C Programming

First let's consider the requirements for the program. It should store and remember configuration items such as login identification, password, telephone number, baud rate, communications port, data format, and modem-command strings. The program should be a multiple-window application with windows to display incoming and outgoing messages. It should remember how the user (me) positions and sizes those windows, and it should preserve those settings between executions. The program should maintain an address book of people to whom I send mail and let me select from that book as a function of addressing messages. The program should hold incoming and outgoing messages in their respective mailboxes until they have been read or sent. An option should permit me to automatically save copies of outgoing messages. The program should allow me to reply to and forward messages that I am reading. There should be user-defined file folders into which I can file mail messages. The mail-creation editor should support the usual text-editing functions. The program should be able to dial the host computer and upload and download all pending mail messages without my interaction or intervention. The program should support automatic message uploads and downloads, as well as interactive logins with the ability to upload and download binary files by using the XModem file-transfer protocol.

Interaction with the host--logging in, reading mail, sending mail, logging off--is managed by a set of scripts. IMail watches for prompting strings from the host and sends UNIX commands accordingly. The scripts are coded in a C++ source-code file named scripts.cpp. By isolating them there, I can change the scripts to work with other mail protocols if I want. They are not scripts in the truest sense of the word, however: They are not interpreted like the scripts of other communications programs. Since this program is for programmers who can be expected to have compilers, the scripts are written to be compiled into the program along with the rest of the code.

IMail uses mailbox classes that store messages and display lists of the messages. Mail-reader classes display the contents of messages. The user can read messages, delete them, and file them in other mailboxes. The program uses two default mailboxes for incoming and outgoing messages: INBOX and OUTBOX. Other mailboxes are implemented as folders into which the user files messages to be saved. The user can establish and delete folders. The program maintains folders as subdirectories under a fixed subdirectory named CABINET. Any subdirectory under CABINET without a file extension is automatically treated as a folder. One such folder is named SENT and optionally holds copies of all the messages that the user sends. Another folder is named GENERAL, to be used as needed. The user may delete the GENERAL folder but not the SENT folder.

Messages in the mailboxes are text-file copies of the messages as written by the user or received from the user's correspondents. Each message is its own text file, and the program assigns a unique filename to each message file. The message filename has this format: MAILnnnn.MSG. The first message written to a folder is named MAIL0000.MSG, the next is MAIL0001.MSG, and so on. When messages are deleted or moved to other folders, their names can be reused for new messages in the folder.

I don't save a lot of mail. If I did, putting every message into its own text file would get expensive. Not only would each message consume a directory entry, but every message would also use disk space in multiples of the file system's cluster factor, which on my computer is 8K. If I were a heavy mail user and saver, I'd build an indexing system and concatenated message files, perhaps even with conversation-thread management.

IMail receives mail messages when it sees the text string "you have mail" in the input stream from the host. The UNIX system displays that message when you log in if mail is indeed waiting. The program waits for the $ prompt and then issues the following command as if you had typed it: cat $MAIL. The UNIX cat command displays a text file on the screen, and the $MAIL environment variable expands to the name of the logged-on user's incoming mail file in the UNIX environment. IMail captures the input stream into a disk file named IMAIL.TMP. When the messages are copied, IMail sends this command to delete the mail text file from the UNIX system: rm $MAIL.

After IMail logs off and hangs up, it converts the messages in IMAIL.TMP into message files in the incoming-mailbox folder. Each received message is stored in the incoming mailbox in this format: a line of text with the sender's mail address, a second line of text with the subject, and a third line with the date the message was written. Subsequent lines of text constitute the message body.

By reacting to the UNIX mail program's format for incoming messages, IMail is able to preserve only those three lines from the typically abundant, verbose message headers that are created by Internet mail transport programs and that are meaningless to most people. This one feature is worth the whole program. It removes one of my principal gripes about Internet mail--all that junk up front that nobody reads. The program parses the first lines of text, watching for lines with the strings "From:", "Subject:", and "Date:" in the first characters of the line. The program extracts the pertinent fields from those lines, ignoring all others until it sees a blank line, which marks the end of the message header and the start of the message text.

For each outgoing message, the IMail script simulates an interactive session with the UNIX mail program. The script sends the "mail" command, a space character, the recipient's mail address, and a carriage return. Then the script waits for a "ject:" string that prompts for the subject, whereupon it sends the subject text and a carriage return. Next, it sends the message text, a final carriage return, and the Ctrl-D character to tell the mail program that the message is complete. When the $ prompt comes back, the script copies the message to the SENT folder if that option is selected and deletes the message from the outgoing message box.

Both scripts have a lot of one-second delays built in between interchanges with the host. These delays reflect what I have determined works best. Without them, some incoming data characters are lost, and the program goes into a timeout loop waiting for an expected string from the host.

To support the dial-up connection, I encapsulated the operations of the serial port and the modem in two classes. The CommPort class includes a lot of low-level stuff to connect to the input interrupt vector, read and write the serial device's data and status ports, and manage things like XON/XOFF protocols. The Modem class manages modem operations--dialing, hanging up, sending modem commands, and watching the carrier-detect signal. An XModem class handles uploads and downloads of binary files with the XModem file-transfer protocol. These operations allow me to use ddj.com for FTP downloads and for exchanging files with the staff at DDJ.

I developed IMail and the D-Flat wrapper classes with an objective: Like the rest of the world, I'm gradually becoming more of a Windows user and less of a DOS user. The architecture of this software leans toward that of a program written under the Microsoft Foundation Classes. Eventually IMail will be a Win32 application.

The source-code files for IMail and the D-Flat libraries are free. You can download them from the DDJ Forum on CompuServe and on the Internet by anonymous FTP; see "Availability," page 3.

If you cannot get to one of the online sources, send a 3.5-inch diskette and an addressed, stamped mailer to me at Dr. Dobb's Journal, 411 Borel Avenue, San Mateo, CA 94402, and I'll send you the source code. Make sure that you include a note that says which project you want. The code is free, but if you care to support my Careware charity, include a dollar for the Brevard County Food Bank.

Copyright © 1995, Dr. Dobb's Journal