netcdf-c: nc_get_var_string and ncdump throw HDF error on an NC_STRING variable with one unlimited dimension and written with an offset

I am able to create this NC_STRING variable with one limited and one unlimited dimension, and write to it some data with an offset without issue. But when trying to read it back with nc_get_var_string, I am getting a NC_EHDFERR (-101). When trying to use ncdump on the file, I also see “NetCDF: HDF error”. However, I am able to read it fine with nc_get_vars_string.

Here are the repro steps (attached here as well: repro.cpp.zip):

#include <iostream>
#include <vector>
#include "netcdf.h"

void checkErrorCode(int status, const char* message){
    if (status != NC_NOERR){
        std::cout << "Error code: " << status << " from " << message << std::endl;
    }
}

int main(int argc, const char * argv[]) {
    

    // set up data
    
    const char** charPointers = new const char*[4];
    std::string* stringData = new std::string[4];
    
    stringData[0] = "a";
    charPointers[0] = stringData[0].c_str();
    
    stringData[1] = "aa";
    charPointers[1] = stringData[1].c_str();
    
    stringData[2] = "b";
    charPointers[2] = stringData[2].c_str();
    
    stringData[3] = "bb";
    charPointers[3] = stringData[3].c_str();
    
    
    int ncid;
    int retval;
    
    // -- WRITE --
    
    retval = nc_create("myfile.nc", NC_NETCDF4, &ncid);
    checkErrorCode(retval, "nc_create");
    
    // Define dimensions
    int dimid_a;
    retval = nc_def_dim(ncid, "a", NC_UNLIMITED, &dimid_a);
    checkErrorCode(retval, "nc_def_dim for 'a'");
    
    int dimid_b;
    retval = nc_def_dim(ncid, "b", 10, &dimid_b);
    checkErrorCode(retval, "nc_def_dim for 'b'");

    int dimids[2] = {dimid_b, dimid_a};
    
    // Define variable
    int varid;
    retval = nc_def_var(ncid, "var", NC_STRING, 2, dimids, &varid);
    checkErrorCode(retval, "nc_def_var");
        
    
    // Put variable
    size_t start[2] = {0, 1};
    size_t count[2] = {2, 2};
    ptrdiff_t stride[2] = {1, 1};
    
    retval = nc_put_vars_string(ncid, varid, start, count, stride, charPointers);
    checkErrorCode(retval, "nc_put_vars_string");
        

    retval = nc_close(ncid);
    checkErrorCode(retval, "nc_close(1)");
    
    
    // -- READ --
    retval = nc_open("myfile.nc", NC_NOWRITE, &ncid);
    checkErrorCode(retval, "nc_open");
    
    // get dimensions
    int ndims, dimids_read[NC_MAX_VAR_DIMS];
    retval = nc_inq_var(ncid, varid, NULL, NULL, &ndims, dimids_read, NULL);
    checkErrorCode(retval, "nc_inq_var");

    // calculate num elements to read
    size_t dimlen = 0;
    size_t num_items = 1;
    for ( int j = 0; j < ndims; ++j ) {
        retval = nc_inq_dimlen(ncid, dimids_read[j], &dimlen);
        checkErrorCode(retval, "nc_inq_dimlen");
        num_items *= dimlen;
    }
    
    // get var
    char** readStrings = new char*[num_items];
    retval = nc_get_var_string(ncid, varid, readStrings);
    checkErrorCode(retval, "nc_get_var_string");

    retval = nc_close(ncid);
    checkErrorCode(retval, "nc_close(2)");
    
    return retval;
}

The output when run:

$ ./a.out 
Error code: -101 from nc_get_var_string
$ ls
a.out*		myfile.nc	repro.cpp
$ ncdump myfile.nc 
netcdf myfile {
dimensions:
	a = UNLIMITED ; // (3 currently)
	b = 10 ;
variables:
	string var(b, a) ;
data:

 var =
NetCDF: HDF error
Location: file <..>/netcdf/ncdump/vardata.c; line 478

As far as I could tell, to reproduce the issue there has to be >1 dimension, one has to be unlimited, and when writing data the offset needs to be in the unlimited dimension. I also only see this with NC_STRING data.

I am seeing this with netcdf 4.7.4 and the above repro steps are run on macOS 10.15.6 (but we are seeing it on other platforms as well).

Is this a known bug or limitation? I know nc_get_var_string has a warning about using it with unlimited dimensions, but this issue seems specific to NC_STRING