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    Subjectutimensat fails to update ctime
    POSIX requires that utimensat/futimens must update ctime in all cases
    where any change is made (it only exempts when both atime and mtime were
    requested as UTIME_OMIT, where the file must exist but no change is made).
    Unfortunately, when atime is specified and mtime is UTIME_OMIT, the
    kernel mistakenly behaves like read(), by updating atime but not ctime.
    This in turn caused a regression in coreutils 8.2, visible through 'touch -a':
    http://lists.gnu.org/archive/html/bug-coreutils/2009-12/msg00171.html

    Here is a simple program demonstrating the failure:
    $ cat foo.c
    #include <fcntl.h>
    #include <unistd.h>
    #include <sys/stat.h>
    int
    main ()
    {
    int fd = creat ("file", 0600);
    struct stat st1, st2;
    struct timespec t[2] = { { 1000000000, 0 }, { 0, UTIME_OMIT } };
    fstat (fd, &st1);
    sleep (1);
    futimens (fd, t);
    fstat (fd, &st2);
    return st1.st_ctime == st2.st_ctime;
    }
    $ gcc -o foo foo.c -D_GNU_SOURCE
    $ ./foo; echo $?
    1

    The exit status should have been 0.

    GNU coreutils will end up working around the bug by calling fstat/[l]stat
    prior to futimens/utimensat, and populating the mtime field with the
    desired value rather than using UTIME_OMIT. But this is a pointless stat
    call, which could be avoided if the kernel were fixed to comply with POSIX
    by updating ctime even when mtime is UTIME_OMIT.

    --
    Don't work too hard, make some time for fun as well!

    Eric Blake ebb9@byu.net


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