CFITSIO supports reading and writing FITS images or table columns containing 64-bit integer data values. Support for 64-bit integers was added to the official FITS Standard in December 2005. FITS 64-bit images have BITPIX = 64, and the 64-bit binary table columns have TFORMn = 'K'. CFITSIO also supports the 'Q' variable-length array table column format which is analogous to the 'P' column format except that the array descriptor is stored as a pair of 64-bit integers.
For the convenience of C programmers, the fitsio.h include file defines (with a typedef statement) the 'LONGLONG' datatype to be equivalent to an appropriate 64-bit integer datatype on each platform. Since there is currently no universal standard for the name of the 64-bit integer datatype (it might be defined as 'long long', 'long', or '__int64' depending on the platform) C programmers may prefer to use the 'LONGLONG' datatype when declaring or allocating 64-bit integer quantities when writing code which needs to run on multiple platforms. Note that CFITSIO will implicitly convert the datatype when reading or writing FITS 64-bit integer images and columns with data arrays of a different integer or floating point datatype, but there is an increased risk of loss of numerical precision or numerical overflow in this case.