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

# [NAME](#NAME)

CURLOPT\_HEADERDATA - pointer to pass to header callback

# [SYNOPSIS](#SYNOPSIS)

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

# [DESCRIPTION](#DESCRIPTION)

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

If [*CURLOPT\_WRITEFUNCTION(3)*](/man/os/curl/CURLOPT_WRITEFUNCTION.3) or [*CURLOPT\_HEADERFUNCTION(3)*](/man/os/curl/CURLOPT_HEADERFUNCTION.3) 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)*](/man/os/curl/CURLOPT_WRITEFUNCTION.3) or [*CURLOPT\_HEADERFUNCTION(3)*](/man/os/curl/CURLOPT_HEADERFUNCTION.3) if you set this option or you might experience crashes.

# [DEFAULT](#DEFAULT)

NULL

# [PROTOCOLS](#PROTOCOLS)

This functionality affects all supported protocols

# [EXAMPLE](#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](#AVAILABILITY)

Added in curl 7.10

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

2026-10-14 libcurl

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