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 statements?

EDIT1: What i'm trying to do is write to kernel using the proc file-system. The print code is

static int proc_fractel_config_write(struct file *file, const char *argbuf, unsigned long count, void *data)
{
    printk(KERN_DEBUG "writing fractel config\n");
    ...

In kern.log, I see the following message when i try to overwrite the file /proc/net/madwifi/ath1/fractel_config (with varying time of course).

[ 8671.924873] proc write 
[ 8671.924919] 

Any explainations?

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.