Listen to a push message?

Question

I have to listen to a web service’s push message indefinitely. The web service I am listening to send soap response messages whenever there is an update in the content. After I received the message I have to parse it and store it in to structs. Following is my code.

CURL *curl;
CURLcode res;
const char *onlineWebServiceRequest = "<?xml version=\"1.0\" encoding=\"utf-8\"?> <soap:Envelope xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\" xmlns:xsd=\"http://www.w3.org/2001/XMLSchema\" xmlns:soap=\"http://schemas.xmlsoap.org/soap/envelope/\">  <soap:Body>    <GetCitiesByCountry xmlns=\"http://www.webserviceX.NET\">  <CountryName>Netherlands</CountryName>     </GetCitiesByCountry>    </soap:Body>    </soap:Envelope>";

if(curl){

    curl_easy_setopt(curl, CURLOPT_URL, "http://www.webservicex.net/globalweather.asmx/GetCitiesByCountry" );

    curl_easy_setopt(curl, CURLOPT_POSTFIELDS, onlineWebServiceRequest);  

    res = curl_easy_perform(curl);       
    // call some other method to parse the res
    curl_easy_cleanup(curl);    
} 

Doubts: Is it the right way to receive push messages? If yes.,

The above code does not check the connection status with web service. How can I check that?

If no, what are all the other option I have probably with open source?

Thanks in advance

Answer 1

You'd better off checking for errors.

To do that you have to enable CURLOPT_ERRORBUFFER option.

curl_easy_setopt(curl, CURLOPT_ERRORBUFFER, errbuf);

And add a buffer (it must be at least CURL_ERROR_SIZE bytes big).

char errbuf[CURL_ERROR_SIZE];

Additionally, you could set CURLOPT_FAILONERROR to true, to force curl to convert all response codes >= 300 to errors.

curl_easy_setopt(curl, CURLOPT_FAILONERROR, 1L);

To get the actual output from the server you need to set a function to write the data (CURLOPT_WRITEFUNCTION) and a chunk of memory (CURLOPT_WRITEDATA).

A complete example adapted from [1] and [2]:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <curl/curl.h>

struct MemoryStruct {
  char *memory;
  size_t size;
};

static size_t
WriteMemoryCallback(void *contents, size_t size, size_t nmemb, void *userp)
{
  size_t realsize = size * nmemb;
  struct MemoryStruct *mem = (struct MemoryStruct *)userp;

  mem->memory = (char *)realloc(mem->memory, mem->size + realsize + 1);
  if(mem->memory == NULL) {
    /* out of memory! */ 
    printf("not enough memory (realloc returned NULL)\n");
    return 0;
  }

  memcpy(&(mem->memory[mem->size]), contents, realsize);
  mem->size += realsize;
  mem->memory[mem->size] = 0;

  return realsize;
}

int main(void)
{
  curl_global_init(CURL_GLOBAL_ALL);

  /* init the curl session */ 
  CURL curl = curl_easy_init();
  if(curl) {
    CURLcode res;

    struct MemoryStruct chunk;

    chunk.memory = (char *)malloc(1);  /* will be grown as needed by the realloc above */ 
    chunk.size = 0;    /* no data at this point */ 

    char errbuf[CURL_ERROR_SIZE];

    /* specify URL to get */ 
    curl_easy_setopt(curl, CURLOPT_URL, "http://example.com");

    /* send all data to this function  */ 
    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, WriteMemoryCallback);

    /* we pass our 'chunk' struct to the callback function */ 
    curl_easy_setopt(curl, CURLOPT_WRITEDATA, (void *)&chunk);

    /* force curl to fail error when http code >= 300 */
    curl_easy_setopt(curl, CURLOPT_FAILONERROR, 1L);

    /* provide a buffer to store errors in */
    curl_easy_setopt(curl, CURLOPT_ERRORBUFFER, errbuf);

    /* set the error buffer as empty before performing a request */
    errbuf[0] = 0;

    /* perform the request */
    res = curl_easy_perform(curl);

    /* if the request did not complete correctly, show the error
    information. if no detailed error information was written to errbuf
    show the more generic information from curl_easy_strerror instead.
    */
    if(res != CURLE_OK) {
      size_t len = strlen(errbuf);
      fprintf(stderr, "\nlibcurl: (%d) ", res);
      if(len)
        fprintf(stderr, "%s%s", errbuf,
                ((errbuf[len - 1] != '\n') ? "\n" : ""));
      else
        fprintf(stderr, "%s\n", curl_easy_strerror(res));
    }
    else {
      /*
       * Now, our chunk.memory points to a memory block that is chunk.size
       * bytes big and contains the remote file.
       *
       * Do something nice with it!
       */ 

      printf("%lu bytes retrieved\n", (long)chunk.size);
    }

    /* cleanup curl stuff */ 
    curl_easy_cleanup(curl);

    free(chunk.memory);

  }

  /* we're done with libcurl, so clean it up */ 
  curl_global_cleanup();

  return 0;
}