Stream

A stream is a connection to a file. The terminal is treated as a special file. A stream can be referenced 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 that is 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 that is identified by FileName already exists, then the file is emptied; otherwise, a file with the name FileName is created.
  • append - Output. This is similar to write, except that the contents of a file will not be lost if the file already exists.

  The list of stream-options is optional. The list can either be empty, or it can include^8.1:

  • 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 the name Atom. A stream-alias can appear anywhere that a stream can occur. A stream can be given multiple names, but an atom cannot be used as the name of more than one stream.

  • eof_action(Atom). Specifies what to do upon repeated attempts to read past the end of the file. Atom can be^8.2:
    • error - raises an error condition.
    • eof_code (the default) - makes each attempt return the same code that the first attempt returned (-1 or end_of_file).

• close(Stream,Options):
• close(Stream): Closes a stream identified by Stream, which is 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 all cases.

• stream_property(Stream,Property): This predicate is true if the stream that is identified by the stream identifier or the stream alias Stream has a stream property Property. Property may be one of the following^8.3:
  • file_name(Name) - the file name.
  • mode(M) - input or output.
  • alias(A) - A is the stream's alias, if any.
  • end_of_stream(E) - E is either at, past, or no, indicating whether reading has just reached the end of file, has gone past the end of file, or has not reached the end of file.
  • eof_action(A) - the action taken upon reading past the end of file.
  • type(T) - T is the type of the file.

• current_input(Stream): This predicate is true if the stream identifier or the stream alias Stream identifies the current input stream.

• current_output(Stream): This predicate is true if the stream identifier or the 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: This predicate is true if the current input stream has reached the end of file, or is past the end of file.

• at_end_of_stream(Stream): This predicate is true if the input stream Stream has reached the end of file, or is past the end of file.