For a read??!!!
You're suggesting that the behaviour of a read() should depend on what
was in the buffer before the read? Gack! Surely you have better
taste than that?
Or are you saying that a read (or write) has a side-effect of altering
some other area of memory besides the buffer you give to read()? That
seems even worse to me.
Then you end up with two system calls to get the data rather than one
(one to send the request and another to read the reply). For
something that needs to be quick that is a suboptimal interface.