You'll have to look in your platform-specific documentation. As you have discovered, the C standard library doesn't cover such large numbers. There might be a large file platform-specific (i.e. non-standard) extension to the FILE stuff, or it could be reimplemented in a different library.
Another approach could be to use redirection, e.g.:
cat bigfile | myprog
then in myprog just do
although that is one-way wandering, not random access.
Yes, that's why I said "You'll have to look in your platform-specific documentation".
I can't do that because you've given no clues as to what your platform is, other than that it's an OS with a filesystem that caters for single file sizes up to at least 100,000 Yottabytes (Yotta=1,000,000 Exa; Exa=1,000,000 Tera; Tera=1,000,000 Mega).