Stream

A stream is a connection to a file. The user's terminal is treated as a special file. A stream can be referred to by a stream identifier or its aliases. By default, the streams user_input and user_output are already open, referring to the standard input (keyboard) and the standard output (screen) respectively.

• open(+FileName,+Mode,-Stream,+Options)
• open(+FileName,+Mode,-Stream)
  Opens a file for input or output as indicated by I/O mode Mode and the list of stream-options Options. If it succeeds in opening the file, it unifies Stream with the stream identifier of the associated stream. If FileName is already opened, this predicate unifies Stream with the stream identifier already associated with the opened stream, but does not affect the contents of the file.

  An I/O mode is one of the following atoms:

  • read - Input. FileName must be the name of a file that already exists.
  • write - Output. If the file identified by FileName already exists, then the file is emptied; otherwise, a file with the name FileName is created.
  • append - Output. Similar to write except that the contents of a file will not be lost if it already exists.

  The list of stream-options is optional and can be empty or a list that includes^5.1:

  • type(text) or type(binary). The default is type(text). This option does not have any effect on file manipulations.

  • alias(Atom). Gives the stream a name Atom. A stream-alias can appear anywhere a stream can occur. A stream can be given multiple names, but an atom cannot be used as names of multiple streams.

  • eof_action(Atom). Specifies what to do upon repeated attempts to read past the end of the file. Atom can be^5.2:
    • error - raises an error condition.
    • eof_code (the default)- makes each attempted return the same code that the first one did ( -1 or end_of_file).

• close(+Stream,+Options)
• close(+Stream)
  Closes a stream identified by Stream, a stream identifier or a stream alias. The Options can include:
  • force(false) - raises an error condition if an error occurs while closing the stream.

  • force(true) - succeeds in any case.

• stream_property(?Stream,?Property) It is true if the stream identified by the stream identifier or stream alias Stream has a stream property Property. Property may be one of the following^5.3:
  • file_name(Name) - the file name.
  • mode(M) - input or outout.
  • alias(A) - A is the stream's alias if any.
  • end_of_stream(E) - where E is at, past or no, indicating whether reading has just reached the end of file, has gone past it or has not reached it.
  • eof_action(A) - action taken upon reading past the end of file.
  • type(T) - T is the type of the file.

• current_input(?Stream) It is true if the stream identifier or stream alias Stream identifies the current input stream.

• current_output(?Stream) It is true if the stream identifier or stream alias Stream identifies the current output stream.

• set_input(+Stream) Sets the stream identified by Stream to be the current input stream.

• set_output(+Stream) Sets the stream identified by Stream to be the current output stream.

• flush_output Sends any output which is buffered for the current output stream to that stream.
• flush_output(+Stream) Sends any output which is buffered for the stream identified by Stream to the stream.

• at_end_of_stream It is true if reading the current input stream has reached the end of file or is past the end of file.

• at_end_of_stream(+Stream) It is true if reading the input stream Stream has reached the end of file or is past the end of file.