Explicit Input and Output Streams

The predicates below are part of the Quintus compatible stream-based I/O package. In this package streams are explicitely created using the predicate open/3. The resulting stream identifier is then passed as a parameter to the reading and writing predicates to specify the source or destination of the data.

open( +SrcDest, +Mode, ?Stream)

  SrcDest is either an atom, specifying a Unix file, or a term `pipe( Command)', just like see/1 and tell/1. Mode is one of read , write or append . Stream is either a variable, in which case it is bound to a small integer identifying the stream, or an atom, in which case this atom will be the stream indentifier. In the latter case the atom cannot be an already existing stream identifier. Examples:

open_null_stream( ?Stream)

  On Unix systems, this is equivalent to open('/dev/null', write, Stream) . Characters written to this stream are lost, but the stream information (see character_count/2, etc.) is maintained.

close( +Stream)

  Close the specified stream. If Stream is not open an error message is displayed. If the closed stream is the current input or output stream the terminal is made the current input or output.

current_stream( ?File, ?Mode, ?Stream)

  Is true if a stream with file specification File, mode Mode and stream identifier Stream is open. The reserved streams user and user_error are not generated by this predicate. If a stream has been opened with mode append this predicate will generate mode write.

stream_position( +Stream, -Old, +New)

  Unify the position parameters of Stream with Old and set them to New. A position is represented by the following term:

'$stream_position'(CharNo, LineNo, LinePos).

It is only possible to change the position parameters if the stream is connected to a disk file. If the position is changed, the CharNo field determines the new position in the file. The LineNo and LinePos are copied in the stream administration.