If a d_foo function is undefined the corresponding operation is unsupported. However, dopen and dclose are unique; when they’re undefined the kernel will automatically define them as follows:
int
nullop(void)
{
return (0);
}
This ensures that every registered character device can be opened and closed.
即在cdevsw结构体中，d_open和d_close是永远不为空的。
/*
* Character device switch table
*/
struct cdevsw {
int d_version;
u_int d_flags;
const char *d_name;
d_open_t *d_open;
d_fdopen_t *d_fdopen;
d_close_t *d_close;
d_read_t *d_read;
d_write_t *d_write;
d_ioctl_t *d_ioctl;
d_poll_t *d_poll;
d_mmap_t *d_mmap;
d_strategy_t *d_strategy;
dumper_t *d_dump;
d_kqfilter_t *d_kqfilter;
d_purge_t *d_purge;
d_mmap_single_t *d_mmap_single;
int32_t d_spare0[3];
void *d_spare1[3];
/* These fields should not be messed with by drivers */
LIST_HEAD(, cdev) d_devs;
int d_spare2;
union {
struct cdevsw *gianttrick;
SLIST_ENTRY(cdevsw) postfree_list;
} __d_giant;
};