Getting Started with QNX Neutrino - QNX Software Systems

© 2009, QNX Software Systems GmbH & Co. KG. Using message passing
Notice that the message transfer has specified an offset of sizeof
(header.io_write) in order to skip the write header that was added by the client’s
C library. We’re assuming here that cache_buffer [index].size is actually 4096
(or more) bytes.
Similarly, for writing data to the client’s address space, we have:
#include
int MsgWrite (int rcvid,
const void *msg,
int nbytes,
int offset);
MsgWrite() lets your server write data to the client’s address space, starting offset
bytes from the beginning of the client-specified “receive” buffer. This function is most
useful in cases where the server has limited space but the client wishes to get a lot of
information from the server.
For example, with a data acquisition driver, the client may specify a 4-megabyte data
area and tell the driver to grab 4 megabytes of data. The driver really shouldn’t need to
have a big area like this lying around just in case someone asks for a huge data
transfer.
The driver might have a 128 KB area for DMA data transfers, and then message-pass
it piecemeal into the client’s address space using MsgWrite() (incrementing the offset
by 128 KB each time, of course). Then, when the last piece of data has been written,
the driver will MsgReply() to the client.
128 KB chunk
128 KB chunk
128 KB chunk
1
2
3
128 KB chunk
Client's address space
Driver's address space
Transferring several chunks with MsgWrite().
Note that MsgWrite() lets you write the data components at various places, and then
either just wake up the client using MsgReply():
MsgReply (rcvid, EOK, NULL, 0);
or wake up the client after writing a header at the start of the client’s buffer:
MsgReply (rcvid, EOK, &header, sizeof (header));
This is a fairly elegant trick for writing unknown quantities of data, where you know
how much data you wrote only when you’re done writing it. If you’re using this
method of writing the header after the data’s been transferred, you must remember to
leave room for the header at the beginning of the client’s data area!
April 30, 2009 Chapter 2 • Message Passing 107