syslog: fix UDP timing problems

- start README file describing the interface
- add 2ms delay before sending the next package

syslog/syslog.c

@@ -16,6 +16,7 @@
 
 extern void * mem_trim(void *m, size_t s);	// not well documented...
 
+#undef SYSLOG_DBG
 #ifdef SYSLOG_DBG
 #define DBG(format, ...) do { os_printf(format, ## __VA_ARGS__); } while(0)
 #else
@@ -25,7 +26,7 @@ extern void * mem_trim(void *m, size_t s);	// not well documented...
 #define WIFI_CHK_INTERVAL 1000	// ms to check Wifi statis
 static os_timer_t wifi_chk_timer;
 
-static struct espconn syslog_espconn;
+static struct espconn *syslog_espconn = NULL;
 static uint32_t syslog_msgid = 1;
 static uint8_t syslog_task = 0;
 
@@ -35,8 +36,10 @@ static syslog_entry_t *syslogQueue = NULL;
 static enum syslog_state syslogState = SYSLOG_NONE;
 
 static void ICACHE_FLASH_ATTR syslog_add_entry(syslog_entry_t *entry);
-static void ICACHE_FLASH_ATTR syslog_chk_wifi_stat(void);
+static void ICACHE_FLASH_ATTR syslog_chk_status(void);
 static void ICACHE_FLASH_ATTR syslog_udp_sent_cb(void *arg);
+static syslog_entry_t ICACHE_FLASH_ATTR *syslog_compose(uint8_t facility, uint8_t severity, const char *tag, const char *fmt, ...);
+
 #ifdef SYSLOG_UDP_RECV
 static void ICACHE_FLASH_ATTR syslog_udp_recv_cb(void *arg, char *pusrdata, unsigned short length);
 #endif
@@ -44,27 +47,70 @@ static void ICACHE_FLASH_ATTR syslog_udp_recv_cb(void *arg, char *pusrdata, unsi
 #define syslog_send_udp() post_usr_task(syslog_task,0)
 
 /******************************************************************************
- * FunctionName : syslog_chk_wifi_stat
+ * FunctionName : syslog_chk_status
  * Description  : check whether get ip addr or not
  * Parameters   : none
  * Returns      : none
 *******************************************************************************/
-static void ICACHE_FLASH_ATTR syslog_chk_wifi_stat(void)
+static void ICACHE_FLASH_ATTR syslog_chk_status(void)
 {
   struct ip_info ipconfig;
-  DBG("syslog_chk_wifi_stat: state: %d ", syslogState);
 
   //disarm timer first
   os_timer_disarm(&wifi_chk_timer);
 
