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[PATCH v2.2][BZ #12515] Improve precision of clock function
- From: Siddhesh Poyarekar <siddhesh at redhat dot com>
- To: Roland McGrath <roland at hack dot frob dot com>
- Cc: Paul Eggert <eggert at cs dot ucla dot edu>, libc-alpha at sourceware dot org
- Date: Thu, 13 Jun 2013 00:57:18 +0530
- Subject: [PATCH v2.2][BZ #12515] Improve precision of clock function
- References: <20130521153442 dot GO8927 at spoyarek dot pnq dot redhat dot com> <519B9A09 dot 6030305 at cs dot ucla dot edu> <20130521161441 dot GQ8927 at spoyarek dot pnq dot redhat dot com> <20130603092604 dot GL2145 at spoyarek dot pnq dot redhat dot com> <20130610083821 dot GD1570 at spoyarek dot pnq dot redhat dot com> <51B66522 dot 1060008 at cs dot ucla dot edu> <20130611135422 dot GK8010 at spoyarek dot pnq dot redhat dot com> <20130611171952 dot EB3CF2C0A2 at topped-with-meat dot com> <20130612050307 dot GU8010 at spoyarek dot pnq dot redhat dot com> <20130612173819 dot 67B272C07E at topped-with-meat dot com>
On Wed, Jun 12, 2013 at 10:38:19AM -0700, Roland McGrath wrote:
> Removing the fallback is OK, but ignoring the result of
> clock_gettime is not.
I reckoned it ought to be OK since the only way to get an unintended
error was through a kernel bug. However I guess we ought to cover
that too. Here's an updated patch that checks the return value and
returns (clock_t) -1 if clock_gettime fails.
Siddhesh
[BZ #12515]
* sysdeps/unix/sysv/linux/clock.c (clock): Use result from
CLOCK_PROCESS_CPUTIME_ID clock instead of __times.
diff --git a/sysdeps/unix/sysv/linux/clock.c b/sysdeps/unix/sysv/linux/clock.c
index 5268d33..25c3b38 100644
--- a/sysdeps/unix/sysv/linux/clock.c
+++ b/sysdeps/unix/sysv/linux/clock.c
@@ -1,4 +1,5 @@
-/* Copyright (C) 1991-2013 Free Software Foundation, Inc.
+/* Return the time used by the program so far (user time + system time).
+ Copyright (C) 1991-2013 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
@@ -19,26 +20,17 @@
#include <time.h>
#include <unistd.h>
-/* Return the time used by the program so far (user time + system time). */
clock_t
clock (void)
{
- struct tms buf;
- long clk_tck = __sysconf (_SC_CLK_TCK);
+ struct timespec ts;
- /* We don't check for errors here. The only error the kernel
- returns is EFAULT if the value cannot be written to the struct we
- pass a pointer to. Otherwise the kernel returns an `unsigned
- long' value which is the number of jiffies since system start.
- But this number can be negative (when read as `long') when the
- system is up for some time. Ignoring errors should therefore
- have no negative impacts but solve the problem. */
- __times (&buf);
+ /* clock_gettime shouldn't fail here since CLOCK_PROCESS_CPUTIME_ID is
+ supported since 2.6.12. Check the return value anyway in case the kernel
+ barfs on us for some reason. */
+ if (__glibc_unlikely (__clock_gettime (CLOCK_PROCESS_CPUTIME_ID, &ts) != 0))
+ return (clock_t) -1;
- return
- (clk_tck <= CLOCKS_PER_SEC)
- ? ((unsigned long) buf.tms_utime + buf.tms_stime) * (CLOCKS_PER_SEC
- / clk_tck)
- : ((unsigned long) buf.tms_utime + buf.tms_stime) / (clk_tck
- / CLOCKS_PER_SEC);
+ return (ts.tv_sec * CLOCKS_PER_SEC
+ + ts.tv_nsec / (1000000000 / CLOCKS_PER_SEC));
}