From 8d0b2dbf2154d5da17ff09b1d4f864d281362ad2 Mon Sep 17 00:00:00 2001 From: Alex Van Brunt Date: Fri, 26 Sep 2014 13:32:47 -0700 Subject: [PATCH] Reimplement clock(3) using clock_gettime(3) Unlike times(), clock_gettime() is implemented as a vDSO on many architectures. So, using clock_gettime() will return a more accurate time and do so with less overhead because it does have the overhead of calling into the kernel. It is also significantly more accurate because it measures the actual time in nanoseconds rather than the number of ticks (typically 1 millisecond or more). Bug: 17814435 Change-Id: Id4945d9f387330518f78669809639952e9227ed9 --- libc/bionic/clock.cpp | 13 +++++++------ tests/time_test.cpp | 8 ++++++++ 2 files changed, 15 insertions(+), 6 deletions(-) diff --git a/libc/bionic/clock.cpp b/libc/bionic/clock.cpp index a2636c67c..053e9e742 100644 --- a/libc/bionic/clock.cpp +++ b/libc/bionic/clock.cpp @@ -30,12 +30,13 @@ #include #include +#include "private/bionic_constants.h" + // http://pubs.opengroup.org/onlinepubs/9699919799/functions/clock.html clock_t clock() { - tms t; - times(&t); - // Although times(2) and clock(3) both use the type clock_t, the units are - // different. For times(2) it's pure clock ticks, but for clock(3) the unit - // is CLOCKS_PER_SEC, so we need to scale appropriately. - return (t.tms_utime + t.tms_stime) * (CLOCKS_PER_SEC / sysconf(_SC_CLK_TCK)); + timespec ts; + if (clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &ts) == -1) { + return -1; + } + return (ts.tv_sec * CLOCKS_PER_SEC) + (ts.tv_nsec / (NS_PER_S / CLOCKS_PER_SEC)); } diff --git a/tests/time_test.cpp b/tests/time_test.cpp index c631b6adb..d2fec9ae9 100644 --- a/tests/time_test.cpp +++ b/tests/time_test.cpp @@ -404,3 +404,11 @@ TEST(time, clock_gettime) { ASSERT_EQ(0, ts2.tv_sec); ASSERT_LT(ts2.tv_nsec, 1000000); } + +TEST(time, clock) { + // clock(3) is hard to test, but a 1s sleep should cost less than 1ms. + clock_t t0 = clock(); + sleep(1); + clock_t t1 = clock(); + ASSERT_LT(t1 - t0, CLOCKS_PER_SEC / 1000); +}