file-position stream position

    file-position stream position-spec success-p

    Arguments and Values

    stream — a stream.

    position-spec — a file position designator.

    position — a file position or nil.

    success-p — a generalized boolean.


    Returns or changes the current position within a stream.

    When position-spec is not supplied, file-position returns the current file position in the stream, or nil if this cannot be determined.

    When position-spec is supplied, the file position in stream is set to that file position (if possible). file-position returns true if the repositioning is performed successfully, or false if it is not.

    An integer returned by file-position of one argument should be acceptable as position-spec for use with the same file.

    For a character file, performing a single read-char or write-char operation may cause the file position to be increased by more than 1 because of character-set translations (such as translating between the Common Lisp #\Newline character and an external ASCII carriage-return/line-feed sequence) and other aspects of the implementation. For a binary file, every read-byte or write-byte operation increases the file position by 1.

     (defun tester () 
       (let ((noticed '()) file-written) 
         (flet ((notice (x) (push x noticed) x)) 
           (with-open-file (s "test.bin" 
                              :element-type '(unsigned-byte 8) 
                              :direction :output 
                              :if-exists :error) 
              (notice (file-position s)) ;1 
              (write-byte 5 s) 
              (write-byte 6 s) 
              (let ((p (file-position s))) 
                (notice p) ;2 
                (notice (when p (file-position s (1- p))))) ;3 
              (write-byte 7 s) 
              (notice (file-position s)) ;4 
              (setq file-written (truename s))) 
            (with-open-file (s file-written 
                               :element-type '(unsigned-byte 8) 
                               :direction :input) 
              (notice (file-position s)) ;5 
              (let ((length (file-length s))) 
                (notice length) ;6 
                (when length 
                  (dotimes (i length) 
                    (notice (read-byte s)))))) ;7,... 
            (nreverse noticed)))) 
     (0 2 T 2 0 2 5 7) 
    or  (0 2 NIL 3 0 3 5 6 7) 
    Side Effects

    When the position-spec argument is supplied, the file position in the stream might be moved.

    Affected By

    The value returned by file-position increases monotonically as input or output operations are performed.

    Exceptional Situations

    If position-spec is supplied, but is too large or otherwise inappropriate, an error is signaled.

    See Also

    file-length, file-string-length, open


    Implementations that have character files represented as a sequence of records of bounded size might choose to encode the file position as, for example, record-number*max-record-size+character-within-record. This is a valid encoding because it increases monotonically as each character is read or written, though not necessarily by 1 at each step. An integer might then be considered “inappropriate” as position-spec to file-position if, when decoded into record number and character number, it turned out that the supplied record was too short for the specified character number.