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CURLOPT_OP...ETFUNCTION(3)  Library Functions Manual  CURLOPT_OP...ETFUNCTION(3)

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 CUR-
     LOPT_OPENSOCKETDATA(3) 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 addi-
     tional  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 sock-
     addr_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_SOCKOPTFUNC-
     TION(3) 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)
     {
       return CURL_SOCKOPT_ALREADY_CONNECTED;
     }

     int main(void)
     {
       CURL *curl = curl_easy_init();
       if(curl) {
	 CURLcode result;
	 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);

	 result = curl_easy_perform(curl);

	 curl_easy_cleanup(curl);
       }
     }

AVAILABILITY
     Added in curl 7.17.1

RETURN VALUE
     curl_easy_setopt(3) returns a CURLcode indicating success or error.

     CURLE_OK (0) means everything was OK, non-zero means an error occurred, see
     libcurl-errors(3).

SEE ALSO
     CURLOPT_CLOSESOCKETFUNCTION(3),	 CURLOPT_OPENSOCKETFUNCTION(3),     CUR-
     LOPT_SOCKOPTFUNCTION(3)

libcurl 			   2026-06-11	      CURLOPT_OP...ETFUNCTION(3)

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