Get file modification time to nanosecond precision

I need to get the full nanosecond-precision modified timestamp for each file in a Python 2 program that walks the filesystem tree. I want to do this in Python itself, because spawning a new subprocess for every file will be slow.

From the C library on Linux, you can get nanosecond-precision timestamps by looking at the st_mtime_nsec field of a stat result. For example:

#include <sys/stat.h>
#include <stdio.h>
int main() {
    struct stat stat_result;
    if(!lstat("/", &stat_result)) {
        printf("mtime = %lu.%lu\n", stat_result.st_mtim.tv_sec, stat_result.st_mtim.tv_nsec);
    } else {
        printf("error\n");
        return 1;
    }
}

prints mtime = 1380667414.213703287 (/ is on an ext4 filesystem, which supports nanosecond timestamps, and the clock is UTC).

Similarly, date --rfc-3339=ns --reference=/ prints 2013-10-01 22:43:34.213703287+00:00.

Python (2.7.3)'s os.path.getmtime(filename) and os.lstat(filename).st_mtime give the mtime as a float. However, the result is wrong:

In [1]: import os
In [2]: os.path.getmtime('/') % 1
Out[2]: 0.21370339393615723
In [3]: os.lstat('/').st_mtime % 1
Out[3]: 0.21370339393615723

—only the first 6 digits are correct, presumably due to floating-point error.

Alternatively you coudl use the cffi library which works with Python 2 with the follwoing code (tested on LInux):

from __future__ import print_function

from cffi import FFI

ffi = FFI()
ffi.cdef("""
typedef long long time_t;

typedef struct timespec {
    time_t   tv_sec;
    long     tv_nsec;
    ...;
};

typedef struct stat {
    struct timespec st_mtim;
    ...;
};

int lstat(const char *path, struct stat *buf);
""")

C = ffi.verify()

result = ffi.new("struct stat *")
p = C.lstat("foo.txt", result)
print("mtime = {0:d}.{1:09d}".format(result.st_mtim.tv_sec, result.st_mtim.tv_nsec))

This is identical in behavior to your C program in your question. This produces the output:

$ ./test.py
mtime = 1381711568.315075616

Which has the same precision as your C program:

$ gcc test.c -o test
$ ./test
mtime = 1381711568.315075616