Questions about GPIO

[limitlesscodes]

Alright, so I kind of figured out pin assignment using the GPIO image on the wiki. I still would like to know what PC, PD, PH, etc. means and when to use them vs. others. Also I would like to know if there is any better way of exporting the pins through C rather than doing something like:

Code:

//near beginning of file
system("echo 71 > /sys/class/gpio/export")

and

Code:

//near end of file
system("echo 71 > /sys/class/gpio/unexport")