curl / libcurl / API / curl_easy_setopt / CURLOPT_OPENSOCKETFUNCTION

CURLOPT_OPENSOCKETFUNCTION explained

Name

CURLOPT_OPENSOCKETFUNCTION - callback for opening socket

Synopsis

#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);

Description

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.

If CURL_SOCKET_BAD is returned by the callback then libcurl treats it as a failed connection and tries to open a socket to connect to a different IP address associated with the transfer. If there are no more addresses to try then libcurl fails the transfer with error code CURLE_COULDNT_CONNECT.

You can get the IP address that curl is opening the socket for by casting address->addr to sockaddr_in if address->family is AF_INET, or to sockaddr_in6 if address->family is AF_INET6. For an example of how that data can be compared against refer to docs/examples/block_ip.c.

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.

Default

The equivalent of this:

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

Protocols

This functionality affects all supported protocols

Example

/* 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 */
  return CURL_SOCKOPT_ALREADY_CONNECTED;
}
 
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, "http://99.99.99.99:9999");
    /* 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);
 
    curl_easy_cleanup(curl);
  }
}

Availability

Added in curl 7.17.1

Return value

curl_easy_setopt returns a CURLcode indicating success or error.

CURLE_OK (0) means everything was OK, non-zero means an error occurred, see libcurl-errors.