CURLOPT\_OPENSOCKETFUNCTION(3) Library Functions Manual CURLOPT\_OPENSOCKETFUNCTION(3)

# [NAME](#NAME)

CURLOPT\_OPENSOCKETFUNCTION - callback for opening socket

# [SYNOPSIS](#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](#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)*](/man/os/curl/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(3)*](/man/os/curl/CURLOPT_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 additional [*setsockopt(2)*](/man/os/curl/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-&gt;addr* to *sockaddr\_in* if *address-&gt;family* is *AF\_INET*, or to *sockaddr\_in6* if *address-&gt;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(3)*](/man/os/curl/CURLOPT_SOCKOPTFUNCTION.3) to signal that it already is connected.

# [DEFAULT](#DEFAULT)

The equivalent of this:

```


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

# [PROTOCOLS](#PROTOCOLS)

This functionality affects all supported protocols

# [EXAMPLE](#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;
}
extern int sockfd; /* the already connected one */
int main(void)
{


  CURL *curl = curl_easy_init();


  if(curl) {


    CURLcode result;


    /* 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](#AVAILABILITY)

Added in curl 7.17.1

# [RETURN VALUE](#RETURN_VALUE)

[*curl\_easy\_setopt(3)*](/man/os/curl/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)*](/man/os/curl/libcurl-errors.3).

# [SEE ALSO](#SEE_ALSO)

[**CURLOPT\_CLOSESOCKETFUNCTION**(3)](/man/os/curl/CURLOPT_CLOSESOCKETFUNCTION.3), [**CURLOPT\_OPENSOCKETFUNCTION**(3)](/man/os/curl/CURLOPT_OPENSOCKETFUNCTION.3), [**CURLOPT\_SOCKOPTFUNCTION**(3)](/man/os/curl/CURLOPT_SOCKOPTFUNCTION.3)

2026-10-01 libcurl

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Document: CURLOPT_OPENSOCKETFUNCTION(3) from curl 8.22.0-r4 (Chainguard OS)
Source: https://github.com/curl/curl.git