Re: [PATCH 00/25] Change time_t and clock_t to 64 bit

Joseph S. Myers
2014-05-15T20:10:16Z

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@codesourcery.com