MyCFunction.c

// a global function; no need to expose this in a header file

void internalDoSomething()
{
	// put your function here.
}

// note that if you were using C++ or Objective-C++ you'd need to put this in an extern closure, like so:
extern "C"
{
	void internalDoSomething()
	{
		// ...
	}
}

MyCFunction.cs

// In a Unity C# class:

public class MyCFunction {
	public static void DoSoemthing() {
		internalDoSomething();
	}
	
	[DllImport ("__Internal")]
	private static extern void internalDoSomething();
}