Some times when i start KDevelope in C mode,the stndard libaries are not found,so i found a fix to it,hold ALT down on the highlighted error that says somthing like “header nor found” or somthing like that,then click The solution that says add custom headers,then add these:
- /packages/gcc-7.3.0_2018_05_01-2/.self/develop/tools/lib/gcc/x86_64-unknown-haiku/7.3.0/include
- /boot/system/develop/headers/posix
Hope this helps