printk log buffer

printk (defined in kernel/printk/printk.c) fills the kernel log buffer with messages. The size of the log buffer is (1<<CONFIG_LOG_BUF_SHIFT)

Each message is associated with a log level:

• KERN_EMERG (0) - System is unusable
• KERN_ALERT (1) - Action must be taken immediately
• KERN_CRIT (2) - Critical conditions
• KERN_ERR (3) - Error conditions
• KERN_WARNING (4) - Warning conditions
• KERN_NOTICE (5) - Normal, but significant condition
• KERN_INFO (6) - Informational
• KERN_DEBUG (7) - Debug-level messages

Linux' kernel coding style has some notes on the preferred message format, such as

• Do not use crippled words like *dont*; use do not or don't instead
• Messages do not have to be terminated with a period.
• Printing numbers in parentheses (%d) adds no value and should be avoided.
• <linux/dev_printk.h> has a number macros that make sure messages are matched to the right device and driver, and are tagged with the right level: dev_err(), dev_warn(), dev_info() etc. (Compare with pr_info(), pr_warn(), pr_err() etc. which are defined in <linux/printk.h>).

It seems as though printk can only be used when PDA is initialized.

/proc/sys/kernel/printk

CONFIG_LOG_BUF_SHIFT

dmesg can be used to examine the kernel ring buffer.