this function is outside the scope of the standards, but logically should behave like the set*id functions whose effects are process-global.
this is a prerequisite for addition of other interfaces that use kernel tids, including futex and SIGEV_THREAD_ID. there is some ambiguity as to whether the semantic return type should be int or pid_t. either way, futex API imposes a contract that the values fit in int (excluding some upper reserved bits). glibc used pid_t, so in the interest of not having gratuitous mismatch (the underlying types are the same anyway), pid_t is used here as well. while conceptually this is a syscall, the copy stored in the thread structure is always valid in all contexts where it's valid to call libc functions, so it's used to avoid the syscall.
clock_adjtime always returns the current clock setting in struct timex, so it's always possible that the time64 version is needed.
the 64-bit time code path used the wrong (time32) syscall. fortunately this code path is not yet taken unless attempting to set a post-Y2038 time.
commit 2b4fd6f75b4fa66d28cddcf165ad48e8fda486d1 added time64 for this function, but did so with a hidden assumption that the new time64 version of struct timex will be layout-compatible with the old one. however, there is little benefit to doing it that way, and the cost is permanent special-casing of 32-bit archs with 64-bit time_t in the public interface definitions. instead, do a full translation of the structure going in and out. this commit is actually a revision to an earlier uncommited version of the code.
presently the kernel does not actually define time64 versions of these syscalls, and they're not really needed except to represent extreme cpu time usage. however, x32's versions of the syscalls already behave as time64 ones, meaning the functions were broken on x32 if the caller used any part of the rusage result other than ru_utime and ru_stime. commit 7e8171143124f7f510db555dc6f6327a965a3e84 made it possible to fix this by treating x32's syscalls as time64 versions. in the non-time64-syscall case, make the syscall with the rusage destination pointer adjusted so that all members but the timevals line up between the libc and kernel structures. on 64-bit archs, or present 32-bit archs with 32-bit time_t, the timevals will line up too and no further work is needed. for future 32-bit archs with 64-bit time_t, the timevals are copied into place, contingent on time_t being larger than long.
the 64-bit/time64 version of the syscall is not API-compatible with the userspace timex structure definition; fields specified as long have type long long. so when using the time64 syscall, we have to convert the entire structure. this was always the case for x32 as well, but went unnoticed, meaning that clock_adjtime just passed junk to the kernel on x32. it should be fixed now. for the fallback case, we avoid encoding any assumptions about the new location of the time member or naming of the legacy slots by accessing them through a union of the kernel type and the new userspace type. the only assumption is that the non-time members live at the same offsets as in the (non-time64, long-based) kernel timex struct. this property saves us from having to convert the whole thing, and avoids a lot of additional work in compat shims. the new code is statically unreachable for now except on x32, where it fixes major brokenness. it is permanently unreachable on 64-bit.