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Subject: Re: How to handle concurrent access to /dev/ttyprintk ?
From:   Samo =?UTF-8?Q?Poga=C4=8Dnik?= <samo_pogacnik@t-2.net>
To:     Petr Mladek <pmladek@suse.com>
Cc:     Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
        Jiri Slaby <jirislaby@kernel.org>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        John Ogness <john.ogness@linutronix.de>,
        linux-kernel@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Date:   Tue, 13 Apr 2021 17:22:46 +0200
In-Reply-To: <YHWrmuZ9tei0VTXN@alley>
References: <20210403041444.4081-1-penguin-kernel@I-love.SAKURA.ne.jp>
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Dne 13.04.2021 (tor) ob 16:32 +0200 je Petr Mladek napisal(a):
> On Tue 2021-04-13 13:10:50, Samo Pogačnik wrote:
> > Dne 13.04.2021 (tor) ob 11:41 +0200 je Petr Mladek napisal(a):
> > > On Mon 2021-04-12 14:41:27, Samo Pogačnik wrote:
> > > > Dne 12.04.2021 (pon) ob 19:39 +0900 je Tetsuo Handa napisal(a):
> > > > > What is the intended usage of /dev/ttyprintk ?
> > > > > 
> > > > 
> > > > The intended use of 'ttyprintk' is to redirect console to /dev/ttyprintk
> > > > via the TIOCCONS ioctl. After successfull redirection, all console
> > > > messages get "merged" with kernel messages and as such automatically
> > > > processed
> > > > (stored/transferred) by the syslog service for example.
> > > 
> > > The same can be achieved by /dev/kmsg that was created by systemd
> > > developers.
> > > 
> > 
> > 'kmsg' and 'ttyprintk' are different types of drivers and as such rather
> > complementary than exclusive. The 'ttyprintk' being a tty driver allows 
> > for a system wide automatic redirection of anything written to the
> > console.
> 
> I might miss something. But how can one setup ttyprintk as the system
> wide console? I do not see any code that would use ttyprintk
> in struct console.
> 

You can compile this simple code below and call:

# ./tioccons /dev/ttyprintk

... from now on all console output interleaves the kernel log  (you can check
dmesg or logs)

# ./tioccons /dev/console

... sets things back they were.

You will be able to recognize console messages by preceding "[U]" tag (meaning
User).

-------------------
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <fcntl.h>
#include <sys/ioctl.h>

int main(int argc, char **argv)
{
	int fd;

	if (argc != 2) {
		printf("Wrong usage!\n");
		exit(1);
	}

	if ((fd = open(argv[1], O_WRONLY)) == -1) {
		perror(argv[1]);
		exit(1);
	}

	if (ioctl(fd, TIOCCONS, NULL) == -1) {
		printf("ioctl: %s\n", strerror(errno));
		exit(1);
	}

	exit(0);
}
--------------------

best regards, Samo