---
name: "CURLOPT_OPENSOCKETDATA"
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_OPENSOCKETDATA"
---

# CURLOPT\_OPENSOCKETDATA(3)

pointer passed to open socket callback

## NAME

CURLOPT\_OPENSOCKETDATA - pointer passed to open socket callback

## SYNOPSIS

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

## 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)](/CURLOPT_OPENSOCKETFUNCTION/3.md)*.

## DEFAULT

NULL

## 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)