Accurate Timestamps using gettimeofday and localtime

I'm attempting to monitor system time elapsed across multiple applications and data paths via gettimeoday and localtime. In this example, I want to grab the system time (microsecond percision) right before my code runs. I'm currently doing it like this:

#include <stdio.h>
#include <sys/time.h>

struct timeval tv;
struct timezone tz;
struct tm *tm;

**code I don't care about**

gettimeofday(&tv, NULL);
tm=localtime(&tv.tv_sec);

**code to watch**    

printf(" %d:%02d:%02d %ld \n", tm->tm_hour, tm->tm_min,
            tm->tm_sec, tv.tv_usec);

From what I understand the localtime call definitely incurs a bit of overhead and can throw off accuracy. I might be totally wrong on this, but should I wait to call localtime until after my code to watch completes? I'm assuming that lo开发者_StackOverflowcaltime is just doing a semi-expensive conversion of gettimeofday's results, and thus it should be placed right before the printf statement.

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If you really need microsecond accuracy, yes. I'd be very surprised if localtime executed in less than a microsecond. But I'd also be surprised if gettimeofday had a microsecond resolution, and even if the actual timer did (highly unlikely), the context switch when returning from the system will probably take well over a microsecond (and could be longer than the call to localtime). The fact is that except when accessing special hardware directly, you can't get anywhere near microsecond resolution.