CURLOPT_IOCTLDATA(3)

✨ NAME ✨

CURLOPT_IOCTLDATA - pointer passed to I/O callback 💅

🌸 SYNOPSIS 🌸

#include <curl/curl.h>

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_IOCTLDATA, void *pointer);

🦋 DESCRIPTION 🦋

Okay so bestie, you pass in a pointer here, and libcurl literally does not touch it — it stays untouched by libcurl — and then it gets passed as the 3rd argument in the ioctl callback that you set with CURLOPT_IOCTLFUNCTION(3) ✨

🎀 DEFAULT 🎀

NULL — that's the default, no pointer, periodt 💖

🌈 PROTOCOLS 🌈

This functionality affects all supported protocols — yes, all of them, no cap ✨

🧚 EXAMPLE 🧚

Here's the whole vibe, bestie:

#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 since 7.18.0. So yeah, this one is deprecated, bestie — noted and flagged 🙈

⭐ AVAILABILITY ⭐

Added in curl 7.12.3 👑

💜 RETURN VALUE 💜

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).

🫶 SEE ALSO 🫶

• CURLOPT_IOCTLFUNCTION(3)
• CURLOPT_SEEKFUNCTION(3)

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Signed, sealed, and delivered by libcurl 💌 dated 2026-10-12, all about CURLOPT_IOCTLDATA(3) — bye bestie! 💖