Commit d0193b41 authored by Mike Hibler's avatar Mike Hibler
Browse files

Explain what makes our tip so special

parent b68c61f9
``Ntip'' aka the Emulab version of tip.
This is a variant of the old UNIX ``tip'' program (specifically, based on the
FreeBSD version) that has been bent to our will. There are two significant
1. Support for Linux. It should now build/run under at least RedHat Linux.
Changes are ifdefed under LINUX which should get automatically defined
in a native build (assuming __linux__ is defined).
2. Support for talking to a socket instead of a tty device. We use this
for connecting to our "capture" console serial line proxy. It is only
useful with that proxy. Ifdefed under USESOCKETS.
There are also a number of significant "subtractions" to make tip more
palatable in our environment:
1. It does not support calling a remote system, all auto-dialing code
has been removed.
2. All the cheezy file transfer support has been removed.
3. Most of the tilde escapes have been removed. Mostly these were the
file transfer related ones. See below for what remains.
4. It ignores 90% of the remote(5) capabilities. You can set the
baud rate (br) and the device (dv). Period.
5. All of tip's variables are still present, but most don't do anything.
It is left as an exercise to the interested user to differentiate.
6. By default, it operates in ``raw'' mode instead of the usual
``cbreak'' mode. This means that all input processing (if any) is
performed by the remote system. Raw mode also disables
``raisechar'' and ``force'' variable interpretation. Yes, you can
actually run emacs on a tip line (modulo the '~' thing).
7. Regular tip is the poster-child for fork-without-exec, creating separate
reader and writer processes executing ``the same code.'' Our tip is a
child of convenience and consists of a single process using select(2).
8. Tip no longer uses uucp(1) style locking. It relies on the TIOCEXCL
ioctl (see tty(4)) to provide ``reasonably mutually exclusive''
access. While it is still technically possible that two parties
could open the same line and both get ``exclusive'' access to it, we
consider this to be the source of amusing anecdotes rather than a bug.
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