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open(++SourceSink, +Mode, ?Stream)

Opens the I/O source or sink SourceSink in mode Mode and associates it with the stream identifier Stream.
Atom, string or structure.
One of the atoms read, write, update, append.
Atom or variable.


This predicate opens an ECLiPSe I/O stream.

The most common use is for opening files. In this case, SourceSink is a file name (atom or string).

Mode is one of the following

    read         open for reading
    write        open for writing
    update       open for reading and writing
    append       open for writing at the end
A file must already exist if it is to be opened in read mode. A file opened in append mode is opened in write mode at the end of the file. If an existing file is opened in write mode, it is truncated to zero size, i.e. its previous content is lost.

If Stream is a variable, it will be instantiated to a system-generated stream handle. This handle can subsequently be used to identify the stream. If Stream is an atom, then this atom will be used as a symbolic alias name for the stream (like with the alias(Name) option of open/4, or set_stream/2). The use of handles should be preferred.

If SourceSink is of the form string(InitialString), then a so-called string stream is opened. A string stream is basically an in-memory file and its initial contents is the string InitialString. A string stream can be used like any other stream, i.e. it is possible to read, write and seek like on a true file. The current contents of a string stream can at any time be retrieved as a whole using get_stream_info(Stream, name, Contents).

If SourceSink is of the form queue(InitialString), then a queue stream is opened. It behaves like a string that can be written at the end and read from the beginning. Seeking is not allowed on queues. The current contents of a queue can at any time be retrieved as a whole using get_stream_info(Stream, name, Contents). Queues are considered to be at end-of-file while they are empty. Queues can be configured to raise an event every time something is written to the previously empty queue (see open/4).

If SourceSink is of the form fd(Integer), then the stream in opened onto an existing operating system file descriptor.

Lifetime of Streams

A stream lives until it is closed. Streams that are only referenced by handle are closed automatically, either on failure across the open/3,4 predicate, or after all copies of their handle become unused and garbage collected. This means that no extra precautions have to be taken to ensure that streams are closed on failure or when aborting. Handle-streams can optionally be closed explicitly if their lifetime is statically known in the program. Streams that have aliases cannot be closed automatically: all aliases must be closed explicitly. NOTE: Stream handles are not normal Prolog terms! They can not be assembled, decomposed, or occur in Prolog text.

Modes and Determinism


(4) instantiation fault
File or Mode is not instantiated.
(5) type error
File is not an atom, string or structure.
(5) type error
Mode is not an atom.
(5) type error
Stream is not an atom or a variable.
(170) system interface error
The operating system cannot open the file.
(192) illegal stream mode
Mode is an atom, but is not a valid mode.


    ?- open(file1, write, S), write(S, foo), close(S).
    S = $&(stream,"")

    ?- open(file1, update, S), read(S,X), write(S,bar), close(S).
    X = foo
    S = $&(stream,"")

    ?- open(file1, append, S), write(S, baz), close(S).
    S = $&(stream,"")

    ?- open(file1, read, mystr), read(mystr,X), close(mystr).
    X = foobarbaz

    ?- open(string("foo"), update, S),
                 read(S,X), write(S,bar),
                 get_stream_info(S, name, Contents), close(S).
    X = foo
    Contents = "foobar"

    ?- open(queue(""), update, S),
                 write(S, hello), read(S, X), close(S).
    X = hello

    ?- event_create(writeln(my_event_handler), Event),
                 open(queue(""), write, S, [event(Event)]),
                 write(S, hello).
    S = $&(stream,7)

       open(Var,update,s).      (Error 4).
       open(file1,Mode,s).      (Error 4).
       open(2,update,s).        (Error 5).
       open(file1,"str",s).     (Error 5).
       open(file1,update,9).    (Error 5).
       open(nonex,read,s).      (Error 170). % no such file
       open(file1,atom,s).      (Error 192). % no such mode

See Also

open / 4, existing_file / 4, close / 1, set_stream / 2, at / 2, at_eof / 1, current_stream / 1, get_stream_info / 3, seek / 2, stream_select / 3, stream_truncate / 1