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Events

An event object is a kernel object that implements traditional events.

Concepts

Any number of event objects can be defined (limited only by available RAM). Each event object is referenced by its memory address. One or more threads may wait on an event object until the desired set of events has been delivered to the event object. When new events are delivered to the event object, all threads whose wait conditions have been satisfied become ready simultaneously.

An event object has the following key properties:

  • A 32-bit value that tracks which events have been delivered to it.

An event object must be initialized before it can be used.

Events may be delivered by a thread or an ISR. When delivering events, the events may either overwrite the existing set of events or add to them in a bitwise fashion. When overwriting the existing set of events, this is referred to as setting. When adding to them in a bitwise fashion, this is referred to as posting. Both posting and setting events have the potential to fulfill match conditions of multiple threads waiting on the event object. All threads whose match conditions have been met are made active at the same time.

Threads may wait on one or more events. They may either wait for all of the the requested events, or for any of them. Furthermore, threads making a wait request have the option of resetting the current set of events tracked by the event object prior to waiting. Care must be taken with this option when multiple threads wait on the same event object.

Note

The kernel does allow an ISR to query an event object, however the ISR must not attempt to wait for the events.

Implementation

Defining an Event Object

An event object is defined using a variable of type k_event. It must then be initialized by calling k_event_init().

The following code defines an event object.

struct k_event my_event;

k_event_init(&my_event);

Alternatively, an event object can be defined and initialized at compile time by calling K_EVENT_DEFINE.

The following code has the same effect as the code segment above.

K_EVENT_DEFINE(my_event);

Setting Events

Events in an event object are set by calling k_event_set().

The following code builds on the example above, and sets the events tracked by the event object to 0x001.

void input_available_interrupt_handler(void *arg)
{
    /* notify threads that data is available */

    k_event_set(&my_event, 0x001);

    ...
}

Posting Events

Events are posted to an event object by calling k_event_post().

The following code builds on the example above, and posts a set of events to the event object.

void input_available_interrupt_handler(void *arg)
{
    ...

    /* notify threads that more data is available */

    k_event_post(&my_event, 0x120);

    ...
}

Waiting for Events

Threads wait for events by calling k_event_wait().

The following code builds on the example above, and waits up to 50 milliseconds for any of the specified events to be posted. A warning is issued if none of the events are posted in time.

void consumer_thread(void)
{
    uint32_t  events;

    events = k_event_wait(&my_event, 0xFFF, false, K_MSEC(50));
    if (events == 0) {
        printk("No input devices are available!");
    } else {
        /* Access the desired input device(s) */
        ...
    }
    ...
}

Alternatively, the consumer thread may desire to wait for all the events before continuing.

void consumer_thread(void)
{
    uint32_t  events;

    events = k_event_wait_all(&my_event, 0x121, false, K_MSEC(50));
    if (events == 0) {
        printk("At least one input device is not available!");
    } else {
        /* Access the desired input devices */
        ...
    }
    ...
}

Suggested Uses

Use events to indicate that a set of conditions have occurred.

Use events to pass small amounts of data to multiple threads at once.

Configuration Options

Related configuration options:

API Reference

group event_apis

Defines

K_EVENT_DEFINE(name)

Statically define and initialize an event object.

The event can be accessed outside the module where it is defined using:

extern struct k_event <name>; 

Parameters
  • name – Name of the event object.

Functions

void k_event_init(struct k_event *event)

Initialize an event object.

This routine initializes an event object, prior to its first use.

Parameters
  • event – Address of the event object.

void k_event_post(struct k_event *event, uint32_t events)

Post one or more events to an event object.

This routine posts one or more events to an event object. All tasks waiting on the event object event whose waiting conditions become met by this posting immediately unpend.

Posting differs from setting in that posted events are merged together with the current set of events tracked by the event object.

Parameters
  • event – Address of the event object

  • events – Set of events to post to event

void k_event_set(struct k_event *event, uint32_t events)

Set the events in an event object.

This routine sets the events stored in event object to the specified value. All tasks waiting on the event object event whose waiting conditions become met by this immediately unpend.

Setting differs from posting in that set events replace the current set of events tracked by the event object.

Parameters
  • event – Address of the event object

  • events – Set of events to post to event

uint32_t k_event_wait(struct k_event *event, uint32_t events, bool reset, k_timeout_t timeout)

Wait for any of the specified events.

This routine waits on event object event until any of the specified events have been delivered to the event object, or the maximum wait time timeout has expired. A thread may wait on up to 32 distinctly numbered events that are expressed as bits in a single 32-bit word.

Note

The caller must be careful when resetting if there are multiple threads waiting for the event object event.

Parameters
  • event – Address of the event object

  • events – Set of desired events on which to wait

  • reset – If true, clear the set of events tracked by the event object before waiting. If false, do not clear the events.

  • timeout – Waiting period for the desired set of events or one of the special values K_NO_WAIT and K_FOREVER.

Returns

  • set – of matching events upon success

  • 0 – if matching events were not received within the specified time

uint32_t k_event_wait_all(struct k_event *event, uint32_t events, bool reset, k_timeout_t timeout)

Wait for any of the specified events.

This routine waits on event object event until all of the specified events have been delivered to the event object, or the maximum wait time timeout has expired. A thread may wait on up to 32 distinctly numbered events that are expressed as bits in a single 32-bit word.

Note

The caller must be careful when resetting if there are multiple threads waiting for the event object event.

Parameters
  • event – Address of the event object

  • events – Set of desired events on which to wait

  • reset – If true, clear the set of events tracked by the event object before waiting. If false, do not clear the events.

  • timeout – Waiting period for the desired set of events or one of the special values K_NO_WAIT and K_FOREVER.

Returns

  • set – of matching events upon success

  • 0 – if matching events were not received within the specified time

struct k_event
#include <kernel.h>

Event Structure