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"Trailers" is an HTTP/1 feature where headers can be passed on at the end of a transfer. This callback is used for when you want to send trailers with curl after an upload has been performed. An upload in the form of a chunked encoded POST.
The callback set with
CURLOPT_TRAILERFUNCTIONwill be called and the function can then append headers to a list. One or many. When done, libcurl sends off those as trailers to the server.