Single Unix specification
Large file support defines a file offset to be of 'extended signed integral type' i.e. signed long long int. This means the standard functions like fseek should support LFS by standard in a c library that supports it. How this translates into syscalls is up to you. I suppose functions in a small file only c library could call one syscall, and those in a large file one could call a different syscall (e.g. fstat/fstat64 in glibc). How you pass a 64-bit value to a syscall is a question you probably should have already answered, e.g. use EAX:EDX or use EAX to point to a 64 bit value in memory.
Regards,
John.