+  DBG("syslog_chk_status: state: %d ", syslogState);
+
   //try to get ip info of ESP8266 station
   wifi_get_ip_info(STATION_IF, &ipconfig);
   int wifi_status = wifi_station_get_connect_status();
   if (wifi_status == STATION_GOT_IP && ipconfig.ip.addr != 0)
   {
-    if (syslogState == SYSLOG_WAIT) {			// waiting for initialization
+    // it seems we have to add an additional delay after the Wifi is up and running.
-      DBG("connected, initializing UDP socket\n");
+    // so we simply add an intermediate state with 25ms delay
+    switch (syslogState)
+      {
+        case SYSLOG_WAIT:
+          DBG("state SYSLOG_WAIT: Wifi connected\n");
+          syslogState = SYSLOG_INIT;
+          os_timer_setfn(&wifi_chk_timer, (os_timer_func_t *)syslog_chk_status, NULL);
+          os_timer_arm(&wifi_chk_timer, 100, 0);
+          break;
+
+        case SYSLOG_INIT:
+          DBG("state SYSLOG_INIT: init syslog\n");
           syslog_init(flashConfig.syslog_host);
+          os_timer_setfn(&wifi_chk_timer, (os_timer_func_t *)syslog_chk_status, NULL);
+          os_timer_arm(&wifi_chk_timer, 100, 0);
+          break;
+
+        case SYSLOG_DNSWAIT:
+          DBG("state SYSLOG_DNSWAIT: wait for DNS resolver\n");
+          os_timer_setfn(&wifi_chk_timer, (os_timer_func_t *)syslog_chk_status, NULL);
+          os_timer_arm(&wifi_chk_timer, 100, 0);
+          syslogState = SYSLOG_READY;
+          break;
+
+        case SYSLOG_READY:
+          DBG("state SYSLOG_READY: start sending\n");
+          syslog_send_udp();
+          break;
+
+        case SYSLOG_SENDING:
+          DBG("state SYSLOG_SENDING: delay\n");
+          os_timer_setfn(&wifi_chk_timer, (os_timer_func_t *)syslog_chk_status, NULL);
+          os_timer_arm(&wifi_chk_timer, 2, 0);
+          syslogState = SYSLOG_SEND;
+          break;
+
+         case SYSLOG_SEND:
+          DBG("state SYSLOG_SEND: start sending\n");
+          syslog_send_udp();
+          break;
+
+        default:
+           DBG("state DEFAULT: %d\n", syslogState);
+         break;
       }
   } else {
     if ((wifi_status == STATION_WRONG_PASSWORD ||
@@ -74,108 +120,12 @@ static void ICACHE_FLASH_ATTR syslog_chk_wifi_stat(void)
       os_printf("*** connect failure!!!\n");
     } else {
       DBG("re-arming timer...\n");
-      os_timer_setfn(&wifi_chk_timer, (os_timer_func_t *)syslog_chk_wifi_stat, NULL);
+      os_timer_setfn(&wifi_chk_timer, (os_timer_func_t *)syslog_chk_status, NULL);
       os_timer_arm(&wifi_chk_timer, WIFI_CHK_INTERVAL, 0);
     }
   }
 }
 
