CD/DVD/ISO images, readcd adding extra bytes at EOF?

[aluminumsulfate at earthlink.net]

   From: "Ben Scott" <dragonhawk at gmail.com>
   Date: Tue, 19 Sep 2006 07:42:49 -0400

     I made an image copy of the DVD using the readcd(1) command:

	     readcd dev=/dev/scd0 f=FC-5-i386-DVD-readcd.iso

   -rw-rw-r-- 1 root   pubadmin 3253698560 Sep 18 21:59 FC-5-i386-DVD-readcd.iso
   -rw-rw-r-- 1 bscott pubadmin 3253669888 Sep 19 00:57 FC-5-i386-DVD.iso

     Anyone know what that extra 30 KB or so at the end of the readcd
   image is, and why this happens?  Is there a way to have produce an
   image without extra stuff at the end of it?

I too have seen something like this... ironically enough only on Red Hat 6.
When copying data cds using dd (with no count= argument), the image file
contained a few more zero (ASCII 0) sectors than the original medium.  Ever
since then, I run an extra "dd skip=" just to make sure it got the right
device length.

$ ls -l image.iso
-rw-------   1 user group     671088640 Sep 19 07:07 image.iso
$ dd if=/dev/cdrom of=image.iso
1310720+0 records in
1310720+0 records out
$ dd if=/dev/cdrom skip=1310720               # should report 0 records in/out
$ dd if=/dev/cdrom skip=1310719 of=/dev/null  # should report 1 record in/out
$ echo $((1310720 * 512))                     # should match file size from ls

I don't know how readcd reads the cd, but I know that dd has a
"conv=sync,noerror" option.  This tells dd, when it encounters a read
error, to pad the read sector with ASCII 0 and to try to keep reading.
But a device EOF should not be mistaken as a media error by ANY
program.  So, maybe the actual spiral of pits pressed into the surface
of the disk is a few sectors shorter than the disc's table of contents
claims it is.  Or it could just be one of tohse Red Hat things. :)