On Sun, 19 Apr 2015 17:19:36 -0700 (PDT), in message
d339042b-1cb8-4ca5-ba9d-3c70a8314cbd@googlegroups.com,
gnuist006@gmail.com wrote:

> emacs does not record the position at which it opened a file.

Indeed, by default it does not.  However, you can set it to do so.
See the EmacsWiki page:

    <http://www.emacswiki.org/emacs/SavePlace>

> If I
> move the cursor and then I say
> 
> C-u SPC, there is "no mark set in this buffer" 

C-u SPC should give you four spaces.  If it doesn't, you've got
something mis-configured somewhere.  C-x SPC sets the mark for
rectangle mode; C-SPC sets the mark.  

> I am sick of C-@ as I tend to forget.

Use C-SPC instead.

> Is there any remedy like this but it does not work, I tried
> 
> (set-mark-command nil)
> (set-mark-command (point))

"set-mark-command" is a function that is bound to a keystroke.  By
default, C-@ and C-SPC.  Either will work.

Have you gone through the Emacs tutorial?  If not, you should;
it's got lots of very useful information.  It's the first entry in
the "Help" menu, or available through "C-h t" if you don't use the
menus.  The Info manual is also very useful, especially the
Concept Index.

Hope this helps.

--Dale

-- 
+----------+  +----------+  +-----------+  +---------+  +-------+
| Space is |  | Space is |  | It's Hard |  | A Place |  | Burma |
|   Big    |  |   Dark   |  |  to Find  |  | to Park |  | Shave |
+----------+  +----------+  +-----------+  +---------+  +-------+