AmigaDOS Packets

Packet passing handles all communication performed by AmigaDOS between processes. The function diagram below shows how packets fit in with the other components of the Amiga operating system.

   +--------------+
   | User Process +-----------------------+
   +-------+------+                       |
           |                              |
           |                              |
    Function Calls                        |
           |                              |
          \|/                             |
 +---------+--------+                     |
 | AmigaDOS Open(), |                     |
 |   Close(), etc.  |                     |
 +---------+--------+                     |
           |                              |
           |                              |
      +-Packets---+--------+     +-----Packets--------+
      |           |        |     |        |           |
     \|/         \|/      \|/   \|/      \|/         \|/
 +----+----+ +----+----+ +-+--+--+-+ +----+----+ +----+----+
 | FFS/OFS | | FFS/OFS | | FFS/OFS | |   CON:  | |   CON:  |
 |   DH0:  | |   DF0:  | |   DF1:  | | Window1 | | Window2 |
 | Handler | | Handler | | Handler | | Handler | | Handler |
 | Process | | Process | | Process | | Process | | Process |
 +----+----+ +-------+-+ +----+----+ +----+----+ +----+----+
      |              |        |           |           |
     \|/            \|/      \|/         \|/         \|/
 +----+--------+  +--+--------+----+ +----+-----------+----+
 |Hddisk.device|  |Trackdisk.device| |    Console.device   |
 +-------------+  +----------------+ +---------------------+

A StandardPacket (defined in <dos/dosextens.h>) is used to send packet commands to a process's MsgPort. The StandardPacket structure contains an Exec Message structure and an AmigaDOS DOSPacket structure:

   struct StandardPacket
   {   struct Message   sp_Msg;
       struct DOSPacket sp_Pkt;
   };

This structure must be longword-aligned, and initialized to link the Message and DOSPacket sections to each other:

   packet->sp_Msg.mn_Node.ln_Name = (char *) &(packet->sp_Pkt);
   packet->sp_Pkt.dp_Link         = &(packet->sp_Msg);

Packets must also be initialized with a ReplyPort which can be created with the amiga.lib function CreatePort():

   if (replyport = (struct MsgPort *) CreatePort(NULL, 0))
       packet->sp_Pkt.dp_Port = replyport;

The DOSPacket portion of the StandardPacket structure is used to pass the packet type and arguments, and to receive the results of the packet. The argument types, number of arguments, and results vary for different packet types and are documented with each packet description. A DOSPacket must be longword-aligned and has the following general structure:

Type	Name	Description
struct Message*	dp_Link	Pointer back to Exec message structure
struct Message*	dp_Port	Reply port for the packet. Must be filled in each send
LONG	dp_Type	Packet type
LONG	dp_Res1	For filesystem calls this is the result that would have been returned by the function; eg. Write("W") returns actual length written.
LONG	dp_Res2	For filesystem calls this is what would have been returned by IoErr()
LONG	dp_Arg1	Argument [1] (depends on packet type)
LONG	dp_Arg2	Argument 2 (depends on packet type)
LONG	dp_Arg3	Argument 3 (depends on packet type)
LONG	dp_Arg4	Argument 4 (depends on packet type)
LONG	dp_Arg5	Argument 5 (depends on packet type)
LONG	dp_Arg6	Argument 6 (depends on packet type)
LONG	dp_Arg7	Argument 7 (depends on packet type)