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

Opens the I/O source or sink SourceSink in mode Mode and associates it with the stream identifier Stream.

+SourceSink: Atom, string or structure.
+Mode: One of the atoms read, write, update, append.
?Stream: Atom or variable.
+Options: List.

Description

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.

Stream is the identifier that is subsequently used to identify the stream. This identifier is either a name (atom) provided by the user, or a system-generated identifier (when the user passes a free variable).

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.

Options is a list of the following stream options:

alias(Name): Make the stream known under an alternative name. Name is an atom. See also set_stream/2.
compress(OnOff): a hint for output operations (e.g. write_exdr/2) to use a more compact output format (output streams only). OnOff is one of the atoms on or off. The default is on (for files, pipes and sockets) or off (for queues and string streams).
end_of_line(CrLf): This option affects only write-streams and determines which end-of-line character sequence is written by predicates like nl/1, writeln/1 and printf/3. Possible values are the atoms lf and crlf. The default for string and queue streams is lf, for other streams it is operating-system dependent.
event(Name): This option is intended for queue streams in embedded applications of ECLiPSe. It configures a read-queue to raise the named event whenever the host program writes something to the previously empty queue. Name must be an atom.
flush(Where): This option affects only write-streams and allows to configure a stream to automatically flush after every line written. Where is one of the atoms end_of_line (flush automatically after every line) or flush (require explicit flushing). The default setting is flush, except for tty streams where the default is end_of_line.
macro_expansion(OnOff): Input streams only. Specifies whether term-macros (see macro/3) will be expanded when reading from this stream using read/2, read_term/3 etc. OnOff is one of the atoms on or off, the default is on.
output_options(OptionList): Write-streams only. Specifies the default output options that will be used when printing terms onto this stream, e.g. using write/2. The format of OptionList is the same as the one accepted by the write_term/2,3 predicates. Note that certain output predicates can override these default settings, e.g. writeq/2, write_term/3, printf/3, etc.
yield(OnOff): This option is intended for queue streams in embedded applications of ECLiPSe. It configures the stream to yield control to the host program whenever a read-queue reaches end-of-file or a write-queue gets flushed. See the Embedding Manual for more details. OnOff is one of the atoms on or off.

Note that streams are not closed on backtracking through the call to open/3.

Fail Conditions

None.

Resatisfiable

No.

Exceptions

(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.

Examples