-static void ICACHE_FLASH_ATTR syslog_udp_send_event(os_event_t *events) {
-  if (syslogQueue == NULL)
-    syslogState = SYSLOG_READY;
-  else {
-    int res = 0;
-    syslog_espconn.proto.udp->remote_port = syslogHost.port;			// ESP8266 udp remote port
-    os_memcpy(&syslog_espconn.proto.udp->remote_ip, &syslogHost.addr.addr, 4);	// ESP8266 udp remote IP
-    res = espconn_send(&syslog_espconn, (uint8_t *)syslogQueue->datagram, syslogQueue->datagram_len);
-    if (res != 0) {
-      os_printf("syslog_udp_send: error %d\n", res);
-    }
-  }
-}
-
-/******************************************************************************
- * FunctionName : syslog_compose
- * Description  : compose a syslog_entry_t from va_args
- * Parameters   : va_args
- * Returns      : the malloced syslog_entry_t
- ******************************************************************************/
-static syslog_entry_t* ICACHE_FLASH_ATTR syslog_compose(uint8_t facility, uint8_t severity, const char *tag, const char *fmt, ...)
-{
-  syslog_entry_t *se = os_zalloc(sizeof (syslog_entry_t) + 1024);	// allow up to 1k datagram
-  char *p = se->datagram;
-  uint32_t tick = WDEV_NOW();			// 0 ... 4294.967295s
-
-  // The Priority value is calculated by first multiplying the Facility
-  // number by 8 and then adding the numerical value of the Severity.
-  p += os_sprintf(p, "<%d> ", facility * 8 + severity);
-
-  // strftime doesn't work as expected - or adds 8k overhead.
-  // so let's do poor man conversion - format is fixed anyway
-  if (flashConfig.syslog_showdate == 0)
-    p += os_sprintf(p, "- ");
-  else {
-    time_t now = NULL;
-    struct tm *tp = NULL;
-
-    // create timestamp: FULL-DATE "T" PARTIAL-TIME "Z": 'YYYY-mm-ddTHH:MM:SSZ '
-    // as long as realtime_stamp is 0 we use tick div 10⁶ as date
-    now = (realtime_stamp == 0) ? (tick / 1000000) : realtime_stamp;
-    tp = gmtime(&now);
-
-    p += os_sprintf(p, "%4d-%02d-%02dT%02d:%02d:%02dZ ",
-		    tp->tm_year + 1900, tp->tm_mon + 1, tp->tm_mday,
-		    tp->tm_hour, tp->tm_min, tp->tm_sec);
-  }
-
-  // add HOSTNAME APP-NAME PROCID MSGID
-  if (flashConfig.syslog_showtick)
-    p += os_sprintf(p, "%s %s %lu.%06lu %lu ", flashConfig.hostname, tag, tick / 1000000, tick % 1000000, syslog_msgid++);
-  else
-    p += os_sprintf(p, "%s %s - %lu ", flashConfig.hostname, tag, syslog_msgid++);
-
-  // append syslog message
-  va_list arglist;
-  va_start(arglist, fmt);
-  p += ets_vsprintf(p, fmt, arglist );
-  va_end(arglist);
-
-  se->datagram_len = p - se->datagram;
-  se = mem_trim(se, sizeof(syslog_entry_t) + se->datagram_len + 1);
-  return se;
-}
-
-/******************************************************************************
- * FunctionName : syslog_add_entry
- * Description  : add a syslog_entry_t to the syslogQueue
- * Parameters   : entry: the syslog_entry_t
- * Returns      : none
- ******************************************************************************/
-static void ICACHE_FLASH_ATTR syslog_add_entry(syslog_entry_t *entry)
-{
-  syslog_entry_t *pse = syslogQueue;
-
-  // append msg to syslog_queue
-  if (pse == NULL)
-    syslogQueue = entry;
-  else {
-    while (pse->next != NULL)
-      pse = pse->next;
-    pse->next = entry;	// append msg to syslog queue
-  }
-
-  // ensure we have sufficient heap for the rest of the system
-  if (system_get_free_heap_size() < syslogHost.min_heap_size) {
-    if (syslogState != SYSLOG_HALTED) {
-      os_printf("syslog_add_entry: Warning: queue filled up, halted\n");
-      entry->next = syslog_compose(SYSLOG_FAC_SYSLOG, SYSLOG_PRIO_CRIT, "-", "queue filled up, halted");
-      if (syslogState == SYSLOG_READY)
-	syslog_send_udp();
-      syslogState = SYSLOG_HALTED;
-    }
-  }
-}
-
 /******************************************************************************
  * FunctionName : syslog_sent_cb
  * Description  : udp sent successfully
@@ -193,10 +143,29 @@ static void ICACHE_FLASH_ATTR syslog_udp_sent_cb(void *arg)
   syslogQueue = syslogQueue -> next;
   os_free(pse);
 
-  if (syslogQueue != NULL)
+  if (syslogQueue == NULL)
-    syslog_send_udp();
+    syslogState = SYSLOG_READY;
-  else
+  else {
+    // UDP seems timecritical - we must ensure a minimum delay after each package...
+    syslogState = SYSLOG_SENDING;
+    syslog_chk_status();
+  }
+}
+
+static void ICACHE_FLASH_ATTR
+syslog_udp_send_event(os_event_t *events) {
+//  os_printf("syslog_udp_send_event: %d %lu, %lu\n", syslogState, syslogQueue->msgid, syslogQueue->tick);
+  if (syslogQueue == NULL)
     syslogState = SYSLOG_READY;
+  else {
+    int res = 0;
+    syslog_espconn->proto.udp->remote_port = syslogHost.port;			// ESP8266 udp remote port
+    os_memcpy(syslog_espconn->proto.udp->remote_ip, &syslogHost.addr.addr, 4);	// ESP8266 udp remote IP
+    res = espconn_send(syslog_espconn, (uint8_t *)syslogQueue->datagram, syslogQueue->datagram_len);
+    if (res != 0) {
+      os_printf("syslog_udp_send: error %d\n", res);
+    }
+  }
 }
 
