I don't know whether anyone has experienced this thing or not.There is a strange thing in Eclipse 11.0. --> If we add one ".c" file and build project is OK now if we add another c file by renaming original c file ( any extension except .c i.e .C__) then also build is ok. --> Now if we rename new file (.c_) and keep original file by change its name (".c") then "undefined reference to main".This error pop up."Clean up" of project doesnot work to eliminate this error --> Deletion of file (then add again by cleaning project)or copy and paste content to new files are solutions i found up til now. This thing are not appeared in Nios II 11 IDE ,seems "Eclipse bug" because after adding new file (changing its name),"Make" file is not able to be updated. if anyone knows about this,please share over here to avoid future confusions for other people.