Getting Started with QNX Neutrino - QNX Software Systems

Writing a resource manager
© 2009, QNX Software Systems GmbH & Co. KG.
typedef struct _resmgr_attr {
unsigned flags;
unsigned nparts_max;
unsigned msg_max_size;
int (*other_func) (resmgr_context_t *ctp, void *msg);
} resmgr_attr_t;
The other_func message handler
In general, you should avoid using this member. This member, if non-NULL,
represents a routine that will get called with the current message received by the
resource manager library when the library doesn’t recognize the message. While you
could use this to implement “private” or “custom” messages, this practice is
discouraged (use either the _IO_DEVCTL or _IO_MSG handlers, see below). If you
wish to handle pulses that come in, I recommend that you use the pulse_attach()
function instead.
You should leave this member with the value NULL.
The data structure sizing parameters
The flags parameter
These two parameters are used to control various sizes of messaging areas.
The nparts_max parameter controls the size of the dynamically allocated iov member
in the resource manager library context block (of type resmgr_context_t, see
below). You’d typically adjust this member if you were returning more than a one-part
IOV from some of your handling functions. Note that it has no effect on the incoming
messages — this is only used on outgoing messages.
The msg_max_size parameter controls how much buffer space the resource manager
library should set aside as a receive buffer for the message. The resource manager
library will set this value to be at least as big as the header for the biggest message it
will be receiving. This ensures that when your handler function gets called, it will be
passed the entire header of the message. Note, however, that the data (if any) beyond
the current header is not guaranteed to be present in the buffer, even if the
msg_max_size parameter is “large enough.” An example of this is when messages are
transferred over a network using Qnet. (For more details about the buffer sizes, see
“The resmgr_context_t internal context block,” below.)
This parameter gives additional information to the resource manager library. For our
purposes, we’ll just pass a 0. You can read up about the other values in the Neutrino
Library Reference under the resmgr_attach() function.
resmgr_connect_ funcs_t connect table
When the resource manager library receives a message, it looks at the type of message
and sees if it can do anything with it. In the base layer, there are two tables that affect
this behavior. The resmgr_connect_funcs_t table, which contains a list of
connect message handlers, and the resmgr_io_funcs_t table, which contains a
similar list of I/O message handlers. We’ll see the I/O version below.
206 Chapter 5 • Resource Managers April 30, 2009