---
c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
SPDX-License-Identifier: curl
Title: CURLOPT_HSTSREADFUNCTION
Section: 3
Source: libcurl
Protocol:
  - HTTP
See-also:
  - CURLOPT_HSTS (3)
  - CURLOPT_HSTSREADDATA (3)
  - CURLOPT_HSTSWRITEFUNCTION (3)
  - CURLOPT_HSTS_CTRL (3)
Added-in: 7.74.0
---

# NAME

CURLOPT_HSTSREADFUNCTION - read 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] */
};

CURLSTScode hstsread(CURL *easy, struct curl_hstsentry *sts, void *clientp);

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HSTSREADFUNCTION, hstsread);
~~~

# DESCRIPTION

Pass a pointer to your callback function, as the prototype shows above.

This callback function gets called by libcurl repeatedly when it populates the
in-memory HSTS cache.

Set the *clientp* argument with the CURLOPT_HSTSREADDATA(3) option or it is
NULL.

When this callback is invoked, the *sts* pointer points to a populated struct:
Copy the hostname to *name* (no longer than *namelen* bytes). Make it
null-terminated. Set *includeSubDomains* to TRUE or FALSE. Set *expire* to a
date stamp or a zero length string for *forever* (wrong date stamp format
might cause the name to not get accepted)

The callback should return *CURLSTS_OK* if it returns a name and is prepared
to be called again (for another host) or *CURLSTS_DONE* if it has no entry to
return. It can also return *CURLSTS_FAIL* to signal error. Returning
*CURLSTS_FAIL* stops the transfer from being performed and make
*CURLE_ABORTED_BY_CALLBACK* get returned.

This option does not enable HSTS, you need to use CURLOPT_HSTS_CTRL(3) to do
that.

The hostname provided to libcurl *should not* have a trailing dot nor leading
dot.

# DEFAULT

NULL - no callback.

# %PROTOCOLS%

# EXAMPLE

~~~c
struct priv {
  void *custom;
};

static CURLSTScode hsts_cb(CURL *easy, struct curl_hstsentry *sts,
                           void *clientp)
{
  /* populate the struct as documented */
  return CURLSTS_OK;
}

int main(void)
{
  CURL *curl = curl_easy_init();
  if(curl) {
    struct priv my_stuff;
    CURLcode result;

    curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");

    /* set HSTS read callback */
    curl_easy_setopt(curl, CURLOPT_HSTSREADFUNCTION, hsts_cb);

    /* pass in suitable argument to the callback */
    curl_easy_setopt(curl, CURLOPT_HSTSREADDATA, &my_stuff);

    result = curl_easy_perform(curl);
  }
}
~~~

# %AVAILABILITY%

# RETURN VALUE

This returns CURLE_OK.
