Question

I have code like

#define ONE 1
#define TWO 2
#define SUM (ONE+TWO)

How do I dump SUM as “3”, the resolved value, in gcc 4.3+?

gcc -dM -E foo.h only seems to dump it as is. How do I get the actual value like it’s inserted on compilation stage?

Answer 1

You can’t. As far as the compiler is concerned, the line printf("%d\n", SUM) before preprocessing is indistinguishable from the line printf("%d\n", 1+2). The compiler just happens to perform an optimization called constant folding, even at the default optimization level (-O0), which turns the result into a constant 3 at runtime.

There’s not really a good way to see the output of these optimizations. You could use the -S option to view the generated assembly code and see what that looks like, but if your program is anything larger than a toy, that will be a lot of manual effort. You could also look at the parse tree by using one of the -fdump-tree options (see the GCC man page).