---
c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
SPDX-License-Identifier: curl
Title: CURLOPT_SOCKOPTFUNCTION
Section: 3
Source: libcurl
See-also:
  - CURLOPT_OPENSOCKETFUNCTION (3)
  - CURLOPT_SEEKFUNCTION (3)
  - CURLOPT_SOCKOPTDATA (3)
Protocol:
  - All
Added-in: 7.16.0
---

# NAME

CURLOPT_SOCKOPTFUNCTION - callback for setting socket options

# SYNOPSIS

~~~c
#include <curl/curl.h>

typedef enum {
  CURLSOCKTYPE_IPCXN,  /* socket created for a specific IP connection */
  CURLSOCKTYPE_ACCEPT, /* socket created by accept() call */
  CURLSOCKTYPE_LAST    /* never use */
} curlsocktype;

#define CURL_SOCKOPT_OK 0
#define CURL_SOCKOPT_ERROR 1 /* causes libcurl to abort and return
                                CURLE_ABORTED_BY_CALLBACK */
#define CURL_SOCKOPT_ALREADY_CONNECTED 2

int sockopt_callback(void *clientp,
                     curl_socket_t curlfd,
                     curlsocktype purpose);

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SOCKOPTFUNCTION, sockopt_callback);
~~~

# DESCRIPTION

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

When set, this callback function gets called by libcurl when the socket has
been created, but before the connect call to allow applications to change
specific socket options. The callback's *purpose* argument identifies the
exact purpose for this particular socket:

*CURLSOCKTYPE_IPCXN* for actively created connections or *CURLSOCKTYPE_ACCEPT*
for FTP when the connection was setup with PORT/EPSV (in earlier versions
these sockets were not passed to this callback).

Future versions of libcurl may support more purposes. libcurl passes the newly
created socket descriptor to the callback in the *curlfd* parameter so
additional setsockopt() calls can be done at the user's discretion.

The *clientp* pointer contains whatever user-defined value set using the
CURLOPT_SOCKOPTDATA(3) function.

Return *CURL_SOCKOPT_OK* from the callback on success. Return
*CURL_SOCKOPT_ERROR* from the callback function to signal an unrecoverable
error to the library and it closes the socket and returns
*CURLE_COULDNT_CONNECT* for *CURLSOCKTYPE_IPCXN* and
*CURLE_ABORTED_BY_CALLBACK* for *CURLSOCKTYPE_ACCEPT*.

The callback function may return *CURL_SOCKOPT_ALREADY_CONNECTED* for
*CURLSOCKTYPE_IPCXN*, to tell libcurl that the socket is already connected and
then libcurl does no attempt to connect. This allows an application to pass in
an already connected socket with CURLOPT_OPENSOCKETFUNCTION(3) and then have
this function make libcurl not attempt to connect (again).

The *CURL_SOCKOPT_ALREADY_CONNECTED* feature does not work for HTTP/3 (QUIC)
connections.

# DEFAULT

NULL

# %PROTOCOLS%

# EXAMPLE

~~~c
/* 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;
    int sockfd; /* our custom file descriptor */
    /* 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%

# 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).
