Implementing A Plugin System in C or C++

Question

What are your tips on implementing a plugin style system?

Answer 1

In C (and I think C++ too although I haven't done it myself), this is most typically done using dynamically loaded modules. The API:s for such are platform-dependent.

On POSIX (Linux), you use the dlopen() family of functions. Basically you build your plugin separately, then load it at run-time, look up its symbols by name, and can then call them.

For Win32, there is LoadLibrary() which does something very similar, you build your code into a DLL.

For a handy wrapper that makes all of these easy and transparent, check out GLib's GModule API.

Answer 2

I have had some success using a fairly naive system:

• Create API Specification for plug-ins
• Use a singleton plug-in manager
• Use LoadLibrary/GetProcAddress based run time dynamic linking
• Implement Inversion of control based event handling for notifying the plug-ins

Answer 3

In the '92/'93 time frame I worked on a plugin architecture for Aldus PageMaker, which was coded in C++. PageMaker was built on a C++ OOP framework called VAMP, which assisted its portability between Mac OS and Windows.

So we tried to use the features of C++ to build a plugin architecture. This proved to be very problematic for C++ classes due to the so-called brittle base class problem. I proceeded to write a paper that was published in journals and that I presented at OOPSLA '93 in a reflection workshop. I also made contact with Bjarne Stroustrup at a Usenix conference in Portland and proceeded to dialog with him for several months, where he championed the issue of dealing with the brittle base class problem on my behalf. (Alas, other issues were deemed more important at that time.)

Microsoft introduced the COM/DCOM system and for that platform that was looked on as a viable solution to the problem. C++ could be used as an implementation language for COM via abstract classes used to define COM interfaces.

However, these days developers shun away from COM/DCOM.

In contrast, NeXT devised a plugin architecture using Objective C in the early 90s in the NeXT Step framework. Today that lives on vibrantly in Mac OS X on Apple's computers and important platforms such as the iPhone.

I submit Objective C enabled solving the plugin problem in a superior manner.

I personally regard the brittle base class problem of C++ to be it's most fatal flaw.

If were building a plugin architecture with the C-based family of languages, would do so using Objective C.

Answer 4

It's best to use a framework like ACE (http://www.cs.wustl.edu/~schmidt/ACE.html) that shields you (as good as possible) from platform specific coding.

ACE contains a plugin framework that is based on shared libraries that you can use to create dynamically assembled applications.

For a higher level abstraction check out CIAO (http://www.cs.wustl.edu/~schmidt/CIAO.html) the Open Source C++ implementation of the CORBA Component Model.

Answer 5

This may not be what you're looking for, but you could embed a scripting language in your application, such as Lua. Lua was designed to be embedded in other programs and used as a scripting language for writing plugins. I believe it's fairly easy to add the Lua interpreter to your program, though I don't know Lua so I can't vouch for how effective of a solution this would be. Others with more experience with Lua, please add comments about your experience with embedding Lua in another application.

This would, of course, mean that your plugins need to be written in Lua. If you don't like Lua then the de-facto standard Perl, Python and Ruby interpreters are all written in C, and can be embedded in a C program. I know of a number of programs that use these languages as scripting language extensions.

However, I don't know what you're looking for, as your question is a little vague. Perhaps more information about what you want people to be able to do with said plugins would be appropriate. For some tasks, a full-blown scripting language may be a bit overkill.

Answer 6

This question is very close from "Creating a module system (dynamic loading) in C". StackOverflow is so rich now that you should really search before asking.

Answer 7

This podcast on plugin architectures might also be interesting.