1 .. SPDX-License-Identifier: GPL-2.0 1 .. SPDX-License-Identifier: GPL-2.0
2 2
3 =========================== 3 ===========================
4 Message logging with printk 4 Message logging with printk
5 =========================== 5 ===========================
6 6
7 printk() is one of the most widely known funct 7 printk() is one of the most widely known functions in the Linux kernel. It's the
8 standard tool we have for printing messages an 8 standard tool we have for printing messages and usually the most basic way of
9 tracing and debugging. If you're familiar with 9 tracing and debugging. If you're familiar with printf(3) you can tell printk()
10 is based on it, although it has some functiona 10 is based on it, although it has some functional differences:
11 11
12 - printk() messages can specify a log level. 12 - printk() messages can specify a log level.
13 13
14 - the format string, while largely compatibl 14 - the format string, while largely compatible with C99, doesn't follow the
15 exact same specification. It has some exte 15 exact same specification. It has some extensions and a few limitations
16 (no ``%n`` or floating point conversion sp 16 (no ``%n`` or floating point conversion specifiers). See :ref:`How to get
17 printk format specifiers right <printk-spe 17 printk format specifiers right <printk-specifiers>`.
18 18
19 All printk() messages are printed to the kerne 19 All printk() messages are printed to the kernel log buffer, which is a ring
20 buffer exported to userspace through /dev/kmsg 20 buffer exported to userspace through /dev/kmsg. The usual way to read it is
21 using ``dmesg``. 21 using ``dmesg``.
22 22
23 printk() is typically used like this:: 23 printk() is typically used like this::
24 24
25 printk(KERN_INFO "Message: %s\n", arg); 25 printk(KERN_INFO "Message: %s\n", arg);
26 26
27 where ``KERN_INFO`` is the log level (note tha 27 where ``KERN_INFO`` is the log level (note that it's concatenated to the format
28 string, the log level is not a separate argume 28 string, the log level is not a separate argument). The available log levels are:
29 29
30 +----------------+--------+------------------- 30 +----------------+--------+-----------------------------------------------+
31 | Name | String | Alias function 31 | Name | String | Alias function |
32 +================+========+=================== 32 +================+========+===============================================+
33 | KERN_EMERG | "0" | pr_emerg() 33 | KERN_EMERG | "0" | pr_emerg() |
34 +----------------+--------+------------------- 34 +----------------+--------+-----------------------------------------------+
35 | KERN_ALERT | "1" | pr_alert() 35 | KERN_ALERT | "1" | pr_alert() |
36 +----------------+--------+------------------- 36 +----------------+--------+-----------------------------------------------+
37 | KERN_CRIT | "2" | pr_crit() 37 | KERN_CRIT | "2" | pr_crit() |
38 +----------------+--------+------------------- 38 +----------------+--------+-----------------------------------------------+
39 | KERN_ERR | "3" | pr_err() 39 | KERN_ERR | "3" | pr_err() |
40 +----------------+--------+------------------- 40 +----------------+--------+-----------------------------------------------+
41 | KERN_WARNING | "4" | pr_warn() 41 | KERN_WARNING | "4" | pr_warn() |
42 +----------------+--------+------------------- 42 +----------------+--------+-----------------------------------------------+
43 | KERN_NOTICE | "5" | pr_notice() 43 | KERN_NOTICE | "5" | pr_notice() |
44 +----------------+--------+------------------- 44 +----------------+--------+-----------------------------------------------+
45 | KERN_INFO | "6" | pr_info() 45 | KERN_INFO | "6" | pr_info() |
46 +----------------+--------+------------------- 46 +----------------+--------+-----------------------------------------------+
47 | KERN_DEBUG | "7" | pr_debug() and pr_ 47 | KERN_DEBUG | "7" | pr_debug() and pr_devel() if DEBUG is defined |
48 +----------------+--------+------------------- 48 +----------------+--------+-----------------------------------------------+
49 | KERN_DEFAULT | "" | 49 | KERN_DEFAULT | "" | |
50 +----------------+--------+------------------- 50 +----------------+--------+-----------------------------------------------+
51 | KERN_CONT | "c" | pr_cont() 51 | KERN_CONT | "c" | pr_cont() |
52 +----------------+--------+------------------- 52 +----------------+--------+-----------------------------------------------+
53 53
54 54
55 The log level specifies the importance of a me 55 The log level specifies the importance of a message. The kernel decides whether
56 to show the message immediately (printing it t 56 to show the message immediately (printing it to the current console) depending
57 on its log level and the current *console_logl 57 on its log level and the current *console_loglevel* (a kernel variable). If the
58 message priority is higher (lower log level va 58 message priority is higher (lower log level value) than the *console_loglevel*
59 the message will be printed to the console. 59 the message will be printed to the console.
