c libcurl CURLOPT_POSTFIELDS strange error - c

Question

i'm a quite new c developer and I have a little trouble with libcurl. I searched a lot in the net before but i didn't managed to find an answer working for me.
This is pure c, no ++.
I have this code and the request is failing
char * write_url(MyData data)
{
char *field = malloc(20 * sizeof(char));
if (strcmp(data.sensorName,"Temperature")==0){
sprintf(field,"&temperature=%.2f",data.measure);
}
CURL *curl;
curl_global_init(CURL_GLOBAL_ALL);
curl = curl_easy_init();
curl_easy_setopt(curl, CURLOPT_VERBOSE, 1);
curl_easy_setopt(curl, CURLOPT_URL, "http://127.0.0.1/");
curl_easy_setopt(curl, CURLOPT_POST, 1);
curl_easy_setopt(curl, CURLOPT_POSTFIELDS, field);
curl_easy_perform(curl);
curl_easy_cleanup(curl);
}
But if I change
curl_easy_setopt(curl, CURLOPT_POSTFIELDS, field);
for
char *data="&temperature=20";
curl_easy_setopt(curl, CURLOPT_POSTFIELDS, data);
It's working.
But I want to pass the data through that funcion!
Thanks!

Answer 1

malloc() will not initialize the memory, therefore you have to do it. But if the if statement isn't taken the memory remains uninitialized. Do this in the else part of the code:
if (strcmp(data.sensorName,"Temperature")==0)
{
sprintf(field,"&temperature=%.2f",data.measure);
}
else
{
strcmp(field,"Unknown") ;
}
And free the memory when you are done:
free(field) ;
Also note that the libcurl calls always return an error value, use it.
curl = curl_easy_init(); //no error checking is perform in your code
curl_easy_setopt(curl, CURLOPT_VERBOSE, 1);
curl_easy_setopt(curl, CURLOPT_URL, "http://127.0.0.1/");
curl_easy_setopt(curl, CURLOPT_POST, 1);