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Inside Macintosh: Interapplication Communication /: Chapter 11 - Program-to-Program Communications Toolbox / Using the PPC Toolbox

Exchanging Data During a PPC Session
After a session begins, each application can send data to and receive data from the other using a sequence of message blocks. The PPC Toolbox treats each message block as a byte stream and does not interpret the contents of the message block. The size of a message block can be between 0 and (232-1) bytes. The PPC Toolbox treats the buffer size as an unsigned long integer.
The PPC Toolbox delivers the message blocks in the same sequence as they are sent and without duplicates. In Figure 11-17, an application transmits message blocks during a session.
Figure 11-17 Transmitting message blocks

For each message block, you specify a block creator, block type, and user data. The first PPCWrite function that you use to create a new message block sets the attributes for the block. The PPCRead function returns the block creator, block type, and user data attributes for the current message block when the call completes.
Although the PPC Toolbox does not interpret these attributes, they can give the receiving application information about how to process the contents of the message block. For example, a database application may specify, in the block creator field, a counter to indicate the block number (block number 20 of 30 total blocks). This application could also specify a code, such as 'DREC', in the block type field to indicate that the information it contains is a database record. In addition, this application could specify, in the user data field, the length of the message block.

Reading Data From an Application
An application can both read from and write data to another application during a session. Use the PPCRead function during a session to read incoming blocks of data from another application.
Once a session is initiated, you should have a PPCRead function pending. You can issue a PPCRead function from inside a completion routine. This provides you with immediate notification if an error condition arises or the session closes.
The blockCreator, blockType, and userData fields are returned for the block you are reading. (These fields are set by the PPCWrite function.) To determine whether there is additional data to be read, check the more field. The value FALSE indicates the end of a message block.
Listing 11-14 illustrates how you use the PPCRead function to read data during a session.
Listing 11-14 Using the PPCRead function to read data during a session
FUNCTION MyPPCRead(thePPCReadPBPtr: PPCReadPBPtr; 
                     theSessRefNum: PPCSessRefNum; 
                     theBufferLength: Size; 
                     theBufferPtr: Ptr): OSErr;
BEGIN
   WITH thePPCReadPBPtr^ DO
   BEGIN
      ioCompletion := NIL;
      sessRefNum := theSessRefNum;  {from PPCStart or PPCInform}
      bufferLength := theBufferLength;
      bufferPtr := theBufferPtr;
   END;
   MyPPCRead := PPCRead(thePPCReadPBPtr, TRUE); {asynchronous}
END;
You should make any calls to PPCRead asynchronously. You can provide a completion routine that will be called when the PPCRead function has completed, or you can poll the ioResult field of the PPC parameter block to determine whether the PPCRead function has completed. A PPCRead completion routine can issue another asynchronous PPC Toolbox call or set global variables. If another PPC Toolbox call is made from a completion routine, then the PPCRead function must use a record of data type PPCParamBlockRec instead of type PPCReadPBRec.
Listing 11-15 illustrates a function that can be used to poll the ioResult field of a record of data type PPCReadPBRec. The function returns TRUE when the PPCRead function associated with PPCReadPBRec has completed.
Listing 11-15 Polling the ioResult field to determine if a PPCRead function has completed
FUNCTION MyReadComplete(thePPCReadPBPtr: PPCReadPBPtr; 
                        VAR err: OSErr): Boolean;
BEGIN
   err := thePPCReadPBPtr^.ioResult;
   MyReadComplete := err <> 1;
END;
Sending Data to an Application
Use the PPCWrite function to send a message block during a session specified by the session reference number.
You should call the PPCWrite function asynchronously. You can provide a completion routine that will be called when the PPCWrite function has completed, or you can poll the ioResult field of the PPC parameter block to determine whether the PPCWrite function has completed. A PPCWrite completion routine can issue another PPC Toolbox call or set global variables. If another PPC Toolbox call is made from a completion routine, then the PPCWrite function must use a record of data type PPCParamBlockRec instead of type PPCWritePBRec. Note that message blocks are sent in the order in which they are written.
Listing 11-16 illustrates how you use the PPCWrite function to write data during a session.
Listing 11-16 Using the PPCWrite function to write data during a session
FUNCTION MyPPCWrite(thePPCWritePBPtr: PPCWritePBPtr; 
                   theSessRefNum: PPCSessRefNum; 
                   theBufferLength: Size; 
                   theBufferPtr: Ptr): OSErr;
BEGIN
   WITH thePPCWritePBPtr^ DO
   BEGIN
      ioCompletion := NIL;
      sessRefNum := theSessRefNum;  {from PPCStart or PPCInform}
      bufferLength := theBufferLength;
      bufferPtr := theBufferPtr;
      more := FALSE;          {no more data to read}
      userData := 0;          {application-specific data}
      blockCreator := '????'; {application-specific data}
      blockType := '????';    {application-specific data}
   END;
   MyPPCWrite := PPCWrite(thePPCWritePBPtr, TRUE); {asynchronous}
END;
The first PPCWrite function that you use to create a new message block sets the block creator, block type, and user data attributes for the block. These attributes are returned to the application when it reads from the message block. Set the more field to FALSE to indicate the end of the message block or set this field to TRUE if you want to append additional data to a message block.
Listing 11-17 illustrates a function that can be used to poll the ioResult field of a record of data type PPCWritePBRec. The function returns TRUE when the PPCWrite function associated with PPCWritePBRec has completed.
Listing 11-17 Polling the ioResult field to determine if a PPCWrite function has completed
FUNCTION MyWriteComplete(thePPCWritePBPtr: PPCWritePBPtr;
                         VAR err: OSErr): Boolean;
BEGIN
   err := thePPCWritePBPtr^.ioResult;
   MyWriteComplete := err <> 1;
END;