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

I'm making an assumption of what you're doing, but if you'd like to supply from the command-line a non-default value for that macro, try this for the macro definition:

#ifndef MACRO
#define MACRO 2
#endif

That way if the MACRO has already been defined (via command-line parameter) it will neither be redefined nor result in an error.