XL C/C++ Runtime Library supports old 31-bit programs and new LP64 programs with a stat structure that contains 4-byte and 8-byte time fields for all five file time values: the POSIX atime, mtime, ctime; and the z/OS UNIX reference time and create time. The old fields could not be expanded in place without changing the offset of all the following fields; new fields were therefore added to the end. When a C program is compiled without LP64, the stat structure is generated with the POSIX names (such as st_atime) on the 4-byte fields; and when it is compiled with LP64, those names coincide with the new 8-byte fields. The unused fields in each compile have dummy names that would not be referenced by the average C program.

There are two separate ru-time libraries, compiled from the same source with and without LP64, so that even the runtime library will not reference both field types at the same time.