-reply:<4B7F7F02.1000004@freebsd.org>

[Thomas Schmitt]

Hi

> I was looking at your Linux code here and thought
> the technique of trying lseek(SEEK_END) might work.

Linux 2.6.18, /dev/hda and /dev/sr0,
with a CD-RW written by write type TAO

lseek(fd, 0, SEEK_END)=     636499968
lseek(fd, -1, SEEK_END)=    636499967
lseek(fd, -300k, SEEK_END)= 636192768

and with a CD-ROM media:

lseek(fd, 0, SEEK_END)=     618311680
lseek(fd, -1, SEEK_END)=    618311679
lseek(fd, -300k, SEEK_END)= 618004480

(No tape drive at hand. Sorry.)


Nevertheless:

SEEK_END brings you to the first unwritten
address of the file object.
With a read-only object, this address is not
necessarily defined.

Further: If you test a CD that was written by
write type TAO, then the last two blocks of the
track are no readable data blocks.
It depends on the implementation of lseek()
whether it will allow you to address bytes in
those two blocks. (Read will fail, of course.
On Linux it even fails shortly before the end
because of the old TAO read-ahead bug.)

So on CD i would test rather lseek() to an
address that is several hundred kB before the
end.
300 kB is the traditional fear-padding of CD
burn programs on Linux.


(On Linux i decide by stat(2). Considered as
seekable are S_IFREG and S_IFBLK. Actually
tested only with optical drives and USB sticks.
To my knowledge ioctl(BLKGETSIZE) works only on
S_IFBLK.)


Have a nice day :)

Thomas