How can I improve/replace sprintf, which I've measured to be a performance hotspot?

Question

Through profiling I've discovered that the sprintf here takes a long time. Is there a better performing alternative that still handles the leading zeros in the y/m/d h/m/s fields?

SYSTEMTIME sysTime;
GetLocalTime( &sysTime );
char buf[80];
for (int i = 0; i < 100000; i++)
{

    sprintf(buf, "%4d-%02d-%02d %02d:%02d:%02d",
        sysTime.wYear, sysTime.wMonth, sysTime.wDay, 
        sysTime.wHour, sysTime.wMinute, sysTime.wSecond);

}

---

Note: The OP explains in the comments that this is a stripped-down example. The "real" loop contains additional code that uses varying time values from a database. Profiling has pinpointed sprintf() as the offender.

Answer 1

If you were writing your own function to do the job, a lookup table of the string values of 0 .. 61 would avoid having to do any arithmetic for everything apart from the year.

edit: Note that to cope with leap seconds (and to match strftime()) you should be able to print seconds values of 60 and 61.

char LeadingZeroIntegerValues[62][] = { "00", "01", "02", ... "59", "60", "61" };

Alternatively, how about strftime()? I've no idea how the performance compares (it could well just be calling sprintf()), but it's worth looking at (and it could be doing the above lookup itself).

Answer 2

You could try filling each char in the output in turn.

buf[0] = (sysTime.wYear / 1000) % 10 + '0' ;
buf[1] = (sysTime.wYear / 100) % 10 + '0';
buf[2] = (sysTime.wYear / 10) % 10 + '0';
buf[3] = sysTime.wYear % 10 + '0';
buf[4] = '-';

... etc...

Not pretty, but you get the picture. If nothing else, it may help explain why sprintf isn't going to be that fast.

OTOH, maybe you could cache the last result. That way you'd only need to generate one every second.

Answer 3

Printf needs to deal with a lot of different formats. You certainly could grab the source for printf and use it as a basis to roll your own version that deals specifically with the sysTime structure. That way you pass in one argument, and it does just exactly the work that needs to be done and nothing more.

Answer 4

What do you mean by a "long" time -- since the sprintf() is the only statement in your loop and the "plumbing" of the loop (increment, comparison) is negligible, the sprintf() has to consume the most time.

Remember the old joke about the man who lost his wedding ring on 3rd Street one night, but looked for it on 5th because the light was brighter there? You've built an example that's designed to "prove" your assumption that sprintf() is ineffecient.

Your results will be more accurate if you profile "actual" code that contains sprintf() in addition to all the other functions and algorithms you use. Alternatively, try writing your own version that addresses the specific zero-padded numeric conversion that you require.

You may be surprised at the results.