CFITSIO is a library of C and Fortran subroutines for reading and writing
data files in FITS (Flexible Image Transport System) data format. CFITSIO
simplifies the task of writing software that deals with FITS files by
providing an easy to use set of high-level routines that insulate the
programmer from the internal complexities of the FITS file format. At
the same time, CFITSIO provides many advanced features that have made
it the most widely used FITS file programming interface in the
astronomical community.

WWW: http://heasarc.gsfc.nasa.gov/docs/software/fitsio/