[GRASS-dev] Segment library zero filling

[Markus Metz]

On Mon, May 20, 2019 at 5:39 PM Vaclav Petras <wenzeslaus at gmail.com> wrote:
>
> Hi MarkusM and all,
>
> I'm trying to understand if the Segment_open() [1] function fills with
zeros or not. I don't think it does since it is calling G_malloc (malloc)
or Segment_format_nofill(). However, it is not completely clear to me what
is supposed to be doing because documentation still says it calls
Segment_format() and I don't understand context of the related commit [2,
3] and the usage of lseek and USE_LSEEK are not clear to me from format.c
[4].

Segment_open() uses Segment_format_nofill() [1], if it can not use the
all-in-memory cache. The documentation has not been updated accordingly
(yet). The advantage of no fill, only lseek, is that it is faster, the
disadvantage is that any "no space left on device" error will be
encountered only later on, and you always need to check the return code of
Segment_put().

HTH,

Markus M

[1] https://github.com/OSGeo/grass/blob/master/lib/segment/open.c#L89

>
> Markus, can you please clarify that for me? I will then update the
documentation with whatever is needed.
>
> Thanks,
> Vaclav
>
> [1]
https://grass.osgeo.org/programming7/segment_2open_8c.html#ae24d2e794c66c0512b67d7cea8b2ba9a
> [2]
https://github.com/OSGeo/grass/commit/7a0d8d749537acd6d5c4baea11dbb6167fdef916
> [3] https://trac.osgeo.org/grass/changeset/73268
> [4] https://github.com/OSGeo/grass/blob/master/lib/segment/format.c
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