How to set structure element at desired offset

Well, this won't win the miss Universe award, but I think it does what you want:

#include <boost/preprocessor/cat.hpp>
typedef unsigned char uchar;
typedef unsigned short ushort;
#define PAD_FIELDS(i_,f_, n_) \
    typedef struct __attribute__((packed)) {f_}             ftype##i_; \
    typedef struct __attribute__((packed)) {f_ uchar t_;}   ttype##i_; \
    f_ uchar BOOST_PP_CAT(padding,i_)[n_ - sizeof (BOOST_PP_CAT(ftype,i_)) * (sizeof (BOOST_PP_CAT(ftype,i_)) != sizeof (BOOST_PP_CAT(ttype,i_)))];

struct sFPGA {
    PAD_FIELDS(1,
    PAD_FIELDS(2,
    uchar   Spare1[0x24];
    ushort  DiscreteInput;
    //CPLD_Version is required to be at offset 0xA0
    , 0xA0)        // First padding
    ushort  CPLD_Version;
    uchar   more_stuff[0x50];
    ushort  even_more[4];
    //NID_Version is required to be at offset 0x10A2
    , 0x10A2)      // Second padding
    ushort  NID_Version;
} __attribute__((packed));

int main() {
    printf("CPLD_Version offset %x\n", offsetof(sFPGA,CPLD_Version));
    printf("NID_Version offset %x\n", offsetof(sFPGA,NID_Version));
}

Let's say you want N=20 padding fields. You have to add N of those PAD_FIELDS(i, in the beginning of your structure, where i runs for example from 1 to 20 (as in my example) or from 0 to 19 or whatever makes you happy. Then, when you need the padding you add for example , 0x80) which means that the next field will be positioned at offset 0x80 from the beginning of the structure.

Upon running this code, it outputs the following text:

CPLD_Version offset a0
NID_Version offset 10a2

The way this macro works is it defines a structure with your fields, it then incorporates your fields, and adds the padding computed according to the structure.

If you don't mind some boost::preprocessor magic, here's a way you can automate the whole PAD_FIELDS(1,PAD_FIELDS(2,PAD_FIELDS(3,PAD_FIELDS(4,... in the beginning:

#include <boost/preprocessor/cat.hpp>
#include <boost/preprocessor/comma.hpp>
#include <boost/preprocessor/repetition/repeat.hpp>
#include <boost/preprocessor/punctuation/paren.hpp>
typedef unsigned char uchar;
typedef unsigned short ushort;
#define PAD_FIELDS(i_,f_, n_) \
    typedef struct __attribute__((packed)) {f_}             BOOST_PP_CAT(ftype,i_); \
    typedef struct __attribute__((packed)) {f_ uchar t_;}   BOOST_PP_CAT(ttype,i_); \
    f_ uchar BOOST_PP_CAT(padding,i_)[n_ - sizeof (BOOST_PP_CAT(ftype,i_)) * (sizeof (BOOST_PP_CAT(ftype,i_)) != sizeof (BOOST_PP_CAT(ttype,i_)))];
#define PADMAC(z,n,s) PAD_FIELDS BOOST_PP_LPAREN() n BOOST_PP_COMMA()
#define PADREP(n) BOOST_PP_REPEAT(n, PADMAC, junk)
#define FORCE_EVAL(...) __VA_ARGS__
#define CONTAINS_PADDING(n) FORCE_EVAL(PADREP(n)
#define SET_OFFSET(o) BOOST_PP_COMMA() o BOOST_PP_RPAREN()

struct sFPGA {
    CONTAINS_PADDING(2);
    uchar   Spare1[0x24];
    ushort  DiscreteInput;
    //CPLD_Version is required to be at offset 0xA0
    SET_OFFSET(0xA0);
    ushort  CPLD_Version;
    uchar   more_stuff[0x50];
    ushort  even_more[4];
    //NID_Version is required to be at offset 0x10A2
    SET_OFFSET(0x10A2);
    ushort  NID_Version;
    )
} __attribute__((packed));

Notice what changed in the usage:

1. In the beginning of the structure you write CONTAINS_PADDING(n) where n is the number of padding elements desired.
2. Right before the end of the structure you have to add a ")".
3. Instead of ,0x0A) to specify the padding you have to write SET_OFFSET(0x0A); (the ; is optional).