Printing¶

`printk`¶

Tip

You probably shouldn't use printk, but the pr_### functions instead.

Source

printk(KERN_INFO "Message: %s\n", arg);

Note

The KERN_### part of the printk call is not a separate argument, it concatenates to the message.

# current, default, minimum, boot
$ cat /proc/sys/kernel/printk
4        4        1        7

Using sysfs:

echo 8 > /proc/sys/kernel/printk

Using dmesg:

dmesg -n 8

Name	String	Alias function
KERN_EMERG	“0”	pr_emerg()
KERN_ALERT	“1”	pr_alert()
KERN_CRIT	“2”	pr_crit()
KERN_ERR	“3”	pr_err()
KERN_WARNING	“4”	pr_warn()
KERN_NOTICE	“5”	pr_notice()
KERN_INFO	“6”	pr_info()
KERN_DEBUG	“7”	pr_debug() and pr_devel() if DEBUG is defined
KERN_DEFAULT	“”
KERN_CONT	“c”	pr_cont()

The pr_### functions utilise an internal function called pr_fmt which you can overwrite locally in your own kernel driver.

For example, you can force every message to be prepended with the module and function name, saving a lot of duplication and potential future change:

#define pr_fmt(fmt) "%s:%s: " fmt, KBUILD_MODNAME, __func__