[netcdfgroup] C++ netcdf interface: NcFile closing issue. Any thoughts?

• To: netcdfgroup <netcdfgroup@xxxxxxxxxxxxxxxx>
• Subject: [netcdfgroup] C++ netcdf interface: NcFile closing issue. Any thoughts?
• From: Bhupendra Raut <bhupendraraut@xxxxxxxxx>
• Date: Tue, 16 Jun 2015 18:11:46 +1000

C++ interface to netcdf developed by Lynton Appel is a good addition to
netcdf libraries. This allows us to write C++ style programs to read and
write netcdf data with lightning speed and hence preferred in some
situations.
1. To make it more customized and simple for our purpose, I have written a
class "write_llt" that only writes a single variable (say rain) of the type
rain(lat, lon, time) with unlimited time axis. This function takes care of
several things like managing frame_number etc. I create an object of the
class as follow

write_llt ncout(with all the variable structure as arguments);

2. and then I call the wrapper function (called write_data(rain,
frame_time)) to write a time-step at single time value.

The write_data function used following function calls from netcdf interface.
dataVar.putVar (startp, countp, rain);
tVar.putVar (startt, frame_time);

3. When I finish writing all the time-steps, and goes out of the loop the
files closes and objects are destroyed properly before the program stops.
Check this link for useful discussion
http://netcdf-group.1586084.n2.nabble.com/NcFile-close-member-function-td7574815.html
This works without any issues with my class write_llt.

4. However, because, I am using my own class with pointer notations to
ncFile class object, when I want to write several files in a loop.

loop for all the N input files
{
write_llt ncout(Nth output file and its structure as arguments);
loop for each time step
{
ncout.write_data(rain, frame_time)
}
//Here When I finish writing the current file, I want it to be closed
properly so that I can
//write the next file, so I need to have a manual close file function.
}

5. When I run my code like this some time it writes a correct file but most
often the file is written without any time axis. ncdump looks like this
netcdf \200307_02_64bit {
dimensions:
    latitude = 256 ;
    longitude = 256 ;
    time = UNLIMITED ; // (0 currently)
variables:
    float latitude(latitude) ;
        latitude:units = "degrees_north" ;
    float longitude(longitude) ;
        longitude:units = "degrees_east" ;
    int64 time(time) ;
        time:units = "seconds since 1970-01-01 00:00:00 UTC" ;
    float rain(time, latitude, longitude) ;
.....
.....
}

The file has to be 22.1MB (when works properly) but it shows only 17.9 MB
(always when it goes wrong).

6. As this works once in a while this looks like an issue of delay in
flushing the memory to hard-disc. Because there is not 'close' function,
when the outside loop closes my object ncout gets destroyed and all the
data gets deleted with it, without being flushed.
7. I have tried calling the destructor ofile->~NcFile() from my write_llt
class which gives segmentation fault.

8. I may have chosen a non-recommended way of using the libraries to begin
with. If you are a netcdf C++ interface user, please share your experience
and possibly a solution to my problem.

The C++ interface is very useful and we can make it better with more
feedbacks. Also, is there any chance of newer version of it? Is there any
active team of researcher using and modifying it?
I would like to share my WritingClass with others once I get it to work
properly.

Thanks and Cheers,

--
Bhupendra Raut
(Research Fellow)
School of Earth, Atmosphere and Environment
9, Rainforest Walk
Monash University
Clayton 3800
Victoria, Australia

PH: +61 3 9905 2269
FAX: +61 3 9905 4403
Website: www.baraut.info