@param fp File handle of the data file whose index is being opened @param fn BAM/CRAM/etc data file filename @param fnidx Index filename, or NULL to search alongside @a fn @return The index, or NULL if an error occurred.
See Implementation
@param fp File handle of the data file whose index is being opened @param fn BAM/CRAM/etc data file filename @param fnidx Index filename, or NULL to search alongside @a fn @return The index, or NULL if an error occurred.