linux-kernel - Re: serial console _after_ boot ?

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Date:	Thu, 03 Jan 2008 15:39:43 +0100
From:	devzero@....de
To:	Jan Engelhardt <jengelh@...putergmbh.de>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: serial console _after_ boot ? - was: Redirect kernel console

fantastic, thanks!

unfortunately......

opensuse103:/home/roland/serialcons # ./mytioccons
ioctl: Device or resource busy

but, it`s not userspace:

opensuse103:/home/roland # lsof |egrep "console|ttyS0"
opensuse103:/home/roland #


in tty_io.c -> ticoccons() there is

        if (redirect) {
                spin_unlock(&redirect_lock);
                return -EBUSY;
        }


but i`m not deep enough into programming to understand this.

-------------------
mytioccons.c

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

int main(void)
{
int fd = open("/dev/ttyS0", O_RDWR);
int errno;

ioctl(0, TIOCCONS, NULL);

if (ioctl(fd, TIOCCONS, NULL) < 0)
        printf("ioctl: %s\n", strerror(errno));
}
-------------------




> -----Ursprüngliche Nachricht-----
> Von: "Jan Engelhardt" <jengelh@...putergmbh.de>
> Gesendet: 03.01.08 14:28:45
> An: devzero@....de
> CC: linux-kernel@...r.kernel.org
> Betreff: 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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