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Re: [PATCH v2][BZ #12515] Improve precision of clock function
- From: Siddhesh Poyarekar <siddhesh at redhat dot com>
- To: Paul Eggert <eggert at cs dot ucla dot edu>
- Cc: libc-alpha at sourceware dot org
- Date: Tue, 21 May 2013 21:44:41 +0530
- Subject: Re: [PATCH v2][BZ #12515] Improve precision of clock function
- References: <20130521145611 dot GM8927 at spoyarek dot pnq dot redhat dot com> <20130521151839 dot GA18430 at domone dot kolej dot mff dot cuni dot cz> <20130521153442 dot GO8927 at spoyarek dot pnq dot redhat dot com> <519B9A09 dot 6030305 at cs dot ucla dot edu>
On Tue, May 21, 2013 at 09:00:09AM -0700, Paul Eggert wrote:
> On 05/21/13 08:34, Siddhesh Poyarekar wrote:
>
> > + return (clock_t) ((ts.tv_nsec * CLOCKS_PER_SEC) / 1000000000
> > + + ts.tv_sec * CLOCKS_PER_SEC);
>
> First, there's no need to cast to clock_t, and in general it's better
> to avoid casts if they're not needed.
OK, cast removed.
> Second, ts.tv_nsec * CLOCKS_PER_SEC might overflow on a machine
> with 32-bit 'long', and afterwards the resulting division by 1000000000
> won't wrap around to the "correct" value.
>
> How about this instead?
>
> return (ts.tv_sec * CLOCKS_PER_SEC
> + ts.tv_nsec / (1000000000 / CLOCKS_PER_SEC));
Thanks, patch updated and tested. OK to go in with this change?
After the PLT fix is reviewed of course.
Siddhesh
2013-05-21 Siddhesh Poyarekar <siddhesh@redhat.com>
Paul Eggert <eggert@cs.ucla.edu>
[BZ #12515]
* sysdeps/unix/sysv/linux/clock.c (clock): Use result from
CLOCK_PROCESS_CPUTIME_ID clock if available.
diff --git a/sysdeps/unix/sysv/linux/clock.c b/sysdeps/unix/sysv/linux/clock.c
index 5268d33..e4e2065 100644
--- a/sysdeps/unix/sysv/linux/clock.c
+++ b/sysdeps/unix/sysv/linux/clock.c
@@ -23,6 +23,14 @@
clock_t
clock (void)
{
+ struct timespec ts;
+
+ if (__clock_gettime (CLOCK_PROCESS_CPUTIME_ID, &ts) == 0)
+ return (ts.tv_sec * CLOCKS_PER_SEC
+ + ts.tv_nsec / (1000000000 / CLOCKS_PER_SEC));
+
+ /* clock_gettime failed. This will only occur if CLOCK_PROCESS_CPUTIME_ID is
+ not supported by the kernel. Fall back to __times. */
struct tms buf;
long clk_tck = __sysconf (_SC_CLK_TCK);