1 .. SPDX-License-Identifier: GPL-2.0
2
3 ===========================
4 Message logging with printk
5 ===========================
6
7 printk() is one of the most widely known funct
8 standard tool we have for printing messages an
9 tracing and debugging. If you're familiar with
10 is based on it, although it has some functiona
11
12 - printk() messages can specify a log level.
13
14 - the format string, while largely compatibl
15 exact same specification. It has some exte
16 (no ``%n`` or floating point conversion sp
17 printk format specifiers right <printk-spe
18
19 All printk() messages are printed to the kerne
20 buffer exported to userspace through /dev/kmsg
21 using ``dmesg``.
22
23 printk() is typically used like this::
24
25 printk(KERN_INFO "Message: %s\n", arg);
26
27 where ``KERN_INFO`` is the log level (note tha
28 string, the log level is not a separate argume
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_
48 +----------------+--------+-------------------
49 | KERN_DEFAULT | "" |
50 +----------------+--------+-------------------
51 | KERN_CONT | "c" | pr_cont()
52 +----------------+--------+-------------------
53
54
55 The log level specifies the importance of a me
56 to show the message immediately (printing it t
57 on its log level and the current *console_logl
58 message priority is higher (lower log level va
59 the message will be printed to the console.
60
61 If the log level is omitted, the message is pr
62 level.
63
64 You can check the current *console_loglevel* w
65
66 $ cat /proc/sys/kernel/printk
67 4 4 1 7
68
69 The result shows the *current*, *default*, *mi
70 levels.
71
72 To change the current console_loglevel simply
73 ``/proc/sys/kernel/printk``. For example, to p
74
75 # echo 8 > /proc/sys/kernel/printk
76
77 Another way, using ``dmesg``::
78
79 # dmesg -n 5
80
81 sets the console_loglevel to print KERN_WARNIN
82 console. See ``dmesg(1)`` for more information
83
84 As an alternative to printk() you can use the
85 logging. This family of macros embed the log l
86 example::
87
88 pr_info("Info message no. %d\n", msg_num);
89
90 prints a ``KERN_INFO`` message.
91
92 Besides being more concise than the equivalent
93 common definition for the format string throug
94 instance, defining this at the top of a source
95 directive)::
96
97 #define pr_fmt(fmt) "%s:%s: " fmt, KBUILD_MO
98
99 would prefix every pr_*() message in that file
100 that originated the message.
101
102 For debugging purposes there are also two cond
103 pr_debug() and pr_devel(), which are compiled-
104 also ``CONFIG_DYNAMIC_DEBUG`` in the case of p
105
106
107 Function reference
108 ==================
109
110 .. kernel-doc:: include/linux/printk.h
111 :functions: printk pr_emerg pr_alert pr_cri
112 pr_fmt pr_debug pr_devel pr_cont
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