官方文档：
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自定义命令
并不是所以的协议都像HTTP那样，通过消息头来告诉服务器如何处理请求。对于FTP，你就要使用另外的方式来处理。
发送自定义的命令到ftp服务器，意味着你发送的命令必须是能被ftp服务器理解的命令（FTP协议中定义的命令，参考rfc959）。
下面是一个简单的例子，在文件传输操作操作之前删除指定文件：
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); // 在数据传输之后操行删除操作
curl_easy_perform(easyhandle); /* transfer ftp data! */ curl_slist_free_all(headers); /* free the header list */
FTP服务器执行命令的顺序，同这些命令被添加到列表中顺序是一致的。发往服务器的命令列表中，只要有一个命令执行失败，ftp服务器就会返回一个错误代码，此时libcurl将直接返回CURLE_QUOTE_ERROR，不再执行剩余的FTP命令。
将CURLOPT_HEADER设置为1，libcurl获取目标文件的信息，并以HTTP消息头的样式来输出消息头。
FTP自定义CUSTOMREQUEST
使用CURLOPT_CUSTOMREQUEST属性，可以向FTP服务器发送命令。”NLST”是ftp默认的列出文件列表的命令。 下面的代码用于列出FTP服务器上的文件列表：
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; }