The fseeko() and ftello() functions are identical to fseek() and ftell() (see fseek(3)), respectively, except that the offset argument of fseeko() and the return value of ftello() is of type off_t instead of long.
On many architectures both off_t and long are 32-bit types, but compilation with
#define _FILE_OFFSET_BITS 64
will turn off_t into a 64-bit type.
These functions are found on SysV-like systems. They are not present in libc4, libc5, glibc 2.0 but available since glibc 2.1.
The fseeko and ftello functions conform to SUSv2.