printk

printk is a function that prints messages and is used in the C Programming Language exclusively for the Linux Kernel. It accepts a string parameter called the format string, which specifies a method for rendering an arbitrary number of varied data type parameter(s) into a string. The string is then printed to the kernel log.

It provides a printf-like abstraction and its parsing of the format string and arguments behave exactly the same way. It acts as a debugging tool for kernel programmers who need this function for logging messages from the kernel.

The printk function prototype is:

int printk(const char *fmt, ...);

In kernel mode, you can't use the standard C library, so printf is not available, hence printk.

Difference with printf

Logging Levels

printk has an optional prefix string: Loglevel.

Loglevel specifies the type of message being sent to the kernel message log. The syntax with loglevel is:

printk(KERN_DEBUG "Debug message shown!");

Different Loglevels are shown here:

KERN_EMERG	Emergency condition, system is probably dead
KERN_ALERT	Some problem has occurred, immediate attention is needed
KERN_CRIT	A critical condition
KERN_ERR	An error has occurred
KERN_WARNING	A warning
KERN_NOTICE	Normal message to take note of
KERN_INFO	Some information
KERN_DEBUG	Debug information related to the program

When a log level is not specified, the default log level is KERN_WARNING.

Loglevels are defined in <linux/kern_levels.h>. Which log levels are printed is configured in the /proc/sys/kernel/printk sysctl file (format: console level, default message level, minimum console level, default console level).

Pointer formats

The %p format is extended to support advanced parameter formatting. For example: %pISpc of a struct sockaddr * would print an IPv4/v6:port in the usual way (eg. "1.2.3.4:12345" or "[1:2:3:4:5:6:7:8]:12345" depending on the address family). Extensive format list in the Format Reference.

Description

The function tries to grab the console semaphore (console_sem). If it succeeds, the output is logged and the console drivers are called. If it is not possible to grab the semaphore the output is placed into the log buffer. The current holder of the console semaphore (console_sem) will notice the new output in release_console_sem and will send it to the consoles before releasing the semaphore.

One effect of this deferred printing is that code which calls printk and then changes console_loglevel may break. This is because console_loglevel is inspected when the actual printing occurs.

printk() can be called from anywhere in the Kernel at any time. It can be called from interrupt or process context. It can be called while a lock is held. It can be called simultaneously on multiple processors, yet it does not require the caller to hold a lock.

References

Format Reference
Linux Kernel Development
Manual page