NAME

       CURLOPT_TRAILERFUNCTION - callback for sending trailing headers

SYNOPSIS

       #include <curl.h>

       int curl_trailer_callback(struct curl_slist ** list, void *userdata);

       CURLcode curl_easy_setopt(CURL *handle, CURLOPT_TRAILERFUNCTION,
                                 curl_trailer_callback *func);

DESCRIPTION

       Pass a pointer to a callback function.

       This  callback  function  is  called  once right before sending the final CR LF in an HTTP
       chunked transfer to fill a list of trailing headers to be sent before finishing  the  HTTP
       transfer.

       You can set the userdata argument with the CURLOPT_TRAILERDATA(3) option.

       The  trailing  headers  included  in  the linked list must not be CRLF-terminated, because
       libcurl adds the appropriate line termination characters after each header item.

       If you use curl_slist_append to add  trailing  headers  to  the  curl_slist  then  libcurl
       duplicates the strings, and frees the curl_slist and the duplicates once the trailers have
       been sent.

       If one of the trailing header fields is not formatted correctly it is ignored and an  info
       message is emitted.

       The  return  value can either be CURL_TRAILERFUNC_OK or CURL_TRAILERFUNC_ABORT which would
       respectively instruct libcurl to either continue with sending the trailers or to abort the
       request.

       If  you  set  this  option  to  NULL,  then  the  transfer  proceeds  as usual without any
       interruptions.

DEFAULT

       NULL

PROTOCOLS

       HTTP

EXAMPLE

       #include <curl/curl.h>

       static int trailer_cb(struct curl_slist **tr, void *data) {
         /* libcurl frees the list */
         *tr = curl_slist_append(*tr, "My-super-awesome-trailer: trailer-stuff");
         return CURL_TRAILERFUNC_OK; }

       CURL *curl = curl_easy_init(); if(curl) {
         /* Set the URL of the request */
         curl_easy_setopt(curl, CURLOPT_URL, "https://example.com/");
         /* Now set it as a put */
         curl_easy_setopt(curl, CURLOPT_PUT, 1L);

         /* Assuming we have a function that returns the data to be pushed
            Let that function be read_cb */
         curl_easy_setopt(curl, CURLOPT_READFUNCTION, read_cb);

         struct curl_slist *headers = NULL;
         headers = curl_slist_append(headers, "Trailer: My-super-awesome-trailer");
         res = curl_easy_setopt(curl, CURLOPT_HTTPHEADER, headers);

         /* Set the trailers filling callback */
         curl_easy_setopt(curl, CURLOPT_TRAILERFUNCTION, trailer_cb);

         /* Perform the transfer */
         res = curl_easy_perform(curl);

         curl_easy_cleanup(curl);

         curl_slist_free_all(headers); }

AVAILABILITY

       This option was added in curl 7.64.0 and is present if HTTP support is enabled

RETURN VALUE

       Returns CURLE_OK.

SEE ALSO

       CURLOPT_TRAILERDATA(3), CURLOPT_WRITEFUNCTION(3)