在FTP服务器中实现“TYPE A”

Question

I'm writing an FTP server (in Qt/C++), and right now I with "200 Ok" to "TYPE A", but I really treat it the same as "TYPE I" - files are sent as-is.

How should I go about properly implementing TYPE A? Would opening the files in text mode instead of binary mode be enough?

Also, I assume that the SIZE method should be more complex than returning the file's size on disk, it should read it and perform the text substitutions, and return the size that way?

Edit: In response to a comment, here's the relevant excerpt from the RFC959 spec :

  3.1.1.1. ASCII TYPE This is the default type and must be accepted by all FTP implementations. It is intended primarily for the transfer of text files, except when both hosts would find the EBCDIC type more convenient. The sender converts the data from an internal character representation to the standard 8-bit NVT-ASCII representation (see the Telnet specification). The receiver will convert the data from the standard form to his own internal form. In accordance with the NVT standard, the <CRLF> sequence should be used where necessary to denote the end of a line of text. (See the discussion of file structure at the end of the Section on Data Representation and Storage.) Using the standard NVT-ASCII representation means that data must be interpreted as 8-bit bytes. The Format parameter for ASCII and EBCDIC types is discussed below. 

Answer 1

For transferring files with ASCII mode (type A) you would need to open the files in text mode and then transfer them with line ending CRLF. If you implement the SIZE command you would be required to report the size according to the transfer type. Because this is obviously too much overhead to scan the whole file just for getting the size right server often return 550 SIZE not allowed in ASCII mode if the command is used not in image mode.