Run by UID 100:
	shmid = shmget(shmemKey, sizeof(struct memory), IPC_CREAT | 0666);
	sharedMemory = (struct memory *) shmat(shmid, (char *) 0, 0);

   Run by UID 101:
	if ((shmid = shmget (key, sizeof (struct memory), 666 )) < 0) {
	    perror("shmget");
	    exit(1);
	}

This fails for me, even when I run both programs as the same uid.
That's because there's a typo in the second program.  I changed:

        if ((shmid = shmget (key, sizeof (struct memory), 666 )) < 0) {

to:

        if ((shmid = shmget (key, sizeof (struct memory), 0666 )) < 0) {

and it worked fine.