CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HTTPHEADER, struct curl_slist *headers);
The linked list should be a fully valid list of struct curl_slist structs properly filled in. Use curl_slist_append(3) to create the list and curl_slist_free_all(3) to clean up an entire list. If you add a header that is otherwise generated and used by libcurl internally, your added one will be used instead. If you add a header with no content as in 'Accept:' (no data on the right side of the colon), the internally used header will get disabled. With this option you can add new headers, replace internal headers and remove internal headers. To add a header with no content (nothing to the right side of the colon), use the form 'MyHeader;' (note the ending semicolon).
The headers included in the linked list must not be CRLF-terminated, because libcurl adds CRLF after each header item. Failure to comply with this will result in strange bugs because the server will most likely ignore part of the headers you specified.
The first line in a request (containing the method, usually a GET or POST) is not a header and cannot be replaced using this option. Only the lines following the request-line are headers. Adding this method line in this list of headers will only cause your request to send an invalid header. Use CURLOPT_CUSTOMREQUEST(3) to change the method.
When this option is passed to curl_easy_setopt(3), libcurl will not copy the entire list so you must keep it around until you no longer use this handle for a transfer before you call curl_slist_free_all(3) on the list.
Pass a NULL to this option to reset back to no custom headers.
The most commonly replaced headers have "shortcuts" in the options CURLOPT_COOKIE(3), CURLOPT_USERAGENT(3) and CURLOPT_REFERER(3). We recommend using those.
There's an alternative option that sets or replaces headers only for requests that are sent with CONNECT to a proxy: CURLOPT_PROXYHEADER(3). Use CURLOPT_HEADEROPT(3) to control the behavior.
Use CURLOPT_HEADEROPT(3) to make the headers only get sent to where you intend them to get sent.
Custom headers are sent in all requests done by the easy handles, which implies that if you tell libcurl to follow redirects (CURLOPT_FOLLOWLOCATION(3)), the same set of custom headers will be sent in the subsequent request. Redirects can of course go to other hosts and thus those servers will get all the contents of your custom headers too.
Starting in 7.58.0, libcurl will specifically prevent "Authorization:" headers from being sent to other hosts than the first used one, unless specifically permitted with the CURLOPT_UNRESTRICTED_AUTH(3) option.
Starting in 7.64.0, libcurl will specifically prevent "Cookie:" headers from being sent to other hosts than the first used one, unless specifically permitted with the CURLOPT_UNRESTRICTED_AUTH(3) option.
CURL *curl = curl_easy_init();
struct curl_slist *list = NULL;
if(curl) {
curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
list = curl_slist_append(list, "Shoesize: 10");
list = curl_slist_append(list, "Accept:");
curl_easy_setopt(curl, CURLOPT_HTTPHEADER, list);
curl_easy_perform(curl);
curl_slist_free_all(list); /* free the list again */
}