The libcoap functions for creating a server and a client require a few more steps as part of formatting and retrieving the URI and the URI query information into/from a CoAP request.
The complete sample applications, client.c and server.c, are in my GitHub repository at https://github.com/RichardChambers/raspberrypi/tree/master/coap .
On the server side the message handler will use an option iterator, a coap_opt_iterator_t struct object, to iterate over the options of the coap_pdu_t struct request object. While iterating, the server will be looking for options of type COAP_OPTION_URI_QUERY which will contain a query argument, usually a keyword equal value pair.
So the function hello_handler() of the posted question could be rewritten with a helper function to obtain the list of URI query items as follows:
/*
* The following helper function, printQueryOption(), is used to
* iterate over the list of options looking for query type options
* to the base URI. This is analogous to a web URL that contains
* a question mark followed by options such as:
* http://www.server.com/hello?type=1,id=2345
*
* We will return a value of 1 (true) if we found something or a value
* of 0 (false) if we do not.
*/
static int
printQueryOption (coap_pdu_t *request, coap_opt_iterator_t * popt_iter )
{
int iRet = 0;
coap_opt_t *option;
// iterate over the options looking for queries. If we find one
// then print it and return a value of 1, true, to indicate we
// found one. If we don't find any then return the default value
// of 0, false.
while (option = coap_option_next(popt_iter)) {
// found an option, is it a query option or not.
if (popt_iter->type != COAP_OPTION_URI_QUERY) continue;
// it is a query option so print out the query text.
char xBuff[128] = {0};
strncpy (xBuff, COAP_OPT_VALUE(option), COAP_OPT_LENGTH(option));
printf (" option len %d\n %s \n", COAP_OPT_LENGTH(option), xBuff);
// indicate that we found a query option.
iRet = 1;
break;
}
return iRet;
}
/*
* The resource handler which the libcoap library will invoke when the registered
* URI is specified. This is a simple request handler which will just display some
* of the information from the request.
*/
static void
hello_handler(coap_context_t *ctx, struct coap_resource_t *resource,
const coap_endpoint_t *local_interface, coap_address_t *peer,
coap_pdu_t *request, str *token, coap_pdu_t *response)
{
static int iCount = 0; // a simple count to provide some kind of response data.
unsigned char buf[3];
const char* response_data = "{\"device\": \"DEV-01-123\", \"item\" : %d }";
char response_buf[256] = {0};
// generate a response to this request. we have a hard coded JSON text that
// we are using as a stub for testing.
response->hdr->code = COAP_RESPONSE_CODE(205);
coap_add_option(response, COAP_OPTION_CONTENT_TYPE, coap_encode_var_bytes(buf, COAP_MEDIATYPE_TEXT_PLAIN), buf);
sprintf (response_buf, response_data, iCount);
coap_add_data (response, strlen(response_buf), (unsigned char *)response_buf);
iCount++; // this count is some type of varying data so that we can see things are working.
if (request != NULL) {
// there is a request URI so lets print out the base URI and then
// iterate over the options looking for the Query type options.
coap_opt_iterator_t opt_iter;
printf (" request %s\n", resource->uri.s);
coap_option_iterator_init (request, &opt_iter, COAP_OPT_ALL);
// iterate over the options of the request printing out any
// query text that may exist.
while (printQueryOption (request, &opt_iter));
} else {
printf (" request - NONE\n");
}
}
On the client side we would build our request adding the queries we want to include in the request with a series of calls of function coap_add_option() as in:
coap_split_uri(server_uri, strlen(server_uri), &uri);
request = coap_new_pdu();
request->hdr->type = COAP_MESSAGE_CON;
request->hdr->id = coap_new_message_id(ctx);
request->hdr->code = get_method;
printf (" Request URI: path %d %s\n", uri.path.length, uri.path.s);
coap_add_option(request, COAP_OPTION_URI_PATH, uri.path.length, uri.path.s);
sprintf (server_query, "item=%d", iItem);
coap_add_option(request, COAP_OPTION_URI_QUERY, strlen(server_query), server_query);
printf (" Query: len %d %s\n", strlen(server_query), server_query);
sprintf (server_query, "device=%s", aszDevName);
coap_add_option(request, COAP_OPTION_URI_QUERY, strlen(server_query), server_query);
printf (" Query: len %d %s\n", strlen(server_query), server_query);
sprintf (server_query, "tempo=%s", aszTempoName);
coap_add_option(request, COAP_OPTION_URI_QUERY, strlen(server_query), server_query);
printf (" Query: len %d %s\n", strlen(server_query), server_query);
These statements create a CoAP request to a particular URI with a set of arguments or query options so that the server's handler of the URI can provide a specific response back to the request. For this example the variables used are hard coded with values like the following:
char *aszDevName = "DEV-01-203";
char *aszTempoName = "TEMPO-12345";
int iItem = 5;
const char* server_uri = "coap://127.0.0.1/hello";
The two sample programs can be run in two separate terminal windows, starting the server first. Trying the client three times, the first with a URI of "hello", the second with a URI of "goodbye", and the third time again with a URI of "hello" we see the following output.
From the server:
pi@raspberrypi:~/Documents/raspberrypi/coap $ ./server
request hello
option len 6
item=5
option len 17
device=DEV-01-203
option len 17
tempo=TEMPO-12345
request hello
option len 6
item=5
option len 17
device=DEV-01-203
option len 17
tempo=TEMPO-12345
From the terminal window of the client:
pi@raspberrypi:~/Documents/raspberrypi/coap $ ./client
Request URI: path 5 hello
Query: len 6 item=5
Query: len 17 device=DEV-01-203
Query: len 17 tempo=TEMPO-12345
Received: {"device": "DEV-01-123", "item" : 0 } -> item = 0
pi@raspberrypi:~/Documents/raspberrypi/coap $ ./client goodbye
Request URI: path 7 goodbye
Query: len 6 item=5
Query: len 17 device=DEV-01-203
Query: len 17 tempo=TEMPO-12345
COAP_RESPONSE_CLASS() unknown.
pi@raspberrypi:~/Documents/raspberrypi/coap $ ./client
Request URI: path 5 hello
Query: len 6 item=5
Query: len 17 device=DEV-01-203
Query: len 17 tempo=TEMPO-12345
Received: {"device": "DEV-01-123", "item" : 1 } -> item = 1
pi@raspberrypi:~/Documents/raspberrypi/coap $
