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CURLOPT_TRAILERFUNCTION(3) Library Functions Manual CURLOPT_TRAILERFUNCTION(3)

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) op-
       tion.

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

       If  you	use  curl_slist_append(3)  to  add  trailing  headers  to  the
       curl_slist   then   libcurl  duplicates	the  strings,  and  frees  the
       curl_slist 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_TRAILER-
       FUNC_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
       This functionality affects http only

EXAMPLE
       extern size_t read_cb(char *ptr,	size_t size,
			     size_t nmemb, void	*userdata);

       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;
       }

       int main(void)
       {
	 CURL *curl = curl_easy_init();
	 if(curl) {
	   CURLcode res;

	   /* 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_UPLOAD, 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
       Added in	curl 7.64.0

RETURN VALUE
       curl_easy_setopt(3) returns a CURLcode indicating success or error.

       CURLE_OK	(0) means everything was OK, non-zero means an error occurred,
       see libcurl-errors(3).

SEE ALSO
       CURLOPT_TRAILERDATA(3), CURLOPT_WRITEFUNCTION(3)

libcurl				  2025-06-03	    CURLOPT_TRAILERFUNCTION(3)

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