Share and access a 2-dimensional array between processes

Question

I have a 2-dimensional array of a structure Data, that has to be initialized. The main program creates 1 server and 4 clients. All the processes should be able to access and modify the data (I already took care of synchronization problems using semaphores). Every part of the program is located in a separate .c file. How can I initialize, share, and access / modify my data (how can I access the fields of the structure, in server() or client()) ? I'm currently using System V but could use Posix if more appropriate.

/*  project.h   */
typedef struct Data {
    int fieldA;
    int fieldB;
    int fieldC;
    int fieldD;
} Data;

extern Data data[4][3];
extern int shm;
extern Data *p;

/*  main.c  */
#include "project.h"

Data *p;
int shm;

void main()
{
    int i;

    shm = shmget(IPC_PRIVATE, 3*4*sizeof(Data), 0666);
    p = (Data *) shmat(shm, NULL, 0);

    for (i=1;i<5;i++) {
        if (fork() == 0) {
            client(i);
        }
    }

    if (fork() == 0) {
        server();
    }
}

/*  server.c    */
#include "project.h"

// I only care about the first field of Data when I initialize.
Data data[4][3] = { {{0},{1},{-1}},
                    {{2},{-1},{-1}},
                    {{5},{0},{3}},
                    {{-1},{6},{-1}} };

void server()                   
{
    /*  read / modify the data  */
}

/*  client.c    */
#include "project.h"

void client(int i)                  
{
    /*  read / modify the data  */
}