void shm_init()
{
        if (!attach_shm())
                create_shm();
}

/* Attach existing shared memory segment */
int attach_shm()
{
        int fd = shm_open(SHM_NAME, O_RDWR, 0);
        if (fd == -1)  return 0;  /* Doesn't exist yet... */

        /* Sanity check, make sure it has the right size ... */
        struct stat st;
        if (stat("/dev/shm/" SHM_NAME, &st) != 0)  fail("/dev/shm/" SHM_NAME);
        assert(st.st_size == sizeof(struct sched_shm));

        shm = mmap(0, shm_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
        if (shm == MAP_FAILED)  fail("mmap");

        assert(shm->magic == SHM_MAGIC);
        assert(shm->size == shm_size);
        assert(shm->ready);
        return 1;
}

/* Create new shared memory segment */
void create_shm()
{
       int fd = shm_open(SHM_NAME, O_RDWR | O_CREAT | O_TRUNC, 0644);
       assert(fd != -1);
       int r = ftruncate(fd, shm_size);  assert(r == 0);
       void *pt = mmap(0, shm_size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
       assert(pt != MAP_FAILED);

       shm = pt;
       memset(shm, 0, sizeof(*shm));
       shm->size = shm_size;
       shm->magic = SHM_MAGIC;

       /* init code ... */
       shm->ready = 1;
}