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.TH CURLOPT_OPENSOCKETFUNCTION 3 "June 09, 2020" "libcurl 7.73.0" "curl_easy_setopt options"

.SH NAME
CURLOPT_OPENSOCKETFUNCTION \- set callback for opening sockets
.SH SYNOPSIS
.nf
#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);
.SH 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 \fIsocket(2)\fP
call. The callback's \fIpurpose\fP argument identifies the exact purpose for
this particular socket. \fICURLSOCKTYPE_IPCXN\fP is for IP based connections
and is the only purpose currently used in libcurl. Future versions of libcurl
may support more purposes.

The \fIclientp\fP pointer contains whatever user-defined value set using the
\fICURLOPT_OPENSOCKETDATA(3)\fP function.

The callback gets the resolved peer address as the \fIaddress\fP argument and
is allowed to modify the address or refuse to connect completely. The callback
function should return the newly created socket or \fICURL_SOCKET_BAD\fP in
case no connection could be established or another error was detected. Any
additional \fIsetsockopt(2)\fP calls can of course be done on the socket at
the user's discretion.  A \fICURL_SOCKET_BAD\fP return value from the callback
function will signal an unrecoverable error to libcurl and it will return
\fICURLE_COULDNT_CONNECT\fP 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
\fICURLOPT_SOCKOPTFUNCTION(3)\fP to signal that it already is connected.
.SH DEFAULT
The default behavior is the equivalent of this:
.nf
   return socket(addr->family, addr->socktype, addr->protocol);
.fi
.SH PROTOCOLS
All
.SH EXAMPLE
.nf
/* 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;
}

curl = curl_easy_init();
if(curl) {
  /* libcurl will internally think 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);
}
.fi
.SH AVAILABILITY
Added in 7.17.1.
.SH RETURN VALUE
Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.
.SH "SEE ALSO"
.BR CURLOPT_OPENSOCKETDATA "(3), " CURLOPT_SOCKOPTFUNCTION "(3), "
.BR CURLOPT_CLOSESOCKETFUNCTION "(3), "