Question

I would like to define a macro that will help me to auto generate offsets. Something like this:

#define MEM_OFFSET(name, size) ...

MEM_OFFSET(param1, 1);
MEM_OFFSET(param2, 2);
MEM_OFFSET(param3, 4);
MEM_OFFSET(param4, 1);

should generate the following code:

const int param1_offset = 0;
const int param2_offset = 1;
const int param3_offset = 3;
const int param4_offset = 7;

or

enum {
  param1_offset = 0,
  param2_offset = 1,
  param3_offset = 3,
  param4_offset = 7,
}

or even (not possible using C-preprocessor only for sure, but who knows 😉

#define param1_offset 0
#define param2_offset 1
#define param3_offset 3
#define param4_offset 7

Is it possible to do without running external awk/bash/… scripts?

I’m using Keil C51

Answer 1

In the comments to your post you mention that you’re managing an EEPROM memory map, so this answer relates to managing memory offsets rather than answering your specific question.

One way to manage EEPROM memory is with the use of a packed struct. ie, one where there is no space between each of the elements. The struct is never instantiated, it is only used for offset calculations.

typedef struct {
    uint8_t param1;
#ifdef FEATURE_ENABLED
    uint16_t param2;
#endif
    uint8_t param3;
} __packed eeprom_memory_layout_t;

You could then use code like the following to determine the offset of each element as needed(untested). This uses the offsetof stddef macro.

uint16_t read_param3(void) {
    uint8_t buf;
    eeprom_memory_layout_t * ee;

    /* eeprom_read(offset, size, buf) */
    eeprom_read(offsetof(eeprom_memory_layout_t, param3), sizeof(ee->param3), &buf);

    return buf;
}

Note that the struct is never instantiated. Using a struct like this makes it easy to see your memory map at a glance, and macros can easily be used to abstract away the calls to offsetof and sizeof during access.