Re: if file exist

[nickroberts@blueyonder.co.uk (Nick Roberts)]

On 23 Sep 2002 06:38:22 -0600, Kilgallen@SpamCop.net (Larry Kilgallen)
strongly typed:

>On some operating systems the existence of a file for which the user
>lacks read permission cannot be detected.  A general solution for
>detecting unreadable files is not possible.

In addition, if you were to do, e.g. on MS-DOS:

   Open(F1,In_File,"COM1:");

You could get failure for a variety of reasons nothing to do with the
existence, non-existence, or readability of files.

In particular, it is quite possible that a file existence detector may say
"COM1:" does not exist (as a file, or as a readable file, or as a readable
text file), yet the above statement may succeed (and be useful).

In full generality, it is not possible for a program to detect the
existence of files for the purposes of determining whether it can
subsequently open them for a particular purpose. On any multi-processing
system, for example, the file could be deleted in between the detection and
the opening.

As far as the Ada program is concerned, the file name "foo" might mean:
hack into the CIA central computer in CIACHQ Langley VA*; select a file at
random with "foo" somewhere in the name; decrypt it using Blowfish and the
key "Oojalum123"; reverse the order of the byte contents; send an e-mail to
Tom Cruise, telling him what a handsome guy he is; insert the names of
known KGB agents into the byte stream at 4KB intervals. The OS can
interpret names how it likes.

In general, I recommend that Ada code simply handles the Name_Error
exception appropriately.

[* properly, now named the George Bush Center for Intelligence]

-- 
Nick Roberts
Per Ardua ad Disastra