This is the mail archive of the
libc-alpha@sourceware.org
mailing list for the glibc project.
Re: [RFC] Make getenv O(1)
- From: OndÅej BÃlka <neleai at seznam dot cz>
- To: Rich Felker <dalias at aerifal dot cx>
- Cc: Alexander Monakov <amonakov at ispras dot ru>, Florian Weimer <fweimer at redhat dot com>, libc-alpha at sourceware dot org
- Date: Mon, 21 Oct 2013 18:19:38 +0200
- Subject: Re: [RFC] Make getenv O(1)
- Authentication-results: sourceware.org; auth=none
- References: <20131014155229 dot GA25159 at domone dot podge> <20131014165411 dot GZ20515 at brightrain dot aerifal dot cx> <20131014172852 dot GA26005 at domone dot podge> <52611F60 dot 3080700 at redhat dot com> <20131018124013 dot GA15803 at domone dot podge> <alpine dot LNX dot 2 dot 00 dot 1310181656380 dot 3212 at monopod dot intra dot ispras dot ru> <20131018132604 dot GA16445 at domone dot podge> <alpine dot LNX dot 2 dot 00 dot 1310181744060 dot 3212 at monopod dot intra dot ispras dot ru> <20131018140705 dot GA18542 at domone dot podge> <20131021152356 dot GQ20515 at brightrain dot aerifal dot cx>
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);})