How can a command-line Tcl application catch and gracefully process signals such as control-C?
It can't--not without an extension.  Signal handling is not part of core Tcl, as of version 8.6.

The most popular signal-aware extensions are [Expect] and [TclX]
(available, incidentally, through the popular [ActiveTcl] distribution). [TWAPI] also has signal handling support [http://twapi.magicsplat.com/console.html#set_console_control_handler] for Windows.
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And what about signal handling from C extensions to Tcl? Say I'm writing some loadable module, and need to register an handler with SIGFPE. What TCL functions are safe to call? Which aren't?

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[TclX] has a signal command: 

    :   '''signal''' ?'''-restart'''? ''action siglist'' ?''command''?

where ''action'' is one of "'''default'''", "'''ignore'''", "'''error'''", "'''trap'''", "'''get'''",
plus the [POSIX] "'''block'''" and "'''unblock'''" actions (available only on
POSIX systems, of course). ''Siglist'' is a list of either the symbolic
or numeric Unix signal (the SIG prefix is optional). ''Command'' is your
error handler (or a simple `{puts stdout "Don't press *that* key!"}` :-)
'''trap''' does what you expect, and I find '''error''' and '''get''' to be extremely
useful in interactive programs which demand keyboard traversal.
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Americus P offers this example of signal usage: 

    package require Tclx 8.0
    set cntrlc_flag 1

    proc mysig {} {
            global cntrlc_flag
            puts stdout "Aborting current routine"
            set cntrlc_flag 0
    }

    signal trap SIGINT mysig

    The procedure that uses the interrupt looks like this:

    proc infinite {} {
        global cntrlc_flag
        set cntrlc_flag 1
        set a 0
        while {$cntrlc_flag == 1} {
            set a [expr $a+1]
            puts "Loop: $a"
        }
    }

[TV] Remember that on various systems, signals can get lost, and repeated signals masked and the order of signal reception not guaranteed. Sending a message over a socket (or pipe) is preferable over using signals, usually, except of course probably when using 'kill' or 'sleep' signals.


On windows, [cygwin] gives you signals but than behaviour could be even more unpredictable, though it is possible to use signals unix-like. - [RS] confirms that the above sample code for SIGINT runs as expected on [Windows] XP, with or without [Cygwin].
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