Pnetcdf is a library providing high-performance parallel i/o while still maintaining file-format compatibility with unidata's netcdf, specifically the formats of cdf-1 and cdf-2. although netcdf supports parallel i/o starting from version 4, the files must be in hdf5 format. pnetcdf is currently the only choice for carrying out parallel i/o on files that are in classic formats (cdf-1 and 2).

in addition, pnetcdf supports the cdf-5 file format, an extension of cdf-2, that supports more data types and allows users to define large dimensions, attributes, and variables (>2b elements).