Getting Started with QNX Neutrino - QNX Software Systems

Using message passing
© 2009, QNX Software Systems GmbH & Co. KG.
Let’s examine the arguments:
coid
sparts and rparts
siov and riov
The connection ID that we’re sending to, just as with
MsgSend().
The number of send and receive parts specified by the iov_t
parameters. In our example, we set sparts to 2 indicating that
we’re sending a 2-part message, and rparts to 1 indicating that
we’re receiving a 1-part reply.
The iov_t arrays indicate the address and length pairs that we
wish to send. In the above example, we set up the 2 part siov to
point to the header and the client data, and the 1 part riov to
point to just the header.
This is how the kernel views the data:
Header
Header
Header and
data chunk
Data chunk
Data chunk
Client's address space
Kernel copying data
Driver's address space
How the kernel sees a multipart message.
The kernel just copies the data seamlessly from each part of the IOV in the client’s
space into the server’s space (and back, for the reply). Effectively, the kernel is
performing a gather-scatter operation.
A few points to keep in mind:
• The number of parts is “limited” to 512 KB; however, our example of 2 is typical.
• The kernel simply copies the data specified in one IOV from one address space into
another.
• The source and the target IOVs don’t have to be identical.
Why is the last point so important? To answer that, let’s take a look at the big picture.
On the client side, let’s say we issued:
write (fd, buf, 12000);
which generated a two-part IOV of:
• header (12 bytes)
• data (12000 bytes)
110 Chapter 2 • Message Passing April 30, 2009