Events

Events groups are synchronization primitives that allow tasks to wait for multiple conditions to be met before proceeding. They are particularly useful in scenarios where a task needs to wait for several events to occur simultaneously. This concept can be particularly powerful in real-time operating systems (RTOS).

Overview

An event group consists of a set of binary flags, each representing a specific event. Tasks can set, clear, and wait on these flags. When a task waits on an event group, it can specify which flags it is interested in and whether it wants to wait for all specified flags to be set or just any one of them.

Configuration Options

CONFIG_SCHED_EVENTS

This option enables event objects. Threads may wait on event objects for specific events, but both threads and ISRs may deliver events to event objects.

Common Events Interfaces

Events Types

  • nxevent_t. Defines one event group entry.

  • nxevent_mask_t. Defines one events mask value.

Notifier Chain Interfaces

int nxevent_init(FAR nxevent_t *event, nxevent_mask_t events)

Initializes an event object, Set of default events to post to event.

Parameters:

  • event – Address of the event object

  • events – Set of events to post to event

int nxevent_destroy(FAR nxevent_t *event)

This function is used to destroy the event.

Parameters:

  • event – Address of the event object

int nxevent_reset(FAR nxevent_t *event, nxevent_mask_t events)

Reset events mask to a specific value.

Parameters:

  • event – Address of the event object

  • events – Set of events to post to event

int nxevent_post(FAR nxevent_t *event, nxevent_mask_t events, nxevent_flags_t eflags)

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 Set events to 0 will be considered as any, waking up the waiting thread immediately.

  • eflags – Events flags

nxevent_mask_t nxevent_wait(FAR nxevent_t *event, nxevent_mask_t events, nxevent_flags_t eflags)

Wait for all of the specified events.

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

Parameters:

  • event – Address of the event object

  • events – Set of events to wait, 0 will indicate wait from any events

  • eflags – Events flags

nxevent_mask_t nxevent_tickwait(FAR nxevent_t *event, nxevent_mask_t events, nxevent_flags_t eflags, uint32_t delay)

Wait for all of the specified events for the specified tick time.

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.

Parameters:

  • event – Address of the event object.

  • events – Set of events to wait, 0 will indicate wait from any events

  • eflags – Events flags

  • delay – Ticks to wait from the start time until the event is posted, If ticks is zero, then this function is equivalent to nxevent_trywait().

nxevent_mask_t nxevent_trywait(FAR nxevent_t *event, nxevent_mask_t events, nxevent_flags_t eflags)

Try wait for all of the specified events.

This routine try to waits on event object event if any of the specified events have been delivered to the event object. A thread may wait on up to 32 distinctly numbered events that are expressed as bits in a single 32-bit word.

Parameters:

  • event – Address of the event object

  • events – Set of events to wait, 0 will indicate wait from any events

  • eflags – Events flags

int nxevent_open(FAR nxevent_t **event, FAR const char *name, int oflags, ...)

This function establishes a connection between named event groups and a task. the task may reference the event group associated with name using the address returned by this call. The event group may be used in a subsequent calls to nxevent_wait(), or nxevent_post(). And the event group remains usable until the event group is closed by a successful call to nxevent_close().

If a task makes multiple calls to event_open() with the same name, then the same event group address is returned.

Parameters:

  • event – Location to return the event group reference.

  • name – Event group name.

  • oflags –

    Event group creation options. This may the following settings:

    • oflags = 0: Connect to the event group only if it already exists.

    • oflags = O_CREAT: Connect to the event group if it exists, otherwise create the event group.

    • oflags = O_CREAT|O_EXCL: Create a new event group unless already exists.

  • ... –

    Optional parameters. When the O_CREAT flag is specified,

    the two optional parameters are expected:

    • mode: The mode parameter is of type mode_t. This parameter is

      required but not used in the present implementation.

    • events: The events parameter is type unsigned. The event group

      is created with an initial value of events.

Returns:

0 (OK), or negated errno if unsuccessful.

int nxevent_close(FAR nxevent_t *event)

This function is called to indicate that the calling task is finished with the specified named event group. The event_close() deallocates any system resources allocated by the system for this named event group.

Parameters:

  • event – event descriptor

Returns:

0 (OK), or negated errno if unsuccessful.