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Notes On Specific Signals

Signals are highly nonportable. Their behavior varies quite a bit from one system to another. Nonetheless, it is possible to state some generalizations about each one.

SIGINT—Software Interrupt Signal

SIGINT is an interrupt signal. It is usually generated by pressing ^C from the keyboard. The specific key can be changed using stty. The signal can, of course, also be generated via the kill command. If the SIGINT handler is set to SIG_DFL, a SIGINT will cause Expect to die without evaluating exit.

By default, Expect traps SIGINT and defines it to call exit. This association is defined with the command "trap exit SIGINT“, which is evaluated when Expect starts. If you redefine the exit procedure, the trap will invoke your exit.[53]

If Expect is in raw mode, the ^C will not automatically generate a SIGINT but will instead be handled like any other character. For example, interact implicitly puts the terminal in raw mode so that a ^C is sent to the spawned process. You can define a pattern to match ^C and generate a SIGINT using kill, but that is not common practice and would be confusing to users.

In Chapter 9 (p. 219), I described how the debugger is enabled if Expect is started with the -D flag. Part of what -D does is to redefine the behavior of SIGINT as follows:

trap {exp_debug 1} SIGINT

Pressing ^C will then invoke the debugger rather than causing Expect to exit. If you want to redefine SIGINT so that it performs some other action (and does not exit), you can ...

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