Problem on capturing system Shutdown

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# 8  
Old 08-27-2013
Originally Posted by hakermania
So you claim that if I make a sync-safe function using the above sync-safe functions, then the handler will be executed?
Not necessarily, depending on how the program is killed. There are signals that cannot be interrupted. Further experimentation is required.

What I would do is create and close file when the program starts. When the signal handler is called, unlink() it. If the program cleans up properly, the file will disappear, if not, it will remain.

And if this is true then how does the system determine:

a) Where in the memory the handler sits for each application
b) If the handler is sync-safe or sync-unsafe
It doesn't "determine" unsafe things. It doesn't "punish" unsafe code. It's the software interrupt itself that is unsafe, really...

It halts your main program, whatever it's doing, finished or not; then does a graceless jump to a temporary, limited stack space. It leaves whatever was happening, dangling. main could be inside printf or malloc already when you try and call them in your interrupt handler. That would be bad.

So to get around this, you have to talk directly to the kernel with system calls, instead of using your handy stdio libraries and such. This avoids tearing out your floor from under you by accident.
What different does the system do instead of sending SIGTERM, waiting for the process to exit, and, after timing out, sending SIGKILL? Because it does something different, it not only sends SIGTERM, because if I send it, it just works.
I'd wonder if your disks are still mounted read-write by the time your program is killed. If they're not, you'll never see any changes even if your interrupt handler works.
# 9  
Old 08-27-2013
So if it doesn't determine whether my function is sync-safe or not, the function should run without many problems most of the time (as you see, the file is accessed nowhere inside the program, a simple sleep is being done)

The only thing that I want my real handler to do is to save one variable into one file that is closed during the whole execution of the program. That is the only place where the file is opened for writing and closed.

The variable is being updated once per second (adding one to itself), so the chance of it being accessed the wrong moment is very slight, and even if it does, I don't really care.

If I understand correctly you are advising me on how to make a function sync-safe, and thanks a lot for that, but that was not my initial question. My initial question was why the handler is not executed while it should. Function being async-unsafe has nothing to do as far as I can understand from your last reply.

And one last thing: If my disk is not writable when I receive the signal, how the heck am I going to do my cleanup? How are other linux applications do stuff like this?
# 10  
Old 08-27-2013
Originally Posted by hakermania
And one last thing: If my disk is not writable when I receive the signal, how the heck am I going to do my cleanup? How are other linux applications do stuff like this?
It may be getting proper cleanup, I'm not sure. Daemons definitely do, I'm not so sure of random user processes that happen to be sitting around.

My suggestion would be to try the suggestion I made earlier, unlinking the file...
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