Problem with reboot(), takes a few seconds before rebooting

**galapogos** · 11-24-2008

Hi,

I'm trying to use a call to reboot() to reboot the system after my program completes, however it seems that it takes a few seconds before the reboot actually happens. Why is this so?

My sample code is as follows:

Code:

#include <stdlib.h>
#include <stdio.h>
#include <memory.h>
#include <time.h>
#include <ctype.h>
#include <unistd.h>
#include <linux/reboot.h>

// Function prototypes
int restart(int delay);
int main(void);

int restart(int delay)
{
	sleep(delay);
	return reboot(LINUX_REBOOT_CMD_RESTART);
}

int main(void)
{
	printf("Rebooting...");
	fflush(stdout);
	restart(0);
	return 0;
}

After the "Rebooting..." is printed out, it takes a few seconds before the system reboots itself, even though I've specified 0 second sleep. Why is this so?

**matsp** · 11-24-2008

That is because "reboot" is not just rebooting the machine, but shutting down processes so that you will have a (more) clean boot when the system starts up again. For example, you may want any disk-caches to be flushed to disk, so that files are complete on disk - that's just one example.

--
Mats

**abachler** · 11-24-2008

the kernel has to shut down every running process in the system, then it has to shut down the daemons, which will entail flushing any virtual memory and disk caches, then it will actually shut down. Not every program will instantly halt execution when told to terminate, some have their own cleanup to do, which mostly involves disk I/O. Just be happy that it takes fewer than 30 seconds like my windows machine does, mostly because I have a lot of background applications running.

**brewbuck** · 11-25-2008

Originally Posted by abachler

the kernel has to shut down every running process in the system, then it has to shut down the daemons, which will entail flushing any virtual memory and disk caches, then it will actually shut down. Not every program will instantly halt execution when told to terminate, some have their own cleanup to do, which mostly involves disk I/O. Just be happy that it takes fewer than 30 seconds like my windows machine does, mostly because I have a lot of background applications running.

Actually, none of that should happen. The kernel doesn't do teardown, init does that as it shifts to the reboot runlevel. A call to reboot() should actually cause an immediate reboot without the normal shutdown.

init itself calls reboot() to do the actual reboot once everything is torn down.

It does seem odd that it takes a few seconds. LINUX_REBOOT_CMD_RESTART should immediately restart, without even syncing the disks. (Which is an important point in itself -- if the disks don't get synced you'll probably lose data and possibly corrupt the filesystem). At the very least call sync() before calling reboot()

**galapogos** · 12-01-2008

Hmm, so which is it? Is it supposed to be an immediate reboot with no concern of running processes/daemons/etc? Or is there some cleaning up to do before an actual reboot?

My program resides in a very simple Linux environment that does not even have init, in fact the init is simply a script that among other things calls the program itself.

**brewbuck** · 12-01-2008

Originally Posted by galapogos

Hmm, so which is it? Is it supposed to be an immediate reboot with no concern of running processes/daemons/etc? Or is there some cleaning up to do before an actual reboot?

It is supposed to be an immediate reboot. In this case "immediate" seems to involve some kinds of cleanup but not the normal kind of teardown you'd see on a desktop system.

You might try building a debug kernel and see what all is happening during the reboot.

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