void *emalloc( size_t nbytes) { // malloc() with exit on error
  void *p = malloc(nbytes); if(!p) { fprintf( stderr, "malloc() failed\n"); exit(1); } return p;
}

typedef struct node { double x; int y; /* ... data ... */ struct node *next; } Node;

#if BLOCK
static Node *head = 0;
static int block = 1000; // number of nodes to allocate at one time
Node *new_node( void) {
  if( !head) { // allocate a block of nodes
    head = emalloc(block*sizeof(Node)); Node *last = head+block-1;
    for( Node *p = head; p < last; ++p) p->next = p+1; // link the nodes
    last->next = 0;
  }
  Node *n = head; head = head->next; return n; // return head of list
}
void free_node( Node *n) {
  n->next = head; head = n; // insert at head of list
}
#else
#define new_node()   emalloc(sizeof(Node))
#define free_node(n) free(n)
#endif