libcoap  4.3.0rc2


coap_async, coap_register_async, coap_async_set_delay, coap_find_async, coap_free_async, coap_async_set_app_data, coap_async_get_app_data — Work with CoAP async support


#include <coap3/coap.h>

coap_async_t *coap_register_async(coap_context_t *context, coap_session_t *session, coap_pdu_t *request, coap_tick_t delay);

void coap_async_set_delay(coap_async_t *async, coap_tick_t delay);

void coap_free_async(coap_context_t *context, coap_async_t *async);

coap_async_t *coap_find_async(coap_context_t *context, coap_session_t *session, coap_binary_t token);

void coap_async_set_app_data(coap_async_t *async, void *app_data);

void *coap_async_get_app_data(const coap_async_t *async);

For specific (D)TLS library support, link with -lcoap-3-notls, -lcoap-3-gnutls, -lcoap-3-openssl, -lcoap-3-mbedtls or -lcoap-3-tinydtls. Otherwise, link with -lcoap-3 to get the default (D)TLS library support.


CoAP server responses can be piggybacked (RFC7252 5.2.1) or separate (RFC7252 5.2.2).

For piggybacked responses, the response packet contains both the status and any data.

For separate responses, there is an initial empty ACK response (Confirmable only - to stop the client re-transmitting the request) followed at a later time by the packet containing the status and any data.

Usually responses are piggybacked, but this man page focuses on separate (async) support.

The coap_register_async() function is used to set up an asynchronous delayed request for the request PDU associated with the context and session. The application request handler will get called with a copy of request after delay ticks which will then cause a response to be sent. If delay is 0, then the application request handler will not get called.

The coap_async_set_delay() function is used to update the remaining delay before the application request handler is called for the async definition. If delay is set to 0, then the application request handler will not get called.

An example of usage here is coap_register_async() sets delay to 0, and then when the response is ready at an indeterminate point in the future, coap_async_set_delay() is called setting delay to 1.

The coap_free_async() function is used to delete an async definition.

The coap_find_async() function is used to determine if there is an async definition based on the context, session and token token.

The coap_async_set_app_data() function is used to add in user defined app_data to the async definition. It is the responsibility of the application to release this data if appropriate.

The coap_async_get_app_data() function is used to retrieve any defines application data from the async definition.


coap_register_async() and coap_find_async() return a pointer to an async definition or NULL if there is an error.

coap_async_get_app_data() returns a pointer to the user defined data.


CoAP Server Non-Encrypted Setup

#include <coap3/coap.h>

 * This example is used to demonstrate how to set up and use a "separate"
 * response (empty ACK followed by data response at a later stage).
static void
hnd_get_with_delay(coap_context_t *context,
              coap_resource_t *resource,
              coap_session_t *session,
              coap_pdu_t *request,
              coap_binary_t *token,
              coap_string_t *query,
              coap_pdu_t *response) {
  unsigned long delay = 5;
  size_t size;
  coap_async_t *async;
  coap_binary_t zero_token = {0, NULL};

   * See if this is the initial, or delayed request
  async = coap_find_async(context, session, token ? *token : zero_token);
  if (!async) {
    /* Set up an async request to trigger delay in the future */
    if (query) {
      const uint8_t *p = query->s;

      delay = 0;
      for (size = query->length; size; --size, ++p)
        delay = delay * 10 + (*p - '0');
      if (delay == 0) {
        coap_log(LOG_INFO, "async: delay of 0 not supported\n");
        coap_pdu_set_code(response, COAP_RESPONSE_CODE_BAD_REQUEST);
    async = coap_register_async(context,
                                COAP_TICKS_PER_SECOND * delay);
    if (async == NULL) {
      coap_pdu_set_code(response, COAP_RESPONSE_CODE_SERVICE_UNAVAILABLE);
     * Not setting response code will cause empty ACK to be sent
     * if Confirmable
  /* async set up, so this is the delayed request */

  /* Send back the appropriate data */
  coap_add_data_large_response(resource, session, request, response, token,
                               query, COAP_MEDIATYPE_TEXT_PLAIN, -1, 0, 4,
                               (const uint8_t *)"done", NULL, NULL);
  coap_pdu_set_code(response, COAP_RESPONSE_CODE_CONTENT);

  /* async is automatically removed by libcoap on return from this handler */



"RFC7252: The Constrained Application Protocol (CoAP)"

for further information.


Please report bugs on the mailing list for libcoap: or raise an issue on GitHub at


The libcoap project <>