Getting Started with QNX Neutrino - QNX Software Systems

Using message passing
© 2009, QNX Software Systems GmbH & Co. KG.
You may have noticed that there are two sizes for every buffer transfer (in the client
send case, there’s sbytes on the client side and rbytes on the server side; in the server
reply case, there’s sbytes on the server side and rbytes on the client side.) The two sets
of sizes are present so that the programmers of each component can specify the sizes
of their buffers. This is done for added safety.
In our example, the MsgSend() buffer’s size was the same as the message string’s
length. Let’s look at the server and see how the size is used there.
Server framework
Here’s the overall structure of a server:
#include
...
void
server (void)
{
int rcvid; // indicates who we should reply to
int chid; // the channel ID
char message [512]; // big enough for our purposes
}
// create a channel
chid = ChannelCreate (0);
// this is typical of a server: it runs forever
while (1) {
}
// get the message, and print it
rcvid = MsgReceive (chid, message, sizeof (message),
NULL);
printf ("Got a message, rcvid is %X\n", rcvid);
printf ("Message was \"%s\".\n", message);
// now, prepare the reply. We reuse "message"
strcpy (message, "This is the reply");
MsgReply (rcvid, EOK, message, sizeof (message));
As you can see, MsgReceive() tells the kernel that it can handle messages up to
sizeof (message) (or 512 bytes). Our sample client (above) sent only 28 bytes
(the length of the string). The following diagram illustrates:
Client
28
bytes
Server
28
bytes
484
bytes
not
written
Transferring less data than expected.
96 Chapter 2 • Message Passing April 30, 2009