使文件修改时间达到纳秒级精度 [英] Get file modification time to nanosecond precision

问题描述

我需要获取遍历文件系统树的Python 2程序中每个文件的完整纳秒精度修改时间戳.我想在Python本身中执行此操作，因为为每个文件生成一个新的子进程会很慢.

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.

在Linux上的C库中，您可以通过查看stat结果的st_mtime_nsec字段来获得纳秒级的时间戳.例如:

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;
    }
}

打印mtime = 1380667414.213703287(/在ext4文件系统上，该文件系统支持纳秒级的时间戳，时钟为UTC).

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

类似地，date --rfc-3339=ns --reference=/打印2013-10-01 22:43:34.213703287+00:00.

Python(2.7.3)的os.path.getmtime(filename)和os.lstat(filename).st_mtime给出mtime作为float.但是，结果是错误的:

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

-仅前6位数字是正确的，大概是由于浮点错误引起的.

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

推荐答案

或者，您也可以使用 cffi 与以下代码一起在Python 2中使用的库(<在LInux上测试 ):

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))

在行为上与问题中的C程序相同. 产生输出:

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

$ ./test.py
mtime = 1381711568.315075616

与您的C程序具有相同的精度:

Which has the same precision as your C program:

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

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