Some of the content might be incorrect, since I am still trying to understand it throughly.

So I started studying the Linux Kernel more in depth, so I decided it would be nice for me to document my findings, so I can read it from time to time. I will be daily updating this post, as I am studying it everyday.

Keep in mind that I am using the x86 architecture.

printk(const char * fmt, ...)

The printk is meant to print a kernel message, which you can later read using the dmesg command. It provides a printf-like abstraction. It can be used as a debugging tool for logging messages from the kernel.

In kernel mode, you cannot use printf.

The * fmt argument is a format string, whereas the ... are variable arguments.

The include/linux/kern_levels.h defines 8 different log levels which specifies the severity of the error message. Those are:

#define KERN_EMERG      KERN_SOH "0"    /* system is unusable */
#define KERN_ALERT      KERN_SOH "1"    /* action must be taken immediately */
#define KERN_CRIT       KERN_SOH "2"    /* critical conditions */
#define KERN_ERR        KERN_SOH "3"    /* error conditions */
#define KERN_WARNING    KERN_SOH "4"    /* warning conditions */
#define KERN_NOTICE     KERN_SOH "5"    /* normal but significant condition */
#define KERN_INFO       KERN_SOH "6"    /* informational */
#define KERN_DEBUG      KERN_SOH "7"    /* debug-level messages */

Comments are pretty clear what each one means. With all that, we can easily call printk like this: printk(KERN_ERR "Something happend). By default, KERN_WARNING is used when nothing is specified, though this can be changed by setting CONFIG_DEFAULT_MESSAGE_LOGLEVEL kernel option (make menuconfig -> Kernel Hacking -> Default message log level).

For convenience, Linux also provides shorthand definition to those calls:

 * These can be used to print at the various log levels.
 * All of these will print unconditionally, although note that pr_debug()
 * and other debug macros are compiled out unless either DEBUG is defined
#define pr_emerg(fmt, ...) \
	printk(KERN_EMERG pr_fmt(fmt), ##__VA_ARGS__)
#define pr_alert(fmt, ...) \
	printk(KERN_ALERT pr_fmt(fmt), ##__VA_ARGS__)
#define pr_crit(fmt, ...) \
	printk(KERN_CRIT pr_fmt(fmt), ##__VA_ARGS__)
#define pr_err(fmt, ...) \
	printk(KERN_ERR pr_fmt(fmt), ##__VA_ARGS__)
#define pr_warning(fmt, ...) \
	printk(KERN_WARNING pr_fmt(fmt), ##__VA_ARGS__)
#define pr_warn pr_warning
#define pr_notice(fmt, ...) \
	printk(KERN_NOTICE pr_fmt(fmt), ##__VA_ARGS__)
#define pr_info(fmt, ...) \
printk(KERN_INFO pr_fmt(fmt), ##__VA_ARGS__)

Meaning instead of calling printk(KERN_EMERG "System is corrupted!") we could call pr_emerg("System is corrupted") which is basically the same thing. Unless you compile your kernel in DEBUG mode, you can also use pr_debug and KERN_DEBUG.

Log level allows the kernel to determine the importance of a message, with that, it can decide whether it should present the message immediately to the user (printing in the console, etc).

The #define console_loglevel (console_printk[0]) is used to compare the log level of the message against this defined variable. If the priority is > than this value, the message will then be printed to the current console. Note that console_loglevel's value comes from console_printk[0] which is defined as an array (extern int console_printk[]). kernel/printk/printk.c defines each index value:

int console_printk[4] = {
	CONSOLE_LOGLEVEL_DEFAULT,	/* console_loglevel */
	MESSAGE_LOGLEVEL_DEFAULT,	/* default_message_loglevel */
	CONSOLE_LOGLEVEL_MIN,		/* minimum_console_loglevel */
	CONSOLE_LOGLEVEL_DEFAULT,	/* default_console_loglevel */

Your current console_loglevel can be found by printing /proc/sys/kernel/printk:

$ cat /proc/sys/kernel/printk
4       4       1       4

From left to right, the meaning of those values are as follow: current, default, minimum, boot-time-default. For example, if you would like to get all messages printed to your current console, you can simply change these values to 8:

$ echo 8 > /proc/sys/kernel/printk

Or by setting the log level through dmesg using the -n argument:

$ dmesg -n 8