Re: how to make symbolic links?

[Philip Guenther]

On Thu, Sep 8, 2011 at 4:30 PM, Mike Shal <address@hidden> wrote:
> On Thu, Sep 8, 2011 at 8:09 AM, Paul Smith <address@hidden> wrote:
>> One last thing: it is NOT necessary that the target of a symlink exist!
>> Just because you can create the link without error doesn't meant that
>> the link can be resolved.  In fact a number of programs use this feature
>> to good advantage, since creating a symlink is an atomic operation on
>> the filesystem, for creating lock files, etc. where the target of the
>> symlink is not a file at all, but rather some useful information related
>> to the lock (etc.)  If the information you want to keep is small this is
>> MUCH more efficient than open/write/close for a real file: it's one
>> (atomic) system call.
>
> Can you point to a specific program that uses symlinks in this way? I
> never thought of using it like that - it sounds pretty inventive so
> I'd like to see it in action :)

On at least OpenBSD and FreeBSD systems, the malloc() implementation
in libc does this for configuration flags given as letters in the
value of the optional symlink /etc/malloc.conf.  So, on those systems,
basically *every* program uses this idea.  For example:

$ ls -l /etc/malloc.conf
lrwxr-xr-x  1 root  wheel  1 Jul  1 15:19 /etc/malloc.conf -> S
$

That 'S' turns on additional checks and operations that aim to
increase the security of the malloc implementation in the face of
application bugs (e.g., better detection of use-after-free and
double-free occurrences).


Philip Guenther