log: add lxc_unix_epoch_to_utc()

Converts a unix time Epoch given by a struct timespec to a UTC string useable
in our logging functions. Maybe expanded to allow for more generic formatting.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>

diff --git a/src/lxc/log.c b/src/lxc/log.c
index 413b381e2aeb67669b43a3593e0a975e3fdddb02..4880f7bac2c2f85eabe48879680a161f0d8f9cbb 100644 (file)
--- a/src/lxc/log.c
+++ b/src/lxc/log.c
@@ -22,9 +22,12 @@
  */
 
 #define _GNU_SOURCE
+#define __STDC_FORMAT_MACROS /* Required for PRIu64 to work. */
 #include <assert.h>
+#include <stdint.h>
 #include <stdio.h>
 #include <errno.h>
+#include <inttypes.h>
 #include <limits.h>
 #include <unistd.h>
 #include <sys/types.h>
@@ -138,6 +141,44 @@ static int log_append_stderr(const struct lxc_log_appender *appender,
 }
 
 /*---------------------------------------------------------------------------*/
+int lxc_unix_epoch_to_utc(char *buf, size_t bufsize, const struct timespec *time)
+{
+       int64_t days, z, era, doe, yoe, year, doy, mp, day, month, d_in_s,
+           hours, h_in_s, minutes, seconds;
+       char nanosec[__LXC_NUMSTRLEN];
+       int ret;
+
+       /* See https://howardhinnant.github.io/date_algorithms.html for an
+        * explanation of the algorithm used here.
+        */
+       days = time->tv_sec / 86400;
+       z = days + 719468;
+       era = (z >= 0 ? z : z - 146096) / 146097;
+       doe = (z - era * 146097);
+       yoe = (doe - doe / 1460 + doe / 36524 - doe / 146096) / 365;
+       year = (yoe) + era * 400;
+       doy = doe - (365 * yoe + yoe / 4 - yoe / 100);
+       mp = (5 * doy + 2) / 153;
+       day = doy - (153 * mp + 2) / 5 + 1;
+       month = mp + (mp < 10 ? 3 : -9);
+       d_in_s = days * 86400;
+       hours = (time->tv_sec - d_in_s) / 3600;
+       h_in_s = hours * 3600;
+       minutes = (time->tv_sec - d_in_s - h_in_s) / 60;
+       seconds = (time->tv_sec - d_in_s - h_in_s - (minutes * 60));
+
+       ret = snprintf(nanosec, __LXC_NUMSTRLEN, "%ld", time->tv_nsec);
+       if (ret < 0 || ret >= __LXC_NUMSTRLEN)
+               return -1;
+       ret = snprintf(buf, bufsize, "%" PRId64 "%" PRId64 "%" PRId64 "%" PRId64
+                                    "%" PRId64 "%" PRId64 ".%.3s",
+                      year, month, day, hours, minutes, seconds, nanosec);
+       if (ret < 0 || (size_t)ret >= bufsize)
+               return -1;
+
+       return 0;
+}
+
 /* This function needs to make extra sure that it is thread-safe. We had some
  * problems with that before. This especially involves time-conversion
  * functions. I don't want to find any localtime() or gmtime() functions or