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

# [NAME](#NAME)

CURLOPT\_OPENSOCKETDATA - pointer passed to open socket callback

# [SYNOPSIS](#SYNOPSIS)

```
#include <curl/curl.h>
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_OPENSOCKETDATA, void *pointer);
```

# [DESCRIPTION](#DESCRIPTION)

Pass a *pointer* that is untouched by libcurl and passed as the first argument in the open socket callback set with [*CURLOPT\_OPENSOCKETFUNCTION(3)*](/man/os/curl/CURLOPT_OPENSOCKETFUNCTION.3).

# [DEFAULT](#DEFAULT)

NULL

# [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)

2026-10-22 libcurl

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Document: CURLOPT_OPENSOCKETDATA(3) from curl 8.23.0-r2 (Chainguard OS)
Source: https://github.com/curl/curl.git