.\" generated by cd2nroff 0.1 from CURLOPT_HSTSREADFUNCTION.md
.TH CURLOPT_HSTSREADFUNCTION 3 "2026-09-10" libcurl
.SH NAME
CURLOPT_HSTSREADFUNCTION \- read callback for HSTS hosts
.SH SYNOPSIS
.nf
#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);
.fi
.SH 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 \fIclientp\fP argument with the \fICURLOPT_HSTSREADDATA(3)\fP option or it is
NULL.

When this callback is invoked, the \fIsts\fP pointer points to a populated struct:
Copy the hostname to \fIname\fP (no longer than \fInamelen\fP bytes). Make it
null\-terminated. Set \fIincludeSubDomains\fP to TRUE or FALSE. Set \fIexpire\fP to a
date stamp or a zero length string for \fIforever\fP (wrong date stamp format
might cause the name to not get accepted)

The callback should return \fICURLSTS_OK\fP if it returns a name and is prepared
to be called again (for another host) or \fICURLSTS_DONE\fP if it has no entry to
return. It can also return \fICURLSTS_FAIL\fP to signal error. Returning
\fICURLSTS_FAIL\fP stops the transfer from being performed and make
\fICURLE_ABORTED_BY_CALLBACK\fP get returned.

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

The hostname provided to libcurl \fIshould not\fP have a trailing dot nor leading
dot.
.SH DEFAULT
NULL \- no callback.
.SH PROTOCOLS
This functionality affects http only
.SH EXAMPLE
.nf
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);
  }
}
.fi
.SH AVAILABILITY
Added in curl 7.74.0
.SH RETURN VALUE
This returns CURLE_OK.
.SH SEE ALSO
.BR CURLOPT_HSTS (3),
.BR CURLOPT_HSTSREADDATA (3),
.BR CURLOPT_HSTSWRITEFUNCTION (3),
.BR CURLOPT_HSTS_CTRL (3)