The problem: Older versions of AMD’s APP (Accelerated Parallel Processing) OpenCL SDK are recommended for use with older Radeon chipsets, but produce segmentation faults in string handling code or other inoperability when used with newer Linux distributions. Version 2.4 through 2.6 exhibit the problem. However, version 2.8 does not support, probably among others, the RV770 chipset used in the HD 4850.
The solution: Use version 2.7 of AMD’s APP when using cgminer with an ATI HD 4850 video card on current Linux distributions.