argument size(of type size_t) has been assigned a default value,hence the last(5th) argument becomes an optional argument that need not be used,a really basic feature of most high level programming languages
I do not agree. The fourth argument shoudl be a cudaChannel Descriptor.
The fourth channel in cuBlas is an integer, which is the size. So something is
not quite correct.
The runtime API (as opposed to the C runtime) has a few convenience functions. One of them uses the type of the texref to set the channel descriptor. See the following function signature in the programming guide 1.1 at D.6.2.2