These functions allow the current read or write subdevice to be changed
to another subdevice that supports streaming input (for read), or
streaming output (for write) asynchronous commands.
They return 0 on success, in which case the comedi_get_read_subdevice()
or comedi_get_write_subdevice() functions will get the updated read or
write subdevice.
Changes are local to the "open file description" that was created by
comedi_open() (actually, by open()), and have no effect on other open
file descriptions created by other calls to comedi_open() (or open())
for the same underlying Comedi device node.
Changes to the read or write subdevice is not currently supported by the
comedi.org version of the Comedi drivers, but is supported by the Linux
"in-tree" Comedi drivers since kernel version 3.19.
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