at(+Stream, -Pointer)

Stream

Stream handle or alias (atom)

Pointer

Integer or variable.

Description

• For file streams, Position is the position in the file (the offset in bytes from the beginning of the file) where the next read/write operation will occur.
• For tty read streams, Position is the number of bytes already read from the tty. For tty write streams, Position is the number of bytes already written to the tty.
• For pipe read streams, Position is the number of bytes already read from the pipe. For pipe write streams, Position is the number of bytes already written to the pipe.
• For socket streams, Position is the number of bytes already read from the socket.
• For string streams, Position is the position in the string (the offset in bytes from the beginning of the string) where the next read/write operation will occur.
• For queue read streams, Position is always 0. For queue write and queue update streams, Position is the number of unread bytes in the queue.
• For the null stream, Position is always 0.

Modes and Determinism

• at(+, -) is det

Exceptions

(4) instantiation fault

Stream is not instantiated.

(5) type error

Pointer is instantiated to a non-integer.

(5) type error

Stream is instantiated to neither to an atom nor a stream handle.

(192) illegal stream mode

Stream is an illegal stream mode.

Examples

Success:
      ?- open(file1,write,S), at(S, P1),
         write(S, 1234567890), at(S, P2), close(S).
      P1 = 0
      P2 = 10


      ?- open(file1,update,S), at(S, P1), read(S, T),
         at(S, P2), close(S).
      P1 = 0
      T = 1234567890
      P2 = 10

Error:
      at(Stream, 4).      (Error 4).
      at("3", Position).   (Error 5).
      at(7, 4.3).         (Error 5).

See Also