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 c

Answer 1

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 ...

Answer 2

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.

Answer 3

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

Answer 4

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;
    ...
}

Answer 5

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

Answer 6

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.