CURLOPT_HEADERDATA(3)

💖 NAME 💖

okay so this is the CURLOPT_HEADERDATA(3) page from the Library Functions Manual, and it's literally about a pointer you pass to your header callback. No cap.

• CURLOPT_HEADERDATA — pointer to pass to header callback

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✨ SYNOPSIS ✨

#include <curl/curl.h>

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

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🦋 DESCRIPTION 🦋

Bestie, you pass a pointer in here, and that pointer is used to write the header part of the received data to. It's giving "custom data delivery service" ✨.

If CURLOPT_WRITEFUNCTION(3) or CURLOPT_HEADERFUNCTION(3) is used, pointer is passed in to the respective callback. So your callback can be like "oh hey, this is my stuff" 💅.

If neither of those options are set, pointer must be a valid FILE * and it is used by a plain fwrite() to write headers to. Literally must be valid — no playing around.

🚨 warning, bestie: if you are using libcurl as a Windows DLL, you must use CURLOPT_WRITEFUNCTION(3) or CURLOPT_HEADERFUNCTION(3) if you set this option or you might experience crashes. Not cute. Not slay. Use the callback.

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🎀 DEFAULT 🎀

NULL — iconic empty-handed start.

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🌈 PROTOCOLS 🌈

This functionality affects all supported protocols. Every single one, periodt.

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🍓 EXAMPLE 🍓

Here's the code. Copy it exactly, don't freestyle the literals:

struct my_info {
  int shoesize;
  char *secret;
};

static size_t header_callback(char *buffer, size_t size,
                              size_t nitems, void *userdata)
{
  struct my_info *i = userdata;
  printf("shoe size: %d\n", i->shoesize);
  /* now this callback can access the my_info struct */

  return nitems * size;
}

int main(void)
{
  CURL *curl = curl_easy_init();
  if(curl) {
    struct my_info my = { 10, "the cookies are in the cupboard" };
    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");

    curl_easy_setopt(curl, CURLOPT_HEADERFUNCTION, header_callback);

    /* pass in custom data to the callback */
    curl_easy_setopt(curl, CURLOPT_HEADERDATA, &my);

    curl_easy_perform(curl);
  }
}

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💜 AVAILABILITY 💜

Added in curl 7.10. Old-school but still iconic.

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👑 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). Fr fr.

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🔥 SEE ALSO 🔥

• CURLOPT_HEADERFUNCTION(3)
• CURLOPT_WRITEFUNCTION(3)
• curl_easy_header(3)

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mwah 💋 this page's footer tea is: source/version libcurl, date 2026-10-12. Stay hydrated and validate your FILE *, bestie 🫶