Where does output of print in kernel go?

Question

I am debugging a driver for linux (specifically ubuntu server 9.04), and there are several printf statements in the code.

Where can I view the output of these statem

Answer 1

Many times KERN_DEBUG level messages are filtered and you need to explicitly increase the logging level. You can see what the system defaults are by examining /proc/sys/kernel/printk. For example, on my system:

# cat /proc/sys/kernel/printk
4       4       1       7

the first number shows the console log level is KERN_WARNING (see proc(5) man pages for more information). This means KERN_NOTICE, KERN_INFO, and KERN_DEBUG messages will be filtered from the console. To increase the logging level or verbosity, use dmesg

$ sudo dmesg -n 7
$ cat /proc/sys/kernel/printk
7       4       1       7

Here, setting the level to 7 (KERN_DEBUG) will allow all levels of messages to appear on the console. To automate this, add loglevel=N to the kernel boot parameters where N is the log level you want going to the console or ignore_loglevel to print all kernel messages to the console.

Answer 2

You might try a higher level than KERN_DEBUG, for example KERN_INFO. Depending on your configuration the lowest priority messages might not be displayed.

Answer 3

It depends on the distribution, but many use klogd(8) to get the messages from the kernel and will either log them to a file (sometimes /var/log/dmesg or /var/log/kernel) or to the system log via syslog(3). In the latter case, where the log entries end up will depend on the configuration of syslogd(8).

One note about the dmesg command: Kernel messages are stored in a circular buffer, so large amounts of output will be overwritten.

Answer 4

dmesg outputs all the messages from the kernel. Finding your desired messages would be difficult. Better use dmesg and grep combination and use a driver specific label in all your printk messages. That will ease in eliminating all the unwanted messages.

printk("test: hello world")

dmesg | grep test

Answer 5

I had this problem on Ubuntu 11.10 and 10.04 LTS, on the former I edited /etc/rsyslog.d/50-default.conf, then restarted rsyslog using "sudo service rsyslog restart" to restart rsyslogd. Then it worked.

Note that Ubuntu uses *r*syslogd, not syslogd.

Answer 6

You'll get the output with the command dmesg