digest auth with libcurl

[Shimmy]

Hello, everyone!I want to connect to my camera using libcurl and RTSP and I've wrote simple function to do it. In case of using basic auth it works fine but I don't know how can I auth in case of digest method. Here is my code:

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

static int
rtsp_describe(CURL *curl, const char *uri, const char *user, const char *pwd)
{     
    char *userpwd = calloc(0, strlen(user) + strlen(pwd) + 2);     
    if (userpwd == NULL) {
        fprintf(stderr, "Error: memory allocation failed!\n");
        return -1;     
    }     
    strcat(userpwd, user);     
    strcat(userpwd, ":");     
    strcat(userpwd, pwd);     
    fprintf(stdout, "RTSP: DESCRIBE %s\n", uri);   
    curl_easy_setopt(curl, CURLOPT_RTSP_STREAM_URI, uri);
    curl_easy_setopt(curl, CURLOPT_RTSP_REQUEST, CURL_RTSPREQ_DESCRIBE);     
    curl_easy_setopt(curl, CURLOPT_HTTPAUTH, CURLAUTH_DIGEST);
    curl_easy_setopt(curl, CURLOPT_USERPWD, userpwd);
    curl_easy_setopt(curl, CURLOPT_WRITEDATA, stdout);
    curl_easy_perform(curl);
    curl_easy_reset(curl);

    free(userpwd);
    return 0;
}

int
main(int argc, char **argv)
{
    if (argc != 4) {
        fprintf(stderr, "Usage: %s addr username password\n", argv[0]);
        exit(EXIT_FAILURE);
    }
    CURLcode ret = CURLE_OK;
    const char *url = argv[1];
    char *uri = malloc(strlen(url) + 32);
    if (uri == NULL) {
        fprintf(stderr, "Error: memory allocation failed!\n");
        exit(EXIT_FAILURE);
    }

    ret = curl_global_init(CURL_GLOBAL_ALL);
    if (ret == CURLE_OK) {
        CURL *curl = curl_easy_init();
        if (curl != NULL) {
            curl_easy_setopt(curl, CURLOPT_VERBOSE, 0L);
            curl_easy_setopt(curl, CURLOPT_NOPROGRESS, 1L);
            curl_easy_setopt(curl, CURLOPT_HEADERDATA, stdout);
            curl_easy_setopt(curl, CURLOPT_URL, url);
            snprintf(uri, strlen(url) + 32, "%s", url);
            rtsp_describe(curl, uri, argv[2], argv[3]);

            curl_easy_cleanup(curl);
        }
    }
    free(uri);
    return 0;
}

If I delete line with CURLAUTH_DIGEST it work fine using basic auth. So, the question is, how can I login using digest auth? How can I get realm and nonce from camera in my code? Sorry, if this question is stupid, but I really stuck on it, and after my long googling I couldn't find anything useful.