了解 time_t 调用

Question

I've written the following to print the local time as a string:

 char time_buffer[25];
 time_t t = time(&t);
 strftime(time_buffer, 25, "%c", localtime(&t));
 printf("Formatted time: %s\n", time_buffer);

However, the code doesn't make too much sense to me (even though I've written it). A few questions on it:

• How does time_t t = time(&t) work? Is this setting up a struct of the localtime? Where is it grabbing the time from?
• What is the difference between the time(&t) and localtime(&t) ? Why are both needed?

Answer 1

time() is implemented in libc. For example, here is the implementation in glibc 2.33 for Linux:

time (time_t *t)
{
  return INLINE_VSYSCALL (time, 1, t);
}

which asks the kernel via a syscall for the time. The kernel, in turn, maintains a variable somewhere with this information. The kernel gets the time on boot from a battery backed clock and often subsequently sync'ed with a reference clock source (on Linux this would be via the Network Time Protocol (NTP)).

time() returns a time_t (on my system that is a typedef of long) which is number of seconds since epoch (this is measure of time is independent of timezone). localtime() returns a struct tm * which has the usual components for of date and time (sec, min, hour, day, month, year etc) in whatever timezone you selected (on Linux via the environment variable TZ).