CURLOPT_APPEND - enable appending to the remote file
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_APPEND, long append);
A long parameter set to 1 tells the library to append to the remote file instead
of overwrite it. This is only useful when uploading to an FTP site.
CURL *curl = curl_easy_init();
curl_easy_setopt(curl, CURLOPT_URL, "ftp://example.com/dir/to/newfile");
curl_easy_setopt(curl, CURLOPT_UPLOAD, 1L);
curl_easy_setopt(curl, CURLOPT_APPEND, 1L);
This option was known as CURLOPT_FTPAPPEND up to 7.16.4
Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.