How to use and modify a public member data which was declared in another fix command class by using pointers?

Hi steve and all,
The LAMMPS version I used is 20Oct11.
I want to add a new FIX command , so I wrote the source and head files just according to fix_addforce command. In the head file “fix_abcd.h”, there are :
#include “fix.h”
namespace LAMMPS_NS {
class FixABCD : public Fix { // ABCD represents the name of my own command.
FixABCD(class LAMMPS *, int, char **);
int setmask();
void setup (int);
// there are also other member functions.
double **efg;
// I think that means I declared a two-dimensional array named efg as public member data.
This array contains some data about atom information. Then, I defined it in “fix_abcd.cpp” with words like these:
void FixABCD::post_force(int vflag)
for (int i = 0; i < nlocal; i++){
for (int j = 0; j < 100; j++){
efg[i][j] = ; // ignore some math functions.
So, what should I do if I want to use and modify this array in another command (not FixABCD) I wrote myself ?
I noticed that when fix_addforce wants to modify the atom positions, you wrote "double **x = atom -> x;"in the source file while including “atom.h”. But the FixABCD class, even the Fix class itself are not the fundamental LAMMPS class which have ptrs stored in “lammps.h”, and, I didnot figure out how the “pointers.h” works, so I couldn’t simply imitate that.
Would you please show me the proper way of, in short, defining a public member data in one command class(especially means a Fix command in this case), and then, use and modify it in another commmand’s source file? Thank you very much!

every fix that your input script creates has an "id" which
you give it. in your script. Modify.cpp has a "find_fix" method. If you
pass it the "id", it will return the index to the fix
and which can then be converted to a ptr, which
any other piece of code can access.

If this doesn't make sense, grep the code for modify->find_fix