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Re: [RFC] Make getenv O(1)


On Mon, Oct 21, 2013 at 11:23:56AM -0400, Rich Felker wrote:
> On Fri, Oct 18, 2013 at 04:07:05PM +0200, OndÅej BÃlka wrote:
> > On Fri, Oct 18, 2013 at 05:46:57PM +0400, Alexander Monakov wrote:
> > > On Fri, 18 Oct 2013, OndÅej BÃlka wrote:
> > > > You could argue that this does not help when likely case is that
> > > > variable is not defined. For that I would need posix say that changing
> > > > variable name via environ pointers is illegal.
> > > 
> > > Indeed; I would also ask if such functions are allowed to query interesting
> > > environment variables once on first call and then cache the result (similar to
> > > how the dynamic linker stashes initial LD_LIBRARY_PATH on the side and calls
> > > to dlopen do not check the environment again).
> > > 
> > > Alexander
> > 
> > This could be done by following macro with global renaming of
> > getenv->GETENV.
> > 
> > Also do we support in libc setenv to change behaviour?
> > 
> > #define GETENV(x) ({ \
> >   static char *__ret; \
> >   static int __cached; \
> >   if (__builtin_constant_p (x) && !__cached) \
> >   { \
> >     __ret = getenv (x); \
> >     __cached = 1; \
> >   } \
> >   __builtin_constant_p (x) ? __ret : getenv (x);})
> 
> I think this is the best approach, either via such a macro or simply
> writing out the caching directly. However the above code performs
> unsynchronized access on __cached and __ret. In particular the
> compiler could store __cached before storing __ret, or due to cache
> line issues, __cached might simply become visible to other cores
> before __ret does.
> 
Why, following does not need synchronization.

#define GETENV(x) ({ \
  static char *__ret = NULL; \
  if (__builtin_constant_p (x) && !__cached) \
    __ret = getenv (x) + 1; \
  __builtin_constant_p (x) ? __ret - 1: getenv (x);})


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