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1.. SPDX-License-Identifier: GPL-2.02 3===========================4Message logging with printk5===========================6 7printk() is one of the most widely known functions in the Linux kernel. It's the8standard tool we have for printing messages and usually the most basic way of9tracing and debugging. If you're familiar with printf(3) you can tell printk()10is based on it, although it has some functional differences:11 12 - printk() messages can specify a log level.13 14 - the format string, while largely compatible with C99, doesn't follow the15 exact same specification. It has some extensions and a few limitations16 (no ``%n`` or floating point conversion specifiers). See :ref:`How to get17 printk format specifiers right <printk-specifiers>`.18 19All printk() messages are printed to the kernel log buffer, which is a ring20buffer exported to userspace through /dev/kmsg. The usual way to read it is21using ``dmesg``.22 23printk() is typically used like this::24 25 printk(KERN_INFO "Message: %s\n", arg);26 27where ``KERN_INFO`` is the log level (note that it's concatenated to the format28string, the log level is not a separate argument). The available log levels are:29 30+----------------+--------+-----------------------------------------------+31| Name | String | Alias function |32+================+========+===============================================+33| KERN_EMERG | "0" | pr_emerg() |34+----------------+--------+-----------------------------------------------+35| KERN_ALERT | "1" | pr_alert() |36+----------------+--------+-----------------------------------------------+37| KERN_CRIT | "2" | pr_crit() |38+----------------+--------+-----------------------------------------------+39| KERN_ERR | "3" | pr_err() |40+----------------+--------+-----------------------------------------------+41| KERN_WARNING | "4" | pr_warn() |42+----------------+--------+-----------------------------------------------+43| KERN_NOTICE | "5" | pr_notice() |44+----------------+--------+-----------------------------------------------+45| KERN_INFO | "6" | pr_info() |46+----------------+--------+-----------------------------------------------+47| KERN_DEBUG | "7" | pr_debug() and pr_devel() if DEBUG is defined |48+----------------+--------+-----------------------------------------------+49| KERN_DEFAULT | "" | |50+----------------+--------+-----------------------------------------------+51| KERN_CONT | "c" | pr_cont() |52+----------------+--------+-----------------------------------------------+53 54 55The log level specifies the importance of a message. The kernel decides whether56to show the message immediately (printing it to the current console) depending57on its log level and the current *console_loglevel* (a kernel variable). If the58message priority is higher (lower log level value) than the *console_loglevel*59the message will be printed to the console.60 61If the log level is omitted, the message is printed with ``KERN_DEFAULT``62level.63 64You can check the current *console_loglevel* with::65 66 $ cat /proc/sys/kernel/printk67 4 4 1 768 69The result shows the *current*, *default*, *minimum* and *boot-time-default* log70levels.71 72To change the current console_loglevel simply write the desired level to73``/proc/sys/kernel/printk``. For example, to print all messages to the console::74 75 # echo 8 > /proc/sys/kernel/printk76 77Another way, using ``dmesg``::78 79 # dmesg -n 580 81sets the console_loglevel to print KERN_WARNING (4) or more severe messages to82console. See ``dmesg(1)`` for more information.83 84As an alternative to printk() you can use the ``pr_*()`` aliases for85logging. This family of macros embed the log level in the macro names. For86example::87 88 pr_info("Info message no. %d\n", msg_num);89 90prints a ``KERN_INFO`` message.91 92Besides being more concise than the equivalent printk() calls, they can use a93common definition for the format string through the pr_fmt() macro. For94instance, defining this at the top of a source file (before any ``#include``95directive)::96 97 #define pr_fmt(fmt) "%s:%s: " fmt, KBUILD_MODNAME, __func__98 99would prefix every pr_*() message in that file with the module and function name100that originated the message.101 102For debugging purposes there are also two conditionally-compiled macros:103pr_debug() and pr_devel(), which are compiled-out unless ``DEBUG`` (or104also ``CONFIG_DYNAMIC_DEBUG`` in the case of pr_debug()) is defined.105 106 107Function reference108==================109 110.. kernel-doc:: include/linux/printk.h111 :functions: printk pr_emerg pr_alert pr_crit pr_err pr_warn pr_notice pr_info112 pr_fmt pr_debug pr_devel pr_cont113