strftime performance vs. snprintf

Question

I have run into an interesting performance conundrum but before I start delving into glibc and entering bugs left right and center I just wanted to get to get any insight that might be out there.

I have code that in one of the functions does this:

gettimeofday( &tv, 0);
localtime_r( &tv.tv_sec, &local_tm );
char result[25];
strftime( result, 24, "%Y-%m-%d %H:%M:%S", &local_tm);

The rest of the code is irrelevant for this question. When I replace it with this:

gettimeofday( &tv, 0);
localtime_r( &tv.tv_sec, &local_tm );
char result[25];
snprintf(result, sizeof(result), "%04d-%02d-%02d %02d:%02d:%02d",
         local_tm.tm_year+1900, local_tm.tm_mon+1,
         local_tm.tm_mday, local_tm.tm_hour, local_tm.tm_min,
         local_tm.tm_sec);

on average I get 20% performance boost.

Has anyone ran into this? Is this OS specific?

Answer 1

POSIX requires strftime to call tzset() (or act as if it did), which on a linux system will likely stat /etc/timezone and other files, which is slow (compared to snprintf). Setting the TZ environment variable will generally give it a great boost.

As was said in the comments it also localizes the message.