C++ How to check the last modified time of a file

Question

I'm caching some information from a file and I want to be able to check periodically if the file's content has been modified so that I can read the file again to get the new content if needed.

That's why I'm wondering if there is a way to get a file's last modified time in C++.

@Jean-FrançoisFabre, it's a unix system call, so it's not cross-platform — SingerOfTheFall, Nov 09 '16 at 10:09

score 42 · Accepted Answer · edited Jul 13 '18 at 21:34

42

There is no language-specific way to do this, however the OS provides the required functionality. In a unix system, the stat function is what you need. There is an equivalent _stat function provided for windows under Visual Studio.

So here is code that would work for both:

#include <sys/types.h>
#include <sys/stat.h>
#ifndef WIN32
#include <unistd.h>
#endif

#ifdef WIN32
#define stat _stat
#endif

auto filename = "/path/to/file";
struct stat result;
if(stat(filename.c_str(), &result)==0)
{
    auto mod_time = result.st_mtime;
    ...
}

edited Jul 13 '18 at 21:34

vallentin

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answered Nov 09 '16 at 10:10

Smeeheey

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So there's nothing that I can use that will work both on UNIX and on Windows? :/ – Mr. Nicky Nov 09 '16 at 10:12
2

Modified to add windows support – Smeeheey Nov 09 '16 at 10:17
1

this worked great after blind coding waiting 10min to let travisci end remote compilation :) – Aquarius Power Apr 20 '18 at 18:23

score 22 · Answer 2 · edited Jun 09 '20 at 04:36

22

since the time of this post, c++17 has been released, and it includes a filesystem library based on the boost filesystem library:

https://en.cppreference.com/w/cpp/header/filesystem

which includes a way to get the last modification time:

https://en.cppreference.com/w/cpp/filesystem/last_write_time

edited Jun 09 '20 at 04:36

serg06

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answered Apr 09 '19 at 04:22

Dylan95

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In order to use std::filesystem on macOS with Apple's developer tools, you must assume Catalina or later. – yig Oct 05 '20 at 10:51
The returned time is unusable portably in C++17. But from C++20, we can cast between chrono clocks, and convert the returned time to a systime which is portable. – Nicolas Dusart Jul 29 '21 at 12:34

score 9 · Answer 3 · answered Nov 09 '16 at 10:18

9

You can use boost's last_write_time for that. Boost is cross platform.

Here's the tutorial link for that.

Boost has the advantage that it works for all kinds of file names, so it takes care of non-ASCII file names.

answered Nov 09 '16 at 10:18

The Quantum Physicist

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since c++17 also as part of standard library https://en.cppreference.com/w/cpp/filesystem/last_write_time – sciamano Oct 28 '18 at 17:33

score 2 · Answer 4 · answered Mar 08 '19 at 19:34

2

Please note that there are some limitations:

... The [time] resolution is as low as one hour on some filesystems... During program execution, the system clock may be set to a new value by some other, possibly automatic, process ...

answered Mar 08 '19 at 19:34

pooya13

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score 1 · Answer 5 · answered Jul 27 '20 at 15:42

1

Also note that after copying a file (on Windows) the copy's last_write_time is the last_write_time of the original file rather than the time the copy was created, as one would naively think.

answered Jul 27 '20 at 15:42

foobar

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score 0 · Answer 6 · answered Oct 05 '20 at 10:50

0

This cross-platform library (Mac, Windows, Linux) is simple to add to a project. It uses #ifdef's to compile the right implementation.

https://github.com/jameswynn/simplefilewatcher

answered Oct 05 '20 at 10:50

yig

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C++ How to check the last modified time of a file

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