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From: Arvid Brodin <>
To: "linux-os (Dick Johnson)" <>
Subject: Re: Print long messages to console from kernel module
Date: Tue, 26 Feb 2008 13:58:55 +0100	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On 2008-02-25 23:27, linux-os (Dick Johnson) wrote:
> On Mon, 25 Feb 2008, Arvid Brodin wrote:
>> I need to write messages > 1023 characters long to the console from a module*. printk() is limited to 1023 characters, and splitting the message over several printk()'s results in a line break and "Month hh:mm:ss host kernel:" being inserted in my text.
>> I tried including <linux/console.h> and using the console_drivers declared there, but get
>> "WARNING: "console_drivers" [<path>/log.ko] undefined!" when compiling and
>> "insmod: error inserting 'log.ko': -1 Unknown symbol in module" when insmodding.
>> I guess this is because non EXPORT_SYMBOL'd symbols are only accessible to statically linked code, and not to modules? I see in printk.c that console_drivers is set up there, and I haven't been able to find any other interface to console_drivers.
>> In short: is there any way to print messages to the console from a kernel module, except printk()? Is opening /dev/tty and writing to it the way to go?
>> * I'm writing an in-memory logger to be included in a module. The log can be several megabytes. The idea is to use SysRq to print the contents of the log to console after a kernel panic or otherwise when writing to disk might not work.
> Write the data to a kernel buffer. Impliment read() or ioctl() and
> poll(). Have a user-mode task sleep in poll, waiting for data to
> become available. That user-mode task can do anything it wants,
> unrestricted, with the data including writing it to files or any
> tty it wants to open.

Thank you for your answer. However, I don't see how a user-mode task will help me print my log after a kernel panic, through SysRq? Please clarify.

What we want is essentially a replacement for printk(), where the messages are instead logged in a big ring buffer, and can be printed with Alt-SysRq-l when need be. And the problem is the actual printing of the buffer to the console, since printk() inserts its timestamp after every linebreak or 1023 characters, whichever comes first.

Arvid Brodin
Enea LCC

  reply	other threads:[~2008-02-26 12:57 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-02-25 17:30 Arvid Brodin
2008-02-25 22:27 ` linux-os (Dick Johnson)
2008-02-26 12:58   ` Arvid Brodin [this message]
2008-02-29 14:58 ` Arvid Brodin

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