CURLOPT_HSTSWRITEFUNCTION(3) Library Functions Manual
NAME
CURLOPT_HSTSWRITEFUNCTION - write callback for HSTS hosts
SYNOPSIS
#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 functionality affects http only
EXAMPLE
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 result;
/* 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);
result = curl_easy_perform(curl);
}
}
AVAILABILITY
Added in curl 7.74.0
RETURN VALUE
This returns CURLE_OK.
SEE ALSO
CURLOPT_HSTSWRITEFUNCTION(3),
CURLOPT_HSTS_CTRL(3)
libcurl 2026-10-18 CURLOPT_HSTSWRITEFUNCTION(3)
curl 8.23.0 - Generated Sun Oct 18 15:53:52 CDT 2026