The <utmp.h> and <utmp.h> headers had a bug that the on-disk structures defined there could change size on some targets when _TIME_BITS was set to 64. This is obviously wrong because the files are not going to magically change their layout because the application accessing them was built in a specific way. We're going to fix this in glibc upstream on the stable release branches, going all the way back to glibc 3.34 (the first release with this kind of time64 support). After the fix, the _TIME_BITS definition will no longer impact struct layout. Usually, that means epoch fields are 32-bits wide, to match co-installable architectures. To extend the usable life-time of these interfaces somewhat, glibc 2.40 changes epoch fields to unsigned types in these structures. This change is specific to the upcoming glibc 2.40 release, I do not plan to backport it. Thanks, Florian