  /*****************************************************************************
@@ -222,9 +191,10 @@ static void ICACHE_FLASH_ATTR syslog_gethostbyname_cb(const char *name, ip_addr_
   struct espconn *pespconn = (struct espconn *)arg;
   (void) pespconn;
   if (ipaddr != NULL) {
+    syslog(SYSLOG_FAC_USER, SYSLOG_PRIO_NOTICE, "SYSLOG",
+          "resolved hostname: %s: " IPSTR, name, IP2STR(ipaddr));
     syslogHost.addr.addr = ipaddr->addr;
-    syslogState = SYSLOG_SENDING;
+    syslogState = SYSLOG_DNSWAIT;
-    syslog_send_udp();
   } else {
     syslogState = SYSLOG_ERROR;
     DBG("syslog_gethostbyname_cb: state=SYSLOG_ERROR\n");
@@ -240,55 +210,176 @@ static void ICACHE_FLASH_ATTR syslog_gethostbyname_cb(const char *name, ip_addr_
  *******************************************************************************/
 void ICACHE_FLASH_ATTR syslog_init(char *syslog_host)
 {
-  if (!*syslog_host) {
   syslogState = SYSLOG_HALTED;
+
+  if (!*syslog_host) {
     return;
   }
-  char host[32], *port = &host[0];
 
-  syslog_task = register_usr_task(syslog_udp_send_event);
+  if (syslog_host == NULL) {
-  syslogHost.min_heap_size = flashConfig.syslog_minheap;
+    // disable and unregister syslog handler
-  syslogHost.port = 514;
+    if (syslog_espconn != NULL) {
-  syslogState = SYSLOG_WAIT;
+      if (syslog_espconn->proto.udp) {
+        // there's no counterpart to espconn_create...
+        os_free(syslog_espconn->proto.udp);
+	  }
+      os_free(syslog_espconn);
+    }
+    syslog_espconn = NULL;
+
+    // clean up syslog queue
+    syslog_entry_t *pse = syslogQueue;
+    while (pse != NULL) {
+      syslog_entry_t *next = pse->next;
+      os_free(pse);
+      pse = next;
+    }
+    syslogQueue = NULL;
+    return;
+  }
 
+  char host[32], *port = &host[0];
   os_strncpy(host, syslog_host, 32);
   while (*port && *port != ':')			// find port delimiter
     port++;
+
   if (*port) {
     *port++ = '\0';
     syslogHost.port = atoi(port);
   }
 
-  wifi_set_broadcast_if(STATIONAP_MODE); // send UDP broadcast from both station and soft-AP interface
+  if (syslogHost.port == 0)
-  syslog_espconn.type = ESPCONN_UDP;
+    syslogHost.port = 514;
-  syslog_espconn.proto.udp = (esp_udp *)os_zalloc(sizeof(esp_udp));
+
-  syslog_espconn.proto.udp->local_port = espconn_port();			// set a available  port
+  // allocate structures, init syslog_handler
+  if (syslog_espconn == NULL)
+    syslog_espconn = (espconn *)os_zalloc(sizeof(espconn));
+
+  if (syslog_espconn->proto.udp == NULL)
+    syslog_espconn->proto.udp = (esp_udp *)os_zalloc(sizeof(esp_udp));
+
+  syslog_espconn->type = ESPCONN_UDP;
+  syslog_espconn->proto.udp->local_port = espconn_port();			// set a available  port
 #ifdef SYSLOG_UDP_RECV
-  espconn_regist_recvcb(&syslog_espconn, syslog_udp_recv_cb);			// register a udp packet receiving callback
+  espconn_regist_recvcb(syslog_espconn, syslog_udp_recv_cb);			// register a udp packet receiving callback
 #endif
-  espconn_regist_sentcb(&syslog_espconn, syslog_udp_sent_cb);			// register a udp packet sent callback
+  espconn_regist_sentcb(syslog_espconn, syslog_udp_sent_cb);			// register a udp packet sent callback
-  espconn_create(&syslog_espconn);   						// create udp
+  syslog_task = register_usr_task(syslog_udp_send_event);
+  syslogHost.min_heap_size = flashConfig.syslog_minheap;
+
+// the wifi_set_broadcast_if must be handled global in connection handler...
+//  wifi_set_broadcast_if(STATIONAP_MODE); // send UDP broadcast from both station and soft-AP interface
+  espconn_create(syslog_espconn);   						// create udp
 
