Add CoAP server example doc. Closes cesanta/dev#4917

PUBLISHED_FROM=ee7728db61721c0628a00ed702a47f7382ad7cd7

docs/coap/server_example.md

@@ -2,4 +2,63 @@
 title: CoAP server example
 ---
 
-TBD
+To create a CoAP server, follow this pattern:
+1. Create a listening connection by calling `mg_bind()` or `mg_bind_opt()`
+2. 2. Call `mg_set_protocol_coap()` for that listening connection.
+3. Create an event handler function that handles the following events:
+- `MG_EV_COAP_CON`
+- `MG_EV_COAP_NOC`
+- `MG_EV_COAP_ACK`
+- `MG_EV_COAP_RST`
+
+Here's an example of the simplest CoAP server. Error checking is omitted for the sake of clarity:
+
+```c
+#include "mongoose.h"
+
+static char *s_default_address = "udp://:5683";
+
+static void coap_handler(struct mg_connection *nc, int ev, void *p) {
+  switch (ev) {
+    case MG_EV_COAP_CON: {
+      uint32_t res;
+      struct mg_coap_message *cm = (struct mg_coap_message *) p;
+      printf("CON with msg_id = %d received\n", cm->msg_id);
+      res = mg_coap_send_ack(nc, cm->msg_id);
+      if (res == 0) {
+        printf("Successfully sent ACK for message with msg_id = %d\n",
+               cm->msg_id);
+      } else {
+        printf("Error: %d\n", res);
+      }
+      break;
+    }
+    case MG_EV_COAP_NOC:
+    case MG_EV_COAP_ACK:
+    case MG_EV_COAP_RST: {
+      struct mg_coap_message *cm = (struct mg_coap_message *) p;
+      printf("ACK/RST/NOC with msg_id = %d received\n", cm->msg_id);
+      break;
+    }
+  }
+}
+
+int main(void) {
+  struct mg_mgr mgr;
+  struct mg_connection *nc;
+
+  mg_mgr_init(&mgr, 0);
+
+  nc = mg_bind(&mgr, s_default_address, coap_handler);
+  mg_set_protocol_coap(nc);
+
+  while (1) {
+    mg_mgr_poll(&mgr, 1000);
+  }
+
+  mg_mgr_free(&mgr);
+  return 0;
+}
+```
+
+See full source code at [CoAP server example](https://github.com/cesanta/mongoose/tree/master/examples/coap_server).