typedef struct curlbuffer {
    char *buff;
    size_t size;
} curlbuffer;

bool loginworks() {
    curlbuffer authtest = curlrequest("https://domain.com/some_url_that_returns_json");
    if(strncmp(authtest.buff, "{\"ping\":\"success\"}", 19) == 0) {
        return true;
    }
    return false;
}

curlbuffer curlrequest(char *url) {
    CURL *curl;
    CURLcode res;

    /* curl delivers 16KiB of data at once, so this buffer should hold the first request */
    curlbuffer curldata;
    curldata.buff = malloc(CURL_MAX_WRITE_SIZE);
    memset(curldata.buff, '\0', CURL_MAX_WRITE_SIZE);
    curldata.size = CURL_MAX_WRITE_SIZE;

    <I do the request and some stuff>

    curl_global_cleanup();
    return curldata;
}