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The PTOP data-exchange calls are master-slave. The master initiates an
exchange by calling PWRITE (to send data to the slave), PREAD
(to receive data from the slave) or PCONTROL. The slave calls
GET to receive the request and ACCEPT to perform the actual
data movement into or out of its buffer. Each of these operations may also
exchange a tag between the master and the slave. The master call sends the
master tag to the slave and returns a tag from the slave. The slave's
GET call receives the master tag. The slave's ACCEPT or
REJECT call returns the slave's tag to the master.
The GET intrinsic can return an indication of the master request (0 =
error, 1 = POPEN, 2 = PREAD, 3 = PWRITE, 4 =
PCONTROL). In many applications, the slave will always know the next
request that it will receive from the master, so the function returned by
GET is superfluous. But in some applications, the slave does not know
in advance what the next master request will be, and so it depends on the
GET function to decide on its response to the request. The
GET call can also return the requested length of data to be sent or
received. These applications typically have a loop with a GET that
receives the master requests and a case statement with cases for each of the
different functions.
Data exchange with NetIPC is peer-to-peer. A process on either side of a
virtual-circuit connection can send or receive data independently from its
partner. The master-slave data exchange of PTOP can be simulated using NetIPC
calls. For example, a PWRITE in the master can be replaced by an
IPCSEND, while the corresponding GET and ACCEPT in
the slave can be replaced by an IPCRECV. Tags can also be exchanged
using sends and receives.
In applications where the sequence of master requests is not known by the
slave, or where the length of data sent to or received from the slave is not
known, some information in addition to the exchanged data and tags may need to
be transmitted. This includes a) a master request indication, b) master
request lengths, and c) the slave accept or reject indication.
Because of the way that NetIPC stream mode operates on the HP 3000, an
IPCRECV may not receive all of the data requested. For this reason,
we recommend that you write a procedure that calls IPCRECV in a loop
to receive chunks of data until the entire requested amount is received. An
example of this is in Chapter 4 "NetIPC Examples" in
this manual.
Exchanging Data: In the Master Program
To convert the PTOP intrinsic listed below, perform the following steps.
Syntax: PREAD
lgth := PREAD ( dsnum, target, tcount [,itag] );
- If the slave requires a master function, send the PREAD request
function (=2) on the virtual circuit to the slave.
- If the slave requires the requested data length, send
tcount on the virtual circuit.
- If a tag is specified, send the master tag on the virtual circuit.
- If the master needs to know the actual data length sent from the slave,
receive the actual data length from the virtual circuit.
- If the slave may call either ACCEPT or REJECT, receive
a one byte accept or reject indication on the virtual circuit from the
slave. Otherwise assume the slave accepted the request.
- If the slave accepted the request, receive the target data from the
slave, using either a predetermined length or the actual data length
received in Step 4.
- If a tag is specified, receive the slave tag from the virtual
circuit.
To convert the PTOP intrinsic listed below, perform the following steps:
Syntax: PWRITE
PWRITE ( dsnum, target, tcount [,itag] );
- If the slave requires a master function, send the PWRITE
request function (=3) on the virtual circuit to the slave.
- If the slave requires the requested data length, send
tcount on the virtual circuit.
- If a tag is specified, send the master tag on the virtual circuit.
- If the slave may call either ACCEPT or REJECT, receive
a one byte accept or reject indication on the virtual circuit from the
slave, Otherwise assume the slave accepted the request.
- If the slave accepted the requested, send the target data on the virtual
circuit to the slave, using the tcount length.
- If a tag is specified, receive the slave tag from the virtual
circuit.
To convert the PTOP intrinsic listed below, perform the following steps.
Syntax
PCONTROL ( dsnum [,itag] );
- If the slave requires a master function, send the PCONTROL
request function (=4) on the virtual circuit to the slave.
- If a tag is specified, send the master tag on the virtual circuit, and
receive the slave tag from the virtual circuit.
Exchanging Data: In the Slave Program
To convert the PTOP intrinsic listed below, perform the following steps.
Syntax
ifun := GET [ ( [itag] [,il] [,ionumber] ) ];
- If the slave requires a master function, receive a one byte master
function number from the virtual circuit from the master.
- If the slave requires the request length, receive the length from the
virtual circuit.
- If a tag is specified, receive the master tag from the virtual
circuit.
To convert the PTOP intrinsic listed below, perform the following steps.
Syntax
ACCEPT [ ( [itag] [,target] [,tcount] ); ]
- Depending on the master function, either known to the application, or
received from the master in Step (1):
PREAD: Send the target data, of length tcount, on the virtual
circuit to the master.
PWRITE: Receive the data into target, using either the known
tcount or the length received from the master in Step (2).
- If a tag is specified, send the slave tag on the virtual circuit.
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