60 60
61 If the log level is omitted, the message is pr 61 If the log level is omitted, the message is printed with ``KERN_DEFAULT``
62 level. 62 level.
63 63
64 You can check the current *console_loglevel* w 64 You can check the current *console_loglevel* with::
65 65
66 $ cat /proc/sys/kernel/printk 66 $ cat /proc/sys/kernel/printk
67 4 4 1 7 67 4 4 1 7
68 68
69 The result shows the *current*, *default*, *mi 69 The result shows the *current*, *default*, *minimum* and *boot-time-default* log
70 levels. 70 levels.
71 71
72 To change the current console_loglevel simply 72 To change the current console_loglevel simply write the desired level to
73 ``/proc/sys/kernel/printk``. For example, to p 73 ``/proc/sys/kernel/printk``. For example, to print all messages to the console::
74 74
75 # echo 8 > /proc/sys/kernel/printk 75 # echo 8 > /proc/sys/kernel/printk
76 76
77 Another way, using ``dmesg``:: 77 Another way, using ``dmesg``::
78 78
79 # dmesg -n 5 79 # dmesg -n 5
80 80
81 sets the console_loglevel to print KERN_WARNIN 81 sets the console_loglevel to print KERN_WARNING (4) or more severe messages to
82 console. See ``dmesg(1)`` for more information 82 console. See ``dmesg(1)`` for more information.
83 83
84 As an alternative to printk() you can use the 84 As an alternative to printk() you can use the ``pr_*()`` aliases for
85 logging. This family of macros embed the log l 85 logging. This family of macros embed the log level in the macro names. For
86 example:: 86 example::
87 87
88 pr_info("Info message no. %d\n", msg_num); 88 pr_info("Info message no. %d\n", msg_num);
89 89
90 prints a ``KERN_INFO`` message. 90 prints a ``KERN_INFO`` message.
91 91
92 Besides being more concise than the equivalent 92 Besides being more concise than the equivalent printk() calls, they can use a
93 common definition for the format string throug 93 common definition for the format string through the pr_fmt() macro. For
94 instance, defining this at the top of a source 94 instance, defining this at the top of a source file (before any ``#include``
95 directive):: 95 directive)::
96 96
97 #define pr_fmt(fmt) "%s:%s: " fmt, KBUILD_MO 97 #define pr_fmt(fmt) "%s:%s: " fmt, KBUILD_MODNAME, __func__
98 98
99 would prefix every pr_*() message in that file 99 would prefix every pr_*() message in that file with the module and function name
100 that originated the message. 100 that originated the message.
101 101
102 For debugging purposes there are also two cond 102 For debugging purposes there are also two conditionally-compiled macros:
103 pr_debug() and pr_devel(), which are compiled- 103 pr_debug() and pr_devel(), which are compiled-out unless ``DEBUG`` (or
104 also ``CONFIG_DYNAMIC_DEBUG`` in the case of p 104 also ``CONFIG_DYNAMIC_DEBUG`` in the case of pr_debug()) is defined.
105 105
106 106
107 Function reference 107 Function reference
108 ================== 108 ==================
109 109
110 .. kernel-doc:: include/linux/printk.h 110 .. kernel-doc:: include/linux/printk.h
111 :functions: printk pr_emerg pr_alert pr_cri 111 :functions: printk pr_emerg pr_alert pr_crit pr_err pr_warn pr_notice pr_info
112 pr_fmt pr_debug pr_devel pr_cont 112 pr_fmt pr_debug pr_devel pr_cont
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