Getting Started with QNX Neutrino - QNX Software Systems

© 2009, QNX Software Systems GmbH & Co. KG. Examples
// 5) if returning data, write it to client
if (nbytes) {
MsgReply (ctp -> rcvid, nbytes, data_string + off, nbytes);
// 6) set up POSIX stat() "atime" data
ocb -> attr -> flags |= IOFUNC_ATTR_ATIME |
IOFUNC_ATTR_DIRTY_TIME;
// 7) advance the lseek() index by the number of bytes
// read if not _IO_XTYPE_OFFSET
if (xtype == _IO_XTYPE_NONE) {
ocb -> offset += nbytes;
}
} else {
// 8) not returning data, just unblock client
MsgReply (ctp -> rcvid, EOK, NULL, 0);
}
}
// 9) indicate we already did the MsgReply to the library
return (_RESMGR_NOREPLY);
Step 1
Here we ensured that the client’s open() call had in fact specified that the device was to
be opened for reading. If the client opened the device for writing only, and then
attempted to perform a read from it, it would be considered an error. In that case, the
helper function iofunc_read_verify() would return EBADF, and not EOK, so we’d
return that value to the library, which would then pass it along to the client.
Step 2
Here we checked to see if the client had specified an xtype-override — a per-message
override (e.g., because while the device had been opened in non-blocking mode, this
specifies for this one request that we’d like blocking behavior). Note that the blocking
aspect of the “xtype” override can be noted by the iofunc_read_verify() function’s last
parameter — since we’re illustrating a very simple example, we just passed in a NULL
indicating that we don’t care about this aspect.
More important, however, is to see how particular “xtype” modifiers are handled. An
interesting one is the _IO_XTYPE_OFFSET modifier, which, if present, indicates that
the message passed from the client contains an offset and that the read operation
should not modify the “current file position” of the file descriptor (this is used by the
function pread(), for example). If the _IO_XTYPE_OFFSET modifier is not present,
then the read operation can go ahead and modify the “current file position.” We use the
variable xtype to store the “xtype” that we received in the message, and the variable off
to represent the current offset that we should be using during processing. You’ll see
some additional handling of the _IO_XTYPE_OFFSET modifier below, in step 7.
If there is a different “xtype override” than _IO_XTYPE_OFFSET (and not the no-op
one of _IO_XTYPE_NONE), we fail the request with ENOSYS. This simply means that
we don’t know how to handle it, and we therefore return the error up to the client.
April 30, 2009 Chapter 5 • Resource Managers 257