[Ada.]Text_IO.Create

[john@assen.demon.co.uk (John McCabe)]

I'd be grateful if anyone would let me know their thoughts, or correct
me on the following matter. I'd also appreciate any replies via email
to mccabe@mmsuk.co.uk.

I am currently trying to implement some very basic file handling on an
embedded VME PC with a floppy disk drive attached. I'm trying to make
it look similar to the Text_IO specification (I know you can get this
with some compilers, and from some third parties probably, but we
don't have it so we're doing it ourselves).

Anyway, I am currently trying to implement a Create function (like in
Text_IO), however it is not clear to me (either from the Ada 83 LRM or
the Ada 95 ARM) exactly what behaviour is expected if Create is called
for a file that already exists. It does state that it "Establishes a
new external file", however this seems a bit wooly.

I then had a look at the a-textio.ads/b and s-fileio.adb files to see
how this case was handled in GNAT, and noted that if GNAT is creating
an Out_File, it uses the open method "w" in a call to fopen which, if
the file already exists, will [probably] truncate it.

This would be the behaviour I personally would expect, and I wonder if
this is actually what is meant by "Establishes a new external file",
however I also wonder whether alternative behaviour such as raising
NAME_ERROR or USE_ERROR would be acceptable, or possibly even
appending to the existing file (but this sounds wrong).

Further, I find the concept of creating an In_File interesting. I
noted from s-fileio.adb that if a call to Create is made with the mode
In_File, the open method "w+" is passed to the fopen function (i.e.
append-ish). I'd appreciate the information if anyone knows whether
this is a typical implementation and whether there is some precedent
or advice anywhere on this matter. I did look to the Ada 95 Rationale
for information but found nothing of use on this subject.

Thanks in advance for any information.


Best Regards
John McCabe <john@assen.demon.co.uk>