I have a text editor written in C using gtk+-3.x and gtksourceview3. I would like to expand the functionality to include the ability to open a text file from the command line and add the text in this file to a new buffer in the previously opened editor.
So the question is, how do I access this text? I can add it to a buffer,scrollwin... that isn't the problem. Thanks.
I'm assuming your problem is what's mentioned in the title: getting an already running instance of a GTK app to do something from the command line.
GtkApplication and it's base class GApplication will give you the tools you need: It allows you to define actions that the new instance can invoke on the original instance (using D-Bus but that's an implementation detail). For your use case take a look at the already defined "open" action in GApplication (see example).
If you are using a an old version of GTK+ that doesn't have GtkApplication, you can use libunique instead -- the API is roughly the same.