Precedence of -D MACRO and #define MACRO

Question

If I have a C file foo.c and while I have given -DMACRO=1 as command line option for compilation. However, if within the header file also I hav

Answer 1

Defines are stored in order the compiler sees them, and when the compiler encounters a new macro with the same name, it overwrites the previous macro with the new one (at least this is the case in gcc). GCC will also give you a warning when this happens.