   if (UTILS_StrToIP((const char *)host, (void*)&syslogHost.addr)) {
-    syslogState = SYSLOG_SENDING;
+    syslog(SYSLOG_FAC_USER, SYSLOG_PRIO_NOTICE, "SYSLOG",
-    syslog_send_udp();
+          "syslogserver: %s:%d", host, syslogHost.port);
+    syslogState = SYSLOG_READY;
   } else {
-    static struct espconn espconn_ghbn;
+    syslog(SYSLOG_FAC_USER, SYSLOG_PRIO_NOTICE, "SYSLOG",
-    espconn_gethostbyname(&espconn_ghbn, host, &syslogHost.addr, syslog_gethostbyname_cb);
+          "resolving hostname: %s", host);
-    // syslog_send_udp is called by syslog_gethostbyname_cb()
+    syslogState = SYSLOG_DNSWAIT;
+    espconn_gethostbyname(syslog_espconn, host, &syslogHost.addr, syslog_gethostbyname_cb);
   }
 #ifdef SYSLOG_UDP_RECV
   DBG("syslog_init: host: %s, port: %d, lport: %d, recvcb: %p, sentcb: %p, state: %d\n",
-		  host, syslogHost.port, syslog_espconn.proto.udp->local_port,
+		  host, syslogHost.port, syslog_espconn->proto.udp->local_port,
 		  syslog_udp_recv_cb, syslog_udp_sent_cb, syslogState	);
 #else
   DBG("syslog_init: host: %s, port: %d, lport: %d, rsentcb: %p, state: %d\n",
-		  host, syslogHost.port, syslog_espconn.proto.udp->local_port,
+		  host, syslogHost.port, syslog_espconn->proto.udp->local_port,
 		  syslog_udp_sent_cb, syslogState	);
 #endif
 }
 
