Re: [PATCH 1/1] Y2038: add function __difftime64

[Albert ARIBAUD <albert.aribaud@3adev.fr>]

Hi Paul,

On Thu, 21 Jun 2018 14:17:24 -0700, Paul Eggert <eggert@cs.ucla.edu>
wrote :

> On 06/20/2018 01:55 PM, Albert ARIBAUD wrote:
> >  
>  [...]  
> >> Since no glibc code calls difftime, can we assume that a later patch
> >> will change will make __difftime64 and difftime both available to user
> >> code? I'm not getting the big picture here.  
> > Actually, this is not an assumption  
> 
> In that case, I'm still puzzled about the specific example of difftime. 
> Are you assuming that glibc can export just one difftime function to 
> 32-bit user code, and that because every 32-bit time_t value fits into 
> __time64_t this single function will work with both time_t and 
> __time64_t? That assumption fails in many platforms (as they use 
> different calling conventions for 32-bit versus 64-bit integers), so in 
> general a glibc implementation on a platform that currently has 32-bit 
> time_t will need to export two difftime entry points.

I am indeed assuming that every 32-bit time_t value will fit into a
64-bit __time64_t, but I don't infer from this that 64-bit as well as
32-bit time user code could use a single function as their difftime.
Actually, if you'll remember my first rounds of RFC patches (which you
did comment), I had initially elected to provide a 64-bit difftime
alongside the original difftime; and later, since wrappers seemed to be
the preference, I replaced the original 32-bit difftime with a wrapper
around the 64-bit time; and in both cases, there were two functions, on
for each time size.

> > What actually happened is, I am getting quite more feedback now than I
> > got with the RFCs.  
> 
> That's a good sign. You're getting feedback now. Many patches languish 
> nearly forever because nobody has the time to review them, unfortunately.
>
> > I'll provide two branches for reviewers  
> 
> Thanks, please let us know when they're ready. That will help me answer 
> questions like the difftime question above.
>
> > We do currently have unsigned 32-bit time_t implementations, but do we
> > have unsigned 64-bit time_t? A signed 64-bit integer __time64_t was
> > a core assumption  
> 
> That can be a core assumption of glibc, but for code shared with Gnulib 
> we'd rather not make assumptions like that. It's little trouble to 
> support unsigned 64-bit (or even wider) time_t in Gnulib code, and we 
> should continue to support this. All we should need to assume is that 
> every time_t value fits into __time64_t. Come to think of it, __time64_t 
> will be a misnomer in Gnulib since __time64_t is not necessarily a 
> 64-bit type, so perhaps code shared with Gnulib should continue to use 
> the name "internal_time_t" (for internal use only, of course) to 
> underscore the fact that it might not be 64 bits.
> 
> > If the proposed patches are OK, then the best is to apply them to
> > master and I will remove the corresponding ones from the Y2038 series
> >  
> 
> I don't think we're ready to start installing these into master, since I 
> still don't fully understand how difftime (much less mktime) will work. 
> Also, I want to make sure that the patches will be compatible with 
> Gnulib. It may make more sense to change Gnulib first, as it is less 
> formal about this sort of update.

Since you want the changes in gnulib first, then I suspect I should
provide branches above gnulib as well as above glibc? If so, what
would you recommend as a good source on setting up a build and test
setup for gnulib, similar to build-many-glibcs.py is for glibc?

Cordialement,
Albert ARIBAUD
3ADEV