CURLOPT_TRAILERFUNCTION(3)                            curl_easy_setopt options

NAME
       CURLOPT_TRAILERFUNCTION - Set 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 will be 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 option.

       The trailing headers included in the linked list must not be CRLF-
       terminated, because libcurl will add 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 will duplicate the strings, and will free the curl_slist
       and the duplicates once the trailers have been sent.

       If one of the trailing headers is not formatted correctly (i.e.
       HeaderName: headerdata) it will be ignored and an info message will be
       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 will free 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 will return 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 request, res will get the return code */
         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

SEE ALSO
       CURLOPT_TRAILERDATA(3),

libcurl 7.77.0                 November 4, 2020     CURLOPT_TRAILERFUNCTION(3)