Add CoAP client example doc

PUBLISHED_FROM=51fd5cd50ae8c9fce43d95d06fec2e7582018d00

docs/coap/client_example.md

@@ -2,4 +2,64 @@
 title: CoAP client example
 ---
 
-TBD
+To create a CoAP client, follow this pattern:
+
+1. Create an outbound connection by calling `mg_connect`
+2. Call `mg_set_protocol_coap` for created 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 client.
+Error checking is omitted for the sake of clarity:
+
+```c
+#include "mongoose.h"
+
+static int s_time_to_exit = 0;
+static char *s_default_address = "udp://coap.me:5683";
+
+static void coap_handler(struct mg_connection *nc, int ev, void *p) {
+  switch (ev) {
+    case MG_EV_CONNECT: {
+      struct mg_coap_message cm;
+
+      memset(&cm, 0, sizeof(cm));
+      cm.msg_id = 1;
+      cm.msg_type = MG_COAP_MSG_CON;
+      mg_coap_send_message(nc, &cm);
+      break;
+    }
+    case MG_EV_COAP_ACK:
+    case MG_EV_COAP_RST: {
+      struct mg_coap_message *cm = (struct mg_coap_message *) p;
+      printf("ACK/RST for message with msg_id = %d received\n", cm->msg_id);
+      s_time_to_exit = 1;
+      break;
+    }
+  }
+}
+
+int main(int argc, char *argv[]) {
+  struct mg_mgr mgr;
+  struct mg_connection *nc;
+
+  mg_mgr_init(&mgr, 0);
+
+  nc = mg_connect(&mgr, s_default_address, coap_handler);
+
+  mg_set_protocol_coap(nc);
+
+  while (!s_time_to_exit) {
+    mg_mgr_poll(&mgr, 1000000);
+  }
+
+  mg_mgr_free(&mgr);
+
+  return 0;
+}
+```
+
+See full source code at [CoAP client example](https://github.com/cesanta/mongoose/tree/master/examples/coap_client).