Returns or changes the current position within a stream.
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.
;;; TODO figure out the hell does this example do (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 (setf 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)
When the position-spec argument is supplied, the file position in the stream might be moved.
The value returned by file-position increases monotonically as input or output operations are performed.
If position-spec is supplied, but is too large or otherwise inappropriate, an error is signaled.
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.