A Terminal.T, or terminal handle, is a file handle that provides
access to a duplex communication channel usually connected to a
user terminal.
INTERFACETerminal ; IMPORT File; TYPE T <: File.T; VAR (*CONST*) FileType: File.Type;
Equal to {\tt Atom.FromText("Terminal").}
END Terminal.Like every
File.T, a terminal handle h has the components
type(h) an atom, equal to FileType
readable(h) a boolean
writable(h) a boolean
A terminal handle is readable, or writable, or both (until it is
closed). If it is readable, it has the component
srcTerm(h) a terminal device
If it is writable, it has the component
snkTerm(h) a terminal device
A terminal device t has the components
seq(t) a sequence of bytes
r(t) a non-negative integer, the index of the next byte to read
w(t) a non-negative integer, the index of the next byte to write
flag(t) a byte reserved to mark the end-of-file in seq(t)
The meaning of the call
h.read(b, mayBlock)
is given by the specification of File.T.read together with these
definitions, where t = srcTerm(h), and k is the number of
occurrences of flag(t) in seq(t) up to r(t)-1:
src(h) = subsequence of seq(t) with all occurrences of flag(t)
deleted
srcCur(h) = r(t)-k
srcEof(h) = (seq(t)[r(t)] = flag(t))
When end-of-file is reported, r(t) is also incremented. This
means subsequent reads can return further data in seq(t).
The meaning of the call
h.write(b)
is given by the specification of File.T.write together with these
definitions, where t = snkTerm(h):
snk(h) = seq(t)
snkCur(h) = w(t)
A specific implementation may provide one or more subtypes of
Terminal.T with additional methods.
The communication channel underlying a terminal handle is necessarily monitored, since the purpose of the channel is to allow asynchronous communication between a program and a user operating a terminal device. However a terminal handle itself should be treated as unmonitored, thus avoiding the question of the unit of atomicity for reads and writes.