Is there a way to control macro expansion order.

Answer 1

If the root name is always the same you could use:

#define SET_COMPOSITE(root) SET(root##_PORT, root##_MASK)

Answer 2

Substitution of parameters in the expansion of function-like macros happens in a set way. All arguments which don't appear after the # operator or either side of a ## are fully macro expanded when they are replaced, not before the function-like macro is expanded.

This means that to make a single macro become two macro arguments a round of macro substitution must occur before the required function like macro is itself expanded.

This means that the solution of a second function like macro that expands to the desired function-like macro is the simplest solution.

i.e. given your original SET definition

#define SET(reg,bits) *((volatile unsigned long*)(& reg+SET_OFF)) = bits

and a macro that expands to two potential arguments

#define STATUS_LED_PORT LATB
#define STATUS_LED_MASK 0x0040;
#define STATUS_LED STATUS_LED_PORT, STATUS_LED_MASK

You have to use another function-like macro to get the substitution that you need.

e.g.

#define SET2(x) SET(x)

Then SET2( STATUS_LED ) expands as follows.

SET( LATB , 0x0040; )

then

*((volatile unsigned long*)(& LATB + 2 )) = 0x0040;

This isn't valid as there are not enough arguments to the SET macro; the parameters are matched to arguments before any expansion of the argument occurs. My compiler generates an error; the behaviour isn't defined.

SET( STATUS_LED )