Dynamic Interface for functions in C

Question

Assume you have a function read_key and normally it does some stuff. You someone should be able to replace it with his function read_key_myfunction and it does other stuff. The general approach would of course be to build an array and register function pointers or using simple switch statements (or both).

But my target is a bit broader: People should be able to write their C-stuff and NOT interfere with my code and it should still register. Of course, I tell them which interface to implement.

What they now basically do is program a library for my software which I dynamically load based on a configuration option. Think of it like OpenSSLs engines: Anyone can write their own engine, compile it as a dll/so and distribute it. They don't need to modify (or know) OpenSSLs code, as long as they stick to the defined interface.

I just want the same (it will in the end be a wrapper for OpenSSL engine functions) for my program.

A colleague suggested I should use the same function in every file and load the libraries dynamically. This sounds like a good solution to me, but I am not quite satisfied since I don't see OpenSSL using any non-engine-specific function in their engine-code.

If some things are unclear here is my specific example:

I am extending a program called sscep which implements a protocol for automatic certificate renewal. A lot of cryptography should take place in HSMs in the future (and right now it should take place within the Windows Key Management (which is accessed by the capi-engine from OpenSSL)).

While OpenSSL already serves a generic interface, there is some stuff I need to do beforehand and it depends on the engine used. I also want to open the possibility for everyone else to extend it quickly without having to dig into my code (like I had from the person before me).

If anyone has any idea, it would be greatly appreciated to see some kind of guideline. Thanks in advance.

Answer 1

What you are describing is commonly called a plugin architecture/plugin framework. You need to combine cross-platform dlopen/LoadLibrary functionality with some logic for registering and performing lookup of exported functions. You should be able to find examples on how to do this on the internet.