nanocoap: add coap_build_empty_ack()

sys/include/net/nanocoap.h

@@ -1956,6 +1956,22 @@ ssize_t coap_build_hdr(coap_hdr_t *hdr, unsigned type, const void *token,
 ssize_t coap_build_reply(coap_pkt_t *pkt, unsigned code,
                          uint8_t *rbuf, unsigned rlen, unsigned payload_len);
 
+/**
+ * @brief   Build empty reply to CoAP request
+ *
+ * This function can be used to create an empty ACK so that a later, separate
+ * response can be sent independently.
+ *
+ * If the request was non-confirmable, this will generate nothing.
+ *
+ * @param[in]   pkt         packet to reply to
+ * @param[out]  ack         buffer to write reply to
+ *
+ * @returns     size of reply packet on success
+ * @returns     -ENOSPC if @p rbuf too small
+ */
+ssize_t coap_build_empty_ack(coap_pkt_t *pkt, coap_hdr_t *ack);
+
 /**
  * @brief   Handle incoming CoAP request
  *

sys/net/application_layer/nanocoap/nanocoap.c

@@ -626,6 +626,18 @@ ssize_t coap_reply_simple(coap_pkt_t *pkt,
     return header_len + payload_len;
 }
 
+ssize_t coap_build_empty_ack(coap_pkt_t *pkt, coap_hdr_t *ack)
+{
+    if (coap_get_type(pkt) != COAP_TYPE_CON) {
+        return 0;
+    }
+
+    coap_build_hdr(ack, COAP_TYPE_ACK, NULL, 0,
+                   COAP_CODE_EMPTY, ntohs(pkt->hdr->id));
+
+    return sizeof(*ack);
+}
+
 ssize_t coap_build_reply(coap_pkt_t *pkt, unsigned code,
                          uint8_t *rbuf, unsigned rlen, unsigned payload_len)
 {