In the /sys/class/gpio folder on my Gumstix Overo board, there are GPIOs that I did not manually export.  I noticed that none of these GPIO folders contain a "direction" file.  I'm assuming that these GPIOs have special purposes and should not be changed by the user; is this correct?  Are there any additional GPIOs whose folders would not contain a "direction" file when exported?  Is there a list of the GPIOs that are free for use?