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
diff --git a/libc/bionic/clock.cpp b/libc/bionic/clock.cpp
index a2636c6..053e9e7 100644
--- a/libc/bionic/clock.cpp
+++ b/libc/bionic/clock.cpp
@@ -30,12 +30,13 @@
#include <sys/sysconf.h>
#include <sys/times.h>
+#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 c631b6a..d2fec9a 100644
--- a/tests/time_test.cpp
+++ b/tests/time_test.cpp
@@ -404,3 +404,11 @@
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);
+}