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CURLOPT_HSTSREADFUNCTION - read callback for HSTS hosts #include <curl/curl.h> CURLSTScode hstsread(CURL *easy, struct curl_hstsentry *sts, void *userp); CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HSTSREADFUNCTION, hstsread); 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 userp argument with the CURLOPT_HSTSREADDATA(3) option or it will be NULL. When this callback is invoked, the sts pointer points to a populated struct: Copy the host name 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 will stop 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. NULL - no callback. This feature is only used for HTTP(S) transfer.

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

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

 result = curl_easy_perform(curl);
}

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