Bug#872556: libc0.1: clock() has wrong scaling factor on hurd

[James Cowgill]

Package: libc0.1
Version: 2.24-14
Severity: normal
Control: affects -1 src:ffmpeg

Hi,

The clock() function on hurd has the wrong scaling factor. It generates
only 100 clock ticks per second, even though CLOCKS_PER_SEC is 1000000.
I has written a small testcase (attached) which illustrates the difference:

amd64:
CLOCKS_PER_SEC = 1000000
clock() = 444
sleeping for 1 second...
clock() = 1000464

hurd-i386:
CLOCKS_PER_SEC = 1000000
clock() = 0
sleeping for 1 second...
clock() = 100

I raised this issue on debian-hurd here:
https://lists.debian.org/debian-hurd/2017/07/msg00054.html

While I appreciate that hurd's clock may not be able to be as precise as
on Linux, but it should still return values which corresponds in some
way to CLOCKS_PER_SEC.

As far as I can tell, this bug is caused by the Debian patch:
debian/patches/hurd-i386/unsubmitted-clock_t_centiseconds.diff

The patch header contains a description about applications failing with
high precision. IMHO those applications are broken. Their bugs should
not be worked around in libc and that patch should be dropped.

This was originally found after debugging ffmpeg. I believe the FTBFS on
hurd is caused by this bug.

Thanks,
James

#include <time.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>

static void busy_wait(int seconds)
{
    struct timespec start, current;
    if (clock_gettime(CLOCK_MONOTONIC, &start) != 0)
        abort();

    for (;;)
    {
        if (clock_gettime(CLOCK_MONOTONIC, &current) != 0)
            abort();

        if (current.tv_sec - start.tv_sec > seconds ||
            (current.tv_sec - start.tv_sec == seconds &&
            current.tv_nsec >= start.tv_nsec))
            break;
    }
}

int main(void)
{
    printf("CLOCKS_PER_SEC = %ld\n", CLOCKS_PER_SEC);
    printf("clock() = %ld\n", clock());
    printf("sleeping for 1 second...\n");
    busy_wait(1);
    printf("clock() = %ld\n", clock());
    return 0;
}

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