---
name: "CURLOPT_HEADERDATA"
section: "3"
platform: "macos"
package: "curl"
version: "166.0.0.0.1"
release: "macOS 26.5"
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 win32 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

All

## EXAMPLE

```
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);
  }
}
```

## AVAILABILITY

Always

## RETURN VALUE

Returns CURLE\_OK

## 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)