copy-file: ok-if-already-exists versus mustbenew

copy-file: ok-if-already-exists versus mustbenew

[Lennart Borgman]

I am trying to enhance my reading capabilities, but I can not understand 
the difference between the arguments ok-if-already-exists and mustbenew 
in the doc for `copy-file'. Can someone please explain this to me?

The only difference I see is that  "never overwrite" is mentioned for 
"mustbenew" - though I hope this holds for the first argument too. 
However it sounds like the second argument should be about modification 
times, but the doc string does not say so.

Re: copy-file: ok-if-already-exists versus mustbenew

[Luc Teirlinck]

Lennart Borgman wrote:

   I am trying to enhance my reading capabilities, but I can not understand 
   the difference between the arguments ok-if-already-exists and mustbenew 
   in the doc for `copy-file'. Can someone please explain this to me?

   The only difference I see is that  "never overwrite" is mentioned for 
   "mustbenew" - though I hope this holds for the first argument too.

I do not immediately see the difference either.  mustbenew was
introduced by the following change:

2005-04-23  Richard M. Stallman  <rms@gnu.org>

	    * fileio.c (Fcopy_file): New arg MUSTBENEW.
	    (Frename_file): Pass new arg to Fcopy_file.

I do not immediately understand why Frename_file could not have used
the ok-if-already-exists arg.

   However it sounds like the second argument should be about modification
   times, but the doc string does not say so.

I do not understand this one.  I assume you mean the second _optional_
argument.  From the docstring:

   Fourth arg keep-time non-nil means give the output file the same
   last-modified time as the old one.  (This works on only some systems.)

Another problem is that the preserve-uid-gid arg is not mentioned in
the `(elisp)Changing Files' description of copy-file.

Sincerely,

Luc.

Re: copy-file: ok-if-already-exists versus mustbenew

[Lennart Borgman]

Luc Teirlinck wrote:
> Lennart Borgman wrote:
>
>    I am trying to enhance my reading capabilities, but I can not understand 
>    the difference between the arguments ok-if-already-exists and mustbenew 
>    in the doc for `copy-file'. Can someone please explain this to me?
>
>
>   
...
>    However it sounds like the second argument should be about modification
>    times, but the doc string does not say so.
>
> I do not understand this one.  I assume you mean the second _optional_
> argument.  From the docstring:
>   
By "second" I meant "mustbenew" here.

Re: copy-file: ok-if-already-exists versus mustbenew

[Richard Stallman]

There are two meaningful non-nil settings for MUSTBENEW: `excl' and
anything else (such as t).

Using t seems to be more or less equivalent to specifying an integer
for OK_IF_ALREADY_EXISTS, but `excl' does something different: it
passes the flag to `open' which causes it to fail if the file exists.
That feature is used in `backup-buffer-copy'.

It would be easy to eliminate the special case for t, since that
does nothing new.  However, the `excl' case does something new.
I see two ways to eliminate it:

* Set the O_EXCL flag for OK_IF_ALREADY_EXISTS = `excl' instead.

* Set the O_EXCL flag when OK_IF_ALREADY_EXISTS is any symbol.

Does anyone see a problem with either of these?