File manipulations in Ada.Text_IO

File manipulations in Ada.Text_IO

[Zheng Wang]

    Hi,

  I would like to ask a simple question about Ada.Text_IO, please.
How could we check if a file exists or not? I mean is there a function
or procedure that takes a File_Type/File_Name and returns true if the
File exists, otherwise return false. 
  Many thanks in advance.

Re: File manipulations in Ada.Text_IO

[Georg Bauhaus]

Zheng Wang wrote:
>     Hi,
> 
>   I would like to ask a simple question about Ada.Text_IO, please.
> How could we check if a file exists or not? I mean is there a function
> or procedure that takes a File_Type/File_Name and returns true if the
> File exists, otherwise return false. 
>   Many thanks in advance.
> 

One way using standard Ada is an attempt to open the file.
If the external file doesn't exist, exception Name_Error
is raised.


-- Georg

Re: File manipulations in Ada.Text_IO

[Jeffrey Carter]

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.

-- 
Jeff Carter
"If a sperm is wasted, God gets quite irate."
Monty Python's the Meaning of Life
56

Re: File manipulations in Ada.Text_IO

[Larry Kilgallen]

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.

Re: File manipulations in Ada.Text_IO

[Randy Brukardt]

"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.

Re: File manipulations in Ada.Text_IO

[Jeffrey Carter]

Larry Kilgallen wrote:
> 
> 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.

That's true in the general case, and something I've never encountered in 
30 years of actual SW development, including software that was intended 
to manipulate the same file from different applications running at the 
same time.

-- 
Jeff Carter
"C's solution to this [variable-sized arrays] has real problems,
and people who are complaining about safety definitely have a point."
Dennis Ritchie
25

Re: File manipulations in Ada.Text_IO

[Larry Kilgallen]

In article <suydneKD44DoEnPfRVn-ug@megapath.net>, "Randy Brukardt" <randy@rrsoftware.com> writes:
> "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.

Yes.  My point is that the utility of checking first before attempting
to open a file is vastly overrated.

Re: File manipulations in Ada.Text_IO

[Gautier Write-only]

Zheng Wang:

>     Hi,
> 
>   I would like to ask a simple question about Ada.Text_IO, please.
> How could we check if a file exists or not? I mean is there a function
> or procedure that takes a File_Type/File_Name and returns true if the
> File exists, otherwise return false.
>   Many thanks in advance.

You mean something like that ?

with Ada.Text_IO;
--
  function Exist(name:String) return Boolean is
    use Ada.Text_IO;
    f: File_Type;
  begin
    Open(f,in_file,name);
    Close(f);
    return True;
  exception
    when Name_Error => return False;
  end Exist;
______________________________________________________________
Gautier     --     http://www.mysunrise.ch/users/gdm/index.htm
Ada programming -- http://www.mysunrise.ch/users/gdm/gsoft.htm

NB: For a direct answer, e-mail address on the Web site!

Re: File manipulations in Ada.Text_IO

[Robert A Duff]

Jeffrey Carter <spam@spam.com> writes:

> Larry Kilgallen wrote:
> > 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.
> 
> That's true in the general case, and something I've never encountered in
> 30 years of actual SW development, including software that was intended
> to manipulate the same file from different applications running at the
> same time.

How do you know?  ;-)  I mean, if somebody writes code that contains
such a race condition, and their customer trips over it once in a while,
the programmer might never find out about it.

The server process crashes once in a while.  Nobody knows why.  So the
solution is to restart the server by hand.  (Yuck.)

Race conditions, by their very nature, are impossible to detect with any
confidence via testing.

- Bob

Re: File manipulations in Ada.Text_IO

[Jeffrey Carter]

Robert A Duff wrote:
> 
> How do you know?  ;-)  I mean, if somebody writes code that contains
> such a race condition, and their customer trips over it once in a while,
> the programmer might never find out about it.

This was internal SW, so I would have found out. I was concerned about 
what might happen, so I included logging to see how often it happened 
and what actually occurred when it did. After a year of use, there were 
no reports of conflicts, so I took the logging out. There might have 
been race conditions in the logging. Conflicts might have started 
happening left and right after I took the logging out. At least I had a 
warm fuzzy feeling.

-- 
Jeff Carter
"Whatever it is, I'm against it."
Horse Feathers
46