Re: A Text_IO Question

[dewar@merv.cs.nyu.edu (Robert Dewar)]

iJohn Herro says, answering a question

ajaskey@gnn.com (Andy Askey) writes:
> The C program writes a ... dummy header to the
> top of the file, process a lot of data, ... moves the
> file pointer back to the top of the file, and overwrites
> the dummy header ... I am just wondering if this is
> possible with Text_IO.

Unfortunately, no.  To achieve random access, you need to instantiate
Direct_IO.  There are two ways to consider solving your problem.  The
first is to instantiate Direct_IO for type Character.  The effect will
depend on the system you're running.  Instantiating Direct_IO for type
Character works great on a PC, and I've done it several times.  On a VAX
running VMS, however, you can create a random access file for type
Character, but you can't later read it as a plain text file.  The file
"attributes" are different, and the system remembers that you have a
random access file and not a plain text file.  DOS doesn't care.


That's poor advice for Ada 95 at least. Using Stream_IO would be far
more appropriate, and will solve the given problem precisely, whereas
Direct_IO is quite inappropriate (it is likely to be slow, and the use
described by John is highly non-portable, there is no guarantee that when
you instnatiate direct_IO on charaer that the records will be single
charaters.