man pages section 3: Extended Library Functions, Volume 1

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Updated: July 2014



dat_evd_dequeue - remove the first event from the Event Dispatcher event queue


cc [ flag… ] file–ldat [ library… ] 
#include <dat/udat.h>

    IN    DAT_EVD_HANDLE    evd_handle,
    OUT   DAT_EVENT         *event



Handle for an instance of the Event Dispatcher.


Pointer to the Consumer-allocated structure that Provider fills with the event data.


The dat_evd_dequeue() function removes the first event from the Event Dispatcher event queue and fills the Consumer allocated event structure with event data. The first element in this structure provides the type of the event; the rest provides the event-type-specific parameters. The Consumer should allocate an event structure big enough to hold any event that the Event Dispatcher can deliver.

For all events the Provider fills the dat_event that the Consumer allocates. So for all events, all fields of dat_event are OUT from the Consumer point of view. For DAT_CONNECTION_REQUEST_EVENT, the Provider creates a Connection Request whose cr_handle is returned to the Consumer in DAT_CR_ARRIVAL_EVENT_DATA. That object is destroyed by the Provider as part of dat_cr_accept(3DAT), dat_cr_reject(3DAT), or dat_cr_handoff(3DAT). The Consumer should not use cr_handle or any of its parameters, including private_data, after one of these operations destroys the Connection Request.

For DAT_CONNECTION_EVENT_ESTABLISHED for the Active side of connection establishment, the Provider returns the pointer for private_data and the private_data_size. For the Passive side, DAT_CONNECTION_EVENT_ESTABLISHED event private_data is not defined and private_data_size returns zero. The Provider is responsible for the memory allocation and deallocation for private_data. The private_data is valid until the Active side Consumer destroys the connected Endpoint (dat_ep_free(3DAT)), or transitions the Endpoint into Unconnected state so it is ready for the next connection. So while the Endpoint is in Connected, Disconnect Pending, or Disconnected state, the private_data of DAT_CONNECTION_REQUEST_EVENT is still valid for Active side Consumers.

Provider must pass to the Consumer the entire Private Data that the remote Consumer provided for dat_ep_connect(3DAT), dat_ep_dup_connect(3DAT), and dat_cr_accept(). If the Consumer provides more data than the Provider and Transport can support (larger than IA Attribute of max_private_data_size), DAT_INVALID_PARAMETER is returned for that operation.

The returned event that was posted from an Event Stream guarantees Consumers that all events that were posted from the same Event Stream prior to the returned event were already returned to a Consumer directly through a dat_evd_dequeue() or dat_evd_wait(3DAT) operation.

The ordering of events dequeued by overlapping calls to dat_evd_wait() or dat_evd_dequeue() is not specified.

Return Values


The operation was successful. An event was returned to a Consumer.


Invalid DAT handle; evd_handle is invalid.


There are no entries on the Event Dispatcher queue.


One of the parameters was invalid for this operation. There is already a waiter on the EVD.


No matter how many contexts attempt to dequeue from an Event Dispatcher, each event is delivered exactly once. However, which Consumer receives which event is not defined. The Provider is not obligated to provide the first caller the first event unless it is the only caller. The Provider is not obligated to ensure that the caller receiving the first event executes earlier than contexts receiving later events.

Preservation of event ordering within an Event Stream is an important feature of the DAT Event Model. Consumers are cautioned that overlapping or concurrent calls to dat_evd_dequeue() from multiple contexts can undermine this ordering information. After multiple contexts are involved, the Provider can only guarantee the order that it delivers events into the EVD. The Provider cannot guarantee that they are processed in the correct order.

Although calling dat_evd_dequeue() does not cause a context switch, the Provider is under no obligation to prevent one. A context could successfully complete a dequeue, and then reach the end of its timeslice, before returning control to the Consumer code. Meanwhile, a context receiving a later event could be executing.

The Event ordering is preserved when dequeueing is serialized. Potential Consumer serialization methods include, but are not limited to, performing all dequeueing from a single context or protecting dequeueing by way of lock or semaphore.


See attributes(5) for descriptions of the following attributes:

Interface Stability
uDAPL, 1.1, 1.2

See Also

dat_cr_accept(3DAT), dat_cr_handoff(3DAT), dat_cr_reject(3DAT), dat_ep_connect(3DAT), dat_ep_dup_connect(3DAT), dat_ep_free(3DAT), dat_evd_wait(3DAT)libdat(3LIB), attributes(5)