Re: Zsh Guide chapter 5 (substitutions)

[Bart Schaefer <schaefer@xxxxxxxxxxxxxxxx>]

On Aug 21,  3:21pm, Richard Curnow wrote:
}
} On page 74, 2nd para from the end,
} 
}   It seems a bit odd to signal a process to restart; why can't the
}   operating system just restart it when you ask? The real answer is
}   probably that signals provide an easy way for you to talk to the
}   operating system without grovelling around in the dirt too much.
} 
} I think it's to provide a way for a process to save and restore state
} across the suspension, by installing handlers for SIGTSTP and SIGCONT.

Saving and restoring state is one reason that there are four signals to
stop a process (STOP, TSTP, TTIN, TTOU).  However, it's not the reason
that a signal is used for SIGCONT; Peter's more on target with that one.

There's no such thing as a SIGCONT handler, by the way.  The process
just picks up where it left off (possibly with system calls interrupted
depending on the operating system).  A typical TSTP handler looks like

    save_state();
    kill(getpid(), SIGSTOP);	/* STOP is not catchable */
    restore_state();

-- 
Bart Schaefer                                 Brass Lantern Enterprises
http://www.well.com/user/barts              http://www.brasslantern.com

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