most efficient way to get current time/date/day in C

What is the most efficient way of getting current time/dat开发者_如何学Goe/day/year in C language? As I have to execute this many times, I need a real efficient way. I am on freeBSD.

thanks in advance.

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/* ctime example */
#include <stdio.h>
#include <time.h>

int main ()
{
  time_t rawtime;

  time ( &rawtime );
  printf ( "The current local time is: %s", ctime (&rawtime) );

  return 0;
}

You can use ctime, if you need it as a string.

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Standard C provides only one way to get the time - time() - which can be converted to a time/date/year with localtime() or gmtime(). So trivially, that must be the most efficient way.

Any other methods are operating-system specific, and you haven't told us what operating system you're using.

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It really depends on what you mean by "many" :-)

I think you'll probably find that using the ISO standard time() and localtime() functions will be more than fast enough. For example, on my "Intel(R) Core(TM)2 Duo CPU E6850 @ 3.00GHz", using unoptimised code, I can call time() ten million times in 1.045 seconds, and a time()/localtime() combination half a million times in 0.98 seconds. Whether that's fast enough for your needs, only you can decide, but I'm hard-pressed trying to come up with a use case that needs more grunt than that.

The time() function gives you the number of seconds since the epoch, while localtime() both converts it to local time (from UTC) and splits it into a more usable form, the struct tm structure.

#include <time.h>
time_t t = time (NULL);
struct tm* lt = localtime (&t);
// Use lt->tm_year, lt->tm_mday, and so forth.

Any attempt to cache the date/time and use other ways of finding out a delta to apply to it, such as with clock(), will almost invariably:

• be slower; and
• suffer from the fact you won't pick up external time changes.

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The simplest is

#include <time.h>
//...
time_t current_time = time (NULL);
struct tm* local_time = localtime (&current_time); 
printf ("the time is %s\n", asctime (local_time));

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You can use gettimeofday() function to get time in seconds & microseconds which is (I think) very fast (as there is a similar function in Linux kernel do_gettimeofday()) and then you can convert it to your required format (might possible to use functions mentioned above for conversion.

I hope this helps.

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Just about the only way (that's standard, anyway) is to call time followed by localtime or gmtime.

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well, in general, directly accessing the OS's API to get the time is probably the most efficient, but not so portable.....

the C time functions are ok.

But really depends on your platform

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Assuming a one second resolution is enough, the most efficient way on FreeBSD (or any POSIX system) is likely

• Install a one second interval timer with setitimer (ITIMER_REAL, ...)
• When it triggers the SIGALRM, update a static variable holding the currrent time
• Use the value in the static variable whenever you need the time

Even if signals get lost due to system overload this will correct itself the next time the process is scheduled.