Recently we've seen a few users in #pidgin (and I've received some emails too) with some strange issues that result from plugins doing things they really shouldn't but can anyway. I'm going to run down a small punch list of items for you, as plugin authors, to do to prevent your users having hard-to-diagnose issues.
- Never use printf() in a plugin. This causes terminal disruption for Finch users and is a bad idea in general. It will probably behave unpredictably on Windows and/or in other UI's too. We provide a debugging API to avoid this kind of stuff. Instead of printf(), use purple_debug_info() or purple_debug_misc(). You can see the debug output (even during initial startup) if you run 'pidgin -d', even on Windows and even if your Pidgin is built without '--enable-debug'. For Finch, you can do 'finch -d 2> finch_debug.txt' to get your debug info.
- Don't lie about UI requirements. If you use GTK+, you're a Pidgin plugin, not a "core" (or libpurple) plugin. If you use GNT (or if you're really brave and use ncurses directly), you're a Finch plugin. There are plenty of examples in the Pidgin and Purple Plugin Pack source trees that show you how to properly declare your UI dependency. The UI requirement is designed specifically so that plugins requiring a specific UI can't be loaded by, and thus cause problems for, other UI's. Sure, it's possible to make a UI-agnostic plugin that requires GTK+, but I guarantee it will cause at least one or two Finch users unnecessary problems.
- Install your binaries to the correct place. If you're installing a Pidgin plugin, install to $prefix/lib/pidgin, not $prefix/lib/purple-2. If you're installing a Finch plugin, install to $prefix/lib/finch. This will prevent other UI's from needlessly trying to probe your plugin and is just general courtesy. (Note that this particular item doesn't apply to plugins for Pidgin on Windows at the moment, but may be important later.)
- If your plugin is a protocol plugin, you absolutely are not allowed to touch UI-specific code at all. That means no GTK+, no GNT, no XUL, etc. If you need UI elements, use the request API libpurple provides. If the request API doesn't cut it, there are other approaches you can take, such as having two plugins, one of which does the actual protocol work and the other providing the UI. A better approach, though, is helping us identify where the request API is lacking so we can improve it. We know our API's aren't perfect, so when someone can show us a concrete reason why we're deficient, we will try to fix that deficiency.