I actually did try writing something at the extended position, although I
removed it before posting the above. Even with a write, it still does not
do as expected. In fact, the results are even stranger:
#lang racket
(define (say . args) (displayln (apply ~a args)))
(define the-path "./test-file")
(when (file-exists? the-path) (delete-file the-path))
(say "before open, file exists?: " (file-exists? the-path))
(let ([the-port (open-output-file the-path #:mode 'binary #:exists
'append)])
(say "after open, file exists?: " (file-exists? the-path))
(say "before repos, file size, position: " (file-size the-path) ", "
(file-position the-port))
(file-position the-port 1000); should extend the file and fill the
intervening space with 0
(say "before flush, file size, position: " (file-size the-path) ", "
(file-position the-port))
(flush-output the-port) ; ensure that the filling 0s have been written,
just to be sure
(say "before write, file size, position: " (file-size the-path) ", "
(file-position the-port))
(write "foo" the-port)
(say "before 2nd flush, file size, position: "
(file-size the-path) ", " (file-position the-port))
(flush-output the-port)
(say "finally, file size, position: " (file-size the-path) ", "
(file-position the-port))
)
Ouptut:
before open, file exists?: #f
after open, file exists?: #t
before repos, file size, position: 0, 0
before flush, file size, position: 0, 1000
before write, file size, position: 0, 1000
before 2nd flush, file size, position: 0, 1005
finally, file size, position: 5, 5
Personally, i find this even weirder. I wrote 3 characters, so why is the
file size 5? (Which it is, on disk.)
On Mon, Jun 24, 2019 at 4:57 PM Jon Zeppieri <zeppi...@gmail.com> wrote:
> On Mon, Jun 24, 2019 at 4:51 PM Jon Zeppieri <zeppi...@gmail.com> wrote:
> >
> > `lseek` docs say:
> >
> > > The lseek() function shall allow the file offset to be set beyond the
> end of the existing data in the file. If data is later written at this
> point, subsequent reads of data in the gap shall return bytes with the
> value 0 until data is actually written into the gap.
> >
>
> And the Windows call `SetFilePosition` is similar:
>
> > It is not an error to set a file pointer to a position beyond the end of
> the file. The size of the file does not increase until you call the
> SetEndOfFile, WriteFile, or WriteFileEx function. A write operation
> increases the size of the file to the file pointer position plus the size
> of the buffer written, which results in the intervening bytes uninitialized.
>
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