file-position (original) (raw)

ANSI Common Lisp 21 Streams 21.2 Dictionary of Streams

Syntax:

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-byteor write-byteoperation 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.

Allegro CL Implementation Details:

None.