On Thu, 15 May 2014, Arnd Bergmann wrote: > Earlier in the thread there seemed to be a rough consensus that > _TIME_BITS=64 wouldn't be a good idea because we wouldn't get everything > changed to use it. For _FILE_OFFSET_BITS=64 that's ok because most > user space doesn't ever want to deal with large files. Well, I'm coming into this in the middle since it isn't on linux-api and noone has tried to work out on libc-alpha what things should look like from the glibc side. _TIME_BITS seemed to make sense when I thought about this previously, however. > Can you elaborate on how the switch to the new default would work? At some appropriate release (probably after _TIME_BITS=64 is widely used in distributions), the glibc headers would change so that _TIME_BITS=64 is the default and _TIME_BITS=32 can be set to get the old interfaces. At some later point _TIME_BITS=32 API support might be removed, leaving the old symbols as compat symbols for existing binaries. > If it's easy, why hasn't it been done for _FILE_OFFSET_BITS already > and what's stopping us from changing the default as soon as the interfaces > are there? If it's hard, what would need to happen before the default > time_t can be set? The distribution side of the change for _FILE_OFFSET_BITS (i.e., moving to building libraries that way so a glibc change to the default wouldn't cause issues for other libraries' ABIs) has gradually been done. The discussion in March on libc-alpha about changing the default tailed off. This is something that needs someone to take the lead with a *careful and detailed analysis of the information from the previous discussion* in order to present a properly reasoned proposal for a change to the default - not scattergun patches, not patches with brief or no analysis of the environment in which glibc is used, not dismissing concerns, but a properly reasoned argument for why the change should be made, along with details of how distributions can determine whether ABI issues would arise from rebuilding a particular library against newer glibc. > > Obviously 64-bit time_t syscalls would be an appropriately narrow set of > > syscalls like those in the generic ABI (so glibc would implement stat for > > _TIME_BITS=64 using fstatat64_time64 or whatever the syscall is called, > > just as the stat functions for generic ABI architectures are implemented > > with newfstatat / fstatat64 rather than lots of separate syscalls. > > This assumes that we'd leave the kernel time_t/timespec/timeval using 'long' > and introduce a new timespec64 using a signed 64-bit type, rather than > changing the kernel headers to the new syscalls and data structures with > new names for the existing ones, right? Yes. I consider it simply common sense that new kernel headers should continue to work with much older glibc, meaning that the API (syscall names etc.) presented by the headers from headers_install should not change incompatibly. (64-bit type only for time_t, of course. There's no need for a 64-bit type for nanoseconds and tv_nsec is explicitly "long" in POSIX, meaning that if the kernel uses a 64-bit type for nanoseconds on systems where "long" is 32-bit in userspace, either it needs to treat the high word as padding or glibc needs to wrap all interfaces passing a struct timespec into the kernel so they clear the padding field. There's even less need for a 64-bit type for microseconds.) -- Joseph S. Myers joseph@xxxxxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
