Getting Started with QNX Neutrino - QNX Software Systems

Using message passing
© 2009, QNX Software Systems GmbH & Co. KG.
Establishing a connection
So, let’s look at these functions in turn. The first thing we need to do is to establish a
connection. We do this with the function ConnectAttach(), which looks like this:
#include
int ConnectAttach (int nd,
pid_t pid,
int chid,
unsigned index,
int flags);
ConnectAttach() is given three identifiers: the nd, which is the Node Descriptor, the
pid, which is the process ID, and the chid, which is the channel ID. These three IDs,
commonly referred to as “ND/PID/CHID,” uniquely identify the server that the client
wants to connect to. We’ll ignore the index and flags (just set them to 0).
So, let’s assume that we want to connect to process ID 77, channel ID 1 on our node.
Here’s the code sample to do that:
int coid;
coid = ConnectAttach (0, 77, 1, 0, 0);
As you can see, by specifying a nd of zero, we’re telling the kernel that we wish to
make a connection on our node.
Sending messages
How did I figure out I wanted to talk to process ID 77 and channel ID 1? We’ll see that
shortly (see “Finding the server’s ND/PID/CHID,” below).
At this point, I have a connection ID — a small integer that uniquely identifies a
connection from my client to a specific channel on a particular server.
I can use this connection ID when sending to the server as many times as I like. When
I’m done with it, I can destroy it via:
ConnectDetach (coid);
So let’s see how I actually use it.
Message passing on the client is achieved using some variant of the MsgSend*()
function family. We’ll look at the simplest member, MsgSend():
#include
int MsgSend (int coid,
const void *smsg,
int sbytes,
void *rmsg,
int rbytes);
MsgSend()’s arguments are:
• the connection ID of the target server (coid),
92 Chapter 2 • Message Passing April 30, 2009