Reads data from the pseudoterminal. The PTD$READW
routine completes synchronously; that is, it returns to the caller
after the data has been read.
For asynchronous completion, use the PTD$READ
routine. The PTD$READ routine is identical to the PTD$READW routine
in every way except that PTD$READ returns to the caller without waiting
for the data to be read.
PTD$READW [efn], chan [.astadr] [,astprm] readbuf, readbuf_len