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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
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Subject: Re: File manipulations in Ada.Text_IO
Date: Mon, 1 Aug 2005 16:12:14 -0500
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Date: 2005-08-01T16:12:14-05:00
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"Larry Kilgallen" <Kilgallen@SpamCop.net> wrote in message
news:67kFmodka+Zb@eisner.encompasserve.org...
> In article <wauHe.7071$0C.1043@newsread3.news.pas.earthlink.net>, Jeffrey
Carter <spam@spam.com> writes:
> > Georg Bauhaus wrote:
> >>
> >> One way using standard Ada is an attempt to open the file.
> >> If the external file doesn't exist, exception Name_Error
> >> is raised.
> >
> > Ada 0X will have Ada.Directories as part of the standard library, which
> > will include such a function.
>
> But such a function for a non-wildcarded filename is always going
> to be susceptible to a race condition if the next step would be
> to open the file.  The file might be deleted or created between
> the test and using the information gained from the test.

True, but that's a general problem with I/O, not specifically with the
directories operations. Generally, if you're doing I/O, you have to handle
exceptions and never assume that they won't be raised, no matter what
pretests you might have done.

For an obvious example, try appending to a file, creating it if it isn't
there. Even a version using exceptions:

    begin
          Open (...);
   exception
         when Name_Error =>
                Create (...);
   end;

has a built-in race condition. That cannot be avoided with the operations
available in Ada - it's best to just do whatever you have to do and deal
with unusual cases with global exception handlers (and avoid pretests as
much as possible).

                               Randy.