Question

Some filesystems (e.g. ext4 and JFS) offer nanosecond resolution of atime/mtime fields. How can I read ns-resolution fields? The stat syscall returns time_t which is a second-resolution.

Answer 1

The second-resolution times are in the fields:

           time_t    st_atime;   /* time of last access */
           time_t    st_mtime;   /* time of last modification */
           time_t    st_ctime;   /* time of last status change */

But “NOTES” section of the man http://www.kernel.org/doc/man-pages/online/pages/man2/stat.2.html says:

Since kernel 2.5.48, the stat structure supports nanosecond resolution for the three file timestamp fields. Glibc exposes the nanosecond component of each
field using names of the form st_atim.tv_nsec if the _BSD_SOURCE or
_SVID_SOURCE feature test macro is defined. These fields are specified in
POSIX.1-2008, and, starting with version 2.12, glibc also exposes these field
names if _POSIX_C_SOURCE is defined with the value 200809L or greater, or
_XOPEN_SOURCE is defined with the value 700 or greater. If none of the
aforementioned macros are defined, then the nanosecond values are exposed with
names of the form st_atimensec.

So, nsec parts of times are in the same “struct stat”: ( /usr/include/asm/stat.h )

 unsigned long st_atime_nsec;

 unsigned int st_mtime_nsec;

 unsigned long st_ctime_nsec;

 #define STAT_HAVE_NSEC 1