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+---
+c: Copyright (C) Daniel Stenberg, <daniel.se>, et al.
+SPDX-License-Identifier: curl
+Title: CURLOPT_HSTSWRITEFUNCTION
+Section: 3
+Source: libcurl
+See-also:
+  - CURLOPT_HSTS (3)
+  - CURLOPT_HSTSWRITEDATA (3)
+  - CURLOPT_HSTSWRITEFUNCTION (3)
+  - CURLOPT_HSTS_CTRL (3)
+---
+
+# NAME
+
+CURLOPT_HSTSWRITEFUNCTION - write callback for HSTS hosts
+
+# SYNOPSIS
+
+~~~c
+#include <curl/curl.h>
+
+struct curl_hstsentry {
+  char *name;
+  size_t namelen;
+  unsigned int includeSubDomains:1;
+  char expire[18]; /* YYYYMMDD HH:MM:SS [null-terminated] */
+};
+
+struct curl_index {
+  size_t index; /* the provided entry's "index" or count */
+  size_t total; /* total number of entries to save */
+};
+
+CURLSTScode hstswrite(CURL *easy, struct curl_hstsentry *sts,
+                      struct curl_index *count, void *clientp);
+
+CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HSTSWRITEFUNCTION, hstswrite);
+~~~
+
+# DESCRIPTION
+
+Pass a pointer to your callback function, as the prototype shows above.
+
+This callback function gets called by libcurl repeatedly to allow the
+application to store the in-memory HSTS cache when libcurl is about to discard
+it.
+
+Set the *clientp* argument with the CURLOPT_HSTSWRITEDATA(3) option
+or it is NULL.
+When the callback is invoked, the *sts* pointer points to a populated
+struct: Read the hostname to 'name' (it is *namelen* bytes long and null
+terminated. The *includeSubDomains* field is non-zero if the entry matches
+subdomains. The *expire* string is a date stamp null-terminated string
+using the syntax YYYYMMDD HH:MM:SS.
+
+The callback should return *CURLSTS_OK* if it succeeded and is prepared to
+be called again (for another host) or *CURLSTS_DONE* if there is nothing
+more to do. It can also return *CURLSTS_FAIL* to signal error.
+
+This option does not enable HSTS, you need to use CURLOPT_HSTS_CTRL(3) to
+do that.
+
+# DEFAULT
+
+NULL - no callback.
+
+# PROTOCOLS
+
+This feature is only used for HTTP(S) transfer.
+
+# EXAMPLE
+
+~~~c
+struct priv {
+  void *custom;
+};
+
+static CURLSTScode hswr_cb(CURL *easy, struct curl_hstsentry *sts,
+                           struct curl_index *count, void *clientp)
+{
+  /* save the passed in HSTS data somewhere */
+  return CURLSTS_OK;
+}
+
+int main(void)
+{
+  CURL *curl = curl_easy_init();
+  if(curl) {
+    struct priv my_stuff;
+    CURLcode res;
+
+    /* set HSTS read callback */
+    curl_easy_setopt(curl, CURLOPT_HSTSWRITEFUNCTION, hswr_cb);
+
+    /* pass in suitable argument to the callback */
+    curl_easy_setopt(curl, CURLOPT_HSTSWRITEDATA, &my_stuff);
+
+    res = curl_easy_perform(curl);
+  }
+}
+~~~
+
+# AVAILABILITY
+
+Added in 7.74.0
+
+# RETURN VALUE
+
+This returns CURLE_OK.