cgtklibcurlcjson

How to use GTK and libcurl at the same time in C?

I'm trying to develop an application in C with GTK for the GUI, libcurl to get data by a web API and cJSON to parse JSON.

My problem is that when I do my request with libcurl after gtk_init, the data that I get can not be parsing in JSON. Otherwise, if I get the data and parse it before gtk_init, the parsing works fine.

I have an example to demonstrate it, the first printf in the main returns the JSON correctly but the second printf which is after gtk_init returns NULL (to be exact, the parsing stops at the first decimal number and fail) :

initString : 

void initString(String * s) {
  s->len = 0;
  s->ptr = malloc(s->len + 1);
  if (s->ptr == NULL) {
    fprintf(stderr, "malloc() failed\n");
    exit(EXIT_FAILURE);
  }
  s->ptr[0] = '\0';
}

writeFunc : 

size_t writeFunc(void *ptr, size_t size, size_t nmemb, String * s)
{
  size_t newLen = s->len + size*nmemb;
  s->ptr = realloc(s->ptr, newLen + 1);
  if (s->ptr == NULL) {
    fprintf(stderr, "realloc() failed\n");
    exit(EXIT_FAILURE);
  }
  memcpy(s->ptr + s->len, ptr, size * nmemb);
  s->ptr[newLen] = '\0';
  s->len = newLen;

  return size * nmemb;
}

My function to get data with libcurl : 

char * getData(gpointer user_data)
{
curl_global_init(CURL_GLOBAL_ALL);
CURL * curl;
char * url = user_data;
CURLcode res;
String s;
struct curl_slist * headers = NULL;
curl = curl_easy_init();

if(curl)
{
    initString(&s);

    headers = curl_slist_append(headers, "Accept: application/json");
    headers = curl_slist_append(headers, "Content-Type: application/json");
    headers = curl_slist_append(headers, "charsets: utf-8");

    //curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers);
    curl_easy_setopt(curl, CURLOPT_FOLLOWLOCATION, 1L);
    curl_easy_setopt(curl, CURLOPT_URL, url);

    curl_easy_setopt(curl, CURLOPT_SSL_VERIFYPEER, FALSE);
    curl_easy_setopt(curl, CURLOPT_SSL_VERIFYHOST, FALSE);
    curl_easy_setopt(curl, CURLOPT_SSLVERSION, 6);

    curl_easy_setopt(curl, CURLOPT_USERAGENT, "libcurl-agent/1.0");
    curl_easy_setopt(curl, CURLOPT_VERBOSE, 1L);
    //write data in a string
    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, writeFunc);
    curl_easy_setopt(curl, CURLOPT_WRITEDATA, &s);

    res = curl_easy_perform(curl);
}

if(res != CURLE_OK)
  fprintf(stderr, "curl_easy_perform() failed: %s\n", curl_easy_strerror(res));

curl_easy_cleanup(curl);
return s.ptr;
}

The main : 

int main(int argc, char ** argv)
{
char * str;
cJSON * json;

str = getData("https://data.culture.gouv.fr/api/records/1.0/search/?dataset=liste-et-localisation-des-musees-de-france&facet=ville&sort=ville&facet=nomdep&refine.nomdep=AIN&rows=1");
json = cJSON_Parse(str);
printf("DATA : %s\n", cJSON_Print(json));//returns the JSON perfectly

gtk_init(&argc,&argv);

str = getData("https://data.culture.gouv.fr/api/records/1.0/search/?dataset=liste-et-localisation-des-musees-de-france&facet=ville&sort=ville&facet=nomdep&refine.nomdep=AIN&rows=1");
json = cJSON_Parse(str);
printf("ERROR : %s\n", cJSON_GetErrorPtr());//returns half of data
printf("DATA : %s\n", cJSON_Print(json));//returns NULL

gtk_main();

curl_global_cleanup();

return EXIT_SUCCESS;
}

I have tried to resolve this by creating threads with g_idle_add, gdk_threads_idle_add, gdk_threads_entry and gdk_threads_leave, pthread_create and pthread_join but nothing worked.

Does someone know how to resolve this problem ?

Thanks.

Solution

  • You need to narrow down your problem. In other words, you need "M" in MVCE. Currently you have three libraries:

    that might interact in arbitrary ways. What you need to check:

    My guess is as follows. gtk_init does several things, amongst them sets locale. Since you fetch data from .fr domain, I assume that you have French locale set on your computer.

    to be exact, the parsing stops at the first decimal number and fail

    In French, decimal separator is comma , instead of period ., so I suppose that after GTK+ changes locale, cJSON starts to look for , in decimal numbers but it finds . and fails.

    Possible solutions