Kernel message logging with printk

Kernel message logging with printk

printk() is one of the most widely known functions in the Linux kernel. It's the standard tool we have for printing messages and usually the most basic way of tracing and debugging. If you're familiar with printf(3) you can tell printk() is based on it, although it has some functional differences:

• printk() messages can specify a log level.

• the format string, while largely compatible with C99, doesn't follow the exact same specification. It has some extensions and a few limitations (no ``%n`` or floating point conversion specifiers). See :ref:`How to get printk format specifiers right <printk-specifiers>`.

All printk() messages are printed to the kernel log buffer, which is a ring buffer exported to userspace through /dev/kmsg. The usual way to read it is using ``dmesg``.

printk() is typically used like this::

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

where ``KERN_INFO`` is the log level (note that it's concatenated to the format string, the log level is not a separate argument). The available log levels are:

+----------------+--------+-----------------------------------------------+
| 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()                                     |
+----------------+--------+-----------------------------------------------+ 

You can check the current console_loglevel with:

$ cat /proc/sys/kernel/printk
4        4        1        7

The result shows the current, default, minimum and boot-time-default log levels.

To change the current console_loglevel simply write the desired level to /proc/sys/kernel/printk.

For example, to print all messages to the console:

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

Another way, using dmesg:

# dmesg -n 5

Reference: https://www.kernel.org/doc/html/v5.12-rc3/core-api/printk-basics.html