Getting Started with QNX Neutrino - QNX Software Systems

Using message passing
© 2009, QNX Software Systems GmbH & Co. KG.
What about the other versions?
// allocate / find cache buffer entries
// fill 3-part IOV with cache buffers
MsgReadv (rcvid, iov, 3, sizeof (header.io_write));
This does the initial MsgReceive() (note that we didn’t use the IOV form for this —
there’s really no need to do that with a one-part message), figures out what kind of
message it is, and then continues reading the data out of the client’s address space
(starting at offset sizeof (header.io_write)) into the cache buffers specified by
the 3-part IOV.
Notice that we switched from using a 4-part IOV (in the first example) to a 3-part IOV.
That’s because in the first example, the first part of the 4-part IOV was the header,
which we read directly using MsgReceive(), and the last three parts of the 4-part IOV
are the same as the 3-part IOV — they specify where we’d like the data to go.
You can imagine how we’d perform the reply for a read request:
1 Find the cache entries that correspond to the requested data.
2 Fill an IOV structure with those entries.
3 Use MsgWritev() (or MsgReplyv()) to transfer the data to the client.
Note that if the data doesn’t start right at the beginning of a cache block (or other data
structure), this isn’t a problem. Simply offset the first IOV to point to where the data
does start, and modify the size.
All the message-passing functions except the MsgSend*() family have the same
general form: if the function has a “v” at the end of it, it takes an IOV and a
number-of-parts; otherwise, it takes a pointer and a length.
The MsgSend*() family has four major variations in terms of the source and
destinations for the message buffers, combined with two variations of the kernel call
itself.
Look at the following table:
Function Send buffer Receive buffer
MsgSend() Linear Linear
MsgSendnc() Linear Linear
MsgSendsv() Linear IOV
MsgSendsvnc() Linear IOV
MsgSendvs() IOV Linear
MsgSendvsnc() IOV Linear
continued. . .
112 Chapter 2 • Message Passing April 30, 2009