[GRASS5] new vector2

Michel Wurtz mw at teledetection.fr
Fri Jan 5 07:41:01 EST 2001

Hi Justin and Radim,

Justin Hickey wrote:
> Hi Radim
> Radim Blazek wrote:
> > My conclusion is:
[... snip...]
> > - write new module for conversion little <=> big
> > - no auto conversion into native format (read only files, files shared
> >   by more platforms)
> > - vector written in one byte order (big for example) will be updated
> >   in that byte order (big) even on machine with some other byte order
> >   (little)
> Hmmm. I don't understand why you need to maintain the byte-order of a
> file, or provide the users with a new module. It seems to me that it
> would be easier (less work) to write a conversion function in the vector
> library that checks the byte-order and converts it to the machine
> byte-order if necessary. Write will always write in native format. Then
> the conversion is only performed when necessary, the user never knows
> the difference, and grass will always be able to read either format
> making the files platform independent. It seems to me that in this case
> the only task here is to write a single "check and convert" function for
> reading vector files. Am I missing something?

I agree with Radim when he says that autoconversion (the "check and
convert" function suggested by Justin) may be a bad idea in case of
file shared (NFS, etc...) by different platform (Ex :Sun + Pentium),
but I think that this may only be a problem if both platforms use the
file at the same time, which is actually possible, but extremelly
dangerous, even with the actual file format : there is no file level
locking mecanism. There is also no multiuser support in Grass : you
can only run one instance of Grass on one machine, even when you work
with two different databases, mainly because the fifos used for the
monitors are in a fixed location, not on a "per instance" location.

Sharing files implies the management of locks at least at the file level,
and this is a major change in sources... so it is imho only usefull
if you add also the possibility of multiple Grass instances running
on the same machine.

I am more sensible to the second argument : read-only files cannot be
converted "automagically"... so I agree with Radim except for his last
proposition : autoconversion can be done, but only for a file open
for update (or read/write)

Michel WURTZ - DIG - Maison de la télédétection
               500, rue J.F. Breton
               34093 MONTPELLIER Cedex 5

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