NetCDF version 3.6 improves large file support, Windows compatibility, ease of installation, and Fortran-90 interface performance. In addition, it fixes a few bugs. Version 3.6.0 is available in source form as a gzipped tar file or compressed tar file with new simplified installation instructions. Prebuilt binary versions are also available for some platforms.
In previous releases it was possible to create netCDF files that exceeded 2 GiB by observing fairly severe constraints on the structure of the data. For example, you could use a very large fixed-size variable as the last variable, or you could use the record dimension for large variables and use a large number of records. Such restrictions were necessary due to the use of 32-bit fields in the netCDF format for offsets pointing to the beginning of data for each variable.
Version 3.6 introduces a new variant of the netCDF file format, while preserving backward compatibility with the existing format. The new variant has 64-bit fields for file offsets, removing many of the size constraints for netCDF files. Greg Sjaardema of Sandia Labs contributed C library code to support 64-bit offsets, and we added changes to increase the maximum variable and record size, to permit use from Fortran, C++, and Java interfaces, to support creation of large files with the ncgen utility, and to check declared variable shapes for conformance with remaining size constraints. The new library will access data transparently in both the classic and 64-bit offset formats.
Version 3.6 is configured to use Large File Support by default if available on the target platform. Building and installing from source has been simplified so that there is no need to set environment variables on most platforms.
Creating a file that uses 64-bit file offsets requires specifing an extra flag on file creation. There are no other changes to the interface, so existing programs will continue to work when linked with the new library and will continue to use the "classic" format by default, used in all previous versions of the netCDF library, when the 64-bit offset flag is not provided. Users are encouraged to continue to use the netCDF classic format unless they need to create very large files, because data in this new 64-bit offset format can't be read by earlier versions of the netCDF library.
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