted with ❤ by GitHub

	cmake_minimum_required(VERSION 3.13) #3.13 was what I had on my WSL install.
	project(SharedLib VERSION 0)
	add_library(SharedLib SHARED "")
	set_target_properties(SharedLib PROPERTIES VERSION ${PROJECT_VERSION})
	set_target_properties(SharedLib PROPERTIES SOVERSION 1) #API Version
	target_sources(SharedLib
	PUBLIC
	SharedLib.h
	SharedLib.cpp
	InterfaceTypes.h
	)

view raw CMakeLists.txt (for SharedLib project) hosted with ❤ by GitHub

	#include "ConsumingProject.h"
	#include <dlfcn.h>
	#include <iostream>
	int main()
	{
	void* handle = dlopen("libSharedLib.so", RTLD_NOW);
	if (!handle)
	{
	std::cout << dlerror() << std::endl;
	}
	void (*HelloWorld)() = (void (*)())dlsym(handle, "HelloWorld");
	char* error = dlerror();
	if (nullptr != error)
	{
	std::cout << error << std::endl;
	}
	HelloWorld();
	dlclose(handle);
	return 0;
	}

view raw ConsumingProject.cpp hosted with ❤ by GitHub

	#include "SharedLib.h"
	#include <iostream>
	void HelloWorld()
	{
	std::cout << "Hello World" << std::endl;
	}

view raw SharedLib.cpp hosted with ❤ by GitHub

	#pragma once
	extern "C"
	{
	void HelloWorld();
	}

view raw SharedLib.h hosted with ❤ by GitHub

Passing an interface from a Linux shared library:

	cmake_minimum_required (VERSION 3.13)
	project (ConsumingProject)
	add_executable (ConsumingProject "ConsumingProject.cpp" "ConsumingProject.h")
	if (UNIX)
	target_link_libraries(ConsumingProject "${CMAKE_DL_LIBS}") #Allows us to include dlfcn.h to link shared (dynamic) libraries
	endif()
	find_library(SHARED_LIB_LOCATION SharedLib PATHS ${CMAKE_BINARY_DIR}) #Assumes library's .so files have been copied into the same directory as this file
	target_link_libraries(ConsumingProject "${SHARED_LIB_LOCATION}")

view raw CMakeLists.txt (for consuming project) hosted with ❤ by GitHub

	cmake_minimum_required(VERSION 3.13) #3.13 was what I had on my WSL install.
	project(SharedLib VERSION 0)
	add_library(SharedLib SHARED "")
	set_target_properties(SharedLib PROPERTIES VERSION ${PROJECT_VERSION})
	set_target_properties(SharedLib PROPERTIES SOVERSION 1) #API Version
	target_sources(SharedLib
	PUBLIC
	SharedLib.h
	SharedLib.cpp
	InterfaceTypes.h
	)

view raw CMakeLists.txt (for SharedLib project) hosted with ❤ by GitHub

	#include "ConsumingProject.h"
	#include "InterfaceTypes.h"
	#include <dlfcn.h>
	#include <iostream>
	int main()
	{
	void* handle = dlopen("libSharedLib.so", RTLD_NOW);
	if (!handle)
	{
	std::cout << dlerror() << std::endl;
	}
	MyInterface theInterface;
	theInterface.GetCustomReturnType = (MyCustomReturnType(*)(MyCustomParameterType))dlsym(handle, "GetCustomReturnType");
	MyCustomParameterType param = 1;
	MyCustomReturnType returnStruct = theInterface.GetCustomReturnType(param);
	char* error = dlerror();
	if (nullptr != error)
	{
	std::cout << error << std::endl;
	}
	else
	{
	std::cout << "Return value: " << returnStruct.Result << std::endl;
	}
	dlclose(handle);
	return 0;
	}

view raw ConsumingProject.cpp hosted with ❤ by GitHub

	#pragma once
	extern "C"
	{
	typedef long MyCustomParameterType;
	typedef struct MyCustomReturnType
	{
	long Result;
	} MyCustomReturnType;
	typedef struct MyInterface
	{
	MyCustomReturnType (*GetCustomReturnType)(MyCustomParameterType);
	} MyInterface;
	}

view raw InterfaceTypes.h hosted with ❤ by GitHub

	#include "SharedLib.h"
	#include <iostream>
	MyCustomReturnType GetCustomReturnType(MyCustomParameterType param)
	{
	std::cout << "Param value: " << param << std::endl;
	MyCustomReturnType returnStruct;
	returnStruct.Result = 123;
	return returnStruct;
	}

view raw SharedLib.cpp hosted with ❤ by GitHub

	#pragma once
	#include "InterfaceTypes.h"
	extern "C"
	{
	MyCustomReturnType GetCustomReturnType(MyCustomParameterType param);
	}

view raw SharedLib.h hosted with ❤ by GitHub