I do not know SDCC and its limitations but I think I would define the union outside the structure. In the structure just put an "int data" member. (((Do check whether it is legal to be using the word "data" as a variable name because it is often a reserved keyword.))).

Then to place a "c" data item into the structure you could use casts like this.

union chr_int {
char c;
int i;
};

struct {
char data_id;
int data;
} message;

...
...

(union chr_int *)&message.data -> c = 0;


Try that or something like it.
Michael Karas