Would like to know a short program of Linux kernel code (not user
space) to show me how to write a character or string output to a disk
file.

I currently use printk for debug purpose and have to look at the
output at the message file, which is a very tedious process.

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Request for Question Clarification by maniac-ga on 10 Feb 2003 16:06 PST

Hello Ga1000,

Are you sure you *really* want to write to a file from kernel mode? A
quick search of the linux-kernel mailing list
  http://www.uwsg.iu.edu/hypermail/linux/kernel/0005.3/0061.html
suggests what you have asked is difficult and prone to bugs. I can
make such an answer, but it may be easier to suggest a few
alternatives including:

- continue to use printk, but start each message with a unique string
(e.g., ga1000:) and then use
  grep ga1000: /var/log/messages
to get just the messages you want or
  grep ga1000: /var/log/messages > ga1000.txt
to extract them into a file named ga1000.txt.
 - revise the syslog configuration file to capture "kernel debug"
messages in a separate file; mark all your printk messages as debug
messages.
 - review other techniques described in Linux Device Drivers, chapter
4
  http://www.xml.com/ldd/chapter/book/

Would you be interested in such a solution?

  --Maniac

---

Clarification of Question by ga1000-ga on 10 Feb 2003 16:52 PST

The system message file is too long and may have messages generated
earlier.

I will settle for a file that has the messages from the latest run
only, with some special strings, like 'flag1', 'flag2'.

By the way, the hardware used in an earlier question is an in-house
product, not ready to use, because of the 'missing prompt sign' bug.

My current approach is to implement XON, XOFF, in case there are
missing characters. That may or may not solve the problem.

From my test, a control Z will bring back the prompt sign. It is
possible that the BASH interpreter waits for and does not the control
Z, which happens to be the EOF.

Control C also can bring the prompt sign back. But that is probably a
forced action, not the character the Shell is looking for.

I have asked another question regarding ASCII character meanings.

Hello Ga1000,

You are asking for a way to gather debugging messages from the "latest
run".

Using the existing syslog facility, add
  kern.debug    /var/log/kern.debug
to capture the debugging messages to the file /var/log/kern.debug.

Restart the syslogd server to make the changes take effect
  killall -HUP syslogd
(or similar method)

In your driver use
  printk ( KERN_DEBUG "-- your message here--" );
to generate the debug messages. Run your test, review the output in
/var/log/kern.debug. Before starting the next run, you can "rotate"
the log file so you can check the progress of separate runs.

To "rotate" the log file, a command such as logrotate can be used
  logrotate -f -s /var/lib/logrotate.kd.status /etc/logrotate.kd
where /etc/logrotate.kd has contents
  /var/log/kern.debug {
    rotate 10
    daily
    postrotate
      /sbin/killall -HUP syslogd
    endscript
  }
which will keep logfiles for the last 10 tests in separate files named
/var/log/kern.debug.1, .2, etc.

If you need to separate data more finely, unique strings would still
work - use grep on the log file to pull out those specific items.

References:
Syslog.conf man page for Linux:
  http://www.die.net/doc/linux/man/man5/syslog.conf.5.html
Example code using printk ( KERN_DEBUG ...)
  http://www.linuxdevices.com/eljonline/misc/6226l1.html
Man page for logrotate
    http://nodevice.com/sections/ManIndex/man0735.html

  --Maniac