---
c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
SPDX-License-Identifier: curl
Title: CURLOPT_HSTSWRITEFUNCTION
Section: 3
Source: libcurl
Protocol:
  - HTTP
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.

# 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.