 /******************************************************************************
+ * FunctionName : syslog_add_entry
+ * Description  : add a syslog_entry_t to the syslogQueue
+ * Parameters   : entry: the syslog_entry_t
+ * Returns      : none
+ ******************************************************************************/
+static void ICACHE_FLASH_ATTR
+syslog_add_entry(syslog_entry_t *entry)
+{
+  syslog_entry_t *pse = syslogQueue;
+
+  DBG("syslog_add_entry: %p", syslogQueue);
+  // append msg to syslog_queue
+  if (pse == NULL)
+    syslogQueue = entry;
+  else {
+    while (pse->next != NULL)
+      pse = pse->next;
+    pse->next = entry;	// append msg to syslog queue
+  }
+  DBG("%p %lu %d\n", entry, entry->msgid, system_get_free_heap_size());
+
+  // ensure we have sufficient heap for the rest of the system
+  if (system_get_free_heap_size() < syslogHost.min_heap_size) {
+    if (syslogState != SYSLOG_HALTED) {
+      os_printf("syslog_add_entry: Warning: queue filled up, halted\n");
+      entry->next = syslog_compose(SYSLOG_FAC_USER, SYSLOG_PRIO_CRIT, "SYSLOG", "queue filled up, halted");
+      if (syslogState == SYSLOG_READY)
+        syslog_send_udp();
+      syslogState = SYSLOG_HALTED;
+    }
+  }
+}
+
+/******************************************************************************
+ * FunctionName : syslog_compose
+ * Description  : compose a syslog_entry_t from va_args
+ * Parameters   : va_args
+ * Returns      : the malloced syslog_entry_t
+ ******************************************************************************/
+LOCAL syslog_entry_t ICACHE_FLASH_ATTR *
+syslog_compose(uint8_t facility, uint8_t severity, const char *tag, const char *fmt, ...)
+{
+  syslog_entry_t *se = os_zalloc(sizeof (syslog_entry_t) + 1024);	// allow up to 1k datagram
+  char *p = se->datagram;
+  se->tick = WDEV_NOW();			// 0 ... 4294.967295s
+  se->msgid = syslog_msgid;
+
+  // The Priority value is calculated by first multiplying the Facility
+  // number by 8 and then adding the numerical value of the Severity.
+  p += os_sprintf(p, "<%d> ", facility * 8 + severity);
+
+  // strftime doesn't work as expected - or adds 8k overhead.
+  // so let's do poor man conversion - format is fixed anyway
+  if (flashConfig.syslog_showdate == 0)
+    p += os_sprintf(p, "- ");
+  else {
+    time_t now = NULL;
+    struct tm *tp = NULL;
+
+    // create timestamp: FULL-DATE "T" PARTIAL-TIME "Z": 'YYYY-mm-ddTHH:MM:SSZ '
+    // as long as realtime_stamp is 0 we use tick div 10⁶ as date
+    now = (realtime_stamp == 0) ? (se->tick / 1000000) : realtime_stamp;
+    tp = gmtime(&now);
+
+    p += os_sprintf(p, "%4d-%02d-%02dT%02d:%02d:%02dZ ",
+		    tp->tm_year + 1900, tp->tm_mon + 1, tp->tm_mday,
+		    tp->tm_hour, tp->tm_min, tp->tm_sec);
+  }
+
+  // add HOSTNAME APP-NAME PROCID MSGID
+  if (flashConfig.syslog_showtick)
+    p += os_sprintf(p, "%s %s %lu.%06lu %lu ", flashConfig.hostname, tag, se->tick / 1000000, se->tick % 1000000, syslog_msgid++);
+  else
+    p += os_sprintf(p, "%s %s - %lu ", flashConfig.hostname, tag, syslog_msgid++);
+
+  // append syslog message
+  va_list arglist;
+  va_start(arglist, fmt);
+  p += ets_vsprintf(p, fmt, arglist );
+  va_end(arglist);
+
+  se->datagram_len = p - se->datagram;
+  se = mem_trim(se, sizeof(syslog_entry_t) + se->datagram_len + 1);
+  return se;
+}
+
+ /*****************************************************************************
   * FunctionName : syslog
   * Description  : compose and queue a new syslog message
   * Parameters   : facility
@@ -374,13 +465,8 @@ void ICACHE_FLASH_ATTR syslog(uint8_t facility, uint8_t severity, const char *ta
   // and append it to the message queue
   syslog_add_entry(se);
 
-  if (syslogState == SYSLOG_READY) {
+  if (syslogState == SYSLOG_NONE)
-    syslogState = SYSLOG_SENDING;
-    syslog_send_udp();
-  }
-
-  if (syslogState == SYSLOG_NONE) {
     syslogState = SYSLOG_WAIT;
-    syslog_chk_wifi_stat();	// fire the timer to check the Wifi connection status
+
-  }
+  syslog_chk_status();
 }

syslog/syslog.h

@@ -20,9 +20,13 @@ extern "C" {
 
 enum syslog_state {
     SYSLOG_NONE,        // not initialized
-    SYSLOG_WAIT,        // initialized, waiting for Wifi
+    SYSLOG_WAIT,        // waiting for Wifi
+    SYSLOG_INIT,	// WIFI avail, must initialize
+    SYSLOG_DNSWAIT,	// WIFI avail, init done, waiting for DNS resolve
     SYSLOG_READY,       // Wifi established, ready to send
     SYSLOG_SENDING,     // UDP package on the air
+    SYSLOG_SEND,
+    SYSLOG_SENT,
     SYSLOG_HALTED,      // heap full, discard message
     SYSLOG_ERROR,
 };
@@ -78,6 +82,8 @@ struct syslog_host_t {
 typedef struct syslog_entry_t syslog_entry_t;
 struct syslog_entry_t {
     syslog_entry_t *next;
+    uint32_t	msgid;
+    uint32_t	tick;
     uint16_t	datagram_len;
     char	datagram[];
 };

