The Harwell-Boeing format is the only well-defined mechanism for text-file exchange of sparse matrix data. Details of the format are provided in (Duff, Grimes and Lewis, 1989). The format has several advantages. First, it includes a detailed header with a variety of descriptive information, such as matrix labels, type, order, number of nonzeros, etc. It also admits a very broad range of formatting for the numerical entries; this is done by providing Fortran format descriptors within the header that describes the numerical entries. Although this greatly eases the work for most contributors, it means that the text file is quite difficult to parse by those using languages other than Fortran.
To help remedy this situation, we have developed a set of C procedures for doing I/O with Harwell-Boeing files. The current implementation provides routines for gathering matrix information from the file header, reading into pre-allocated or dynamically allocated storage vectors, and writing to files according the Harwell-Boeing specification from storage vectors. A link to these routines is provided on the Matrix Market home page.