I read on stackoverflow.com that it is more portable. I have no particular must, just curiosity, I am also reading about POSIX ones. Oh, I haven't noticed that fact. Good to know. Ok, I will remember that. Do you mean that below code is wrong?

Code:

int shm;
key_t key;
game_state *gsbuf;

if((key = ftok(".",'s')) < 0) ERR("ftok"); // still don't know why here is 's', copied from some webpage
shm = makeshm(sizeof(gsbuf), key);
if((gsbuf = shmat(shm, NULL, 0)) == (void *) -1) ERR("shmat");

/* then some forking */

(...)

/* outside main */
int makeshm(size_t size, key_t key) {
    int shm;
    if((shm = shmget(key,size,IPC_CREAT|S_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP|S_IROTH|S_IWOTH))<0)
         ERR("Create shared memory");
    return shm;
}

I think that is my problem. What does it mean big enough? Server doesn't know how much clients would have. The main point is how to organize this shared memory to grow as new games are started. Good idea, thanks
I am not sure if I understand well. Could you propose any sketch?