A file larger than 2GB is called large file on HP/UX UFS. Handle large file is different from handle file size under 2GB.

First, user needs to turn on the large file support on the fs:
fsadm -F hfs -o largefiles /dev/you_fs_entry

To write a program to handle large file, you will need to use this flag in your opne() system call: O_LARGEFILE, if you are writing from user space, use k_off_t instead of unsigned long if you are reading or writing from within the kernel.