FTP client Programming

about the FTP protocol;
FTP command;
FTP return value;
FTP programming structure;
About the FTP protocol:
About the FTP protocol, I think all probably understand, so do not explain. The following is an introduction to RPC959:
The objectives of FTP are 1) to promote sharing of files (computer
Programs and/or data), 2 to encourage indirect or implicit (via
programs) Use of the remote computers, 3) to shield a user from
Variations in file storage systems among hosts, and 4) to transfer
Data reliably and efficiently. FTP, though usable directly by a user
At a terminal, are designed mainly for use by programs.
FTP command:
Perhaps a lot of people have used ftp command, but there may be many people and I have made the same mistake, mistook the ftp command. I used to play FTP under the Win2000 console, then I went to the FTP prompt, and then I used many characters with "Windows" features of the command, such as "dir", "CD", see it in accordance with their own ideas in the running, thinking that they have mastered the ftp command. I am very confident to open the programming tool, write a socket, and then connect to the FTP server. Of course, the server returned the message is a successful connection, I began to send some of the commands just used, can return most of the information is "x ': Command not understood."
Finally, I sent the help command, and I was amazed by the results I had, and most of the commands I used. This is the real FTP command. Windows inside the FTP-only client tool, which commands are not FTP commands at all. There is a lot of information on the Internet is also the Windows FTP client commands and FTP command mixed together. The Real FTP command should refer to the instructions in the RFC959 documentation (in fact, some FTP tools such as CuteFTP also display the commands). The commands which are frequently used include the following:
Abor: Suspended;
CWD Directory Name: Change existing directory;
Cdup: Returns to the parent directory;
REIN: Reinitialize server state;
QUIT: Exit;
PORT: Set up data transfer ports;
PASV: Passive mode
TYPE a/i: transmission mode;
Mode s/b/c: transmission mode? ；
RETR filename: Download file;
STOR filename: Upload file;
APPEND append local files to the server;
RNFR: Modify the original filename (file name before modification)
Rnto: modified filename;
DELE filename: Delete file;
RMD Directory folder: delete folders;
MKD directory: New directory;
PWD current working directory;
List: Lists the items in the directory;
Syst: View server operating system information;
Stat view FTP parameter status;