LddNonBlockingIo < Training2

%SECTION0{name=LddNonBlockingIo}% 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:*

<verbatim>
wait_queue_head_t
</verbatim>

_linux/wait.h_


*Declaration:*

<verbatim>
/* compile time */
DECLARE_WAIT_QUEUE_HEAD(name); 

/* run time */
wait_queue_head_t q;
init_waitqueue_head(&q);
</verbatim>

*Sleeping:*

<verbatim>
wait_event(queue, condition)
wait_event_interruptible(queue, condition)
wait_event_timeout(queue, condition, timeout)
wait_event_interruptible_timeout(queue, condition, timeout)
</verbatim>

*waking up sleeping processes:*

<verbatim>
void wake_up(wait_queue_head_t *queue);
void wake_up_interruptible(wait_queue_head_t *queue);
</verbatim>


*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:*

<verbatim>
void sleep_on(wait_queue_head_t *q);
void interruptible_sleep_on(wait_queue_head_t *q);
</verbatim>
5.3.5. The read and write system calls		1. Denx Training Topics		5.3.7. =poll= and =select=
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