optimize converting UNIX timestamp to localtime

Make a huge difference (cuts the time in half to process the same amount of
lines) on atleast glibc 2.30 on Void Linux.  Seems to make no difference on
OpenBSD.

- This removes atleast one heap allocation per line (checked with valgrind).
  This is because glibc will strdup() the environment variable $TZ and free it
  each time, which is pointless here and wasteful.
- localtime_r does not require to set the variables like tzname.

In glibc-2.30/time/tzset.c in __tz_convert is the following code and comment:

  /* Update internal database according to current TZ setting.
     POSIX.1 8.3.7.2 says that localtime_r is not required to set tzname.
     This is a good idea since this allows at least a bit more parallelism.  */
  tzset_internal (tp == &_tmbuf && use_localtime);

This makes it always tzset() and inspect the environment $TZ etc. While with
localtime_r it will only initialize it once:

	static void tzset_internal (int always) {
	[...]
	if (is_initialized && !always)
		return;

1 files changed, 2 insertions, 2 deletions

diff --git a/sfeed_plain.c b/sfeed_plain.c
index 42f9626..1b11a71 100644
--- a/sfeed_plain.c
+++ b/sfeed_plain.c
@@ -16,7 +16,7 @@ static void
 printfeed(FILE *fp, const char *feedname)
 {
 	char *fields[FieldLast];
-	struct tm *tm;
+	struct tm rtm, *tm;
 	time_t parsedtime;
 	ssize_t linelen;
 
@@ -27,7 +27,7 @@ printfeed(FILE *fp, const char *feedname)
 
 		parsedtime = 0;
 		if (!strtotime(fields[FieldUnixTimestamp], &parsedtime) &&
-		    (tm = localtime(&parsedtime))) {
+		    (tm = localtime_r(&parsedtime, &rtm))) {
 			if (parsedtime >= comparetime)
 				fputs("N ", stdout);
 			else