1.. SPDX-License-Identifier: GPL-2.0 2 3=========================== 4Message logging with printk 5=========================== 6 7printk() is one of the most widely known functions in the Linux kernel. It's the 8standard tool we have for printing messages and usually the most basic way of 9tracing 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 the 15 exact same specification. It has some extensions and a few limitations 16 (no ``%n`` or floating point conversion specifiers). See :ref:`How to get 17 printk format specifiers right <printk-specifiers>`. 18 19All printk() messages are printed to the kernel log buffer, which is a ring 20buffer exported to userspace through /dev/kmsg. The usual way to read it is 21using ``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 format 28string, 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 whether 56to show the message immediately (printing it to the current console) depending 57on its log level and the current *console_loglevel* (a kernel variable). If the 58message 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/printk 67 4 4 1 7 68 69The result shows the *current*, *default*, *minimum* and *boot-time-default* log 70levels. 71 72To change the current console_loglevel simply write the desired level to 73``/proc/sys/kernel/printk``. For example, to print all messages to the console:: 74 75 # echo 8 > /proc/sys/kernel/printk 76 77Another way, using ``dmesg``:: 78 79 # dmesg -n 5 80 81sets the console_loglevel to print KERN_WARNING (4) or more severe messages to 82console. See ``dmesg(1)`` for more information. 83 84As an alternative to printk() you can use the ``pr_*()`` aliases for 85logging. This family of macros embed the log level in the macro names. For 86example:: 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 a 93common definition for the format string through the pr_fmt() macro. For 94instance, 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 name 100that 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`` (or 104also ``CONFIG_DYNAMIC_DEBUG`` in the case of pr_debug()) is defined. 105 106 107Function reference 108================== 109 110.. kernel-doc:: kernel/printk/printk.c 111 :functions: printk 112 113.. kernel-doc:: include/linux/printk.h 114 :functions: pr_emerg pr_alert pr_crit pr_err pr_warn pr_notice pr_info 115 pr_fmt pr_debug pr_devel pr_cont 116