no way to reverse a file with tac

[Dan Jacobson]

None of tac's options can turn
$ seq 3|od -c
0000000   1  \n   2  \n   3  \n
into \n 3 \n 2 \n 1
on the stdout.

You see, there is no tool to read backward until an I/O error.
E.g. cat /cdrom/file > x
is great for the part before the error,
but we also want to see how far we can get reading from the end of the file
until we bump into the error.  All we can do at present is give dd(1)
various skip= values until we know we are beyond the error... but we don't know 
how far beyond
unless we skiped with increments of 1 byte until no problems occurred.