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+---
+c: Copyright (C) Daniel Stenberg, <daniel.se>, et al.
+SPDX-License-Identifier: curl
+Title: CURLOPT_HEADERDATA
+Section: 3
+Source: libcurl
+See-also:
+  - CURLOPT_HEADERFUNCTION (3)
+  - CURLOPT_WRITEFUNCTION (3)
+  - curl_easy_header (3)
+---
+
+# NAME
+
+CURLOPT_HEADERDATA - pointer to pass to header callback
+
+# SYNOPSIS
+
+~~~c
+#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) or 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 win32 DLL, you **MUST** use a
+CURLOPT_WRITEFUNCTION(3) or CURLOPT_HEADERFUNCTION(3) if you set
+this option or you might experience crashes.
+
+# DEFAULT
+
+NULL
+
+# PROTOCOLS
+
+All
+
+# EXAMPLE
+
+~~~c
+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