Write an Objective C wrapper class for your C library functions.
You can make the functions into Objective C class methods, if there is no resonable mappiing to object instance methods.
I had a fair bit of Objective C++ code in my app and there is no wrapper support for C++.
I recommend not to use Swift if you intend to use C or C++. It is just troublesome, use Objective-C or Objective-C++ instead and you do not have to write (a lot of) wrappers. Just under very special circumstances you may need a wrapper.