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

# [NAME](#NAME)

CURLOPT\_IOCTLDATA - pointer passed to I/O callback

# [SYNOPSIS](#SYNOPSIS)

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

# [DESCRIPTION](#DESCRIPTION)

Pass the *pointer* that is untouched by libcurl and passed as the 3rd argument in the ioctl callback set with [*CURLOPT\_IOCTLFUNCTION(3)*](/man/os/curl/CURLOPT_IOCTLFUNCTION.3).

# [DEFAULT](#DEFAULT)

NULL

# [PROTOCOLS](#PROTOCOLS)

This functionality affects all supported protocols

# [EXAMPLE](#EXAMPLE)

```
#include <unistd.h> /* for lseek */
struct data {


  int fd; /* our file descriptor */
};
static curlioerr ioctl_callback(CURL *handle, int cmd, void *clientp)
{


  struct data *io = (struct data *)clientp;


  if(cmd == CURLIOCMD_RESTARTREAD) {


    lseek(io->fd, 0, SEEK_SET);


    return CURLIOE_OK;


  }


  return CURLIOE_UNKNOWNCMD;
}
int main(void)
{


  struct data ioctl_data;


  CURL *curl = curl_easy_init();


  if(curl) {


    curl_easy_setopt(curl, CURLOPT_IOCTLFUNCTION, ioctl_callback);


    curl_easy_setopt(curl, CURLOPT_IOCTLDATA, &ioctl_data);


  }
}
```

# [DEPRECATED](#DEPRECATED)

Deprecated since 7.18.0.

# [AVAILABILITY](#AVAILABILITY)

Added in curl 7.12.3

# [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\_IOCTLFUNCTION**(3)](/man/os/curl/CURLOPT_IOCTLFUNCTION.3), [**CURLOPT\_SEEKFUNCTION**(3)](/man/os/curl/CURLOPT_SEEKFUNCTION.3)

2026-10-14 libcurl

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Document: CURLOPT_IOCTLDATA(3) from curl 8.23.0-r0 (Chainguard OS)
Source: https://github.com/curl/curl.git