Macro as a parameter to another macro

Question

I'm trying to pass the parameters to macro SETBIT with another predefined macro like this:

#define SETBIT(ADDRESS,BIT,N) {(N) ? (ADDRESS &= ~(1<<BIT)) : (ADDRESS |= (1<<BIT))}
#define DAC_SYNC PORTB,3,POS
SETBIT(DAC_SYNC);

However I receiver error:

macro SETBIT requires 3 parameters only 1 given

There is an article with the following recommendations:

to prevent misnesting of arithmetic operations: #define foo (a,b) or #define bar(x) lose((x))

But even though I still have an error. BTW, reading the article I've indicated I can make the following conclusion: preprocessor expands ALL macroses appearing. But actually it looks like macro #define DAC_SYNC PORTB,3,POS is not expanding by preprocessor.

Could anyone make more clear how the GCC's preprocessor works?

ooga · Accepted Answer · 2014-04-11 19:38:45Z

10

This works:

#define SETBIT2(ADDRESS,BIT,N) ((N) ? (ADDRESS &= ~(1<<BIT)) : (ADDRESS |= (1<<BIT)))
#define SETBIT(PARAMS) SETBIT2(PARAMS)
#define PORTB 5
#define POS 7
#define DAC_SYNC PORTB,3,POS

int main() {
  int a = SETBIT(DAC_SYNC);
  return 0;
}

answered Apr 11, 2014 at 19:38

ooga

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4 Comments

Roman Matveev Over a year ago

That's worked!!! But actually I didn't get the point: why this solved the problem? Is there any clear article on the topic?

Chris Dodd Over a year ago

I would suggest #define SETBIT(...) SETBIT2(__VA_ARGS__) -- this way you can call SETBIT with a macro that expands to 3 args or with the 3 args directly.

Chris Dodd Over a year ago

@RomanMatveev: It works by adding an extra indirection that causes the DAC_SYNC macro to be expanded before parsing the arguments to SETBIT2. C macro expansion works by first parsing the arguments, then expanding any macros in the arguments, then substituting the argments into the body, then rescanning the result for more macros to expand. The order is very important!

Roman Matveev Over a year ago

OMG! Now declaration SETBIT(PORTA,2,0) not working :(((((((

Mali Remorker · Accepted Answer · 2014-04-11 19:52:46Z

5

Just for the sake of completeness, that same manual you are linking to also states:

The number of arguments you give must match the number of parameters in the macro definition. When the macro is expanded, each use of a parameter in its body is replaced by the tokens of the corresponding argument.

So ooga's example is a nice demonstration of how macro expansion works recursively, first the outer macro gets expanded, then the argument.

answered Apr 11, 2014 at 19:52

Mali Remorker

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