SNAX/APN Application Programming Manual

The SNALU Interface
SNAX/APN Application Programming Manual—420111-001
4-8
Other File-System Procedures
•
readcount must equal 12
When the read operation completes, the byte count is returned in the word area
following the SNAX header, as shown in Figure 4-4:
For this type of read completion, the QRI control bit is also set in the SNAX header of
the incoming byte count record.
For example, an SLU process on a Tandem system might use a two-step read to accept
an incoming BIND request during session initiation. A maximum-sized BIND image
could take up as much as 512 bytes of buffer space (beyond that required by the SNAX
header). Instead of tying up that much system buffer space while waiting for the BIND
request, the SLU could use the QRI bit and a 12-byte read buffer. When the BIND
eventually arrives, the SLU then issues a second read operation, specifying the exact
number of bytes required for accepting the BIU from the BIND request.
Other File-System Procedures
As well as WRITEREAD, a SNALU application can use the following other file-system
procedures to interact with the SNALU interface.
•
The AWAITIO Procedure
When opening an LU, one of the things your application must specify is whether to
use wait or nowait I/O:
•
With wait I/O, each operation initiated by a file system procedure call within the
application must be completed before control passes to the next instruction or
statement.
With nowait I/O, certain procedure calls initiate the specified operation and then pass
control immediately to the next instruction or statement. Such I/O calls are logically
completed by a corresponding AWAITIO call later in the program.
Note. This is the only case in which
write-count can equal 6.
Figure 4-4. Byte Count Returned
Note
Although not required, it is highly recommended that all LUs be opened for nowait I/O.
When doing so, use AWAITIO calls to logically complete all operations initiated by
WRITEREAD calls.
404CDT .CDD
SNAX Header Byte Count
9 Bytes 1 2 Bytes