linux-kernel - Re: serial console _after_ boot ?

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Date:	Thu, 3 Jan 2008 14:27:57 +0100 (CET)
From:	Jan Engelhardt <jengelh@...putergmbh.de>
To:	devzero@....de
cc:	linux-kernel@...r.kernel.org
Subject: Re: serial console _after_ boot ? - was: Redirect kernel console

On Jan 3 2008 13:43, devzero@....de wrote:
>
>hi !
>
>i was wondering how to make kernel messages appear on /dev/ttyS0 without a reboot, i.e. kernelparam "console=ttyS0"

The solution is simple... the following piece of code is inside
opensuse-10.3/src/sysvinit-2.86-115.src.rpm#showconsole-1.08.tar.bz2#showconsole-1.08/blogd.c

    (void)ioctl(0, TIOCCONS, NULL);  /* Undo any current map if any */
    if (ioctl(pts, TIOCCONS, NULL) < 0)
        error("can not set console device to %s: %s\n", ptsname, strerror(errno));

so I suppose that's it. Write up a new program that calls the ioctl
on a tty, and you should be done. IOW:

#include <sys/ioctl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <fcntl.h>

int main(void)
{
	int fd = open("/dev/ttyS0", O_RDWR);
	ioctl(0, TIOCCONS, NULL);
	ioctl(fd, TIOCCONS, NULL);
}
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