Why is `%!xxd -r` result wrong while `xxd -r` from console with the same input works correctly?

Question

I have the following file named asterix:

0000000: 15 00 1f c1 13 31 49 80 02 64 01 08 42 48 ef 28             
0000010: 2d cd 0c 02 00 07 f1 e9 1b 04 63 30 c7 08 20

When I open this file in vim, I do %!xxd -r, then %!xxd -g1. I get this:

0000000: 15 00 1f d0 b0 13 31 49 e2 94 80 02 64 01 08 42  ...1I...d..B
0000010: 48 d0 9e 28 2d d0 bc 0c 02 00 07 d0 af d0 98 1b  H..(-...
0000020: 04 63 30 d0 b3 08 20 0a                          .c0... .

But if in bash I do xxd -r < asterix | xxd -g1, I get the expected result:

0000000: 15 00 1f c1 13 31 49 80 02 64 01 08 42 48 ef 28  ...1I..d..BH.(
0000010: 2d cd 0c 02 00 07 f1 e9 1b 04 63 30 c7 08 20     -...c0..

I've tried doing e ++enc=c in vim to prevent possible interference of encoding, but this doesn't seem to change anything.

So what's going on? How do I make vim work correctly with binary data (up to newline at the end, of course)?

I'm using VIM 7.4 (2013 Aug 10, compiled Jan 2 2014 19:39:59) on Ubuntu 14.04. When I run vim with env -i vim asterix, it works as expected. But if I run it as usually, but logging in as a different user (the one without custom vim config, by e.g. su -), the above described behavior persists.

I've now noticed that env -i works because it resets locale. My default locale is this:

LANG=en_US
LC_CTYPE=ru_RU.UTF-8
LC_NUMERIC="en_US"
LC_TIME=en_GB.UTF-8
LC_COLLATE="en_US"
LC_MONETARY="en_US"
LC_MESSAGES="en_US"
LC_PAPER="en_US"
LC_NAME="en_US"
LC_ADDRESS="en_US"
LC_TELEPHONE="en_US"
LC_MEASUREMENT="en_US"
LC_IDENTIFICATION="en_US"
LC_ALL=

Setting LC_CTYPE=C, I get Vim to work correctly (it appends 0a to the end, but it's OK). Still I'd like to know why locale interferes with calling xxd -r.

Answer 1

Edit: notice after posting there's already this answer in the comments.

Run vim in binary mode as vim -b asterix. I think the problem has something to do with conversions caused by encoding/fileencoding settings.

If your file is already loaded, you can switch to binary mode without leaving vim. Save the file, then do :e ++bin.