DAT_EP_RECV_QUERY(3DAT)DAT_EP_RECV_QUERY(3DAT)NAMEdat_ep_recv_query - provide Endpoint receive queue consumption on SRQ
cc [ flag... ] file... -ldat [ library... ]
IN DAT_EP_HANDLE ep_handle,
OUT DAT_COUNT *nbufs_allocated,
OUT DAT_COUNT *bufs_alloc_span
Handle for an instance of the EP.
The number of buffers at the EP for which comple‐
tions have not yet been generated.
The span of buffers that EP needs to complete arriv‐
The dat_ep_recv_query() function provides to the Consumer a snapshot
for Recv buffers on EP. The values for nbufs_allocated and
bufs_alloc_span are not defined when DAT_RETURN is not DAT_SUCCESS.
The Provider might not support nbufs_allocated, bufs_alloc_span or
both. Check the Provider attribute for EP Recv info support. When the
Provider does not support both of these counts, the return value for
the operation can be DAT_MODEL_NOT_SUPPORTED.
If nbufs_allocated is not NULL, the count pointed to by nbufs_allocated
will return a snapshot count of the number of buffers allocated to
ep_handle but not yet completed.
Once a buffer has been allocated to an EP, it will be completed to the
EP recv_evd if the EVD has not overflowed. When an EP does not use SRQ,
a buffer is allocated as soon as it is posted to the EP. For EP that
uses SRQ, a buffer is allocated to the EP when EP removes it from SRQ.
If bufs_alloc_span is not NULL, then the count to which bufs_alloc_span
pointed will return the span of buffers allocated to the ep_handle. The
span is the number of additional successful Recv completions that EP
can generate if all the messages it is currently receiving will com‐
If a message sequence number is assigned to all received messages, the
buffer span is the difference between the latest message sequence num‐
ber of an allocated buffer minus the latest message sequence number for
which completion has been generated. This sequence number only counts
Send messages of remote Endpoint of the connection.
The Message Sequence Number (MSN) represents the order that Send mes‐
sages were submitted by the remote Consumer. The ordering of sends is
intrinsic to the definition of a reliable service. Therefore every send
message does have a MSN whether or not the native transport has a field
with that name.
For both nbufs_allocated and bufs_alloc_span, the Provider can return
the reserved value DAT_VALUE_UNKNOWN if it cannot obtain the requested
count at a reasonable cost.
The operation was successful.
The DAT handle ep_handle is invalid.
The requested Model was not supported by the
If the Provider cannot support the query for nbufs_allocated or
bufs_alloc_span, the value returned for that attribute must be
An implementation that processes incoming packets out of order and
allocates from SRQs on an arrival basis can have gaps in the MSNs asso‐
ciated with buffers allocated to an Endpoint.
For example, suppose Endpoint X has received buffer fragments for MSNs
19, 22, and 23. With arrival ordering, the EP would have allocated
three buffers from the SRQ for messages 19, 22, and 23. The number
allocated would be 3, but the span would be 5. The difference of two
represents the buffers that will have to be allocated for messages 20
and 21. They have not yet been allocated, but messages 22 and 23 will
not be delivered until after messages 20 and 21 have not only had their
buffers allocated but have also completed.
An implementation can choose to allocate 20 and 21 as soon as any
higher buffer is allocated. This makes sense if you presume that this
is a valid connection, because obviously 20 and 21 are in flight. How‐
ever, it creates a greater vulnerability to Denial Of Service attacks.
There are also other implementation tradeoffs, so the Consumer should
accept that different RNICs for iWARP will employ different strategies
on when to perform these allocations.
Each implementation will have some method of tracking the receive buf‐
fers already associated with an EP and knowing which buffer matches
which incoming message, though those methods might vary. In particular,
there are valid implementations such as linked lists, where a count of
the outstanding buffers is not instantly available. Such implementa‐
tions would have to scan the allocated list to determine both the num‐
ber of buffers and their span. If such a scan is necessary, it is
important that it be only a single scan. The set of buffers that was
counted must be the same set of buffers for which the span is reported.
The implementation should not scan twice, once to count the buffers and
then again to determine their span. Not only is it inefficient, but it
might produce inconsistent results if buffers were completed or arrived
between the two scans.
Other implementations can simply maintain counts of these values to
easily filter invalid packets. If so, these status counters should be
updated and referenced atomically.
The implementation must never report n buffers in a span that is less
See attributes(5) for descriptions of the following attributes:
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
│Interface Stability │ Standard: uDAPL, 1.2 │
│MT-Level │ Unsafe │
SEE ALSOdat_ep_create(3DAT), dat_srq_create(3DAT), dat_srq_free(3DAT),
dat_srq_query(3DAT), dat_ep_set_watermark(3DAT), libdat(3LIB),
Jul 16, 2004 DAT_EP_RECV_QUERY(3DAT)