Messages in this thread | | | Date | Thu, 17 Dec 2009 22:38:53 -0700 | From | Eric Blake <> | Subject | utimensat fails to update ctime |
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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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