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Date: Fri, 06 Nov 2009 06:42:44 -0600
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] [PATCH 0/4] net-bridge: rootless bridge support
	for qemu
References: <1257294485-27015-1-git-send-email-aliguori@us.ibm.com>	<20091105163702.GC21630@shareable.org>	<4AF30129.7080203@us.ibm.com>
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To: Jamie Lokier <jamie@shareable.org>
Cc: Mark McLoughlin <markmc@redhat.com>, Anthony Liguori <aliguori@us.ibm.com>, Arnd Bergmann <arndbergmann@googlemail.com>, Arnd Bergmann <arnd@arndb.de>, Juan Quintela <quintela@redhat.com>, Dustin Kirkland <kirkland@canonical.com>, qemu-devel@nongnu.org, Michael Tsirkin <mst@redhat.com>, Avi Kivity <avi@redhat.com>

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Jamie Lokier wrote:
> Avi Kivity wrote:
>   
>>> I know this has been discussed before, but isn't this why there are 
>>> things like vfork()? 
>>>       
>> vfork() doesn't work with threads - it requires that the calling process 
>> be halted until exec() is called.
>>     
>
> On Linux (NPTL), vfork() is good for this.  It only halts the calling
> thread.  Other threads continue to run, and when the vfork'd thread
> called exec*(), it doesn't affect the other threads.  Of course this
> behaviour isn't portable, but then again, neither is KVM.
>   
Yup, see attached program.  Here's the output which confirms this is the 
case.

tick
sleeping
tick
tick
tick
tick
tick
tick
tick
tick
tick
done sleeping
vfork() returned
hello world

Regards,

Anthony Liguori

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 filename="vfork.c"

#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <pthread.h>

static void *tick_thread(void *unused)
{
    while (1) {
        pthread_testcancel();
        printf("tick\n");
        sleep(1);
    }
    return NULL;
}

int main(int argc, char **argv)
{
    pid_t pid;
    pthread_t tid;
    void *ret;

    if (pthread_create(&tid, NULL, tick_thread, NULL) < 0) {
        printf("pthread_create failed\n");
        return 1;
    }

    pid = vfork();
    if (pid == 0) {
        printf("sleeping\n");
        sleep(10);
        printf("done sleeping\n");
        execl("/bin/echo", "echo", "hello world", NULL);
        exit(1);
    } else if (pid > 0) {
        printf("vfork() returned\n");
        waitpid(pid, NULL, 0);
    } else {
        printf("vfork failed\n");
        return 1;
    }

    pthread_cancel(tid);
    pthread_join(tid, &ret);

    return 0;
}

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