As the size of the dataset should be extensible, the simplest solution is to use one file for each sparse data set, and store a the name of this file in the group. Each integral can be a line in the file: i j k l x which can be read with "%10ld %10ld %10ld %10ld %24.16e". The offset can be used with fseek(69L*offset, SEEK_SET)

We need to declare the number of rows of the dataset as UNLIMITED. This requires to use the Chunked storage, and the chunks should absolutely not be larger than 1MB.

To extend the storage, see : https://support.hdfgroup.org/HDF5/doc1.6/UG/10_Datasets.html (figure 17)

If the offset+num > nmax, we need to extend the dataset.