Re: Fourth draft of the Y2038 design document

[Joseph Myers <joseph at codesourcery dot com>]

On Thu, 23 Jun 2016, Arnd Bergmann wrote:

> Is there a policy about what justifies such an ABI break? I.e. is it

The clear implication from the discussions around C++11 support in 
libstdc++, which resulted in the complicated ABI tag infrastructure, is 
that changing the libstdc++ SONAME would now be too costly, which implies 
the same for the libc SONAME.  That is, incompatible ABIs may be 
introduced with new ports (including fundamentally different 
function-calling ABIs on an existing architecture), so under appropriate 
SHLIB_COMPAT conditionals, but we don't expect to make such changes on 
existing architectures (although as developers, it would be beneficial if 
we could change the libc and libstdc++ SONAMEs say once a decade and get 
rid of accumulated ABI peculiarities at that point - the problem is the 
effects of such transitions on users).

> possible to add a build-time configuration flag later that disables
> 32-bit time_t support for an existing architecture while setting
> a different SONAME, assuming there is sufficient user interest for
> this feature? I see a couple of different options that change SONAME
> on https://sourceware.org/glibc/wiki/ABIList, but those are all
> for fundamental incompatibilities, not for voluntary ABI breaks.

We got rid of --enable-oldest-abi as being extremely bitrotten.  I don't 
think new options for such incompatibility are likely, at least absent 
clear evidence of maintainability (and maybe builtbots using them, etc.).  
It is of course possible to use function lists developed in the course of 
designing and implementing _TIME_BITS=64 to produce a list of ABIs 
involving 32-bit time_t, which could be used to validate that a binary 
doesn't use such ABIs.  And a configuration option to turn the symbols 
into compat symbols wouldn't break the ABI - it would be more like 
--enable-obsolete-rpc.

-- 
Joseph S. Myers
joseph@codesourcery.com