linux-kernel - RE: printf internals

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Message-ID: <D83235F0F3C86D4D889D8B9A0DA8C6D7012AF540@corpexc01.corp.networkrobots.com>
Date:	Wed, 19 Dec 2007 14:33:15 -0800
From:	"Siva Prasad" <sprasad@...io.net>
To:	"Clemens Koller" <clemens.koller@...gramm.de>,
	"David Newall" <david@...idnewall.com>
Cc:	<linux-kernel@...r.kernel.org>
Subject: RE: printf internals

Thank you very much for your response Clemens.

I tried strace on a regular system. It does not show which tty, etc., as it uses the stdout (fd = 1) and write(1, ...) to it.

This is not a student project. I am trying to build my own kernel and ramdisk. Kernel boots fine to a point where it starts accessing ramdisk and executes init scripts. From there on nothing gets printed. I did some debugging and found that prints of user land programs are not coming to the serial console, while kernel prints are working fine. I found all the programs getting executed, by placing a printk in execve routine and printing the arguments.

So, I wanted to trace down the path from user program to the kernel and see why it is not printing messages from user program. I placed a printk in drivers/char/tty_io.c:tty_write() and it is not getting called from my file system. I tried the same thing on my good system (say regular PC) and it works as expected.

Any clues that can help debug this issue is highly appreciated.
How can I get access to the same printf string inside kernel.

Thanks
Siva

-----Original Message-----
From: Clemens Koller [mailto:clemens.koller@...gramm.de] 
Sent: Friday, December 14, 2007 8:16 AM
To: David Newall
Cc: Siva Prasad; linux-kernel@...r.kernel.org
Subject: Re: printf internals

David Newall schrieb:
 > Siva Prasad wrote:
 >> I am looking at how exactly does the printf in user programs succeeds in
 >> displaying characters to the serial console.
 >
 > Is it a student assignment?  This is so not the right mailing list.

Come on, are we playing hide and seek here?

You can use strace to follow a typical hello world example and
see what device it opens to feed the hello out.
I.e. it can be to the current /dev/ttyX and not to /dev/console
or if you are logged in via SSH to a machine, the device
is again different... or if you use screen, ...

Work your way from there and then use a Linux Source code
Cross Reference (lxr, ask Google for one) and follow the code.

Regards,
-- 
Clemens Koller
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