Byte offset
Scott Sheffield
I have been tasked to write a program that takes an source document and
converts it to and ascii text file and writes it to a disk. I must build
a table that identifies the byte offset for parts of the ascii file as
well as a byte count for that part of the file. I am writing other
applications relating to this task in lisp and would like to write this
application in lisp also, but I unfamiliar with the functions i need to
accomplish this task. Any help in pointing me in the right direction (or
leading me by the nose) would be greatly appreciated.
Scott
Scott Sheffield
I never received an answer to my question about getting a byte offset
from lisp. I need to no if there is a lisp command that will give me a
byte offset of a point in a text file stream. I also need to no the best
way to get a byte count for a section of the text stream. Theres got to
be a lisp guru who knows this. Oh great lisp guru speak to me.....
Scott
Erik Naggum
* Scott Sheffield
man, you make it hard to answer you.
have you looked at `file-position'? does it fail to give a useful answer?
#\Erik
--
1,3,7-trimethylxanthine -- a basic ingredient in quality software.
Marco Antoniotti
Scott Sheffield <l10...@lmtas.lmco.com> writes:
>
> I never received an answer to my question about getting a byte offset
> from lisp. I need to no if there is a lisp command that will give me a
> byte offset of a point in a text file stream. I also need to no the best
> way to get a byte count for a section of the text stream. Theres got to
> be a lisp guru who knows this. Oh great lisp guru speak to me.....
>
>
> Scott
'file-position' should do it. here is what the on line ANSI doc
coming with GCL tells us. Of course I assume you are using Common Lisp.
===============================================================================
`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::
..........
*Note file-length:: , *Note file-string-length:: , *Note 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.
--
Marco Antoniotti - Resistente Umano
===============================================================================
International Computer Science Institute | mar...@icsi.berkeley.edu
1947 Center STR, Suite 600 | tel. +1 (510) 643 9153
Berkeley, CA, 94704-1198, USA | +1 (510) 642 4274 x149
===============================================================================
...it is simplicity that is difficult to make.
...e` la semplicita` che e` difficile a farsi.
Bertholdt Brecht