在C中的函数内部使用宏时出错

Question

He everyone,

I'm trying to use this macro in c:

#define CONTROL_REG(num_device) CONTROL_DEV_##num_device

But it doesn't work. This is my code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>

#define CONTROL_DEV_0 0x00  /* control register for device 0 */
#define CONTROL_DEV_1 0x01  /* control register for device 1 */
#define CONTROL_DEV_2 0x02  /* control register for device 2 */
#define CONTROL_DEV_3 0x03  /* control register for device 3 */

#define CONTROL_REG(num_device) CONTROL_DEV_##num_device

void func(int num_device)
{
    printf("Value: %d\n", CONTROL_REG(num_device));
}

int main(int argc, char **argv)
{
    func(0);
    func(1);
    func(2);
    func(3);
}

Any suggestion?

Best regards.

Answer 1

Since you require a run-time mapping for values, use a proper function that maps via an array:

int control_reg(int num_device) {

   static int ret[] = {
     CONTROL_DEV_0,
     CONTROL_DEV_1,
     CONTROL_DEV_2,
     CONTROL_DEV_3,
   };

   assert(num_device >= 0 && num_device <= 3);
   return ret[num_device];
}

You can't expand a macro by a run-time value.

Answer 2

Please remember that macros are completely implemented by the preprocessing step. When the actual compiler sees the code, all macros (definitions and uses) are gone.

So, this can never work.