// file-backed
int fd=open("filename", O_RDWR|O_CREAT);
ftrundate(fd, length);
char *mem=mmap(NULL, length, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);

// not file-backed
char *mem=mmap(NULL, length, PROT_READ|PROT_WRITE, MAP_ANON, 0, 0);

char *shared_mem;
sem_t sem;

void child_stuff(void)
{
        while(1)
        {
                sem_wait(&sem);
                shared_mem[0]++;
                sem_post(&sem);
        }
}

int main(void)
{
       shared_mem=mmap(...);

       // Initialize shared semaphore to 1.  It can be locked once, further attempts
       // will block.
       sem_init(&sem, 1, 1); 
       while(1)
       {
               pid_t pid;
               int fd=accept(...);
               if(fd < 0) continue;

               pid=fork();
               if(pid == 0) // child code
               {
                       // A forked process gets a copy of everything the main process
                       // has open.  99% of it the child probably doesn't need, so
                       // close it.
                       //
                       // You can also set files auto-close fork with an ioctl, look for 
                       // close-on-exec.
                       close_positively_everything_except_what_child_needs;
                       child_stuff();
                       exit(0); // DO NOT FORGET!  Don't want child doing parent stuff after child_stuff finishes for some reason
               }
               else if(pid > 0) // parent stuff
                      add_to_list_of_children(pid);
               else
               {
                       perror("fork failed for some reason");
                       break;
               }
       }
}