string attributes not supported?

[Chrismarsh]

I have a C++ code that loads a netcdf file and accesses the units attribute like this:

    netCDF::NcFile _data;
// open file [...]
    auto a = _data.getVar("valid_time").getAtt("units");
    SPDLOG_DEBUG(a.getType().getName());  // prints string
    std::string epoch;
    a.getValues(epoch); // throws exception

which results in an exception with the following error:

NetCDF: Attempt to convert between text & numbers

This netcdf has string attributes and are Copernicus CDS ERA5 netcdf files. ncdump shows:

	double datetime(datetime) ;
		datetime:_FillValue = NaN ;
		string datetime:standard_name = "time" ;
		string datetime:long_name = "time" ;
		string datetime:units = "seconds since 1970-01-01T00:00:00" ;
		string datetime:calendar = "proleptic_gregorian" ;

If I have a non-string attr like:

	double datetime(datetime) ;
		datetime:_FillValue = NaN ;
		datetime:standard_name = "time" ;
		datetime:long_name = "time" ;
		datetime:units = "seconds since 1970-01-01T00:00:00" ;
		datetime:calendar = "proleptic_gregorian" ;

the above C++ code works as expected.

This problem appears to be the same issue noted in this netcdf-fortran issue Unidata/netcdf-fortran#181 (comment)

However, as I understand it, string attributes is supposed to work with netcdf-c. Is this a limitation to the C++ api?

Versions:

[email protected]
[email protected]

[amstokely]

@Chrismarsh, I recently encountered a similar issue, but with reading string variables. When working with string variables, you need to allocate a temporary char** buffer for the number of strings you want to read. Here's the process I followed:

Allocate the first dimension of the char** buffer (NetCDF manages the individual string memory).
Pass the buffer to the getVar method.
After reading, copy the buffer data into a std::vectorstd::string or similar container.
Free the buffer using NetCDF’s string memory management functions.
Deallocate the buffer memory you allocated.
For reference, here is an example of reading a string array variable in C++.

https://github.com/NCAR/obs2ioda/blob/101e9c66e5190a51279f6aa04cc1c207c5e7b698/obs2ioda-v2/src/cxx/netcdf_variable.cc#L199

While this example deals with string variables, I assume the process for string attributes might be similar.