block_map
static int block_map(struct open_file *f, daddr_t file_block,
error = block_map(f, (daddr_t)0, &disk_block);
error = block_map(f, file_block, &disk_block);
static int block_map(struct open_file *, daddr_t, daddr_t *);
rc = block_map(f, file_block, &disk_block);
rc = block_map(f, (daddr_t)0, &disk_block);