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From: Kilgallen@SpamCop.net (Larry Kilgallen)
Newsgroups: comp.lang.ada
Subject: Re: if file exist
Date: 27 Sep 2002 17:09:41 -0600
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In article <3D94D418.5010604@attbi.com>, Mark Biggar <mark.a.biggar@attbi.com> writes:
> steve_H wrote:

>> But the above is not logical. If your function return FALSE, then one
>> does not know if this means the file actually does not exist, or that 
>> the function was not able to determine if it exist or not becuase of 
>> permission issues.  The user might want to know this.
> 
> No, from a computer security point of view, this is exactly what is
> wanted.  A user should see absolutely no difference between "file does
> not exist" and "you don't have permission to see the file".  Otherwise,
> you have introduced a covert information channel.

Beyond security, there are other possibilities like "this file is
shelved and all the unshelving drives are currently busy".  And as
has been mentioned by others, "this file exists but someone else
will delete it between now and when you try to open it" :-)