DESCRIPTION

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

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 a packet containing the status and any data.

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

NOTE: If a server is providing Proxy support, then the server code should return from the request handler with no response code set (i.e. respond with empty ACK) and then send back the response as provided by the upstream server when received, so no need to use the async support.

FUNCTIONS

Function: coap_register_async()

The coap_register_async() function is used to set up an asynchronous delayed request for the request PDU associated with the 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 until coap_async_trigger() or coap_async_set_delay() is called.

Function: coap_async_trigger()

The coap_async_trigger() function is used to expire the delay for the async definition, so the application request handler is almost immediately called.

Function: coap_async_set_delay()

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. Alternatively, coap_async_trigger() can be called.

Function: coap_free_async()

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

Function: coap_find_async()

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

Function: coap_async_set_app_data2()

The coap_async_set_app_data2() function is used to define a app_data pointer for the async which can then be retrieved at a later date. There is an additional callback app_cb (if set) to be used if the data is to be released when the async is deleted. If this is a subsequent call for the async, then the existing data is returned, and it is the responsibility of the caller to release this previous data. On the first call, NULL is returned.

The app_cb handler function prototype is defined as:

/**
 * Callback to free off the app data when the cache-entry is
 * being deleted / freed off.
 *
 * @param data  The app data to be freed off.
 */
typedef void (*coap_app_data_free_callback_t)(void *data);

Function: coap_async_get_app_data()

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

RETURN VALUES

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_async_set_app_data2() returns a previously defined pointer or NULL.

EXAMPLES

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_session_t *session,
                   coap_resource_t *resource,
                   coap_pdu_t *request,
                   coap_string_t *query,
                   coap_pdu_t *response) {
  unsigned long delay = 5;
  size_t size;
  coap_async_t *async;
  coap_bin_const_t token = coap_pdu_get_token(request);

  /*
   * See if this is the initial, or delayed request
   */

  async = coap_find_async(session, 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_info("async: delay of 0 not supported\n");
        coap_pdu_set_code(response, COAP_RESPONSE_CODE_BAD_REQUEST);
        return;
      }
    }
    async = coap_register_async(session,
                                request,
                                COAP_TICKS_PER_SECOND * delay);
    if (async == NULL) {
      coap_pdu_set_code(response, COAP_RESPONSE_CODE_SERVICE_UNAVAILABLE);
      return;
    }
    /*
     * Not setting response code will cause empty ACK to be sent
     * if Confirmable
     */
    return;
  }
  /* async is set up, so this is the delayed request */

  /* remove any stored app data associated with 'async' here */

  /* Send back the appropriate data */
  coap_add_data_large_response(resource, session, request, response,
                               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 */
}

SEE ALSO

coap_handler(3) and coap_supported(3)

FURTHER INFORMATION

See

"RFC7252: The Constrained Application Protocol (CoAP)"

for further information.

BUGS

Please raise an issue on GitHub at https://github.com/obgm/libcoap/issues to report any bugs.

Please raise a Pull Request at https://github.com/obgm/libcoap/pulls for any fixes.

AUTHORS

The libcoap project <libcoap-developers@lists.sourceforge.net>