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CURLOPT_HSTSWRITEFUNCTION - write callback for HSTS hosts #include <curl/curl.h> CURLSTScode hstswrite(CURL *easy, struct curl_hstsentry *sts, struct curl_index *count, void *userp); CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HSTSWRITEFUNCTION, hstswrite); 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 userp argument with the CURLOPT_HSTSWRITEDATA(3) option or it will be NULL. When the callback is invoked, the sts pointer points to a populated struct: Read the host name 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's 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. NULL - no callback. This feature is only used for HTTP(S) transfer.

{
 /* set HSTS read callback */
 curl_easy_setopt(curl, CURLOPT_HSTSWRITEFUNCTION, hstswrite);

 /* pass in suitable argument to the callback */
 curl_easy_setopt(curl, CURLOPT_HSTSWRITEDATA, &hstspreload[0]);

 result = curl_easy_perform(curl);
}

Added in 7.74.0 This will return CURLE_OK. CURLOPT_HSTSWRITEDATA "(3), " CURLOPT_HSTSWRITEFUNCTION "(3), " CURLOPT_HSTS "(3), " CURLOPT_HSTS_CTRL "(3), "