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Re: [PATCH v2 05/10] Use clock_gettime to implement time.
Florian Weimer wrote:
time can definitely return the
value of a variable that is incremented periodically from the timer
interrupt.
Is that variable the one that CLOCK_REALTIME_COARSE uses? If so, and if we're
going to replace calls to 'time' with calls to 'clock_realtime', we can do
either of the following:
* Use CLOCK_REALTIME_COARSE. This takes less CPU time and its behavior better
matches what the current glibc does.
* Use CLOCK_REALTIME. This will lessen bugs due to naive code (quite possibly
some code within glibc!) which assumes that 'time (0)' and clock_gettime
(CLOCK_REALTIME, ...)' use the same clock.
It sounds you're leaning towards (1) and I'm inclined to agree. However,
shouldn't the manual say that 'time' does not necessarily agree with
CLOCK_REALTIME? The current behavior is a trap for the unwary.
To illustrate the problems with naive code, see the attached program. On my
Fedora 30 x86-64 desktop, './a.out' outputs this:
clock_gettime (CLOCK_REALTIME, ...) yielded a seconds count of 1567026298;
then time (0) yielded a seconds count of 1567026297, which was 1 s earlier.
Time warp!
In contrast, './a.out 1' (which uses CLOCK_REALTIME_COARSE) finds no
discrepancies in 2**32 attempts.
#include <time.h>
#include <stdio.h>
int
main (int argc, char **argv)
{
int clock = argc == 1 ? CLOCK_REALTIME : CLOCK_REALTIME_COARSE;
for (unsigned int i = 1; i; i++)
{
struct timespec ts;
if (clock_gettime (clock, &ts))
{
perror ("clock_gettime");
return 1;
}
time_t x = time (0);
if (x < ts.tv_sec)
{
printf ("clock_gettime (%s, ...)"
" yielded a seconds count of %ld;\n"
"then time (0) yielded a seconds count of %ld,"
" which was %ld s earlier.\n"
"Time warp!\n",
(clock == CLOCK_REALTIME
? "CLOCK_REALTIME"
: "CLOCK_REALTIME_COARSE"),
(long) ts.tv_sec, (long) x, (long) (ts.tv_sec - x));
return 1;
}
}
return 0;
}