Using libcurl for a POST request - c++

Question

I'll preface this by saying I'm still a new C/C++ programmer, so please excuse me for what may be a redundant question.
I'm writing a program in C/C++ to interact with this website: http://www.youtube-mp3.org/.
From what I understand, to get my program to download a link for me I'll have to send a POST request to the server containing the URL I want to convert, then find a way of getting it to follow the URL that is generated allowing me to download the file. I also understand that libcurl is a good way of doing this sort of thing in C/C++.
I've tried using the POST examples on the libcurl website (http://curl.haxx.se/libcurl/c/simplepost.html and one other) but neither seems to work. In addition, I'm not sure how to then get my program to follow the link that appears saying 'Download' . I've tried sending a POST request, then telling my program to get the html source of the page and store this in a file, but that file doesn't seem to contain any download link. When this is done through a browser, the page source definitely includes a working download link.
Would really appreciate some help, as I'm not sure whether I've got completely the wrong idea!
EDIT: My question wasn't very clear at all. Here is the relevant code I'm using for the POST request:
static const char *postthis="http://www.youtube.com/watch?v=KMU0tzLwhbE";
CURL *curl;
CURLcode res;
curl = curl_easy_init();
if(curl) {
curl_easy_setopt(curl, CURLOPT_URL, "http://www.youtube-mp3.org/");
curl_easy_setopt(curl, CURLOPT_POSTFIELDS, postthis);
curl_easy_setopt(curl, CURLOPT_POSTFIELDSIZE, (long)strlen(postthis));
/* Perform the request, res will get the return code */
res = curl_easy_perform(curl);
/* Check for errors */
if(res != CURLE_OK)
fprintf(stderr, "curl_easy_perform() failed: %s\n",
curl_easy_strerror(res));
}
And for writing the html source to file:
static size_t write_data(void *ptr, size_t size, size_t nmemb, void *stream)
{
int written = fwrite(ptr, size, nmemb, (FILE *)stream);
return written;
}
{
static const char *filename = "head.txt";
FILE *htmlfile;
curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_data);
// open the file
htmlfile = fopen(filename,"w");
if (htmlfile == NULL) {
curl_easy_cleanup(curl);
return -1;
}
curl_easy_setopt(curl,CURLOPT_WRITEDATA, htmlfile);
curl_easy_perform(curl);
/* close the header file */
fclose(htmlfile);
/* always clean up */
curl_easy_cleanup(curl);
}

Answer 1

Your code does not work because you are assuming the wrong logic to begin with.
http://www.youtube-mp3.org does NOT use POST, in fact its download form doesn't even submit to a server-side URL at all. When you click on the "Convert Video" button, a client-side JavaScript is invoked to process the input URL, download the relevant information from YouTube, and modify the calling page's HTML to display the actual download link and video preview image. This is why you don't see the download link when you simply retrieve the HTML - you are not invoking the JavaScript that performs the actual work of preparing the download link. And you will not be able to do that from an application (without a LOT of extra work), it has to be done inside of a web browser that has a real JavaScript engine and a real DOM for the script to manipulate.