These notes are for the benefit of future hackers and maintainers.
The following sections are both functional and narrative, read from
beginning to end.
History
TCPtty was initially conceived while I was using the
Palm OS Emulator
(POSE). POSE 3.2 and higher has a feature to map serial I/O to a TCP
socket instead of one of the host's serial ports. I was doing network
development for Palm OS and thus had to setup a PPP server for POSE to
communicate with. Without using the Serial over TCP feature I had to
connect a null modem cable between two serial ports and run a PPP server
off of one of them. If you wanted to run 5 Emulator sessions at the same
time then you better have a lot of serial ports.
I decided that I wanted to run PPP over TCP. Thus I created TCPtty as
a means to spawn a copy of pppd(or any program) everytime a new connection
came in and remap pppd's stdin/stdout/stderr to the slave side of a pseudo
terminal (pty) pair, while the master side communicated with the TCP socket.
Remapping a program's stdin/stdout/stderr to a pseudo terminal is nothing
new and examples of how to do this can be found in
Advanced Programming in the
Unix Environment by Stevens. Specifically one should read Chapter 19. The
source file "ptyfuncs.c" is based on his sample code.
A similar program that does not allow multiple simulateneous connections is
Socket by Juergen Nickelsen.
Buffer size and Transmission interval?
There is a default transmission interval of 4 milliseconds and
a default transmission buffer size of 16 bytes. The interval and
buffer size are there for a very good reason. As mentioned above I wrote
TCPtty to connect a pppd server to POSE. POSE (as of this writing) does
not regulate the speed with which it delivers data to the emulated Palm
when using Serial over TCP. Thus the emulated Palm's serial receive buffers
are easily overrun by a TCP socket on a 100MB/s network. To correct for
this problem it was necessary to only transmit data every so often and to
only transmit a relatively small amount of data. For my development
environment I found that transmitting 16 bytes of data every 4 ms allowed
the emulated Palm to not be overrun.
Internationalization
Internationalization is handled using GNU gettext (see the file
ABOUT_NLS in the source distribution). This places some
minor constraints on the code.
Strings that must be subject to translation should be wrapped with GT_()
or N_() -- the former in function arguments, the latter in static
initializers and other non-function-argument contexts.
I have not performed any translations for the program. That is something
I hope to do later.
Lessons learned
Write once, compile everywhere, debug everywhere
To date TCPtty has only been tested on Redhat Linux 7.1 and FreeBSD 4.3.
It appears to work on both. The testing process for a new distribution
can be tedious. In the future I may develop a Java version of this utility.
Credits
Special thanks go to Eric Raymond, Richard Stevens (rest in peace), and
others who provided solid examples of how to write portable code and how
to package an open source project for distribution. Where possible these
individuals have been acknowledged in the source.
Other significant contributors to the code have included Dave Bodenstab
(report.c code and --syslog).
Conclusion
This is the first release of TCPtty and it will likely undergo severe
revision as I am sure people will find plenty of bugs and better ways to
do things.
Stuart C. Eichert <tcptty@copera.com>
