Re: [RFC 1/7] y2038: Introduce struct __timespec64

[Lukasz Majewski <lukma at denx dot de>]

Hi Paul, Joseph,

> >> However, as long as __time64_t and __time_t are not visible to user
> >> code when _TIME_BITS=64, this will address most of my own practical
> >> concerns and should be OK as a compromise.  
> > 
> > To sum up:
> > ----------
> > 
> > 1. __time64_t and __time_t are "exported" (visible
> > in /usr/include/*) always (similar to __off_t and __off64_t).  
> 
> No, the compromise is that these types are "exported" only when
> _TIME_BITS<64. Exporting them when _TIME_BITS==64 is unnecessary and
> confusing. (It's also unnecessary and confusing when _TIME_BITS<64,
> but I'm willing to compromise there because the _TIME_BITS<64 case is
> obsolescent and will eventually go away.)
> 
> > 2. The "exported" struct timespec would look like:
> > 
> > struct timespec
> > {
> > /* Use the original definition for 64-bit arches
> >     or when 64-bit-time by default has *not* been requested */
> > #if __WORDSIZE > 32 || ! defined(__USE_TIME_BITS64)
> >    __time_t tv_sec; /* Seconds. */
> > #else
> >    __time64_t tv_sec;
> > #end
> >    __syscall_slong_t tv_nsec;
> > };  
> 
> Sorry, I'm not following this. Why can't this be the natural 'struct
> timespec { time_t tv_sec; ...; }'? The two forms are equivalent,
> regardless of whether __WORDSIZE > 32 || ! defined(__USE_TIME_BITS64)
> and regardless of what we do about (1), so why use the
> more-complicated and more-confusing definition?

Ok, time_t shall be used for tv_sec.

> 
> As Joseph mentioned, there may need to be ifdeffery around tv_nsec
> due to the issue of endian-dependent padding around tv_nsec.

It is not only the padding. 
The problem is with the size of this struct passed to the kernel as
syscall.

1. __TIMESIZE = 64 -> No problem with kernel ABI
	- sizeof(tv_sec) = 64 bit
	- sizeof(tv_nsec) = 64 bit

	Kernel looks for tv_sec and tv_nsec, each 64 bit

2. __TIMESIZE = 32 (no, Y2038 support) ->  No problem with kernel ABI
	- sizeof(tv_sec) = 32 bit
	- sizeof(tv_nsec) = 32 bit

	Kernel looks for tv_sec and tv_nsec, each 32 bit (those
	syscalls would be removed from the kernel)

3. __TIMESIZE = 32 (Y2038 support) -> ABI problem 
	- sizeof(tv_sec) = 64 bit (time_t)
	- sizeof(tv_nsec) = 32 bit
	- needed padding (tv_pad) 32 bit

	Kernel expects tv_sec and tv_nsec to be both 64 bits.

Exported (installed at /usr/include) struct timespec (to work in above
three cases):

struct timespec
 {
   time_t tv_sec;   /* Seconds.  */
#ifdef __USE_TIME_BITS64
# if BYTE_ORDER == BIG_ENDIAN
   int tv_pad: 32;  /* Padding named for checking/setting */
   __syscall_slong_t tv_nsec; /* Nanoseconds */ 
# else
   __syscall_slong_t tv_nsec; /* Nanoseconds */
   int tv_pad: 32;  /* Padding named for checking/setting */ 
# endif
#else
   __syscall_slong_t tv_nsec;	/* Nanoseconds.  */
#endif
 };


and internal (in ./include/time.h) definition of struct __timespec64:

#if __TIMESIZE == 64
# define __timespec64 timespec
#else
struct __timespec64
{
	__time64_t tv_sec;         /* Seconds */
	__time64_t tv_nsec;         /* Nanoseconds */
};
#endif



> However,
> there does not need to be ifdeffery around tv_sec. Just use time_t
> there; problem solved.
> 
> > 3. Put the internal struct __timespec64 ...
> > into include/time.h and don't introduce separate
> > include/bits/types/struct___timespec64.h
> > 
> > The same idea would be applied to struct timeval  
> 
> Yes.




Best regards,

Lukasz Majewski

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