How can I get my C program running time in ms not s on Windows10?

Question

#include <time.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/timeb.h>
#include <Windows.h>

struct timeb timenow;

int main() {
    ftime(&timenow);
    printf("%ds %dms\n", timenow.time, timenow.millitm);
    Sleep(1503);
    ftime(&timenow);
    printf("%ds %dms\n", timenow.time, timenow.millitm);
    return 0;
}

Why the program always print 0 ms ？ The time is OK but the millitm is always 0.

Answer 1

According to ftime(3) - Linux manual page , the time member of struct timeb has the type time_t .

This type may be 64-bit long to support date beyond the year 2038.

On the other hand, typical int is 32-bit long.

The %d format specifier is for printing int , and passing time_t to that will invoke undefined behavior by passind data having wrong type.

Unfortunately printf doesn't seem to have proper format specifier to print time_t , so you should cast that to some type that can be safely printed via printf .

On the other hand, the millitm member is unsigned short and it will be promoted to int , so using %d fot that is fine.

Try this:

#include <time.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/timeb.h>
#include <Windows.h>
#include <inttypes.h>

struct timeb timenow;

int main() {
    ftime(&timenow);
    printf("%" PRId64 "s %dms\n", (int64_t)timenow.time, timenow.millitm);
    Sleep(1503);
    ftime(&timenow);
    printf("%" PRId64 "s %dms\n", (int64_t)timenow.time, timenow.millitm);
    return 0;
}