---
name: "CURLOPT_OPENSOCKETFUNCTION"
section: "3"
platform: "linux"
package: "curl"
version: "8.23.0-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux", "macos"]
url: "https://new.carta.tech/CURLOPT_OPENSOCKETFUNCTION"
---

# CURLOPT\_OPENSOCKETFUNCTION(3)

callback for opening socket

## 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)](/socket/2.md)* 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)](/CURLOPT_OPENSOCKETDATA/3.md)* 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)](/setsockopt/2.md)* 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(3)](/CURLOPT_SOCKOPTFUNCTION/3.md)* 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;
}
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

Added in curl 7.17.1

## RETURN VALUE

*[curl\_easy\_setopt(3)](/curl_easy_setopt/3.md)* returns a CURLcode indicating success or error.

CURLE\_OK (0) means everything was OK, non-zero means an error occurred, see *[libcurl-errors(3)](/libcurl-errors/3.md)*.

## SEE ALSO

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