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set_stream(+Alias, +Stream)

The symbolic stream name Alias is associated with the stream Stream.
Stream handle or alias (atom)


This predicate is used to create new symbolic names (aliases) for streams, or to redirect existing symbolic stream names to other streams.

If Alias is a new user-defined stream name, then that new name is associated with the stream denoted by the Stream. Stream can be given in the form of a stream handle or an already existing alias name.

If Alias is an already existing stream name (including one of the symbolic system stream names like input, output, error, warning_output, log_output), then that stream is redirected to Stream. Note that the setting of the 'input' and 'output' aliases determines the streams used by the I/O predicates that have no stream argument.

Any previous association of the name Alias is forgotten. Other alias names for the same physical stream are not affected by redirection.

When a user-defined symbolic stream is closed, the associated physical stream is closed and the association forgotten. Note that it is not enough to close the stream only via a handle, because the association of symbolic an physical stream remains (even though the physical stream is closed) until the stream alias is closed as well. A system-defined stream alias however will be automatically redirected back to its default when the associated physical stream is closed.

An anonymous stream handle associated with a symbolic stream name can be obtained using get_stream/2 or get_stream_info/3.

An alternative way of creating a stream alias is to specify it during stream creation, i.e. as the Stream-argument to open/3, socket/3, etc., or by using an alias(Alias) option in open/4.

The standard stream aliases stdin, stdout, stderr and null cannot be redirected.

Modes and Determinism


(4) instantiation fault
Either Alias or Stream is uninstantiated.
(5) type error
Either Alias is not an atom or Stream is not a stream handle or symbolic stream name.
(193) illegal stream specification
Stream is an illegal stream specification.
(196) trying to modify a system stream
Stream is fixed system stream.


        % suppress standard output temporarily:
        ?- set_stream(output, null).
        Yes (0.00s cpu)
        ?- writeln(hello).
        Yes (0.00s cpu)

        % set standard output back to default:
        ?- set_stream(output, stdout).
        Yes (0.00s cpu)
        ?- writeln(hello).
        Yes (0.00s cpu)

        % alias the names s and output:
        ?- open(file1,update,s), set_stream(output,s),
           writeln(output,hi), flush(output).
        ?- seek(s,0), read(s,X).
        X = hi

        set_stream(a, S).        (Error 4).
        set_stream(1.0, S).      (Error 5).
        set_stream(a, nonex).    (Error 193).

See Also

open / 4, get_stream / 2