warning: implicit declaration of function 'open'

Mostly, "warning: implicit declaration of function " tell you some header are not correctly include in you c code.

By check man menu, it will tell you these tree headers are required.

 #include <types.h>
 #include <stat.h>
 #include <fcntl.h>

However, to fix the problem you have to add "#include <unistd.h >" to fix the problem.