21.2.27 file-position [Function]

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.

Description::

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.

Examples::

 (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))))
⇒  tester
 (tester)
⇒  (0 2 T 2 0 2 5 7)
OR⇒ (0 2 NIL 3 0 3 5 6 7)
OR⇒ (NIL NIL NIL NIL NIL NIL)

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

Notes::

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.