syslog/syslog.md

@@ -0,0 +1,110 @@
+syslog
+======
+
+The lib implements a RFC5424 compliant syslog interface for ESP8266. The syslog
+message is send via UDP.
+
+syslog messages are queued on heap until the Wifi stack is fully initialized.
+If the remaining heap size reaches a given limit, syslog will add a final obituary
+and stop further logging until the queue is empty and sufficient heap space is
+available again.
+
+The module may be controlled by flashconfig variables:
+
+* **syslog_host: host[:port]**
+
+    **host** is an IP-address or DNS-name. **port** is optional and defaults to 514.
+DNS-Resolution is done as soon as the Wifi stack is up and running.
+
+* **syslog_minheap: 8192**
+
+    **minheap** specifies the minimum amount of remaining free heap when queuing up
+syslog messages. If the remaining heap size is below **minheap**, syslog will insert
+an obituary message and stop queuing. After processing all queued messages, the
+logging will be enabled again.
+
+* **syslog_filter: 0..7**
+
+    **syslog_filter** is the minimum severity for sending a syslog message. The filter
+is applied against the message queue, so any message with a severity numerical lower
+than **syslog_filter** will be dropped instead of being send.
+
+* **syslog_showtick: 0|1**
+
+    If **syslog_showtick** is set to **1**, syslog will insert an additional timestamp
+(system tick) as "PROCESS" field (before the users syslog message).
+The value shown is in ms, (1µs resolution) since (re)boot or timer overflow.
+
+* **syslog_showdate: 0|1**
+
+    If **syslog_showdate** is set to **1**, syslog will insert the ESPs NTP time
+into the syslog message. If "realtime_stamp" (NTP 1s ticker) is **NULL**, the
+time is derived from a pseudo-time based on the absolute value of systemticks.
+
+    Some syslog servers (e.g. Synology) will do crazy things if you set **syslog_showdate** to **1**
+
+
+The syslog module exports two functions:
+
+```
+syslog_init(char *server_name);
+syslog(uint8_t facility, uint8_t severity, const char *tag, const char *fmt, ...);
+```
+
+syslog_init
+-----------
+usage: `syslog_init(char *server_name);`
+
+**syslog_init** expects a server name in format "host:port" (see **syslog_host** flashconfig).
+
+If **server_name** is **NULL**, all dynamic allocated memory (buffers, queues, interfaces)
+are released and the syslog state is set to "SYSLOG_HALTED".
+
+If **server_name** is **""**, syslog state is set to "SYSLOG_HALTED", without clearing
+the queue.
+
+Otherwise, syslog_init will allocate all required structures (buffers, interfaces) and
+send all collected syslog messages.
+
+syslog is self-initializing, meaning the syslog_init(server_name) is called on first
+invocation. The syslog_init function is only for convenience if you have to stop or disable syslog functions.
+
+
+syslog
+------
+usage: `syslog(uint8_t facility, uint8_t severity, const char *tag, const char *fmt, ...);`
+
+* **facility**
+
+    the message facility (see syslog.h, **enum syslog_facility**).
+
+* **severity**
+
+    the message severity (see syslog.h, **enum syslog_severity**)
+
+* **tag**
+
+    user defined tag (e.g. "MQTT", "REST", "UART") to specify where the message belongs to
+
+* ** const char *fmt, ...**
+
+    the desired message, in printf format.
+
+Examples
+========
+
+    showtick=0, showdate=0
+    Invocation: syslog()
+output:
+
+    showtick=1, showdate=1
+    Invocation: syslog()
+output:
+
+    showtick=1, showdate=1, NTP not available
+    Invocation: syslog()
+output:
+
+    showtick=1, showdate=1, NTP available
+    Invocation: syslog()
+output: