[PATCH v7 0/3] y2038: Linux: Introduce __clock_settime64 function

[Lukasz Majewski]

Hi Joseph,

> On Wed, 18 Sep 2019, Alistair Francis wrote:
> 
> > +#include <endian.h>
> > 
> >  /* POSIX.1b structure for a time value.  This is like a `struct
> > timeval' but has nanoseconds instead of microseconds.  */
> >  struct timespec
> >  {
> >    __time_t tv_sec;             /* Seconds.  */
> > +#if __WORDSIZE == 64 \
> > +  || (defined __SYSCALL_WORDSIZE && __SYSCALL_WORDSIZE == 64)
> >    __syscall_slong_t tv_nsec;   /* Nanoseconds.  */
> > +#else
> > +# if BYTE_ORDER == BIG_ENDIAN
> > +  __int32_t tv_pad;           /* Padding */
> > +  __syscall_slong_t tv_nsec;  /* Nanoseconds */
> > +# else
> > +  __int32_t tv_nsec;          /* Nanoseconds */
> > +  __syscall_slong_t tv_pad;   /* Padding */
> > +# endif
> > +#endif  
> 

Just one more question - am I correct that the above code will increase
the overall sizeof(struct timespec) to 12 for machines with __WORDSIZE
== 32 ?

I guess that now such machine have sizeof(struct timespec) = 8 ?

However, this shouldn't be a problem (unless some user space SW uses
this data in an unusual way...).

> The padding must be an *unnamed bit-field* so that { tv_sec, tv_nsec
> } initializers (common in practice even if not officially supported
> by the standards) continue to work.  Also, I think you should just
> use "long int" for tv_nsec in the case where there is padding, as the
> standard-defined type (and then the padding can be "int: 32", so
> avoiding any dependence on whether compilers support non-int
> bit-fields).  Certainly the choice of types for tv_nsec and padding
> should not depend on the endianness (the patch above is using
> __int32_t for the first field and __syscall_slong_t for the second,
> regardless of which is tv_nsec and which is padding).
> 
> There are namespace issues when changing installed headers.  You
> can't use macros such as BYTE_ORDER or BIG_ENDIAN because they aren't
> in the standard-reserved namespaces.
> 
> Unfortunately the definitions of __LITTLE_ENDIAN and __BIG_ENDIAN are
> in <endian.h> (__BYTE_ORDER is in the architecture-specific
> <bits/endian.h>), and while the non-reserved names therein are all
> conditional on __USE_MISC, I don't think we really want to start
> exporting them from every header that uses struct timespec.  My
> inclination would be to have a separate bits/ header that only
> defines the __LITTLE_ENDIAN / __BIG_ENDIAN / __PDP_ENDIAN macros (or
> that defines those and includes the architecture-specific header for
> __BYTE_ORDER), so that other headers can test endianness without
> bringing in all the other __USE_MISC endian-related macros from
> <endian.h>, but Zack might advise on how such changes would fit into
> his header cleanups.
> 




Best regards,

Lukasz Majewski

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