---
name: "CURLOPT_HEADERDATA"
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_HEADERDATA"
---

# CURLOPT\_HEADERDATA(3)

pointer to pass to header callback

## NAME

CURLOPT\_HEADERDATA - pointer to pass to header callback

## SYNOPSIS

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

## DESCRIPTION

Pass a *pointer* to be used to write the header part of the received data to.

If *[CURLOPT\_WRITEFUNCTION(3)](/CURLOPT_WRITEFUNCTION/3.md)* or *[CURLOPT\_HEADERFUNCTION(3)](/CURLOPT_HEADERFUNCTION/3.md)* is used, *pointer* is passed in to the respective callback.

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.

If you are using libcurl as a Windows DLL, you **must** use a *[CURLOPT\_WRITEFUNCTION(3)](/CURLOPT_WRITEFUNCTION/3.md)* or *[CURLOPT\_HEADERFUNCTION(3)](/CURLOPT_HEADERFUNCTION/3.md)* if you set this option or you might experience crashes.

## DEFAULT

NULL

## PROTOCOLS

This functionality affects all supported protocols

## EXAMPLE

```
struct my_info {
  int shoesize;
  const 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) {
    CURLcode result;
    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);
    result = curl_easy_perform(curl);
    curl_easy_cleanup(curl);
  }
}
```

## AVAILABILITY

Added in curl 7.10

## 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\_HEADERFUNCTION**(3)](/CURLOPT_HEADERFUNCTION/3.md), [**CURLOPT\_WRITEFUNCTION**(3)](/CURLOPT_WRITEFUNCTION/3.md), [**curl\_easy\_header**(3)](/curl_easy_header/3.md)