How do I merge two C header files, such that re- #defines are removed?

Question

Supposing I have:

sdk.h:

#define foo 1
#define bar 1

app.h:

#define foo 0

How can I generate a merged.h that looks like this, given I want app.h to redefine anything already defined in sdk.h:

#define foo 0
#define bar 1

One approach might be "don't do this, just let the preprocessor do it for you". I've tried that and it's not going to work for my situation I don't think. I'm really looking for an approach using a basic shell script.

Answer 1

If the order of the contents in the header files is not important, you could do something like this:

$ awk '/#define/{a[$2]=$3; next}1;END{for(i in a){print "#define",i,a[i]}}' sdk.h app.h 
#define foo 0
#define bar 1

Note that any lines not containing #define will be printed first.