从上次在OS X中使用C修改文件以来获得时间？

Question

I have the following code that is suppose to print time passed since file was last modified:

int main(int argc, const char * argv[]) {

    struct stat fileInfo;
    char * dir = "/Users/jenna/Desktop/Random.rtf";
    stat(dir, &fileInfo);

    printf("Last modified time: %s\n", ctime(&fileInfo.st_mtime));

    time_t mytime;
    mytime = time(NULL);
    printf("Current time: %s\n", ctime(&mytime));

    double diff = difftime(ctime(&mytime), ctime(&fileInfo.st_mtime));
    printf("diff : %f\n", diff);

}

Unfortunately, this code gives the following output:

Last modified time: Wed Apr  6 00:15:30 2016

Current time: Wed Apr  6 00:17:29 2016

diff : 0.000000

When I change the format flag in the printf to %d , I get the following output:

Last modified time: Wed Apr  6 00:15:30 2016

Current time: Wed Apr  6 00:18:34 2016

diff : 1983419808

Neither output is correct since I am expecting the difference of just several minutes( expressed in seconds). What am I doing wrong here ? Thanks for any help.

Answer 1

You should get into the practise of enabling compiler warnings. This code should be giving you some very helpful ones. Specifically, you're passing char* values into difftime .

Since ctime always returns a pointer to the same static memory, the two calls to ctime return the same pointer. Now you are implicitly casting these to time_t (compiler warning!!!), and you get a difference of zero.

In the second case, you pass a double into printf and tell it you passed an int , which is undefined behaviour.

What you actually want is to pass time_t values into difftime like this:

double diff = difftime( mytime, fileInfo.st_mtime );
printf( "diff : %f\n", diff );

And get into the habit of reading documentation for functions that you are unfamiliar with:

• Reference for difftime
• Reference for ctime