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This page will teach you how to set Emacs editor to work on the latest source of GIMP from git. The information below assume that you have installed the following tools:
Configuring Netbeans to work on GIMP
Direct compilation: Yes, with M-x compile and M-x flymake-mode
Code completion: Kind of, Emacs supports tab-completion for symbols in all buffers, so if I have the relevant headers open, I have tab-completion for e.g. all gtk_ functions. Not the best code completion, but you come pretty far with it.
Documentation browser: I use exuberant-ctags for symbol definition lookup, so to get to the documentation of a function, I usually go to the declaration/definition of it using the exuberant-ctags indexes. I also occasionally use devhelp, but that's an external program.
Debugging: Yes, M-x gdb
Refactoring: No, unfortunately not.
Thanks to Martin Nordholts for the above configuration