官方文檔：
FTP Custom Commands
Not all protocols are HTTP-like, and thus the above may not help you when you want to make, for example, your FTP transfers to behave differently.
Sending custom commands to a FTP server means that you need to send the commands exactly as the FTP server expects them (RFC 959 is a good guide here), and you can only use commands that work on the control-connection alone. All kinds of commands that require data interchange and thus need a data-connection must be left to libcurl's own judgement. Also be aware that libcurl will do its very best to change directory to the target directory before doing any transfer, so if you change directory (with CWD or similar) you might confuse libcurl and then it might not attempt to transfer the file in the correct remote directory.
A little example that deletes a given file before an operation:
headers = curl_slist_append(headers, "DELE file-to-remove");
/* pass the list of custom commands to the handle */ curl_easy_setopt(easyhandle, CURLOPT_QUOTE, headers);
curl_easy_perform(easyhandle); /* transfer ftp data! */
curl_slist_free_all(headers); /* free the header list */
If you would instead want this operation (or chain of operations) to happen _after_ the data transfer took place the option to curl_easy_setopt(3) would instead be called CURLOPT_POSTQUOTE and used the exact same way.
The custom FTP command will be issued to the server in the same order they are added to the list, and if a command gets an error code returned back from the server, no more commands will be issued and libcurl will bail out with an error code (CURLE_QUOTE_ERROR). Note that if you use CURLOPT_QUOTE to send commands before a transfer, no transfer will actually take place when a quote command has failed.
If you set the CURLOPT_HEADER to 1, you will tell libcurl to get information about the target file and output "headers" about it. The headers will be in "HTTP-style", looking like they do in HTTP.
The option to enable headers or to run custom FTP commands may be useful to combine with CURLOPT_NOBODY. If this option is set, no actual file content transfer will be performed.
FTP Custom CUSTOMREQUEST
If you do want to list the contents of a FTP directory using your own defined FTP command, CURLOPT_CUSTOMREQUEST will do just that. "NLST" is the default one for listing directories but you're free to pass in your idea of a good alternative.
中文：
FTP自定義命令
并不是所以的協議都像HTTP那樣，通過消息頭來告訴服務器如何處理請求。對于FTP，你就要使用另外的方式來處理。
發送自定義的命令到ftp服務器，意味著你發送的命令必須是能被ftp服務器理解的命令（FTP協議中定義的命令，參考rfc959）。
下面是一個簡單的例子，在文件傳輸操作操作之前刪除指定文件：
headers = curl_slist_append(headers, "DELE file-to-remove"); /* pass the list of custom commands to the handle */ curl_easy_setopt(easyhandle, CURLOPT_QUOTE, headers); //curl_easy_setopt(easyhandle, CURLOPT_POSTQUOTE, headers); // 在數據傳輸之后操行刪除操作
curl_easy_perform(easyhandle); /* transfer ftp data! */ curl_slist_free_all(headers); /* free the header list */
FTP服務器執行命令的順序，同這些命令被添加到列表中順序是一致的。發往服務器的命令列表中，只要有一個命令執行失敗，ftp服務器就會返回一個錯誤代碼，此時libcurl將直接返回CURLE_QUOTE_ERROR，不再執行剩余的FTP命令。
將CURLOPT_HEADER設置為1，libcurl獲取目標文件的信息，并以HTTP消息頭的樣式來輸出消息頭。
FTP自定義CUSTOMREQUEST
使用CURLOPT_CUSTOMREQUEST屬性，可以向FTP服務器發送命令。”NLST”是ftp默認的列出文件列表的命令。 下面的代碼用于列出FTP服務器上的文件列表：
int main(int argc, char **argv) { curl_global_init(CURL_GLOBAL_WIN32); CURL *easy_handle = curl_easy_init(); curl_easy_setopt(easy_handle, CURLOPT_URL, "ftp://127.0.0.1/");curl_easy_setopt(easy_handle, CURLOPT_CUSTOMREQUEST, "NLST");curl_easy_perform(easy_handle); curl_easy_cleanup(easy_handle); curl_global_cleanup(); return 0; }