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++.

Quarrier

what's wrong with good old C stat call? – Vierra

@Jean-FrançoisFabre, it's a unix system call, so it's not cross-platform – Corbicula

@Corbicula that's why I did not answer. – Vierra

Answer 1

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;
    ...
}

Neel

So there's nothing that I can use that will work both on UNIX and on Windows? :/ – Quarrier

Modified to add windows support – Neel

this worked great after blind coding waiting 10min to let travisci end remote compilation :) – Frontolysis

Answer 2

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

Cockney

In order to use std::filesystem on macOS with Apple's developer tools, you must assume Catalina or later. – Mantissa

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. – Indefectible

Answer 3

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.

Irmgardirmina

since c++17 also as part of standard library en.cppreference.com/w/cpp/filesystem/last_write_time – Harmonica

Answer 4

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 ...

Dymoke

Answer 5

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.

Deice

Answer 6

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

Mantissa