curl / libcurl / API / curl_easy_setopt / CURLOPT_OPENSOCKETFUNCTION



CURLOPT_OPENSOCKETFUNCTION - callback for opening socket


#include <curl/curl.h>
typedef enum  {
  CURLSOCKTYPE_IPCXN,  /* socket created for a specific IP connection */
} curlsocktype;
struct curl_sockaddr {
  int family;
  int socktype;
  int protocol;
  unsigned int addrlen;
  struct sockaddr addr;
curl_socket_t opensocket_callback(void *clientp,
                                  curlsocktype purpose,
                                  struct curl_sockaddr *address);
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_OPENSOCKETFUNCTION, opensocket_callback);


Pass a pointer to your callback function, which should match the prototype shown above.

This callback function gets called by libcurl instead of the socket(2) call. The callback's purpose argument identifies the exact purpose for this particular socket. CURLSOCKTYPE_IPCXN is for IP based connections and is the only purpose currently used in libcurl. Future versions of libcurl may support more purposes.

The clientp pointer contains whatever user-defined value set using the CURLOPT_OPENSOCKETDATA function.

The callback gets the resolved peer address as the address argument and is allowed to modify the address or refuse to connect completely. The callback function should return the newly created socket or CURL_SOCKET_BAD in case no connection could be established or another error was detected. Any additional setsockopt(2) calls can of course be done on the socket at the user's discretion. A CURL_SOCKET_BAD return value from the callback function signals an unrecoverable error to libcurl and it returns CURLE_COULDNT_CONNECT from the function that triggered this callback. This return code can be used for IP address block listing.

If you want to pass in a socket with an already established connection, pass the socket back with this callback and then use CURLOPT_SOCKOPTFUNCTION to signal that it already is connected.


The equivalent of this:

   return socket(addr->family, addr->socktype, addr->protocol);


This functionality affects all supported protocols


/* make libcurl use the already established socket 'sockfd' */
static curl_socket_t opensocket(void *clientp,
                                curlsocktype purpose,
                                struct curl_sockaddr *address)
  curl_socket_t sockfd;
  sockfd = *(curl_socket_t *)clientp;
  /* the actual externally set socket is passed in via the OPENSOCKETDATA
     option */
  return sockfd;
static int sockopt_callback(void *clientp, curl_socket_t curlfd,
                            curlsocktype purpose)
  /* This return code was added in libcurl 7.21.5 */
int main(void)
  CURL *curl = curl_easy_init();
  if(curl) {
    CURLcode res;
    extern int sockfd; /* the already connected one */
    /* libcurl thinks that you connect to the host
     * and port that you specify in the URL option. */
    curl_easy_setopt(curl, CURLOPT_URL, "");
    /* call this function to get a socket */
    curl_easy_setopt(curl, CURLOPT_OPENSOCKETFUNCTION, opensocket);
    curl_easy_setopt(curl, CURLOPT_OPENSOCKETDATA, &sockfd);
    /* call this function to set options for the socket */
    curl_easy_setopt(curl, CURLOPT_SOCKOPTFUNCTION, sockopt_callback);
    res = curl_easy_perform(curl);


Added in curl 7.17.1

Return value

Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.

See also


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