The macro NULL is defined in the stdio.h header file. This header file is included in the get_dscr.c, get_infc.c, get_strd.c, and get_cnfg.c source files. The include statement is of the form
Since the compiler did not complain about not being able to find stdio.h, it may be that there is another stdio.h header file in the search path that does not define the NULL macro or the get_xxxx.c files have been modified and they no longer include stdio.h header file. One could add this Macro definition to each file but it would be better to find out why it is not being defined by the header file that normally resides in C:\Keil\C51\INC.