NAME
dd_driver - Data descriptors globus_xio uses data descriptors to
associate meta data with the data being writen or the data read.
Data descriptors flow into the drivers read and write interface
functions by way of the operation structure. If the driver is
interested in viewing the data decriptor it can request it from the
operation structure via a call to
globus_xio_driver_operation_get_data_descriptor() and it can view any
driver specific data descriptor via a call to
globus_xio_driver_data_descriptor_get_specific(). The driver can modify
values in the data descriptor by setting values before passing the
request down the stack. Several functions are available to modify the
data descriptors. There is no need to ’set()’ the data descriptors back
into the operation. The functions for manipluating the values in a DD
affect the values xio has directly.
Data descriptors flow back to the driver in the callbacks for the data
operations. When calling finished operation on a data operation the
driver must pass in a data descriptor. It should get this data
descriptor from the io operation callback.
Life Cycle:
Passing in a data descriptor: A data descriptor is first created by the
globus_xio user. The user can add driver specific data descriptors to
it. Once the usre has created and set the attributes on its data
descriptor to their liking they pass it into a globus_xio data
operation (either read or write). When the data descriptor is passed on
globus_xio will make an internal copy of it. It does this by first
coping the user the level data descriptor and then walkinging through
the list of driver specific data descriptor contianed in to and
requesting the the driver make a copy of the driver specific data
descriptor. If ever a driver specific data descriptor is NULL
globus_xio need not call into its drivers dd_copy function. If ever the
user level data descriptor is NULL globus_xio need not deal with the
data descriptor functionality at all.
A data descriptor coming back up the stack Once an io operation reachs
the transport driver (the bottom of the stack) it takes on a slightly
different role. On the way in it is describing what is requested to be
done with the data, on the way out it is describing what has actually
been done. Once the transport driver performs the operation it should
adjust the data descriptor to reflect what has actually happened (few
drivers will need to worry about this). Each driver on the way up can
adjust the data descriptor and its driver specific data decriptor. When
xio reachs the the top of the stack it calls a user callback. When that
callback returns all memory associated with the data descriptor is
cleaned up. The interface function
globus_xio_driver_data_descriptor_free() is used for this.