CMake Beginner's Question: project with 3rd party library gets not compiled - c++

Question

So I am struggeling with cmake for some time now. I want to use the xmlrpc-c library from here. So I started a new project with main.cpp and CMakeLists.txt and copied the xmlrpc-c as a subdirectory into my project (since xmlrpc-c is unfortunately not a cmake library):
My code is exactly a example from here and looks like this:
#include <iostream>
#include <string>
#include "xmlrpc-c/include/xmlrpc-c/base.hpp"
#include "xmlrpc-c/include/xmlrpc-c/registry.hpp"
#include "xmlrpc-c/include/xmlrpc-c/server_abyss.hpp"
using namespace std;
class hello : public xmlrpc_c::method
{
public:
void execute(const xmlrpc_c::paramList& params, xmlrpc_c::value* retval)
{
string msg(params.getString(0));
params.verifyEnd(1);
cout << msg << endl;
*retval = xmlrpc_c::value_string("XMLRPC server says hello!");
}
};
int main(int argc, char** argv)
{
xmlrpc_c::registry registry;
registry.addMethod("hello", new hello);
xmlrpc_c::serverAbyss server(xmlrpc_c::serverAbyss::constrOpt().registryP(&registry).portNumber(8080));
server.run();
return 1;
}
CMakeLists.txt looks like this
cmake_minimum_required(VERSION 3.16)
project(xmlrpc_c_server C CXX)
add_executable(xmlrpc_c_server main.cpp)
target_link_libraries(xmlrpc_c_server -lxmlrpc++ -lxmlrpc_server++ -lxmlrpc_server_abyss++ -lxmlrpc_util++)
The problem I have is that my build-process fails with a linker-error: as far as I understand is the header file registry.hpp not included correctly. If I comment out the code line registry.addMethod("hello", new hello);, I can compile the program without any errors.
====================[ Build | xmlrpc_c_server | Debug ]=========================
/usr/bin/cmake --build /mnt/c/Users/valentin.ackva/CLionProjects/xmlrp-c-server/cmake-build-debug --target xmlrpc_c_server -- -j 9
Scanning dependencies of target xmlrpc_c_server
[ 50%] Building CXX object CMakeFiles/xmlrpc_c_server.dir/main.cpp.o
[100%] Linking CXX executable xmlrpc_c_server
/usr/bin/ld: CMakeFiles/xmlrpc_c_server.dir/main.cpp.o: in function `main':
/mnt/c/Users/struppel/CLionProjects/xmlrp-c-server/main.cpp:31: undefined reference to `xmlrpc_c::registry::addMethod(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, xmlrpc_c::method*)'
collect2: error: ld returned 1 exit status
make[3]: *** [CMakeFiles/xmlrpc_c_server.dir/build.make:84: xmlrpc_c_server] Error 1
make[2]: *** [CMakeFiles/Makefile2:76: CMakeFiles/xmlrpc_c_server.dir/all] Error 2
make[1]: *** [CMakeFiles/Makefile2:83: CMakeFiles/xmlrpc_c_server.dir/rule] Error 2
make: *** [Makefile:118: xmlrpc_c_server] Error 2
What is missing?

Answer 1

Since the library you want to use doesn't have a cmake project you need to handle this on your own. I would suggest using add_subdirectory and creating a sub-project, where you build the library (either as shared or static depending on your needs).
In addition you need to point cmake to the location of the headers. Try manually adding what's missing to the add_executable or use include_directories and adjust your #includes accordingly.
You can also use some unofficial cmake version of it like this one. If you are using git you can add the repo as a submodule and integrate the code from that repo into your main project.

Answer 2

Since you are new to CMake maybe it's worth looking at a tutorial instead?
https://cliutils.gitlab.io/modern-cmake/
I think it would help your confusion a lot.