5.3.6. Non-Blocking IO

• normal (blocking) behavour
  • read: process blocks if no data is available and may return less than requested
  • write: process blocks if output buffer is full. write returns once free space becomes available in output buffer. Partial write possible.

• non-blocking behaviour
  • open with O_NONBLOCK
  • both read and write return -EAGAIN
  • allows polling device

API: waitqueues

type:

wait_queue_head_t

linux/wait.h

Declaration:

/* compile time */
DECLARE_WAIT_QUEUE_HEAD(name); 

/* run time */
wait_queue_head_t q;
init_waitqueue_head(&q);

Sleeping:

wait_event(queue, condition)
wait_event_interruptible(queue, condition)
wait_event_timeout(queue, condition, timeout)
wait_event_interruptible_timeout(queue, condition, timeout)

waking up sleeping processes:

void wake_up(wait_queue_head_t *queue);
void wake_up_interruptible(wait_queue_head_t *queue);

specialities:

• exclusive waits: wake only one process on queue (avoids thundering herd). wake_up wakes up all non-exclusive waiters and one exclusive waiter.

• wake_up_nr, wake_up_interruptible_nr: wake up to X exclusive waiters

• wake_up_interruptible_sync: avoid (possible) immediate rescheduling so that wake_up will return.

deprecated:

void sleep_on(wait_queue_head_t *q);
void interruptible_sleep_on(wait_queue_head_t *q);