Question

I’m trying to write a portable function in c that compares the last modified times of 2 files. The files are tiny and written one right after the other, so I need finer granularity than 1 second (milliseconds).
There seems to be a plethora of time/date functions…

Answer 1

The C standard does not have any functions for this, but the Posix specification does. The 2008 edition even provides sub-second timestamps. #define _POSIX_C_SOURCE 200809L

The following code should give you an idea how to use it.

#define _POSIX_C_SOURCE 200809L
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>

#include <stdio.h> // for printf
#include <stdlib.h> // for EXIT_FAILURE

int main(int argc, char **argv)
{
    for (int i = 1; i < argc; ++i) {
        struct stat st = {0};
        int ret = lstat(argv[i], &st);
        if (ret == -1) {
            perror("lstat");
            return EXIT_FAILURE;
        }

        printf("%s: mtime sec=%lld nsec=%lld\n", argv[i],
               (long long) st.st_mtim.tv_sec, 
               (long long) st.st_mtim.tv_nsec);
    }

    return 0;
}