CFITSIO supports reading and writing FITS images or table columns containing non-standard 64-bit integer data values. Under this convention FITS 64-bit images have BITPIX = 64, and the 64-bit binary table columns have TFORMn = 'K'. CFITSIO also supports the non-standard '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 eqivalent 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.
The Fortran interface to CFITSIO does not currently directly support reading or writing 64-bit integer values in FITS files (except by implicit conversion to a more standard datatype) because the conversion macros in the cfortran.h file which are used by CFITSIO do not support 64-bit integers.