[Pw_forum] Compatibility among different versions

[Paolo Giannozzi]

On Oct 6, 2006, at 1:40 , Tianshu Li wrote:

> I am curious if there might be a compatibility problem in file I/O  
> among
> different versions, because when I used the post-processing code, for
> instance, pp.x in the latest version (v.3.1.1) to read the .save file
> generated in the previous version (v.3.0), it complained it could not
> find the .save file.

this is definitely a compatibility problem!

> The message is like:
>
> from pp_check_file : error #         1
> file  **********************/**.save not found.
>
> I noticed that in the latest version the new function "pp_check_file"
> was added to the xml_io_base.f90. This might be what gives the  
> problem,
> but I am not very sure of that. Thanks

in principle with the new format there shouldn't be any longer any
problem of incompatibility, because the file format is extensible, i.e.
if something more is needed, one just adds a new field, without
breaking the rest. Or at least, so goes the theory. In practise:

- one may realise that the way things are done is not smart and that
it needs to be changed, so that a field name or the content of a field
may no longer be what it used to be. This happens especially at the
beginning, when the file format has to stabilize.

- if something is added to the file format, the code reading it should
be aware that something may be missing from files produced by earlier
versions, For instance, this "pp_check_file" stuff was added to read
a tag that is written at convergence. In the first version, the code  
simply
stopped if there was no such tags, thus making all files produced by
previous versions unreadable. Now this should be corrected.

In the future we hope that there will be no incompatible changes.
For the past ... the past is gone. The specific error you have looks
very strange and I am not sure it is related to the file format. Does
it work if you try to read files produced with the same version of
the code?

Paolo