One of the primary uses of objects is to implement plugins. A plugin is a dynamic load library or DLL, also called a shared library on Unix platforms.

Felix can load DLLs at run time. However plugins are special DLLs with a specific structure.

Loading Plugins

To load a plugin named “ob_implementation” we do this:

var joe =
     ( dll-name="ob_implementation", setup-str="")

The load-plugin-func1 loads a plugin with DLL basename “ob_implementation”, initialising global memory by calling a function setup_ob_implementation and passing it the empty string “”, then it calls the entry-point function ob_implementation which accepts a value of type string and returns a value of type ob_t.

The DLL will be searched for on the standard OS search path, given by the environment variables LD_LIBRARY_PATH on Linux, DYLD_LIBRRY_PATH on MacOSX, and PATH on Windows. The standard extension for each platform is appended, that will be .so on Linux, .dylib on MacOSX, and .dll on Windows.

The load operation uses a wrapper around the OS load function: dlopen on Linux and MacOSX, and LoadLibrary on Windows.

The system then uses the symbol finding function to find setup. It uses dlsym on Linux and MacOSX and LoadAddress on Windows. It must be a C function, not a C++ function, and it must accept a C++ string as an argument and return an int.

Then the system find the entry point, which must be a C function which accepts a value of type string, and returns a value of type ob_t. The return type and parameter type of the function are given in square brackets in the call to load-plugin-func1.

The important thing here is that the type ob_t as to be known to both the plugin code as well as the client code. Therefore it is usual to put the interface specification in a separate file and include it in both places, in just the same manner as header files in C and C++. There is no type checking done on loading, so it’s important if the interface changes to rebuild both the plugin and the client.

Because plugins typically provide a lot of functions, not just one, we typically provide just one function, and object factory, which will return a set of function packed into a record when called.