Export boot timestamp, update docs and examples to use it

tutorials/udp/sntp-time-sync/main.c

@@ -1,62 +1,32 @@
 // Copyright (c) 2022 Cesanta Software Limited
 // All rights reserved
 //
-// Synchronize time with remote SNTP server
-// A working time() call is required by TLS, so an embedded device without
-// a clock that wants to use TLS, must sync time via SNTP.
+// Synchronize time with a remote SNTP server.
+// After a successful sync, mg_now() returns wall-clock time in milliseconds
+// since Unix epoch and mg_boot_timestamp_ms is set to a non-zero value.
+// Useful on embedded devices that lack a real-time clock but need accurate
+// time for TLS certificate validation or timestamping.
 
 #include "mongoose.h"
 
-// The UNIX epoch of the boot time. Initially, we set it to 0.  But then after
-// SNTP response, we update it to the correct value, which will allow us to
-// use time(). Uptime in milliseconds is returned by mg_millis().
-static time_t s_boot_timestamp = 0;
-
-// SNTP client connection
-static struct mg_connection *s_sntp_conn = NULL;
-
-// On embedded systems, rename to time()
-time_t my_time(time_t *tp) {
-  // you can just return mg_now() / 1000;
-  time_t t = s_boot_timestamp + mg_millis() / 1000;
-  if (tp != NULL) *tp = t;
-  return t;
-}
-
-// SNTP client callback
-static void sfn(struct mg_connection *c, int ev, void *ev_data) {
-  if (ev == MG_EV_SNTP_TIME) {
-    // Time received, the internal protocol handler updates what mg_now() returns
-    uint64_t curtime = mg_now();
-    MG_INFO(("SNTP-updated current time is: %llu ms from epoch", curtime));
-    // otherwise, you can process the server returned data yourself
-    {
-      uint64_t t = *(uint64_t *) ev_data;
-      s_boot_timestamp = (time_t) ((t - mg_millis()) / 1000);
-      MG_INFO(("Got SNTP time: %llu ms from epoch, ", t));
-    }
-  } else if (ev == MG_EV_CLOSE) {
-    s_sntp_conn = NULL;
-  }
-  (void) c;
-}
-
-// Called every 5 seconds. Increase that for production case.
-static void timer_fn(void *arg) {
-  struct mg_mgr *mgr = (struct mg_mgr *) arg;
-  if (s_sntp_conn == NULL) { // connection issues a request
-    s_sntp_conn = mg_sntp_connect(mgr, NULL, sfn, NULL);
-  } else {
-    mg_sntp_request(s_sntp_conn);
-  }
-}
-
 int main(void) {
-  struct mg_mgr mgr;        // Event manager
-  mg_mgr_init(&mgr);        // Initialise event manager
-  mg_log_set(MG_LL_DEBUG);  // Set log level
-  mg_timer_add(&mgr, 5000, MG_TIMER_REPEAT | MG_TIMER_RUN_NOW, timer_fn, &mgr);
-  for (;;) mg_mgr_poll(&mgr, 300);  // Infinite event loop
-  mg_mgr_free(&mgr);                // Free manager resources
+  struct mg_mgr mgr;
+  mg_mgr_init(&mgr);
+  mg_log_set(MG_LL_DEBUG);
+
+  uint64_t timer = 0;
+  for (;;) {
+    // Retry every second until we have a valid time, then sync once per hour
+    uint64_t period = mg_boot_timestamp_ms == 0 ? 1000 : 3600 * 1000;
+    if (mg_timer_expired(&timer, period, mg_millis())) {
+      mg_sntp_connect(&mgr, NULL, NULL, NULL);
+    }
+    if (mg_boot_timestamp_ms > 0) {
+      MG_INFO(("Current time: %llu ms from epoch", mg_now()));
+    }
+    mg_mgr_poll(&mgr, 1000);
+  }
+
+  mg_mgr_free(&mgr);
   return 0;
 }