Where does output of print in kernel go?

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?

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.

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.

You'll get the output with the command dmesg

Abhisheietk

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

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.

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.

In centos (Atleast in centos 6.6) the output will be in /var/log/messages

来源：https://stackoverflow.com/questions/4518420/where-does-output-of-print-in-kernel-go

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Linux

Ubuntu

linux-kernel

printk