Is it possible to use libcurl without file descriptor based socket? - c

Question

I wrote the following libcurl code with my custom write and read socket functions:
//...
CURL *curl;
FILE *fp;
CURLcode res;
char outfilename[FILENAME_MAX] = "file";
curl = curl_easy_init();
/* no progress meter please */
//curl_easy_setopt(curl, CURLOPT_NOPROGRESS, 1L);
curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_data);
curl_easy_setopt(curl, CURLOPT_WRITEDATA, &sharedValue);
curl_easy_setopt(curl, CURLOPT_READFUNCTION, read_data);
curl_easy_setopt(curl, CURLOPT_READDATA, &sharedValue);
curl_easy_setopt(curl, CURLOPT_OPENSOCKETFUNCTION, opensocket);
curl_easy_setopt(curl, CURLOPT_OPENSOCKETDATA, &sharedValue);
curl_easy_setopt(curl, CURLOPT_CLOSESOCKETFUNCTION, closecb);
curl_easy_setopt(curl, CURLOPT_CLOSESOCKETDATA, &sharedValue);
curl_easy_setopt(curl, CURLOPT_URL, hostname.c_str());
curl_easy_setopt(curl, CURLOPT_SOCKOPTFUNCTION, sockopt_callback);
curl_easy_setopt(curl, CURLOPT_VERBOSE, 1);
res = curl_easy_perform(curl);
curl_easy_cleanup(curl);
I didn't know what to do with CURLOPT_SOCKOPTFUNCTION so I did sockopt_callback which is
static curl_socket_t opensocket(void *clientp,
curlsocktype purpose,
struct curl_sockaddr *address)
{
return 0;
}
But I get
* Trying 212.183.159.230:80...
* Could not set TCP_NODELAY: Socket operation on non-socket
* connect to 212.183.159.230 port 80 failed: Socket operation on non-socket
libcurl wants to close 0 now
because probably it's trying to write to the socket 0. Apparently I can't prevent it from calling options on the socket, for example setting TCP_NODELAY and most important, connect.
Is it possible to make libcurl do not call the socket file descriptor ever?
One idea I had was to pass an existing socket to be a dummy for libcurl and then still use my write and read functions. However, libcurl simply send things through the socket, not the write and read functions

Answer 1

I thought write_data was a way to redirect the socket calls but it has nothing to do with it, it actually is a function that writes what the client receives.
However, by passing a socketpair, as sugested by #domen, I made it work!
Sketch:
//...
static curl_socket_t opensocket(void *clientp,
curlsocktype purpose,
struct curl_sockaddr *address)
{
auto *sharedValue = static_cast<SharedValue *>(clientp);
return sharedValue->curl_socket;
}
int socket_vector[2];
if (0 != socketpair(AF_UNIX, SOCK_STREAM, 0, socket_vector))
{
std::cout << "problem creating socketpair" << std::endl;
std::exit(2);
}
sharedValue.curl_socket = socket_vector[0];
sharedValue.vpn_socket = socket_vector[1];
//writes the result to a file
curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, write_data_file);
curl_easy_setopt(curl, CURLOPT_WRITEDATA, destFile);
curl_easy_setopt(curl, CURLOPT_OPENSOCKETFUNCTION, opensocket);
curl_easy_setopt(curl, CURLOPT_OPENSOCKETDATA, &sharedValue);
curl_easy_setopt(curl, CURLOPT_CLOSESOCKETFUNCTION, closecb);
curl_easy_setopt(curl, CURLOPT_CLOSESOCKETDATA, &sharedValue);
curl_easy_setopt(curl, CURLOPT_SOCKOPTFUNCTION, sockopt_callback);
curl_easy_setopt(curl, CURLOPT_URL, hostname.c_str());
//curl_easy_setopt(curl, CURLOPT_VERBOSE, 1);
//Start a thread here that will read/write to the socket pair on the other endpoint (in my case vpn_socket)
res = curl_easy_perform(curl);
curl_easy_cleanup(curl);
if (res)
{
std::cout << "libcurl error code: " << res << ", libcurl error: " << curl_easy_strerror(res) << std::endl;
return 1;
}