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From: Paolo Bonzini <pbonzini@redhat.com>
Date: Sun, 15 Jan 2012 11:42:36 +0100
Message-ID: <jeuais$bsg$1@dough.gmane.org>
References: <20120114124210.E43483DE@gandalf.tls.msk.ru>
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In-Reply-To: <20120114124210.E43483DE@gandalf.tls.msk.ru>
Subject: Re: [Qemu-devel] [PATCH] rework daemonizing logic in qemu-nbd
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To: qemu-devel@nongnu.org

On 01/14/2012 01:39 PM, Michael Tokarev wrote:
>           if (pid == 0) {
> -            close(stderr_fd[0]);
> -            ret = qemu_daemon(0, 0);
> -
> -            /* Temporarily redirect stderr to the parent's pipe...  */
> -            dup2(stderr_fd[1], STDERR_FILENO);
> -            if (ret == -1) {
> +            int nullfd = open("/dev/null", O_RDWR);
> +            if (nullfd<  0 || setsid()<  0) {
>                   err(EXIT_FAILURE, "Failed to daemonize");
>               }

This is forking only once.

> -
> -            /* ... close the descriptor we inherited and go on.  */
> -            close(stderr_fd[1]);
> -        } else {
> -            bool errors = false;
> -            char *buf;
> -
> -            /* In the parent.  Print error messages from the child until
> -             * it closes the pipe.
> +            /* redirect stdin from /dev/null,
> +             * stdout (temporarily) to the pipe to parent,

This is a bit of a hack.

> +    /* now complete the daemonizing procedure.
> +     */
> +    if (device && !verbose) {
> +        if (chdir("/") < 0) {
> +            err(EXIT_FAILURE, "unable to chdir to /");
> +        }
> +        /* this redirects stderr to /dev/null */
> +        dup2(STDIN_FILENO, STDERR_FILENO);
> +        /* this redirects stdout to /dev/null too, and closes parent pipe */
> +        dup2(STDIN_FILENO, STDOUT_FILENO);
> +    }
> +

Half of this is already done in client_thread, and that would be the 
place where you should add dup2(0, 1).  Also, the chdir can be moved 
earlier, after bdrv_open.

Paolo