Problem on capturing system Shutdown Post: 302847699

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Top Forums Programming Problem on capturing system Shutdown Post 302847699 by Corona688 on Tuesday 27th of August 2013 03:04:25 PM

08-27-2013

Registered User

Quote:

I also have this simple question: Why not use async-unsafe functions after all? The worse thing that could happen would be the application to crash or to be in a loop easily terminated with SIGKILL.

I think you just answered your own question.

Quote:

The application was going to forcefully terminate after all. At least give it a chance to do a proper cleanup.

You can give it more than a chance -- you can actually do so. Async-safe system calls include open(), close(), read(), and write() among many other things. All you need.

fopen, printf() and its relatives in particular are not signal safe because they allocate memory. You might get away with it by accident but it's system-dependent, compiler-dependent, library-dependent, and luck-dependent. Your code may work all the time on your system and crash all the time on someone else's.

To write code that'll work both here and there, use signal-safe things. That's what they're for.

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LEARN ABOUT CENTOS

dracut-shutdown.service

DRACUT-SHUTDOWN.S(8)						      dracut						      DRACUT-SHUTDOWN.S(8)

NAME

       dracut-shutdown.service - unpack the initramfs to /run/initramfs

SYNOPSIS

       dracut-shutdown.service

DESCRIPTION

       This service unpacks the initramfs image to /run/initramfs. systemd pivots into /run/initramfs at shutdown, so the root filesytem can be
       safely unmounted.

       The following steps are executed during a shutdown:

       o   systemd switches to the shutdown.target

       o   systemd starts /lib/systemd/system/shutdown.target.wants/dracut-shutdown.service

       o   dracut-shutdown.service executes /usr/lib/dracut/dracut-initramfs-restore which unpacks the initramfs to /run/initramfs

       o   systemd finishes shutdown.target

       o   systemd kills all processes

       o   systemd tries to unmount everything and mounts the remaining read-only

       o   systemd checks, if there is a /run/initramfs/shutdown executable

       o   if yes, it does a pivot_root to /run/initramfs and executes ./shutdown. The old root is then mounted on /oldroot.
	   /usr/lib/dracut/modules.d/99shutdown/shutdown.sh is the shutdown executable.

       o   shutdown will try to umount every /oldroot mount and calls the various shutdown hooks from the dracut modules

       This ensures, that all devices are disassembled and unmounted cleanly.

       To debug the shutdown process, you can get a shell in the shutdown procedure by injecting "rd.break=pre-shutdown rd.shell" or
       "rd.break=shutdown rd.shell".

	   # mkdir -p /run/initramfs/etc/cmdline.d
	   # echo "rd.break=pre-shutdown rd.shell" > /run/initramfs/etc/cmdline.d/debug.conf
	   # touch /run/initramfs/.need_shutdown

AUTHORS

       Harald Hoyer

SEE ALSO

       dracut(8)

dracut								    09/12/2013						      DRACUT-SHUTDOWN.S(8)