Question

I have this #define statement in legacy code I’m inspecting in C.

#define STEP(x)         case x: STEP ## x : WPAN_Startup_Step = x;

This is a macro to replace cases in a very big switch state machine.
I can’t understand what’s going on in this macro. What does it expand to ?

Answer 1

## does a concatenation, this means that the result will be something like this:

STEP(1)

case 1: STEP1: WPAN_Startup_Step = 1;

or another example:

STEP(v)

case v: STEPv: WPAN_Startup_Step = v;

this macro does not make to much sense to me, since it generates x: STEPx:
maybe a usage example would clarify this.

if you want to see the expansion of a macro use: gcc -E program.c

also a good place to learn about macros: http://gcc.gnu.org/onlinedocs/cpp/Macros.html