The netcdf-c library filters API in version 4.7.4 has been deprecated in favor of a modified version that unfortunately may cause incompatibilties for users.

The initial reason for the incompatible changes was to support the use of filters in the new NCZarr code. The changes were not completely thought out so it was decided to remove them and revert to previous behaviors. At some future point, the filter mechanism will be extended to support filters for NCZarr, but these will be proper extensions: the existing, reverted, filter API will continue to be supported with no user-visible modifications.

Unfortunately, some advanced users of netcdf filters may experience some compilation or execution problems for previously working code because of these reversions. In that case, please revise your code. Apologies are extended for any inconvenience. Note that it is possible to detect which mechanism is in place at build time.

In summary, the changes are of the following kinds:

• Some functions were renamed for consistency.
• Revert the way that the function nc_inq_var_filter was indicating no filters existed.
• Some auxilliary functions for parsing textual filter specifications have been moved to netcdf_aux.h.
• All of the "filterx" functions have been removed.
• The undocumented function nc_filter_remove was deleted.

See the Github document https://github.com/Unidata/netcdf/blob/master/NUG/filters.md for details.

This document proposes adding compression to the netcdf-3 (aka classic) file format. The proposal has numerous limitations because of the nature of the existing netcdf-3 format.

The algorithm and data format proposed here requires re-writing an existing netcdf-3 file to move it to the new format. In effect, the re-written file becomes archival (read-only).