CURLOPT_TRAILERFUNCTION - callback for sending trailing headers

#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);

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(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_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.



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");
int main(void)
  CURL *curl = curl_easy_init();
  if(curl) {
    CURLcode res;
    /* Set the URL of the request */
    curl_easy_setopt(curl, CURLOPT_URL, "");
    /* 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, trailer_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);

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

Returns CURLE_OK.


2024-05-22 libcurl