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Inside Macintosh: Networking /

Chapter 7 - Datagram Delivery Protocol (DDP) / DDP Reference

Routines / Sending DDP Datagrams

PWriteDDP

The PWriteDDP function sends a DDP datagram to another socket.

FUNCTION PWriteDDP (thePBptr: MPPPBPtr; async: Boolean): OSErr;

thePBptr

A pointer to an MPP parameter block.

async

A Boolean that specifies whether the function should be executed asynchronously. Specify TRUE for asynchronous execution.

-->	ioCompletion	ProcPtr	A completion routine.
<--	ioResult	OSErr	The result code.
-->	csCode	Integer	Always writeDDP for this function.
-->	socket	Byte	The number of socket to send data from.
-->	checksumFlag	Byte	The checksum flag; nonzero to compute checksum.
-->	wdsPointer	Ptr	A pointer to write-data structure.

Field Description

socket

The number of the socket from which you want to send data. See the description of the POpenSkt function for information on the assignment of socket numbers.

checksumFlag

The checksum flag. If you set this field to a nonzero value and if DDP uses a long header for the datagram (that is, if the destination socket has a network number different from that of the source socket), then the PWriteDDP function calculates a checksum for the datagram and includes it in the datagram header. Set this field to 0 if you do not want the PWriteDDP function to calculate a checksum.

wdsPointer

A pointer to a write-data structure. The write-data structure provides the destination address and the data for the datagram.
The DDP write-data structure is described in "Creating a DDP Write-Data Structure" on page 7-12.

DESCRIPTION

Before you call the PWriteDDP function, you must prepare a write-data structure.
The write-data structure, shown in Figure 7-4 on page 7-13, includes a pointer to the destination address and pointers to buffers containing the data you wish to send.
You can use the BuildDDPwds procedure to build a write-data structure.

Set the checksum flag field when you call the PWriteDDP function to have the function calculate the checksum and include it in the packet header. Note, however, that only
long packet headers include a checksum field, and that whether the checksum is used
for error checking depends on how the socket listener code at the destination socket is implemented.

The PWriteDDP function is equivalent to calling the PBControl function with a value of writeDDP in the csCode field of the parameter block.

SPECIAL CONSIDERATIONS

Memory used for the write-data structure belongs to DDP and must be nonrelocatable until the PWriteDDP function completes execution, after which you can either reuse the memory or release it.

ASSEMBLY-LANGUAGE INFORMATION

To execute the PWriteDDP function from assembly language, call the _Control trap macro with a value of writeDDP in the csCode field of the parameter block. You must also specify the .MPP driver reference number. To execute the _Control trap asynchro-
nously, include the value ,ASYNC in the operand field.

RESULT CODES

noErr	0	No error
ddpSktErr	-91	Bad socket number
ddpLenErr	-92	Datagram data exceeds 586 bytes
noBridgeErr	-93	Could not find router to forward packet

SEE ALSO

For a description of the DDP write-data structure, see "Creating a DDP Write-Data Structure" on page 7-12.

If you are programming in Pascal or C, see the description of the BuildDDPwds procedure that follows for help in creating a write-data structure.