File Format Compatibility between different compilers

File Format Compatibility between different compilers

[casado]

Imagine this scenario on one computer:

There are two different Ada compilers, C_A and C_B;
There is a source program to create a direct file F, P_C.
There is a source program to read a direct file F, P_R.

P_C is compiled by C_A to obtain a object program P_C_A.
P_C is compiled by C_B to obtain a object program P_C_B.

P_R is compiled by C_A to obtain a object program P_R_A.
P_R is compiled by C_B to obtain a object program P_R_B.

P_C_A is executed to produce the file F_A.
P_C_B is executed to produce the file F_B.

F_A is read by P_R_A and no problem is detected. This is normal.
F_B is read by P_R_B and no problem is detected. This is normal.

F_A is read by P_R_B and  problems are detected. Is this  normal?
F_B is read by P_R_A and  problems are detected. Is this  normal?

The problems detected were to read meaningless data when the file
has been created by a program that was compiled by a different compiler.

Has anybody tried this scenario?

I did it when I compiled a Pretty_printer that one friend had modified.

The compiler utilized by my friend and the one I utilized come from
different manufacturer.

He created several parameters files for his Pretty_Printer.

My Pretty_printer read these files, but the data contained in them were
meaningless so my Pretty_printed aborted the execution.I had to create
these files with the programs compiled by my compiler.

Later, I searched in the LRM, and in <14.1:7> you can read:

The language does not define what happens to external files after the
completion of the main program (in particular, if corresponding files
have not been closed). The effect of input_output for access types is
					   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
implementation-depended.
^^^^^^^^^^^^^^^^^^^^^^^

I have concluded that this kind of subtleties must be normalized if we
want to mix tools coming from different manufactures. I do not want to
imagine how many time the people is wasting on this kind of problems.

Do you think that this must be normalized?

Luis M. Casado

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Re: File Format Compatibility between different compilers

[Robert I. Eachus]

In article <313@enstb.enst-bretagne.fr> casado@enstb.enst-bretagne.fr (casado) writes:

> The problems detected were to read meaningless data when the file
> has been created by a program that was compiled by a different compiler.

> Has anybody tried this scenario?

    This is a very good question, but not an easy one to answer. What
an external file looks like is not in the domain of the Ada standard
(there are other ANSI and ISO standards on the subject), and therefore
it is difficult for the RM to mandate a bit layout in an external
file.  Most Ada compiler vendors do a good job of mapping TEXT_IO onto
the standard text file formats for a particular architecture, but when
it come to DIRECT_IO and SEQUENTIAL_IO the situation gets much
messier.

    The easiest thing for a compiler to do is to provide generic IO
packages which copy the internal representations of objects to
external files unchanged.  However different vendors will layout the
same record type differently even on the same hardware, but the bigger
differences are in two different ways to handle discriminated records
and unconstrained array objects.

     Defining a record type EXACTLY with representation clauses is
often enough to make files portable between compilers, but I have had
trouble getting ANSI standard text files from a VAX running VMS to a
VAX running Ultrix, even when using ANSI standard labelled tapes.
--

					Robert I. Eachus

with STANDARD_DISCLAIMER;
use  STANDARD_DISCLAIMER;
function MESSAGE (TEXT: in CLEVER_IDEAS) return BETTER_IDEAS is...